top of page

CALCULATION OF HARDNESS WITH   EDTA 

  • Water hardness can be measured using a titration with ethylenediaminetetraacetic acid (EDTA). 
        EDTA dissolved in water forms a colourless solution.

     

  • At pH 10, calcium and magnesium ions form colourless, water soluble, complexeswith EDTA: 
        calcium ion complexed by EDTA : CaEDTA2- 
        magnesium ion complexed by EDTA : MgEDTA2-

     

  • An indicator, known as a metal ion indicator, is required for the titration. 
        This indicator can be used to detect the presence or absence of free EDTA ions in solution.

     

  • Endpoint of the titration is when all the Ca2+ and Mg2+ ions have been complexed by the EDTA. 
    Before the endpoint, Ca2+ and Mg2+ are in excess, there is no free EDTA in solution. 
    Immediately after the endpoint, there is an excess of EDTA.

     

     

     

     

     

     

     

     

     

The Equations

The balanced chemical equation for the reaction between calcium and magnesium ions and EDTA is:

 

Ca2 + H2EDTA2- → CaEDTA2- + 2H+

 

Mg2 + H2EDTA2- → MgEDTA2- + 2H+

 

So if we let M2+ be the total of Ca2+ and Mg2+ ions in solution then:

 

M2 + H2EDTA2- → MEDTA2- + 2H+

 

Note that 1 mole of M2+ reacts with 1 mole of H2EDTA2-. 
At the equivalence point of the titration:

 

moles M2+ = (moles Ca2+ + moles Mg2+) = moles H2EDTA2-

bottom of page