Monograph
Conductometry – Conductivity Measurement Peter Bruttel
Conductometry – Conductivity Measurement
Peter A. Bruttel
All rights reserved, including those of translation. Printed by Metrohm Ltd., CH-9101 Herisau, Switzerland 8.028.5003 – 2004-11 Conductometry – Conductivity Measurement
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Metrohm Monograph 8.028.5003
Table of Contents 4
Foreword
4
Terms / definitions
21
Measuring instruments
22
Conductivity measuring cells
23
Calibration
26
Practical applications
41
Literature
42
Tables
Conductometry – Conductivity Measurement
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Foreword The measurement of the electrical (electrolytic) conductivity – conductometry – can look back on a long tradition. Conductivity measurements were already being made about 125 years ago. For approximately 50 years Metrohm has provided measuring instruments and accessories for its customers. This monograph is concerned only with the measurement of conductivity in solutions. Although conductivity measurements are carried out frequently, they have never achieved the wide range of application of potentiometry (e.g. pH, ion, redox measurements). The reason for this may be that in conductometry there is no generally valid equation (such as, for example, the Nernst equation) that permits the appropriate calculations. On the contrary, in conductometry experience, experiments and comparative measurements come to the fore instead of calculations. This monograph is intended to provide a basic familiarization course in conductometry in a short time. Help is provided by the alphabetically arranged terms and definitions as well as the practical examples.
Terms / Definitions Activity a In water and polar solvents ionic compounds dissociate into their single ions (e.g. NaCl → Na+ + Cl–). At normal concentrations, strong electrolytes (e.g. HCl, K2SO4) are completely dissociated, but weak electrolytes (e.g. NH3, CH3COOH) are always incompletely dissociated. However, strong electrolytes behave as if an incomplete dissociation were present. This is caused by the fact that interactions (associations) occur between the oppositely charged ions and these reduce the chemical potential. For this reason the electrical conductivity is not a linear function of the concentration. The effective concentration (c) is known as the activity a. (Of course, these interactions also occur with weak electrolytes, but can normally be neglected because of the low ionic concentration.) Activity coefficient In real ionic solutions the interaction (association tendency) causes the «mass action» of the ions to be smaller than would be expected from the weight present. The stoichiometric concentration c must therefore be multiplied with a correction factor so that the mass action law continues to remain valid. This concentration-independent factor is known as the activity coefficient fi, the effective concentration as the activity a (see above). Amount-of-substance concentration c(X) = amount-of-substance concentration of substance X in mol/L, e.g. c(KCl) = 0.01 mol/L. Ash The ash content of sugars can be determined by measuring the electrical conductivity. This method is much more rapid than the ashing process itself and provides usable results. The theoretical basis is that the majority of the ash components consist of electrolytes, whereas the sugar itself is a non-electrolyte. This means that the electrical conductivity of sugar solutions is practically only determined by their ash content.
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5.0 g sugar is dissolved in dist. H2O, the solution made up to 100 mL and mixed. The electrical conductivity is measured at 20 ±0.1 °C → K5 The electrical conductivity of the water used is measured at the same temperature → KW K5 = electrical conductivity in µS/cm of the 5% sugar solution
% ash = K5 – (0.9 x KW) x 0.0018 Calibrated Reference (767) With the 2.767.0010 Calibrated Reference, Metrohm can provide users of conductivity meters with a handy solar-cell-powered device for checking their instruments. The built-in resistors are certified. This means that it is possible to check the conductometer displays for electrical conductivity as well as the temperature (Pt 100 or Pt 1000 temperature sensor) and validate the instruments in this way. The same instrument is also suitable for checking pH/ mV meters, potentiometric and Karl Fischer titrators. See also the Calibration section. Calibration See the Calibration section. Calibration solutions These solutions are used for calibrating conductivity cells, i.e. for determining the cell constant. See also the Calibration section. Calibration solutions are solutions whose electrical conductivity γ is known exactly. It is best to use so-called secondary standards. These are certified and can be directly traced to standard reference materials (e.g. National Institute of Standards and Technology – NIST, USA – www.nist.gov). Of course, you can also make up your own calibration solutions by dissolving the corresponding salts in ultrapure water or by diluting concentrated solutions (e.g. c(KCl) = 0.1000 mol/L – Metrohm no. 6.2301.060). However, for electrical conductivities <100 µS/cm we recommend that you purchase these standards (they are extremely difficult to prepare by ordinary means). Sources are, for example: – Hamilton (5 µS/cm...100 µS/cm, accuracy ±1%) – Reagecon, Shannon (Ireland) – www.reagecon.com – (1 µS/cm…100 µS/cm). The following table shows the electrical conductivity of KCl solutions at two different temperatures: c(KCl) 0.001 mol/L 0.010 mol/L 0.100 mol/L
20 °C 133 µS/cm 1.28 mS/cm 11.67 mS/cm
25 °C 147 µS/cm 1.41 mS/cm 12.90 mS/cm
Cell constant In principle most instruments for measuring the electrical conductivity are instruments for measuring the resistance RX or the conductance GX = 1/RX of a measuring cell filled with the sample. Conductometry – Conductivity Measurement
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The relationship to the electrical conductivity γ, the real subject of interest, is provided by the cell constant c, which depends on the geometrical dimensions of the measuring cell: γ = c/RX = c x GX (S/cm) In a two-plate measuring cell the cell constant c is obtained from the area F and the distance d between the plates: c = d/F (cm/cm2) [c] = cm–1 Because the distribution of the lines of current is not ideal, this calculated value does not agree exactly with the effective cell constant of the measuring cell. The effective value of c can only be determined by calibration; see below in the «Calibration» section. Charge number The charge number zi gives the charge of the particular ion including its sign. Examples: NaCl → Na+ + Cl– CaCl2 → Ca
2+
z+ = +1; z– = –1
+ 2 Cl
–
K3PO4 → 3 K+ + PO43–
z+ = +2; z– = –1 z+ = +1; z– = –3
Conductance The conductance G is the reciprocal of the electrical resistance (1/R) and has the unit Siemens (S) = Ω–1. The electrical conductivity is obtained by multiplying it with the cell constant. Conductivity titrations only require the conductance G. Conductivity cells See section on Measuring cells/conductivity cells. Conductivity standards See Calibration solutions. Conductivity titration See conductometric titration Conductivity water Term (introduced by W. Ostwald) for highly pure water with a very small self-conductivity. It expresses the fact that such a water is suitable for even demanding studies in the field of electrical conductivity (e.g. preparation of Conductivity standards). The term also applies to the ultrapure water used in the pharmaceutical industry. USP Monograph 645 defines the specifications for such a water as <1.3 µS/cm at 25 °C. Conductometer See Instruments section. Conductometric titrations In addition to colored indicators, measuring the conductivity is one of the oldest methods for indicating titration endpoints. 6
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As at each point in the titration, all the ions present contribute to the conductivity, typical V-shaped titration curves are obtained. Example: Titration of HCl with NaOH H+ + Cl– + Na+ + OH– = H2O + Cl– + Na+ Thin lines contributions of the individual ions to the conductivity Bold lines total conductivity (produces titration curve)
Contact-free methods As the name says, in these methods (for measuring the electrical conductivity) there is no contact between the measuring cell electrodes and the sample. These methods are frequently called «electrode-free» which is, of course, incorrect. Two basic methods are used: 1. Capacitive method See under the term High-frequency measurement. 2. Inductive method In this case the sample forms a coupling loop between two windings of a transformer that are magnetically shielded from each other. Measuring frequencies of 50 Hz...500 Hz are used in this case. There is a relationship between the electrical conductivity of the sample and the voltage transferred from the primary to the secondary winding throughout a very wide range. This means that it is possible to evaluate the measured values by using the cell constant. Contact-free methods have the advantage that the measured values can never be falsified by polarization. There are also no corrosion or contamination problems with the electrodes. However, the cost and complexity of the necessary apparatus is considerably higher when compared with those for «normal» instruments. For this reason such methods are used, if at all, for industrial applications. Convection One of the three mechanisms (convection, migration, diffusion) by which ion transport can take place is convection – ions travel as a result of liquid flow, e.g. thermal convection (temperature gradient). Degree of dissociation Quantity that describes the extent to which dissociation occurs. It represents the ratio between the free ions and the total molecules present in the solution and is given either relatively or as a percentage (1 or 100% means complete dissociation, 0.5 or 50% means that only half the molecules are dissociated).
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Strong electrolytes such as HCl, HNO3, H2SO4, NaOH, KOH and their salts are always completely dissociated in aqueous solution. Dependency of electrical conductivity The electrical conductivity of a solution depends on: • The number of ions: The more ions a solution contains, the higher its electrical conductivity. • In general on the ionic mobility, which in turn depends on: – The type of ion: The smaller the ion, the more mobile it is and the better it conducts. H3O+, OH–, K+ and Cl– are good conductors. If hydration occurs (the ion surrounds itself with water molecules that make it larger) then the conductivity is reduced. – The solvent: The more polar a solvent is, the better the dissolved compounds it contains can ionize. Water is an ideal solvent for ionic compounds. In alcohols ionization decreases as the chain length increases (methanol > ethanol > propanol). In nonpolar organic solvents (e.g. chlorinated and non-chlorinated hydrocarbons) practically no ionization occurs. – The temperature: In contrast to solids, in solutions the electrical conductivity increases as the temperature increases by 1...9% per K, depending on the ion. – The viscosity: As the viscosity increases, the ionic mobility and therefore the electrical conductivity decreases. Dielectric constant D In electrochemistry the dielectric constant is important wherever opposite charges act on each other. Examples are dissociation or ionic interaction. The larger the relative dielectric constant (Dvacuum = 1), the better the ionic compounds dissociate in the particular solvent. A few examples: Solvent
D (20 °C)
Formamide
110
Water
80
Methanol
34
Ethanol
25
Acetone
21
Propanol
19
Chloroform
4.8
Hexane
1.9
Diffusion One of the three mechanisms (convection, migration, diffusion) by which ion transport can take place is diffusion – ions travel as a result of differences in chemical potential (concentration gradients). Diffusion is described quantitatively by Fick’s law.
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Dissociation In solvents with a high dielectric constant (polar solvents) ionic compounds break down into freely mobile single ions, e.g. KCl → K+ + Cl– A different type of dissociation occurs when a chemical compound with a heteropolar bond is dissolved in a protic solvent, e.g. CH3COOH + H2O → CH3COO– + H3O+ The most important solvent for dissociation is water. It has a high dielectric constant and is polar. Ionic solutions are electrically conductive and decompose when a direct current is applied (electrolysis). Ionic solutions are also known as electrolytes. Both strong and weak electrolytes occur; these are differentiated by their degree of dissociation. Dissociation constant This quantity is used to describe the ionic equilibrium in aqueous solutions of weak electrolytes and is defined by the following equation: K = (CA x CC) / CCA CA and CC are the concentrations of the anions and cations formed by dissociation, CCA is the concentration of the molecules that remain undissociated. K increases as the degree of dissociation increases and therefore represents a usable quantity for describing the strength of a weak acid or weak base. The negative common logarithm of K is also known as the pK value; pK = –log K. As all dissociation equilibria are temperature-dependent this also applies to K and pK. Dosimat Metrohm term for motor-powered piston burets. Dosimats are high-precision universal dosing devices that can be controlled manually or remotely. Dosimats are equipped with so-called Exchange Units. Dosino Metrohm term for a stepper-motor-controlled drive for precise dosing in small spaces. Dosing Units are attached to the dosing drive by a quick-action coupling and screwed directly onto the reagent bottle. Electrical conductivity The electrical conductivity γ is equal to the reciprocal of the electrical resistance (conductance G) multiplied by the cell constant c: γ R G l A c
Conductometry – Conductivity Measurement
= = = 1/R = = = l/A
conductivity resistance conductance length of measuring path cross sectional area cell constant
unit: S cm–1 (S m–1) unit: Ω unit: S (Siemens) = Ω–1 unit: cm (m) unit: cm2 (m2) unit: cm–1 (m–1) 9
The electrical conductivity is normally given in µS/cm or mS/cm (12.88 mS/cm = 1288 mS/m; 5 µS/cm = 500 µS/m). In American usage the terms mhos and µhos are frequently encountered. Definitions according to EN 27888 (1993) or ISO 7888. Electrolytes Electrolytes are substances that, in solutions or melts, undergo heterolytic dissociation into ions that conduct the electrical current. Electrolytes include acids, bases and salts. Strong electrolytes dissociate completely, weak electrolytes only partially. Emulsions Emulsions («water in oil» or «oil in water») normally belong to the group of non-electrolytes. Migration of ions in an electric field and the resulting electrical conductivity do not occur. However, a technical effect can be used for measurement – this is charge transport in an electric field. According to Coehn’s law, two phases that are immiscible are characterized by the occurrence of surface charges. The phase with the higher dielectric constant (water) assumes a positive charge, that with the lower one (oil) a negative charge. At maximum viscosity the stability of the emulsion is also at its maximum, but also has the lowest «electrical conductivity». This means that under defined measuring conditions conclusions can be drawn about the stability of emulsions. Reference: Dahms, G.H., Jung, A., Seidel, H. Predicting emulsion stability with focus on conductivity analysis Cosmetics & Toiletries Manufacture Worldwide 2003, p. 223–228
Equivalent conductivity The equivalent conductivity Λ* is a quantity that is primarily used in theoretical studies. It can be obtained from the molar conductivity Λ and the electrochemical valency ne: Λ* = Λ / ne The electrochemical valency ne can be calculated for a molecule that dissociates into ν– anions and ν+ cations with the corresponding valencies z– und z+ as follows:
ne = ν– x z– = ν+ x z+
Examples: NaCl → Na+ + Cl– AlCl3 → Al
3+
ν+ = ν– ne = 1
+ 3 Cl
–
ν+ = 1 and z+ = 3 ne = 3
Four-electrode measuring technique In particular when materials other than platinized platinum (e.g. steel) are used for electrodes, interference occurs and incorrect measured values are obtained because of polarization. The four-electrode measuring technique was introduced to prevent this. It is somewhere in the middle between the classical two-electrode method and contact-free methods. It uses two current and two voltage electrodes. The resistance RX between the two voltage electrodes, which is proportional to the conductance G, and the voltage drop resulting from RX are measured (high-impedance), amplified and compared against a reference voltage Uref in a controller. If there is a difference between the two voltages then this is 10
Metrohm Monograph 8.028.5003
compensated by the controller altering the oscillator potential Uvar. The current J flowing in the sensor area is then a measure of the required resistance RX or the conductance G, respectively: J = Uref/RX = Uref x GX Thanks to advances made in the fields of measuring and electrode technology we have been able to establish that, despite its advantages, this method with its extra costs for apparatus cannot replace the classical two-electrode measuring technique and that the extra costs involved for laboratory applications are not justified. High-frequency measurement This term is used in connection with capacitive, contact-free methods for measuring the electrical conductivity. It refers in particular to the relatively high frequencies (3 MHz...100 MHz) used for this type of measurement. The special measuring cells are constructed so that two ring-shaped metal electrodes are attached to the outer sides of a non-metallic cell body (e.g. glass beaker). The electric field penetrates the cell body and is influenced by the properties of the sample that it contains. A major component of all instruments used for measuring the high-frequency conductivity is an oscillating circuit. The contact-free measuring cell is located parallel to the rotary condenser of the oscillating circuit. If resonance tuning is carried out during the measurement (maximum value of voltage U), then evaluation is carried out by the reactive component method. In contrast, if after resonance tuning has been carried out the value of U is divided by the conductance G, the evaluation takes place according to the active component method. Both types of evaluation require calibration with different conductance standards and also produce different calibration curves. In comparison to the classical method, high-frequency measurement has practically no advantages – this is why it has never been able to establish itself on the market. Hydration In an aqueous solution all the ions are surrounded by a sheath of oriented water dipoles (hydration sheath). This phenomenon is known as hydration. Interionic interaction In infinitely dilute solutions no electrostatic attraction can occur between the oppositely charged anions and cations of a dissolved electrolyte. This condition would occur for an ion i at the hypothetical concentration ci = 0, which can only be achieved by extrapolation. As the concentration increases the ions move closer together. This causes an interaction between the ions in which each cation is surrounded by a cloud of oppositely charged anions. In the same way each anion is surrounded by a cloud of cations. Interionic interaction is particularly important for strong electrolytes. Ionic mobility Ii The product of the migration speed ui and the Faraday constant F is the ionic mobility. ui refers to a uniform field with a field strength of 1 V/cm. The distance covered by an ion i is given in cm. The ionic mobility depends on the temperature and the concentration. It usually Conductometry – Conductivity Measurement
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refers to 25 °C and an infinite dilution (measurement series extrapolated to ci = 0). Some examples: Cation
I+ (cm2 / Ω)
Anion
I– (cm2 / Ω)
H+
350
OH–
199
Li
39
Cl–
76
50
NO
71
+
Na
+
– 3
K+
74
CH3COO–
41
1⁄2 Mg2+
53
1⁄2 SO42–
80
1⁄2 Ca
60
1⁄2 CO
69
2+
2– 3
Ionic product of water Water undergoes a self-dissociation known as autoprotolysis: 2 H2O → H3O+ + OH– As a result of this self-dissociation, pure water has an electrical conductivity of 0.055 µS/cm at 25 °C or 0.039 µS/cm at 20 °C. Please note the high temperature coefficient of 5.8% per °C! Ionic strength The ionic strength is a measure of the interionic interaction occurring in the solution of an electrolyte. It is determined solely by the concentration ci and charge zi of the ions, and not by their characteristic features. The following applies for the ionic strength: J = (1/2) Σci x zi2 The calculation of the ionic strength J of a known molar concentration ci of a particular electrolyte is simplified by the fact that there is a multiplication factor for each type of electrolyte. It is calculated for 1-molar solutions and can then be used generally: Type of salt 1,1 1,2 2,2 1,3
Example KCl K2SO4 MgSO4 K3PO4
Factor 1 3 4 6
The ionic strength is obtained by multiplying the particular molar salt concentration by this factor. Example: c(MgSO4) = 0.0025 mol/L J = 4 x 0.0025 mol/L = 0.01 mol/L Ions are positively or negatively charged atoms or molecules that are formed by dissociation from compounds with ionogenic or heteropolar bonds. In an electric field the positively charged ions (cations) migrate to the cathode (negative pole), negatively charged ions (anions) to the anode (positive pole). 12
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In dilute solutions anions and cations migrate independently without any interactions in an electric field. Kohlrausch cells See section on Measuring cells/conductivity cells. Kohlrausch’s square-root law This law for strong electrolytes links the molar conductivity ΛC with the square root of the concentration c according to: ΛC = Λ0 – A The equation states that the molar conductivity extrapolated to infinite dilution decreases as the square root of the concentration. The constant A depends on the type of electrolyte. Limiting conductivity Λ0 is the limiting equivalent conductivity or limiting conductivity for short. Λ0 is the sum of the limiting conductivities of the cations (λ0+) and anions (λ0–) (law of independent ionic migration). For Λ see the definition under equivalent conductivity. Limiting conductivities of some ions in water at 25 °C Cations
Limiting conductivity (S x cm2 x mol–1)
Anions
Limiting conductivity (S x cm2 x mol–1)
H3O+
349.8
OH–
197.0
NH4+
73.7
SO42–
80.8
K
78.4
73.5
Br
Ba2+
63.2
I–
Ag
+
62.2
Cl
76.4
Ca2+
59.8
NO3–
71.5
Mg2+
53.1
ClO4–
68.0
Na
50.1
F
55.4
38.6
CH3COO–
+
+
Li+
–
76.5 –
–
40.9
Mass fraction w(X) mass fraction of the substance X in %, e.g. w(NaOH) = 25% Mass concentration
ρ(X) mass concentration of the substance X in g/L, e.g. ρ(NaCl) = 2.5 g/L Measuring cell
See section on Measuring cells/conductivity cells.
Measuring frequency For conductivities the measuring frequency has a decisive influence on the correctness of the measured conductance G. Interfering polarization effects can be eliminated by Conductometry – Conductivity Measurement
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increasing the measuring frequency. At the same time the usable measuring range is extended. On the other hand, it must also be taken into consideration that at high frequencies a «cable error» could occur as a result of capacitive «shunting». The following time-proven compromises apply: – At low conductances (not electrical conductivity) measurements are made at a frequency of 300 Hz. – At high conductances measurements are made at a frequency of 2.4 kHz (712 Conductometer). Measuring range The usable measuring range depends on the type of conductivity cell (platinized/nonplatinized), the cell constant and the measuring frequency. Unfortunately no universal measuring cell exists for the whole usable range. This is why Metrohm offers measuring cells with different cell constants. The following tables should make your choice easier: Cell constant
Measuring range
0.1 cm–1
0.1 µS/cm...20 µS/cm
1 cm–1
1 µS/cm...10 mS/cm
10 cm–1
10 µS/cm...100 mS/cm
100 cm–1
100 µS/cm...10 S/cm
Solution
Measuring cell
Cell constant
Measuring frequency
<20 µS/cm
non-platinized
small
300 Hz
<1 mS/cm
platinized
medium
300 Hz
>1 mS/cm
platinized
large
2.4 kHz
Migration One of the three mechanisms (convection, migration, diffusion) by which ion transport can take place – ions travel (or are transported) in an electric field (field gradient). Migration speed Electrical conductivity in aqueous solutions takes place by the charge transport of the ions. This means that anions and cations can be differentiated by their direction of movement (in a direct current field). The migration speed wi of an ion is obtained from the force Ki acting on the ion and the friction Ri inhibiting the migration. The force Ki is produced by electrostatic attraction (Coulomb’s law). On the other hand, the friction Ri depends on the ionic radius ri and the viscosity η of the solution. If E is the field strength and if the ion zi carries one electron charge e0, then the following applies for the migration speed wi: wi = Ki/Ri = (zi x e0 x E) / 300 x 6π x η x ri) If wi is divided by the field strength E, then for a field of 1 volt/cm we obtain for the migration speed ui: ui = wi/E 14
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Typical values for ui are about 10–4 cm/s. If ui is multiplied by the Faraday constant F we obtain the ionic mobility Ii. The sum of the ionic mobilities of the anions and cations of an electrolyte produces the molar conductivity Λ. Molar conductivity The molar conductivity Λ is defined as the quotient of the specific conductivity γ and the concentration c (mol/L) of the dissolved substance:
Non-electrolyte In contrast to electrolytes, non-electrolytes release no freely mobile anions and cations and therefore make no contribution to the electrical conductivity in aqueous solutions. Typical representatives of this group are e.g. alcohols, urea, sugar (raw sugar), nonionic surfactants and emulsions. Electrolytic contaminations may contribute to the electrical conductivity – see Ash. However, higher concentrations of non-electrolytes may influence the viscosity of the solution, which also affects the ionic mobility. Oscillometry See under high-frequency measurement. Ostwald’s dilution law This law applies for strong electrolytes and links the molar conductivity ΛC with the square root of the concentration c as follows: ΛC = Λ0 – A The equation states that the molar conductivity extrapolated to infinite dilution decreases as the square root of the concentration. The constant A depends on the type of electrolyte. See also Kohlrausch’s square-root law. Platinization Platinization is the deposition of finely dispersed platinum (platinum black) on smooth platinum electrodes. It is an important feature of all classical conductivity cells and is used to avoid polarization effects (and the measuring errors they cause), particularly at high electrical conductivities. Platinized measuring cells are not suitable for industrial applications. Application Bulletin no. 64 provides information about the pretreatment and platinization of measuring cells (the 712 Conductometer has a built-in 20 mA DC source). Polarization In the measurement of the electrical conductivity a number of technical measurement effects that produce incorrect measured values are grouped together under polarization. Their source lies at the electrode/solution boundary. Of chief interest here are the Conductometry – Conductivity Measurement
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polarization resistance, which is in series with RX (resistance of the solution), and the polarization capacity, which is also in series. Interference by polarization usually results in too low electrical conductivities being measured. Polarization depends mainly on the current density at the electrode. This current density can be kept negligibly small if platinized measuring cells are used together with a suitable, not too low measuring frequency. Reference temperature The electrical conductivity measured at a given temperature is converted to a reference temperature. The conversion is carried out (usually automatically) by using the temperature coefficient. The normal reference temperatures are 20 °C and 25 °C. Salinity Applications exist in which it is not the electrical conductivity that is of interest, but rather the total content of the dissolved salts. Separation into the individual ions cannot be achieved by measuring the conductivity, as each type of ion makes a different contribution to the total conductivity. This is why when the salinity is measured it is the electrical conductivity of the sample solution which is compared with those of pure NaCl solutions and the corresponding NaCl concentration is given. The salinity is given in mg/L or g/L NaCl as the TDS (Total Dissolved Solids). Solvents, non-polar Non-polar (also known as aprotic) solvents have no self-dissociation. Molecules with ionic bonding only decompose in them to form ions on rare occasions. Examples of acidic non-polar solvents are: pyridine, dimethylformamide (DMF), dimethylsulfoxide (DMSO) Examples of neutral polar solvents are: acetone, methyl isobutyl ketone (MIBK), acetonitrile, nitrobenzene, ethers, hydrocarbons and chlorinated hydrocarbons. «Normal» conductometers are not suitable for non-polar solvents such as chlorinated and non-chlorinated hydrocarbons, insulation oils or petrochemical products. In order to measure the electrical conductivity (insulation properties) of such products, special instruments are required. These work with voltages in the kV range and with special measuring cells. Solvents, polar Polar (also known as amphiprotic) solvents have an appreciable self-dissociation. In such solvents molecules with ionic bonding decompose by dissociation to form ions (the higher the dielectric constant, the better). The most polar solvent is water: 2 H2O
H3O+ + OH–
Alcohols also belong to the protic solvents, e.g. 2 CH3OH 16
CH3OH2+ + CH3O– Metrohm Monograph 8.028.5003
Examples of acidic polar solvents are: formic acid, glacial acetic acid, cresols, phenol Examples of basic polar solvents are: ethylenediamine, benzylamine, butylamine Examples of neutral polar solvents are: methanol, ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether Specific conductivity Earlier term for the electrical conductivity γ (EN and ISO standard). It was known as κ and had the same units, i.e. S/cm (from conductance G x cell constant c). Specific resistance The specific resistance ρ is the reciprocal of the specific conductivity κ – electrical conductivity γ – with the unit Ω x cm. In earlier days it was usual in water purification plants to give the water quality in these units. 1 µS/cm corresponds to a specific resistance of 1 MΩ x cm. Temperature coefficient
25 and
°C are temperatures at which the electrical conductivities have been measured.
The temperature coefficient can be given as reciprocal Kelvin or % per °C. The temperature coefficient depends primarily on the ions contained in the solution and shows seldom a linear behavior. We recommend that it is determined automatically by the 712 Conductometer. Temperature compensation See under the term Reference temperature. Temperature dependency The temperature dependency of the electrical conductivity can be explained by observations concerning the migration speed of the ions in an electric field. See also Walden’s rule. This can be used to explain at least the positive temperature coefficient. However, the quantitative relationships are complicated and calculations are therefore practically impossible. Even with uniform substances (e.g. KCl solutions) the temperature coefficient changes with the concentration. In mixtures all the ions contribute to a new, mixed temperature coefficient. If high accuracy is required the temperature coefficient must therefore be determined experimentally. See also under the terms Reference temperature, Temperature coefficient and Temperature compensation. Conductometry – Conductivity Measurement
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Titrando Metrohm name for the most modern titration system on the market. The Titrando provides flexibility at the user interface and dosing system. The basic unit can be extended to form a fully automated «supertitrator». Titrino Metrohm name for a whole range of titrators. The range of titrators covers everything from a simple endpoint titrator up to potentiometric and Karl Fischer titrators for universal use. Transference number The transference numbers n+ und n– describe the current fraction transported by the cation and anion respectively. n+ = λ+/Λ = u+ / (u+ + u–) n– = λ–/Λ = u– / (u+ + u–) Experimental determination of the transference numbers allows the calculation of the ionic conductivities and ionic mobilities. Validation Validation is the systematic checking of analysis procedures and/or measuring devices with the aim of ensuring that if defined SOPs (= Standard Operating Procedures) are observed then reliable and reproducible measurements and results will be obtained. See also the Calibration section. Viscosity The (dynamic) viscosity η is the property of a liquid to resist (by internal friction) the mutual laminar displacement of two neighboring layers. For Newtonian liquids at a given temperature, η is a material constant with the SI unit Pascal per second (Pa s–1). In connection with the measurement of conductivity there is the fact that, as the viscosity increases, the ionic mobility and therefore the electrical conductivity decreases and vice versa. See also Walden’s rule. Walden’s rule In its general form this rule states that the product of the ionic mobility Ii of an ion and the viscosity η of a solvent is constant: Ii x η = K An increase in the viscosity results in a reduction of the ionic mobility (and therefore the electrical conductivity) and vice versa. Water, self-conductivity Kohlrausch already recognized about 150 years ago that even the most overdone purification of the water by distillation resulted in a self-conductivity that could not be lowered any further. The cause is the self-dissociation of the water, which is also known as autoprotolysis: 2 H2O 18
H3O+ + OH– Metrohm Monograph 8.028.5003
This self-dissociation depends very strongly on the temperature. Examples are shown in the following table: °C
µS/cm
0
0.010
18
0.038
25
0.060
34
0.090
50
0.170
The very large temperature coefficient is conspicuous. Between 18 °C and 25 °C it is 8.3% per °C!
Conductometry – Conductivity Measurement
19
Instruments Instruments for measuring the electrical conductivity are called conductometers. They are instruments for measuring complex resistances with changing potentials. (In contrast to the measurement of purely ohmic resistances of metallic conductors, in liquids together with the measuring cell a whole network of resistances and capacitances exist.) It makes sense to have at least two frequencies available for the applied alternating potentials (see under Measuring frequency and Polarization). However, by the correct choice of the measuring frequency, the cell constant and the material of the measuring cell, purely ohmic relationships can be achieved. Under these conditions the electrical conductivity can be determined from the measured resistance. This means that an instrument for measuring the conductivity must be able to measure the electrical conductivity of liquids. The demands placed on such instruments for laboratory use have increased year by year. With the 712 Conductometer Metrohm provides the user with an instrument of the highest performance class that leaves nothing to be desired. The possibilities of this instrument are listed below: Types of measurement – Electrical conductivity – Standard (calibration solutions) – TDS (Total Dissolved Solids), expressed as mg/L NaCl or g/L NaCl – salinity – Titration (conductivity titration) – Temperature (of solution) Sensor connections One input each for conductivity measuring cell and temperature sensor (Pt 100 or Pt 1000) Measuring ranges – Electrical conductivity (automatic range switching): 0...2000 µS/cm and 0...20’000 mS/cm. Resolution: max. 41⁄2 digits are shown. – Temperature: –170.0...500.0 °C. Resolution: 0.1 °C. Compensation ranges, e.g. for titrations, background suppression – Electrical conductivity: 0...2000 µS/cm and 0...2000 mS/cm (0...2 S/cm) – Temperature: –170.0…500.0 °C Measuring frequencies 300 Hz or 2.4 kHz; automatic switching to the most suitable frequency or manual setting. Temperature coefficient Automatic determination with connected temperature sensor; TC = f(T) is calculated as a polynomial. Can also be entered manually. Range 0.00...9.99% per °C. 20
Reference temperature For automatic temperature compensation. Freely selectable, 20 °C or 25 °C are normal. Cell constant Automatic via determination – dialog-guided with automatic calculation, or manual input. Range 0.001...500/cm. Platinization DC output for platinizing conductivity cells. I = 20 mA. Interface RS 232 For connection to printer or PC. «Remote» I/O lines For connecting a sample changer or laboratory robot or for triggering any type of alarm. Analog output for electrical conductivity Connection of a lab recorder or titrator. Output signal 0...2000 mV with a resolution of 0.5 mV (12 bit). Analog output for temperature E.g. for connection of a lab recorder. Output signal 0...2000 mV with a resolution of 0.5 mV (12 bit). Instrument diagnosis The instruments carries out a self-diagnosis when switched on. The built-in diagnosis program allows the user to localize or exclude instrument faults.
Metrohm Monograph 8.028.5003
Conductivity measuring cells These measuring cells, which are also known as Kohlrausch cells, normally have two platinized platinum electrodes. By selecting the area of and the distance between the two electrodes it is possible to vary the cell constant of such electrodes throughout a wide range. Platinizing the electrodes greatly reduces the risk of obtaining incorrect measured values because of polarization. This also has a favorable effect on the usable measuring range. This means that a conductivity cell with a cell constant of c = 1 cm–1 can be used at a measuring frequency of 1 kHz from 10 µS/cm to 100 mS/cm. Smooth, i.e. non-platinized measuring cells should only be used for low electrical conductivities (<20 µS/cm). However, platinization also has its disadvantages. Platinized measuring cells are susceptible to encrustation, inclusion and also growth of algae, bacteria or mold. They also dry out during long storage or the platinization slowly breaks down. As a result of these effects the cell constant changes and has to be redetermined from time to time (and, of course, after every replatinization process). Some suggestions for treating and preparing the conductivity cells: – Place measuring cells that have been stored dry in acetone for approx. 30 min. Then thoroughly rinse with dist. H2O and place in dist. H2O for 2...3 h. – Frequently used measuring cells should be stored in dist. H2O. – Less frequently used measuring cells should be stored in 70% ethanol or stored dry (prevents biological growth). – Always thoroughly rinse the measuring cell with dist. H2O after use. Examples of measuring cells
Measuring cell with built-in Pt 1000 temperature sensor in PP shaft
Immersion measuring cell (c = 0.1...20 cm–1)
Conductometry – Conductivity Measurement
Flow-through measuring vessel: The illustration shows the 6.1420.100 flow-through measuring vessel. Together with the 6.0912.110 conductivity cell (screwed into the lid) a flow-through measuring cell is obtained
21
Calibration GLP (Good Laboratory Practice) requires, among other things, that the accuracy and precision of analytical instruments are checked regularly by using SOPs (Standard Operating Procedures). Calibration is used to validate the whole system that is used for making the measurements. In our case this means that instrument and the measuring cell(s) used must be calibrated from time to time. The following test procedures should be considered as guidelines. The limits they contain are merely examples. Depending on the accuracy required for the measuring system it may be necessary to redefine these limits in your own SOPs. Literature: Application Bulletin no. 272
A) Checking the instrument (using the 712 Conductometer as an example) Required accessories: – 2.767.0010 Calibrated Reference – 2 x 6.2106.020 cable (2 x plug B) – 1 x 6.2104.080 electrode cable (with plug B) Parameter settings at the Conductometer (712) for the two following tests: > cond/parameters cell constant
1.000 / cm
meas. temp.
25.0 °C
ref. temp.
25.0 °C
TC selection:
const.
TC const.
0.00% / °C
frequency:
auto
meas. type:
standard
The following equation applies: γ=Gxc γ = electrical conductivity (µS cm–1) G = conductance (µS) c = cell constant (cm–1)
At the 767 Calibrated Reference the cover over the solar cells is closed. The two banana plugs of the electrode cable are inserted into the input sockets (measuring cell) of the conductometer and the electrode plug screwed onto socket 5 or socket 6 of the 767. The two 6.2106.020 cables are now used to connect the two input sockets of the 712 with sockets 1 and 2, or 2 and 3, respectively, of the 767.
22
Metrohm Monograph 8.028.5003
Examples of measured value Socket(s)
Actual value µS/cm
Theoretical value µS/cm
Difference µS/cm
Error tolerance (±0.5%) µS/cm
5
70.07
70.01
0.06
±0.35
6
2.168
2.17
–0.002
±0.011
1&2
10010
10008
2
±50
2&3
999.2
999
0.2
±5
Evaluation: The checked instrument is OK. In order to check the temperature display, the two 6.2106.020 cables are used to connect both red input sockets of the des 712 with sockets 1 and 2 (Pt 100) or 2 and 3 (Pt 1000) of the 767. Examples of measured value Socket(s)
Actual value °C
Theoretical value °C
Difference °C
Error tolerance °C
1&2
–0.2
–0.20
00
±0.1
2&3
00.1
00.17
–0.07
±0.1
Evaluation: The checked instrument is OK. B) Checking the conductivity cell with calibration solution It is assumed that the conductivity cell is in a well-conditioned state. The measuring vessel is thoroughly rinsed, first with ultrapure water and then with the calibration solution. It is then filled with the calibration standard and thermostatted at 25 °C. The measuring cell is first immersed several times in the calibration solution and then positioned so that the upper openings at the sides are completely immersed in the liquid. Any air bubbles that are present inside the measuring cell can be removed by swirling and tapping. The Conductometer is switched on and the corresponding value for the cell constant (printed on the measuring cell) is entered in the 712. The measuring and reference temperatures are also entered in the 712. 712 parameters: > cond/parameters cell constant meas. temp. ref. temp. TC selection: TC const. frequency: meas. type:
value printed on electrode (e.g. 0.78 / cm) 25.0 °C 25.0 °C const. 0.00% / °C auto standard
Conductometry – Conductivity Measurement
23
Wait until the temperature is constant and then read off the measured value. If the KCl calibration solution from Metrohm (6.2301.060) is used then the electrical conductivity must be 12.90 ±0.15 mS/cm. New cell constant = theoretical conductivity / measured conductance Example c(KCl) = 0.1000 mol/L, 25 °C Electrical conductivity
γ
= 12.90 mS/cm
Displayed conductance
G
= 16.13 mS
New cell constant
c
= 12.90 mS/cm / 16.13 mS = 0.80 cm–1
24
Metrohm Monograph 8.028.5003
Practical applications 1. Determination of the cell constant c Metrohm conductivity cells are supplied with the cell constant printed on them. The use of the cells may cause the cell constant to change. This is why, if high-precision measurements are to be made, the cell constant must be checked or redetermined from time to time. KCl solutions of a known concentration are used for these determinations. For reasons of precision (weighing errors) a stock solution with c(KCl) = 0.1000 mol/L is either prepared or purchased and lower concentrations are prepared from it by dilution with conductivity water. The electrical conductivity of conductivity water is ≤1 µS/cm. c(KCl) = 0.1000 mol/L (Metrohm no. 6.2301.060) KCl reagent grade (e.g. Merck no. 104936) is dried at 105 °C for 2 h and allowed to cool down in a desiccator. 7.456 g is dissolved in conductivity water and made up to 1000 mL. The conductivity of this solution is: – at 20 °C: 11.67 mS/cm – at 25 °C: 12.88 mS/cm c(KCl) = 0.010 mol/L 10.0 mL c(KCl) = 0.1 mol/L is diluted to 100 mL with conductivity water. The conductivity of this solution is: – at 20 °C: 1.28 mS/cm – at 25 °C: 1.41 mS/cm c(KCl) = 0.001 mol/L This solution should only be prepared immediately before use. It cannot be stored. The conductivity water must be freed from carbon dioxide before use by passing nitrogen through it. 10.0 mL c(KCl) = 0.01 mol/L is diluted to 100 mL with degassed conductivity water. The conductivity of this solution is: – at 20 °C: 133 µS/cm – at 25 °C: 147 µS/cm Certified conductivity standards with a wide range (1 µS/cm...100 mS/cm) are sold, e.g., by Messrs Reagecon, Chemical Measurement Specialists, in Shannon Free Zone, Shannon, Co. Clare, Ireland; www.reagecon.com
Conductometry – Conductivity Measurement
25
Example of a determination at 20 °C, 712 Conductometer Instruments and accessories: – – – –
2.712.0010 Conductometer, 2.728.002X Magnetic Stirrer Conductivity cell, e.g. 6.0907.110 Printer and printer cable, thermostat, nitrogen from gas cylinder 6.1414.010 titration vessel lid, 6.1418.250 titration vessel with thermostat jacket and 6.1440.010 gas inlet and overflow tube
Reagents: – c(KCl) = 0.0100 mol/L – Acetone and conductivity water Procedure The measuring cell is placed in acetone for 30 min. It is rinsed with conductivity water and then allowed to stand in it for at least 2 h, preferably overnight. The titration vessel is first rinsed with c(KCl) = 0.01 mol/L and then filled with this solution. The measuring cell is also rinsed with c(KCl) = 0.01 mol/L and inserted in the titration vessel. The thermostat is used to bring the liquid in the titration vessel to 20.0 °C under stirring while nitrogen is bubbled through the solution for at least 10 min. The nitrogen is then allowed to blanket the solution. Care must be taken that there are no gas bubbles inside the measuring cell. The following settings are made on the 712 Conductometer (see also parameter report): Cell constant Measuring temperature Reference temperature TC selection: TC const. Frequency Meas. type: Range limit: Standard
1.0 / cm 20.0 °C 20.0 °C const. 0.0% / °C auto Standard off 1.278 mS/cm
The automatic determination is now started at the 712. If no thermostat is available then the following procedure can be used: The measuring cell and the solutions are prepared in exactly the same way. The use of nitrogen in the closed measuring cell and titration vessel is also identical. However, instead of a thermostat a Pt 1000 or Pt 100 temperature sensor (6.1110.100 and 6.1103.000 respectively) is connected. All the 712 Conductometer settings remain the same with the exception of the temperature coefficient, which is entered as 2.11% / °C. Calculation: New cell constant = theoretical conductivity / measured conductance Example: Theoretical conductivity = 1.28 mS/cm (0.010 mol/L KCl, 20 °C) Measured conductance = 1.533 mS 1.28 mS/cm / 1.533 mS = c = 0.835 cm–1 Example with 6.0907.110 conductivity cell: c = 0.8195 ±0.007 cm–1 (n = 10) 26
Metrohm Monograph 8.028.5003
2. Determination of the temperature coefficient α of c(Na2SO4) = 0.05 mol/L As explained in the theoretical section, the conductivity of ionic solutions is extremely temperature-dependent and this dependency is rarely linear. This is why we recommend that the TC in the temperature range of interest is recorded automatically at the Conductometer. Instruments and accessories: – 2.712.0010 Conductometer and 2.728.002X Magnetic Stirrer – e.g. 6.0912.110 conductivity cell with built-in temperature sensor (Pt 1000) – Printer and printer cable, thermostat – 6.1414.010 titration vessel lid, 6.1418.250 titration vessel with thermostat jacket Reagents: – c(Na2SO4) = 0.05 mol/L. 7.10 g Na2SO4 or 16.11 g Na2SO4 x 10 H2O is dissolved in conductivity water and made up to 1000 mL. – Conductivity water Procedure The conditioned measuring cell is rinsed with conductivity water and then with Na2SO4 solution. Sufficient Na2SO4 solution is filled into the measuring vessel and the measuring cell inserted and made bubble-free. The following settings are made on the 712 Conductometer (see also parameter report): Start temperature Stop temperature Reference temperature Frequency Temperature coefficient Cell constant
40.0 °C 15.0 °C 20.0 °C auto 0.00% actual value
The stirrer is switched on and the solution is brought to approx. 45 °C by using the thermostat. The automatic determination is started and the solution is allowed to cool down slowly (not faster than 1 °C / min). Example: Measured value; TC = 2.35 ±0.09% / °C (n = 8) 3. General remarks concerning conductivity measurements The measuring cell – The cell constant should match the liquid to be measured. Low conductivities require the use of a small cell constant, high conductivities a high one: c ≈ 0.1 cm–1
for poorly conducting solutions such as fully or partially deionized water
c ≈ 1 cm–1
for moderately conducting solutions such as drinking water, surface water, ground water and wastewater
c ≈ 10 cm–1
for solutions with good conductivity such as seawater, rinsing water, physiological solutions, etc.
Conductometry – Conductivity Measurement
27
c ≈ 100 cm–1
for solutions with very good conductivity such as brine, acids, alkalis, electroplating baths, etc.
– The measuring cell must be well conditioned/prepared. Measuring cells stored dry must be placed in acetone for 30 min. They are then rinsed in conductivity water and allowed to stand in it for at least 2 h, preferably overnight. Frequently used measuring cells are stored in conductivity water or 20% ethanol (to prevent the growth of microorganisms). Measuring cells that are only used occasionally should be stored dry. – Contaminated measuring cells cannot be used for conductivity measurements and must be cleaned. After cleaning they should be thoroughly rinsed with conductivity water. Possible sources of contamination: • Deposits of calcium carbonate or barium sulfate: Rinse with HCl. For BaSO4 immerse the cell overnight in a solution of w(Na2EDTA) = 10% in c(NaOH) = 0.1 mol/L under stirring. • Fat and oil residues: Rinse with acetone. If this does not help, saponify with ethanolic c(NaOH) = 1 mol/L at approx. 40 °C. • Algae or bacterial growth: Place in hot chromosulfuric acid. • Protein: Place in w(pepsin) = 5% in c(HCl) = 0.1 mol/L for 1...2 h. The instrument – Set the measuring frequency to «auto» – this ensures that the measurement is made at the optimal measuring frequency. Generally: Measure low conductivity at a low measuring frequency and high conductivity at a high one. – Temperature coefficient: If known then set on instrument, otherwise determine it. A further possibility is to use a thermostat and to perform the measurement at the reference temperature, which means that this setting is not necessary. – Temperature: It is best to use a measuring cell with a built-in temperature sensor. Otherwise connect a separate temperature sensor or use a thermometer – in the latter case the measuring temperature must be entered at the instrument. – Cell constant: Enter the cell constant marked on the measuring cell or determine the cell constant again. – Reference temperature: This is normally 20.0 °C; for some applications 25.0 °C is stipulated or preferred (water samples or, e.g., measurements in tropical countries). Any reference temperature can be entered at the 712 Conductometer. The measurement Only use well-conditioned measuring cells. It is best to rinse them with the solution to be measured before the measurement. Immerse the measuring cell in the measuring solution to an adequate depth and make sure it is bubble-free. The lateral openings must be fully immersed. Remove and re-immerse the measuring cell a few times. Wait until the temperature of the measuring cell and solution become constant.
28
Metrohm Monograph 8.028.5003
4. Conductivity measurements in water A) Wastewater, ground, mineral, surface and drinking water The reference temperature is normally 25.0 °C. In order to avoid errors by incorrectly selecting the temperature coefficient (TC) it is either recommended or stipulated that the sample solution is thermostatted at 25.0 °C. If this is not required then a TC can be entered as given in the table below, or TC Ident: «DIN» can be selected. The latter setting is suitable for water that contains mainly calcium and bicarbonate ions as well as small amounts of magnesium, sulfate, chloride and nitrate ions. Sample temperature
TC in % / °C
5...10 °C
2.62
10...15 °C
2.41
15...20 °C
2.23
20...25 °C
2.08
25...30 °C
1.94
30...35 °C
1.79
Examples: Tap water in Herisau, 25 °C
γ = 512.5 ±8.3 µS/cm (n= 10)
Mineral water, 25 °C
γ = 1.813 ±0.013 mS/cm (n= 10)
B) Deionized water Because of possible interferences, a special procedure must be selected for water with a conductivity of <5 µS/cm. This applies to water with a conductivity <1 µS/cm in particular! The most important interferences result from: – Absorption of CO2 (or other «conducting» gases) from the atmosphere. – Leaching of traces of Na and Ca from the glassware used. Both interferences cause the settings to drift and finally high-bias conductivities are measured. We recommend the following procedures to eliminate such interferences to as great an extent as possible: • Version 1 Perform flow-through measurements. Because of the small volume of the measuring setup we recommend to use the 6.0912.110 conductivity cell (PP shaft, built-in Pt 1000 temperature sensor) screwed into the 6.1420.100 flow-through vessel. The water is allowed to flow through the measuring setup and the conductivity is determined in the usual way. • Version 2 The measurement is carried out in as large a volume as possible. Nitrogen or argon should be passed through and over the solution, which should also be stirred. If possible, use a closed or covered setup. Please note the high TC (approx. 5.8% / °C) of such water samples!!
Conductometry – Conductivity Measurement
29
5. Salinity Applications exist in which it is not the electrical conductivity that is of interest, but rather the total content of the dissolved salts. Separation into the individual ions cannot be achieved by measuring the conductivity, as each type of ion makes a different contribution to the total conductivity. For this reason, when the salinity is measured it is the electrical conductivity of the sample solution that is compared with that of pure NaCl solutions and the corresponding NaCl concentration is given. The 712 Conductometer carries out this conversion automatically if the measuring mode TDS (Total Dissolved Solids) is selected. TDS appears on the 712 as mg/L or g/L NaCl in the dialog display instead of the cell constant. Examples: Tap water Herisau, 25 °C
TDS = 245.6 ±4.2 mg/L NaCl (n = 10)
Mineral water, 25 °C
TDS = 894.8 ±6.7 mg/L NaCl (n = 10)
6. Conductometric titrations For conductometric titrations the cell constant does not normally need to be known. A double platinum sheet electrode is used for the measuring electrode as the titration is usually carried out in a beaker. This electrode is easy to clean. Thermostatting is not required for simple titrations – room temperature is adequate. Alterations in conductivity caused by temperature variations are of virtually no consequence. Instruments and accessories: – 2.712.0010 Conductometer – Titrino with 2.728.0040 Magnetic Stirrer – 6.3026.220 Exchange Unit – 6.0309.100 double Pt-sheet electrode with 6.2104.080 electrode cable – 6.2116.000 connecting cable 712 – Titrino – PC with connecting cable for Titrino, printer and printer cable – E.g. Metrodata «VESUV 3.0 Light» for the manual evaluation of L- or V-shaped titration curves (VESUV = Metrodata Application software; Verification Support for Validation). Reagents: – c(AgNO3) = 0.01 mol/L as titrant (in Exchange Unit) – c(KCl) = 0.1000 mol/L, e.g. Metrohm no. 6.2301.060 – Conductivity water and acetone Procedure The instruments are connected together as described in the 712 Instructions for Use. 0.25 mL c(KCl) = 0.1 mol/L and 30 mL each of acetone and conductivity water are placed in a beaker. The electrode and buret tip are immersed and the titration with c(AgNO3) = 0.01 mol/L is started after the following instrument parameters have been set:
30
Metrohm Monograph 8.028.5003
712 Conductometer >cond/parameters cell constant 1.0/cm meas. temp. 20.0 °C ref. temp. 20.0 °C TC selection: const. TC const. 0.0% / °C frequency: auto meas. type: Titration limits: off >cond/analog output status: on polarity + 1 V range: 1 mS/cm 0 V at: 0 µS/cm offset: 0 mV >cond/limits status: off >cond/plot margins left: 0 µS/cm right: 1000 µS/cm
799 Titrino MET U Cond. titr. >titration parameters V step 0.10 mL dos. rate max. mL/min signal drift 50 mV/min equilibr. time 26 s start V: off pause 0s >stop conditions stop V: abs. stop V 4.0 mL filling rate max. mL/min >evaluation EP criterion 30 mV EP recognition all
Example of a titration (VESUV 3.0):
Evaluation L- and V-shaped titration curves are evaluated as follows: Tangents are applied to the two legs of the curve. The titration endpoint is defined as the intercept of the tangents. In the Metrodata VESUV 3.0 program this is carried out as follows: Move Tangent 1 along the curve with the key and the left-hand mouse key, Tangent 2 with the key and the left-hand mouse key. Conductometry – Conductivity Measurement
31
7. Conductivity measurements in deionized water according to USP guidelines Summary The electrical conductivity of ultrapure water is measured according to USP Method 645 (US Pharmacopeia; USP 23, Vol. 5) in a flow-through cell with a cell constant of c = 0.1 cm–1. Sample Ultrapure water from a «Barnstead Nanopure» unit, Application Lab, Herisau. Reagents Hamilton conductivity standard. 5 µS/cm ±5%; WO 1197985, P/N 238926 Instruments and accessories 712 Conductometer Conductivity cell PP, c = 0.091 cm–1 Conductivity cell glass, c = 0.082 cm–1 Flow-through cell 767 Calibrated Reference 728 Magnetic Stirrer Titration vessel Titration vessel lid 683 Pump Thermostat bath Cable 712 – PC (RS 232) Cable 712 – PC (RS 232) Metrodata VESUV 3.0
2.712.0010 6.1420.100 2.767.0010 2.728.0010 6.1418.250 6.1414.010 2.683.0010 6.2125.060 6.2125.010 6.6008.140
The schematic shows the connections.
32
Metrohm Monograph 8.028.5003
Fig. 2 Parameter settings at the 712 712 Conductometer OP1/106 712.0015 date 2001-03-07 time 15:28:11 conductivity >cond/parameters cell constant 0.091 / cm meas.temp. 25.0 °C ref.temp. 25.0 °C TC selection: const. TC const. 0.0 %/°C frequency: auto meas.type: standard range limit: OFF > cond/analog output status: OFF > cond/limits status: OFF > cond/plot margins left: 0 µS / cm right: 0 µS / cm ---------------------------------------config > config / print id. 1 id. 2 print header: always date & time ON send to: IBM > config / print meas. values print crit.: time time interval 300.0 s stop time 12000 s date & time ON > config / report type orig. cal. Report: OFF cal. Report format: short > config / Pt sensor Pt id Pt correction
Conductometry – Conductivity Measurement
0.0 °C
33
> config / auxiliaries run number date
2002-03-07
time
15:28:15
dialog:
english
device label program
712.0015
> config / RS 232 settings baud rate:
9600
data bit:
8
stop bit:
1
parity:
none
handshake:
HWs
RS control:
ON
---------------------------------------Sample preparation Water is taken from the «Nanopure» unit. The first 500 mL is rejected. 1 Liter is then filled into an amber glass bottle (thoroughly rinsed) and the bottle placed in a thermostat bath for 1 h at 25 °C. Preparing the measuring setup The 712 Conductometer is validated with the 767 Calibrated Reference as described in Application Bulletin No. 272. The protocols shown below provide an overview: 712 Conductometer: protocol of measuring amplifier check Instrument: 1.712.0010
Serial/Fabr.-No: 16162
ID-No (if available): ...................
Test agent:
Serial/Fabr.-No: 01102
ID-No (if available): ...................
Cal. date: 12/2001
checked by: S. Jung ..........................................
Calibrator (master)
Type .......................................................
SCS Cal. Serv. Reg. No.
Certificate No 8.767.3003 ......................
2.767.0010
Check carried out on: 2002.02.25 Checked instrument meets requirements:
Yes x
Name: Christian Haider no
Signature:..........................................................
Decision:
Place Calibrated Reference on bench near sensor. The cover remains always closed and therefore the light is not of importance. The electrode cap must not necessarily be firmly screwed onto sockets (4), (5), (6) of the 767; plugging it in is sufficient.
34
Metrohm Monograph 8.028.5003
Prepare the 712 Conductometer: Remove all cables except the mains cable. Switch on instrument. , LCD display: conductivity >cond/parameters : Note down the following parameters (see LCD display) in list below and then enter the following standard values: cell constant
............... /cm
→
1.0 /cm
meas. temp.
............... °C
→
20°C
ref. temp.
............... °C
→
20°C
TC selection:
...............
