Complexometric assay for water hardness: an interactive lab experiment.

Abstract

Water hardness is due to the presence of multivalent metal ions. In natural waters, the most abundant ions are calcium(II) and, in less amount, magnesium(II). Cations such as Fe(III), Al(III), Mn(II) and other metal ions also contribute to hardness, although their concentration level is much below the concentration level of calcium. Water hardness is a parameter of interest in water analysis, since it has adverse consequences in a great deal of human activities related to the use of this natural resource. The most widely employed assay for water hardness is complexometry. When hardness in water is determined by complexometry, the obtained value accounts for all multivalent metal ions present in the sample. Very often, the result is given as calcium carbonate equivalent per liter. In this simulated lab experiment, the complexometric titration of hardness in a water sample is carried out. All steps of the titration are covered, from the weighing of the titrant (EDTA as disodic salt) to the final calculation of the hardness parameter. The analyst can titrate the sample several times and enter the obtained values of the calculated mg/L of calcium carbonate equivalent for each titration. A final table shows and compares both the values calculated by the system and those calculated and entered by the analyst. An introduction shows the chelating properties of EDTA and a simulation of the the theoretical background of the titration is presented.