Micro-chemical test for sulfate

Author: Christine Bläuer

back to Micro-chemical testing

More information about the necessary equipment and materials see Article Micro-chemistry.

Procedure

SO4 test using calcium chloride solution

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

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

Samples containing as well calcium as sulfate

In test solutions containing as well Ca2+ as SO42-, 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 respectively, it can be estimated if the salt solution contains the same amount of calcium and sulfate or if one of the two ions is predominant. This estimation needs to be exercised with known salt mixtures.

SO4 test using silver nitrate solution

The drop to be tested is acidified with very little diluted nitric acid and then a drop of silver nitrate solution is added. If sulfate is present typical silver sulfate crystals are formed.

SO4 test using barium chloride solution

The drop to be tested is acidified with very little diluted hydrochloric acid and then a drop of barium chloride solution is added. If sulfate is present finely crystalline, white barium sulfate (BaSO4) is precipitated.

Literature

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