Complexometric determination of zinc

general remarks

Zinc is determined by EDTA titration in almost the same way magnesium is - in the pH 10 against Eriochrome Black T. If the solution initially contains also different metal ions, they should be removed or masked, as EDTA react easily with most cations (with the exception of alkali metals).

reaction

Reaction taking place during titration is

Zn²⁺ + EDTA^4- → ZnEDTA^2-

sample size

For 0.01 M titrant and assuming 50 mL burette, aliquot taken for titration should contain about 0.35-0.45 millimoles of zinc (23-29 mg). If preparation of such sample is difficult, we can use different EDTA concentration.

end point detection

End point of zinc titration is easily detected with Eriochrome Black T.

solutions used

To perform titration we will need titrant - 0.01 M EDTA solution and ammonia pH 10.0 buffer. We will also need indicator - either in the form of solution, or ground with NaCl - 100 mg of indicator plus 20 g of analytical grade NaCl.

procedure

Procedure to follow doesn't differ from the one used for the magnesium titration.

• Transfer zinc solution to Erlenmeyer flask.
• Dilute to about 100 mL with distilled water.
• Add 2 mL of pH 10 ammonia buffer solution.
• Add a pinch of Eriochrome Black T ground with sodium chloride (100 mg of indicator plus 20 g of analytical grade NaCl).
• Titrate with EDTA solution till the color changes to blue.

result calculation

Calculation of EDTA titration results is always easy, as EDTA reacts with all metal ions in 1:1 ratio:

Zn²⁺ + EDTA^4- → ZnEDTA^2-

That means number of moles of zinc is exactly that of number of moles of EDTA used.

sources of errors

In general this is a simple titration, with no other problems then those listed as general sources of titration errors.