George B. Kauffman and Richard A. Houghten, Jr.
Colifornio State College at Fresno Fresno
I
Syntheses and Titrations of Unknown Acids A general chemistry laboratory experiment
Although most general chemistry laboratory manuals include as separate experiments a typical inorganic synthesis and a determination of the equiva, lent weight of an unknown acid, a combination of the two into one experiment is not usually encountered in such manuals. It is well known that when separate experiments are combined "in series," the whole is greater than the sum of its parts. For example, when a student synthesizes a compound and then investigates his own product, the interest aroused is greater than that generated when the two experiments are performed separately. We have used this synergistic effect to good advantage by having our general chemistry students prepare a solid unknown acid from an unknown solid starting material and then determine its equivalent weight by titration with standard base. Student results for equivalent weights fall within 0.5-2yo of the true values. If a pH meter is used, ionization constants may also be determined for weak acids. After the student has reported his results, he is informed of the identity of his unknown starting material, and he is then required to write and balance all the pertinent equations for thesynthesis and titration and to calculate his percentage yield for the synthesis and the percentage error of his equivalent weight. He thus has an opportunity to utilize a considerable amount of his knowledge of theoretical chemistry and to expand his knowledge of descriptive chemistry. (Many freshmen are completely unacquainted with the interesting polyprotic complex acids and heteropoly acids.) We believe that the results, both in student interest and enthusiasm and in the techniques learned, more than justify the extra time and effort required in assembling and preparing the materials. The syntheses have been adapted and simplified from those appearing in the literature. To be suitable, the acid to be prepared should be (1) solid, (2) nonhygroscopic, (3) titratable by standard base, either directly or after addition of complexing agents, (4) relatively nontoxic, and (5) preparable without too much difficulty from easily obtainable, relatively inexpensive starting materials. We have found the following acids to be most convenient and satisfactory: Hezacvanderric (II) Acid, HiFe(CNh-a strong tetraprotic aeid, prepared fro& potass&m ferrocyanid; &Fe(CN)a,3H%O(1-4). Hexa~anocobaltic(III) Acid, HaCo(CN)6.1/2HzO-a strong triprotic acid, prepared from potassium hexacyanocobaltate(III), K3Co(CN)a (5, 6) obtained from our inorganic preparations course (7-9). Aminomethanexulfonic Acid, NH,CH2S03H-a weak monoprotic acid, prepared from technical grade sodium hydrogensulfite, NaHSOa (10-14). 408
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12-Tungstosilicic Acid, H.:SiW120ro~7Hz0-a strong tetraprotic acid, prepared from sodium tungstate, N%wOr.2HzO (15,16). Boric A&, H3BOa-a weak triprotic acid, prepared from borax, NaBa0~.10H~O (17). The acid is titrated as a monoprotic acid after addition of invert sugar solution (18). Benzmc Acid, C6H6COOH-a weak monoprotic acid, prepared from sodium benzoate, C6HsCOONa. Phthalic Acid, C6H4(COOH)z-a weak diprotic acid, prepared from potassium hydrogenphthalate, C6H4(COO)%HK. The last two syntheses are so simple and straightforward that both are assigned to one student. Copies of detailed student directions for the above seven compounds are available from the senior author (G. B. K.). I n addition to the above acids, other suitable compounds might include iodic acid, HIOs (19,20) ; periodic acid, HJOs (21) ; selenious acid, H2SeO3(22) ; telluric acid, H6Te06 (28) ; triphosphoric acid, H5PaOm(24); and sulfamic acid, HOSOzNHz(26). Acknowledgment
The authors gratefully acknowledge the experimental assistance of Dee Dalton and Me1 Zernow and the financial assistance of the NSF (Undergraduate Research Participation Program Grant GY 372). They are also indebted to the entire Fall 1966 Chemistry 1A class who served as "guinea pigs" and to Drs. Dale C. Burtner and Barrett W. Benson for valuable discussions. Work-study funds were furnished by the U.S. Government as provided by the Economic Opportunity Act of 1964. Literature Cited (1) BRAUER,G., "Handbook of Preparative Inorganic Chemistry," (2nd ed.), Academic Press, New York, 1965, Vol. 2, p. 1509. (2) BILTZ,W., Z. amrg. allgem. Chem., 170, 161 (1928). A,, AND KUSB, E., 2. Elektrochem., 34, 159 (3) MIITAB~H, (* 1- 9- 2R,>. ~ (4) BILTZ,H., AND BILTZ,W., "Ltbbomtory Methods of Inorganic Chemistry," (2nd ed.), John \V!ley & Sons, Inc., New Yark, 1928, p. 146. (5) Reference (I), p. 1543. (6) DOBROLYWBKIY, 0. K., Zhw. Priklad. Khin., 26, 1185, 1233 (1953). (7) BIGMW,J. H., Inorg. Syn., 2 , 225 (1946). (8) Reference ( I ) , p. 1542. (9) SCHLESSINGER, G. G., "Inorganic Laboratory Prepsrstions," Chemical Publishing Co., New Yark, 1962, p. 84. (10) REINKING, K., DEHNEL,E., AND LABHARDT, H., Bw.,38, 1077 (1905). (11) R~scaro,F., AND PRAHL, W., Ann. e h . , 448, 265 (1926). H., Rec. trav. chim., 52, 454 (12) BACKER,H. J., AND MIIULDER, (1933).
PILMER, U-.G., "Experimenlal Inorganic Chemistry," Cambridge Vnivemity Press, Cambrcdge, England, 1954, 0.359. SCHLESSINOER, G. G., Inorg. Syn., 8, 121 (1966); Reference (Y),p. 136. NORTH,E. 0.. Inorg. S!,n., 1 , 129 (1939). ADAM%D. bI., AND RAYNOR, J . B., "Advanced Practical Inorganic Chemistry," John Wiley & Sons, Inc., New Pork, 1965, p. 22. MELMR. J . W.,"A Comprehensive Treatise on Inorganic and Theoretical Chemi&ly," Val. 5, Longmens, Green and C O . London, ~ 1924, p. 49.
(18) KOLTHOFF, I. M., AND SANDELGE. B., "Textbook of Quantitative Inorganic Analysis," (3rd ed.), The Macmillan Co., New York, 1952, p. 534. (19) Reference ( I ) , Vol. 1, p. 317. (20) Reference (S),p. 148. Inorg. Sun., 1, 172 (1939). (21) WILLARD,H.H., (22) KING,A,, "Inorganic Preparations," (mu.ed.), George Allen and Unwin, Ltd., London, 1950, p. 119. (23) Reference (I), Vol. 1, p. 451. (24) JOLLY,W.L.,"Synthetic Inorganic Chemistry," PrentieeHall, Inc., Englewood Cliffs, N. J., 1960, p. 147. L. F., Inorg. (25) SISLER,H.H., BUTLER,M., AND AUDRIETH, Sun., 2, li6 (1946).
Volume 44, Number 7, July 1967
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