This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Not for all beverages are the preceding tests for metallic contamination applicable; in some cases it is desirable to evaporate the liquid carefully to dryness, ignite the residue, and test for the metals in the resultant ash. The evaporation should be conducted in porcelain. About a half pint of such liquids as carbonated and small beer, cider or vinegar, will usually suffice for the examination, but sometimes the use of considerably larger volumes is desirable. Towards the end of the evaporation an addition of strong nitric and sulphuric acids should be made, the quantity used depending on the amount of organic matter to be destroyed. The evaporation is then carefully completed, and the residue ignited at a low, red heat. After cooling, the ash is moistened with nitric acid, and one drop of sulphuric acid, and' again ignited. It is then again treated with a few drops of nitric acid, which is evaporated off cautiously, the process being stopped directly the acid fumes ceased to be copiously evolved.
The residue is then treated with hot water, and the solution filtered, when the following scheme of analysis should be followed: The aqueous solution may contain copper, zinc, iron, etc. Add excess of ammonia, and filter. The precipitate may contain iron, phosphate, etc. The filtrate, if blue, contains copper. Divide into two portions, then take one portion and acidulate with acetic acid, and add potassium ferrocyanide; a brownish precipitate or coloration is indicative of copper. Heat a second portion to boiling, and add potassium ferrocyanide; a white precipitate or turbidity indicates zinc. The residue of the liquid may contain lead, tin, etc. "Wash, and pour boiling solution of ammonium acetate in the filter. Acidulate the solution with acetic acid, and add potassium chromate; a chrome yellow precipitate indicates lead. With the residue, first ignite the filter paper, fuse the ash in a porcelain crucible with potassium cyanide, dissolve the product in water, filter, boil insoluble residue with strong hydrochloric acid, dilute, and heat clear solution with mercuric chloride; a white silky precipitate of mercu-rous chloride is due to tin.