In a dry test tube, a crystal of ferric ammonium sulfate and a crystal of potassium thiocyanate are carefully ground together with a glass stirring rod. The iron hexathiocyanatoferrate that is formed sticks to the stirring rod.
In another test tube, dissolve 30 mg or 3 drops of the unknown in a minimal amount of toluene. The stirring rod bearing the iron hexathiocyanatoferrate solid is used to stir the unknown. If the solid dissolves and a reddish purple color develops, the compound contains oxygen.
Production of a solid which subsequently dissolves and a reddish purple color develops, then compound contains oxygen. This constitutes a positive test.
Hydrocarbons, alkyl halides, diaryl ethers, and other high molecular weight ethers give a negative test. Most compounds in which the oxygen is in a carbonyl group in conjugation with an aromatic ring or a double bond produce a negative test. Such compounds include cinnamic acid, aromatic acids, aromatic esters, aromatic anhydrides, or aromatic ketones. Phenols also give a negative test. However, aromatic aldehydes give a positive test. Other compounds that produce a positive test are aliphatic acids, aliphatic aldehydes, aliphatic ketones, aliphatic esters, aliphatic anhydrides, and aliphatic ethers.
This test should be used in conjunction with other tests. If the unknown produces a positive test, but yields a negative test for an aldehyde, a ketone, an ester, a carboxylic acid, or an anhydride, then the compound is an aliphatic or a low molecular weight ether.
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