Is there a simple test for K⁺ ions?

My lab group was given an unknown compound. It is a solid with square crystals. We were given 15 compounds with which to compare it. We have tested it's solubility in water, HCl, acetone, toluene, and NaOH. It matched the tests ran on potassium chloride. It looks like it also. Are there any more tests we can run to confirm the presence of potassium or chloride ions? We don't have extensive testing equipment.
Renee 4/26/98

Hi, Renee. If you heat an ionic salt in a clear or light blue burner flame, some of them (including potassium) show a characteristic color to the eye (all of them give unique colors if you look through a spectroscope). In the case of potassium, the color is lavender.

However: you need to have a CLEAN wire loop (platinum or nickel-chromium most common) and a way to hold it (not your fingers, please) in the flame. Wash the loop in concentrated hydrochloric acid (caution--nasty stuff, wear safety equipment) and rinse in distilled or de-ionized water. Put the loop in the hot part of the burner flame. If there is a burst of color, the wire is not clean enough (clean it again). When the wire loop is clean, heat it and touch a bit of your unknown. This should transfer enough to the loop to carry out the flame test. If it doesn't, make a few drops of saturated solution using the unknown in distilled water.

The main difficulty is getting the wire clean enough. Sodium ion (yellow flame test)is much brighter than potassium and is a common contaminant. A little bit of sodium will mask a lot of potassium.

Look in the Handbook of Chemistry and Physics for a list of flame test colors. You may also want to look into a process called the "borax bead" test. Have fun!

Author: Garcia Oliver garicao@capaccess.org

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