THE SELECTION OF TITRATION UNKNOWNS CHARLES R. CONARD Harvard College, Cambridge, Massachusetts
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N A COURSE open to students presenting chemistry for college entrance we have an experiment on titration. I n addition to the usual comparison of solutions and the determination of the concentration of an "unknown solution,'' each student determines the equivalent weight of a solid acid. The selection of these solids has been a little difficult, for we wanted them t o be inexpensive, inert enough t o be packaged in paper envelopes ( 2 4 grams), stable enough to keep from year t o year, unaffected by atmospheric moisture and, finally, soluble enough in water t o allow fairly rapid titration.' We have found that the following substances answer these specifications and give good results in the hands of inexverienced students.
employed. The acid must be of low equivalent weight in order to keep the equivalent weight of the mixture within reasonable limits (we set 200 as a maximum). Furthermore, the substance should not cake after being powdered. We have worked with malic, citric, oxalic,andsuccinic acids, and as neutral material we have used sugar or potassium chloride. Malic and citric acids absorbed water or caked too much to allow thorough mixing of the powdered materials. Oxalic acid, a t first, seemed satisfactory. However, both the samples mixed with sugar and those mixed with potassium chloride turned brown on standing over the hot summer months, and in some cases a black sticky tar had formed. Although the oxalic acid mixtures apvarentlv remain unchanged s ~ ~ ~F I~ ~~ ~~ L ~ for ~ several ~ weeks a t 2o0c:,-the ch;nge takes place in a few hours a t 45'C. Sueeinie acid C%HdCOnHh 59 oralie*"id* (COIH)I.ZHIO 63 The mixtures of succinic acid and potassium chloCitric acid CaHI(C0.H). 64 M=I~C acid (dl) C~H.O(CO~H), 67 ride, however, are quite stable t o any atmospheric Phthalie anhydride* C&HdCO)nO 74 changes in temperature and humidity. The mixtures C.HIO
