pH determination of phosphate buffer by Harned cell

Project Description

The project was performed in order to demonstrate and document the capability of the participants to measure the pH of a phosphate buffer by the primary measurement procedure for pH (Harned cell). Good agreement of the reported results was observed. The sample was very similar to the one used in the comparison CCQM-K9[1]. PTB acts as coordinator in CCQM-K9 and in this EUROMET comparison. Therefore it is possible to link the result of NCM in the bilateral EUROMET comparison (table 1) to CCQM-K9. The pH values given in table 1 are calculated by PTB for an ionic strength of the buffer I = 0.100 mol÷kg-1 from the acidity function at zero chloride molality, pa0, reported by the participants. PTB applied coulometric titration (H+) to determine the molality of HCl, mHCl, which was used to determine the potential of the Ag/AgCl electrodes. The standard uncertainty was estimated to u(mHCl) = 1÷10-5 mol÷kg. No information was given by NCM on how mHCl was obtained. A standard uncertainty of u(mHCl) = 2.1÷10-6 mol÷kg was declared by NCM.

Further Information

Table 1. Result of the bilateral comparison between NCM, BG and PTB, DE. Given is pa0, the acidity functionat zero chloride molality, mCl = 0 and its standard uncertainty, the slope of the acidity function, the pH and U the expanded uncertainty of pH with a coverage factor k = 2.