SELECTION OF THE SALT BRIDGE ELECTROLYTE SUITABLE FOR MEASUREMENTS OF HYDROGEN-ION CONCENTRATIONS OR PH

Liquid-junction potentials are examined between strong acid solutions or pH-standards and various salt bridge electrolytes. Salt bridge electrolytes with cation transference numbers smaller than that of KC1 are recommended for measurements of the hydrogen ion concentrations in acid solutions because they reduce changes in liquid-junction potentials resulting from the hydrogen ions. Apparent response slopes of a glass-reference electrode pair determined with two pH-standards (phthalate and neutral phosphate) were approximately proportional to the transference numbers of cation in salt bridge electrolytes. The well-known requisite that the transference numbers of symmetrical electrolytes should be nearly 0. 5 in order to minimize liquid-junction potentials in the pH region near to neutral was confirmed on the basis of both the liquid-junction potentials in acid solutions and the apparent slopes of pH-response. A mixture of KC1 (3.03 mol.dm-3) and CH3CO2Na (0.97 mol.dm-3) is suitable for general measurements in the pH region between 4 and 9 because the ideal Nernstian slope is obtained with three standards (phthalate, neutral phosphate, and borate) at 25°C. A 3mol.m-3 solution of KNO3 is also useful as the salt bridge of single electrolyte because a straight calibration line is obtained with three standards within an experimental error although its response slope is lower than the Nernstian one. The salt bridges with KCl-CH3CO2Na or KNO3 are useful for the pH of carbonate or oxalate standard, respectively, as well as the above-mentioned three standards, if an error of about 0.01 pH unit is allowed. The KNO3 salt bridge is superior for practical use to the KC1 salt bridge which has been widely utilized because it gives good linearities in the calibration with standards. © 1990, The Chemical Society of Japan. All rights reserved.