NOTES
1776
Vol. 64
I n the same range of pH and of concentrations of Cr(I1) and oxalate, but with the Cr(I1) and oxalate in approximately equimolar amounts, the AH. The agreement with the higher of the t,wo values initial rapid reduction of oxalate is soon inhibited calculated by Miller is extremely good considering by the formation of Cr(II1)-oxalato complexes. The tailing off in the rate of Cr(I1) consumption is the extrapolations involved. illustrated by the following typical result. In a solution which was initially 0.975 X 10+ M in M in oxalic acid, and which Cr+2and 1.00 X THE REDUCTION OF OXALATE BY had a pH of 1.6,' 29% of the Cr(I1) was oxidized CHROMIUM(II)'b2 after 6 minutes, but only 51% after 10 hours. The BY RONALD M. MILBURN AND HENRY TAUBE amount of oxalate reduced by the initial fast reaction can be greatly increased by introduction into Georee Herbert Jones Laboratoru, Universatj of Chicago, Chzcago, Illznozs the solution of other chelates, such as pyrophosRecewed June 3. 1960 phate or ethylenediaminetetraacetate, which, while It has been shown that free oxalate is rather ra- not reduced by Cr(II),8 compete for coordination pidly reduced to glycolate by solutions of Cr(1I) per- positions on the Cr(II1). In some typical cases chlorate. Oxalate bound to Cr(II1) is reduced only up to 70% reduction of Cr+2 was achieved in the very slowly by Cr(I1). The observation that oxa- first few minutes. late and Cr(I1) undergo an oxidation-reduction Experimental reaction is in agreement with M ~ i s s a n who , ~ reOrdinary distilled water was further purified by refluxing ported that on preparation of Cr(I1) oxalate, part with alkaline permanganate for about 12 hours, and redistilling from a tin apparatus. Solutions of Cr+2 and Cr.,+3 of the Cr(I1) was always oxidized. prepared by standard methods.9 Except where otherWe find that in the pH range 1-2, with oxalate in were wise indicated, perchlorate was used as the anion in solutions. moderate excess over Cr(I1) and with concentra- Reactions of Cr f 2 were conducted a t room temperature tions of the order of 1/100 AI, most of the Cr(I1) is (-23") under an atmosphere of nitrogen. Concentrations oxidized within a few minutes a t room temperature. of Cr+2 were estimated by addition of a deaerated solution of excess iodine (prepared as needed from standard iodate, The Cr(II1) produced is violet-red, of similar and iodide and acid) followed by potentiometric titration of color to Cr(III)-oxalato complexes. Some tailing the remaining iodine with thiosulfate, using platinum and off in the rate of Cr(I1) consumption is observed. calomel electrodes. Glycolate was determined quantitatively by addition to 1 In a solution which was initially 1.77 X loT2M of solution, !.5 X 10r4.to5.0 X M in glycolate, of in Cr+2 and 9.1 X 10-2 AI in oxalic acid, and which ml. 10 ml. of a solution containing 0.01 g. of 2,7-dihydroxynaphhad an initial pH of 1.2, the consumptions of Cr(I1) thalene per 10 ml. of concentrated sulfuric acid. The resultat -23" were: t = 0, 0%; t = 0.10 hr., 91.4%; ing sample was heated for 1 hour at 90" together with samt = 1.0 hr., 91.2%; t = 24 hr., 97.2%. A solution ples prepared from a solution of known concentration in glyOn cooling the solutions the optical densities at 540 of the same composition was allowed to react to colate. were measured and compared. Beer's law was found to essential completion (> 98% Cr(I1) oxidized), mp be obeyed throughout the specified concentration range. and then was exposed to the atmosphere, heated solution before the potassium hydroxide treatment failed to reveal with potassium hydroxide to free ligands attached the presence of free glyoxal. to the Cr(III), and filtered while hot and again after (7) This solution was buffered with HSOd- and SO'-5 Similar cooling in an ice-bath.* Tests made on portions of tailing off was observed with unbuffered solutions. (8) Although some oxidation of Cr(I1) by ethylenediaminetetraacet,he filtrate to determine the nature of the reduced tate was observed, the amount of oxidation in a few minutes was negspecies gave no indication of chloride, formate, ligible. formaldehyde, glyoxalate or glyoxal, but gave posi(9) H. Taube and H. Myers, J . Am. Chem. Soc., 1 6 , 2103 (1954). tive evidence for the presence of glycolate by the deep red-violet color of the 2,7-dihydroxynaphthalene test.5 Comparison of' the absorbance a t T H E HEAT OF COMBUSTION OF 540 mp of the red-violet product with the absorDICYANOBCETYLENE bancies given by solutions of known glycolate conBYGEORGE T. ARMSTRONG AND SIDNEY MARANTZ centrations indicated that the original filtrate conThe National Bureau of Standards, Washington, D . C. tained 95% of the amount to be expected for quanReceived June 6 , 1060 titative reduction of oxalate to glycolate by the Cr(I1) .'j Cyanogen, dicyanoacetylene and the dicyano(1) Supported by the A.E.C. under Contract AT(ll-1)-378. polyacetylenes are highly unstable thermodynami(2) The observations reported were made in the course of a study of cally. When they are burned with the proper the catalysis b y C r + + of the formation of Cr(II1) complexes. Alproportions of oxygen, carbon monoxide and though the research failed in its purpose with oxalate as ligand, a nitrogen may be the principal products of comstriking effect was demonstrated with pyrophosphate as ligand. (3) H. Moissan, Compt. rend., 92, 1051 (1881); Ann. chim. et p h y ~ . , bustion. These facts lead to the possibility of [5] !Xi, 401 (1882). obtaining very high temperatures of combustion, a (4) Much of the perchlorate was removed as KClO4; large amounts possibility which has been borne out by experiments of perchlorate interfered with some of the tests made. of Conway, Smith, Liddell and Grossel on the ( 5 ) On acidification with nitric acid and addition of silver nitrate, no cloudiness from silver chloride was observed. The qualitative combustion of cyanogen. Kirshenbaum and tests for the organic compounds were taken from F. Feigl, "Spot Testa Grosse2have calculated that even higher temperain Organic Analysis,"' D. Van Nostrand Co., Inc., Princeton, N. .I., QR,, =
6.3 f 0.6
where the limits are those of the derived value of
5th edition, 1954. (6) The potasaium hydroxide treatment may have resulted in at least partial conversion of any glyoxal to glycolate. Tests on the
(1) J. B. Conway, W. F. R. Smith, W. J. Liddell and A. V. Grosses J . Am. Chem. Soc.. 17, 2026 (1955). (2) A. D. Kirshenbaum and A. V, Grosse, ibid., 78, 2020 (195D).
