SOP: Determination of Sulphates in Water

GIFSQ — Global Institute for Food Safety & Quality

Principle

Sulphates are precipitated from the sample as barium sulphate by the addition of a known excess of barium chloride. The excess of barium left unprecipitated is back-titrated with standard EDTA solution using Eriochrome Black T as indicator. The alkalinity and the hardness of the sample must be determined before, to calculate the sulphates present.

Reagents

a. Ammonia–ammonium chloride buffer: mix 67.5 g of AR ammonium chloride (NH₄Cl) with 670 ml of AR ammonia solution, and dilute to 1 litre.

b. Standard barium chloride solution: dissolve 2.443 g of AR barium chloride in water and make up to 1 litre (1 ml of BaCl₂ solution = 1 mg of CaCO₃).

c. 0.01 M standard EDTA: dissolve 4 g of the disodium salt of EDTA (sodium versenate) in 1000 ml of water and standardize against barium chloride; adjust the EDTA concentration such that 1 ml = 1 mg of BaCl₂. The standard EDTA solution prepared for estimating hardness may be used. (Note: the book prints “1 ml = 1 mg of BaCl₂” here, while the calculation proceeds on 1 ml of EDTA being equivalent to 1 mg as CaCO₃ — the same equivalence as the hardness EDTA the book permits using; carried as printed with the inconsistency flagged.)

d. Eriochrome Black T indicator: mix together 0.5 g of Chrome Black T and about 100 g of NaCl to prepare a dry powder mixture.

Procedure

Use the sample taken for the determination of alkalinity; after the determination of alkalinity, add a little acid in excess

↓

If a fresh sample of water is taken, add methyl orange indicator and a slight excess of HNO₃ to 100 ml; boil to expel dissolved CO₂

↓

Pipette 10 ml (or more) of standard barium chloride solution to the boiling solution

↓

Allow the mixture to simmer, cool and make up the volume to 150 ml

↓

When the barium sulphate precipitate has settled, pipette 50 ml of the clear supernatant liquid into a conical flask

↓

Add 0.5 to 1 ml of buffer solution and an appropriate amount of Eriochrome Black T indicator

↓

Titrate with EDTA solution until a permanent blue colour is produced which does not change on further addition

Calculation

mg of SO₄ per litre = ((ml of BaCl₂ solution added as mg of CaCO₃ − ml of EDTA titre × 3) × 0.96 × 1000) / (ml of sample taken)

Where,

• ml of BaCl₂ solution added as mg of CaCO₃ = volume of standard BaCl₂ added × 1 mg CaCO₃ per ml
• ml of EDTA titre = ml of standard EDTA used for the 50-ml aliquot (× 3 for the full 150 ml, since 1 ml of EDTA ≡ 1 mg as CaCO₃)
• 0.96 = mg of SO₄ equivalent to 1 mg of CaCO₃
• ml of sample taken = volume of the water sample taken

Note: the printed formula is too garbled in the source OCR to reproduce verbatim; it is carried here in the standard equivalent form (total BaCl₂ added as CaCO₃, minus the unprecipitated BaCl₂ back-titrated with EDTA, converted to SO₄) and flagged. The 0.96 factor is the stoichiometric equivalent (1 mg as CaCO₃ ≡ 0.96 mg of SO₄) — a reconstruction, not a legible source value — flagged accordingly. If the sulphates exceed 100 mg/litre, the volume of barium chloride solution added must be increased.

References

Taylor, E.W., Examination of Water and Water Supplies, 7th edn., J.A. Churchill Ltd., London, p. 234 (1958).