This test for alum is applicable in the presence of phosphates.
Mrs. Richards.—Cover some logwood chips (they must be pure) with water and bring to a boil. Repeat this four times, saving only the last decoction. Shake some of the sample (a couple of teaspoonfuls) in a beaker half full of water. When it ceases effervescing, strongly acidify with acetic acid. Add a few drops of the logwood extract, and if alum is present a bluish-red color will appear.
CREAM OF TARTAR
Cream of tartar is bitartrate of potassium and is obtained from the lees deposited in wine casks. If gypsum has been used to clarify the wine, it will be present in the cream of tartar as calcium tartrate.
Other adulterants of cream of tartar are acid calcium phosphate, starch, gypsum, and alum.
TARTARIC ACID
Free or Combined
If the sample is known to be free from starch the following test may be made:
Mix a bit of the powder with a small quantity of dry resorcin and add a few drops of concentrated sulfuric acid. Heat slowly, and if a rose-red color forms, which disappears when diluted with water, there is present either tartaric acid or a tartrate.
When the sample contains starch, shake about 4 or 5 grams of it a number of times with 250 cc. of cold water in a large flask. Pour on a filter after the insoluble material has settled and evaporate the filtrate to dryness. The residue is to be tested for tartaric acid and tartrates, the same as when starch was absent.