Determination of Sulphate

Without access to instrumental ion chromatography, the low-cost, fall-back has to be the classical analytical method of precipitation with a barium salt.

It is a method that should be robust, and authoritative, but has gained a reputation for poor and inconsistent recoveries. The truth of the matter is that you need to be experienced in quantitative precipitation methods if it is to yield the excellent results of which it is capable.

The method is basically as per Vogel’s Quantitative Analysis, section IV, 35 adapted to low umolar amounts of sulphate per aliquot. There are details that must be adhered to if reproducible results are to be obtained. The reasoning behind such details is covered by Vogel in section I, 54

Proceed as follows –

  1. 25ml aliquot of sample
  2. add 100ml of 0.05M hydrochloric acid
  3. heat to boiling on hotplate/stirrer, and back-off slightly
  4. add approx. 0.5ml of 0.2M barium chloride, slowly, drop-wise
  5. stir for 5mins
  6. continue adding barium chloride drop-wise over the period of 10mins with constant stirring
  7. heat on steam-bath for 2hrs at 95ºC with occasional swirling
  8. stand overnight to cool
  9. filter through 103 (vacuum) and wash x3 with cold 0.05M hydrochloric acid
  10. return filter paper to the original flask
  11. add 50ml DIW
  12. add 0.5ml conc. ammonia (30%)
  13. add exactly 5ml or 10ml of standardised 0.01M EDTA depending upon the quantity of precipitate
  14. heat on steam-bath for 2hrs at 95ºC with occasional swirling
  15. add 1.5ml pH 10 ammonia/ammonium chloride buffer
  16. add 2 drops of eriochrome black T
  17. titrate against standard 0.01M magnesium chloride

gmoles of SO4 in aliquot = gmoles of EDTA added – gmoles Mg(NO3)2 titrated.

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

Barium Chloride Solution 0.2M:

  • weigh out 24.43g of AR barium chloride
  • make up to 500ml with DIW

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

Notes:

  • the following details are necessary to ensure quantitative precipitation of BaSO4.
    • large starting volume to dilute possible sample interferences
    • acidic conditions to discourage super-saturation
    • boiling conditions to discourage super-saturation
    • very slow addition of BaCl2 and holding for 5 minutes at low concentration to seed crystal formation before allowing the concentration to rise too fast
    • digestion at 95ºC for 2 hours to allow precipitate size distribution to to equilibrate toward larger particles.
    • wash the precipitate with the same acid concentration as used for precipitation to avoid formation of colloidal BaSO4. This is critical for good, reproducible recoveries.
  • the washing of the precipitate and original flask must be exacting to avoid introduction of traces remaining from the considerable amount of excess barium that was used.
  • the filter paper will bind a small amount of Mg++ even in the presence of EDTA, and thus will contribute a positive blank. For this reason it is worth running a blank from the stage where the filtrate is digested in EDTA/NH3. Typical blank figures are 1umole to 1.5umole of Mg++