Traceability of electrolytic conductivity measurements for ultra pure water

In quality control, one of the most commonly used parameters for the determination of the overall ionic purity of water is the electrolytic conductivity. The goal of the Istituto Nazionale di Ricerca Metrologica (INRiM) is to produce electrolytic conductivity values traceable to the International System of Units (SI) especially in the field of ultra- pure water, as required by many industrial sectors. About pure water, the most significant difficulties regard the pollution of the solution with the air CO2, the temperature monitoring and the resistance measurement that should not be affected by parasitic phenomena. European, Japanese and American Pharmacopoeia demand that the purity of the water is defined by electrolytic conductivity measurements with uncertainty of 1%. ASTM Designation prescribe the use of a flow system for solutions with a conductivity of less than 50 µScm-1. In order to carry out measurements of solutions with low electrolytic conductivity, at INRiM a new primary cell with extra devices has been developed and built. The measurement capability of the new primary cell has been verified taking part in the international Key Comparison CCQM-K92 “Electrolytic Conductivity at 0.05 S/m”. Moreover, a Pyrex glass closed circuit consisted also by a secondary cell and an expansion chamber has been realised. Measurements were carried out using solutions at different electrolytic conductivity values: the repeatability of measurements has been improved, the drift over time using the flowing system decreases and the expanded uncertainty value has been reduced. From preliminary measurement, it was observed that the frequency value corresponding to the electrolytic conductivity value of ultra pure water is less than 1 Hz. Since the LCR commercial bridges do not reach frequencies below 20 Hz, an impedance spectrometer which allows the measurement of impedance spectra in the frequency range from mHz to kHz has been developed. Working in the direction of lower conductivity values, measurements on solutions containing water, KCl and ethanol have been carried out. This research activity is conducted under the patronage of the European project EMRP - ENG09 Metrology for biofuels, by means of participation in the international comparison EURAMET Study 1202 "Study on electrolytic conductivity reveals measurements of bioethanol". Closed circuit is also used to perform calibration of the secondary cell for comparison on the primary cell. The traditional method of calibration by substitution is affected by errors in the definition of the conductivity value due to the drift in temperature and concentration of the sample transfer, as it is measured in two separate cells. Using solutions with low electrolytic conductivity (eg < 50 μScm-1 ), the errors generated by this method are too large. To reduce the uncertainty the reference cell and the cell under calibration are inserted in the closed circuit and the measurement is performed using the same solution (that flows within the circuit). In this way it is possible to avoid problems related to the difference of the solution drift and the temperature. By means of this calibration method by comparison it is possible to improve the value of the expanded uncertainty of the measurements of an order of magnitude . Several activities related to the measurement of solutions with high conductivity values have been carried out: these research activities took place in the European project EMRP - ENV05 "Equivalence ratio of conductance measurement result of seawater". They were related to the international pilot comparison CCQM-P142 on aqueous solutions with a high content of KCl and marine waters (different dilutions) and to the international key comparison CCQM-K105 on measures of KCl solutions with a nominal value of electrolytic conductivity of 5.3 S/m, for which INRiM had the role of coordinating laboratory.