Rapid Tests for Anions and Cations

When trying to determine the cause of corrosion it is common to examine a sample in the SEM and analyse the corrosion products.  However, this only detects elements and cannot tell if the sulphur is present as sulphide or sulphate, for example.

There is a quick and simple way to test for many species using spot tests.  All of the ones below were taken from "Spot Tests in Inorganic Analysis" by F Fiegl, published by Elsevier, 2nd edition, 1958.  There are probably more recent editions.  All you need to do is to scrape a small sample of corrosion product onto a glass slide, add a drop or two of the test solution and observe under a low power microscope (X10 or X20).

SULPHIDE - This is readily detected with the sodium azide test and it is not only specific for sulphide, it is also very sensitive.  The rate at which nitrogen bubbles evolve in the solution gives an indication of the quantity of sulphide, from a trace to a large quantity.

CHLORIDE - If you suspect chloride has entered a system where it was not expected, the silver nitrate test is also very sensitive to chloride and the amount of white precipitate indicates how much chloride is present.

AMMONIA- this is a more fiddly test and must be done very quickly after removal as ammonia is so volatile.  However, I have used this to demostrate ammonia-induced crevice corrosion in copper-nickel heat exchanger tubes.

There are lots of tests for other anions and cations and many are very specific and they are all quick to do.  Make sure you test the solution on something of known composition before using, as some of the test solutions can go off, when exposed to daylight, or just with time.

Posted on: 21st May 2017

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Image (top left) by Agnieszka