alkyl pyridinium iodide. Methyl iodide reacts quantitatively at room temperature. This indicated t h a t i t might be possible to resolve methoxyl and ethoxyl mixtures. In this investigation, the receiving Basks wcrc chilled during the reaction. One pyridinc solution was titrated while still a t a low temperature; a second solution was boiled, cooled, then titrated. The first titration indicated the methoxyl content; the second, the total alkoxyl content, and the ethoxyl content was obtained by difference. Imprecise results were noted in all variations of the technique tried. The temperature of the receiving solution is the critical factor. If the temperature is held too low, reaction of the methyl iodide is incomplete; if held too high, small amounts of ethyl iodide react. It seems entirely possible that sufficiently sensitive conditions could be established so that precise quantitative results could be obtained. Evcn so, qualitative distinction between methoxyl and higher alkoxyl groups can easily be realized by this means. DISCUSSION
That iodine and sulfides do not intcrfere was established by adding iodine crystals to one reaction misture, iodine and ferrous sulfide to a second reaction mixture. The blank value from each of these reactions was the same as that obtained with these additives absent. This was tested further by adding ferrous sulfide and iodine to a reaction mixture in the analysis of methylcellulose. The result for methoxy1 content was identical to that obtained with the interferents absent and corresponded to the theoretical value. Phenol and acetic anhydride or
propionic anhydride were tried in conjunction with hydriodic acid in the hydrolysis of the alkoxyl group. Phenol could not be used, as any phenol entering the receiver titrated simultaneously with the alkyl pyridinium iodide. The anhydrides could be used without the phenol, as the acid formed and carried into the receiver titrated separately from both the hydriodic acid and the iodide. However, no particular increase in efficiency was noted when the anhydrides were used. Xylene not only dissolves the test compound, but also boils at a sufficiently high temperature to aid in carrying the alkyl iodide t o the receiver. The presmce of xylene in the mixture also minimizes the fuming of hydriodic acid. It was not necessary to purify further the hydriodic acid as obtained commercially or to use special storage precautions. The more dilute commercial hydriodic acid solutions, with and without preservative, can also be used, although the reaction time must be increased. The procedure was not evaluated with respect to the highly volatile compounds such as diethyl ether, although it is believed that if they are weighed in gelatin capsules and xylene is used in the reaction mixture, additional precautionary steps are unnecessary. If this should prove unsuitable, substitution of a similar apparatus with a water condenser, for use during the hydrolysis, should ensure quantitative results with this type of compound. One distinct advantage of this procedure is that only 2 hours are required for determination of the propoxyl and butoxgl groups. The efficient pro-
cedure of Shaw @), for example, requires a minimum of 3 hours for quantitative conversion of the butoxyl group; whereas other procedures require longer reaction periods and even more extensive modification of the apparatus. ACKNOWLEDGMENT
The authors thank The Dow Chemical Co. for supplying the cellulose ether samples and the analyses on these samples. They also thank A, J. Sensabaugh for preparation of the figures, and Norman VanHoy for technical assistance. LITERATURE CITED
(1) C u n d 8 R. H., Markunas, P. C., ANAL. &EM. 30, 1450 (1958). (2) Cundiff, R. H., Markunas, P. C., Anal. Chtm. Acta 20,506 (1959). (3) Elek, A., “Organic Analysis,” Vol. I, pp. 67-125, Interscience, New York,
1953. (4) Fukuda, M., Milcrochim. Acta 1960, 448. (5) Ki al, A., Buhn, T., Monatsh. 36,853 (1918 (6) Kratk, K.,Gruber, K.,Ibid., 89, 618 (1958). ( 7 ) Pregl, F. Grant, J. “Quantitative Organic Ivficroanalysis,’” 5th English ed., pp. 182-94, Blakiston, Philadelphia, 1951.
(S,-Samsel, E. P., McHard, J. A,, IND. ENO.CHEM., ANAL. ED.14, 750 (1942). (9) Shaw, B. M., J. SOC.Chem. Znd. 66, 147 (1947). (10) Sobue, H., Halano, A., Tosliaki, A. J. SOC. Textile Cellulose Ind. ( J a p a n ) 1 5 . 21 (1959). (1:i, Viebock-’F., Brecher, G., Ber. 63B, 3207 (19301. (1:2) Zeisel, S., Monatsh. 6,989 (1885). I
~~
\ -
RECEIVEDfor review January 11, 1961. Accepted March 20, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
Rapid Determination of Organic Hydroxyl Groups with 3,5-Dinitrobenzoyl Chloride W. T. ROBINSON, Jr., R. H. CUNDIFF, and P. C. MARKUNAS R . 1. Reynolds Tobacco Co., Winsfon-Salem, b 3,5-Dinitrobenzoyl chloride, commonly used for the identification of alcohols, reacts rapidly and quantitatively with organic hydroxyl groups in pyridine. This reaction, followed b y a visual or potentiometric titration of the reaction products, provides another simple procedure for the determinotion of primary and secondary alcohols. Polyols, sugars, phenols, primary and secondary amines, and some oximes may also b e determined. By employing a longer reaction period, a
1030
ANALYTICAL CHEMISTRY
N. C.
much wider range of tertiary alcohols may b e determined than b y existing procedures. Ketones do not interfere nor do aldehydes unless they exceed 40% of the alcohol being determined.
M
(8) has reviewed tlic literature through 1952 on the determination of hydroxyl groups by the most commonly used methods. Among the methods discussed are: acetylation, phthalization, formylation, EHLENBACHER
bromination, periodate osidation, coupling, and determination of active hydrogen. The major objections t o t h e acetylation and phthalization procedures are the length of time necessary for quantitative reaction, interference from aldehydes and ketones, and their applicability only to primary and secondary hydroxyl groups. Recently, Fritz and Schenk (7) reported a highly improved procedure for the determination of organic hydroxyl groups. The method is rapid
of 3,5-dinitrobenzoic acid and ethyl 3,5-dinitrobenzoate. In the first titration the color changed from yellow to brilliant red a t the potentiometric end point. In the second titration addition of one drop of titrant produced the same red color. Tlius, the hydroxyl contcnt may be realized in three ways: by measurement of the dinitrobenzoyl chloride consumed as determined by a visual titration, or by potentiometric titration through the first end point, or a differentiating potentiometric titration in which the dinit,robenzoate formed is actually measured.
+
too
I
1.0 Y L
I
Figure 1. Titration of components in determination of hydroxyl groups a. Ethyl 3,5-dinitrobenzoate
b. Reoctlon mixture of 3,5-dinltrobenzoyl chloride and ethanol c.
3,5-Dinitrobenzoyl chloride
and merely involves acetylation at room temperature in ethyl acetate using perchloric acid as catalyst, However, this procedure, like the earlier methods, is not applicable to tertiary hydroxyl groups, nor can i t be used in the presence of aldehydes. The limited study by Beresin (I), which showed that some primary and secondary alcohols react quantitatively with 3,5-dinitrobenzoyl chloride in pyridine, and the work reported from these laboratories on the determination of 3,5-dinitrobenzoates (IO) and the resolution of acid mixtures (6) suggested that it should be possible t o determine hydroxyl groups simply and specifically by the use of 3,5-dinitrobenzoyl chloride. When this suggested scheme of analysis was tried, it was found that the hydroxyl content could be measured either by a differenthting potentiometric titration of the reactants and the dinitrobenzoate formed, or by the amount of dinit,robenzoyl chloride consumed. METHOD
The reaction of 3,5-dinitrobenzoyl chloride with an alcohol in pyridine solution is shown in Equation 1,
ROH
+W I!
NO,
C
l
+ CrHsN
-+
The excess dinitrobenaoyl chloride is hydrolyzed by water, as indicated by Equation 2.
yo* W
0
C
l
+ HOH + GHsN
-+
I
W
H
+ CrH&T.HCl
(2)
I
N 01
As indicated in curve b, Figure 1, the pyridinium hydrochloride and the dinitrobenaoic acid titrate simultaneously as strong acids, represented by the first inflection in the potentiometric curve; whereas the dinitrobenzoate titrates as a weak acid, represented by the second inflection in the curve. The amount of dinitrobenzoate formed is a measure of the organic hydroxyl content. The color of the reaction mixture changes from yellow to red a t the first equivalence point, which provides the basis for a visual titration. That this is a valid end point mas demonstrated by titration of separate pyridine solutions
Reagents and Apparatus. Tetrabutylammonium hydroxide, 0.2N in 7 t o 1 benzene-methanol, Prepare as described (4), doubling the reactants and increasing the amount of methanol to 300 ml. Anion exchange at the rate of 7 to 10 ml. per minute. Pyridine. Flash-distill technical grade pyridine from barium oxide, reflux the distillate over fresh barium oxide for 3 hours, and distill through a 50-cm. upright, air-cooled column, protected from moisture. This material contains 0.02 to 0.04% water. 3,5-Dinitrobenaoyl chloride, 98 to loo%, EK-2654; Distillation Products Industries. Finely grind in a mortar and store in a desiccator. Precision Shell Dual Titrometer or equivalent p H meter. Alcohol samples. Most liquid samples were distilled once; solid samples were analyzed as received. Estimated purity of all samples was 97 to 100%. The remainder of the apparatus and reagents have been described ( 4 ) . Procedure. For each series of analyses prepare a fresh 0.2M solution of 3,5-dinitrobenzoyl chloride by dissolving with gentle heating 1.15 grams in 25 ml. of pyridine. DO not expose this solution t o moist air unnecessarily. To determine liquid samples, pipet approximately 4 meq. of the hydroxyl compound into a tared 10-ml. volumetric flask containing 3 ml. of pyridine. T o avoid error in weighing liquid samples, take care not to wet the neck of the flask with the sample. Reweigh and dilute to volume with pyridine. Pipet into a 125-ml. '$ Erlenmeyer flask, 4.0 ml. of the dinitrobenzoyl chloride solution, then 1.0 ml. of the sample dilution. Stopper tightly, swirl, and allow to stand 5 to 15 minutes at room temperature, Unstopper the flask and add 7 t o 10 drops of water. To determine solid samples, accurately weigh 0.4 meq. of the hydroxyl compound directly into a 125-ml. Erlenmeyer flask, add 4.0 ml. of the dinitrobenzoyl chloride solution, stopper the flask, swirl gently to dissolve, and allow to stand 5 to 15 minutes a t room temperature. Unstopper the flask and add 7 to 10 drops of water. Prepare a blank solution by pipett+g 4.0 ml. of the dinitrobenzoyl chloride solution into a flask and immediately adding 7 to 10 drops of water. VOL. 33, NO. 8, JULY 1961
1031
EXPERIMENTAL
Table 1.
Analysis of Pure Samples by Visual and Potentiometric Titrations
Compound
Visual
Ethanol 2-Propanol 2-Methyl-2- ropanolc ZMeth 1 2 gutanold l-octadrianol Pentaerythritol' Mannitol/ Triethylene glycol Cholesterol Dextrone - .... - Sucro8e C clohexanone oxime T l y mol 3,4Dimethylphenol Isobutvlarmne Dipheiylamine
99.33 100.24 99.06 97.43 99.50 99.04 99.37 100. 19 99.94 99 30
99.88 99.55 100.39 100.31 99.29 99.85
Purity Found, % Potentiometric YE - VI' v* - V'b 99.33 100.40 99.35 97.03 99.39 100. 19 99.12 100.01 100.17 99.30 99.21 99.55 100.39 100.54 99.01 100.29
99 * 59 100.94 100.21 97.64 99.50 100.32 99.12 100.25 100.17 99.07 99.69 100. 00
Based on difference in volumes of blank and first end point of reaction mixture. Based on difference in volume from first to second end pointa of reaction mixture. e Reaction time 24 hours a t room temperature. Reaction time 48 hours at room temperature. Tetrabasic. Hexabasic. e
Table 11. Analysis of Other Compounds by Visual Titration Technique
Purity
Compound
Found, '3~
Methanol 1-Propanol 1-Butanol Isobutyl alcohol Isopentyl alcohol 2-Propen-1-01 Benzyl alcohol Furfuryl alcohol I-Tetradecanol 1-Hexadecanol (cetyl alcohol) Solanesol %Butanol C clohexanol I-Lent hol Stigmasterol Solareola *Terpineolb Terpinol hydrateb>c Glycerol Propylene glycol
99.24 98.91 98.31 gg*l8 99.00 99.05
97.79 96.91 96.80 100.15 99.32 97.99 97.35 99.92 99.03
2-Hydroxy-2,5,5,8a-tetra-
99.64 99.44 99.77 98.86
methyl-1 (2-h droxyethy1)decah dronapithalened 99.77 Tris( h &oxymethyl)amino98.29 metianeb 98.77 1( -)-Sorbose* 97.90 Fructose' 98 82 1-Napht hol Benzylamine 98.32 100.30 Acetone oxime Reaction time, 96 hours a t room temperature. Dibasic. Alternative procedure. Heating proceaa repeated 4 times. 0 Contains two tertiary hydroxyl groups. Only one esterified. d Glycol of sclareolide. Contains one gimary and one tertiary hydroxyl group. nly primary hydroxyl group esterified m 15 minutes a t room tern rature. Only 4 of 5 hydroxyygroups esterified. Must not be heated or allowed to react longer than 5 minutes a t room temperature. 0
1032
ANALYTICAL CHEMISTRY
Visual Titration. Add 40 ml. of pyridine to the reaction mixture, heat nearly to boiling, Cool, then titrate with 0.2N tetrabutylammonium hydroxide to the first definite and permanent red color. T h e titration is best performed with t h e titrant and solution protected from moisture and air and the tip of the buret immersed in the titrating solution, Titrate the blank in exactly the same manner. Use the difference in volumes between the blank and sample to calculate the per cent hydroxyl. Potentiometric Titration. Add 25 ml. of pyridine to the reaction mixture and heat nearly to boiling. cool and transfer to a 250-ml. electrolytic beaker* Rinse the flask with two loml. portions of pyridine and add the washings to the beaker, Titrate tentiometrically under nitrogen, using the manual titrator. If a blank is determined, titrate only through the first inflection, using t h e difference in volumes between end points of the blank and samples to the per cent hydroxyl. If no blank is determined, titrate tflrough both inflections and use the volume between the first and second end Doints to calculate per cent hydroxyl. In the potentiometric titration, proceed slowly through the first end noint, to avoid a false inflection. Alternative Procedure. After addition of the 3,5-dinitrobenzoyl chloride and sample solution t o t h e reaction flask, stopper the flask tightly and gently heat on a hot plate for 30 t o 60 seconds (Caution!), remove, and allow t o cool. Repeat three t o four times if necessary, then follow the remainder of the procedure as described above. The alternative procedure cannot be applied to most tertiary alcohols or the keto sugars. '
.
I
Figure 1 shows typical potentiometric titration curves of the various components involved in the determination of hydroxyl groups by the proposed procedure. All titrations were made in pyridine solution using tetrabutylammonium hydroxide titrant. Dinitrobenzoyl chloride and dinitrobenzoic acid titrate as dibasic acids with one inflection in their potentiometric curves. Several concentrations of 3,5-dinitrobenzoyl chloride in pyridine were used to determine the minimum molar requirement for quantitative esterification of an alcohol. A minimum of 40% molar excess was required; however, because of possible interference from water, a 75 to 100% molar excess was used. The data in Table I demonstrate that comparable results may be obtained by using either the visual or potentiometric end points. The values listed in Table I as well : in all other tables are the average of :tt least two determinations by each means of titration. Phenols and amines could not be determined by the difference in volumes between the two potentiometric end points of their reaction mixtures, because the dinitrobenzoate moieties of these compounds do not titrate quantitatively. Table I1 lists the results of additional compounds determined by the visual titration method. All hydroxyl groups in the glycols, with the exception of the one noted, were esterified by the reagent. Standard deviation for the procedure is 0.18 as determined by nine replicate analyses of octadecanol. Table I11 lists results from determination of ethanol by the visual method in the presence of the typesof compoun~s which most often interfere in hydroxyl determination. Reaction time was 5 minutes at room temperature Othrwisenoted.
Table 111. Determination of Ethanol in Presence of Other Compounds
(0.4 mmole ethanol taken for each analysis) CornV".-.
Compound Present Mmole 2-Methyl-2-propanol 0 . 1 0.3 Tri henylmethanol Trigemylamine Acetone Cyclohexanone Benzaldehyde Benzaldehyde (23 minutes)
'
Ethanol Recovered, 70
0.3 0.4
101.8 101.8102.7 99.8 99.8 99.5 100.3 99.3 100.2 97.6 92.7
0.4
92.7
0.3 0.3 0.1 0.3 0.4 0.1
Proportionately large amnunts of benzaldehyde interfere. However, since no interference was noted with 40% bcnzaldehyde, it can be assumed that the aldehyde interference is negligible. Interference can be expected from any compound that will react with the reagent. This a,ccounts for the slightly high recovery noted in the presence of 2-methyl-2-propanol in the 5-minute reaction period. Extremely hindered tertiary alcohols, such as triphenylmethanol, will not interfere. As indicated in Table IV, only 20 to 25% watcr can be tolerated in analysis of ethanol-water misturcs by the described procedure. Usc of larger amounts of 3,5-dinitrobenzoyl c,hloride would apprcciably increase this tolernnrc.
Of t'lie compounds tested containing both :tctive hydrogen and carbonyl groups, only the sugars could be quantitatively estcrified by the proposed procedure. Carbonyl interference was notrd with acctamide, isovalcramide, sucvinimide, p-hydroxybenzoic acid, 3hydroxy-Znaphthoic acid, benzoin, vanillin, a-benzoin oxime, and a-furil dioxime. Although the sugars were quantitatively determined, only four of the five hydroxyl groups in the two keto sugars, fructose and sorbose, were esterified, whereas all hydroxyl groups in the aldo sugam and disaccharides were esterified. Fritz and Schenk (7) and Christensen and Chrke (9) are the only previous investigators to report quantitative determinations of fructose by acetylation. They also noted the acetylation of only four of the five hydroxyl groups in fructose. Fritz and Schenk (7) also reported the quantitative acetylation of benzoin and a-benzoin oxime, but noted wrying degrees of acetylation with other oximes. Interference from carbonyl groups appears to be more uniform with the 3,5-dinitrobenzoyl chloride reagent than the acetylation reagents. If this observation should prove true for all compounds Containing both active hydrogen and carbonyl groups, the benzoylation reagent should be useful for st'ructuralstudies. Results by this procedure were compared with those obtained by the acidcatalyzed acetylation procedure of Fritz and Schenk (7) and the phthalization procedure of Elving and Warshowsky (6) (Table V). DISCUSSION
The mechanism of the reaction that produces the red color a t the first end point in the titration is not completely understood, although it is probably a type of quinoidal structure, similar
to those described by Ca,nbn.ck (R) and Porter (9). The red color which develops a t the end point of R. visual titration is easily and accurately determined ; however, i t is not a sudden change from yellow to red. Rather, it is a transitional change from yellow to orange to red. The true end point, therefore, is taken as the first definite and permanent red color. At the end point, the intensity
Table
V.
Table IV.
Analysis of Ethanol-Water Mixtures
Added, Mmole Ethanol water 0.42 0.11 0.41 0.25 0.43 0.38 0.47 0.53
Ethanol, Ethanol Weight Recovered,
%
%
90.17 80.80 74.42 69.17
99.57 99.37 85.42 63.91
Comparison of Benzoylation with Acetylation and Phthalization Procedures
Compound Ethanol 1-Octadecanol Pentaerythritol Triethylene glycol Methanol 1-Butanol %Butanol I-Pro anol Cycloiexanol 1-Hexadecanol
Visual Method 99.33 99.50 99.04 100.19 99.24 98.31 97.99 98.91 97.35 100.15
Potentiometric V B- V I Vs - VI 99.59 99.33 99.50 99.39 100.32 100.19 100.25 100.01
of the red color is directly proportional to the concentration of the titrant. For this reason, 0.2N, tetrabutylammonium hydroxide proved more satisfactory than more dilute titrants. The reaction time as stated for primary and secondary hydroxyl groups is dependent upon the alcohol being determined aa well as the reactivity of the dinitrobenzoyl chloride. For example, with one lot of dinitrobenzoyl chloride, pentaerythritol was completely esterified within I O minutes, thymol in 15 minutes, and ethanol in 5 minutes. With another much more reactive lot of dinitrobenzoyl chloridc the reaction time was cut to less than one minute with many primary and secondary alcohols. I n general, it will be necessary for the analyst to test the reactivity of each lot of dinitrobenzoyl chloride in order to establish thc necessary time of reaction. The 15-minute period should be the maximum timc required. With less reactive dinitrobenaoyl chloride use of the alternative procedure should prove bcneficial in analysis of many primary and secondary alcohols. Benzoylation of a11 sugars was complcte within 5 minutcs a t room temperature. Thc keto sugars could not be heated or allowed to react longer than 5 minutes bcfore dilution, without sample degradation occurring. The reaction mixture of the keto sugars should be titrated by the visual method, bccause extrcmely weak inflections are realized in potentiometric titration of these compounds, making potentiometric equivalence point determinations unreliable. High molecular weight carbohydrate polymers, such as dextran or cellulose, cannot be accurately de-
Acetylation (7)
99.85 99.80 101 * 28 99.58 99.26 98.16 100.50
Phthalization (6) 99.70 100.09 98.10 100.37 99.54 98.78 97.73 98.43 96.31
termined because of their limited solubility in pyridine. Although not all tertiary alcohols can be stoichiometrically determined, enough such compounds can be determined to make application of this procedure worthwhile. The noted times required for quantitative reaction of those tertiary alcohols listed in Tables I and I1 are maximum and were obtained using a lot of less reactive dinitrobenzoyl chloride. Use of a more reactive lot of dinitrobenzoyl chloride yielded quantitative results for 2-methyl-2-propanol after 3 hours, 2-methyl-2-butanol and 2-methyl-Zpentanol after 5 hours. The only two tertiary alcohols investigated which showed incomplete esterification were 3-ethyl-3-pentanol and triphtmylmethanol. 3-Ethyl-3-pcntnnol showed only 60% rcaction after 100 hours when the less reactive acid chloride was uscd, over 80% reaction after only 1,5 hours with the more reactive acid chloride. Triphenylmethanol showed practically no reaction after 100 hours with the less reactive reagent. With the exception of a-terpineol and terpinol hydrate, the alternative procedure cannot be applied to tertiary alrohols because, if hcated, they will dehydrate in the presence of the dinitrobenaoyl chloride. Because of the difference in reactivity of primary or secondary alcohols and tertiary alcohols a ith 3,5-dinitrobenzoyl chloridc, it should be possible to determine primary or secondary nlcohols and tertiary alcohols simultaneously. I n so doing, however, the reactivity of the dinitrobenzoyl chloride being used must be taken into consideration. The application of the procedure VOL. 33, NO. 8, JULY 1961
1033
to. such biologically important cornpounds as the sterols as well as to the essential oils is evident. Because the essential oils usually contain aldehydes, interference is minimized in the determination of the hydroxyl content of these compounds. Linalobl, citronellol, borneol, and geraniol analyzed only 75 to 80% pure by this method; however, determinations are quantitative for essential oils that can easily be obtained in the pure hydroxyl form. Ethers do not react with the reagent
under the described conditions of the procedure. LITERATURE CITED
(1) Berezin, I. V., Doklady Akad. Nauk S.S.S.R. 99,663 (1954).
(6) Elving, P. J., Warshoweky, B., IND.
ENQ.CKEM., ANAL,ED. 19,1006 (1947). (7) Fritz, J. S., Bchenk, Q. H., ANAL CHEM. 31 1808 (1959). (8) M:;lenbacher, V. C., "Organic AnalVol. I, p. 1-65, Interscience, York 1958 (9) Porter, 6.' C., ANAL. CHEM.27, 805
&??;
( 2 ~ ~ 2 ~ ~ ~ $ 4***e ' ; ~ (1g6 ) 955)* ~Robinaon, ~ W. T., Jr., Cundiff, R. H.,
Rsvf/ 163, (3) christenen,B. E., Clarke, R. A., IND. ENQ. CHF,M., ANAL.ED. 17, 265
(1945). (4) CunditT, R. H., Markunaa, P. C., ANAL.CHEX 30,1450 (1958). (5) Cundiff, R H., Markunaa, P. C., Anal. Chim. Acta 20, 600 (1959).
SenSabaUgh, A. J V Markunas, p. c-, Tala& 3,307 (1960). RECEIVEDfor review January 11, 1961. Accepted March 20, 1961. Division of Analytical Chemist 139th Meeting, ACS, St. Louis, Mo.,%arch 1961.
Water Determination by Reaction with 2,2-Dimethoxypropane F. E. CRITCHFIELD and E. T. BISHOP Development Department Technical Center, Union Carbide Chemicals Co., South Charleston,
b Water reacts quantitatively with 2,2-dimethoxypropane, using methanesulfonic acid catalyst, to form acetone and 2 moles of methanol. The infrared absorption at approximately 5.87 microns of the acetone formed can be used to determink water in materials that cannot be analyzed b y Karl Fischer reagent because of interferences. The method can be used to determine as low as 0.05% water in various organic solvents. It has been applied to the determination of water in dodecanethiol and several Inorganic hydrates.
T
HE KARL FISCHER METHOD for determining water ia by far the most versatile of the numerous methods developed to date. However, there are cases where this method cannot be applied readily because of interferences, and alternate procedures are required. Interferences in the Karl Fischer method are generally restricted to compounds that readily undergo oxidation-reduction reactions with the reagents, or organic compounds that readily add iodine. Organic reducing agents, such as mercaptans, hydrazine, and ascorbic acid, and inorganic salts, such as stannous chloride, ferric chloride, and cupric chloride, interfere in the method. This paper describes an alternate method for determining water that is based upon its rapid and quantitative acid-catalyzed reaction with 2,2dimethoxypropane (I): OCH,
HsO
+ CHI-&-C&
H+
-+
bCH8
1034
ANALYTICAL CHEMISTRY
8
CHj CHa
+ 2CHaOH
Acetone formed in the reaction is determined from its infrared absorbance a t 5.87 microns (carbonyl band). Although the infrared method can be used for the direct determination of water in certain cases, special calcium fluoride optics must be used and matrix effects are sometimes serious. Also, other hydroxylic compounds may interfere. EXPERIMENTAL
Apparatus. Beckman IR-5 infrared spectrophotometer or equivalent, equipped with a 0.1-mm. cell. Reagents. D r y Carbonyl-Free Methanol. Distill 3 gallons of methanol from a mixture containing 50 grams of 2,4-dinitrophenylhydrazine and 15 ml. of reagent grade hydrochloric acid. Reflux for 4 hours and collect the distillate until the head temperature reaches 84.8" C. Dry the methanol by allowing it to stand over '/Isinch pellets of Linde Molecular Sieves Type 4A for at least 24 hours. Occasionally mix during this period. Pass the methanol through a column acked with Linde Molecular Sieves %ype 4A ( owder) The water content of the rnetEanol should be below 0.05% as determined by Karl Fischer reagent. Methanesulfonic Acid. 0.1N in dry carbonyl-free methanol. Prepare from Eastman Organic Chemicals grade methanesulfonic acid. Carbon Tetrachloride. Merck reagent grade (for spectrophotometric use). 2.2-Dimethomrouane. The Dow -- Chemical Co. Procedure. PiDet 5.0 mi. of the 0.1N methaneeultonic acid and 2.0 ml. of the 2,2-dimethoxypropane into
W. Vu.
each of two 25-ml. volumetric flasks. Reserve one of the flasks as a blank. Provide a third volumetric flask for a carbonyl blank on the sample (sample blank). Accurately weigh or pipet an amount of sample containing no more than 0.1 gram of water into the sample and sample-blank flasks only, adding the same amount, within *2%, to each. Dilute each of the three flasks to volume with carbon tetrachloride. Determine the absorbance of each solution a t approximately 5.87 microns in a 0.1-mm. cell using a base line technique. Correct the sample for carbonyl content, and, from a reviously prepared calibration curve, letermine the grams of water corresponding to the net absorbance of the sample. Calibration Curve. Follow the procedure outlined above and prepare standards containing 0.02, 0.05, 0.08, and 0.10 gram of water. Omit t h e sample blank. DISCUSSION
Under the conditions of the method, the reaction of 2,2dimethoxypropane with water is practically instantaneous to form the theoretical amount of acetone. This was checked by preparing calibration curves . from acetone and water. In the absence of a strong mineral acid, however, the reaction is not quantitative in a reasonable length of time. Methanesulfonic acid waa selected for this particular application because it is a strong mineral acid that is soluble in organic solvents and is commercially available as the anhydrous material. Because carbonyl compounds present in the sample interfere, it is necessary to provide, in addition to the customary
