Checking anion-cation charge balance of water quality analyses: Limitations of the traditional method for non-potable waters

Abstract

The factors affecting the sound application of an anion-cation balance check to water quality analyses are investigated. It is shown that the carbonate contribution to the total negative charge (in meq/l) is satisfactorily calculated from the formula [HCO3] = (Total Alkalinity)/50 below about pH 10 in samples in which carbonate is the only titratable component. It is also shown that ignoring the presence of titratable organic components may significantly affect this. It is shown that the contributions of H+ and OH- to the total positive and negative charge respectively become important at an 0.1 meq/l level below about pH 4 and above about pH 10. Examples are presented that highlight the importance of knowing detailed speciation of ligands that protonate and of metal ions that hydrolyse. In each case, the contribution to the total negative or positive charge is significantly altered from that of the deprotonated ligand or unhydrolysed metal cation because the effective average charge on the predominant species is modified. Furthermore, strong complex formation between protonated ligands and metal cations and between hydrolysed metal cations and strongly binding ligands can significantly alter the charge that might be deduced from simplistic equilibrium distributions that ignore this binding.