joeyckn Posted January 16, 2008 Share Posted January 16, 2008 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 Link to comment Share on other sites More sharing options...
Fuzzwood Posted January 29, 2008 Share Posted January 29, 2008 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. Link to comment Share on other sites More sharing options...
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