seconds. If no isocyanate is present, no color will develop.
100,
I
DISCUSSION AND RESULTS
The accuracy of the quantitative method was verified by determination of the per cent isocyanate in weighed samples of pure 3,3'-diisocyanato-4,4'dimethylcarbanilide and toluene-2,4diisocyanate alone and in the presence of a typical urethane-based polymer. The 95% confidence limits for the average of a duplicate determination for the determination of less than 0.600% free iqocyanate in a cured elastomer was =t0.03370, calculated on the basis of quadruplicate determinations of four samples by two and y s t s on two different days. Recovery of -NCO from Standard Sample
(Toluene'-2,4-diisocyanate)
Theory,
0 1 1 0
Mole 40 X 50 X 1 0 P 99 X 56 X
Found, Mole 0 43 X 1 60 X 2 30 X 0 62 x
si
Recovery 107 10i 115
110
At the level of 0.5 pniole of butylamine per ml. of solution a variation of i1% in the transmittance reading corresponded to a &:S% change in per cent recovery. The precision of the photometric equipment is estimated to be about *I% and therefore the per cent recovery values were in the expected range. Kater. weak amines, ureas, and urethanes did not interfere with the quantitative method. Strong and weak tertiary amines and amines that might be present in urethane-based polymers, such as o-toluidine, p-toluidine, and m-toluenediamine, which have ioniza-
.,
1.00 2.00 3.00 4.00 MICROMOLES n-BUTYLAMINE / 5 ml. TETRAHYDROFURAN
Figure 2. Calibration curve for decolorization of malachite green with nbutylamine 5 ml. of 0.1 7 0 malachite green in pyridine
tion constants less than 1 X did not react with malachite green. n-Butylamine concentration in tetrahydrofuran diminished upon long standing. This has been tentatively attributed to the reaction of peroxides formed in tetrahydrofuran (or peroxide decomposition products) with n-butylamine. The qualitative isocyanate test reagent will detect less than 0.005 mmole of isocyanate per gram of sample. Polymer samples, if correctly formulated, showed no change in color after standing a t least 6 days. The qualitative isocyanate test reagent was not sensitive to oxygen or carbon dioxide. Addition of approximately 5% water generated color. The reagent also generated color if allowed to stand 3 to 5 minutes in the atmosphere, owing to solvent evaporation or atmospheric moisture. Strong mineral acids such as sulfuric acid formed the colored basic dye from the qualitative test reagent. Weaker acids,
such as acetic, formic. and hydrofluoric. did not form a color. Free mineral acids are not normally present in urethane polymers. The reagent became sluggish as it aged and therefore should be freshly prepared before use. Contamination with strong amines causes the reagent to become less sensitive. The equation for the malachite green-n-butylamine reaction was determined to be as follom:
The product, C23H25S2SHClH9, n as colorless and upon reaction with ieocyanate formed a colored product. The chemistry of the malachite green-n-butylamine reaction used in the quantitative method \vas generally applicable to other basic dyes (crystal violet, brilliant green, methyl violet) and other strong primary and secondary amines (ionization constant greater than 1 X 10-6) (anhydrous ammonia, methylamine, dimethylamine, benzylamine). It is possible that by modification of the dye structure a selective method of determining amines of other base strengths could be developed. S o work of this nature has been attempted a t this laboratory. LITERATURE CITED (1)
Diltey, W,, Wizinger, R., Be?. 59, 1856
(1926). (2) Kulberg, L. M.,Mustafin, I. S., Zhur. ilnal. Khim. 7, 84 (1952). (3) Siggia, S., Hanna, J. G., >$SAL. CHEar. 20, 1084 (1948).
RECEIVED for review Xovember 30, 1956. Accepted February 6, 1957. Division of Analytical Chemistry, 131st Meeting ACS, Miami, Fla., April 1957. Contribution 219, Jackson Laboratory, E. I. du Pont de Nemours & Co., Inc.
Determination of Acetone in Aqueous and Benzene Solution by Messinger's Iodoform Method G. E. GOLTZ' and D. N. GLEW? Department of Chemistry and Chemical Technology, University of Natal, Durban, South Africa
The accuracy of Messinger's iodoform method for the determination of acetone in aqueous and benzene solution has been investigated. For aliquots of aqueous solutions containing between 7 and 41 mg. of acetone the method indicates 102.1 =!= 0.270 of the true acetone content; for benzene solutions, 99.9 f 1.1% of the true
816
ANALYTICAL CHEMISTRY
content, when special precautions are taken. Critical accounts of the factors affecting the accurate determination of acetone in aqueous and aromatic hydrocarbon solution are given.
T
of dilute solutions of acetone in water and benzene became necessary in connection with other HE AXALYSIS
work on liquid-liquid extraction, involving the benzene-acetone-water system. 1 Present address, Department of Chemical Engineering] Imperial College, Londm, S. W. 7, England. * Present address, Dow Chemical of Canada, Sarnia, Ontario, Canada.
Because of the ease and simplicity of the analytical procedure, the hydroxylamine hydrochloride method (6, 8) appeared suitable for acetone determination in both liquids. Investigation of eight aqueous solutions Containing between 140 and 250 mg. of acetone showed that the content could be determined n-ith a standard error of 12.27, or 957, confidence level error of 1 5 . 2 % . The analysis of eight solutions containing betneen 16 and 37 mg. of acetone in benzene by the same method led to a standard error of 12.97, and a 9570 confidence limit error of 1 6 . 9 7 , . The rhief source of error in the hydroxylamine hydrochloride method, even with the more concentrated aqueous solutions, derives from the color matching of the acid-base indicators a t pH 4.0. The large standard errors are attributed to the slon- color change with both screened methyl orange and bromophenol blue indicators, which appears to be due to the buffering of the solution by the oxime hydrolysis. The hydroxylamine hydrochloride method, although possibly suitable for large concentrations of acetone, fails in solutions containing less than 250 mg. of acetone and has been abandoned by the authors in favor of the Jlessinger iodoform method (4, 6 , 9 , IO). The Messinger method of determining acetone in solution depends upon the reaction of a n alkaline solution of acetone n-ith a n excess of iodine to form iodoform, according to the equation CHpCOCHp 3SaI
+ 312 + 4NaOH = CHI3 + + CH3COOSa + 3H20 (1)
At the completion of the reaction, the unreacted iodine excess is liberated from the alkaline solution with acid and estimated n ith sodium thiosulfate. I n the work described it was found possible to analyze solutions containing between 7 and 41 mg. of acetone in water and benzene with standard errors of 1 0 . 2 and *1.1%, respectively, on a single determination. MATERIALS
British Drug Houses AnalaR acetone, dried with barium oxide, was distilled over fresh barium oxide through a 2-foot column, packed with Sichrome wire rings 5 mm. in diameter, using a 20 to 1 reflux ratio. Fractional distillation took place at atmospheric pressure, and a magnetically controlled still head was used, protected from atmospheric moisture by desiccant tubes. The acetone taken for all experiments was the middle fraction of samples which had been refluxed overnight over barium oxide and had a normal boiling point of 56.1" i 0.1"C. The benzene was BDH crystallizable grade taken without further purification, and the water was laboratory oncedistilled water. KOparticular precautions n-ere taken with the tlvo solvents;
a? in the liquid-liquid extraction process, less pure grades of benzene and m-ater were t o be used. ACETONE DETERMINATION IN AQUEOUS SO LUTION
Procedure. Standard solutions of acetone in TT ater were prepared by breaking thin-walled glass capsules of known weight and acetone content in a ITeighed quantity of water. Pipets were used t o transfer different volumes of t h e standard solutions, t h e accurate size of the aliquot being determined by meight. Aliquots of standard acetone solutions were added to tared 150-nil. glassstoppered flasks containing 5 ml. of 5 5 sodium hydroxide solution and the flasks !?ere reweighed to obtain the acetone content. Then 50 ml. of approximately 0.1S iodine solution were added, and the flasks were thoroughly shaken and allowed to stand for at least 10 minutes. Blanks, prepared in the same way without acetone, were given similar treatment. After completion of the reaction, 5.25 ml. of 5-Y sulfuric acid or its equivalent were added and the flasks were vigorously shaken to liberate all the excess iodine, which was titrated with standard sodium thiosulfate, using starch at the end point. The difference of the blank and acetone solution thiosulfate titers gave the iodine used in the iodoform reaction. Experimental Data a n d Discussion. Table I shows five sample results from 20 determinations on aqueous solution containing betlveen 7 and 41 mg. of acetone. Three standard acetone B, and C-containing solutions--4, 0.9821, 1.748, and 2.794 mg. of acetone per gram of solution, mere prepared independently and weighed aliquots were taken for analysis. From the average of 20 determinations it is found that the Messinger iodoform method, performed under the conditions specified, indicates 102.107Gof the true acetone content, with a standard error of =t0.2094 and a 95% confidence limit error of 10.427, on a single determination. The deviation in the final column from the theoretical value of 100.007G indicates t h a t the reaction of acetone with iodine is predominantly that shonn in Equation 1, accompanied by a small
Table 1.
Acetone Acetone Taken, M g . Soln. 7.227 13.49 17.87 C 30.32 B 41.20 C
6
a
amount of side reaction requiring niore than three molecules of iodine per molecule of acetone. The consistency of the results over a sixfold acetone concentration range indicates that systematic errors have been eliminated by standardization of conditions, and that the side reaction is reproducible under these conditions. During preliminary investigation undertaken to determine the experimental conditions under n-hich the iodoform method would give reproducible results of the desired accuracy, a number of features n-ere esamined esperimentally. Aliquots of standard iodine solutions n hich had been dissolved in alkali to give colorless solution. containing iodide. hypoiodite, and iodate ions did not regenerate all the original iodine content on the addition of acid. Because the Jlessinger method utilizes this same reliberation of iodine from alkaline solution by acid, it was important to examine nliether the loss of iodine was a constant value, proportional to the iodine content of the solution, or dependent on the quantity of acid used for the reliberation. Investigation showed that a slight excess of acid was necessary for the reproducible reliberation of iodine and that stoichiometric neutralization produced low and variable regenerations. The quantitative loss of iodine due to the consecutive alkaline and acidic conditions was assessed using 20- and 50-ml. samples of standard iodine solution, which on direct titration required 12.36 and 30.90 ml. of thiosulfate, and, after treatment with 5 ml. of 5-1- sodium hydroxide and then with 5.25 ml. of 5 S sulfuric acid, required 12.26 and 30.80 ml. of thiosulfate, respectively. Identical losses were found n-ith solutions treated with acid and then left for 3 hours before titration Jvith thiosulfate solution. As the iodine losses are constant over a twofold change of concentration, blanks are run with each acetone determination to allow for this reneutralization error. Haughton (6) showed that the Jlessinger iodoform method was dependent on the rate of addition of the iodine solution to the alkaline acetone, and that slow dropwise additions gave more consistent results and indicated higher percentages of the true acetone contents.
Determination of Acetone in Aqueous Solution
Acetone Estd. by Eq. Acetone Backa Normality 1, M g . Found, yo 25.24 0,1114 7.387 102.2 28.70 0,1111 13.77 102.1 24.40 0.1114 18.21 101.9 12.35 0.1106 30.88 101.8 2.39 0.1114 41.95 101.8 Mean of 20 102.10 =k 0.20 reaction with aqueous acetone.
XaS2OJ Titer, MI.
Blank 32.09 41.50 41.29 41.19 41.29
Iodine remaining after iodoform
Na2S30a
VOL. 29,
NO. 5, MAY 1957
817
Table II.
Acetone Taken, Mg. 7.801 13.71 15.19 19.79 31.14
Acetone Soln. B’ D‘ C’ D‘ A’
To examine this effect under the authors’ experimental conditions two pipets were used, the first giving a normal fast delivery and drainage of 50 ml. in 50 seconds and the second giving a continuous slow delivery of 50 ml. in 300 seconds. Tests were conducted on standard solutions containing 16.523 and 23.233 mg. of acetone: The fast addition of iodine to the two solutions indicated 102.07 and 101.75% of the true acetone contents, whereas the slow addition indicated 100.51 and 99.53y0 of the true values. These results differ from those of Haughton, probably because of the different methods of effecting the slow addition. I n view of the greater analytical convenience, the fast delivery was used in all subsequent experiments. Because the Messinger method utilizes a reaction complicated by side reactions, it is necessary for highest precision to calibrate with k n o m acetone solutions, using conditions under which it is subsequently to be performed. Although the effect of variation of all conditions has not been studied, experience has indicated that the following factors are important: concentration of alkaline solution, concentration and rate of addition of iodine solution to alkaline acetone solution, length of time allowed for iodoform reaction, quantity of acid used to liberate iodine from alkaline solution, and in benzene solutions, temperature and absence of light. ACETONE DETERMINATION IN BENZENE SOLUTION
The determination of acetone in benzene by the Messinger method is further complicated by the photooxidation of iodoform in benzene solution (1-3, Y), which produces free iodine. Gross and Schwarz (5) suggested an opaque titrating vessel with a clear space to observe the end point, kept in a closed box when not viewed. I n this investigation black felt bags with a small window covered by a flap were used to protect the titrating flasks from light. Following the procedure for aqueous solutions with the flasks protected from light, determination of acetone in benzene a t room temperature (approximately 25’ C.) gave low and inconsistent results: A 818
ANALYTICAL CHEMISTRY
Determination of Acetone in Benzene Solution
Na~S208
Blank 45.74 45.77 45.74 45.98 45.81
Titer, M1. Back 38.29 32.88 31.92 27.15 17.01
Na2S20s Normality 0.1097 0.1097 0.1097 0.1096 0.1097
considerable improvement was obtained by cooling the flasks in ice water during the iodoform stage of the analysis. The reason for this improvement is not completely clear, although lower temperatures give a more favorable distribution coefficient with higher concentrations of acetone in the aqueous phase where the iodoform reaction takes place.
Procedure. Standard solutions of acetone in benzene, prepared as for aqueous solutions, were weighed into tared 150-ml. flasks containing 5 ml. of 5 N sodium hydroxide. To these flasks, protected from light by felt bags, 50 ml. of approximately 0.1N iodine solution were added, and the contents were shaken, transferred to a light-free sink containing ice water, and allowed to react for a t least 30 minutes with occasional vigorous shaking. Blanks containing the same volume of benzene without acetone were treated in the same manner. After reaction, the flasks were removed, dried, replaced in the felt bags, and treated with 5.25 ml. of 5N sulfuric acid to liberate the iodine excess, which was titrated with standard thiosulfate solution, the flap of the bag being opened only momentarily to check the progress of the titration. The end point was determined in the aqueous phase, using 2 ml. of starch as indicator, and the solution was vigorously shaken to remove the last traces of iodine from the benzene layer. The weight of acetone estimated was obtained, using Equation 1, from the difference of the iodine in the blanks and in the solutions after iodoform reaction. Experimental Data. Five specimen results of the determination of between 8 and 31 mg. of acetone from 22 benzene solutions are shown in Table 11. The samples were obtained by weighing aliquots of four independently prepared acetone standard solutions in benzene-A‘, B’, C’, and D‘-containing 3.553, 1.781, 3.465, and 3.265 mg. of acetone per gram of solution, respectively. Statistical analysis of the results in benzene shows that the Messinger method, performed under the conditions described, indicates 99.89% of the true acetone content, with a standard error of i=1.11’% or a 95% confidence limit error of +2.30% on a single determina-
Acetone
Acetone, Found, yo 101.4 99.8 96.6 100.9 98.2 Mean of 22 99.89 f 1.11
Estd. by Eq. 1, Mg. 7.911 13.69 14.68 19.98 30.58
tion. A number of low results were obtained, but there was no reason to neglect these in the statistical analysis; they were probably due to some unnoticed photooxidation. No significance is to be attributed to the nearly 100.OO~oestimation of acetone present in the benzene, which is probably due to cancellation of errors. Using a less pure Iscor entrainer benzene, the mean of ten estimations indicated 101.4% of the true acetone content, with a similar standard error. The larger standard errors observed for the determination of acetone in benzene solutions can be adequately explained by the nonquantitative extraction of acetone from the benzene layer, the photooxidation of iodoform, and the less distinct and less easily observed end point.
ACKNOWLEDGMENT
One of the authors (G.E.G.) is indebted to the South African Council for Scientific and Industrial Research for the award of a research bursary which make this work possible.
LITERATURE CITED
(1) Dubrisay, R., Emschwiller, G., Compt. rend. 198, 263 (1934). (2) Emschwiller, G., Bull. SOC. chim. 6 , 561 (1939). (3) Emschwiller, G., Compt. rend. 207, 1201 (1938). (4) Godwin, L. F., J . Am. Chem. SOC.42, 39 (1920). (5) Gross, P., Schwarz, K., Mmatsh. 5 5 , 287 (1930). (6) Haughton, C. O., IND.ENG.CHEM., ANAL.ED. 9, 167 (1937). (7) Kistiakowski, G. B., “Photochemical Processes,” p. 206, Chemical Catalog Co., hTew York, 1928. (8) Marasco, M., Znd. Eng. Chem. 18, 701 (1926). (9) Marriott, W. M., J . Biol. Chem. 16, 281 (1913/14). (10) Messinger, J., Ber. 21,3366 (1888). RECEIVEDfor review July 23, 1956. Accepted February 6, 1957.
