Field and laboratory measurement of pH in low-conductivity natural waters

Abstract

The accurate measurement of pH is fundamental to most environmental and hydrogeological studies. Many mineral and ion exchange equilibria are controlled by acidity levels and establishment of saturation indices requires accuracies to within 0.1 pH units. Correspondingly, in studies of the impact of acidic deposition, accurate pH measurement of rainfall, surface runoff and unsaturated zone/groundwater it is essential to establish the proton sources and sinks in the various hydrological pathways. Measurement of pH both in the field and laboratory almost universally involves electrode systems based on silver-silver chloride and calomel cells; these devices, although convenient, inexpensive and portable, are in many instances not sufficiently accurate for low-conductivity waters such as rainfall and many upland streams. Numerous studies for biological and inorganic systems as well as inter-laboratory comparisons have demonstrated this problem (Illingworth, 1981; Tyree, 1981; Mason, 1984; Covington et al., 1985a); hydrogen ion activity/concentration discrepancies of up to an order of magnitude are observed even under laboratory conditions (pH = −log H + activity and for very low ionic strengths H + activity ⋍ H + concentration). Such inaccuracies will be exacerbated in the more testing environment of field measurement. Improved electrodes based on a free-diffusion liquid junction and flowing sample are being designed (Covington et al., 1983, 1985b), however, these are not readily available and may not be suitable for field use. A description of the sources of these errors is given and suggestions are made for pragmatic solutions to the problems encountered. It is concluded that a fresh approach to field and laboratory measurement of the pH of low-conductivity waters is urgently needed.