An assessment of factors affecting the reliability of total alkalinity measurements

Abstract

The accuracy and precision of different methods for detecting the endpoint of the total alkalinity titration and the typical accuracy and precision obtained for alkalinity analyses by a large sample of laboratories were investigated. The optimum, endpoint for total alkalinity titrations decreased from pH5.0 at 10mgl−1 alkalinity to pH4.2 for 300mgl−1 alkalinity or more. The appropriate color changes for bromocresol green-methyl red (BG-MR) and methyl orange (MO) indicators also varied with the initial total alkalinity of samples. Despite differences in pHs at endpoints for samples of different alkalinities, when the best endpoint pH, best color of BG-MR and MO, or the endpoint of methyl purple were used in titrations of standard solutions, there were few differences between measured alkalinities and standard alkalinities — the accuracy was better than ±3mgl−1. Results of spike and recovery tests on aquaculture pond water samples also revealed that an accuracy of ±3mgl−1 alkalinity could be achieved on either unfiltered or filtered samples by all four methods of acceptable endpoint detection. Although precision of measurements could not be consistently maintained below ±1mgl−1, coefficients of variation for repeated measurements usually were less than 3% for all methods of endpoint detection. Nevertheless, this degree of precision was adequate to achieve good accuracy that is the major concern in water analysis.Variations in alkalinity measurement that could result from improper selection of endpoint pH (or color) were rather small — usually not more than ±5mgl−1. In an interlaboratory comparison of alkalinity determinations on standard solutions, most laboratories reported inaccurate alkalinities. These inaccuracies were greater than possible endpoint variations. The most likely sources of error contributing to poor results by the different laboratories were improperly standardized acid for alkalinity titrations, reliance on burets or other titrating devices with coarse calibrations, and use of excessively small sample volumes. Moreover, it was clear that most of the participating laboratories did not have a satisfactory method of quality control. Statement of relevanceAlkalinity is important as a source of carbon for photosynthesis, as a buffer, and as a general index of productivity. Thus, it is often measured, but by several different methods of endpoint detection. This study provides useful information of the reliability of alkalinity analyses and recommends how to maintain acceptable precision and accuracy of the procedure.