Electrolytic conductivity at pure water level final report

• Electrolytic conductivity in aqueous solutions is one of the most common electrochemical measurement techniques in industry. Since it is sensitive to the amount content of dissolved ions in a solution, a limiting value for conductivity is a clear and simple quality criterium for the ionic purity of water. The relevant measuring range for pure water applications is roughly between 0.055 μS cm-1 (ultrapure water) and 150 μS cm-1 at 25 °C. For instance, the European, Japanese and United States (USP) Pharmacopoeia have specified the requirements for purified water, highly purified water and water for injection for pharmaceutical use based on conductiv-ity. Sectors that also use conductivity limits for water purity are electrical power production, food industry, electronic industry and analytical laboratories. At low conductivity levels it is not feasible to circulate water samples for comparison measure-ments, since the conductivity value is instable due to inevitable ionic contamination. The main contamination results from carbon dioxide in ambient air that dissolves in water and builds H3O+ and hydrogen carbonate ions. The contribution of these ions to conductivity is around 1 μS cm-1. Hence, it is impossible to provide stable samples having usable uncertainties in the conductivity range of interest. EURAMET 1271, performed in 2013, was the first successful comparison measurement of pure water conductivity. In the meanwhile, more NMIs, the majority of which is situated in Europe, have built measurement capabilities in the pure water range. EURAMRET 1271 covered a measurement range up to 50 μS cm-1, whereas more and more customers request conductivity cell calibration in the range up to 150 μS cm-1. Consequently, this comparison intends to extend the measurement range and to enable more NMIs to get support for potential CMCs. Therefore, this comparison is additionally intended being a supplementary CCQM comparison. A commercial conductivity measurement meter, including a conductivity measurement cell, was used for the comparison in a Round-Robin scheme. The devices were provided by PTB and were sent from one institute to another. Each institute had to measure the conductivity of a reference solution using the conductivity meter. The reference solution could either be pure water or a measurement standard solution that was reasonably stable in the range of interest. In the first scheme, the cell had to be integrated in a closed pure water flow though system to minimize impurification by CO2. An adequate fixture for this setup was provided by PTB. In the second scheme, the cell was immersed into the measurement standard solution under tem-perature-controlled conditions. Essentially, the institutes had to report the conductivity values indicated by the conductivity meter and the conductivity reference value assigned to the water in the flow though system or that of the measurement standard solution, respectively. The co-ordinating institute calculated adjusted cell constants for the cell from the reported values, which were used to calculate linking conductivities, the actual quantities to be finally compared. The results showed good equivalence in all conductivity ranges, with only a few inconsistent values. Adequate comparison reference values are suggested that can serve to calculate robust degrees of equivalences for the participants usable to support respective CMC claims. 

Nyckelord

Carbon dioxide

Cells

Cytology

Electric conductivity measurement

Electronics industry

Flow of water

Flowmeters

Ions

Laboratories

Purification

Water levels

Analytical laboratories

Comparison measurement

Conductivity measurements

Degrees of equivalence

Electrical power production

Electrochemical measurements

Electrolytic conductivity

Electronic industries

Solution mining

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