Spot-Test for Sulphide

Iodine-Azide Test

The reagent used is made up as per Vogel’s Macro and SemiMicro Qualitative Inorganic Analysis, 4th edition, Section IV, 6, 8


The reagent is prepared by dissolving 3g of AR sodium azide in 100ml 0.1N iodine.

The iodine solution is prepared by dissolving 10g of AR potassium iodide and 3g of AR iodine in 250ml of deionised water.


A sharp scalpel can be used to remove a tiny scraping of the deposit to be tested (approx. 10ug). The scraping is transferred to a microscope slide, and covered with a cover-slip before introducing the reagent under the cover-slip in the usual manner.

The sample can be viewed most conveniently with magnifications of x10 to x100.

Within several minutes of reagent introduction, if the material is a metal sulphide, gas bubbles will be seen nucleating, then growing, on the surfaces of the sample.

Natural iron pyrite (FeS2) crystals showing positive reaction to iodine-azide reagent.

The great advantage of this reaction is that sulphide is not consumed to any significant extent. Although not strictly correct, the action of the sulphide can viewed as that of a catalyst rather than a conventional reactant. Thus an insoluble sulphide can be tested directly, without the need for acid digestion, thereby avoiding the difficulties of trapping and assaying minute amounts of hydrogen sulphide.

The reaction is specific to reduced sulphur species, which, in the SRB corrosion environment, means the various metal sulphides, most likely mackinawite (FeS), but possibly intermediate forms such as greigite (Fe3S4) through to pyrite (FeS2).

The rate of reaction with a bulk metal sulphide is slow compared with what happens if say freshly precipitated FeS is used. Presumably this is simply a matter of the surface area presented.

The same rate of reaction rate is observed for flakes of putative mackinawite as for pyrite. The video below gives an idea of the typical rates observed. Observation over a period of several minutes is required.