V O L U M E 23, NO. 4, A P R I L 1 9 5 1 it was titrated while boiling against standard potassium permanganate solution. Both methods avoid the interference of chloride ion in the permanganate titration and gave identical results. This method was then used to investigate the conversion of aconitic acid to itaconic acid. In Figure 2 the results of experiments a t various concentrations are reported, showing the determination with the bromine method and the new method. The regular and plausible course of the curve in the new method
589
and the irregular course in t,he case of the bromine method are evidence of the usefulness of the proposed method. LITERATURE CITED
(1) Ambler, J. A., Roberts, E. J., and Weissborn, F. W., Bur. Agr. Ind. Chem., U. S. Dept. Agr., Rept. AIC-132 (1946). (2) Boeseken, J., Rec. trau. chim., 47,683 (1928). (3) Friedkin, M.,IND.ENG.CHEM.,ANAL.ED.,17,637 (1945). (4) Roberts, E. J., and Ambler, J. A.,Ibid., 19,118 (1947). RECEIVED March 17, 1950.
Titration of Acids in Nonaqueous Solvents JAMES S. FRITZ AND N. 31. LISICKI V’ayne University,tDetroit I , Mich.
Although a great many organic cornpounds possess acid properties, existing methods for the acidimetric titration of such compounds are limited in scope. This paper describes a simple, rapid method which permits the accurate titration of a large variety of organic compounds as acids. The substance to be determined is dissolved in a suitable organic solvent and titrated with 0.1 N sodium methoxide in benzene-methanol. Most carboxylic acids, acid chlorides, acid anhydrides, enols, amine salts, and some mercaptans and imides can be titrated in either butylamine or benzene-methanol using thymol blue indicator. Colored or very weakly acidic compounds such as phenol can be determined potentiometrically in butylamine using a pH meter with antimony and glass electrodes.
A
LMOST forty years ago Folin and coworkers pointed out that many acids can be titrated accurately in benzene, toluene, chloroform, and carbon tetrachloride ( I ). The titrant employed was a 0.1 N solution of sodium ethylate or sodium amylate in the corresponding absolute alcohol. Phenolphthalein served as the indicator. Benzene solutions of organic acids are almost completely nonionized, as shown by conductivity measurements, yet the end points obtained were much sharper than when the same titrations were carried out in alcohol or water. Ruehle ( 6 ) employed solvent mixtures of various alcohols and ethers for the potentiometric titration of acidic constituents in asphalts and pitches. Lavine and Toennies ( 4 ) employed a methanol solution of sodium methylate for the differential titration of perchloric and acetic acids in acetonitrile. Moss, Elliott, and Hall ( 5 ) showed that weakly acidic compounds behave as strong acids when titrated in a basic solvent such as ethylenediamine. Excellent end points were obtained potentiometrically using either hydrogen-calomel or antimonyantimony electrode combinations. Several indicators were tried but failed to give satisfactory visual end points. Previous work by one of the authors has demonstrated the feasibility of titrating bases in a wide variety of organic solvents (8,3). The object of the present work is to show that many types of organic compounds may be determined by titration as acids in nonaqueous solution. The titrant employed consists of a 0.1 to 0.2 N solution of sodium methylate in a benzene-methanol mixture. The solvent employed depends on the compound being titrated. For the titration of most carboxylic acids and other moderately acidic compounds, benzene-methanol is the preferred solvent. Most weakly acidic compounds are best titrated in butylamine. Occasionally the use of acetonitrile, ether, pyridine, acetone, dimethylformamide, or toluene is advantageous. By choosing the proper solvent, carboxylic acids, acid anhydrides, acid chlorides, enols, and amine salts may be quickly and accurately titrated to the thymol blue end point. Some mercaptans (thiols), imides, and aliphatic nitro compounds may also be titrated. For colored compounds and very weakly acidic com-
pounds such as phenol, the end point can be detected with a standard pH meter equipped with a glass-antimony electrode combination. REAGENTS AND SOLUTIONS
Acid samples, commercial samples (98 to 100% purity) analyzed as received. Benzene, purified grade. Benzoic acid, primary standard grade. Butylamine, Sharples. Methanol, absolute, as purchased commercially. Benzene-Methanol. hIix 3 volumes of benzene with 1 of methanol. Sodium Methylate Solution. Wash about 6 grams of sodium in methanol and dissolve immediately in 100 ml. of methanol. Protect the solution from carbon dioxide while the sodium is dissolving; if necessary, cool the solution in cold water to prevent the reaction from becoming too violent. When all of the sodium has reacted, add 150 ml. of methanol and I500 ml. of benzene and store the reagent in borosilicate glassware protected from carbon dioxide. Standardize this solution by titration against benzoic acid dissolved in benzene-methanol. hlthough the reagent is reasonably stable it should be restandardized every few days. Thymol Blue dolution. Dissolve 0.1 gram of thymol blue in 100 nil. of methanol. PROCEDURE
In order to conserve chemicals, a 10-ni1. buret is recommended instead of the usual 50-ml. buret. Dissolve the sample in 20 to 30 nil. of the solvent chosen, add indicator, and titrate to a deep blue color. For titrations carried out in basic solvents, the solution being titrated must be protected from carbon dioxide. This may be conveniently done by carrying out the titration in a small f h k fitted with a one-hole sto er to admit the buret tip, or in a small beaker covered with carxtoard Use of a magnetic stirrer adds to the convenience of the titration. Unless the solvents employed are known to be free of acid impurities, a blank should be run. This is particularly important in the case of basic solvents. The suggested procedure in this case is to add indicator and exactly neutralize the solvent, then add the sample and carry out the titration. For titrations carried out in benzene-methanol, the blank was almost nonexistent and t h e r e fore did not have to be determined before each titration.
A N A L Y T I C A L CHEMISTRY
590 -~
~
were somewhat erratic and several per cent low. This is possibly due to partial decomposition of nitromethane caused by reaction with the butylamine solvent. Acid Gases. Although no quantitative results were obtained, carbon dioxide, sulfur dioxide, and hydrogen sulfide gave excellent end points rvhen titrated in butylamine. M e r c a p t a n s . Thiophenol and 2mercaptobenzothiazole are sufficiently acidic to permit titration in butylamine. Oximes. Apparently ordinary oximes cannot be titrated. Keither or-benzoinoxime or dimethylglyoxirne gave an acidic reaction to thymol blue in butylamine solution. S a l t s . S t r y c h n i n e sulfate and Bbromoethylaniine hydrobromide were titrated directly in benzene-methanol Hydroxylamine hydrochloride is insoluble in benzene-methanol but w&s titrated successfully in pyridine. The free base liberated during the titrations does not interfere with the thymol blue end point Data for the titration of the compounds listed above are given in Table I.
'Table I . Titration of Acidic Compounds Using Visual End Point Solvent Benzene-methanol 2-11Prcaptohenzothiazole
Biitylnniine
m-Sitrabcnzoir acid Succinimide
Piperidine Benzene-methanol Butylamine Butylamine
3,5-Dinitrobenzoyl chloride
Benzene
Phthalic anhydride
Benzene-met hanol
Salicylic acid
Benzene-methanol Acetonitrile
Malonic acid
Butylamine
Acetylacetone
But ylamine
Benzoyl chloride
Benzene-met hanol
Brornoetbylamine hydrobromide
Benzene-methanol
hlethone
Benzene-methanol
Strychnine sulfate
Benzene-methanol
Hydroxylamine hydrochloride Pyridine Thiophenol
Butylamine
Taken, G. 0.0694 0.0857 0.0842 0.0950 0.1012 0,1060 0.1089 0,1033 0.1020 0.0610 0 1099 0 1261 0,1169 0.1411 0.6175 0.6134 0.7086 0.5087 0.5215 0.5111 0.2776 0.2167 0.2513 0.3503 0.3366 0.1816 0.3174 0.4389 0.4761 0.5122 0,3529 0.4123 0.1380 0.1413 0.0612 0.0762 0.1733 0.1027 0.0388 0 0532 0.0232
Base, .M1. 5.06 6.24 6.22 5.84 6.19 4.30 4.43 4.19 6.79 4.02 2.89 3.32 4.87 5.88 46.00 45.70 52.74 25.38 26.10 25.54 54.30 42.42 49.10 35.82 34.24 18.50 32.24 23.50 25.52 27.46 13.22 15.42 5.27 5.39 3 43 4 28 3 51 2 08 4 39 6 04 1 61
'
N of Base 0.0965 0.0965 0.0965 0.0965 0.0965 0.1446 0.1446 0,1446 0.1146 0.1446 0.1644 0.1644 0.1625 0.1625 0.0965 0.0965 0.0965 0.1440 0,1440 0,1440 0.0969 0.0969 0.0969 0.0969 0.0969 0.0969 0.0969 0.1297 0.1297 0.1297 0.1297 0.1297 0.1270 0.1270 0 1270 0 1270 0 1270 0 1270 0 1270 0 1270 0 1278
Purity,
% 96.6 96.4 96.3 99 2 98.7 98.0 98.3 98.0 95.4 94.5 99.7 99.8 100.3 100.0 99.3 99.3 99.2 99.2 99.5 99.4 98.6 98.6 98.5 99.2 98.7 98.8 98.8 97.6 97.7 97.7 99.7 99.5 99.4 99,3 99.8 100.0 98.7 98.7 99.9 100.3 97.7
POTENTIOMETRIC TITRATIONS
Potentiometric titrations were carried out using a Beckman line-operated pH meter equipped with glass and antimony electrodes. This is very convenient to
TITRATIONS USING VISUAL END POINT
Carboxylic Acids. Benzene-methanol appears to be the most desirable solvent for the titration of carboxylic acids. Although very sharp end points are obtained in benzene, butylamine, acetone, and other solvents, a gelatinous precipitate forms in many instances during the course of the titration. Salicylic, m-nitrobenzoic, p-aminobrnzoic, and stearic acids all give very sharp end points in benzene-methanol. .\laIonic acid gives a poor end point in this solvent., probably owing to the weakly acidic nature of the second carboxjl group. In butylamine, hoffever, malonic acid cm be titrated sharply as a dibasic acid. Acid Chlorides and Anhydrides. In benzene or benzenemethanol solution, acid chlorides and anhydrides behave as strong monobasic acids and may be titrated very accurately as such. Results are reported for 3,5-dinitrobenzoyl chloride, benzoyl chloride, and phthalic anhydride. Enols. Many enols are too weakly acidic to be titrated sharply in benzene-methanol. I n such cases, butylamine may be used to advantage as a solvent. Excellent results were obtained in the titration of acetylacetone in butylamine. Methone (dimethylcyclohexanedione) possesses sufficient acid strength to be titrated accurately in benzene-methanol. Imides. Succinimide behaves as a moderately strong acid in butylamine and can be titrated fairly accurately using thymol blue. In less basic solvents, it is too weak an acid to be titrated. Phthalimide gave very erratic results when titrated under the same conditions. Nitro Compounds. The acid strength of nitromethane is sufficiently enhanced by basic solvents to permit titration. Using thymol blue indicator, the titration is carried out in butylamine until a deep blue color is obtained. Quantitative results obtained
I
0
I
I
I
I
I
I
1
2
3
4
5
6
ML. TITRANT Figure 1. Titration of Benzoic Acid (A) and Benzoic Acid-Phenol (B) in Butylamine
7
59 1
V O L U M E 2 3 , N O . 4, A P R I L 1 9 5 1
point. Further progress, however, is hindered by the lack of :L wider choice of suitable indicators, especially for titrations carried out in basic solvents. Development of new indicators for this use would be a very valuable contribution. Interferences are not numerous. When titrations are carrictd out in basic solvents, provision must, be made for protecting the solution from carbon dioxide in the atmosphere. The presc.ncv of much water or other very weakly acidic substances ail1 irit,erfere with titrations in basic solvents by seriously decreasing the Pensitivity of the end point. Esters may interfere, owing t o 1):irtial hydrolysis.
.60-
.50 -
5;
I-
w .30
,zot2 0
1
2
3
4
5
6
7
ML. TITRANT Figure 2. Titration of Nitromethane (A), Carbon Dioxide (B), and Acetylacetone (C) in Butylamine
use and does not require special apparatus. The antimonyglass electrode combination is c h i d y for use in basic solvents; it cannot be used in benzene or benzene-methanol, (Titrations in benzene-methanol may be carried out potentiometrically using a pH meter with antimony and calomel electrodes. Lithium chloride is added to the solution to decrease the resistance.) I n basic solvents, the antimony electrode appears to serve as the indicator electrode, while the glass electrode acts as the reference electrode.
0
Taken, G. 0.0581 0 0436 0 0330
Base, MI. 4 78 3 58 4.38
N of Base 0.1298 0,1298
0.1298
Purity,
%
100.0 100.3 100.9
I
I
I
I
I
2
3
4
5
6
7
ML. TITRANT Figure 3.
Table 11. Potentionietric Titration of Phenol in Butylamine
I
1
Titration of Phenol (A), Succinimide ( B ) , and Thiophenol (C) in Butylamine
The formation of a gelatinous precipitate during the course of some titrations is a source of error. Such precipitates may often be avoided, however, by proper selection of t,he solvent. Benzoic acid, for esample, gives :t very bad gel when t.itrated in benzene alone; in benzene-methanol no precipitate forms. LITERATURE CITED
111 Figures 1, 2, and 3 titration curves of various acid compounds in butylamine solution are given. I n the titration of nitrornethane and succinimide, thymol blue indicator changes slightly before the true end point, as indicated potentiometrically. Thymolph thalein and phenolphthalein change a t essentially the sanw point in the curve; alizarin yellow changes a t a slightly loner potential. The latter indicator is useful in titrating benzoic a(-idin the presence of phenol. X o indicator tested was found to be satisfactory for the titration of phenol. However, it may lie titrated potentiometrically with an accuracy of 1% or better. Data are given in Table IT. DISCUSSION
By following the procedures described above, a large number of acidic compounds may be accurately titrated using a visual end
Flanders, J . A m . Chem. Soc., 34, 774 (1912). (2) Fritz, ANAL.CHEM.,22, 578 (1950). (3) Ibid., p. 1028. (4) Lavine and Toennics. A M .J . J l e d . Sci., 185, 302 (19331 (5) Moss, Elliott, a n d Hall. ANAL.C‘HEY., 20, 784 (19481. (6) Ruehle, ISD.ZSG.CHM.. A s . 4 1 ~ED.. . 10, 130 (19381. (1) E’olin arid
RECEIVEDAugust 23, 1950. Presented before the Division of Analytica Chemistry at the 118th Meeting of t h e A V E R I C A SC H E M ~ C . SOCIETY, AL Chicago, Ill.
