Ion Balance in Water Analyses — The Effect of Added Silica on the Carbonate-Bicarbonate Titration

Abstract

Anion-cation balances based on the ions Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, Cl^-, HCO₃^- and CO₃^2- (in milli-equivalents per litre) for over 1500 analyses of natural waters have been examined. While most waters showed an ion balance which was within ± 0.10 of the total of these ions, there was a lack of ion balance associated with and expected of acidic waters and waters with low ionic concentrations. In addition, waters which were alkaline and which contained soluble silica generally showed an excess of anions.

The effects of added silica in various forms on the alkalinity titration of the carbonate-bicarbonate system have been investigated. The presence of silicate anions leads to an apparent excess of cations in the ion balance, while the presence of silicic acid or silica gel leads to an apparent excess of anions. These apparent ionic imbalances result from reactions between soluble and insoluble silica with carbonate/bicarbonate ions, which cannot be detected either by the alkalinity titration or by the determination of soluble silica.