International Comparison of Carbonate pH Primary Measurement Method

The process of the Chinese aqueous solution acidity (pH) primary standard participating in three international comparisons of carbonate pH measurements is briefly described. A systematic error in the benchmark was found in the 2006 CCQM.K18 and 2007 CCQM.K18.1 comparisons. It was concluded that the negative systematic error was caused by the continuous drift of the electrode potential due to the structure of the primary standard’s Harned-Cell and the low equilibrium potential obtained by using slope inversion. In 2009, the pH primary standard device was modified and improved with a new Harned-Cell structure, and in the international comparison of APMP.QM-K18.2016 in 2020, the international equivalence agreement was achieved, and the degree of equivalent DoE(d_i, U(d_i)) was DoE(−0.0014, 0.0030) .