Polarography in Anhydrous Organic Solvents Acid Halides in Acetone PAUL ARTHUR AND HAROLD LYONS' Department of Chemistry, Oklahoma Agricultural and Mechanical College, Stillwater, Okla.
Polarography in anhydrous organic media involves difficulties found to be due chiefly to the high resistances of such solutions and the unusually great interferences encountered from oxygen. Special techniques were developed to overcome these difficulties and were thoroughly tested on solutions of lead and cadmium salts in anhydrous methanol, ethanol, and acetic acid. The procedure was then applied, with good results, to the polarography of several acid halides in acetone solutions. It was found that the half-wave potentials of the aliphatic acid halides tested were almost identical with each other, though different from that of benzoyl chloride. T w o reference calomel electrodes made in acetone instead of water and using lithium chloride instead of potassium chloride were tested and the use of one of these is recommended when acetone is being used as the solvent in polarographic investigations.
A
LTHOUGH the advantages of having a large number of tested anhydrous solvents and well-developed techniques for their use in polarography are obvious, only a few articles have appeared in the literature on this subject. Bachman and Astle (2, 3 ) obtained normal waves for a number of cations in glacial acetic acid; Lewis, Quackenbush, and PeVries (8) studied peroxides in rancid fats using a solvent composed of a mixture of methanol and benzene, while Hall (6) reported the successful determination of oxygen in petroleum fractions using a similar mixture. Ethylene glycol ( 6 ) and glycol ethers have been suggested as solvents, Parks and Hansen (9) describing the use of ethylene glycol monomethyl ether (a Cellosolve) in a direct determination of tetraethyllead in gasoline, while methanol ( 1 , 4, 10) and ethanol (1, 4 ) have been investigated, both anhydrous and in mixtures Kith water. The purpose of this research was (1) to extend such studies to an investigation of any special techniques needed for consistently successful polarography in anhydrous media, and (2) to determine the suitability of acetone (which is a good solvent for many organic substances) as a solvent for use in polarography. APPARATUS AND MATERIALS
Polarographs. Both a Sargent Model X I 1 and a Sargent Model X X I polarograph were employed in this research. Reference Electrode and Cell. A quiet pool of mercury was used in obtaining all curves in the work reported here. For acetone solutions, however, the potential difference between this pool and an especially designed nonaqueous electrode was measured at various points along each curve, SO that critical values could be referred to either electrode. These potential differences were measured by means of a Leeds and Northrup student-type potentiometer. Much of the exploratory work involved in this investigation was done in an E. H. Sargent Co. No. S-29306 polarographic cell. Later, however, it was found necessary to develop and make use of the special H-type cell illustrated in Figure 1. The need for a suitable reference electrode that would not involve the bringing of water into contact with anhydrous acetone solutions led to attempts to make a calomel electrode in acetone. It was learned immediately that when acetone saturated with lithium chloride is brought into contact with calomel, part of the I
Present address, General Electric Co., Pittsfield, Mass.
mercurous mercury is reduced to the metal and part is ovidized to a mercury complex that is soluble in the acetone. When a paste of calomel and mercury was placed on a mercury pool and covered with acetone that had been saturated with lithium chloride a t 30" C., however, the system quickly reached e uilibrium and the electrode so prepared proved to be both stabye and reproducible. A cell made of ti\-o such electrodes was tested and found to have the following characteristics: ( 1 ) Its potential was zero (measured to 1 mv.) before and after passage of currents up to 50 pa. for as long as 1 hour; (2) with one electrode freeh!y prepared and the other several weeks old, the potential was again found to be zero; and (3) the relationship between current and applied voltage was a straight-line function over a range of 0 to 2 volts. These three tests showed the electrode to be reasonably reversible, nonpolarizable, stable, and. reproducible-thus suitable for use in making polarographic comparisons. This electrode, sealed to prevent evaporation of the acetone, was used to check the potential of the quiet pool anode whenever polarograms were made on acetone solutions. The need for such an electrode was quickly confirmed, for during the making of a single polarogram the potential difference betlyeen the pool and the acetone calomel electrode (.4.C.E.) often varied from a value as high as 0.320 volt to as Ion- as 0.230 volt, the pool being negative with respect to the acetone calomel electrode. After the polarographic work reported here was completed it was found that the potential of the acetone calomel electrode used would have changed if evaporation of acetone had not been prevented. Another electrode which did not possess this difficulty was developed, therefore, and put through numerous tests of the type described for the acetone calomel electrode. This electrode (the A.S.C.E., or acetone-saturated calomel electrode) was made like the first, except that the paste contained solid lithium chloride in addition to calomel and mercury and the electrolyte solution was made separately by allowing an e x c w of lithium chloride and calomel to come to equilibrium in acetone. By trial it was found that 6 grams of calomel and 1.5 grams of lithium chloride in each 100 ml. of acetone vere sufficient to prepare the electrolyte solution. A comparison of these two electrodes showed the acetonesaturated calomel electrode to be 0.027 volt more negative than the acetone calomel electrode, at both 25" and 30" C. Solvents. Reagent grade Baker and Adamson methanol, absolute ethanol from the U. S. Industrial Alcohol Co., and glacial acetic acid from P u Pont were found to be sufficiently anhydrous to be used x-ithout further purification. Acetone, c.P., from the RIallinckrodt Chemical Works TI as dried a t least 24 hours over anhydrous potassium carbonate and redistilled according to the procedure described by Weisbberger and Proskauer ( 1 1 ) . The salts used were all Baker and Adamson reagent grade. These were oven-dried a t 110' C. for a t least 24 hours and then stored in desiccators over concentrated sulfuric acid for several days before use. The acid halides, with the exception of the acetyl chloride, 11-ere all white-label Eastman Kodak reagents, the acetyl chloride being obtained from Merck & Co., Inc. The nitrogen used for the removal of oxygen was commercial water-pumped nitrogen purified by passing it through a train consisting of (in that order) a bubbling tower filled with concentrated sulfuric acid, an empty trap, a tower containing alkaline pyrogallol solution, another empty trap, a tower containing concentrated sulfuric acid, then another empty trap. The nitrogen was then passed through a tower filled with the solvent-carrier solution in order to saturate the gas with solvent vapors. A fritted-glass gas dispersion tube was used to introduce the gas into the solution to be deoxygenated. EXPERIMENTAL PROCEDURE AND RESULTS
Special Techniques. I n earlier work, Arthur, Allison, and Black (1, 4 ) found that polarograms of nickel and lead could be made in methanol solutions but that reproducible results were obtained only when a certain definite procedure was followed.
1422
1423
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 One of the first problems of this research, therefore, was to determine which of the techniques involved in this procedure were critical acd which were not. One of the difficulties encountered was predictable and therefore easily recognized. When either the cell arrangement or the solution is such that the internal resistance is more than a few hundred ohms, the voltage drop through the solution ( I R )becomes significant. Commercially available automatic polarographs plot the current as a function of applied voltage and do not correct for the ( I R ) drop; consequently, as the current incieases and the effective voltage and the applied voltage differ more and more, the plotted curve flattens so much that waves may not be readily discernible. Other difficulties encountered were not so easily understood as that caused by high resistance. Sometimes excellent polarograms would be obtained. At other times, the current would start at zero applied potential and go off scale a t -0.1 to -0.2 volt, or spurious waves would appear a t voltages around -0.8 to -1.0 volt, and sometimes a t -0.3 to -0.5 volt as well. The fact that these potentials were suggestive of those obtained with mercurous ion, oxygen, and hydrogen peroxide, and that the wave a t -0.4 volt seemed to decrease in height with further degassing finally led to the conclusion that the difficulty lay in the degassing procedures employed. -4s a result of surh studies it was found that the following techniques are essential t o success in nonaqueous polarography: The solution must be degassed before the mercury pool is added, then the solution must be subjected to a final minute of degassing to remove any oxygen inadvertently admitted during the addition of the mercury. Unless this procedure is fo!lowed, spurious Tvaves 11 ill almost invariably be obtained. The removal of oxygen must be more thorough than would be required for aqueous solutions. This requires that the tank nitrogen used be purified with unusual care, and that either prolonged degassing times or efficient degassing techniques be used. K i t h proper purification and the use of a fritted-glass dispersion tube, 10 to 15 minutes' degassing is usually sufficient. The electrode and cell arrangement must be such that the Internal resistance will not exceed 8000 ohms. I n this research the best results were obtained when a quiet pool of mercury in the bottom of the cell was used as the anode, the dropping mercury electrode being placed so its lower end was only 5 t o 6 mm. above the surface of the pool. I n methanol and ethanol it was possible to use greater distances; in acetone solutions, however, this minimum spacing was necessary; for even then resistances of the order of 7500 ohms were encountered. Closer spacing, on the other hand, resulted in erratic polarograms, the evidence indicating that the observed surges in current were caused by cathodic reduction of products formed a t the anode and brought to the dropping mercury electrode by the stirring action of the falling droplets of mercury. When these three techniques were employed, excellently formed, reproducible polarograms were obtained consistentlj with solutions of lead in methanol and ethanol and with cadmium i n methanol, ethanol, and glacial acetic acid (see Table I )
Table I.
Half-Wave Potentials of Lead and Cadmium in Various Solvents
Cation 0 002 iM P b
Carrier a n d Solvent +
+
0,002 ,M P b 0 005 .II Cd T T 0 005 M Cd 0 001 M C d - + Corrected for IR + +
+
+
1 M LiCl in methanol
1 M LiCl in ethanol 0.1 M LiCl in methanol 0 1 M LiCl in ethanol 0.1 .If LiCl in acetic acid
Pool', Volt 36 - 0 36 -0 6 6 -0 63 -0 7 6
Eli P v s .
-0
drop where significant.
Acid Halides. Attempts to study cations in acetone failed. Of the salts tried, only cadmium chloride was sufficiently soluble, and the waves obtained with this salt were so erratic and so close to the carrier wave that no good measurements could be obtained. I t was decided, therefore, to study some representative acid
halides instead; for by this means not only could the suitability of acetone as a solvent in polarography be ascertained, but some useful information on a previously unstudied class of compounds could also be obtained. Acetyl chloride, propionyl chloride, butyryl chloride, isovaleryl chloride, benzoyl chloride, P-phenylpropionyl chloride, acetyl bromide, propionyl bromide, and benzoyl bromide were chosen for this purpose, each being run in acetone saturated with lithium chloride and a t 30" C. The cell used was that illustrated in Figure 1. The reference electrode was an acetone calomel electrode, though the actual c,lectrolysis anode was a quiet pool of mercury.
D.M.E. INLET sl
U
Figure 1.
Cell with Trap
-kt the beginning of each run, the funnel in the bridge was filled with acetone saturated with lithium chloride, the solution was degassed, then the cock was opened to fill the bridge. In this way, any solution entering from the acetone calomel electrode should (its specific gravity being greater than that of the other ~ h t i o n s )collect a t the bottom of the trap and, if necessary, could be drawn off occasionally and flushed out with fresh solution. K i t h this arrangement interference from calomel was never observed; without it, trouble was usually encountered. After the bridge was filled, more acetone-lithium chloride solution was placed in the cell. This solution was degassed for 15 minutes, the mercury pool was added, then the solution was degassed 1 minute more. With nitrogen flowing over the solution and out the top of the cell, a weighed sample of the acid chloride in a small glass-stoppered weighing bottle was introduced and rinsed into the cell. hlixing was accomplished by bubbling nitrogen through the solution for 15 seconds. The polarogram was then run, the mercury pool being used as the anode. Because of the observed drift in potential of the mercury pool during the making of polarograms, the potential difference between the pool anode and the acetone calomel electrode was checked just before each run and a t intervals during the run. The two sets of data thus obtained made it possible to refer vahes to both the pool and the acetone calomel electrode, and later (see Table 11) to the acetone-saturated calomel electrode.
ANALYTICAL CHEMISTRY
1424 ,Table:If. Half-Wave Potentials and Diffusion Currents of Acid Halides in Lithium Chloride-Acetone Solutions" Acid Hdiat Acetyl chloride Propionyl chloridt Butyryl chloride Isovaleryl chloride 6-Phenylpropionyi chloride Acetyl bromide Propionyl bromidt Benzoyl chloride
Ei/r (Volts) Versus A.C.E. A.S.C.E
Concn. of Halide, Molarity
Pool
0.003 0.007 0.003
0,006
-1.03 -1.01 -1.00 -1.01
0.005 0.004 0.005 0 003
-1.00 -0.94 -1.01 -0.83
-1.28 27 -1.27 -1.27
-1.28 -1.24 -1.24 -1.24
-1.27 -1.21 -1.27 -1.10
-1.24 -1.18 -1.24 -1.07
-1
a A t -1.50 volt vtrbus A.C.E. mZ'*i''p value for D.M.E. iived \vas 2.13. reported were corrected for IR drop.
Id
(4
Mmole/L.) 1.08
..
.. 0:6G 0.78 0.82
All
valilr,
fiist expected that the products formed by reducing the acid halides would be aldehydes or some condensation product thereof, the hydrogen required being supplied by the solvent itself or by traces of water present. Analysis with modified Karl Fiqcher reagent shoTTed that the acetone contained small amounts (0.1% or less) of mater. That this water was not essential to the electrode reaction, however, was indirated by the fact that when acetic mhydnde n n s added to destrm tht. lnqt of the nater, no difference \$as obseived i n the results Finallj a. graph made
of R us.
1 : for propionyl chlo-
ld
In general, the wavw obtained were well formed, though the limiting currents rose somewhat more steeply than would usually be the case for inorganic ions in aqueous solutions. Kithin the limits of error (0.001 volt) the half-wave potentials were constarit for acetyl chloride for solutions varying between 0.0034 Jf and 0.014 '1.1;this was true also for propionyl bromide in concentrationsfrom0.005iMto0.017M. Studiesof diffusion current vemus concentration showed a linear relationship to exist, but even Kith extraordinary precautions against the admission of moisture, individual measurements frequently fell as much as 5% below the best line on these graphs. With benzoyl bromide no limiting current was ever obtained. In every case the benzoyl bromide seemed t o have the effect of increasing the drop time rapidly and erratically, and by the time the applied voltage reached -1.2 volts, mercury was actually running from the capillary in an almost unbroken stream. A similar phenomenon observed in early experiments ITith bcnzoyl chloride was traced and found to be due to the presence of small amounts of benzoyl acid. Since the acetone used in these experiments was found, by analysis with modified Karl Fischer reagent, t o contain small amounts (0.101, or less) of water, it is not impossible that some of the benzoyl bromide might have hydrolyzed t o produce some benzoic acid and that the latter caused the difficulty observed. DISCUSSION OF RESULTS
Degassing Technique, KO definite effort was made to determine experimentally the reason for the need t o degas these solutions before they are brought into contact with mercury It has been observed for aqueous solutions (Y),however, that when ions capable of forming stable complexes or very insoluble salts with mercury are present, metallic mercury will react with these ions and oxygen t o form the mercury complex or salt together with hydrogen peroxide. Similar reactions niight occur in organic solvents, either organic peroxides or (especially if traces of moisture are present) hydrogen peroxide being formed. If such were the case, neither the mercury complexes nor the peroxides would be removed by the passage of nitrogen through the solution, and waves due t o these substances would be expected to appear The potentials a t which spurious waves appeared were compatible with the concept of their being due to oxpgvri, peroxides, and compounds of mercury. The great care required in degassing these solutions has been explained by Vitek (IO)for methanol as being due t o the greater solubility of oxygen in this solvent. That this is not the only factor, however, is indicated by the fact that even after such solutions were degassed for as much as an hour with unpurified tank nitrogen, strong oxygen waves were obtained, whereas the same nitrogen would degas aqueous solutions within 5 minutes sufficiently for no traces of the oxygen wave t o be evident. The difference was so great that it seemed highly probable that the oxygen wave, in the organic solvents used in this research, was actually enhanced. Nature of Electrode Reactions of Acid Halides. It was a t
2
ride and for iuovaleryl chloride gave slopes of 0.066 and 0.081, respectively, indicating that such reactions are 1-electron reactions. An examination of the various possibilities in the light of these data would therefore lead to the conclusion that the electrode reaction produced free radicals. Reference Electrodes. Although it was definitely not an objective of this research to develop reference electrodes for use in nonaqueous polarography, the drifts observed in the potential of the quiet pool anode during the making of polarograms justific,d the xork done on the acetone calomel electrodes described. Such changes in the pool potential are not surprising if one considers that at first there are no anodic products in the Polutiori in contart with the pool, but as the electrolysis proceeds, such products form in increasing concentrations. In aqueous chloride solutions, the concentration of dissolved anodic product is limited by rapid saturation of the solution with the very slightly soluble mercurous chloride. In acetone, however, the conip1t.s anodic reactions involved probably never reach equilibrium during any one run; consequently, the electrode never reaches its limiting potential-a potential which should be, theoretically, equal t o that of the acetone calomel electrode as the two would be essential identical electrodes. Actually the potential difference between the pool and the acetone calomel electrode w a p never found to be lower than 0.230 volt. Because the pool potential cannot be trusted to remain constant, the use of an external electrode for significant measurements of potential in acetone solutions is essential. Both of the tn-o electrodes described proved to be suitable for polarographic work, although the acetone-saturated calomel electrode, which docs not change potential if some of the acetone is lost by evaporation, is, in this respect only, the better of the two. Applications. From the standpoint of analytical chemistry the implications of the research extend far beyond a consideration of the few cases studied. The techniques found here to be essential to polarography in nonaqueous solvents should open the field t o investigation of many organic substances which cannot be studied in aqueous solutions. This should be particularly valuable t o oil, fat, and petroleum chemists. As for the acid halides, the results of this research show that Ivhile aliphatic acid chlorides can be identified a8 a class and can i)e distinguished from benzoyl chloride, their half-wave potent,ials lie too close together t o make it feasible to distinguish tietween thorn. With proper protection from moisture it is possible to determine quantitatively any one of those in Table I1 in acetone-lithium chloride solutions, although errors as high as 5% probably should be expected. The future of polarography in anhydrous organic solvents \vould be greatly aided if commercial polarographs designed to plot current as a function of effective voltage rather than applied voltage were made available. Such equipment should be a boon t o chemists working with petroleum products or animal and vegetable fats and oils, as well as to organic chemists in general.
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2
1425
LITERATURE CITED
Lewis, W. R., Quackenbush, F. M., and DeVriea, T.,
( 1 ) Alison, B., “Polarographic Studies of Lead in Methanol Solution,” unpublished M.S. thesis, Oklahoma A . and M. College, 1950. (2) Bachman, G . B.. and Astle, 11.J., J . Am. Chcm. SOC.,64, 1303 (1442). ~ - _-, - . (3) Ibid., p. 2177. (4) Black, H., “Polarographic Studies in Organic Solvents,” unpublished M.S. thesis, Oklahoma A. and hI. Collerre. - 1948. (5) Gentry, C. H. R., Aroture, 1 5 7 , 4 7 9 (1946). (6) Hall, M. E., ANAL. CHEX,2 3 , 1382 (1951). ( 7 ) Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 111, New York, Interscience Publishers. 1941.
.bi.41,.
CHEW,2 1 , 7 6 2 (1949).
Parks, T. D., and Hansen, J. A., Ibid., 22, 1268 (1950). Vitek, V., ColEeetion Csechoslov. Chem. Commum., 7 , 537 11935). Weissberger, A., and Proskauer, E., “Organic S o l v e n t 3 , ” p. 143, London, Oxford Press, 1935. RECEIVED for review February 21, 1952. Accepted Jurtr. 113, 19.52. D a t a taken from a thesis presented b y Harold Lyons in partial fulfillment of t h e requirements for t h e P h . D . degree at Oklahoma A. and M. College, 1931. P a r t s of this material were presented before the Division of Analytical Chemistry a t t h e Seventh Southwest Regional Meeting, Aimtin, rex
Determination of Reactive Hydrogen in Organic Compounds A Review ELIZABETH D. OLLEIIZAN’ Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa. Methods for quantitatively determining reactive hydrogen in organic compounds have been reviewed to find the most suitable procedures for estimating the reactive structural units in coal degradation products. In the past, reaction with methylmagnesium iodide or lithium aluminum hydride has been used to determine total reactive hydrogen in organic compounds with an accuracy o f 1 3 to 5%. The types of compounds which have been determined are: alcohols, phenols, enols, mercaptans, acids. amines, amides, and water. Acylation methods. including acetylation, phthaloylation, benzoylation, ETHODS for quantitatively determiningreactive hydrogen in organic compounds-Le., hydrogen attached tonitrogen. oxygen, or sulfur and exceptionally reactive carbon-hydrogen structures-have been reviewed to find the most suitable procedures for estimating the reactive oxygen- and nitrogen-containing structures in coal degradation products. The chemical literature from 1927 to 1949, inclusive, has been thoroughly searched, and some earlier and more recent references have been consulted The literature previous to 1931 was reviewed by Meyer (93). Methyl Grignard reagents and lithium aluminum hydride will react quantitatively R ith reactive hydrogen atoms in organic compounds t o form methane and hydrogen, respectively. The gaa formed in each case is a memure of the “artive hydrogen.” Other reagents have been reported for the determination of active hydrogen but are generally less satisfactory. Some of the active hydrogen atoms can also be replaced b i a(-ylgroups, and compounds containing such hydrogen atoms are said to contain “acylatable hydrogen” or usually “acetylatable hydrogrn.” ““wylatable hydrogen” and ‘ acetylatable hydrogen” are not strictly correct t e i m ~ inaqmuch , as the hydrogen is not acylated but rather replaced by an acyl group. However, these are convenient ternis which distinctly differentiate methods which involve acylation of reactive groups from the methods IT hich determine total “active hydrogen.” Different reagents and conditions cause quantitative acylation of different functional groups, and consequently the terms “acylatable hydrogen” and “acetylatahle hydrogen” have little meaning unless the reagent and conditions are specified. 1
Pa
Present addreas, Verona Research Center Koppers Co , Inc , Verona,
formylation, and stearylation, vary greatly among themselves and give results depending on reagents and conditions. Variations of the most common method-i.e., acetylation using acetic anhydride and pyridine at approximately 100’ C.-have been used to determine primary and secondary alcohols, “unhindered” phenols, and primary and secondary amines. Other acylation reagents may be advantageous if tertiary alcohols or pyrroles must be determined or selectivity of reaction is desired. Applicability of different acylation methods for determining functional groups has been tabulated. The methods of deterniining reactive hydrogen have thus been divided into two major groups-determinations of active hydrogrn and determinations of acylatable hydrogen.
DETERIIIINATION OF ACTIVE HYDROGEX In 1902, Tschugaeff (168) reported that Grignard reagents reacted quantitatively with free hydroxyl groups of acids, alcohols, phenols, and oximes, and when methylmagnesium iodide was used, the amount of hydroxyl could be calculated from the volume of methane gas formed. Shortly thereafter, Hibbert, and Sudhorough (63) and Zerewitinoff (185) published similar methods for the determination of hydroxyl, amine, imine, and amide groups. This analytical technique has been called the Zerewitinoff determination, probably as a result of his continued investigat,ion of this method and the caonipoiind.;; which react with Grignard reagents (186-189). Recently, a new reagent-lithium aluminum h y d r i d e w h i c h is very similar to Grignard reagents in its chemical behavior has heen applied in the determination of active hydrogen. Other reagents have also been suggested. GRIGh i R U R E \GENTS
Reaction. Xethylniagnesium halides react with compounds containing active hydrogen according to the following equation:
RH
+ CH3MgX --+ RMgX + CH, t
Alcoholfi, phenols, mercaptans (thiols), inorganic aa well as carboxylic and sulfonic acids, secondary amines including pyrrole (41), and monosubstituted amides liberate 1 mole of methane;
