VOL. 3. NO. 11
TEE?S&N~ITIVITY OP Q U A L I T A ~ ~ ~TB s s ~
1303
THE SENSITMTY OF QUALITATIVE TESTS BYRON A. SOULB,UNNBRSITYOP MICHICAN, ANNARBOR, MICHIGAN The average student has little, if any, conception of the sensitivity of the tests commonly used in qualitative analysis. Though solubility tables are readily accessible, he does not seem to recognize them as a possible source of such information. Therefore, with the idea in mind of giving the beginner a tool with which he can quickly and easily determine the delicacyespecially the comparative delicacy-of a test under ordinary laboratory conditions, the following experiment was worked out and has been used with marked success in the author's classes:' Put 1 cc. of N / 4 silver nitrate in a 25-cc. graduated cylinde~.~Dilute to 20 cc., then shake to mix thoroughly. Pour 10 cc. into a clean test tube. To this half add a few drops of hydrochloric acid. If a precipitate forms, dilute the part remaining in the graduate to 20 cc., shake, again transfer 10 cc. to a clean test tube and add hydrochloric acid. Continue diluting and testing until no precipitate is obtained. This is best determined by comparing the tested portion with 10 cc. of distilled water in a similar test tube. Hold the two tubes close together and look down through them from the top toward the bottom. The data secured in this experiment may be put to a variety of uses, e. g., the student may calculate (a) the quantity of silver in the last dilution giving a precipitate, (b) the quantity of silver chloride in the next succeeding sample. He may then compare this figure with the solubility of silver chloride as given in a table of solubilities. The numbers will probably not agree. Here is an excellent opportunity to bring up such points as: common ion effect, solubility of silver chloride in hydrochloric acid, carelessness in volume measurements, etc. A factor not usually considered in this connection is the effect of hydrochloric acid on the rate of precipitation of silver chloride, especially from a dilute solution. It may, for example, require hours to obtain maximum opalescence if ammonium chloride is used as the source of chloride ions, whereas with hydrochloric acid a much shorter time will suffice. Perhaps the most important application of this experiment is to show the comparative sensitivity of various tests using the same ion. Students are astonished to find, after obtaining 10-12 with silver nitrate, that they get not more than one dilution when testing lead nitrate. The knowledge thus obtained is not quickly forgotten and no trouble is encountered in explaining the occurrence of lead in both Group I and Group 11. The method is a modification of that suggested by Treadwell and Hall, "Analytical Chemistry." John Wilep and Sons,5th English ed., 1921, Vol. 1, pp. 78-9. a The silver nitrate may he dispensed from a buret, although the author prefers the use of a 1-cc. pipet.
Another opportunity to use the method comes when the confirmatory tests for iron are being considered. If five dilutions are obtained in the ferrocyanide test, then approximately fifteen will be obtained using thiocyanate, which corresponds fairly well with the published figures for the ratio of the sensitiveness of the two tests. By suitable modification as regards determination of the end-point, the experiment may be applied to tests involving the formation of a precipitate, a change in color, or the evolution of a gas.
