JOURNAL OF CHEMICAL EDUCATION
THE USE OF STARCH-IODATE PAPER IN QUALITATIVE ANALYSIS JOHN A. LUNDIN Macalester College, St. Paul, Minnesota
IN EVERY class of semimicro qualitative analysis a large number of students seem to have trouble in distinguishing between the sulfite ion and the carbonate ion when gaseous methods are used for their detection. Many such methods have been developed which, if carried out according to directions, will detect each ion nicely. However, they do have some disadvantages. In the methods where the formation of Pmssian blue is used to detect the sulfite ion, contamination from previous experiments may cause the decomposition of the ferric ferricyanide which many mistake for a positive test. The disadvantage of precipitating the two above ions as their calcium or barium salts lies in their identical physical appearance. In many other techniques the apparatus used is bulky and cumbersome. A simple answer to the above problems proved to be the use of "starch-iodate" paper in conjunction with the above tests. The paper turns blue on contact with the sulfur dioxide. The "starch-iodate" paper is prepared simply by moistening a piece of filter paper with a drop of each of the following solutions: starch, potassium iodate, and dilute nitric acid.
A few drops of the solution to be tested is placed in a 5-om. test tube and acidified with dilute hydrochloric acid. The test paper is placed over the mouth of the test tube and the tube heated in a hot water bath. The other anions which give gases upon acidification and interfere with the test are the sulfide and the thiosulfate ions. The sulfide ion may be removed easily by precipitating with cadmium nitrate and applying the test to the centrifugate. The presence of thiosulfate necessitates the removal of the sulfite ion with strontium nitrate and the testing of a portion of the precipitate. As little as 0.3 mg. of sulfite ion can be detected by this method. The "starch-iodate" paper may also be used as a preliminary test for the reducing anions. In this case, a drop of the test solution is merely placed upon the acidified paper. A positive test was obtained with the following anions: S-- (0.1), 8 0 3 - (0.1), SpOa-(lO.O), As03--- (1.0), Fe(CN)s---- (10.0), I- (0.1), NOa- (10.0), and SCN- (0.05). (The number in the parentheses indicates the number of milligrams of ion per milliliter of solution.)
