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            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 reaction

We determine the “hardness” of
the water a a function of the concentration of Ca2+ in ppm. The
endpoint for all titrations done in the lab is determined by ecriochrome black
T (EBT). The reaction of EBT (expressed as H3In in formula) reacts
and forms compexes with the 2 metals as follows:

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The Min- is a wine red
colour while the product HIn2- is blue in colour and this is how we
determine 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 and
the red colour will only change when the rest of the magnesium has been
reacted. During this experiment we added around 10-12 drops of the MgY2-
complex because for samples that do not contain magnesium, the colour change is
quite slow and errors can arise in the endpoint detection.

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