I'm sure that all of you know the experiment to find Ca2+ in ppm in a sample of hard water by titrating with edta using Eriochrome Black T as an indicator.

 

During titration: H2Y2- (aq) + Ca2+ (aq) à CaY2- (aq) + 2 H+ (aq)

H2Y2- (aq) + Mg2+ (aq) à MgY2- (aq) + 2 H+ (aq)

At end point H2Y2- (aq) + MgIn- (aq) à MgY2- (aq) + HIn2- (aq) + H+

Wine-red sky-blue

 

1.The indicator reaction has to occur after the free Ca2+ or any free Mg2+ react. Why?

 

2.Hardness is traditionally expressed in mg CaCO3/L of water. Suppose that a water sample

you analyzed contained mainly Mg2+, how would the calculation of hardness be affected?

Explain.

 

3. Why [Mg-indicator]- is less stable than [Mg(edta)]2- and in turn less stable than [Mg-indicator]- ??

 

THX

1. If it happens before, would you measure any calcium/magnesium?

 

2. If there isn't a lot of Ca²⁺, there wont be a lot of CaCO₃.

 

3. You are contradicting yourself there.