Micro-chemical test for sulfate

Author: Christine Bläuer

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Micro-chemical test for sulfate (SO₄^2-)

For more information on the required equipment and materials see the Micro-chemistry article.

Procedure

SO₄^2- test using calcium chloride solution

Add a drop of CaCl₂ solution (1% aqueous solution) to the test drop. Heat the drop gently until the edge of the drop starts to dry (do not dry completely!). Observe the crystals forming at the fringe of the drop under the microscope with a magnification of about 100x. When the solution contains sulfate, gypsum crystals form that can easily be recognized by their short and twinned shape (image 1).

Image 1: Short, typically twinned gypsum crystals at the border of a drying test drop.

Samples containing both calcium and sulfate

In solutions containing both Ca²⁺ and SO₄^2-, gypsum crystals can be observed at the fringe of a test drop while drying. These have very distinct shapes if the drop was previously acidified with 2M hydrochloric acid, or 2 M nitric acid, respectively. When comparing the amount of gypsum formed in drops with or without addition of calcium chloride solution, it can be estimated whether the salt solution contains the same amount of calcium and sulfate or if one of the two ions is predominant. For this purpose, the test has to be repeated with known salt mixtures.

SO₄^2- test using silver nitrate solution

The drop to be tested is acidified with a very small drop of dilute (2M) nitric acid and then a drop of silver nitrate solution is added. If sulfate is present typical silver sulfate crystals are formed.

SO₄^2- test using barium chloride solution

The drop to be tested is acidified with a small drop of dilute (2M) hydrochloric acid and then a drop of barium chloride solution is added. If sulfate is present, finely crystalline white barium sulfate (BaSO₄) precipitates out.