Determination of Calcium and Magnesium

Confirmatory Method

In most instances, the compleximetric method described elsewhere is the one I usually employ. However, despite the elaborate clean-up, the possibility of interference sometimes leaves a nagging doubt in my mind. So, when an unusual result turns-up, it is good to have a more authoritative method to use as a sanity check.

The method below uses two unique properties, one of calcium, and the other of magnesium, to extract the electrolytes of interest from the sample prior to their determination. Calcium is precipitated out as the oxalate, and magnesium as the hydrogen phosphate. The method is adapted from various methods described by Vogel.

  1. 10ml aliquot of the sample
  2. add 100ml 0.05M HCl
  3. add 2 drops methyl red indicator
  4. heat to near boiling and then back-off to approx. 80ºC
  5. add 140mg of ammonium oxalate in 10ml DIW dropwise
  6. add 0.5M ammonium hydroxide dropwise until v. faint pink remains
  7. stir for 5min
  8. add 0.5M ammonium hydroxide dropwise until full yellow obtained
  9. stand overnight at RT
  10. vacuum filter through sintered glass G4 funnel
  11. wash x3 with DIW
  12. set precipitate aside for Ca analysis
  13. add 30ml 1.0M HCl
  14. add 2.5g of di-ammonium hydrogen phosphate
  15. add conc. ammonia soln. dropwise with stirring until full yellow
  16. stir for 5min
  17. add 3ml conc. ammonia solution
  18. stir 20mins
  19. stand overnight
  20. vacuum filter through 103
  21. wash x4 with 1.0M ammonium hydroxide
  22. wash x1 with methanol to clear walls of the funnel
  23. transfer filter-paper to the original flask
  24. add 50ml DIW
  25. add 10ml 0.01M EDTA
  26. add 1ml conc. ammonia soln.
  27. heat on steam-bath for 2hr at 95ºC
  28. stand to cool overnight
  29. add 1ml pH10, ammonia buffer
  30. add 2 drops eriochrome black T indicator
  31. titrate against 0.01M magnesium nitrate

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

With the calcium oxalate precipitate, proceed as follows –

  1. dissolve/wash the oxalate from the sintered glass funnel with successive amounts of 2M sulphuric acid up to the total volume of 50ml
  2. add 100ml DIW
  3. slowly titrate against 0.1N potassium permanganate using first persistent pink as the end point

gmoles of Ca = 2 x gequivalents of KMnO4 titrated