Determination of total alkalinity in waters by using two different procedures: a colour indicator and the Gran plot curve

The total alkalinity determination by two titration procedures was carried out in water samples collected at the Guarapiranga Reservoir, Sao Paulo, Brazil. For this, a dye indicator (0.1% methyl orange plus 0.1% blue anylin, 1:1) and the Gran plot titration methods were confronted. Results were different for all the ten samples selected in the assay, being higher up to 92% of the cases, when the indicator method was employed. To assess for the accuracy of the analysis, a quality control standard solution constituted of diluted NaOH was prepared and titrated by using both procedures. Related to the Gran titration, high correlation coefficients were obtained for all samples using only three points in the potentiometric curves. Based on this approach, an Excel worksheet is available, aiming that the determination of total alkalinity by Gran titration procedure can be carried out on routine basis with accuracy and feasibility.