+ +
H3CCOT\'(CJ&,)z HzO + CH3COOH (CsHs)?NH (1) O"(CBHs)?
+ HNOi Ht0 + (CEHS)Z" +
(2)
may likewise be brought about by heating the diphenylamine derivatives with the monohydrate of manganous sulfate, which gives off its water of hydration in the range 150" to 200°C. (1)and thus functions as the water donor. The formation of quinoidal blue dyestuffs by fusion with oxalic acid can be employed for the microdetection of diphenylamine and its derivatives. This test is far more reliable than the blue color produced by treating diphenylamine and its derivatives with concentrated sulfuric acid containing alkali nitrate (5). This color reaction fails with acetyldiphenylamine, as-diphenylurea, ethyl-S,N-diphenyl carbamate, and several derivatives of carbazole. I n contrast, these com-
pounds, even in micro amounts, yield blue or blue-green products when fused with oxalic acid. Procedure. A micro test tube is used. A little of the solid or the evaporation residue of a drop of the liquid solution is heated with several centigrams of hydrated oxalic acid in a glycerol bath preheated to 160' C. The temperature is raised t o 180" to 190' C. A positive response is indicated by the development of a blue color within several minutes. The test revealed the following in microgram amounts: diphenylamine (0.5), N-methyldiphenylamine (0.5), N-acetyldiphenylamine (2), phenyl-lnaphthylamine (15), as-diphenylurea (3), 4,4-diphenylsemicarbazole (3), ethyl-N,N-diphenyl carbamate (5), Nphenylanthranilic acid (5), N,N-diphenyl formamide (2.5), Ar,hr-diphenylbenzidine (5), tropaeolin 00 [sodium p - (2 - hydroxy - 1 - naphthy1azo)benzenesulfonate ] (5), 1,l-diphenylhydra-
zine chloride (1.2), carbazole (0.5), N ethyl-4-diethylaminocarbazole (l), A'toluylsulfonecarbazole (1.5), and Nethyl-4-aminocarbazole (1). A positive response was also obtained with: p-nitrosodiphenylamine, 4-methyldiphenylamine, diphenylamine-4-sulfonic acid and its barium salt, 4-hydroxydiphenylamine, and p-butoxydiphenylamine. LITERATURE CITED
( 1 ) Duval, C., "Inorganic Thermogravimetric Analysis," p. 189, Elsevier,
Amsterdam, 1953.
(2) Feigl, Fritz, Angew. Chem. 70, 166
(1958).
RECEIVED for review December 28, 1959. Accepted February 18, 1960.
Colorimetric Determination of Secondary Alcohols by Conversion to Ketones FRANK E. CRITCHFIELD and J. A. HUTCHINSON Development Department, Union Carbide Chemicals Co., Division o f Union Carbide Corp., South Charleston, W. Va.
,The colorimetric determination of secondary alcohols in the presence of primary alcohols is based upon the oxidation of the sample with acidic potassium dichromate to form ketones from secondary alcohols. The amount of ketone formed is determined by a 2,4-dinitrophenylhydrazine colorimetric method. Primary alcohols are oxidized by the dichromate reagent to acids and do not interfere. Reaction conditions are presented for the determination of 10 secondary alcohols. Data are presented for the determination of 0.04 to 2.6% 2-propanol in ethanol and 0.4 to 1.8% isopropanolamine in ethanolamine. The method i s also applicable to the determination of ketones in the presence of aldehydes.
T
HE DETERMISATION of low concentrations of secondary alcohols in the presence of primary alcohols is usually difficult. Of the several methods proposed for this determination (3, 5-7) most are applicable to macro concentrations only. The method described here is based upon the selective oxidation of secondary alcohols to ketones using acid
862
ANALYTICAL CHEMISTRY
potassium dichromate. The amount of ketone formed in the reaction is determined by a modification of existing colorimetric 2,4-dinitrophenylh~-drazine methods ( 8 , 4 ) . Therefore, this investigation was primarily concerned with combining the two procedures. DEVELOPMENT
OF
METHOD
Many variables were studied. Because acetone is the oxidation product of 2-propanol, these two compounds were used for all of the preliminary investigations. Dichromate Oxidation of Alcohols. The extent of oxidation of organic compounds with acidic potassium dichromate depends considerably upon the acid strength of the reagent. The reagent of Aguhlon ( I ) , which contains 1.5% potassium dichromate and is approximately 1 2 s in nitric acid, has been used as an oxidizing agent for organic compounds in this laboratory for several years. The reagent is selective in that it will oxidize many secondary alcohols to ketones, aldehydes to acids, and primary alcohols to acids, and will not oxidize carboxylic acids. These generalizations do not hold for many bifunctional compounds.
For example, 1,2-glycols are cleaved regardless of whether the hydroxyls are primary or secondary. Also, oxalic acid and glyoxal are oxidized to carbon dioxide. Because of its selectivity this system was initially chosen for oxidizing secondary alcohols to ketones. However, the Aguhlon reagent was not satisfactory because excess dichromate ion must be reduced before the 2,4-dinitrophenylhydrazine reaction. When a reducing agent is added t o accomplish this, some of the nitric acid is reduced to nitrite ion which decomposes to nitrous oxide in the acid medium. This causes an objectional liberation of a gas and requires excessive reducing agent to react with the potassium dichromate. For this reason sulfuric acid was substituted for nitric acid in the reagent. The normality of the acid x a s not changed appreciably. Reduction of Excess Dichromate Ion. T o determine the amount of ketone formed by the oxidation of a secondary alcohol, the excess potassium dichromate must be reduced. This is necessary because dichromate ion will oxidize 2,4-dinitrophenylhydrazine and methanol used as a solvent.
~
Among the many reducing agents investigated for this purpose, only hypophosphorous acid and sodium arsenite rapidly and effectively reduced the excess dichromate ion. Hypophosphorous acid was selected as the more satisfactory reagent because i t is available as a 50% solution and does not require the addition of a large volume of water as in the case of aqueous sodium arsenite. Using 1.0 ml. of 50% hypophosphorous acid, potassium dichromate is quantitatively reduced in slightly more than 5 minutes. K h e n dichromate ion is not completely reduced, high blanks are obtained. This occurs because the unreduced dichromate partially oxidizes methanol, used as a diluent, t o formaldehyde. The stability of ketones to hgpophosphorous acid is a n important consideration in using the reagent for reducing dichromate. Acetone is not affected by the reagent for a period of 10 hours; however, after 24 hours reduction of acetone does occur. 2,4-Dinitrophenylhydrazine Reaction. T h e 2,4-dinitrophenylhydrazine method used in this investigation is a modification of t h e methods of Lappin and Clark ( 4 ) and Bohme and Winkler ( 2 ) . Keither is applicable t o the determination of ketones in the reaction mixture from the potassium dichromate oxidation. Although one difficulty was eliminated by reducing the dichromate ion with hypophosphorous acid, other factors that affect the 2,4dinitrophenylhydrazine reaction are the acidity and water content of the potassium dichromate reagent. The amount of sulfuric acid in a 3-ml. aliquot of the reduced reaction mixture completely inhibits the color formation in the 2,4-dinitrophenylhydrazine method. The acid must, therefore, be neutralized prior t o the 2,4-dinitrophenylhydrazine reaction. This was accomplished with 3 nil. of aqueous 4 5 potassium hydroxide. X1though the use of the concentrated reagent causes a considerable generation of heat, which is undesirable, the aniount of water introduced into the system must be kept t o a minimum because it decreases the sensitivity of the end determination. i l n additional 107" increase in water content in the system will decrease the sensitivity of the method by 30%. Excessive heating occurs when 4N potassium hydroxide is used as a neutralizing agent. Calibration curves obtained by allowing the reaction flask to heat up and by cooling during the neutralization showed a constant displacement of 0.02 absorbance unit. Also, the former calibration curve did not go through the origin. When heating occurred the blanks were 0.02 unit higher than when the blanks were
cooled. Samples containing acetone were unaffected by the heat. This is probably the result of a small loss of acetone by volatilization. I n the case of the blanks, heat of neutralization causes a small amount of methanol to be oxidized t o formaldehyde. This same reaction was observed when methanolic potassium hydroxide was used for the neutralization. I n this case extremely high blanks were obtained. Under the conditions of the 2,4-dinitrophenylhydrazine method, as modified for this application, the color developed is stable for a t least 1 hour. The enhanced stability obtained in this modification is apparently due to the fact t h a t more water is present. This constitutes a major advantage over other 2,4-dinitrophenylhgdrazine methods. Because of the large amount of salt present in the colorimetric step, the solution must be filtered prior t o the optical measurement. The large concentration of salt cannot be eliminated because the potassium dichromate reagent must be strongly acid for quantitative oxidation of alcohols and the acid niust be neutralized for quantitative reaction of the ketones with 2,4-dinitrophenylhydrazine. The salt cannot be solubilized with mater because of the decreased sensitivity t h a t is obtained a t high mater concentrations.
Toble I. Reaction Conditions for Determination of Secondary Alcohols Secondary Alcohol, Oxidation Mg., Time, Compound Maximum Minutes 2 92 60 to 120" 2-Butanol 6 165 5 to 60 3-Heptanol 5 365 5 to 75 4-Heptanol 1 47 30 to 60 2.5-Hexanediol 3 63 5 to 60 2-Hexanol 1 96 5 to 600 2-Propanol Isopropanolamine 2 17 120 to 210°~c 4 06b 10 to 60 2-Octanol 5 475 30 t o 100 4-Octanol 6 66 5 to 60 3-Pentanol a At room temperature. b Dilute in acetonitrile or use as cosolvent to effect solution in oxidation step. c Allow 17 hours for quantitative reaction with 2,i-dinitrophenylhydrozine.
end of the specified time, add 100 nil. of distilled mater and 10 ml. of 15% potassium iodide to the sample and blank flasks. Titrate immediately with standard 0 . W sodium thiosulfate to a greenish yellow. Add a few milliliters of a 1% starch solution and continue to titrate t o the disappearance of the starch-iodine color. The end point is a chromic blue. The optinium oxidation time should be selected as the time a t which no further dichromate is consumed. Calibration Curve. I n t o a 100ml. volumetric flask, add 50 nil. of distilled water (or acetonitrile if EXPERIMENTAL specified in Table I). Transfer 100 times t h e maximum sample specified Reagents. Acid Potassium Dichroin Table I t o the volumetric flask mate (approximately 0.3-47). Disa n d dilute t o volume n i t h the approsolve 15 grams of reagent grade potaspriate solvent a n d mix. Transfer 5.0-, sium dichromate in 500 ml. of dislo&, 15.0-, and 20.0-nil. aliquots tilled water. Slowly a d d 360 ml. of of t h e dilution into separate 100-ml. concentrated sulfuric acid a n d cool volumetric flasks and dilute to volume t o prevent excessive heating. Dilute with the solvent. Determine the abt o 1 liter with distilled water a n d mix. sorbance of each of these standards by Hypophosphorous Acid. Baker and using 5-ml. aliquots of the standards in Adamson technical grade, joy0solution. place of the sample in the procedure Carbonyl-Free Rlethanol. Reflux 3 described below. gallons of methanol containing 50 Procedure. Pipet 15.0 ml. of t h e grams of 2,4-dinitrophenylhydrazine potassium dichromate reagent into and 15 ml. of concentrated hydrochloric each of three 50-ml. volumetric flasks, acid for 4 hours and collect the distillate reserving one of the flasks for a blank until the head temperature reaches determination. I n t o each of the other 64.8' C. flasks, transfer sufficient sample so Pyridine Stabilizer. Pyridine-water t h a t t h e secondary alcohol content solution, SO to 20 v./v. does not exceed t h e amount specified 2,4-Dinitrophenylhydrazine Reagent. in Table I. Prepare a dilution in water Suspend 0.05 gram of reagent grade 2,4or redistilled acetonitrile if the optinium dinitrophenylhydrazine crystals in 25 sample size is too small to be weighed ml. of carbonyl-free methanol. Add 2 accurately. The total alcohol content ml. of concentrated hydrochloric acid of the sample should not exceed 3.8 and mix to effect solution. Dilute to 50 meq. as determined by the volumetric ml. with carbonyl-free methanol. procedure previously described. If the Optimum Oxidation Time for P u r e sample is iveighed directly and is inAlcohols. Pipet 15.0 nil. of t h e potassoluble in the reagent, add sufficient sium dichromate reagent into each of redistilled acetonitrile to effect solution. eight 250-ml. Erlenmeyer flasks. ReAdd the same volume to the blank. serve four flasks for blank determinaAllow the samples and blanks t o stand a t tions. I n t o each of t h e other flasks 0" C., unless otherwise the temperature introduce 2.0 t o 3.0 meq. of t h e alcohol specified in Table I until the alcohols t o be oxidized. If a n aliquot of a are oxidized quantitatively dilution of t h e sample is used, introThe optimum times for secondary duce t h e same volume of solvent into alcohols are listed in Table I; the optithe blank flasks. Allow one sample and mum times for primary alcohols can be one blank flask to stand for 30, 60, 90, determined by the volumetric procedure. and 120 minutes, respectively. At the VOL. 32, NO. 7 , JUNE 1960
863
Table 11.
Compound
Absorbance per Equivalent
Structure OH
2-Butanol
tion of secondary alcohol from the calibration curve.
Effect of Structure on Sensitivity
CH3-C-CtHb
0.266
I
3-Heptanol
H ?H c~H,&-c,H,
0.188
4-Hep tan01
H ?H CjHr-C-CsHi
0.216
I
H OH
OH
2,5-Hexanediol
0.402
2-Hexanol
0.281 H OH
2-Propanol
CH3-C-CHs
0.308
I
H OH I
Isopropanolamine
CH3-C-CH2--SH2
0.347
H OH 2-Octanol
CHs-c-csH~s I
0.319
H OH I
4-Octanol
0.238
3-Pentanol
0.132
Table 111.
2-Propanol ethanol
Analysis of Mixtures
in
0.04
0.16 1.25 Isopr?panol0.43 amine in 0.57 ethanolamine 1 . 0 2 Acetoneinacet- 0 08 aldehyde 0.16 0.22
0.02 0.18 1.23
0.47 0.59
0.97 0.09 0.17 0.24
50
113 98 109 104 95 112 106 109
Immerse the flasks in a n ice-water bath and pipet 1.0 ml. of the hypophosphorous acid into each flask, swirling the flasks during the addition. Remove the flasks from the bath and immerse them in a water bath at room temperature for 15 minutes. Dilute the contents of the flask. to volume with carbonyl-free 864 * ANALYTICAL CHEMISTRY
methanol and mix. Transfer a 3.0-m!. aliquot of each dilution to separate 50ml. volumetric flasks. Immerse the flasks in a n ice-water bath and pipet 3.0 ml. of 4N potassium hydroxide into each flask. Remove the flasks from the bath and allow the contents to warm to room temperature. Pipet 3.0 ml. of the 2,4dinitrophenylhydrazine reagent into each flask, mix, and allow the flask to stand at room temperature for 30 minutes unless otherwise specified in Table I. Pipet 15.0 ml. of the pyridine stabilizer into each flask. Transfer 3.0 ml. of freshly prepared 10% methanolic potassium hydroxide to each flask, stopper, and mix. Allow the flasks to stand a t room temperature for 5 minutes and filter the solution through No. 40 Whatman filter paper. Collect the filtrate in separate 2.3-ml. glass-stoppered graduated cylinders and allow the cylinders to stand for 10 minutes. Using a suitable spectrophotometer, measure the absorbance of the sample us. the blank at 480 mfi using 1-em. cells. Read the concentra-
DISCUSSION AND RESULTS
Reaction rate studies and a calibration curve were obtained for each secondary alcohol investigated. Table I list's the recommended reaction conditions for secondary alcohols to which this method has been applied. In all cases the maximum sample size is based on the amount of pure alcohol introduced in the oxidation step that corresponded to a n absorbance of 0.6 under the conditions of the method. Table I shows that the Oxidation time required for quantitative oxidation to the ketone varies from 5 to 120 minutes depending upon the alcohol oxidized. I n most cases it was necessary to conduct the oxidation a t 0" C. to inhibit further oxidation of the ketone. The optimuni oxidation time selected for the determination of a secondary alcohol in the presence of a primary alcohol will depend upon the time necessary to oxidize both alcohols quantitatively-i.e., secondary alcohols to ketones and primary alcohols to acids. The optimum time for oxidat'ion of any primary alcohol, sample matrix, can be conveniently determined by the volumetric 1:rocedure. When acetonitrile is used as a cosolvent for samples insoluble in the potassium dichromate reagent, a redistilled grade should be employed and the same volume should be incorporated in the blank. With the exception of isopropanolamine, the ketones formed by the oxidation react quantitatively with 2j4dinitrophenylhydrazine in less than 30 minutes. I n the case of isopropanolamine, approximately 17 hours are required for quantitative reaction. h l though this reaction time is long, the reaction can be conducted conveniently overnight. I n general, a separate calibration curve must be obtained for each secondary alcohol being determined. The data in Table I1 show the effect of the structure of the alcohol on the color reaction. I n this table the sensitivity is eqressed in terms of the absorbance obtained 1. er microequivalent of secondary alcohol present as the ketone in the color reaction step. The differences in sensitivities obtained are due to the different, sensitivities of the color reactions of the corresponding ketones. This was established by preparing calibration curves from the ketones. I n all cases the same calibration curve was obtained from the ketone as from the alcohol. I n general, ketones that contain a methyl group adjacent t o the carbonyl give the most sensitive color reaction. The same effect was obtained with the corresponding alcohols. The most sensitive color reaction was ob-
tained from 2,5-hexanediol. I n this case the ketone would contain a methyl group adjacent to each carbonyl. This method has been applied to the determination of 2-propanol in ethanol (Table 111). The lower limit of determination by this method is dependent upon the sample matrix. For example, if the matrix is methanol, the sensitivity of the method is lower than for ethanol because methanol is oxidized to carbon dioxide by a three-step oxidation. Therefore, considerably more dichromate ion is consumed by methanol than in the two-step oxidation of ethanol to acetic acid. I n general, the sample should not consume more than 85% of the dichromate ion. Because of this restriction on sample size, the lower limit of determination of 2-propanol in ethanol is approximately 0.02970. An example of the determination of isopropanolamine in ethanolamine is also shown in Table 111. This analysis
would be difficult t o perform by other methods. This method is not applicable t o bifunctional secondary hydroxyl compounds in which the hydroxyls are separated by less than four carbon atoms. I n general, the method is not applicable to the determination of cyclic secondary alcohols or highly branched aliphatic alcohols. These compounds are not oxidized t o ketones but t o acids. Any compound that is oxidized t o a carbonyl and resists further oxidation will interfere in the method. Nost ketones interfere quantitatively; however, suitable corrections can be made by utilizing the 2,4-dinitrophenylhydrazine method without the oxidation step. This method is also applicable to the determination of l o r concentrations of ketones in the presence of aldehydes because the latter compounds oxidize t o acids under the conditions of the
method (Table 111). The determination of acetone in acetaldehyde is the only application that has been investigated to date; however, this type of determination should find considerable npplicahilitj-. LITERATURE CITED
(1) hguhlon, H., Bull. SOC. chim. France 9.881 11911). ( 2 ) 'Bohme, H., Winkler, O., 2. a d . Chem. 142, l(1954). (3) Etienne, H., Ind. chim. helge 17, 455 (1952). (4) Lappin, G. R., Clark, L. C., ANAL. CHEX.23.541 (1951). (5) Koetzel; O., Z . Untersucli. Lebensm 53, 388 (1927 ). (6) Stanley, R. D., J . Assoc. Ofic. Agr. Chemists 22, 594 (1939). ( 7 ) Starche, .F.) Martienssen, E., 2. Lehenstti.-Untersztch. u.-Forsch. 104, 339 (1956). RECEIVED for reviety Sovemher 16, 1959. Accepted March 9, 1960.
Colorimetric Determination of Low Concentrations of Primary and Secondary Alcohols DELWIN P. JOHNSON and FRANK E. CRITCHFIELD Development Department, Union Carbide Chemicals Co., Division o f Union Carbide Corp., South Charleston, W. Vu.
F A general method for determining low concentrations of primary and secondary monohydroxy alcohols is based on reaction of the alcohols with 3 3 dinitrobenzoyl chloride and the subsequent treatment of the esters with a solution of dimethylformamide and propylenediamine to produce a red quinoidal ion. The intensity of the color, which exhibits maximum absorption at 525 mp, is a function of the alcohol concentration. Concentrations ranging from 2 to 100 y of hydroxyl can b e determined in the presence of acetals, vinyl ethers, and other acidhydrolyzable compounds with an accuracy of =t2% of the contained amount. Interferences are generally restricted to compounds which react to consume the reagent,
C
for determining lo^ concentrations of primary and secondary monohydroxy alcohols are relatively few because of the limited ability of the hydroxyl groups to form satisfactory colorproducing derivatives. The majority of these methods are based on the oxidation of the hydroxyl group in various OLORIMETRIC hPETHODS
degrees, but this approach suffers numerous limitations. Oxidation of the alcohol to the corresponding carbonyl compound and subsequent determination of the latter by specific methods have been used ( d ) , but because the stability of various alcohols toward oxidation varies widely, it is difficult to establish a general procedure. Henry et al. ( 3 ) further oxidized ethanol t o acetic acid and determined the latter by reaction with p-hydrosybiphenyl to produce a colored product. An acid solution of potassium dichromate is the most generally used system for oxidizing the alcohols and methods have been based on the measurement of the resulting blue chromic ion. Williams and Reese (9) modified this technique by measuring the e x e s dichromate ion that forms a violet color with s-diphenylcarbazide. A11 of these methods are subject to interference from aldehydes and various other reducing compounds, as well as from substances which hydrolyze in acid media to form interfering products. Pesez (6) determined hydroxyl compounds by forming colored esters nith 5 - (p - s u 1fa in y 1p h e n y 1a z 0 ) sa 1i c y 1i c acid; however, interference from amines
was reported. Reid and Solomon ( 8 ) utilized the red coordination complex between the hydroxyl group and ceric ammonium nitrate for determining low concentrations of alcohols, but this method also requires acid reaction conditions which preclude its application in the presence of acid-sensitive substances. The need for a specific method for determining trace quantities of alcohols in the presence of acid-hydrolyzable substances prompted an investigation to find a procedure requiring basic reaction conditions. Berezin ( I ) had reported the macrodetermination of alcohols by reacting them with 3 3 dinitrobenzoyl chloride (3,5-D) in pyridine and subsequently titrating the excess reagent with a standard base. Previous work had also shown that, in pyridine medium, 3,5-D reacts rapidly with active hydrogen atoms and that the products form highly colored quinoidal ions in certain basic nonaqueous media. This principle provided the basis for the present method. EXPERIMENTAL
Reagents. 3,5-Dinitrobenzoyl chloVOL. 32, NO. 7, JUNE 1960
865
