Question #93356

1 Answer
Feb 14, 2018

Direct: Mg²⁺, Zn²⁺, Cd²⁺, Co²⁺, Ni²⁺, Cu²⁺, Fe³⁺, Pb²⁺, Ca²⁺, Ba²⁺, Sr²⁺, Bi³⁺, Th⁴⁺. Indirect : it depends what do you mean by "indirect".


Direct titrations
Mg²⁺, Zn²⁺, Cd²⁺ with Eriochrome black T;
Co²⁺, Ni²⁺, Cu²⁺ with murexide;
Fe³⁺, with sujphosalycilic acid or Tiron;
Pb²⁺ with methylthymol blue;
Ca²⁺ with calcein, calcon, calcein, calcon carboxylic acid;
Ba²⁺, Sr²⁺ with phthalein purple;
Bi³⁺, Th⁴⁺ with xylenol orange or pyrocatechol violet

substitution titration example: Mn²⁺,
This kind of titration is used with strong chelating metal ions if there is no known metal indicator for the ion to be titrated.

A known excess of Zinc or Magnesium complexes with EDTA are added to the metal ion unknown solution. The metal ion will replace Zn²⁺ or Mg²⁺. The liberated ions are then directly titrated with EDTA/Eriochrome black T.

Back titration examples: Al³⁺, Co²⁺, Ni²⁺,
This kind of titration is used when the metal ions to be titrated don't react with any known indicator or in case the metal ion to be titrated interfere with the indicator remaining chelated to it with it (e.g. Al³⁺ with dithizon or xylenol orange).

An excess of EDTA is added to the metal ion solution to be titrated. The remaining free EDTA is then titrated with a standardized solution of Zn²⁺ or Mg²⁺ of equal molarity with Eriochrome black T as the indicator.

Indirect titration of anions examples: SO₄²⁻ with Ba²⁺, CN⁻ with Ni²⁺.
The excess of a known amount of precipitating cation (e.g. Ba²⁺ with SO₄²⁻) or chelating cation (e.g. Ni ²⁺ with CN⁻) is titrated with EDTA. From the difference you can determine the amount of precipitated - chelated anion.

Indirect titration of cations example: Ag⁺.
A metal ion, as Ni²⁺, is substituted from its complex (e.g. from [Ni(CN)₄]²⁻) by another metal ion, as Ag⁺, which has to be determined. The substituted metal Ni²⁺ is then titrated with EDTA and its amount in moles will be equal to the amount of Ag⁺.

I hope your kind of "indirect" titration is included among these examples.