Complexometric determination of calcium

general remarks

Calcium can be determined by EDTA titration in solution of 0.1 M sodium hydroxide (pH 12-13) against murexide. Just like during determination of magnesium all metals other than alkali metals can interfere and should be removed prior to titration. Magnesium in that high pH precipitates as Mg(OH)₂ and is not complexed by EDTA, thus its presence can be ignored. Note, that if the amount of magnesium is huge, calcium can coprecipitate with Mg(OH)₂. Presence of ammonium salts is undesired, as they lower pH and make end point less sharp. To get rid of ammonia, solution can be heated after NaOH was added.

reaction

Reaction taking place during titration is

Ca²⁺ + EDTA^4- → CaEDTA^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 calcium (14-18 mg). If preparation of such sample is difficult, we can use different EDTA concentration.

end point detection

End point of calcium titration is easily detected with murexide, assuming pH is high enough. Murexide solutions are not stable and should be not stored longer than a week.

solutions used

To perform titration we will need titrant - 0.01 M EDTA solution and 1 M sodium hydroxide solution. 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 is mostly identical with the one used for the EDTA standardization.

• Transfer calcium solution to Erlenmeyer flask.
• Dilute to about 100 mL with distilled water.
• Add 10 mL of 1 M sodium hydroxide solution.
• Add a pinch of murexide ground with sodium chloride (100 mg of indicator plus 20 g of analytical grade NaCl).
• Titrate with EDTA solution till the color changes to violet.

result calculation

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

Ca²⁺ + EDTA^4- → CaEDTA^2-

That means number of moles of calcium 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. In the presence of huge amounts of magnesium there is a risk that some calcium will precipitate together with Mg(OH)₂, thus introducing negative error to the titration result.