|MadSci Network: Chemistry|
Dear Miss Claudia; The flame test is by far the best, simplest, and most sensitive and specific method for the detection of Sodium. Unfortunately, it is sometimes too sensitive, it is hard to find something that will not give a positive flame test for Na. In the usual system of qualitative analyis, Na and K are tested for only after all other metal ions are removed from the unknown. The poorly soluble triple acetate of soium, zinc, and uranium is perhaps the simplest test. Try this concurrently with a known Na compound. [A] First, get rid of some of the interferences that may be present. To a portion [about 2 grams] of the sample add about 10 ml water, stir, then slowly add double the same volume of concentrated HCl. Observe what happens - if anything. Now evaporate the mixture to dryness. Add more HCl and dry again. The Na should be present now as NaCl. [B] Add water, stir,and filter off any insoluble portion [save it]. Any Na should be in the filtrate. Evaporate the filtrate to dryness again [to remove any excess HCl] observe the nature of the evaporate [NaCl crystals are cubic, may be hopper-shaped] and re-dissolve in a minimum of water. [C] The Reagent: Take 10 grams of Uranyl acetate, 10 grams of Zn acetate, 15 ml glacial acetic acid, 80 ml water. After preparation, leave overnight - a yellowish ppt. will appear, due to traces of Na present in reagents, the glass srface, etc. [D] 1. To 1 drop of the test solution in a watch glass or spot plate, add 8 drops of the reagent. Slowly a yellowish ppt of the salt will form if sodium is present. The crystals will be tetrahedra, examine under a microcope. 2. The Na triple acetate precipitate is fluoescent, but the reagent itself is not. [E] There are many interferences with this method. K and Li form a similar ppt. It is best to go through the usual qualitative analysis scheme to remove interferences before running the above test. Identification of the anion[s] present in the samplewill hewlp narrow down the possibilities. Charlie Crutchfield
Try the links in the MadSci Library for more information on Chemistry.