The general shape of the titration curves obtained by titrating 10.00 mL of a 0.01M solution of a metal ion M with 0.01M EDTA solution & relevent F A Q s

The apparent stability constant of various metal-EDTA complexes are indicated at the extreme right of the curves. It is evident that the greater the stability constant, the sharper is the end point provided the pH is maintained constant (the apparent stability constant is dependent on the pH)

In EDTA titrations, the end point is generally detected by a metal ion sensitive indicator (metalochromic indicator). Such indicators (which contain type of chelated groupings and generally posses resonance systems typical; of dyestuff) form complexes with specific metal ions, which differ in color from the free indicator and produce a sudden color change at the equivalence point.

Constant ph in EDTA titration is also important to maintain the proper function of many metalochromoc indicators because the color exhibited by them is dependant on pH.
eg: Eriochrome Black T indicator (EBT). This is represented as H3In

This reaction will proceed if the metal indicator complex [M-In] is less stable than the [metal-EDTA]. [M-In] dissociate during the titration and the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the [M-In] complex to leave the free indicator In.

a)Determination of Calcium and Magnesium in a mixture

Procedure

Pipette 25.00mL of the mixture into a titration flask, dilute with 25 mL of distilled water, add 5 mL pH =10 buffer solution and mix by swirling. Add 6 drops of EBT indicator. Titrate with the 0.05M EDTA solution to a pure blue end point.

Color change at the end point : red to blue

b)Determination of the Calcium

Procedure

Pipette two 25.00mL portions of the mixture into two separate titration flasks and dilute each with about 25 mL of distilled water. To the first flask add 4mL of 8 M KOH solution (a precipitate of magnesium hydroxide may be noted here), and allow to stand for 3-5 minutes with occasional swirling. Add about 50 mg of the Patton and Reeders indicator mixture and titrate with 0.05M EDTA until the color changes from red to blue. Run in to second flask from a burette a volume of EDTA solution equal to that required to reach the end point less 1mL. now add 4 mL of the KOH solution, mix well and complete the titration as with the first sample; record the exact volume of EDTA solution used.

I. Determine the concentration of magnesium and calcium ions in solution separately.

II. In experiment b, why can’t we use EBT instead of Patton and Reeder’s indicator?

All magnesium ions are precipitated as magnesium hydroxide by addition of 8M KOH. Only calcium ions remain in the solution. Calcium ions form a relatively stable complex with EDTA. With calcium ions alone no sharp end point can be obtained with EBT and the transition color from red to pure blue is not observed.

III. In experiment b, we should add KOH solution first and then the indicator. Why is this?

To determine calcium ion concentration, magnesium ions must be precipitated and removes first. Then add Patton and Reeder’s indicator. It works above pH 12

IV. Explain temporary hardness and permanent hardness. Give a method to determine total hardness and calcium hardness.

Temporary hardness is due to calcium and magnesium ions which are dissolved as bicarbonates.
Permanent hardness is due to magnesium, calcium, ferric, chloride and sulphate ions.

V. What would be the color change that you would observe if the method (a) is altered as follows
To a 25.00 mL of the test sample added 50.00mL of EDTA, 5mL of pH=10 buffer, 6 drops of EBT indicator and titrated with 0.05 M standard magnesium solution.
Purple to blue. (Red color is seen through blue)

VI. With calcium alone, no sharp end point can be obtained with EBT and a common procedure is to add a small amount of magnesium chloride to the EDTA solution before it is titrated. Explain why is that?

Calcium forms a relatively stable complex with EDTA but magnesium forms a less stable complex with EDTA.

VII. What is meant by a back titration? Explain why we determine Ni 2+ generally by a back titration

Some metal ions precipitate in the given pH or form inert complexes or suitable indicators are not available, so have to go for back titrations.
Nickel complexation reaction is very slow so it need to be titrated via a back titration.