Micro-chemical test for calcium

Author: Christine Bläuer

back to Micro-chemical testing

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

Procedure

Add one drop of 1M sulfuric acid to the test drop of the aqueous salt solution on a flat object slide. 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.

Positive test result

If Ca²⁺ is present sheaf like bundles of elongated needles of gypsum can be observed at the fringe of the test drop. Detection limit for Ca²⁺ = 0.05 µg.

Samples containing both calcium and sulfate

In test solutions containing both Ca²⁺ and SO₄^2-, gypsum crystals can be observed to form at the fringe of a test drop while drying. Better crystals will be formed if the drop is previously acidified with 2M hydrochloric acid or 2 M nitric acid prior to drying it. Depending on the acid used the shape of the gypsum crystals will vary. To become familiar with the possible shapes that gypsum crystals will take in different acid preparations, known sample solutions should be prepared and observed. A common feature of all crystallization forms of gypsum formed during drying of a solution is the formation of twinned crystals (see image 1).

When comparing the amount of gypsum formed in drops with or without addition of sulfuric acid 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 requires to run parallel tests with known salt mixtures for comparison.

Further observations

When using HCl for the test indications for the presence of Na⁺ or K⁺ it will be seen that as the drop dries typical isotropic cubes of NaCl or likewise isotropic rectangular, cruciform structures of KCl will form at the edge of the drop.