New Technique for Polarography of Water-Insoluble Compounds G. E. 0. PROSKE' Defence Research Laboratories, Department of Supply, Victoria, .4ustralia OLdROGRAPHIC studies, particularly of organic comPpounds, are often difficult because the material is not soluble in water. Most of the polarographic work on such compounds has been done in mixtures of water and organic solvents. In nonaqueous media such as acetic acid (1, 2 ) , forniamide, ethyl alcohol, methanbl, glycerol (fl), and ethylene glycol (6) waves are generally lower and in some cases more drawn out than those obtained for the same concentration in water. Keverthelesq, much useful work has been done in this field. Lewis, Quackenbush, and de Vries (7) recently showed that oxidized fats can be determined when dissolved in a mixture consisting of equal volumes of absolute methanol and benzene in v-hich 0.3 JI lithium chloride is dissolved. Bovey and Kolthoff (3) determined organic peroxides dissolved in a mixture of 58% benzol, 37% methanol, and 5% of an 0.05 hl aqueous solution of ammonium acetate. The author determined elementary sulfur and organic sulfur compounds dissolved in a mixture of pyridine and an aqueous buffer solution (9, 10). There are compounds, however, that are not soluble in watermiscible solvents. This paper describes methods of dissolving such compounds SO that the solutions are suitable for polarographic examination,
4 PERCEJ
4GE
OF AEROSOL. MA.
Figure 1. Relation between Wave Height of Anthraquinone and Concentration of Aerosol MA I.
11. 111.
centration is maintained constant, the relation between nave height and concentration of compound is linear. EXPERIMENTAL
Equipment and Procedure. The polarographic curves Tvere obtained in the usual manner by means of a Heyrovsk9-Shikata polarograph, Type VIII. The m and t values for the capillary were determined at 25" C. with an open circuit and with the tip dipping in water. The values of ni and t were 2.10 m . per second and 3.10 seconds, respectively. Therefore, m 2 3 . t 1 / e was 1.98. The electrolysis cell used was an H-cell of the type described by Lingane and Laitinen with an external saturated calomel half-crll(8). .ill experiments were done a t 25" i 0.1' C. Before the oolaroeranis w r p recorded purified nitrogen n s bubbled for '15 mrnutes through the solutions to remove dissolved OXYgen. Unless otherwise dated, half-wave potentials are referred to the saturated calomel elertrode. Solubilization for Polarographic Pur oses. It was found thate!t tlvo commercial wetting agents Aerosol M A and Aerosol AY (American Cyanamid Co.), which are stated to be dihexd and diamyl sodium sulfosuccinate, respectivelv, gave satisfactorv solubilization of certain or0.4 0.8 i . 2 VOLTS. ganic compounds. Figure 2. Polarographic Wave A solution containing 20% by weight of Aeroof Anthraquinone in so1 MA R'as free from Solubilized Chloroform turbidity, colorless, and s a t i s f a c t o r y p o l a r ographically up to potentials of -1.3 volts. A 20% solution of Aerosol AY in water had similar properties but it gave a n-ave starting a t -1.2 volts and this solution was not miscible with water in all proportions without turbidity appearing. Other soaps and wetting agents m r e tried and found to be unsuitable because they gave polarographic waves a t low potentials. These included Igepon T, Wetsit, Daxad S o . 11, Ultrawet -4, Soaxit, sodium cetylsulfonate, casein, and potassium oleate.
F
Table I shows the quantities of Aerosols RIA and AY necessary to solubilize various organic solvents. The figures of Table I were obtained by adding the solution of Aerosol in water from a buret to 1 ml. of the solvent, until clear colorless solutions were obtained on shaking. The solubilized aniline, butyl alcohol, and ethyl acetate obtained with Aerosol M A could be diluted with water in any proportion.
Without additional supporting electrolyte With 10% a m m o n i a buffer With a m m o n i a buffer a n d 10% of ethyl alcohol
Table I. -4queous Solutions of Organic Solvents Obtained by Solubilization with Aerosols MA and .4Y
I t is one of the striking properties of soap solutions that they can dissolve many compounds that are insoluble or only slightly soluble in water. JIanJT soaps can be used for solubilization purposes but most are not suitable for polarographic work because they or their contaminants affect the current-voltage curves. The work described, however, cites two materials that are suitable for use over certain voltage ranges. Wave heights obtained are lower than those given by the usual solutions because of the s1ov.w diffusion of the micelles, but, provided the solvent con1
Solvent Butyl alcohol Ethyl acetate Benzyl alcohol Chloroform Aniline Trichloroethylene Carbon tetrachloride Toluene
Present address, South Australia Rubber Mills, Adelaide, .4ustralia.
1834
-4erosol Solutions Kecessary for Solubilization of 1 MI. of Solvent, MI. l.Oyo MA 20% MA 20% AY In water In water in water 0.8 Miscible in any proportion 3.1 0.4 2.4 3.2 1.0 1.2 3.8 0.8 1.6 6.9 3.5 3.2 20 3.3 22 24 6.4 hlore than m 34 8.0 Mo% than 50
1835
V O L U M E 24, NO. 1 1 , N O V E M B E R 1 9 5 2 Determination of Anthraquinone. .4nthraquinone is insoluble in water but chloroform dissolves 0 . 6 5 a t 20" C. Ethyl alcohol dissolves only 0.05% at 18" C. and a 0.01 -11solutlon in a water-miscible solvent could not be obtained. Solutions of anthraquinone in chloroform were readill- rolubilized with Aerosol MA and these solutions \\-ere examined polarographically in three ways: without a supporting electrolyte, with a supporting electrolyte, and xith other solvents added. Curve I in Figure 1 shows results obtained by electrolyzing solutions containing 5 ml. of 0.01 -11 anthraquinone in chloroform, 0 to 15 ml. of 20% Aerosol 11-4in water, and water added to make the volume 25 ml. At and belov- a concentration of 8% of Aerosol MA, two phases u-ere formed, chloroform and watrr. Above 8% only one phase was present, and the viscosity of the solution increased until gels were formed when the concenti ation of A4erosolM A exceeded 12%. Curve I1 of Figure 1 represents the results obtained in the presence of ammonia buffer as supporting electrolytr. The shape of the waves improved and gels did not form until 15% of herosol 1f.4 was used. The solutions used to obtain this curve had the compositions: 5 nil. of 0.01 31 anthraquinone in chloroform, 2.5 of 0.3 A' ammonia plus 0.25 A- ammonium chloride, 0 to 15 ml. of 20y0 Aerosol 1L4 in n-ater, made up to 25 ml. with water. Curve 111 of Figure 1 is a plot of results obtained by electrolyzing solutions containing 5 ml. of 0.01 31 anthraquinone in chloroform, 0 to 15 ml. of 20% A4erosol31.4 in water, 2.5 ml. of ammonia buffer, 2.5 ml. of ethyl alcohol, and water to make a volume of 25 ml. The considerable increase in m v e height is caused by the ethyl alcohol which probably increases the mutual solubility of the soap micelles and the water. In Figure 2 a polarogram obtained with such a solution is given. The relation between wave height and concentration of anthraquinone has been determined R ith solutions of the follou ing composition: 5 ml. of chloroform or solution of anthraquinone in chloroform, 12.5 ml. of 20% A4erosol11.4 in water, 2.5 ml. of ethyl alcohol, 2.5 ml. of ammonia buffer, made up to 25 ml. with water. Such solutions correspond to that giving curve I11 of Figure 1. The relationship is linear up to such relatively high concentrations of anthraquinone as 800 mg. per liter.
In this work, the half-\\-aye potentials shifted to somewhat more negative values. Furman and Stone report a half-wave potential of -0.65 volt for anthraquinone in ammonia buffer (pH 9.0) containing 40% of dioxane ( 4 , 5 ) . The solubilized solution of anthraquinone in chloroform gave half-wave potential 3 of -0.75 volt (not corrected for the ZR drop of the cell or liquid junction potentials).
Figure 4. Effect of -4dditional Organic Solvents on Wave Height of Sulfur in Solubilized Toluene
Determination of Sulfur. I t x a s found that aqueous solutions containing 3 to 4'3 by volume of solub il i z e d t o l u en e could retain sulfur in solution, and, on electrolysis, well shaped and high waves for the reduction of the sulfur were ohtained. Figure 5. Polarogram of Oxidized The relation b e Lard in Solubilized Chloroform, tween nave height with Aerosol .4Y a8 Solubilizing Agent and concentration A. qithout lard of sulfur was linB . With lard ear if the solution always containrd the same proportion of toluenr as provided in the formulation below and provided that the concentration of sulfur did not exceed 4% by volume (Figure 3). X = ml. of toluene-sulfur solution, n here X can be equal to or lea than 1 ml. 1-X=ml. of toluene to be added. 12.5 ml. of Aerosol MA 20(r, in water. 2.5 ml. of 0.33 31 acetic acid 0.35 .If sodium acetate made up to 25 mi. with water.
+
MG/L
OF S U L P H U R
Figure 3. Relation between Wai-e Height and Concentration of Sulfur in Solubilized Toluene A. B.
Obtained with a constant quantity of toluene Obtained without providing constant concentration of toluene
Curve A in Figure 5 shows results obtained by electrolyzing eolutions made up to this formulation. When the solution of sulfur in toluene was added to the aqueous phase without maintaining a constant percentage of toluene, curve B , which is not linear, was obtained. This curve was obtained with solutione
ANALYTICAL CHEMISTRY
1836 in which the (1- X ) volume of toluene in the above formula was replaced with water. No appreciable reaction occurred between the mercury and the sulfur directly, as the wave height was constant for more than 1 hour; the half-wave potential of sulfur was -0.54 volt. The concentration of toluene must not exceed 4% by volume under the above reported conditions; otherwise a part of it is not solubilized, but only emulsified. The sulfur in this emulsified solvent is not recorded by the polarograph. Thus, the wave height decreases on adding toluene. On the other hand, the wave height was slightly increased by ethyl alcohol and considerably by benzyl alcohol. This is shown in Figure 6 which was obtained with a solution containing 1.0 ml. of sulfur (0.5%) in toluene, 12.5 ml. of Aerosol MA 20% in water, 2.5 ml. of 0.33 M acetic acid plus 0.33 M sodium acetate, 0 to 5 ml. of additional solvent, and made up to 25 ml. with water. Determination of Peroxides in Fats. Lard was oxidized by heating at 90" to 95" C. in an oven for several hours. I t was found that the best solution for polarographic purposes was one containing 5 ml. of a 10% solution of oxidized lard in chloroform, 5 ml. of benzyl alcohol, and made up to 25 ml. with Aerosol AY (20% in water). I n this case, Aerosol .4Y is not only the solubilizing agent, but has to be also the supporting electrolyte, as any additional electrolyte decreases the amount of solubilization. Benzyl alcohol was used because it was found that it improved the shape and height of the wave considerably, probably because it increases the mutual solubility of the soap micelles and the water (Figure 4). Under the above conditions, the peroxides in oxidized lard give a wave a t -0.32 volt (see Figure 5, B ) and a linear relation between wave height and content of peroxide was established; the peroxide numbers of the oxidized fats were determined by the Wheeler method ( l a ) . Determination of tert-Butyl Hydroperoxide. Some liquid organic compounds which are reducible a t the dropping mercury electrode may be solubilized without using an organic solvent. tert-Butyl hydroperoxide is only slightly soluble in water and normally cannot be determined polarographically unless dissolved in a suitable solvent. Aerosol MA can be used to solubilize this material so that it can be determined in aqueous solutions. terf-Butyl hydroperoxide, 0.45 gram, was dissolved in 10 ml. of a 20% aqueous solution of Aerosol MA and a 0.05 M solution was prepared by making up to 100 ml. with water. The solutions electrolyzed contained 0 to 2.5 ml. of 0.05 M tert-butyl hydroperoxide in 2% aqueous solution of Aerosol MA, 3 ml. of Aerosol MA 20% in water, 2.5 ml. of 0.3 N ammonia plus 0.25 N ammonium chloride, and were made up to 25 ml. with water. Well-shaped polarograms were obtained only in the presence of more than 2% of Aerosol MA as provided in the above formulation. As the peroxide gave a wave a t approximately the
2
4
8
P E R C E N T A G E OF A E R O S O L M A
Figure 6.
Relation between Wave Height and Concentration
A . AerosolMA B . tert-Butyl hydroperoxide (corrected)
same voltage as .4erosol ,Ilh the wave height had to be corrected (Figure 6). Exactly the same relation between the wave height and the concentration of tert-butyl hydroperoxide was obtained when ethyl alcohol was used as solvent for the nonsolubilized peroxide, but the reduction potentials were different, being -1.28 volts in the Aerosol MA solution and - 1.09 volts in ethyl alcohol. ACKNOWLEDGMENT
The author is indebted to the Chief Scientist, Department of Supply, Australia, for permission to publish this paper. LITERATURE CITED
(1) Bachmann, G. B., and Astle, RI. J., J . A m . Chem. Soc., 64, 1303 (1942). (2) Ibid., p. 2177. (3) Bovey, F. A , , and Kolthoff, I. M., Ibid., 69, 2149 (1947). (4) Furman, N. H., and Stone, K. G., Ibid., 70, 3055 (1948). (5) Ibid., p. 3062. (6) Gentry, C. H. R., S a t u z , 157,479 (1946). and de T'ries. Th., ANAL. (7) Lewis, W,R., Quackenbush, F. W,, CHEM.,21,762 (1949). (8) Lingane, J. J., and Laitinen, H. h., ISD. EVG.CHEM.,A N ~ L . ED.,11, 504 (1939). (9) Proske, G. E. O., Angew. Chem., 59, 121 (1947). (10) Proske, G. E. O., Gummi-Ztg. u. Kautschuk, 1, 339 (1948). (11) Sanko, A. M., and JIanussova, F. -$., J . Gen. Chem. (U.S.S.R.), 10,1171 (1946). (12) Wheeler, D. H., Oil and Sonp, 9 , 8 9 (1932). RECEIVED for review Noveniher 19. 1931. Accepted Augiist 20, 1 9 2
New Slit Drive for Beckman IR-2 Infrared Spectrophotometer W. E. TOLBERG AND H. M. BOYD Research Department, General Mills, Znc., Minneapolis 13, M i n n . OR routine spectral scanning with a Beckman IR-2 infrared Fspectrophotometer, it is desirable to compensate for the exponential decrease in the intensity of the energy from the source over the wave-length range from 2 to 15 microns in order to maintain a constant level of background energy. Since the intensity of the energy decreases, an increase in the area of the beamthat is, a widening of the entrance slits as the wave length is increased-compensates for the decrease. Ordinarily a complete spectrum taken from the IR-2 or similar instrument having no device for continuously widening the en-
trance slits consists of a number of sections of the spectral range. Each section is recorded at a constant slit width and in such a way as to overlap part of the range covered by the preceding section. I n order to obtain a complete spectrum in terms of per cent transmittance, the background absorption, the sample absorption, and the zero transmittance curves are recorded. The per cent transmittance of the sample is calculated from the ratio of the distance from the zero line to the sample curve and from the zero line to the background curve a t the particular wave length. Obviously this method is tedious and leaves a great deal to be desired when
