576
ANALYTICAL CHEMISTRY
Comparison of the values obtained by the authors' method with those obtained using the Knorr alkalimeter indicate that the proposed method is accurate within the limits of its reproducibility. DISCUSSION
The reaction of the ethanolamine carbonates with excess sodium hydroxide, when heated, can be represented as follows, using monoethanolamine as an example:
+ 2iYaOH --+ Xa2C03 + 2HOCH2CH2SH30H HOCH2CHJVH3HC03 + 2NaOH --+ NaZCO:, + HOCHZCHZNH~OH + H20 (HOCH2CH2iTHs)J3Os
In either reaction 2 moles of sodium hydroxide are equivalent t o 1 mole of carbon dioxide, whether the carbon dioxide is present as carbonate or bicarbonate. In back-titrating the excess sodium hydroxide, both hydrolysis of sodium carbonate and ionization of the amine must be repressed. Since ethanolamine solutions in ethyl alcohol are neutral to alizarin yellow, the basic effects of the amine are eliminated by performing the titration in a predominantly alcoholic solution. The low solubility of sodium carbonate in the alcoholic solution is further decreased by the addition of sodium acetate, thus decreasing the concentration of hydroxyl ion. The addition of sodium acetate appears to
decrease the sharpness a t the end point slightly, but still increases the reliability of the method by suppressing hydrolysis of sodium carbonate. .4lthough the end point is slightly indefinite, errois from this source are minimized by titrating the sample to thr same indicator color as that obtained in the standardization of the acetic acid. For a 5-ml. sample a 0.1-ml. net titration represents about 0.02 gram of carbon dioxide per 100 ml. of solution. Results are computed to the nearest 0.01 gram per 100 ml. Even though the percentage error may be high at low concentrations, the ahsolute value can be expected to have a probable error of about +0.04 gram of carbon dioxide per 100 nil. of sample. R'here a very high degree of accuracy is not desired, the eimplicity of titrimetric analyses makes this method desirable for thr estimation of carbon dioxide in ethanolamine solutions LITERATURE CITED
(1) (2) (3) (4) (5)
Dixon, R. E., and Williams, R. A., Analyst, 74, 360 (1949). Biochem. J., 14,451 (1920). Foreman, F. W., Fritz, J. S., - 4 x a ~CHEY., . 24,306 (1952). Lovgren, S., 2. anal. Chem., 64, 457 (1924). Reed, R. M., and Wood, W. R., Trans. Am. lnst. Chem. E?tgrs.. 37, 363 (1941).
RECEIVED for review M a y 26, 1953. Accepted November 12, lQ53. T h e views expressed in this paper are those of the authors and are not t o be construed as the official views of the S a v y Department.
Test for Acyclic Carboxylic Acid Anhydrides DAVID DAVIDSON Department o f Chemistry, Brooklyn College, Brooklyn 10,
x
u
OFTEN reported characteristic test for or-amino acids consists of their reaction with p-nitrobenzoyl chloride in the presence of bases, which results in the appearance of a blue or purple color (3,4,6, 7 ) . The chemistry of this test has been elucidated by Karrer and Keller ( 5 ) ,who showed that in the case of leucine the test depends upon the successive formation of an 5-p-nitrobenzoy1 derivative ( I ) and its azlactone (11). The latter is a weak acid which possesses indicator properties, its anion (111) being blue in pyridine solution. The present author had also reached this conclusion from theoretical considerations and was engaged in a study of this reaction with David Perlman when the work of Karrer and Keller (6) appeared. At that time it was found that the blue color obtained by Waser's procedure ('?), which ordinarily fades more or less rapidly, could be regenerated not only by adding more pnitrobenzoyl chloride but also by adding acetic anhydride. This suggested that the fading and reappearance of the blue color was due to the opening and closing of the azlactone (I1 I).
RCHCOOH I
RC-CO
RCH-CO I
I
\
\
kHCO
I
N. Y. necessary any stronger base than the pyridine which might conveniently be employed as the solvent for the reagent. This expectation was realized. PROCEDURE
To 1 ml. of a 3% solution of or-(p-nitrobenzoy1)-amino-@toluic acid (4)in pyridine add 30 ml. or 1 drop of the substanor to be tested. With acyclic carboxylic acid anhydrides a blue color develops within a few seconds. This color slowly fades. Acids as strong as salicylic acid inhibit the color formation, but this may be overcome by adding a few drops of triethylamine. Under these conditions carboxylic acid chlorides also respond t o the test if a droplet of water is added. The transitory nature of the color makes this test unsuitable for quantitative work ( 8 ) . Positive results were obtained with acetic, propionic, butyric, isobutyric, capric, lauric, myristic, palmitic, stearic, benzoic, anisic, p-bromobenzoic, phenylacetic, 8-phenylpropionic, cinnamic, a-amylcinnamic, polyadipic, and polyazelaic anydrides. Negative results were obtained with maleic, succinic. glutaric, phthalic, and 3-nitrophthalic anhydrides. The azlactones ( a type of anhydride) thus appear to stand between the acyclic and cyclic anhydrides. The lower reactivity of cyclic anhydrides is also evidenced by their lower heats of hydration ( 1 ) . Positive tests were also given by aspirin and o-benzylouybenzoic acid, an anomaly which is discussed elsewhere (9). LITERATURE CITED
(1) Conn, J. I
s
0 2
I
I1
I11
It seemed likely that if R in the a-amino acid were an acidifying group such as phenyl, the corresponding p-nitrobenaoyl derivative would be a useful reagent for detecting acid anhydrides since the increased acidity of the azlactone (11) would make un-
B.,Kistiakowsky, G. B., Roberts. R. AT., and Smith,
G. A , J . Am. Chtm. &c., 64, 1747 (1942). (2) Davidson, D., and Auerbach, L.. [bid., 75, 5984 (1953). ( 3 ) Edlbacher, S., and Litvan. Fr., 2. physiol. Chem., 265, 241 (1940). (4) Ingersoll, A W., and iidams, R., J Am. Chem. SOC.,44, 2930 (1922). ( 5 ) Karrer, P., and Keller, R., H d u . Chim. -Ida, 26, 50 (1943). (6) Kurts. A. C., Proc. SOC.Ezpt2. Bid. M c d . , 46, 339 (1941). (7) Waser, E., Mitt. Gebiete Lebensm. u . Hug., 20, 260 (1929). (8) Weiss, XI., private communication. RECEIVED for review August 20, 1953.
Acoepted November 3, 1953.
