Measurement of pH, alkalinity and acidity in ultra-soft waters: comments on

The paper is a very welcome addition to improving these measurements of natural waters, not only because of the techniques used but also because the approach is based on a detailed analysis of the fundamentals of carbonate chemistry in these waters. However, the paper appears to overlook the significance of errors introduced by not considering the oversaturation of carbon dioxide that is typical of natural surface waters. In particular, the loss of this excess carbon dioxide during sample handling, storage (if any), and laboratory measurement. Further, consideration of the loss of excess carbon dioxide suggests that not all of the drift in measuring pH is due to probe problems; rather it is at least in part due to the loss of carbon dioxide. Other issues raised include: • The effect of the choice of value for the partial pressure of CO2 in air in determining the concentration of the standard bicarbonate solution. • Limitations in the accuracy of measuring total inorganic carbon in some analysers. • The desirability of automating the Gran titration method. • Whether the Standard Methods double endpoint method for measuring alkalinity might be sufficiently accurate. WaterSA Vol.28(3) 2002: 346-347