Micro-chemical test for sodium and potassium ions

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Author: Christine Bläuer

back to Micro-chemical testing

Micro-chemical test for sodium and potassium (Na+, K+)

For more information on the required equipment and materials see Micro-chemical testing.

Procedure

Slightly adapted from [Chamot.etal:1946]Title: Handbook of chemical microscopy - Chemical methods and inorganic qualitative analysis. Vol. II
Author: Chamot, Emil Monnin; Mason, Clyde Walter
Link to Google Scholar p. 62 Preparation of the reagent solution (needs to be prepared fresh for each test series but the preparation can be used to perform several tests within one day):

  • Put 1 small drop of conc. H2SO4 in the cavity of a microscope slide.
  • Add 1 very small particle (max. 1 mm3) of bismuth nitrate to the drop.
  • Stir briskly for 1 to 2 minutes with a glass rod (bismuth nitrate dissolves very slowly).

When the solution is saturated, the acidic drop becomes white, stiff and swells visibly.

  • Add 1 drop of deionized water and stir vigorously. When the white salt is dissolved the solution should be colorless or perhaps opalescent.
  • Add 1 drop of HNO3 2M.

The reagent solution is now ready to be used.

Test for Na+ and K+:

1 drop of the reagent solution is added to a previously dried test drop on an object slide; or a drop the reagent solution is put on an object slide and some grains of the salt sample are added to it. The reaction products have to be observed under the microscope at a magnification of about 100x.

Na+ forms usually very quickly typical needles of a sodium-bismuth-sulfate {3Na2SO4.2Bi2(SO4)3.2H2O};

K+ forms (generally only after very gentle heating) hexagonal plates of potassium-bismuth-sulfate {K2SO4.Bi2(SO4)3.2H2O}, these plates can be iridescent.

Ions that may inhibit the test or affect its sensitivity:

Ca, Ba, Sr, Pb, Ag, Tl and Hg can interfere or completely hinder the reaction. If these ions are present in the sample, they have to be eliminated through precipitation with diluted H2SO4 but avoiding an excess of this acid. This acid solution is evaporated to dryness with gentle heat and then extracted with a small amount of water (the insoluble sulfates of the above ions remain behind). The solution is again evaporated to dryness and then tested for sodium and/or potassium.

Literature

[Chamot.etal:1946] Chamot, Emil Monnin; Mason, Clyde Walter (1946): Handbook of chemical microscopy - Chemical methods and inorganic qualitative analysis. Vol. II, John Wiley & Sons Inc. Republ. 2nd ed. 1989 McCrone Inst., ChicagoLink to Google Scholar
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