Micro-chemical test for magnesium

Author: Christine Bläuer

back to Micro-chemical testing

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

Procedure

Mg test using ammonium hydrogen phosphate and ammoniac

Acidify the sample drop with a small drop of dilute hydrochloric acid (2M). Add to this a drop of ammonium hydrogen phosphate solution. Invert the slide and hold the drop over an open bottle of concentrated ammonium hydroxide (ammonia vapor). Alternatively a drop of 2M ammonium hydroxide solution can be added to the test drop, but the first procedure usually produces the better results. If Mg2+ is present typical crystals of magnesium ammonium phosphate (MgNH4PO4.6H2O) can be observed in the centre of the test drop. Detection limit for Mg2+ = 0.05 µg.

Ions that may inhibit the test or affect its delicacy:

If calcium ions are present these form a white, flocculent precipitate of calcium phosphate which can hamper or even inhibit the detection of the magnesium ammonium phosphate.

Mg test using cesium chloride and sodium hydrogen phosphate

Acidify the sample drop with a small drop of dilute hydrochloric acid (2M). Add to this a drop of sodium hydrogen phosphate solution. Add one crystal of cesium chloride to the solution. If Mg2+ is present very small, octahedral crystals of cesium magnesium phosphate (CsMgPO4.6H2O) are forming very quickly near the dissolving grain of CsCl. Detection limit for Mg2+ = 1 µg.

Ions that may inhibit the test or affect its delicacy:

Ions of the elements Sr, Ba, Zn, Cd, Sn, Pb, Ag and Cu disturb or mask the reaction. calcium ions can form a white, flocculent precipitates of calcium phosphate which can hamper or even inhibit the detection of the cesium magnesium phosphate formed.

Mg test using Titan yellow after [1]

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

Ions that may inhibit the test or affect its delicacy:

The presence of calcium leads to a slightly orange to reddish staining of the solution which cannot be confounded 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 disturb the reaction [2]

Literature

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