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

Conventional characterisation of low alkalinity waters via pH measurement a nd titration of total alkalinity to a prescribed end-joint invariably leads to large errors. These errors result from instability of the pH probe and an unknown titration end-point. In this paper two indirect methods (termed the "double Gran function" and the "blend" method) for the characterisation :bf such waters are.. evaluated critically. A blend composed of the raw wat er, sodium chloride (to increase conductivity), and standard bicarbonate (t o increase buffering capacity) was titrated with standard strong acid in tw o pH regions: 6.3 < pH < 7.0, and 3.5 > pH < 4.0. In both methods, total al kalinity was determined using the latter set of points, and the first Gran function. In the double Gran function method. the upper set of titration po ints was used to determine, CO2, acidity using the second Gran Function. In the "blend" method, equilibrium chemistry data were used to calculate tota l acidity for each point based on the known total alkalinity,,pH reading,te mperature and ionic strength. The two methods gave excellent results (in te rms of both repetition and accuracy) us compared to characterisation based on total alkalinity and inorganic carbon analysis. A detailed procedure for the execution of the two approaches is given in an appendix.