Determination of Total Calcium and Magnesium Ion Concentration

Introduction

This method, called a complexometric titration, is used to find the total calcium and magnesium content of milk, sea water and various solid materials. It can also be used to determine the total hardness of fresh water provided the solutions used are diluted. The combined concentration of calcium and magnesium ions is considered to be the measure of water hardness.

The method uses a very large molecule called EDTA which forms a complex with calcium and magnesium ions. EDTA is short for ethylenediaminetetraacetic acid. A blue dye called Eriochrome Black T (ErioT) is used as the indicator. This blue dye also forms a complex with the calcium and magnesium ions, changing colour from blue to pink in the process. The dye–metal ion complex is less stable than the EDTA–metal ion complex. For the titration, the sample solution containing the calcium and magnesium ions is reacted with an excess of EDTA. The indicator is added and remains blue as all the Ca2+ and Mg2+ ions present are complexed with the EDTA.

A back titration is carried out using a solution of magnesium chloride. This forms a complex with the excess EDTA molecules until the end-point, when all the excess EDTA has been complexed. The remaining magnesium ions of the magnesium chloride solution then start to complex with ErioT indicator, immediately changing its colour from blue to pink.

The main reaction is:

Ca2+ + EDTA4- --> [Ca-EDTA]2-

Back titration:

EDTA4- + Mg2+ --> [Mg-EDTA]2-− −

Indicator reaction: Note: ErioT is blue and ErioT-Mg is pink

ErioT + Mg2+ --> ErioT-Mg

Method

To download a printable version of this experiment (in pdf format) use the link below.

Calcium and Magnesium Ion Concentration (PDF 140Kb)

Please note that Outreach support for these experiments is limited to NZ school students and teachers, and we are unable to answer queries from overseas.

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