Problems in assessments of amalgam electrodes for standardising or certifying the corresponding ion selective electrodes.
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.
Falciola
Luigi
L
Department of Physical Chemistry and Electrochemistry, University of Milano, Via Golgi 19, 20133 Milano, Italy.
Fusi
Patrizia
P
Mussini
Patrizia R
PR
Mussini
Torquato
T
D006863:Hydrogen-Ion Concentration; D017736:Ion-Selective Electrodes / Q000592:standards*; D011199:Potentiometry / Q000379:methods; D017186:Titrimetry / Q000379:methods