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VOl. 57
HARRYLETAW,JR.,AND ARMINH. GROPP
POLAROGRAPHIC STUDIES IN NON-AQUEOUS MEDIA. I. FORMAMIDE’-3 BY
HARRY LETAW,JR.,* AND ARMINH. GROPP
Chemistry Department, University of Florida, Gainesville, Florida Recmoed M a y 6,1865
The polarographic half-wave potent,ials of acetophenone, anisaldehyde, benzalacetone, benzaldehyde, benzophenone, fluorenone, furfural, vanillin, lead( II), thallium( I) and zinc were determined in formamide solutions. Benzalacetone and furfural produced two waves. The diffusion currents of all species investigated were found to be proport,ional to their concentrations. Formamide was found to be a uReful polarographic solvent at potentials less than -1.60 v. VS. S.C.E. because of its stability, solvent properties and the ease of removal of oxygen from its solutions.
Introduction One previous study of the polarographic behaviors of formamide solutions has been reported. Zan’ko and Manusova5 determined the half-wave potentials of Cd++, Pb++, Sn++, Zn*+ as the sulfate, nitrate and chlorides, respectively. They stated that no supporting eIectroIyte was added because of the presence of a sufficient quantity of NH&l in the formamide. They attributed its presence to decomposition or t o an impurity from the synthesis of the formamide. It was concluded that there is little change in the half-wave potential of a substance determined in formamide as compared t o that determined in water. It was found that the diffusion current in formamide is less than that in water. In an effort to extend the usefulness of this solvent in polarography, the present research was designed to study the conclusions of Zan’ko and Manusova with respect to inorganic compounds and to investigate the behavior of various reducible organic compounds in the same medium. Experimental Materials.-Ultra-pure ZnSOc supplied by the Mallinckrodt Chemical Works and Baker and Adamson reagent grade Pb(N0& were used without further purification. Dr. F. H. Health of this Laboratory prepared and recrystallized TlpSOd for this research. Recrystallized fluorenone was provided by Dr. F. E. Ray, Director of the Cancer Research Laboratories of the University of Florida. Redistilled benzaldehyde, furfural and acetophenone and recrystallized benzalacetone, benzophenone and vanillin were given to us by Dr. C. B. Pollard of this Laboratory. Eastman Kodak anisaldehyde was used without further urification. Dried reagent grade KCland KNOs were useam s u p porting electrolytes. Metal Salts Corporation triple-distilled mercury was used without further purification. Eimer and Amend chemically pure grade formamide, after being placed under a pressure of less than 1 mm. for two hoursl wa8 found to be polarographically pure in the range studied. All solutions except those of TIBOt were prepared in a stock solution of 0.1 M KCI in formamide. A 0.1 M KNO, solution in formamide was used for the TlzSO4 solutions. TI804 and ZnSOd are sparingly soluble in formamide. Concentrations of these compounds were 0.0043 and 0.080 M , respectively. Dilutions were prepared from saturated solutions of each in formamide with the suitable supporting electrolytes. (1) Abstracted from a the& presented by Harry Letaw, Jr., to the Graduate Council of the University of Florida in Dartid fulflllrnent of the requirements for the M.S. degree, February, 1961. b (2) Supported in part by a grant from the Rese&rch Corporation. (3) Presented in part at the meeting of the Florida Academy of Sciences on December 2, 1950, at Lakeland, Florid& (4) A.E.C. Predoctoral Fellow, 1960-1952. Rbs+ dt unlvemity of Illinois.
Apparatus.-Polarogram were obtained using a SargentHeyrovsky Model XI1 Polarograph. Potentials were measured on a Gray “Queen” Potentiometer calibrated with a Westop Standard Cell. A 0.1 N calomel referen? electrode equipped with an agar-KC1 bridge was used. in order that transference effects across the interface be mmimized. All potentials are referred to the saturated calomel electrode. The mercury reservoir was 60 crn. above the level of the solutions. It was attached by neoprene tubing to a glass capillary of Corning “marine barometer” tubing 5.4 cm. long with an internal diameter of 0.05 mm. The capillary constant was 2.81 mg.z/a sec.-‘/Z a t -0.40, 2.78 rng.’/a sec.-’/a a t -1.00, and 2.56 mg.z/jJsec.-’/z at -1.40 applied volts in 0.1 M KC1 in formamide. Tank nitrogen was bubbled through a tower of alkaline pyrogallol and then through a tower of formamide before being introduced into the Heyrovsky-type electrolysis cell for the purpose of sweeping out atmospheric oxygen. The formamde tower eliminated errors in concentrations caused by the removal of solvent from the electrolysis cell by dry nitrogen. The te2perature range throughout the experiments was 25 f: 1
.
Results The supporting electrolyte solutions in formamide were found to have a polarographic decomposition potential of - 1.69 =k0.02v. This limited the halfwave potentials of materials studied in this solvent to a value less than -1.60 v. The half-wave potentials found in this research are reported in Table I. The best values obtainable for half-wave potentials of these compounds determined in other solvents are also included in Table I. Average id/Cm’/8t1/Ovalues are reported TABLE I HALF-WAVEPOTENTIALS Forrnarnide
No. Other media deter- volts Edz, minations Solvent - 1 . 4 8 zk 0 . 0 2 7 - 1,156 1 : 1 EtOH-HOH’ - 1 . 4 4 i .01 6 - 1.50 1 : 1 EtOH-HOHa 1.12 1: 1 EtOH-HOH‘ - 1 .OS f .OO 6 - 1 . 2 8 f .03 5 -1.36 1 : l EtOH-HOHa 7 - 1.34 1 :1 EtOH-HOHa Benzaldehyde - 1 . 3 4 f .03 Benzophenone 1 . 2 6 f ,01 7 - 1 . 4 8 1: 1 Diox-HOHb - 0 . 8 7 i .02 7 -0.99 i-PrOH-HOHO Fluorenone Furfural - 1 . 3 1 f .02 6 -1.52 PH~.~O(HOH)~ -1.55* ,132 5 -1.72 pH6.5O(HOHjd -1.73 1 : l EtOH-HOHa 7 -1.44f. .02 Vanillin -0,396 HOH‘ 5 -0.38+ .02 ‘Lead(I1) ‘Thallium(1) -0.35f .01 13 -0.46 HOW HOHa 8 -0.995 -0.97 f. .O1 Zino(I1) El
ana.
S.A.E., volts
Compound or ion Acetophenone Anisaldehyde Benzalacetone
-
-
a H. Adkins and F. W. Cox, J . Am. Chem. Soc., 60,1151 (1938). r, S. Wawzonek, H. A. Laitinen and S. J. Kwiata H. Baker and J. kowski, ibid., 66, 827 (1944). @chafer, ibid., 65, 1075 (1943), I. A. Korshunov and S. A; Ermolaeva J. cfm. Chsm. (U.8: 8. R.):’17 181 (1947); ‘‘I. M. l&!thoff abnd ,J. J. ane, Poiarogra hy, &cond Edits- Xae$ersc~encePubksers, Inc., New N. Y,, 196%
%ark,
Dee., 1953
POLAROGRAPHIC STUDIES IN FORMAMIDE SOLUTIONS
in Table 11. The concentration region studied for materials other than T1+ and Zn++ was 0.1255.88 mm./l. These ions were investigated in the range of 0.0267 t o 0 478 mm./l. and 0.114 t o 13.36 mm./l., respectively. TABLE I1 AVERAGE i d / C n t 2 / a l ’ / 6VALUES Acetophenone 2.10 Furfural Anisaldehyde 2.37 Benaalacetone 0.98 Vanillin 0 07 Lead(I1) Benzaldehyde 1.63 Thallium(1) Benzophenone 1.86 Zinc Fluorenone 1.85
1.26 1.45 1.84 1.71 4.92 1.38
Because of the ill-defined nature of the double waves obtained for furfural and benzalacetone, the half-wave potentials of those substances were considered t o be the points of inflection as determined t)y the graphical method of Zimmerman and Gropp.6
Discussion Kolthoff and Lingane7 state that formamide is tiot reduced a t the dropping mercury electrode. Formamide-KC1 solutions continued to present reproducible decomposition potentials after standing for eight weeks in standard taper glassware. Although the agar bridge of the reference electrode was inserted into a given solution many times, the potentials were always reproducible. The introduction of water by means of the agar bridge would cause a gradual drift in this potential if the hydrolysis materials were not already present in reasonable concentration. Korshunov, Kuznetsova and Schennikova8 have reported the half-wave potential of formic acid to be in the neighborhood of - 1.80 v. They found that the absolute value of the half-wave potential decreases with decreasing concentration of formic acid. On the basis of these facts, it may be concluded that formic acid reduction constituted the limiting useful potential of the formamide used. This material would be expected to be present in excess of the ammonium formate because of the removal of ammonia in the purification procedure. The half-wave potentials of Zn++ and Pb++ are in good agreement with the accepted values determined in aqueous solutions. The half-wave potentials of these ions previously determined in formamide are reported to be -0.51 and - 1.03 v . ~ (6) H. K. Zimmerrnnn, Jr., and A. H. G i o p r , THISJOURNAL,64, 764 (1950). (7) I. M. Kolthoff and J. J. Lingane, “Polarograyliy,” Interscience Publishers, Inc., New York, N. Y.,1946, p. 361. ( 8 ) I. A. Korshunov, Z. B. Kuznetsova and M. K Schennikova, Zhur Fiz. Khim.. 23, 1292 (1949).
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Comparison of the data obtained for the organic compounds t o that available in other solvents is rather complex. Differences in dielectric constants, viscosities and solvation tendencies introduce numerous uncertainties. The dielectric constant of formamide a t 25” is 109.5,g while that of water is 82. The viscosities of the two at 25” are 3.359 and 0.895 centipoises, respectively. On the basis of the data recorded in Table I for the inorganic ions, it would seem that these factors either cancel one another in an extremely fortuitous manner or that they do not affect half-wave potentials. It is likely that the general similarity of the two solvents results in differences in behavior which are too small t o be considered significant, except in the somewhat anomalous case of T1+. It is seen that the opposite situation pertains with respect to organic compounds in formamide as contrasted to those in water-alcohol solutions. Both the dielectric constants and the viscosities of the two systems differ greatly. The shifts in halfwave potentials in going from formamide to the other systems are all in the direction of more negative potentials except in the case of benzaldehyde in which there was no shift. The opposite direction of shift was observed by Sartori and Giacomellolo in the case of Li+ in water as contrasted to methanol solutions. The magnitudes of the shifts in the present research are apparently not subject to correlation. In general, linearity was found in id us. c graphs obtained in this research. A tendency for the id/C ratio t o increase a t concentrations in the neighborhood of loR4m./l. was observed. The latter phenomenon has been found in a large number of polarographic systems investigated in this Laboratory. An analysis of the Tl+ and Pb++ waves shows good agreement with the criteria of reversibility. The waves of Zn++, however, do not indicate that a reversible process is occurring at the electrode. It is believed that polarographic half-wave potentials obtained in formamide are of great theoretical value. It should be feasible to identify them, for comparative purposes, as the potentials which would be observed in water for organic materials which are insoluble in that medium. Although extension of the polarographic range of formamide is probably possible, it is extremely difficult t o protect this solvent from hydrolysis by atmospheric moisture. This limits the classes of compounds which can be studied with ease in formamide. On the other hand, the ease of removal of oxygen from formamide constitutes a definite advantage over the commonly used alcohols for non-aqueous investigations near the polarographic zero of potential. (9) G. R. Leader, J . A m Chenz. SOC.,73, 856 (1951). (10) G. Sartori and G. Giacomello, Uazz. chim. itat., TO, 178 (1940).
