Direct traceability for ultra-pure water conductivity Speaker: Hans D. Jensen Author(s): Hans D. Jensen and Carsten Thirstrup, Danish Fundamental Metrology, Matematiktorvet 307, DK-2800 Lyngby, Denmark, Tel.: +45 4593 1144, Fax: +45 4593 1137, email: info@dfm.dk Abstract: DFM has established a calibration setup with direct SI traceability for conductivity sensors and measurement systems for conductivity of pure to ultra-pure water. Electrolytic conductivity is a widely used parameter for the characterization of purity of water, due to its high sensitivity to ionic content e.g. from contaminants. The Pharmacopoeias (US, EU, etc.) specify (traceable) conductivity measurements as the method for documenting compliance with requirements of Water for Injection (WFI), and other regulations on pure water quality also rely on conductivity as the quality parameter. Conductivity sensors are presently calibrated either using reference materials with conductivity many orders of magnitude different from the level of measurement, and/or in a matrix different (sometimes very different) from pure water. Some users rely on indirect properties such as the temperature coefficient of water as a quality control parameter. DFM has developed a geometrically characterised measurement cell, hence a primary standard, relevant for ultra-low conductivity. Combined with a bulk resistance derived from impedance spectroscopy, also traceable to international standards, it allows direct measurement of low conductivity (less than 1 ÂľS/cm). A calibration setup with comparison to the primary cell establishes direct traceability to SI, without the need for assumptions on scaling properties or insignificant matrix effects. The setup has been validated through international measurement comparisons at the NMI level. 1. Introduction Electrolytic conductivity is used widespread in industry as a control and quality parameter in systems where salts or gases are in contact with liquids, either as part of a solution, as contaminants or trace elements, and where a quick, non-specific indication of concentration is warranted. From ultrapure water (UPW) to Cleaning-In-Place (CIP), conductivity is a relevant parameter for characterisation or control, and is routinely used over a range of 7-8 orders of magnitude. Specifically for low conductivity, the parameter is an excellent tool to detect contamination, because conductivity in UPW rises sharply when adding ionic contaminants (e.g. 4% for 1 ppb of NaCl, Light [1])
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Water used in the pharmaceutical industry is subject to quality requirements set in the Pharmacopeias, which for the past 10+ years have specified these requirements in the form of a limit value for conductivity. However, there is an essential problem: There is presently no direct traceability for conductivity measurements at the pure and ultrapure water level! This deficiency has bothered users seeking well-established documented traceability: practical “work-arounds” for cell calibration due to the necessity to scale from calibration at conductivity values 3-4 magnitudes higher than use and indirect methods to validate ultrapure water systems, e.g. control measurements of the temperature dependence of pure water output. It is the source of the somewhat unclear or metrologically unsatisfactory requirements in the Pharmacopoeias on conductivity measurements for WFI. 2. Traceability, primary standards and conductivity Metrological traceability is defined by the VIM [2] as “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty”. The “ reference” can in principle be an in-house standard, but to ensure comparability to anyone else, the optimal reference is the SI – also, nobody will get confused when you use e.g. ‘Ω’ and ‘m’ as units for your measurement quantities. A “primary measurement standard” (again according to VIM) is used to realise a measurement quantity solely on the basis of other measured quantities and not by calibration of the same quantity. So when we wish to establish traceability to the SI for conductivity – in units S/m, or the reciprocal, resistivity, in Ω∙m – we must establish traceability to resistance (Ω) and a geometric factor (m). Conductivity is given as
( )
, where L is the effective length, A is the effective cross
sectional area of the conductivity cell – defined by the placement of its electrodes – and R is the measured resistance; the factor
, is known as the ‘cell constant’ with unit m-1.
The resistance is meant to be a constant bulk resistance, but it is not ordinarily possible to measure such a bulk resistance directly, due to polarisation effects at the electrodes. Hence, in reality one is required to use an AC signal, measure impedance and from that infer a bulk resistance. This opens an extra full degree of freedom to specify how conductivity is to be measured: Can I use a fixed frequency? Which? Should I use the real part of the impedance or some combination? If I use another signal type (e.g. a square wave), what is the connection between the parameter extracted and the bulk resistance? Basically, an equivalent circuit model is required to specify the parameter ‘resistance’ for the conductivity calculation. The choice of model and its applicability affects the calibration as well 2013 NCSL International Workshop and Symposium
as the subsequent measurement of an unknown conductivity – especially if the value measured differs from the value at which the measurement system was calibrated (a ‘substitution measurement’ versus a ‘ratio measurement’). To realise a primary standard for conductivity, it must thus comprise of: Traceable measurements of the geometry of a conductivity cell plus bulk resistance inferred from traceable impedance measurement. This system can be used to calibrate reference solutions for conductivity, which then again is used to calibrate conductivity measurement systems.
3. Conductivity solutions In practise, conductivity measurement systems are calibrated using a conductivity reference material, a solution – usually aqueous – with an assigned conductivity value. These may be produced by the user him- or herself using ‘recipes’ from the standards literature or acquired as (certified) reference materials available from many different providers. However, note that recipes and values assigned are not always consistent in different standards and an estimate of the uncertainty of the conductivity value achieved is seldom available. The applicability will also depend on the uncertainty level required for the measurement system. There are two important issues to note: 1) The reference solutions mentioned all have conductivities far above the level of UPW and e.g. USP limits. The best characterised primary solutions are those specified in the IUPAC Technical Report [3], start at a value of 1409 µS/cm (0.01 mol/kg KCl(aq)). Other standards quote secondary solutions – from dilutions of primary solutions – in e.g. ASTM D1125, ISO 7888 and IEC 60754 with a lowest value of 75 µS/cm. A practical approach to producing in-house reference solutions, and estimating uncertainty, is given in [4]. 2) The measurement method used to realise the conductivity value quoted in the standard is only in a few cases specified – with Ref. [3] as an important exception. It must be considered, that the value of conductivity assigned to a reference material is in fact dependent on the method used, among others due to the issue of impedance, so to reproduce the conductivity value of a standard solution accurately, the same measurement method as originally used to assign the value, should be used. This information is rarely available. The primary reference solutions specified by the IUPAC Technical Report [3] were characterised by NIST in the 1990’ies – and compared to the results obtained in the 30’ies and 50’ies by other investigators. NIST use a very specific measurement procedure to derive the bulk resistance: They measure the impedance in the frequency range 1 kHz to 5 kHz, and infer the bulk resistance from an extrapolation of the effective parallel resistance (RP(f) = 1/Re[1/Z(f)] ) to infinite frequency (i.e. the intercept of a plot of RP vs. 1/f).
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The basic model behind this approach works well in the medium-to-high conductivity range, in the audio frequency range and for cells with electrodes of a suitable size, where electrode polarisation effects are significant. For lower conductivities, however, the characteristics of the system changes: Polarisation is significant in a certain conductivity-frequency range. Going to lower conductivities reduces this significance, and the same method becomes un-applicable. For accurate measurements, it is necessary to change the method used to derive the bulk resistance from impedance data! This issue has been explored in the recent review article by Seitz [5]. The issue of scaling measurements from the “high” conductivity of standard reference solutions to low conductivity in pure and ultrapure water becomes nontrivial when “crossing into” between the region of the two methods. The obvious solution is to measure conductivity directly in ultrapure water with a primary measurement cell. 4. Establish traceability at UPW conductivity level At low conductivity, a relevant geometry is coaxial placement of electrodes, but to obtain a geometric measureable configuration with sufficient accuracy, it is necessary to avoid, e.g. by guarding, or compensate any fringe fields. A primary cell has been developed at DFM. It is configured as a guarded, coaxial, two electrode cell with the main wetted parts made from PVC and stainless steel SUS316L. To achieve the guarding, the outer electrode is divided into three parts, the two outer parts are held at reference potential (0 V), while the center part is at low potential (which an auto-balancing LCR meter, Agilent 4980E, holds at reference potential). The inner electrode is held at high potential, usually 0.5 V. The inner electrode is a tube with nominal length 110 mm, outer diameter 25 mm and 2 mm wall thickness. The outer electrode is also made from stainless steel, nominal outer diameter 40 mm and wall thickness 2 mm. The two guard parts have nominal length 30 mm each, the center part has nominal length 50 mm. The inner surface of the outer electrode and the outer surface of the inner electrode have been polished to Ra 0.1 μm. The stainless steel tubes are mounted in a PVC fixture with a 6 mm deep slot, and the steel tubes accommodates EDPM O-rings to seal the assembly. The three outer electrode parts are separated by a PTFE foil cut into shape of the tube cross section. A thermometer probe in placed inside the cell, in close contact with the inner wall of the inner electrode. The cell is held together by two end plates which are connected mechanically and which also accommodates the connectors for the fluid. The top plate accommodates the electrical connections, two BNC connectors. Fluid enters the cell at the bottom parallel to the axis in the center of the space between the electrodes, and leaves the cell at the top 180° from the entry. 2013 NCSL International Workshop and Symposium
Because the electric field in this region is purely axial, it is possible to calculate the cell constant K from ⁄ where do is the outer electrode diameter, and di is It is possible to expand the formula to take into account any misalignment of the coaxial arrangement and the effect of the thickness of the isolating foil. Based on measurement performed at the Danish reference laboratory for geometric measurements, we find the cell constant of our cell, Kprim = 1.190 56 m-1 with a standard uncertainty of u(K) = 0.000 69 m-1. The primary cell is used in a measurement setup which allows for circulation of temperature and conductivity regulated solution through the primary cell and one or more measurement cells to be calibrated. Typical calibration points are 0.055 µS/cm, 0.5 µS/cm, 1.3 µS/cm and 15 µS/cm. The measurement uncertainty for the direct measurement of conductivity using the primary cell in the low range (< 20 µS/cm) is summarised in Table 1. Table 1. Uncertainty budget Uncertainty component Cell constant Dispersion of measurements Impedance measurement Inference of bulk resistance Temperature Total Expanded uncertainty
Rel. standard uncertainty 0.06 % 0.03 % 0.05 % 0.04 % 0.05 % 0.10 % 0.2 %
At the UPW conductivity value (0.055 µS/cm), the measurement uncertainty is further expanded. Because a primary standard is not calibrated against the same measurement quantity, the only way to validate the measurement capability is by comparison to another primary standard. In the framework of the European metrology organisation, EURAMET, a series of metrology research and development projects have been (and still are) conducted. Among these, collaboration between PTB, Germany, DFM, Denmark and INRiM, Italy, has explored the realisation of primary standards for ultrapure water conductivity. An initial measurement comparison has been performed in February 2013 between DFM and PTB using the DFM primary standard and a primary standard developed at PTB using a design based on a double coaxial cell described by Seitz []. 2013 NCSL International Workshop and Symposium
The results of the comparison, which will be fully available shortly, Seitz [], gave the following results for the difference in conductivity measured in the two systems:
From these results, we conclude that the measurement capability of the DFM system has been validated, and the formal international recognition is underway. We therefore expect the issue on SI traceable measurements for electrolytic conductivity in pure and ultrapure water to have been resolved and fully SI traceable calibrations . 5. Conclusion Electrolytic conductivity is a widely used measurement quantity e.g. used to monitor and document purity of water. In contrast to electrolytic conductivity at medium-to-high values (i.e. larger than 50 µS/cm), there is presently no widely available direct traceability chain to SI for the parameter. However, a number of national metrology institutes are presently establishing measurement systems which realise direct traceability to SI for electrolytic conductivity at the ultrapure water level, as well as the international metrology framework to ensure that these measurement services are globally recognised. In this article one of these systems have been presented including the initial results of the international comparability achievable. The system allows for calibration of conductivity sensors and systems in the range from 0.05 µS/cm to 15 µS/cm with an expected uncertainty of about 0.5 % 6. Acknowledgements Part of the work leading to the results presented has been financed by EURAMET joint research project receives funding from the European Community’s Seventh Framework Pro- gramme, ERA-NET Plus, under Grant Agreement No. 217257.
7. References 1. T.S. Light, S Licht, A.C. Bevilacqua, and K.R. Morash, “The Fundamental Conductivity and Resistivity of Water”, Electrochemical and Solid-State Letters, Vol 8, No. 1, pp. E16-E19, 2005. 2. International vocabulary of metrology – Basic and general concepts and associated terms (VIM), 3rd edition, JCGM 200, BIPM, 2012 (available at the www.bipm.org website)
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3. K.W. Pratt, W.F. Koch, Y.C. Wu, P.A. Berezansky, “Molality-based Primary Standards Of Electrolytic Conductivity (IUPAC Technical Report)”, Pure Appl. Chem., Vol. 73, No. 11, pp. 1783–1793, 2001 4. R.H Shreiner, “Preparation and Uncertainty Calculations for the Molality-Based Primary Standards for Electrolytic Conductivity”, American Laboratory, February 2004, pp. 28-32, 2004. 4. S. Seitz, A. Manzin, H.D. Jensen, P.T. Jakobsen, P. Spitzer, “Traceability of electrolytic conductivity measurements to the International System of Units in the sub mS m−1 region and review of models of electrolytic conductivity cells”, Electrochimica Acta, Vol. 55, pp 6323– 6331, 2010 5.
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