The main purpose behind this experiment is to determine the amount of Magnesium and Calcium contained within a water sample provided. Due to the fact that EDTA forms extremely stable complexes in a 1:1 stoichiometry with these 2 metals in the following reactionWe determine the “hardness” ofthe water a a function of the concentration of Ca2+ in ppm. Theendpoint for all titrations done in the lab is determined by ecriochrome blackT (EBT). The reaction of EBT (expressed as H3In in formula) reactsand forms compexes with the 2 metals as follows:The Min- is a wine redcolour while the product HIn2- is blue in colour and this is how wedetermine the end point for the reaction (at a pH=10). Since the Ca2+has a much higher Kf value it will therefore be titrated first andthe red colour will only change when the rest of the magnesium has beenreacted.
During this experiment we added around 10-12 drops of the MgY2-complex because for samples that do not contain magnesium, the colour change isquite slow and errors can arise in the endpoint detection.