TECHNICAL PAPERS. Ulrich Breuel, Barbara Werner, Petra Spitzer and Hans D. Jensen

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1 Experiences with Novel Secondary Conductivity Sensors within the German Calibration Service (DKD) Ulrich Breuel, Barbara Werner, Petra Spitzer and Hans D. Jensen Abstract: International efforts concentrate on the traceability of electrolytic conductivity at the field level having small associated measurement uncertainties. Although the measurement of conductivity at the primary level has been widely developed during the last decade, the dissemination of the small measurement uncertainty to the field level is lagging. There is a lack of easy to handle and reliable secondary calibration methods and transfer standards.this paper describes a procedure for determination of the electrolytic conductivity on the secondary level appropriate for calibration laboratories. The procedure was developed within a joint project of the German calibration laboratory (ZMK ANALYTIK GmbH, DKD-K-06901) together with the Physikalisch-Technische Bundesanstalt and the Danish Metrology Institute. Altogether a chain of five measuring cells has been used so that it is possible to measure the conductivity over a wide range from 2 µs/cm up to 100 ms/cm.experiences and first results using the five secondary cells are given including evaluation of comparisons with metrological institutes and partners from industry.the results obtained by the ZMK are in good agreement with the results obtained by the metrological institutes. 1. Introduction 1.1 Measuring Devices for Electrolytic Conductivity In general to determine the electrolytic conductivity at a certain level of the metrological hierarchy (national and international) the followed measuring devices are used: 1. Primary measuring cells. 2. Special designed standard measuring cells. 3. Commercial conductivity measuring instruments (with 2- pole and 4-pole cells). Ulrich Breuel Barbara Werner Zentrum für Messen und Kalibrieren -ANALYTIK- GmbH D Wolfen, Germay info@zmk-wolfen.de Petra Spitzer Physikalisch-Technische Bundesanstalt (PTB) D Braunschweig, Germany Hans D. Jensen Danish Institute of Fundamental Metrology (DFM) DK-2800 Kgs. Lyngby, Denmark 1.2 Metrological Hierarchy The metrological hierarchy is the basis for the metrological infrastructure that is valid nationally and internationally Primary Measuring Cells Primary measuring cells are a part of national standards at the highest level of a calibration hierarchy in a country. For the development of a calibration procedure in the DKD-K-06901, these cells were important because there are commonalities in the measurement with primary cells and standard cells. (e.g., the determination of the conductance with impedance measurement). The measurement of electrolytic conductivity with primary cells is traceable to the International System of Units (SI). There is a differential measurement principle. Changes of cells constants can be measured very exactly by dimensional measurements. [1 4] Fringe effects can be canceled out. The cell constant can be determined by geometrical measurements only Secondary Measuring Cells The measurement of electrolytic conductivity by means of secondary cells is also carried out in a large scale in national metrological institutes. From the Danish Institute of Fundamental Metrology (DFM) several standard measuring cells were developed. [5, 6, 7] 62 MEASURE

2 Primary cells (of the national metrology institute) Transfer standards: Certified reference solutions for electrolytic conductivity In equation (1), j is the imaginary unit. Similarly, the conductance can be described as the complex conductance, Y, consisting of a real, G, and an imaginary part, B: Y = G + j B. (2) Reference standards (Standard cells) Commercial measuring instruments and devices for electrolytic conductivity Calibration objects Figure 1. Metrological hierarchy of electrolytic conductivity. The aim of the project work done in the DKD-K was the development of a metrological concept to measure the electrolytic conductivity in a wide range from 2 µs/cm to 100 ms/cm. At the beginning only one standard measuring cell was available in the laboratory. This cell was applicable only for a limited measuring range from 100 µs/cm to 12 ms/cm. In order to be able to measure in the whole range of interest commercial devices and sensors were used. By means of this procedure, it was not possible to realize the low target uncertainty the customers are asking for. On the other hand, an obvious limitation of the commercial instruments was the non-linearity. The metrological hierarchy for electrolytic conductivity is shown in Fig. 1. The standard cells in the middle of the pyramid are the result of the development project. In the level of the secondary standards commercial measuring instruments and devices with several sensors are used (e.g., 2-pole and 4-pole cells from WTW or Radiometer) Measuring Principle The measurement principle described in the following is valid likewise for primary cells and the standard cells. The complex conductance, G, or its reciprocal the complex resistance, is evaluated from a measurement at different frequencies. For a dc measurement, either of these two quantities would be enough. But in general the measurement is carried out with alternating current by means of an LCR-meter. The detected complex resistance during a measurement with ac is the impedance, Z, consisting of a real part, resistance, R, in ohms, and an imaginary part, X, the so called reactance. The relationship between these quantities is shown in equation (1): Z = R + j X. (1) The quantity B is referred the susceptance. In all measurements of electrolytic conductivity with primary and standard cells, the susceptance is part of the measured impedance. Therefore the conductance of the solution at infinite frequency has to be separated from the complex result. Because an infinite frequency is not realizable, the conductance at different frequencies is measured and plotted as a function of the reciprocal frequency (see Fig. 2). There is a good linearity between the conductance and the reciprocal of the frequency over a given range. The intersection with the ordinate corresponds with the conductance at an infinite frequency. This value is used for the determination of the cell constant of the standard measuring cells if the conductivity of the reference solution is known (e.g., certified reference solutions from DFM, National Institute of Standards and Technology (NIST) or Physikalisch-Technische Bundesanstalt (PTB)). After the cell constant is known, this value is used for the calculation of the electrolytic conductivity of solutions to be calibrated. For a precise evaluation, the choice of a suitable frequency range is very important. [8] A good linearity is not given using the whole frequency range. There are limits in the linearity at determined frequencies, but the limits depend on the electrolytic conductivity of the solution. In general it is possible to say that it is better to use low frequencies for solutions with low electrolytic conductivity and high frequencies for solutions with high electrolytic conductivity. During the work in the development project, different cell constants were realized for the whole measuring range from 2 µs/cm to > 100 ms/cm. The sig- 1 Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately describe the experimental procedure. Such identification does not imply recommendation or endorsement by the author or NCSL International, nor does it imply that the materials or equipment identified are the only or best available for the purpose. Vol. 3 No. 2 June 2008 MEASURE 63

3 y = 970.1x R 2 = Description of the Calibration Procedure As a result of the development project in the DKD-K a calibration procedure was developed that is based on the use of standard measuring cells. Now it is possible to calibrate reference solutions for electrolytic conductivity over a range from 2 µs/cm to > 100 ms/cm. G, µs nificant frequencies were determined. 1.4 Temperature Measurement Constant temperature of the measuring cells is an important precondition for the determination of reproducible measurement results with low uncertainties. The homogeneity of the thermostatic bath must be assured. Usually thermometers with platinum elements of 100 Ω (Pt100) or 25 Ω (Pt25)resistance 1/f, Hz 1 Figure 2. Conductance as a function of the reciprocal value of frequency. G, µs Figure 3. Determination of the cell constant. 1/f, Hz 1 y = x R 2 = are used as temperature sensors. The indication of the results is carried out with resistance bridges (e.g., IsoTech Model TTI-2). It is important that the measurement uncertainty of the bath temperature is not higher than U = ± 5 mk (k = 2). 2.1 First Measurements with Standard Measuring Cells At the beginning, there was one standard measuring cell available in the calibration laboratory, DKD-K Because of its cell constant (K = 1.67 cm -1 ), this measuring cell was suitable for the measurement in the range above 100 µs/cm. The question arose as to whether it is possible to also measure lower electrolytic conductivities with this cell. With decreasing electrolytic conductivity increasing deviations from a strong linearity were found for the conductance as a function of the reciprocal value of the frequency. Because of these strong deviations from a good linearity this standard measuring cell was not suitable for low conductivities in the pure water range. The development of a second cell with a lower cell constant was necessary. The cell constant of this second cell (K = cm -1 ) was determined with a certified reference solution (see Fig. 3). During the development project, it was apparent that the two standard measuring cells are not enough to carry out a measurement over such a wide range of electrolytic conductivity from 2 µs/cm to > 100 ms/cm. Altogether five standard measuring cells with different cell constants were used for the measurement of electrolytic conductivity. 2.2 Standard Measuring Cells The standard measuring cells are special products (Sensortechnik Meinsberg GmbH) manufactured on the basis of a metrological conception. For a clear differentiation, each cell got a letter from A to E and a measuring range. All five standard measuring cells consist of a glass tube with two circular measuring electrodes (platinum, diameter 20 mm). The distance between the measuring electrodes is different from cell to cell. The measuring cells are fixed in a retaining device. A cover plate is made of metal and has grips to set the cell into the thermostatic bath. Connec- 64 MEASURE

4 The ranges where a measurement of electrolytic conductivity is possible for each cell are overlapping. For a clear assignment, the used ranges are smaller. Figure 6 gives an overview over the standard measuring cells. The measuring device consists of: 1. Five standard measuring cells. 2. LCR-meter (Agilent Model 4284A). 3. Precision thermostatic bath (oil bath, Lauda Proline Model PV 36). 4. Temperature measuring device (temperature sensor Pt25 and temperature indication instrument IsoTech TTI-2). Figure 4. Standard measuring cell D (example) for the determination of electrolytic conductivity. 2 connections (for LCR-meter) Openings for filling and emptying 2 connections (for LCR-meter) 2.3 Impedance Measurements with the LCR-meter Impedance measurements in the calibration laboratory DKD-K are carried out with the LCR-meter connected to the standard measuring cells in 4-wire circuit. The cables used have a better electromagnetic shielding in comparison with the original cables from the manufacturer. With the LCR-meter, frequencies in the range from 20 Hz to 1 MHz are realizable, but, for our measurements, only a part of this range, from 20 Hz to 5 khz, was used. Measuring electrodes (Pt plate) Pt plate Solder connection (gold) Figure 5. Schematic image of a standard measuring cell in the DKD-K tion to the LCR meter is realized with four connector sockets. Another connector socket is used for an earth ground cable (see Figs. 4 and 5). The realization of different cell constants was reached by changing the distances and a treatment of the surface (platinising) of the measuring electrodes. With it measurements in different ranges of electrolytic conductivity are possible. 2.4 Stabilized Standard Measuring Cells and Temperature Measurement During the measurement the standard measuring cells are in the precision thermostatic bath. It is important that the time of tempering before the start of measurement is long enough. The temperature sensor (SPRT Pt25) used for the temperature measurement was calibrated at temperature fixed points. It is used together with a temperature indication instrument TTI-2. For the realization of low measuring uncertainties it was necessary use a thermostatic bath with a low temperature inhomogeneity for the tempering. The spatial inhomogeneity of the bath was determined with platinum resistance thermometers. One of these thermometers is the sensor Pt25 also used for the temperature control during the measurement. The sensor was fixed on a reference position in the centre between two measuring electrodes. Another resistance thermometer was positioned on two other measuring posi- 4 mm 6 mm 20 mm 60 mm 60 mm Plantinization Cell A Cell B Cell E Cell C Cell D Operating range 2 µs/cm 15 µs/cm 100 µs/cm 1 ms/cm 20 ms/cm 100 ms/cm Real measuring ranges are overlapping Figure 6. Operating ranges of the standard measuring cells in the DKD-K Vol. 3 No. 2 June 2008 MEASURE 65

5 H H Reference position (Pt 25) during the determination of spatial inhomogenity D Position of the electrodes (cell C and D) during a measurement D W W Figure 7. Positions of the temperature sensors for the determination of spatial inhomogeneity (H, W, D are dimensions height, width and depth of the used volume). tions one after another. A spatial inhomogeneity of 2 mk was determined. Furthermore a time stability of the thermostatic bath of 3 mk was found. An additional temperature control is given by two calibrated liquid-in-glass thermometers with a resolution of 0.01 K. The thermometers are positioned in direct near of the standard measuring cells (see Fig. 7). 2.5 Metrological Traceability/ Validation of the Calibration Procedure The standard measuring cells are traceable to the primary measuring cells of the National Institute of Standards and Technology (NIST), the Physikalisch-Technische Bundesanstalt (PTB) or the Danish Institute of Fundamental Metrology (DFM) by certified reference solutions as transfer standards. In the frame of validation, extensive national and international comparisons were carried out. The recognition in national and international comparisons is the highest level of validation of the developed metrological procedure and is an important condition for an international offering of reference solutions of electrolytic conductivity as high-quality products. 3. Conclusions Finally it is possible to say that the conception/creation of different standard measuring cells with accommodation to defined measuring ranges is a new variant of the metrological trace-ability of electrolytic conductivity. The aim of this work is to supply reference solutions to users in the range from 2 µs/cm to > 100 ms/cm, thereby providing users metrological traceability to the national level. The standard measuring cells represent the level of reference standard in the metrological hierarchy of the procedure. With it the calibration of reference solutions of electrolytic conductivity is possible without a break in the given measuring range. With this development for the industry/laboratories, there is a new possibility to purchase reference solutions for the monitoring of the processes and to save the accuracy of the measuring results. 4. Acknowledgments We thank the Landesförderinstitut Sachsen-Anhalt (Germany) for the financial support of the development project. The authors would like to acknowledge Dr. Reinhard Lange and Mr. Frank Seifert from Sensortechnik Meinsberg GmbH for their suggestions for the development of the standard measuring cells. 5. References [1] P. Spitzer and U. Sudmeier, Electrolytic conductivity a new subject field at PTB, PTB-report PTB-ThEx-15, PTB, Braunschweig, pp , [2] K.W. Pratt, W.F. Koch, Y.C. Wu, and P.A. Berezansky, Molality-based primary standards of electrolytic conductivity, Pure App. Chem., vol. 73, no. 11, pp , [3] R.H. Shreiner and K.W. Pratt, Standard reference materials: Primary standards and standard reference materials for electrolytic conductivity, NIST Special Publication , [4] Y.C. Wu, W.F. Koch, and K.W. Pratt, Proposed new electrolytic conductivity primary standards for KCl solutions, J. Res. Natl. Stand. Technol., vol 96, pp , [5] H.D. Jensen and J. Sørensen, Electrolytic conductivity at DFM results and experiences, PTB-Bericht PTB- ThEx-15, Braunschweig, pp. 3 8, [6] H.D. Jensen and C. Verdier, Towards an improved primary standard for electrolytic conductivity, presentation of the Danish Institute of Fundamental Metrology at the NCSLI Workshop and Symposium, [7] H.D. Jensen and N.-E. Dam, DFM measurement capability: Electrolytic conductivity, DFM-report DFM-04- R81, Lyngby, pp. 1 7, January [8] K. Rommel, Leitfähigkeitsmessungen in Elektrolyten Die Wahl der richtigen Frequenz, off print from the Fachzeitschrift für Labortechnik, vol. 12, [9] Guide to the Expression of Uncertainty in Measurement (GUM), ISO/IEC Guide 98, International Organization for Standardization, Geneva, MEASURE