What is the Test for Sulphate Ions

In an attempt to answer the question, what is the test for sulphate ions, this article covers the area of qualitative analytical methods to identify sulphate (SO₄^2-) ions. Almost all of these test methods involve visual observations, by allowing sulphates to react with other chemical reagents. Precipitation methods are widely used in the analysis of sulphate ions. Some of the metallic precipitates possess unique properties such as insoluble in dilute hydrochloric acid and dilute nitric acid; that of carbonates, phosphates and sulphites do not behave in the same way. Those methods can be used to distinguish sulphates from others. Some of these test methods are very sensitive. They can be identified even if they are present in small quantities.

Tests for Sulphate Ions

Test for Sulphate Ions in Water – Solubility of metal sulphates in water

Sulphates of Barium (BaSO4), Strontium (SrSO4), and Lead (PdSO4) are insoluble in water, those of Calcium (CaSO4) and Mercury (HgSO4) are partially soluble in water. Most of the other metallic sulfates are soluble in water. Some of the basic sulphates such as Mercury, Bismuth, and Chromium are insoluble in water, but those sulphates dissolve in dilute nitric or hydrochloric acids.

Barium Chloride Test for Sulphate Ions

When barium chloride (BaCL₂) is added to a sulphate solution, a white coloured precipitate is formed. It is insoluble in warm dilute hydrochloric acid and dilute nitric acid, but soluble when boiling.

Ba₂+ + SO₄^2- —>  BaSO4 ↓(White)

This test is usually carried out in an acidic medium, by adding dilute hydrochloric acid. Carbonates (CO3^2-), Sulphites (SO3^2-) and Phosphates (PO4^3-) do not precipitate under this condition.

Potassium Permanganate Test for Barium Sulphate

If barium sulphate is precipitated in the presence of potassium permanganate (KMnO4), it turns to a pink (violet) colour after absorbing some of the permanganate. The adsorbed permanganate on the surface cannot be reduced using common reducing agents. Even hydrogen peroxide is not an effective reagent for the reduction. The excess potassium permanganate which are not adsorbed on the surface readily reacts with the common reducing agents. Thus, the pink barium sulphate is clearly visible in the colourless medium.

Lead Acetate Test for Sulphate Ions

When lead acetate is added to a sulphate solution, a white coloured precipitate of lead sulphate is formed. It is soluble in hot concentrated sulfuric acid, solutions of ammonium acetate and hydroxide solutions.

Pb²⁺ + SO4^2- —> PbSO4 ↓(White)

In the last case (PbSO4 with NaOH), sodium tetrahydroxoaluminate (II) is formed. On acidification with hydrochloric acid, lead chloride crystallizes in the medium.

PbSO4↓ + NaOH —>  Na₂[Pb(OH)₄] + SO4 ^2-

Silver Nitrate Test for Sulphate Ions

When silver nitrate is added to a sulphate solution, a crystalline precipitate of silver sulphate is formed. This occurs only in concentrated solutions (solubility of Ag₂SO₄ = 5.8 gl^-1 at 18 ⁰C).

Ag⁺ + SO4 ^2- —>  Ag₂SO4 ↓

Mercury Nitrate Test for Sulphate Ions

When mercury nitrate is added to a solution of sulphate ions, a yellow precipitate of basic mercury sulphate (HgSO4) is formed.

3Hg²⁺ + SO4 ^2- + 2H₂O —> 4H⁺ + HgSO4.2HgO ↓(yellow)

This is a sensitive test and can observe even from the suspensions of barium or lead sulphates.

What is the Test for Sulphates – Summary

Sulphates form precipitates with some of the metal ions with unique solubility properties. It does not form precipitates with all of the metal ions, but with Barium, strontium and lead (BaSO4, SrSO4, PbSO4), it forms a water-insoluble precipitate. They are not soluble in warm dilute hydrochloric acid or nitric acid, but are soluble when warming. Some sulphates (Bi₂(SO4)3, Cr2(SO4)3) are insoluble in water, but soluble in dilute HCl and dilute HNO3 acids. Some sulphates (HgSO4, CaSO4) are partially soluble in water. Mercury nitrate gives a yellow coloured precipitation in the presence of sulphate ions; this can be used to identify even if the concentration is fairly low. Barium chloride forms a pink coloured precipitate with sulphates, when potassium permanganate is in the reaction medium.