Complexometric determination of magnesium

general remarks

Magnesium can be easily determined by EDTA titration 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). Calcium can be precipitated as carbonate or oxalate, although presence of oxalates may make end point detection difficult.

reaction

Reaction taking place during titration is

Mg²⁺ + EDTA^4- → MgEDTA^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 magnesium (8.5-11 mg). If preparation of such sample is difficult, we can use different EDTA concentration.

end point detection

End point of magnesium 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 much from the one used for the EDTA standardization.

• Transfer magnesium 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:

Mg²⁺ + EDTA^4- → MgEDTA^2-

That means number of moles of magnesium 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. The most likely problem is spotting the end point, which is not always sharp.