Micro-chemical test for calcium

Author: Christine Bläuer

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back to Micro-chemical testing

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

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 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, but within those two acids they do not form bundles of long needles. 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 needs to be exercised with known salt mixtures.

Further observations

When using HCl for the test indications for the presence of Na+ or K+ can be seen in the drying drop, by forming typical isotropic cubes of NaCl or likewise isotropic rectangular, cruciform structures of KCl near the fringe of the drop.

Literature

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