Determination of measurement uncertainty in the titrimetric and colorimetric analysis of carbonate-hydrogen carbonate-hydroxide mixtures

Abstract

It is broadly known that the titration of carbonate mixtures using visual indicators is prone to serious errors. The aim of this study was to quantify the measurement uncertainty (MU), select the highest contributor and investigate the relationship between the calculated measurement uncertainty and the corresponding concentration of the analyte. Neutralisation titrations, using hydrochloric acid were carried out to determine the concentration of the constituents of different binary and ternary mixtures, which were made up of variable concentrations and combinations of sodium carbonate, sodium hydrogen carbonate and sodium hydroxide. Each base was titrated separately and in a mixture. Ten replicate readings for each titration were recorded in order to obtain the measurement uncertainty. A spreadsheet was used to calculate all the concentrations and MU of each mixture. It was shown that the MU varies according to the concentration of the analyte. For 0.1 M mixtures of sodium carbonate and sodium hydrogen carbonate the MU for each constituent was identical and equal to ± 0.02mol r1. For 0.05M sodium carbonate O. lM sodium hydrogen carbonate mixture the uncertainties were ±0.012 and ±0.024 mol r1 respectively. The highest MU was obtained for O.lM sodium hydrogen carbonate 0.3M sodium carbonate mixture where the MU for sodium carbonate was± 0.16 mol L-1. It was also found that the MU of a constituent in a mixture varies considerably when compared to the uncertainty of the titration of the analyte alone. The MU of the ternary mixture was found to be one of the highest MUs obtained. In this study the indicator, thymolphthalein was proposed for the sodium carbonate sodium hydrogen carbonate titration since the commonly used phenolphthalein was found to increase the error of the titration.