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Determination of Magnesium by Direct Titration using Eriochrome Black T as indicator

How can you Determine the Magnesium ion by complexometric titration

The concentration of Magnesium ions in a sample can be determined quantitatively by complexometric direct titration with EDTA. For Mg2+, the complexation reactions with Y4‒ and corresponding formation constants Kf are:

Determination of Magnesium

The large magnitude of this formation constant indicates that the above reactions will go to completion if the EDTA is present in its completely deprotonated form Y4‒. Furthermore, the great stability of the complex
would lead to a sharper end point. To ensure that the titrant is in its completely deprotonated Y4‒ 
form, pH value greater than 12 is necessary. Unfortunately, the precipitation of Mg2+ as Mg(OH)2 precludes
 the titration at such high pH. Instead, the titration can be carried out at pH 10. At this pH, a portion of 
the EDTA is in the HY3‒ form and the additional complexation reactions and conditional formation constants at this pH are:

As EDTA and its metal ion complexes are colorless, a metal ion indicator that undergoes a color change when transformed from the metal ion–bound to the unbound form is needed. Eriochrome black T is used as an indicator; it is Blue when it is free (Hln2-) and wine-red when complexed with magnesium. As the EDTA titrant is added to sample, free Mg2+ ions are captured first. After complexation of all the free ions, EDTA reacts with the small amount of bound metal ion–indicator complex. The metal ions are effectively removed from the indicator as EDTA binds more tightly to these ions. This victory in the competition for the metal ion is only possible because of the value of the formation constant for the Mg2+- Eriochrome black T complex,MgIn‒, is smaller than that for the Mg-EDTA complex, MgY2‒. The color change during titration:


1- Magnesium ion solution 
2-pH10 buffer solution
3- Eriochrome black T indicator  
4- 0.01 M EDTA solution 

1-     Pipette 25.0 mL of Magnesium ion solution in a 250.0 mL conical flask and dilute to about 200.0 mL of distilled water
2-     Add 1 to 2 mL of pH10 buffer and 3 to 4 drops of Eriochrome Black T indicator to each.
3-     Titrate with 0.01 M EDTA until the color changes from wine-red topure blue. Record the used volume (as VEDTA).
4-     Repeat the titration twice.