Water alkalinity: an acid-base titration interactive exercise

Abstract

The aim of this interactive exercise is to get a deeper knowledge of acid-base titrations and to improve the user's abilities in stoichiometric calculations. An unknown 50 mL sample is titrated using sequentially the indicators phenolphthalein and methyl orange. The sample solution may contain the basic species carbonate, hydrogenocarbonate (bicarbonate) and free hydroxyl ions (from a strong alkali) individually or as a compatible binary mixture of them. The sample can be a natural water sample, but not necessarily has to be so (a lab prepared solution can mimic the natural sample). The titrant is a solution of hydrochloric acid. The required volumes of titrant are V1 (phenolphthalein endpoint) and V2 (methyl orange endpoint). From the V2/V1 ratio, the analyst must find out the qualitative composition of the sample. Then he has to calculate the amount of the species (one or more) present in the sample. To do so, it is always necessary to set up a quantitative relationship with the use of stoichiometric factors which convert the millimoles of titrant to millimoles of analyte. The calculated concentration is finally entered to be checked by the computer. All data are randomly generated and the problem composition is not given beforehand.