Problems in assessments of amalgam electrodes for standardising or certifying the corresponding ion selective...

The primary method for standardizing or certifying an Mz+-ISE (Ion Selective Electrode for Mz+ where Mz+ = alkali or alkaline-earth cation) implies comparing the latter with the corresponding M-Amalgam electrode in the cell Pt/M-Amalgam/Mz+ Solution/Mz+ ISE/Pt, whose potential difference is obviously independent of Mz+ concentration. Assessment of the potential of the M-Amalgam electrode requires the precise determination of the mole fraction x of the M metal in amalgam, which is customarily performed by decomposing an M-Amalgam sample in excess HCl and titrating the HCl excess with standard NaOH solution. There arises the problem of choosing the correct end-point in pH-metric titrations of strong acids with carbonate-contaminated NaOH standard solutions, which is of frequent occurrence both in research and routine laboratory practice. This topic is either overlooked or insufficiently treated in textbooks: thus the interpretation of the above experimental pH-metric titration curves is often misled and the results may be affected by significant errors. Some recent misleading suggestions are here re-analyzed critically in order to focus correct, recommended methodological schemes.