Determination of Calcium and Magnesium

The classic methods of complexometric titration against EDTA, as described variously in section IV of Vogel’s Quantitative Inorganic Analysis, can exhibit a number of interference problems when employed in the analysis of wall-wash samples.

The following lengthy procedure has proved adequate for the vast majority of samples. However, where the analysis shows signs of interference, the fall-back method is to determine calcium by precipitation as the oxalate, and magnesium by precipitation as the ammonium phosphate.

Process two aliquots as follows –

  1. 10ml aliquot of sample
  2. evaporate to dryness on hotplate at 150ºC
  3. add 1ml conc. hydrochloric acid
  4. add 3ml conc. nitric acid
  5. evaporate to dryness on hotplate at 280ºC
  6. add 0.5ml conc. hydrochloric acid
  7. evaporate to dryness on hotplate at 150ºC
  8. heat on hotplate at 150ºC a further 2 hrs
  9. take-up in approx. 5ml DIW acidified with 2 drops of conc. hydrochloric acid
  10. warm for 5mins at 50ºC on hotplate
  11. add DIW to approx. volume of 25ml in total
  12. add 2 drops conc. ammonia (30%)
  13. swirl and stand for 60mins
  14. filter through 103 (gravity)
  15. wash x3 with DIW
  16. make filtrate up to approx. 100ml with DIW

Notes:

  • wall-wash samples very often contain small amounts of organic material, perhaps polyphenols from algae, that seriously interfere with the eriochrome black T indicator. They are oxidised by steps 3, 4 & 5
  • some samples, particularly where hydrothermal activity is present, contain sufficient soluble silicate to be a problem. Dehydration and precipitation as silica is provided by steps 6, 7 & 8.
  • some samples, particularly where hydrothermal activity is present, contain substantial quantities of iron and aluminium. Even small traces can block the eriochrome black T indicator. They are removed as the hydroxides at neutral pH via steps 12, 13 & 14.

Determination of calcium and magnesium combined:

  1. add 2ml of ammonia/ammonium chloride buffer pH 10
  2. add exactly 5ml of standardised 0.01M EDTA
  3. add 2 drops of eriochrome black T indicator
  4. titrate against standard 0.01M magnesium nitrate

gmoles of Ca and Mg combined = gmoles of EDTA added – gmoles of Mg(NO3)2 titrated

Notes:

  • back-titration of EDTA is used because it is more robust against blocking of the eriochrome indicator. The excess of EDTA ensures that any blocking species are held out of solution, thereby protecting the indicator.

Determination of calcium alone:

  1. add 8ml of 8M potassium hydroxide
  2. add a few mg of P&R indicator
  3. titrate against standardised 0.10M EDTA

gmoles of calcium = gmoles of EDTA titrated

Notes:

  • Even when using AR grade potassium hydroxide, its addition can contribute a significant blank and should be allowed for. Typical blank value is +6umole of Ca++

Magnesium Nitrate Solution 0.01M:

  • weigh out close to 2.568g of AR magnesium nitrate dihydrate
  • make up to 1L with DIW
  • not ideal as a primary standard because of slight deliquescence but is sufficiently accurate for the work in hand

EDTA Solution 0.01M:

  • weigh out close to 2.92g of AR EDTA
  • add 1.3g of AR sodium hydroxide
  • add 500ml of DIW and wait until it has dissolved
  • make up to 1L with DIW
  • standardise against 0.01M magnesium nitrate

Ammonia/Ammonium Chloride Buffer pH 10:

  • weigh out 17.5g of AR ammonium chloride
  • add 142ml of AR 30% ammonia solution
  • make up to 250ml with DIW

Eriochrome Black T Indicator:

  • weigh out 0.2g of eriochrome black T indicator
  • add 15ml of AR triethanolamine
  • add 5ml of absolute alcohol

P & R Indicator:

  • Patton & Reeders indicator. Also called calcon carboxylic acid.
  • weigh-out 0.5g of dyestuff
  • add 50g of AR anhydrous sodium sulphate
  • grind together in mortar until uniform colour is obtained