sodium carbonate solution varying between 0.006414 and 0.02566 mmole were analyzed with an average deviation of 7 parts per thousand. The molarity of the sodium carbonate solution, prepared by weight, was 0.03207 and the average value from these determinations was 0.03198. Similarly, titrations of sodium acetate were carried out in G-H solvent (2) (50% ethylene glycol and 50% isopropyl alcohol) containing 0.5M sodium perchlorate. The average error was 0.87,. I n all titrations it is important to run a preliminary sample, about the same size as the samples to be titrated, to plot the titration curve and determine the pH of the end point and thus the p H to which the titration medium must be preset.
The cation exchange membranes appeared very stable in methanol but in G-H solvent they began to discolor after several days. An analogous attempt to titrate acids using anion exchange membranes (ARX-44 from Ionics, Inc.) was' not successful. This was due t o the presence of tertiary amine groups in what was supposedly a quaternary ammonium-type exchange resin. The tertiary amine groups were protonated by the acid and therefore results were always low. However, if a strong anion exchange resin membrane is used, there should be no difficulty in carrying out acid titrations by this technique. The success of these acid-base titrations using ion exchange membranes as an ion barrier between the half cells indicates that these membranes
should be very adaptable to other coulometric procedures. In contrast to the uee of sintered disks and salt bridges for separating half cells the problems of hydrostatic flow and nonpermselectivity are eliminated. Furthermore, the low electrical resistance of these membranes (about 70 ohms) should simplify maintaining constant current during a titration with less sophisticated current supplies. LITERATURE CITED
( 1 ) Hanselman, R. B.,Streuli, C. A., ANAL. CHEM.28, 916 (1956). (2) Palit, S. R., IND.ENQ.CHEY.,ANAL. ED. 18, 246 (1946). (3) Reilley, C. N., Adam, R. N., Furman. N. H., ANAL.C ~ M 24, . 1044 (1952). RECEIVED for review June 10, 1959. Accepted August 21, 1959.
Determination of Organic Nitro Compounds by Controlled- Pote ntia I Cou Io met ry JURGEN M. KRUSE Eastern Laboratory, E. 1. du Ponf de Nernours and Co., Gibbstown, N. 1.
b The application of controlled-potential coulometry to the determination of nitro compounds was studied. The electroreduction of the nitro compounds in a variety of organic and semiaqueous solvents indicated that a background current posed the chief obstacle to the determination of traces of nitro groups. Methods were developed to reduce and compensate for this background current, so that satisfac!ory measurement of as little as 20 p.p.m. of o nitro body in an organic sample could be accomplished.
T
HE quantitative determination of small amounts of nitro impurities in organic compounds, and particularly of negatively substituted aromatic compounds. is often difficult, as most of the cv&ting methods for the quantitative analysis of organic nitro bodies require either the prior identification or the isolation of the suspected impurity. hmong the methods which have been iised are quantitative reduction ( I , 1. 10).diazotization after reduction ( 6 , l e ) , liolarography either in aqueous (a. 9 , 12 ) o r Iionaqutwus s o l ~ c n t s( I O ) , and various c,olorimetric met hods. Invcstigation of chemical reduction procedures for the drtermination of ric.gativel~-substituted aromatic nitro rompounds indicatccl that compounds of this typr are not rrduccd quantitatively
1854
ANALYTICAL CHEMISTRY
even by such reducing agents as zinc amalgam in acid, stannous chloride, tin-acid couples, or titanous chloride. Reduction methods therefore did not appear attractive for the measurement of an unknown impurity. Both polarographic and colorimetric methods require the isolation or identification of the unknown impurity and had to be ruled out. The use of coulometry, especially in a nonaqueous or semiaqueous system, circumvents the requirement of ing the specific nitro impurity and obviates the need for a standard. s u c h a method of coulometric reduction of nitro compounds was described recently ( I O ) and demonstrated the usefulness of coulometric reduction in nonaqueous systems. I n the present work, which was carried out prior to the appearance
Figure 1 . 1. 2.
of this paper, the coulometric reduction of nitro compounds in some nonaqueous and semiaqueous systems was studied. Solubility considerations indicated that a nonaqueous system was necessary. The high electrical resistance and ditrerent nature of this type of solvent presented a number of problems which had to be studied in detail. APPARATUS AND REAGENTS
The cell design and coulometric apparatus used were essentially those described by Lingane (7). Line Voltage was controlled with a variable transformer and then rectified. The circuit design is depicted schematically in Figure1. suggested by ~ i a single ~ compartment cell with a silver-silv(,r chloride reference electrode was used; this electrode was positioned as closely
Circuit for controlled-potential coulometry
Variac Rectifler 3. 100 ohm potentiometer, 10 watts 4. 500-ohm potentiometer A,. 0-1 0 n o . ammeter A?. 0-1 00 ma. ammeter
AB. C. EC. V.
G. RE.
0-500 ma. ammeter Coulometer Electrolysis cell 0-1.5 volt precision voltmeter Galvanometer Reference electrode
~
~
as possible to the mercury pool cathode to minimize the e.m.f. drop between cathode and reference cathode. A silver wire anode similar to the one described by Ehlers. and Sesse ( 4 ) was used. A titration 2oulometer (8) was used to measure the total number of coulombs. The following reagents were used as test compounds after being purified to 99% purity or better by repeated disl-nitrotillation or crystallization: propane, %nitropropane, a-nitrepxylene, nitrocyclohexane, nitrobenzene, nitroisophthalic acid, nitroterephthalic acid, nitrodurene, and pnitrobenzoic acid. All other reagents were C.P. grade.
Table 1.
30
Variac Settings 60 70 Initial Current, Ma,
80
50
40
Background 2.7 2.8 3.0 2.9 3.0 3.4 samplea 5.8 6.5 7.1 9.2 14 18 Sample, nitrobenzene (-1 mg.); potential, -0.90 volt us. S.C.E.
+
Table 11.
90 3.6 19
Study of Coulometric Solvent Systems
Potential us. Ag-AgC1 Electrode~Solventa
-0.50
0 . 1 M KC1 3 : l Me0B::vater 0 1N K';l 4.1 'MeOIl Mater u 1N L'C I D M F '0 LV Et'NBr) DMF (0 1 Y LiCl) DMSO (0 1.V LiCl) Form amide
BACKGROUND CURRENT CORRECTION
The major problem met in the controlled-potential coulometry of organic compounds was the necessity for making a background correction. The use of organic solvents introduced or greatly magnified a definite background current -i.e., a current which was observed at the desired potential in the absence of any sample. A high background current is, of course, highly undesirable in any coulometric study because it introduces the need for a correction in the measured number of coulombs; where low concentrations of the desired electroactive constituent are being measured, this correction can be a large portion of the total number of coulombs, and even a small error in the correction can cause a significant error in the total measurement. T o minimize the effect of the background current, several different inethods of reducing this current without affecting the over-all electrolysis c-urrent were investigated. I n accordance with polarographic experience, sweeping of the sample solution with oxygen-free nitrogen or carbon dioxide was tested for the removal of any oxygen. This sweeping or outgassing of the solution prior to the addition of the sample reduced the initial background current; outgassing during the electrolysis had only a minor effect but helped to stir the solution and to keep the background current low. Because the background current and the maximum attainable electrolysis current appeared to be proportional, ways of increwhg the electrolysis current without affecting the background current would serve to minimize the relative importance of the background. An increase in the rate of stirring the mercury pool and sample solution had such :in effect (Table I) up to a limiting stiriing speed where diffusion apparently is no longer rate-controlling. The next remedial measure tried for the reduction of the background current was the electrolysis of the supporting dectrolyte solution (solvent) prior to the addition of the sample. The pre-elec-
Effect of Stirring Speed on Electrolysis Current
a
0 0 0 2 0 2 0 2
-0.70
-0.90
Background Current,b Ma. 0 0
-1
1
0
0
0.3 0.2
0 2
0 2
0 3
11 10 7 3 5
24 10 8 5
20 7
*
DMF, dimethylformamide; DMSO, dimethyl eulfovide After 45 minutes of pre-electrolysis.
trolysis procedure lowered the background current to tolerable values, but had the disadvantage of also lowering the maximum attainable electrolysis current. As this phenomenon was encountered even with purified solvents and on successive determinations in the same solvent, reduction of impurities in the electrolyte was highly unlikely, although pre-electrolysis will circumvent the effect of such impurities in the solvent. As a similar effect could also be obtained by using a silver chloridecoated silver anode, pre-electrolysis apparently reduced the background current because of the formation of such a coating. Although not the ideal remedy, the pre-electrolysis of the supporting electrolyte solution was used in all further studies. Generally, an electrolysis period of about 15 minutes was sufficient to reduce the background current to a final steady value. If, after the usual pre-electrolysis, the background current was still above 6 ma., as was sometimes the case with new or freshly cleaned anodes, the pre-electrolysis was continued at a higher current (about 60 ma.) until the background current dropped to the desired value. During the electrolysis of aliphatic nitro compounds, the background current changed during the electrolysis, so that a background correction based on the average background current Initial
-+ final background current 2
proved more accurate than the use of the final background. The characteristics of the anode and of the electrolysis cell finally appeared to be the limiting factors for the background current when sweeping of the solution with nitrogen, rapid stirring,
and pre-electrolysis of the solvent system were used. When a platinum anode was used in place of the usual silver anode, even pre-electrolysis did not lower the background current below 5 ma.; pretreatment of the solvent was therefore held to 5 minutes. T h r only way in which the .background current obtained with a platinum anode could be minimized was by working 'at the lowest possible reduction potential. In general, a silver anode proved more desirable because the background current could be reduced to lower values. CHOICE OF SOLVENT SYSTEM
The ideal solvent for the controllcdpotential coulometry of organic compounds would be a Liquid which would dissolve all samples, have a low internal resistance, and allow the use of an inert electrolyte. Polarographic reduction of organic compounds showed that simple alcohols, dioxane. dimethylformamidc, formamide, dimethyl sulfoxidc, and acetonitrile approacherl several of these specifications. However, preliminary coulometric studies soon eliminated dimethylformamide, formamide, dioxane, and acet>onitrile. The dimcthylformamide, formamide, antl dimcxthyl sulfoxidc solutions had high internal resistances and did not permit reduction of the background current t,o sufficiently low values (Table 11). Dioxane reacted with the electrode system, and the use of acetonitrile rrsultcd in incompl(9tr reductions. Methanol, and especially solutions of methanol containing some water, wri'e very satisfactory solvent systcms. A 4-to-1 methanol-to-water system, 0.1.If in lithium chloride, therefor(,, was used in most of the reductions investigat'ed, although 340-1 methanol-to-water conVOL. 31, NO. 1 1 , NOVEMBER 1959
* 1855
Table 111.
Meq. of nitro compound =
Reduction of Aliphatic Nitro Compounds
(for aromatic nitro compounds)
BackInert Material Also Present Cyclohexane Cyclohexanone 11.47 Cyclohexane 43.10 Cyclohexanone a-Nitro-p-xylene 25.77 p-Xylene 1-Nitropropane 25.02 None After correction for background current. Compound Nitrocyclohexane
Wt., Mg. 1.03
Wt., Mg.
500 1000 1000 lo00 800
Codombs Calcd. Found0 3.05 4.00 3.05 2.42 33.77 33.12 128.8 127.9 65.8 66.1 108.4 103.3
::& :tion 0.18 0.32 0.10 33.3 10.0 21.1
BackCompound Nitrobenzene
2-N i tro-m-xylene Nitroterephthalic acid
0.39 0.39 1.55 19.52 52.79 22.17 55.43 0.504
Inert Materiala None Benzene None Benzene None m-Xylene m-X y lene None
Coulombs Calcd. Foundb 1.83 1.84 1.83 1.95 7.31 7.17 91.80 92.48 248.0 208.3 84.92 83.76 212.3 219.1 1.38 1.63
tion 0.19 0.12 1.2 1.2 10.0 3.0 4.5 0.12
2.52 40.10 50.28
Terephthalic acid Tere hthalic acid IsopKthalic acid
6.90 110.0 137.8
6.31 115.4 127.4
0.15 14.5 10.4
97.4 123.6
103.2 122.9
6.2 11.2
Wt., Mg.
Nitroisophthalic acid 30.13 p-Xylene Nitrodurene Benzoic acid 35.67 Nitrobenzoic acid Present in at least tenfold excess. I, Corrected for background.
or net coulombs 386,000 (for aliphatic nitro compounds)
Meq. of nitro compound =
For the reduction of polynitro compounds, or for that matter, any electre reducible compound, the more genersll formula
yoreducible compound
Reduction of Aromatic Nitro Compounds
Table IV.
coulombs -net579,000
=
(net coulombs)(mol. wt. of reducible compound) (wt. sample)(no. electrons per group)(96.5) applies. The reduction of a n aromatic nitro group requires six electrons, while reduction of an aliphatic nitro group requires four electrons. Regenerate the silver anodes of the electrolysis cell and the coulometer by using them as cathodes us. platinum anodes in an electrolysis of water at a current of about 50 ma. Continue electrolysis until the electrodes appear clean (light gray). After washing, the electrodes are ready for reuse. RESULTS
0
taining 0.1M potassium chloride was equally satisfactory. PROCEDURE
Fill the electrolysis cell nith 250 ml.
of electrolysis solution (4-to-1 methanolto-0.5N aqueous lithium chloride), and adjust the p H to 2.0 with 1N hydrochloric acid. Add enough mercury to form a 1-cm. layer on the bottom of the cell, and place a stirring bar on the mercury pool. Place the cell on a magnetic stirrer, stir a t high speed, and bubble oxygen-free nitrogen through the solution. Place the anode in the cell and start the pre-electrolqsis. For the reduction of aromatic nitro compounds, set the reference potential voltmeter a t -0.90 volt L I S . the silversilver chloride electrode (-0.95 volt for aliphatic nitro compounds), turn on the electrolysis current, and increase the current until the gnlvanometer is balanced. Continue the electrolysis until the current drops to a steady value ("3 ma. for aromatic, 4 ma. for aliphatic nitro compounds) and record this current as the initial background current. Shut off the electrolysis current, but continue the outgassing and stirring. Adjust the pH of the coulometer to 7.0 rt 0.5 and record this p H to the nearest 0.1 pH unit. Place the coulometer in series with the main circuit, with the platinum cathode of the coulometer connected to the anode of the electrolysis cell. Add the sample to the electrolysis cell. Preferably, the sample should re-
1856
0
ANALYTICAL CHEMISTRY
quire between 100 and 150 coulombs, but samples having a reducible content as low as 2 coulombs have been measured successfully. Turn on the electrolysis current, record the starting time, and increase the current until the galvanometer is balanced or until 3 current of 80 ma. is attained. If the galvanometer is balanced, maintain this by reducing the electrolysis current as the sample is reduced. If the initial current exceeds 80 ma., hold to 80 ma. until the galvanometer comes into balance, and then decrease the current as required to maintain this balance. When the electrolysis current remains steady a t a value below 5 ma. for 10 minutes, or decreases only 0.1 ma. during 15 minutes, terminate the reduction by recording the final current and time of termination, turning off the input, and disconnecting both the coulometer and the reference electrodes. Remove and wash the electrodes of the coulometer. Titrate the coulometer solution to its original pH with standard 0.03N hydrochloric acid. Calculate the total number of coulombs used by: total coulombs = ml. HC1 X N HC1 X 96.5 coulombs/ meq. Correct this value for the background contribution by subtracting the product of the electrolysis time (in seconds) and the final background current (in amperes) in the case of aromatic nitro compounds. With aliphatic nitro compounds, use the product of time and average background current as correction. Calculate the nitro body content in the sample from the corrected (net) value of coulombs.
The foregoing procedure was applied t o the reduction of a number of aliphatic and aromatic nitro compounds in the presence of relatively much larger amounts of other organic materials, including aldehydes and ketones. The carbonyl compounds were electrically inert under the experimental conditions described. Tables I11 and 1V list some results obtained by this procedure. These data show that quite accurate measurements of nitro body content can be made in the presence of large excesses of other organic materials; also, the results obtained with the nitro acids show that this type of compound can be analyzed readily by the proposed method. Although all of the results shown were obtained by the reduction of nitro compounds, similar ones can be obtained by reduction of related functional groups, such as nitrite, nitroso, or nitrate. If only nitro compounds are to be determined, these related compounds will introduce significant errors. Also, in the presence of any other material which is reduced a t a potential the same cr lower than that a t which the nitro group is reduced results will be high. I n general, the technique of controlled-potential coulometry has proved very useful for the measurement of nitro compounds in other organic materials, and especially so for the determination of aromatic nitro acids, which are difficult to determine by conventional methods. At optimum sample concentrations, an accuracy of =kl% (standard deviation, computed from four or more
replicate runs) waa attained with many of the compounds investigated, and, although with poorer accuracy, very high sensitivities were demonstrated. The best solvent system for the reduction of mononitro compounds was 4-to-1 methanol-to4.5N aqueous lithium chloride solution, but other methanolwater solutions also proved acceptable. Although the main problem encountered in this study-the background current of the solvent systemcould not be eliminated, o u t g ~ i n g , rapid stirring, and pre-electrolysis of the
supporting electrolyte solution reduced the interference to tolerable limits. REFERENCES
R. s., Fuman, N* H.,ANAL. CHEY.27, 1182 (1955). (2) Bran& w. w., DeVnes, J.E.7 Gmtz, E.,Ibid., 27, 392 (1955). (3) DeVries, T., Ivett, R. W.; IND. ENG. CHEM.,ANAL.ED.13,339 (1941). (4) Ehlers, V. B., Seage, J. W., ANAL. CHEM.31, 16 (1959). (5) Findeis, A., Dissertation Abslr. 17, (1)
1909 (1957).
(6) Koniecki, W., Linch, A., ANAL.C ~ M 30, 1134 (1958). (7) Lingahe, J. J., IND.ENG. CHEY., ANAL.ED.16, 150 (1944). (8) Lingane, J. J., Small, L. A., ANAL. CHEM.21, 1119 (1949). (9) Tirouflet, J., Bull. SOC. chim. Frame 1956, p. 274. (10) Vanderzm, C. E., mgell, W. F., ANAL.CHEM.22,573 (1950). (11) waller, J., Chm. (LO&) 72, 787 (1955). (12) wolthuis, E., s., L., ANAL. 263 1238 (1954).
RECEIVED for review March 27, 1959. Accepted August 13, 1959.
Application of Constant Current Potentiometry to Nonaqueous Titrations of Weak Acids IRVING SHAIN and GLENN R. SVOBODA Chemistry Department, University o f Wisconsin, Madison, Wis.
b Nonaqueous titrations of weak acids were investigated using the general techniques of constant current potentiometry. Two platinum indicator electrodes were polarized by a constant 1 -pa. current, and the potential between the two electrodes was measured with a vacuum tube voltmeter or with a pH meter. In most cases typical peakshaped titration curves were obtained which permitted direct location of the end point from the meter readings. Although the potential-determining electrode reactions are complex, the electrode system gave reproducible results without pretreatment. Several different weak acids were titrated with tetrabutylammonium hydroxide, using acetone as the solvent.
I.o
R
investigations of the nonaqueous titrations of weak organic acids have led to many new applications and refinements of the technique. Reviews have been included in several papers (2, 3, 11). Almost all of the work reported involved the measurement of the potential between glass and calomel electrodes with a p H meter using the general techniques of oonventional p H titrations (2,3, 6). However, other electrode systems have also been investigated using zero current potentiometry such as platinum-calomel and platinum-platinum (6),glass-silver (f4), and platinum (10% rhodium)graphite (8). The results of Harlow, Noble, and Wyld (6) indicated that both preanodized and precathodized platinum electrodes respond to changes in effective acidity in nonaqueous solvents.
C
VOLUME TITRANT
Figure 1. Titration curves salicylic and benzoic acids A.
ECENT
2F
6. C.
of
Potential of platinum anode with respect to S.C.E. Potential of platinum cathode with respect to S.C.E. Potential between the two platinum electrodes
VOLUME TITRAM
Figure 2. Titration curves of several weak acids using two polarized platinum electrodes A.
Although these authors did not postulate any mechanism through which these pretreated electrodes would become sensitive to acidity changes, it seemed reasonable to assume that the preanodized electrode was a platinumplatinum oxide electrode (?', I 2 ) , and that the precathodized electrode was a type of hydrogen electrode. (The actual electrode processes are considerably more complex.) The sensitivity to changes in acidity was not permanent, however, and Harlow, Noble, and Wyld indicated that the electrodes had to be reconditioned before each titration. It was expected that this disadvantage could be overcoine if a small constant
Benzoic acid
6. Acetic acid C. Dichloraacetic acid
D. Salicylic acid E. p-Nitraphenol F. m-Nitrophenol G. Methyl salicylate
current were passed through the solution during the titration. The results of such a study are presented in this paper. APPARATUS
The titrations were carried out using the general techniques of constant current potentiometric titrations (10). The current source consisted of two 45volt batteries in series with the proper VOL. 31,
NO. 1 1 ,
NOVEMBER 1959
a
1857
