Before the eq. point, the Ca 2+ conc. Is nearly equal to the unchelated Ca 2+. At the eq. point and beyond, pCa is determined from the dissociation of the chelate at the given pH. The more stable the chelate (K f large), the farther to the right will be the equilibrium of the reaction, and the larger will be the end point break.
The highly stable chelates can be titrated at low pH values. This is important because it allows the titration of some metals in the presence of others whose EDTA chelates are too weak to titrate at the lower pH.
Eriochrome Black T: o It contains 3 ionizable protons. o Used for the titration of Mg 2+ with EDTA. o It forms a red complex with part of the Mg 2+. o The color of the uncomplexed indicator is blue. o As soon as all the free Mg 2+ is titrated, the EDTA displaces the indicator from the magnesium, causing a change in the color. MgIn - + H 2 Y 2- MgY 2- + In - + 2H + red colorless colorless blue
The metal-indicator complex must be less stable than the metal-EDTA complex, or the EDTA will not displace it from the metal. Also, it must not be too weak or the EDTA will start replacing it at the beginning of the titration. The metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex.