1084
A N A L Y T I C A‘L C H E M IS T R Y
To the original spent acid sample 1.570 nitric acid was added. After the nitrous acid was destroyed 1.4570 nitric acid was found by Procedure B.
LITERATURE CITED
(1)
ACKNOWLEDGMENTS
(2)
The author wishes t o express his gratitude to H. Bray for assistance in the experimental work. Appreciation is expressed to the Oesterreichische Stickstoffwerke AG, Linz, for permission to publish this work.
(4) (5)
(3)
Kolthoff, I. M., Sandell, E. B., a n d Moskovits, B., J . Am. Chem. SOC.,55, 1454 (1933). Leithe, W., Mikrochemie, 33, 48 (1946). I b i d . , 3 3 , 8 5 (1947). Ibid., 33,210 (1947). Peloure a n d Fresenius, Ann., 106, 217 (1857).
RECEIVED January 13, 1948.
Determination of Organic Isocyanates Is0thiocyanates SIDNEY SIGbIA AND J. GORDON HANNA, General Aniline & Film Corporation, Easton, P a . A method has been developed for determining isocyanates and isothiocyanates by reaction with excess butylamine and acidimetric titration of the excess amine. The procedure has an average precision of better than ”0.47‘.
I
SOCYAKATES and isothiocyanates, both aliphatic and aromatic, react with amines to yield substituted ureas and R’NH2+ fhioureas according to the formulas: RS=C=O RHNCONHR’ and RN=C=S R’KH2 -+ RHNCSNHR’. As amines form strongly basic solutions, they can be titrated with standard acids. By adding a known amount of amine and titrating the excess after reaction with the sample, the amount of isocyanate or isothiocyanate present in the sample can be determined.
+
+
Table I.
Analytical Results % 98.6 98.7 98.2 98.7 98.6 98.8 99.0 99.6 100.2 100.9 100.0 98.6 98.7 98.4 98.5 98.1
cr-xaphthyl isocyanate Phenyl isocyanate Phenyl isothiocyanatea Ethyl i s ~ t h i o c y a n a t e ~ (45 minutes a t room temperature) Methyl isothiocyanate (45 minutes a t room temperature)” Ethyl Isocyanate Phenyl Isocyanate 70 calcd. % found % calcd. found 41.4 41.42 62.35 C 62.3 5.8 5.59 3.60 H 3.7 15.83 10.22 16.1 K 10.4 36.90 23.42 36.8 S 23.7 a Analyses for C, H, N, and S check figures shown.
Methyl Isocyanate % calcd. % ’ found 32.9 32.78 4.1 4.18 19.30 19.2 43.8 43.22 ,
Acidic or basic impurites in the samples act as interferences, since the final analysis is a neutralization. The free acid or base in the samples should be determined, and the final analysis should be corrected t o obtain the correct values for isocyanate or isothiocyanate. REAGENTS
Butylamine Solution. Dilute 25 grams of mono-n-butylamine to 1 liter with dioxane, which has been dried over potassium hydroxide pellets. Standard Acid = 0.1 N sulfuric acid. PROCEDURE
A sample containing approximately 0.002 mole of isocyanate or isothiocyanate is weighed in a small glass-stoppered weighing bot-
tle. Very volatile samples are weighed in sealed glass ampoules, The stopper from the weighing bottle is removed, and the weighing bottle containing the sample is placed in a 250-ml. Erlenmeyer flask. To the flask are added 20 ml. of the butylamine solution, and the flask is swirled to mix the reactants. Alkyl isocyanates and alkyl isothiocyanates are allowed to stand 45 minutes a t room temperature for complete reaction. Aromatic compounds react more rapidly with the butylamine, and the reaction mixture can be titrated immediately after mixing the reactants. Then 25 ml. of distilled water are added, and the solution is titrated to the methyl red end point with 0.1 N sulfuric acid. A blank is run on 20 ml. of the butylamine solution. From the difference between the two titrations, the amount of isocyanate or isothiocyanate in the sample can be calculated (Table I). All the samples were Eastman Kodak White Label chemicals used without further purification. Dioxane was chosen as the solvent because isocyanates and isothiocyanates react significantly with water and alcohols. As dioxane is miscible with water, aqueous standard acid can be used for titration of the excess amine after completion of the reaction. Bn aliphatic amine was chosen as the reagent, because these amines are sufficiently basic to be titrated with standard acid. Butylamine was specifically chosen because its boiling point is high enough to avoid trouble with evaporation. (Ethylamine boils a t 16.6” C., propylamine a t 48.7”, and butylamine at 77.8”.) Aniline, methylaniline, and P-naphthylamine react quantitatively with isocyanates and isothiocyanates, but the reaction is somewhat slower than the reaction with butylamine. These amines are more difficult to determine than aliphatic amines. Other primary aliphatic amines, such as n-amyl and n-pentyl amines, could be used as well as butylamine, but the reactivity decreases slightly as one goes up the series; when octadecylamine is used, the reaction takes about 3 hours for completion. Water and alcohols should be entirely absent in the reagent as these materials react readily with isocyanates and isothiocyanates. Because or the acidimetric nature of the final analysis, any free acidic of basic materials present in the sample should be determined on a separate sample, and the final analysis should be corrected accordingly. RECEIVED February 4, 1948.
COI~RECTIOX. In the article “Nomograph for Particle Size Determination with the Sharples Supercentrifuge” [Saunders, E., 4 i x . 4 ~ .CHEW,20, 379 (1948)], the feed rate is given on page 381, third paragraph under heading “Sample Calculation” a s 100 cc. per minute, whereas it should be 1000 cc. per minute.
