Conductivity Basic principles The specific electrical conductivity and the electrical conductance are a measure of the ability of a solution, a metal or a gas - in brief all materials - to conduct an electrical current. In solutions, the current is carried by cations and anions whereas in metals it is carried by electrons. If a substance has a high electrical conductance G, the electrical or ohmic resistance R is low. The electrical conductance G is the reciprocal of the resistance G = 1 R The unit of R is the Ohm and the unit of G is the Siemens. At this point, it would be useful to consider the measuring technique. To measure the electrical conductance, a voltage is applied to the electrode pairs and the current that flows is measured. During this process, the cations migrate to the negative electrode, the anions to the positive electrode and the solution acts as an electrical conductor. A conductor is defined by its length and crosssection. The smaller the electrode gap l and the larger the electrode area A, the larger the measurable current at the same electrolyte concentration and same voltage. + - A A - L The electrical conductance G is given by the equation: G = γ A l = 1 ρ A l where A is the electrode area, l the electrode gap, γ the specific conductivity and ρ the specific resistance. γ and ρ are material constants with the units S/m and Ω m. This equation also illustrates the relation between the specific conductivity γ and the conductance G. Page 1 of 18
As well as γ, σ and κ are also customary symbols used for specific conductivity. The quotient of the length and area is the cell constant K (resulting in the unit m -1 ). K = l A If the cell constant is known, the specific conductivity can be correspondingly determined from the measured conductance and depicts the result of a conductivity measurement. Conductivity measurement cells Basically, conductivity measuring cells consist of electrode pairs to which a voltage is applied. The current that flows is measured and the conductivity is calculated from it. This is a very rough approximation. The voltage applied is an alternating voltage to reduce polarization effects. Polarization of a conductivity measuring cell includes the effects that occur at the junction between the metal and liquid when a current flows and apparently causes the conductivity of the solution to change. If a voltage is applied to an electrode, a capacitor layer (double layer) is created because the electrode attracts inversely charged ions. e - e - e - e - e - e - e - e - voltage drop With increasing depth of the electrode in the solution, the effective voltage continues to drop further. Polarization effects can be reduced or prevented by applying an alternating voltage and by optimizing the electrode areas. In an alternating field, unequal charge distribution as shown in the diagram above cannot form so easily because the ions are alternately attracted by the two electrodes. Cations and anions oscillate about their location at the cycle of the applied frequency. This effect can be compared to a tug-of-war between two equally strong teams. Page 2 of 18
The higher the applied frequency, the lower the polarization effects that can be expected. Because the measuring frequency at high conductivity is restricted by instrument engineering, a suitable electrode material must be used, e.g. usually graphite or platinumplated platinum. The selection depends on the required measuring range of the conductivity measurement. The "classical" conductivity measuring cell consists of an electrode pair. A further development is provided by the four-electrode TetraCon measuring cell from WTW. It has a voltage electrode pair in addition to the current electrode pair. As a result, the ohmic voltage drop is determined in the solution with current flowing through it. The conductance of the solution results from the known current intensity and measured voltage drop. This measuring method does not detect polarization resistances. Moreover, a four-electrode cell is insensitive to measuring errors that result from contamination. + - The cell constant K cannot be determined by purely "measuring the electrode gap and area. Although the field lines in the area directly between the electrodes appear linear, at the edges scatter fields form and the electrode area relevant for the measurement is larger than the actual geometric area. This effect is encountered when the cells are calibrated. The cell constant is determined using a calibration solution with known conductivity, usually a 0.01 mol/l KCl solution. This is described in more detail later. Page 3 of 18
Temperature Compensation Conductivity is a parameter that is heavily dependent on temperature. A 0.01 molar potassium chloride solution is presented here as an example. The conductivity of this solution at 20 C is 1278 µs/cm whereas, at 25 C, it is 1413 µs/cm. As a result, values of the same sample that were measured at different temperatures cannot be compared with one another in practice. For this reason, the reference temperature was introduced. It is usually 20 C or 25 C. Conductivity measurements are normally conducted as follows. The measuring instrument records the actual conductivity and temperature, converts it to the reference temperature using a temperature compensation function and displays the conductivity at the reference temperature. Conductivity measurements and temperature measurement are intrinsically related and, thus, modern WTW conductivity measuring cells have temperature probes integrated in them. Depending on the type of sample used, different temperature compensation functions must be used: Linear function Non-linear function (nlf) for natural waters according to EN 27 888 (DIN 38 404) Non-linear function for purest water (additional consideration of the intrinsic conductivity of the water) Non-linear functions for special solutions No compensation The following example illustrates the measuring principle of conductivity measurements for a sample (e.g. waste water) using nlf temperature compensation. C onductivity nlf T ref =20 C T ref =25 C Actual Conductivity Temperature Linear functions are used, e.g. for saline solutions, acids and leaching solutions. However, the linear dependency is not suitable for many aqueous liquids under test. The temperature dependency can only be described by non-linear functions such as the nonlinear function for natural waters, i.e. for ground water, surface water, drinking water and waste water. High performance conductometers from WTW enable an automatic determination of non-linear functions for special samples by the user. To do this, the Page 4 of 18
sample to be tested is heated or cooled and the conductometer records the conductivity in dependency on the temperature and performs the corresponding curve adaptation. Selected values are listed in the following table as an example of typical conductivity: Conductivity at 25 C Purest water 0.055 µs/cm De-ionized water 1 µs/cm Rainwater 50 µs/cm Drinking water 500 µs/cm Industrial wastewater 5 ms/cm Seawater 50 ms/cm 1 mol/l NaCl 85 ms/cm 1 mol/l HCl 332 ms/cm Even the purest water has a conductivity! This has its origin in the intrinsic dissociation of water that, according to the solubility product, forms oxonium and hydronium ions. H 2 O H 3 O + OH - H 2 O Page 5 of 18
Calibration and analytical quality assurance Calibration First a preliminary remark. The terms calibration and adjustment are often used synonymously. Strictly speaking, calibration describes a comparison with set points. The determined values must lie within specified tolerances. Adjustment means an active change of settings. As opposed to ph measurement and oxygen measurement, the measuring cell does not change and is not depleted by the measurement procedure in conductivity measurements. At least, this is the case if the cell is used correctly. The measuring cell is made of stainless steel, platinum, platinum-plated platinum or graphite electrode pairs that are chemically resistant and whose geometry is determined by the cell constant. The measuring cells are delivered from the factory with a specified and tested cell constant and the user can start measuring without any need for determine the cell constant. The production-dependent tolerances of the cell constant lie between 1.5% and 2% depending on the measuring range. If more precise measurements are required, the cell constant can be precisely adjusted using potassium chloride solutions. This basically applies to new cells or when a cell is changed. To do this, the use of commercially available KCl solutions is recommended as there is no fear of error as a result of dilution or contaminated substances (KCl or water). Calibration of the conductivity cell is to be understood in the sense of checking - and not in the sense of adjustment - as any change of the cell constant is usually caused by contamination. It is not logical to adjust the cell to the current contamination. It is better to clean the cell. Page 6 of 18
The EN 27 888 recommends regular testing at least every six months. The standard calibration solution that corresponds to the cell constant must be used to do this. The solutions to be used are potassium chloride solutions of different concentrations. Measuring range Electrode material Recommended standard solution c (KCl) [mol/l] 200 µs/cm Steel, platinum 0.001 0.147 200 µs/cm Plated platinum, 0.01 1.413 graphite 2 ms/cm Plated platinum, platinum, graphite 0.01 1.413 Electrical conductivity at 25 C [ms/cm] 300 ms/cm Platinum, graphite 3 approx. 300 300 µs/cm Platinum, graphite For the measuring range up to 2 µs/cm, (only the purest water such as boiler feed water or de-ionized water) can no contamination be expected. Standard solutions for this conductivity measuring range would be very unstable. It would also be problematic to use this type of standard calibration solution because contact with air must be completely excluded. Carbon dioxide found in the air dissolves in water to form hydrogen carbonate or carbonate ions. These ionic components cause dramatic changes to the measured conductivity and, consequently, produce meaningless results. CO 2 H 2 O CO 2 H 2 CO 3 H + HCO 3 - H + CO 3 2- Page 7 of 18
Monitoring the effect of carbon dioxide also applies to calibration as well as to the actual measurement. It must be performed with air excluded or using a protective gas. To check a purest water measurement cell, a simple functional test is the most useful. If the cell provides plausible values for de-ionized water, interference can be essentially ruled out. Checking cells for a conductivity range smaller than 200µS/cm A differentiation must be made according to the electrode material: Polished platinum or steel electrodes polarize at high conductivities so that a 0.001 mol/l KCl solution must be used. A 0.001 molar KCl solution can be produced by 1+9 dilution of commercially available 0.01 mol/l KCl solution (pay attention to the quality of the water!). Commercial 0.01 mol/l KCl solution available from WTW is recommended for platinumplated platinum or graphite electrodes because, as a result, any sources of error in the production can be excluded. For the measuring range up to 2 ms/cm The use of commercially available 0.01 mol/l KCl solution is also possible and additional sources of error in the production of the test solution can be excluded. In the measuring range up to 300 ms/cm Only a simple check can be made here where the 3 molar, silver chloride-free KCl solution for the ph measurement is used. The result should lie within the order of magnitude of 300 ms/cm. Page 8 of 18
Adjusting and checking the cell constant Conductivity measurements are extremely sensitive to ionic contamination. Therefore, the cleaned or new conductivity cell should be rinsed with standard solution before it is submersed in the standard solution. For laboratory measurement cells such as the TetraCon 325, it is recommended to submerse the sensor directly in the freshly opened calibration standard bottles to prevent contamination by the sample vessel. If this is not possible, the vessels to be used should be rinsed with standard solution. A special case is presented by stray field cells. The electrodes are attached to the front of the shaft (e.g. TetraCon 700 for online measurement). The cell constant is dependent on the size and type of the stray field. The stray field is changed in turn by limitations such as the base of the vessel or of the mounting tube. As a result, the adjustment or checking of the sensor in a mounting tube must be performed while it is installed. In other cases, a minimum gap to the vessel wall must be maintained. The measured value must be independent of the size of the vessel or the volume of standard solution. If the cell constant is adjusted to raise the measuring accuracy (not to compensate for contamination!), it is important to check whether the conductometer has an automatic calibration function as is the case in modern WTW conductometers. If it does, it is sufficient to submerse the cell in 0.01 mol/l KCl solution and to start the adjustment process. The background to this is that the temperature compensation function of the standard calibration solution is stored in the instrument. During this process, the instrument measures the conductivity and changes the calculated cell constant in such a way that the corresponding conductivity of the standard solution is reached at the selected reference temperature. After this is completed, the adjusted cell constant is displayed. In instruments without automatic calibration, this process is performed by the user. In this case, the user must ensure that adjustment takes place in the thermostatted state. The control solution and measuring cell must be at the reference temperature. The background to adjusting the cell constant is the same. The cell constant is changed so that, at the reference temperature, the corresponding conductivity is achieved as closely as possible. As closely as possible because the resolution of the cell constant is not often high enough to enable an adjustment to the exact value (e.g. 1413 µs/cm for 0.01 molar KCl solution at 25 C). Automatic calibration function No automatic calibration function 0,01 n KCl 0,01 n KCl T = constant T = T Ref Page 9 of 18
Note: If the conductivity measuring cells continue to be submersed in the standard calibration solution after the adjustment is complete and the instrument is switched to the normal measuring mode using, e.g. an nlf temperature compensation function, this usually results in a different measured value. (Reason: A different temperature compensation function!) If a control measurement only is performed with the standard calibration solution and without adjustment of the cell constant, the correct temperature compensation function must be set. The value of the linear temperature compensation for the 0.01 molar KCl solution is 1.9%/K at a reference temperature of 25 C and 2.1% /K at a reference temperature of 20 C. Alternatively, the control solution can be thermostatted precisely to the reference temperature where adjustment of the corresponding temperature compensation function is no longer required. If the conductivity measuring cell is changed, the new cell constant must be transferred to the conductivity measuring instrument! Page 10 of 18
Testing the conductometer The primary process variable of the conductance is the resistance. The function of a conductometer can be tested using certified test resistances. To do this, WTW provides a set of six different resistances that are connected to the instrument instead of the measuring cell. If the measured values displayed lie within the tolerance specified in the certificate, the functional safety is met. If the displayed values lie outside the tolerance, the instrument must be sent for repair. The test set offered by WTW enables testing within the framework of the ISO 9000. It enables the restoring of the measured values to national standards. Page 11 of 18
Measurement and analytical quality assurance Which measuring cell is required for which application As in ph measurement, the selection of the conductivity cell used depends on the application. It is also true that the effort required is greater, the cleaner the sample to be inspected. A number of examples have been included to illustrate this but it also important to note that a great number of special cells are available: The standard TetraCon 325 conductivity measuring cells have graphite electrodes in four conductor technology and are, thus, insensitive to contamination and easy to clean; their application is in routine analysis where the measuring range lies between 1µS/cm and 2 S/cm. For partially aqueous and highly aggressive media, the use of the TetraCon 325 Pt with a measuring range between 1 µs/cm and 1 S/cm is recommended. The electrodes are made of platinum and are, thus, extremely resistant to aggressive media. Page 12 of 18
If a measurement is to be made in purest water such as boiler feed water or ion exchange water, it is necessary to use a flowthrough vessel. Contact with the air must be excluded. The reason for this is the carbon dioxide in the air that in turn forms carbonic acid in water and leads to a change in the conductivity. Two electrodes are available depending on the measuring range required. LR 325/01 (0.001 µs/cm 300 µs/cm) LR 325/001 (0.0001 µs/cm 30 µs/cm) Special tip and just to repeat: Two points are often overlooked in the measurement of conductivity: The result is usually referred to the reference temperature. Mostly, this is 25 C! If the reference temperature is set to 20 C in the instrument, a very different value is obtained that has nothing to do with incorrect measurement, but simply has a different reference. To calculate the conductivity at the reference temperature, the instrument converts from the current temperature of the material under test using a characteristic function. If this function does not correspond to the sample, an incorrect value is obtained. If, for example, a saline solution is to be tested, a linear function is required and not the non-linear function for natural waters. These two points should be taken into consideration in every measurement, set correctly and also documented according to the AQA. Page 13 of 18
Drift control Drift control, as in the ph and oxygen parameters, makes no sense in the case of conductivity measurement as the displayed measured value is the current measured value. No time delay results from the measurement technique. Cleaning the cells Warm water with some household detergent has proved itself as a cleaning agent for organic contamination. Alcohol may also be used. Calcium deposits are best cleaned using ten-percent citric acid. It is possible to clean graphite, steel or polished platinum electrodes mechanically using a soft brush. However, make sure that the surface of the electrodes is not scratched! Never use hard objects such as screwdrivers, etc. Even the use of a brush should be carefully considered. Platinum-plated platinum electrodes should only be cleaned chemically because even the softest brush would damage the platinum coating of the electrodes. It is possible to regenerate a damaged or heavily contaminated electrode surface by platinum plating the electrode surfaces. Information on how to do this is given in WTW application report 243. Storing the measuring cells In the same way as there are different methods of cleaning the measuring cells, there are various points that must be observed when storing the cells. Cells made of polished platinum, steel or graphite must be stored in a dry place. Platinum-plated platinum electrodes, however, must not be allowed to dry out. They must be stored in de-ionized water. Page 14 of 18
Practical exercises Preparation All practical experiments should be carried out in a suitable laboratory to guarantee working safety. While this is a general recommendation, however, in the trials of conductivity measurement described below no hazards arise from the chemicals used as KCl solution is non-toxic and can only leave a washable salt ring. Nevertheless, in the analytical field, the safety regulations should be routinely observed. Safety instructions General rules of conduct when handling chemical substances When working at a work place at which chemicals are handled, the following rules must be observed:! Follow the instructions on the chemical bottles! Always wear protective garments (goggles, gloves,...)! Never point open containers towards other persons! Do not eat, drink or smoke! Ensure the satisfactory disposal of chemicals! Carefully remove or clean up any spilled chemicals! Contact specialist personnel if any serious problems arise We provide these short instructions in the hope that they lead to successful and safe practical studies. Page 15 of 18
The following instruments and equipment must be available: Washable tables Resistant floor coverings Running water Eye bath Measuring station checklist 1 conductivity measuring instrument e.g. LF 330/340, inolab Cond 1 conductivity measuring cell Suitable for the measuring instrument 1 calibration standard 0.01 mol/l KCl solution 1 stand To hold the electrode 1 magnetic stirrer with agitator 2 beakers (150ml) Fresh distilled water Cleaning bottle with distilled water Protective goggles, gloves and apron Ballpoint pen and pad, calculator, lintfree cloths Standard calibration solutions must be beyond any doubt!! That is to say: Do not use any buffer solutions of unknown origin or history! Ideally, use freshly opened commercial standard calibration solutions that will be disposed of afterwards. Page 16 of 18
Practical exercises 1. Calibrating the measuring cell Rinse the clean sensor with standard calibration solution (0.01 mol/l KCl) and then submerse it in the solution Check the selected cell constant Set the reference temperature to 25 C and linear temperature compensation to 2.1%/K. Compare the displayed value with the nominal value of 1413mS/cm. Manual adjustment of the cell constant Change the cell constant while monitoring the change in the conductivity. (Caution: Here, thermostatting has not been used because the cell constant is automatically calibrated in the next step! If the cell constant can only be adjusted manually, it must be thermostatted to the reference temperature!) Automatic adjustment of the cell constant using the operating manual 2. Effect of the reference temperature Determine the conductivity of 0.01 mol/l KCl at a reference temperature of 20 C Linear temperature compensation with 1.9%/K Determine the conductivity of 0.01 mol/l KCl at a reference temperature of 25 C Linear temperature compensation with 2.1%/K 3. Effect of the adjusted temperature compensation function The values measured for tap water (reference temperature 25 C) should be determined using the nlf, a linear function (2.1 %/K) and no temperature compensation (0.0 %/K) and then compared. 4. Effect of carbon dioxide on the measurement of ion-deficient samples Freshly distilled water should be stirred using a magnetic agitator for 15 minutes. While doing so, the conductivity should be continuously monitored. (nlf function and 25 C reference temperature) Page 17 of 18
Bibliography [1] DIN EN 27888, Bestimmung der elektrischen Leitfähigkeit, 11/93 Page 18 of 18