the Bureau International des Poids et Mesures (BIPM).
The described national standard of pH value is used for calibration of pH meter-electrode assemblies. The calibration consists of the following steps:
estimation of electrical resistance RE of the tested glass electrode;
calibration of the tested glass electrode;
calibration of the pH meter;
calibration of the temperature compensator.
Electrical resistance of the glass electrode is determined as the slope of the voltammogram (current vs. potential) within the range of 0.0 ÷ -1.0.V. The RE should be not higher than 1109 Ohm.
Calibration of the glass electrode is performed by measuring emf values with this electrode simultaneously with the reference hydrogen electrode in the same cell and buffer solutions. The experimental slope SlH of the linear calibration curve “E-pH” for buffers solutions measured with hydrogen electrode, and the similar slope SlG of the tested glass electrode, are determined by the regression analysis of all results of the measurements for each electrode. The SlH and SlG values should be not less than 53 mV/pH and not greater than 59.5 mV/pH at 25o C . The ratio (SG / SH)100% should be not less than 90%.
In order to make a pH meter calibration independent of the vagaries of electrode performance, the calibration is conducted by the pH meter comparison with known d.c. potentiometer  traceable to the national standard of volt. Potentiostat/galvanostat is using as d.c. voltage source in the range of applied potentials difference ± 2000 mV.