Evaluation of the Stability of Solutions of Low Electrolytic Conductivity by Primary Measurements

Abstract

Electrolytic conductivity (EC) at low levels (1.0–100 µS·cm−1) is an important measurement parameter of quality for evaluation of the quality of either aqueous or non-aqueous solutions. Certified reference materials (CRMs) are used to calibrate conductivity meters to provide measurements of EC with metrological reliability. Inmetro—the Brazilian National Metrology Institute—has produced CRMs of low EC. An important study is concerned with stability monitoring, since these CRMs generally have short expiration date due to the difficulties usually associated with the stability of low value EC solutions. Inmetro has been performing studies of stability monitoring of low EC CRMs over the last few years by using a primary system of EC. The main goal of this work is to present the results of monitoring the stability of low EC solutions of 1.5, 5, and 50 µS·cm−1 by primary measurements with uncertainties within the characterization uncertainty range of the three CRMs. The results demonstrated that these CRMs remained stable at 25.0 °C during a period of 19 to 20 months.