Micro-chemical test for sodium and potassium ions

Author: Christine Bläuer

back to Micro-chemical testing

More information about the necessary 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
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 1 to 2 minutes strongly 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 strongly. When the white salt is dissolved the solution should be colorless or perhaps opalescent.
Add 1 drop of HNO₃ 2M.

With this the reagent solution is 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 delicacy:
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 H₂SO₄: for this purpose the sample solution is evaporated to dryness with gentle heat and then extracted with little water. The water extraction is again evaporated to dryness and then H₂SO₄ is added avoiding a surplus of the acid.

Bibliography

[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., Chicago