Micro-chemical test for magnesium

Author: Christine Bläuer

back to Micro-chemical testing

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

Procedure

Mg²⁺ test using ammonium hydrogen phosphate and ammonia

Acidify the sample drop with a small drop of dilute (2M) hydrochloric acid. Add a drop of di-ammonia hydrogen phosphate solution (NH₄NH₂NO₃ 10% aqueous solution). Invert the slide and hold the drop over an open bottle of concentrated ammonium hydroxide to expose it to ammonia vapor. Alternatively a drop of 2M ammonium hydroxide solution can be added to the test drop, but the first procedure usually gives better results.

If Mg²⁺ is present typical crystals of magnesium ammonium phosphate (MgNH₄PO₄.6H₂O) can be observed in the center of the test drop. Detection limit for Mg²⁺ = 0.05 µg.

Ions that may inhibit the test or affect its sensitivity: If calcium ions are present, these form a white, flocculent precipitate of calcium phosphate which can interfere or even inhibit the formation of magnesium ammonium phosphate

Mg²⁺ test using cesium chloride and sodium hydrogen phosphate

Acidify the sample drop with a small drop of dilute hydrochloric acid (HCl 2M). Add to it a drop of di-sodium hydrogen phosphate solution (10% aqueous solution). Add one crystal of cesium chloride to the solution.

If the amount of Mg²⁺ present is very small, octahedral crystals of cesium magnesium phosphate (CsMgPO₄.6H₂O) will very quickly form near the dissolving grain of CsCl. Detection limit for Mg²⁺ = 1 µg.

Ions that may inhibit the test or affect its sentivity: Ions of the elements Sr, Ba, Zn, Cd, Sn, Pb, Ag and Cu interfere with or mask the reaction. Calcium ions form a white, flocculent precipitate of calcium phosphate that interferes or even inhibits the formation of the cesium magnesium phosphate formed.

Mg²⁺ test using Titan yellow, a triazin dye, after [1]

Put 1 drop of the test solution on a spot plate, acidify with diluted hydrochloric acid (HCl 2M) and 1 drop of the titan yellow dye solution. Add 1 or 2 drops of sodium hydroxide solution and stir with a glass rod. If Mg²⁺ is present a red, flocculent precipitate will form. Detection limit for Mg²⁺ ca. 1.5 µg

Ions that may inhibit the test or affect its sentivity: The presence of calcium leads to a slightly orange to reddish staining of the solution which cannot be confused with the red, flocculent precipitation. Control tests with known substances will help to recognize a positive reaction for Mg²⁺ unambiguously.

Nickel, zinc, manganese and cobalt ions interfere with the reaction [2]