Determination of Aconitic and Itaconic Acids KARL LACER AND SA4D M. MAKkR I n s t i t u t e of Chemical Technology, Farouk I Unirersity, Alenondria, E g y p t
KO methods are known for rapid determination of a great number of samples of molasses or crude aconitates and itaconic acid. Determination by decarboxylation takes many hours, and other methods are not accurate enough. A new and relatively rapid method consists in the titration of solutions containing the acids and a slight excess of sulfuric acid with a solution of permanganate at boiling temperature. The permanganate solution is standardized against a known sample of the acids. Under the conditions given, the results are correct and reproducible. The method can serve for very rapid determinations of aconitic or itaconic acid in molasses, solutions from fermentations, and cmde aconitates.
D
URING a systematic study of Egyptian cane sugar molasses,
the need for a rapid and accurate method for the quantitative determination of aconitic acid initiated the present work. DETERMINATION O F ACONITIC ACID
hconitic acid in molasses, crude aconitates, etc., is usually determined by heating, whereby one molecule of carbon dioxide is split off per molecule of aconitic acid: C3H3(COOH)a--tC3H4(
C0OH)t
+ COz
bond-Le., HOOC-CH*-C( COOH)=CHCOOH-and this ruggests methods of determination making use of reagents which add to the double bond. Bromine was found to add readily to aconitic acid, but the quantitative results were unsatisfactory. The iodine value was totally erroneous. Titration with potassium permanganate finally gave satisfactory r e s u h . Titration against Potassium Permanganate. This was carried out in acid solut,ions. Experiment3 to stop the react,ion at the hydroxylation step (2): CJ&(COOH)s
+ PO~I~CjH,(OH)*(COOH),
by titrating a t the low temperature of 0" to 4" C. gave no sharp end point because of the separation of manganese dioxide. At higher temperatures, up to 80" C. no manganese dioxide was precipitated, but after a few moments the color of the solution faded so that t'he end point was not sharp. Titration became possible and in good agreement with the carbon dioxide method, when it was carried out against a standard potassium permanganate solution during constant boiling. .4t these high temperatures the titration leads to t,otal oxidation of aconitic acid t,o carbon dioxide and water: CsHCOG Table I.
+ SO---tGCO, + 3HZO
Titration against Potassium Permanganate
Aconitic Acid Used
Standard KMnOl
G.
.ill,
0.1308 0.2996 0 . 30JOa
9.61 22.02 22.65
Aconitic Acid Eqriiv. of 1 MI. R M n O l G. 0.0136 0.0136 0.0136
Aconitic *aid F o u n d
G.
%
99.9 99.9 101 Mixture of 0.1742 gram of p u r e aconitic acid and 0.3519 grain of teclinical calcium rnagnesiurn aconitate irith 38.3% aconitic acid.
0.0
0.2
0.4
0.6
0.8
I.o
Normality of K M n O i
Figure 1.
Titration of Aconitic Acid
This carbon dioxide is determined in the usual manner, either gravimetrically or volumetrically ( 4 ) . Precautions essential for a good result include absolute absence of carbon dioxide from all chemicals, solvents, etc., and tightness of apparatus. This makes the method very sensitive, requiring much time and care, and scarcely serviceable for mass determinations. The authors attempted an attack on the problem from a different angle. The molecule of aconitic acid contains one double
0.1307 0.2994 0.3081
Theoretically 9 oxygrn atoms are required for each molecule of acid. However, the actual consumption of oxygen was found to vary according to the concentration of the permanganate used. With 0.05 to 0.1 S potassium permanganate solution, 15 oxygen atoms per molecule of aconitic acid were used. With 0.7 N potassium permanganate solution, the oxygen consumption was 13 at'oms per molecule of acid, and on further increase of potassium permanganate concentration the oxygen consumption decreased until it reached the theoretical amount-i.e., 9 oxygen atoms per molecule of acid-in 1 S permanganate solution. Thus the amount of oxygen used depends upon the concentration of potassium permanganate solution, as Figure 1 shows, but not upon the concentration of aconitic acid in solution. The new method is an empirical one and the potassium pcrmanganate solution used must be standardized against a pure known sample of aconitic acid. The standardized solution is then used with varying concentrations of aconitic acid or with technical aconitates; the results were found to be satisfactory, ae Table I shows. Standardization of Potassium Permanganate Solution against Pure Aconitic Acid. About 0.03 gram of pure aconitic acid dissolved in 25 ml. of distilled water and acidified with about 5 ml. of concentrated sulfuric acid is heated to boiling and titrated while boiling against a solution of 0.1 N . potassium permanganate. The end point is indicated by a distinct and lasting rose color. The weight in grams of aconitic acid used, divided by the number of milliliters of potassium permanganate consumed, gives the standard titer.
ANALYTICAL CHEMISTRY
588 Technical Aconitates. To about 0.1 gram of technical calcium ma nesium aconitate after removal of coloring matter by washing wit% ethyl alcohol, about 25 ml. of distilled water and 10 ml. of concentrated sulfuric acid are added, heated to boiling, and titrated with a freshly standardized potassium permanganate solution. The slight amount of calcium sulfate precipitated does not interfere with the titration. Aconitic Acid in Molasses. A known amount of molasses (about 200 to 300 grams) is diluted with about 5 parts of water and deproteinized with tungstic acid in the usual manner. The filtrate and combined washings are used for the determination as follows: A part of the filtrate, containing about 0.03 to 0.1 gram of aconitic acid, is treated with neutral lead acetate solution until no more precipitation occurs, and then is filtered, and washed well with distilled water. The filter paper is perforated and the precipitate is washed quantitatively into a small beaker. The lead is precipitated by means of hydrogen sulfide, filtered, and washed. The combined filtrates and washings are strongly acidified with concentrated sulfuric acid and titrated against standard potassium permanganate solution while boiling. Table I1 shows the aconitic acid content of molasses from different regions of Egypt estimated by both the new method and the carbon dioxide method. The results were the same for duplicates.
Table 11. Aconitic Acid in Molasses Molasses Armant 1948 Armant 1949 Kom-Ombo 1948 Naga-Hamadi 1949
Aaonitia Aaid Content, G. % COa method KMnO4 method 6.47 6.41 6.56 6.48 6.00 5.99 4.49 4.50
During their study of the two acids, the authors found that the mercurous itaconitate is readily soluble in water, unlike the lead salt, which is sparingly soluble and can be easily precipitated with neutral lead acetate. The corresponding salts of aconitic acid are both practically insoluble under the same conditions. The new method for quantitative separation and estimation of aconitic and itaconic acids is based upon these observations. 55
50
45
’i-
40
K 35
Q
a
.- 30 4 iP5 c
:
3 PO
.-
I
cP 15 10
DETERMINATION OF ITACONIC ACID
Itaconic acid may be quantitatively estimated either acidimetrically (3) or by addition t o the double bond. The permanganate method used with aconitic acid was found t o be satisfactory with itaconic acid. These methods, however, did not give satisfactory results when the samples to be analyzed were impure or when the estimation was made in presence of aconitic acid. DETERMINATION OF MIXTURE OF ACONITIC AND ITACONIC ACIDS
The technically important conversion of aconitic acid to itaconic acid by heating in the presence of mineral acids, leading to CH 2C00H LII C O O H
~HCOOH
+
r:z II
+ con
&H*
is relatively slow and the determination of the progress of the conversion is not simple. Repeating the methods described in the literature for the determination of itaconic acid in a mixture of aconitic and itaconic acids gave wrong results. Friedkin (3) claimed that the bromine method is not influenced by the presence of aconitic acid, if the concentration of the latter is kept below 15%. The authors have found that aconitic acid is not totally inactive toward bromine, as a certain reaction with bromine should be expected. Control experiments with mixtures of crude aconitic and itaconic acids isolated from the molasses showed the uselessness of this method, as 5, 10, and 20 ml. of the same solution gave nearly identical values of bromine consumption. Ambler, Roberts, and Weissborn ( 1 ) used the simple titration for rapid determination of both acids, as in large scale operations this was more convenient tban the quantitative estimation of the carbon dioxide split off. This gave too high results; as the results obtained paralleled the true values, they were useful for their purpose.
5
0
5
10
15
PO
Concentration of Aconitalr,
Figure 2.
25
30
% by WeisM
Conversion of Aconitic Acid to Itaconic Acid
The basic condition for this method is the insolubility of the mercurous aconitate under the working conditions. This waa proved by the following experiment: Pure aconitic acid (0.0222 gram) was dissolved in 20 ml. of distilled water and heated t o boiling, and from a buret a saturated solution of mercurous nitrate was added dropwise, until no more precipitate was formed. The solution was cooled, filtered, and washed with distilled water. The filter paper waa erforated and the precipitate washed quantitatively into a beater. The volume was made up to 25 ml. with distilled water, 5 ml. of concentrated sulfuric acid were added, then 2 ml. of concentrated nitric acid in order to oxidize the mercurous ion, and after 1 hour the solution was titrated against standardized potassium permanganate solution. The results showed 0.0222 and 0.02209 gram of aconitic acid, corresponding t o 99.5% of the original aconitic acid in solution. In R parallel estimation 0.0222 gram of aconitic acid and 0.0148 gram of itaconic acid were dissolved in 20 ml. of water, treated as before, filtered, and washed. The filtrate and washings were collected and the recipitated mercurous aconitate was deterestimate the itaconitate, the filtrate and mined as before. washings were divided into two equal portions. From a microburet 0.1 N hydrochloric acid was added dropnise to the first portion of the solution, until no more mercurous chloride precipitated, Some Ziemermann-Reinhardt solution was added, to prevent interference of chloride ion, and the boiling solution was titrated against standard potassium permanganate solution. The other half of the filtrate was treated with hydrochloric acid in the same way, and the surplus hydrochloric acid was removed by drops of 0.01 N silver nitrate solution. After filtering
50
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.
