Interlaboratory comparisons of the determination of pH in poorly buffered fresh waters

Ten participants gathered at one location to compare measurements, made in the field and laboratory, of the pH of quiescent solutions of natural waters and dilute acids. The total error associated with a single routine measurement in the laboratory can be controlled to within ±0.2 pH, comprising a standard deviation of 0.05 pH and a bias of 0.1 pH. Inaccurate preparation of standard solutions is an appreciable source of bias. Larger errors observed for field measurements made with commercial equipment arise from poorly defined analytical procedures, the use of inappropriate electrodes, and probably dampness affecting the electronics. Stirring dilute solutions depresses the pH. This bias error, which is similar for all dilute solutions, depends on the type of liquid junction used in the reference electrode. Combination electrodes produce the largest stirring shifts (>0.2 pH) and separate electrodes, especially those with free-flowing junctions, the smallest. Correct choice of equipment and use of well-defined proven procedures are essential for the accurate and precise determination of pH.