relation between true diameters and the Observed Coulter diameters regard1ess Of the numerical value. Such a calibration depends on the chemical composition of the particles under study, and hence physical properties in addition to size and shape. This last conclusion was previously noted for another method of size measurement (6).
ACKNOWLEDGMENT
The author thanks c. F. Callis for helpful discussions and W. ]$r. bIorgenthaler for making Some of the measurements. LITERATURE CITED
(1) Am. Soc. Testing Materials, Phil-
adelphia, Pa., htethod E 2&51T.
( 2 ) Ames, D. P., Irani, R. R., Callis, C. F.,
J. Phuc. Chem. 63, 531 (1959). (3) Berg, R. H., Am. SOC.Testing Materials, Spec. Tech. Publ. 234, 245 (1959). (4)Irani, R. R., J . Phys. Chem. 639
1603 (1959). (5) Irani, R. R., Ames, D. P., Zbid., submitted for publication. (6) Irani, R. R., Callis, C. F., ANAL. CHEW31, 2026 (1959). RECEIVED for review November 19, 1959. Accepted February 29, 1960.
Tetraphenylstibonium Sulfate as a Reagent for the Qualitative Analysis of Organic Acids HAROLD E. AFFSPRUNG and HUBERT E. MAY Department o f Chemistry, The University o f Oklahoma, Norman, Oklo.
b A new reagent, tetraphenylstibonium sulfate, is used for the qualitative identification of organic acids. The methods of preparation of both the reagent and the derivatives of the organic acids are given. The derivatives are small needle crystals, obtained in good yields. They are salts of the tetraphenylstibonium ion and are prepared by adding an 0.05M aqueous solution of tetraphenylstibonium sulfate to aqueous solutions of the acids. The solutions must be slightly acid before precipitation is made. The salts are recrystallized from n-hexane, water, or water-ethyl alcohol mixtures. The melting points are sufficiently sharp and far enough apart for easy differentiation of many organic acids.
A
of reagents have been described for the identification and preparation of derivatives of organic acids. Very few general methods involve the simple salts as deriva, tives, as their melting points or solubilities are suitable only in specific cases. The methods, in general, must be carried out in the absence of water, or in the case of the benzylthiouronium salts, in alcohol-water mixtures. Fairly high concentrations of acid and reagent are normally required for the preparation of the derivative. Recently, onium-type compounds, which contain as the central atom an element other than nitrogen, have been used as precipitation reagents, and might be expected to give suitable derivatives with organic acids for qualitative organic analysis. However, the salts formed were too soluble or had unsatisfactory melting points. The salts of tetraphenylstibonium ion are an ex-
ception, as all except the sulfate are insoluble in water and consequently might form suitable derivatives with organic acids. Compounds containing the tetraphenylstibonium ion were first prepared by Chatt and Mann (1). Willard, Perkins, and Blicke (6) described a method of preparing tetraphenylstibonium chloride, and Willard and Perkins (4) reported the use of this salt as an analytical reagent for gravimetric determinations. Potratz (3) reported some solubilities of inorganic salts of tetraphenyhtibonium ion and described the use of tetraphenylstibonium sulfate as a reagent for the gravimetric determination of fluoride. The use of tetraphenylstibonium sulfate as a reagent for the preparation of organic acid derivatives for qualitative analysis is presented here.
LARGE NUMBER
1164
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Salts of the tetraphenylstibonium ion cannot be purchased a t present and must be prepared. Triphenylstibine and triphenyhtibine dichloride are available commercially, so that only the last reaction with the Grignard reagent is necessary if these compounds are used &s the starting material. The reagent with slight modifications was prepared according to the method of Willard, Perkins, and Blicke (6). Reagent. Antimony trichloride was treated with an excess of phenylmagnesium bromide and the resulting triphenylstibine was chlorinated and again reacted with an excess of phenylmagnesium bromide. Only one reaction with the Grignard reagent is needed if antimony pentachloride is used, but the yield is not as good ( 2 ) . The addition product was de-
composed with dilute sulfuric acid. The resulting solution of. tetraphenylstibonium sulfate was treated with dilute ammonium hydroxide to precipitate tetraphenylstibonium hydroxide, which was filtered and washed. The hydroxide is very insoluble and can be purified easily by reprecipitation. It was also boiled with distilled water to remove the biphenyl produced in the Grignard reaction. The high solubility of the sulfate, greater than 60 grams per 100 ml. of water (S), makes it very difficult to prepare directly in a reasonably pure form. Also, the equivalent weight serves as a simple criterion of purity. The hydroxide was reprecipitated and washed until the equivalent weight indicated a purity of 98%. If desired, the hydroxide may be recrystallized from an alcohol-water mixture. Tetraphenylstibonium hydroxide cannot be dried in an ordinary drying oven a t 110' C. without partial conversion to the oxide; therefore it was dried in a vacuum desiccator before each equivalent weight determination. The equivalent weight was determined by dissolving a weighed amount of the hydroxide in an excess of standard sulfuric acid and back-titrating excess acid with standard sodium hydroxide. The end point was determined with a pH meter. The end point coincided with the first appearance of a faint cloudiness in the solution because of the precipitation of the hydroxide. This appearance of precipitate is satisfactory as an indicator for ordinary titrations and the results can be used for general reagent preparation. Finally, a weighed quantity of the dried tetraphenylstibonium hydroxide was dissolved in an equivalent amount of standard sulfuric acid and diluted with distilled water t o a concentration of 0.05M. Procedure. All of the derivatives of the tetraphenylstibonium ion obtained from organic acids ITere prepared in the same manner. About
5 to 10 ml. of a solution of the salt of the acid, made slightly acid with dilute sulfuric acid, were added to about 4 t o 5 ml. of the reagent and a precipitate usually formed immediately. With water-insoluble acids, a buffer solution was prepared by adding the acid to dilute sodium hydroxide until a p H of 5 or 6 was indicated by pH paper or until the solution was definitely acid to litmus; this solution was then filtered into the solution of tetraphenylstibonium sulfate. If a precipitate did not appear a t room temperature, it usually appeared when the solution was cooled in an ice bath. In each case the precipitate was filtered and washed with a little cold water. The low molecular weight acid derivatives-that is, the acetate and propionate-were then recrystallized from n-hexane. The formate was recrystallized from water. After one or two recrystallizations the melting points remained constant and sharp throughout additional recrystallizations. In general, the n-hexane solutions were not saturated a t the boiling point during the recrystallizations, and the hot solutions were filtered. Although no insoluble material remained visible, usually fewer recrystallizations were required to give good melting points if this filtration was made. No artificial cooling was used; the solutions were allowed to cool to room temperature on the desk and then filtered. The yields of the crude salts were all good, generally greater than 80%, calculated on the basis of the organic acid. The recovery of the salts on recrystallization was usually 70 to 80% in the case of the lower molecular weight acids. No yields were determined on salts of higher molecular weights than the propionate, except for the benzoate, although qualitatively the yields all appeared to be the same. Except for the benzoate, which was recrystallized from water, the salts of higher molecular weight than the propionate were recrystallized from water-alcohol mixtures. The salt was dissolved in hot ethyl alcohol, distilled water was added until a permanent cloudiness was observed, and then more alcohol was added until the solution became clear. Care was taken to avoid adding too much alcohol, as these salts are very soluble in it and reprecipitation will not occur if too much is used. The derivative salt was filtered on paper with suction and allowed to dry a t a temperature of about 60' C., and then the melting points were taken in capillaries. Each salt was recrystallized until the melting points remained constant (usually two recrystallizations were sufficient) and each salt was prepared independently a t least two times. In general, the heating rate of the bath averaged 0.5' t o 1.0' C. per minute a t the melting point. The solubilities of the tetraphenylstibonium salts of formic, acetic, propionic, and benzoic acids were determined. In each case a nearly saturated solution was prepared in water near the
~
Table 1.
Properties of TetraphenylstiboniumSalts
Soh..~ . ~ ~
Composition, yo inbility Water, M. P., Capillary G./100 G. Theory Found Tetraphenylstibonium Heating Rate a t 25.0' TetraDhenvlstibonium 25.0," HydroHvdroHydroHvdroSalt Silt 0.5Oper Min. f 0.1' 0.5Ope; 0.1 Carbon gen ken Carbon gen Formate Decomposes
-
Acetate Propionate Benzoate Acetate and propionate mixed in equal quantities
120.5-21.5 131.5-33 133.5-34.5 161.0-62.5 129 -30
0.85 1.10 0.55 0.09
boiling point and then placed in a water bath held a t 25.0' f 0.1" C. A41iquots were taken and evaporated a t temperatures not exceeding 60' C. until all excess water was removed. Finally the salts were dried to constant weight in a vacuum desiccator and solubilities calculated. A few samples were then placed in an 110' to 115' C. drying oven for several hours with no further loss in weight observed in any case, indicating that no simple hydrates were formed and that the desiccator drying was adequate. RESULTS AND DISCUSSION
Table I gives a few properties of the tetraphenylstibonium derivatives of the first three fatty acids and of benzoic acid. The derivatives have good melting points and relatively low concentrations of the respective acids, approximately O . O O I M J may be used. The carbon and hydrogen contents of the derivatives agree very well with the theoretical values. The formate decomposes a t its melting point with the evolution of a gas; however, the decomposition apparently takes place after the compound has melted and the melting point is sharp. I n Table I1 the melting points of the citrate and hydrogen oxalate salts are also given as decomposition points, with a gas being given off. This indicates that decomposition occurs immediately after the compound has melted, as in the case of the formate. It is unfortunate that the melting points of the propionate and acetate salts, as well
63.18 63.81 64.43 67.53
4.45 4.74 5.01 4.57
63.04 63.80 64.43 67.55
4.29 4.78 5.12 4.46
as the mixed melting point, lie so close together. However, reasonable care will allow one to distinguish them without undue difficulty. Table I1 presents a number of melting points of derivatives prepared from tetraphenylstibonium sulfate. It was not necessary to analyze all of the salts for carbon and hydrogen as the precipitations proceeded rapidly, which indicates an ionic reaction; also, there are practically no other products possible from the reactants. Equivalent weight determinations were made of the salts of the dibasic and tribasic acids given in Table I1 to establish which salt was formed. A comparison of the melting point of the p-toluate salt with that of the acid is interesting. The melting point of the salt is close to that of the acid, which melts a t 180" C. However, a large mixed melting point depression shows definitely that it is the derivative and not the acid. Tetraphenylstibonium hydroxide is very insoluble and will precipitate a t pH values greater than 7. It may be recrystallized from water-alcohol mixtures in the same manner as the derivatives of the organic acids. The melting or decomposition point of tetraphenylstibonium hydroxide, when the temperature is rapidly attained, is 213" to 217' C. If the temperature is increased slowly it appears to start to decompose a t about 190" C. If care is taken to ensure that the solution is acid to litmus or shows a pH of about 6 with pH paper,
Table II. Melting Points of Tetraphenylstibonium Salts of Organic Acids Tetraphenylstibonium Melting Point Tetraphenylstibonium Melting Point n-Butyrate 105-06 Citrate" (dec.) 216-18 Isobutyrate 110.5-11.5 a-Naphthoate 145.0-45.5 n-Valerate 92-93.5 Salicylate 175.0-75.5 Isovalerate 118-20.5 o-Toluate 164.5-65.5 Hydrogen oxalate" (dec.) 229.5-33,5 m-Toluate 153-54 Hydrogen phthalate" 186.0-87.0 181-82 p-Toluate Benzene sulfonateb (dec.) 187 (initial) Pyruvate* (dec.) 120 (initial) p-Nitrophenoxide (dec.) 143-44 Formula established by equivalent weight determination. Very long temperature range of decomposition.
VOL. 32, NO. 9, AUGUST 1960
1165
the derivatives may be prepared without danger of precipitating the hydroxide. Also, if a minimum of alcohol is used in reprecipitating, any hydroxide that is coprecipitated will not redissolve in the first recrystallization of the derivative. Tetraphenylstibonium sulfate overcomes many of the difficulties now encountered with the qualitative analysis of organic acids. It is particularly good for the low molecular weight, watersoluble fatty acids. The salts of the acid and tetraphenylstibonium ion pre-
cipitate from dilute solutions with a minimum of preliminary preparations, and because the compound formed is a salt, the danger of oils forming is small. The salt derivatives are stable, insoluble in water, and easy to recrystallize. I n most cases the melting or decomposition points obtained are sufficiently sharp and far enough apart for the differentiation of many organic acids.
(2) Perkins, L. R., Ph.D. thesis, University of Michigan, 1947. (3) Potratz. H. A,. ANAL. CHEM. 28. ' 1356 (1956). (4) Willard, H. H., Perkins, L. R., Ibid., 25. 1634 (1953). (5) 6-illar& H. H., Perkins, L. R., Blicke, F. F., J. Am. Chem. Sac. 70, 737 (1948).
RECEIVED for review February 8, 1960. Accepted May 31, 1900. Presented in
part at the Southwest Regional Meeting
LITERATURE CITED
(1) Chatt, J., Mann, F. G., J. Chem. SOC. 1192, 1940.
of the American Chemical Society, Baton Rouge, La., October 1959. Work supported by the National Science Founda-
tion.
Titrimetric Determination of Carboxylic Acid Chlori de L. J. LOHR Eastern laboratory, Explosives Deparfment, E. 1. du Pont de Nernours & Co.,lnc., Gibbstown,
)Aromatic and aliphatic carboxylic acid chlorides dissolved in tetrahydrofuran are titrated directly with cyclohexylamine dissolved in tetrahydrofuran. The end point of the titration is determined by a sudden change in potential measured with glass-calomel electrodes. Carboxylic acids d o not interfere with the titration, and the carboxylic acid in the acid chloride does not have to b e determined. Free hydrochloric acid also titrates, and the acid chloride titration must b e corrected for the free hydrochloric acid present.
P
and ammonia have been utilized indirectly in various methods of determining carboxylic acid chlorides. In the method of Pesez and Willemart (3), one sample of the acid chloride is reacted with aniline in dioxane t o form an anilide and aniline hydrochloride, and another sample is hydrolyzed with water. The acid chloride is calculated from the difference between the base consumed in the titration of the hydrolyzed sample and in the titration of the sample reacted with aniline. I n the method of Bauer (a), the fatty acid chloride in mixtures of the acid chloride and the fatty acid is converted to the neutral anilide. After acidification with hydrochloric acid, the excess free hydrochloric acid and the aniline hydrochloride are washed from the anilide and the fatty acid is titrated. The per cent fatty acid chloride is calculated by difference. Ackley and Tesoro ( 1 ) similarly analyzed mixtures of fatty acid chlorides and fatty acids by converting the acid RIMARY AMINES
1 166
ANALYTICAL CHEMISTRY
chloride to the acid amide with ammonia. In the more direct method of Stahl and Siggia (6),the carboxylic acid and acid chloride are determined after reaction of the sample with m-chloroaniline. A resulting aqueous solution of m-chloroaniline hydrochloride and the carboxylic acid is titrated differentially with aqueous base. Free hydrochloric acid, if present, must be determined by a nonaqueous titration of a separate sample ( 4 , 6 ) . These methods are unsuitable for determining carboxylic acid chlorides which have an easily hydrolyzed group such as -CH2C1, -CHC12, or -Cch, or for determining carboxylic acid chlorides in mixtures which contain other compounds having these unstable groups. This paper describes a direct nonaqueous titration of simple and complex aliphatic and aromatic carboxylic acid chlorides. The method was developed mainly for determining aromatic acid chlorides having an easily hydrolyzed group such as -CH,Cl, -CHC12, or -CC13, and for determining aromatic difunctional carboxylic acid chlorides, such as terephthaloyl and isophthaloyl chlorides, Khich may contain impurities having these groups. The method is general and can be used to determine other simple carboxylic acid chlorides such as lauroyl chloride. The determination of aromatic difunctional acid chlorides such as terephthaloyl chloride, or of aromatic acid chlorides having a group which is easily hydrolyzed in basic solution, such as -CH2C1, -CHCL, or -Cch, has not been discussed in the literature. The acid chloride dissolved in tetrahydrofuran is titrated r i t h a standard
N. 1.
solution of cyclohexylamine also dissolved in tetrahydrofuran. The end point is determined by a sudden potential drop measured with glasscalomel electrodes. Carboxylic acids do not titrate, and consequently do not interfere with the titration of the acid chloride. The acid chloride titration must be corrected for any free hydrochloric acid in the sample. Free hydrochloric acid is accurately determined by titration of a separate sample with tripropylaniine (4, 5 ) . DETAILS OF M E T H O D
This titrimetric method is based upon the general Reaction A. "",-"GI
U where R = aliphatic or aromatic. As the titration proceeds, most of the products of the reaction separate from the solution. The stoichiometry was proved by Reaction B.
181
6)6-0e-rg 6"'' +
+
1
L
COC,
O H T.rephfhololl C h orld,
C,:I~h.Womine
U
N,N'-D
