Macro- and Semimicrodetermination of Aldehydes and Ketones by Reaction with Hydroxylammonium Formate JAMES E. RUCH, JAMES B. JOHNSON, and FRANK E. CRITCHFIELD Development Department, Technical Cenfer, Union Carbide Chemicals Co w. vu.
b The hydroxylammonium formate method for the determination of aldehydes and ketones has been revised to provide an improved procedure which yields accurate results without interference from organic acids or easily hydrolyzed compounds such as acetals, ketals, and vinyl ethers. Of particular significance is the application of the method in dilute solution, wherein oximation with hydroxylammonium formate has been found both sensitive and accurate. In general, concentrations of 0.003 to 0.370 aldehyde or ketone can b e determined with an accuracy which is within 5% of the contained amount. NEED frequently arises to determine small amounts of aldehydes and ketones, either for process control or for estimation of product quality. Higuchi and Barnstein (2) reported a hydroxylammonium acetate method which employs an acetic acid solvent and is based on the titration of excess reagent. The reagent is relatively unstable, and the method does not afford sharp end points because the oximes formed (particularly ketoximes) are sufficiently basic to interfere. Fritz, Yamamura, and Bradford ( I ) have devised a nonaqueous hydroxylamine oximation procedure which uses a titrant of perchloric acid in methyl Cellosolve to obtain sharp end points in titrating excess reagent. Purity data by this method are excellent; however, the strong acid induces interferences by acetals, ketals, and vinyl ethers when these compounds are major components of the sample. Pesez (3) described an oximation procedure in methanol using hydroxylammonium formate as the reagent (which is adequately stable), perchloric acid in dioxane as the titrant, and thymol blue as the indicator. In methanolic medium, formic acid is neutral to thymol blue indicator; therefore, the course of this reaction can be followed by the direct titration of unreacted hydroxyIammonium formate. However, purity determinations by this method on some compounds (e.g.,
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ANALYTICAL CHEMISTRY
., Division of
acetone) were 1 to 2% low. Presumably, most of these difficulties encountered with the Pesez method are due to the formation of peroxides and carbonyl compounds in the perchloric aciddioxane titrant. Large amounts of acetals and ketals also interfered with the method. Finally, the use of methanol as the medium is not desirable because of its relatively poor differentiating powers in the presence of large amounts of reagent and reaction product. The hydroxylammonium formate method is attractive because it is simple to apply and because the reaction proceeds rapidly. Modifications of the basic procedure of Pesez have produced a macro method which is reliable for purity determinations and which does not suffer from interferences by acetals, ketals, and vinyl ethers. From these studies, a semimicro method was developed for determining milligram quantities of carbonyl compounds. The development and application of these procedures will be discussed. MODIFIED MACRO METHOD
The composition of both the reagent and the titrant have been altered so that quantitative results can be obtained by the procedure and former interferences are eliminated. The titration medium was changed to a 2 to 1 mixture of methyl Cellosolve and methanol. Methyl Cellosolve is an excellent differentiating solvent and is neutral to thymol blue indicator. Methanol is added because it brightens the indicator color transition. Methyl Cellosolve is also used as a titrant medium because large quantities of this solvent have no effect on the titration, whereas large quantities of methanol tend to shift the indicator end point. Perchloric acid has been replaced by nitric acid after consideration of the properties desired. Of the many acids tried, hydrochIoric, perchloric, and nitric acids are the only ones which produced satisfactory titrations of the reagent. The first two were rejected because they are so strong that they
Union Carbide Corp., South Charleston,
readily hydrolyze acetals. Although nitric acid does not provide the sharpest end point, its methyl Cellosolve solution is stable, and the specificity of the method is increased because of the diminished acid strength of this titrant. Urea is added to destroy any nitrous acid and p-diethoxybenzene may be used to retard peroxide formation. The method of titrant standardization is important. In time, weak acids accumulate which could affect the standardization. Tris(hydroxymethy1)aminomethane is a n excellent primary standard for this use, but it is a stronger base than hydroxylamine; hence, in methyl Cellosolve medium, the equivalence point lies in a region below the thymol blue transition point and subject to interference from weak acids. Both difficulties have been averted by using a propylene glycol-methanol medium wherein the equivalence point and the indicator change coincide and are unaffected by relatively large amounts of organic acids (up to 2 ml. of acetic acid). Reagents. Hydroxylammonium formate, approximately 0.5M in methyl Cellosolve. Place 32 grams of reagent grade potassium hydroxide pellets in 350 ml. of methyl Cellosolve. Add 20 ml. of concentrated (90%) formic acid to help effect solution and stir. Neutralize the solution to a phenolphthalein end point by addition of more formic acid, then add 2 or 3 more pellets of potassium hydroxide and stir until dissolved. The result is a solution of potassium formate containing a small excess of potassium hydroxide, as shown by the color of the phenolphthalein indicator in the solution. Prepare a second solution by dissolving 34 grams of hydroxylamine hydrochloride in 650 ml. of methyl Cellosolve. Mix the two solutions, chill to 15" C., and filter to remove precipitated potassium chloride. (The reagent is chilled to remove as much potassium chloride as possible and prevent later deposition of this salt as a solid material.) The reagent is stable for a t least 2 weeks, but should be discarded when the blank titration becomes less than 35 ml. Nitric acid, 0.5.47 in methyl Cellosolve. Add 33 ml. of concentrated nitric acid to 500 ml. of methyl Cellosolve. Add and dissolve 1.0 gram of urea and
0.1 giam of p-diethoxybenxene (Eastman Chemical Products Co.), dilute to 1 liter with additional methyl Cellosolve, and mix thoroughly. To standardize, dissolve 1.5 gram of tris(hydroxymethy1)aminomethane (primary standard grade) in 50 ml. of hot methanol. ildd 100 ml. of propylene glycol and titrate to thymol blue indicator until the color changes from yellow to a definite orange. Run a blank on the solvents. Thymol blue indicator, 0.37, solution in dimethylformamide. Macro Procedure. Pipet exactly 50 ml. of the 0.5N hydroxylammoniuin formate reagent into each of two 250-ml. flasks and reserve one flask for a blank determination. Into the other flask, introduce a n amount of sample containing no more than 15 meq. of reactive carbonyl compound. Allow blank and sample to stand a t room temperature for the length of time specified in Table I. To each flask add 50 ml. of methanol, 75 ml. of methyl Cellosolve, and 5 or 6 drops of thymol blue indicator. Titrate the blank with standard 0.5N nitric acid in methyl Cellosolve until the color changes from yellow to a definite orange. Titrate the sample until the color matches that of the blank, approaching the end point dropwise. The difference between blank and sample titrations is a measure of the carbonyl content.
of the concentration limits to which the procedure is applicable. SEMIMICRO METHOD
The macro method employs 0.5N reagent and titrant. For the determination of small amounts of aldehydes and ketones, suitable reductions in strength of both reagent and titrant
Table 1.
Comparable data on the purity of a number of aldehydes and ketones were obtained by hydroxylammonium formate and hydroxylamine hydrochloride methods. The average result, the precision attained, and the number of determinations for each sample are shonninTable I. Basic substances (such as amines and salts of organic acids) will interfere quantitatively, as will mineral acids; therefore, a suitable correction must be applied if the sample is not neutral to thymol blue indicator. Acid anhydrides also interfere and require a correction when present. Notable compounds which do not interfere include acetals, ketals, vinyl ethers, and acids with ionization constants less than lo+. To illustrate, a sample was prepared to contain 19.7% acetic acid, 23.7% diethyl acetal, 33.07, methyl Cellosolve, and 23.6% acetaldehyde. By analysis, 23.4% acetaldehyde was found for a recovery of 99.2%. A second sample was prepared to contain 51.9% acetic acid, 46.8% diethyl acetal, and 1.28% acetaldehyde. A valu? of 1.3Oy0acetaldehyde was found, which corresponds to a recovery of 102% The modified procedure is accurats and simple to perform Also, thy nirdiuir, i b b powerful solvent io: cJiganic samp~rs,tile reaction is s ~ i i , brlci tht LDL point is good, Thew factors, yiis the comparative freeduii. iiurr1 ~ ~ ~ t t ~ , t l t ~let1 i ( to e b furtiirr , SLUUJ~
Purity Determinations of Aldehydes and Ketones
Purity, by Wt.0 Reaction Hydroxylammonium Hydroxylamine Compound Timeb formate hydrochloride Acetaldehydec 15 99.0 f 0 . 0 ( 5 ) 98.4 f 0 . 2 ( 3 ) Butyraldehydec 15 97.2 f 0 . 2 ( 3 ) 9 7 . 2 f O . l (3) Crotonaldehyde 15 97.8 f 0 . 2 ( 5 ) 97.6 & 0 . 0 ( 2 ) 2-Ethylbutyraldehyde 15 98.8 f 0 : 2 ( 4 ) 98.9 =t0 . 3 ( 3 ) 2-Ethylhexaldehyde 30 99.9 f 0 . 2 ( 4 ) 99.7 =t0 . 3 ( 3 ) 2-Et hyl-3-propylacrolein 60 98.9 k 0 . 3 (3) 99.0 f 0 . 5 ( 3 ) Formaldehyde 120 37.15 =t0.05 (5) 37.15 f 0.03 (2) 2,4-Hexadienal 15 99.3 f 0.1 (4) 99.6 f 0 . 1 (2) Acetonec 15 98.9 f 0.1 (5) 98.8 f 0 . 0 ( 3 ) Diacetone alcohol 15 99.2 f 0.1 (3) 98.8 & 0 . 2 ( 3 ) Ethyl butyl ketone 15 98.7 f 0 . 4 ( 4 ) 98.8 f 0 . 0 ( 2 ) Methyl ethyl ketone 15 99.1 f 0 . 1 (3) 99.0 * 0 . 1 ( 3 ) Methyl isobutyl ketone 15 99.4 k 0 . 0 (2) 99.3 f 0 . 0 ( 2 ) Number in parentheses represents number of determinations. * Minutes at room temperature. Use an aliquot of a dilution in methyl Cellosolve to avoid evaporation loss. Table II.
DISCUSSION
must be made. Early attempts to scale down both titrant and reagent uncovered a difficulty hitherto not found. Trace amounts of carbonyl impurities in the methyl Cellosolve used as a titrant medium caused a fading end point. Furthermore, these same impurities in the methyl Cellosolve reagent diluent decreased the strength of the diluted reagent below the exqJected
Recovery Data for Small Amounts of Aldehydes and Ketones
(Using 0.1N reagent and 0.02N titrant) Compound and Taken Found, Deviation, Fkcovery, Reaction Time0 Mg. Mg. Mg. % Butvraldehvde (30 minutes) 1 20 1.16 -0.04 97 1.36 +O 16 ii3 1.32 +o 12 110 1.26 +O 06 105 1.26 +0.06 105 1.26 + O . 06 105 1.23 +0.03 103 1.30 1.39 co.09 107 1.43 i o . 13 i io 1.39 +0.09 107 2.60 2.49 -0.11 96 2.68 $0.08 103 2.59 -0.01 100 26.0 25.7 -0.3 99 26.0 0.0 100 Formaldehyde (240 minutes) 4.25 4.14 -0.11 98 4.02 -0.23 95 4.19 -0.06 99 4.27 +o .02 100 7.45 7.19 -0.26 97 7.19 -0.26 97 2-Ethylbutyraldehyde (90 3.78 3.76 -0.02 100 minutes) +o. 10 3.86 102 +0.10 3.86 102 Glyoxal (30 minutes) 1.03 1.06 +0.03 103 3.02 2.94 -0.08 98 2.88 -0.14 96 2.99 -0.03 99 2.99 -0.03 99 Acetone (15 minutes) 2.44 2.47 +0.03 101 2.51 +0.07 103 2.48 f0.04 102 2.47 +0.03 101 2.47 +O .03 101 Av. 0.09 Av. 101 a Reaction times based on maximum amount of carbonyl compound (0.3 meq.) contttined in 10 ml. of hydroxylic solvent, using 5 ml. of 0.1Y reagent and 0.02N titrant. I
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~
VOL. 33, NO. 1 1 , OCTOBER 1961
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YILLILITcns
Figwre 1. Comparative potentiometric curves with 0.02N and 0.5N titrants (Titration of samples, determination of acetone
value. Carbonyl-free methanol has been very satisfactory as a diluent for both titrant and reagent in preparing solutions for the oximations in dilute solution subsequently discussed. I n addition to the reagents previously described, the following are used in the semimicro method: Carbonyl-free methanol. To loo0 ml. of C.P. methanol, add 10 grams of 2,4-dinitrophenylhydrazine and a few drops of concentrated hydrochloric acid. Reflux for 2 hours and recover the methanol by distillation through any suitable column. Stored in a tightlystoppered bottle, the methanol will remain carbonyl-free indefinitely. Sitric acid, 0.02N in carbonyl-free methanol. Prepare by volumetric dilution of the standard 0.5N nitric acid in methyl Cellosolve with carbonyl-free methanol. Prepare this solution fresh weekly. Hydroxylammonium formate, 0.lN in carbonyl-free methanol. Prepare by dilution of the 0.5N reagent with carbonyl-free methanol. Semimicro Procedure. All glassware must be clean and rinsed with carbonyl-free methanol before drying. Pipet exactly 5 ml. of 0.1N hydroxylammonium formate reagent into each of two 250-ml. flasks and reserve one flask for a blank determination. Into Reagents.
Table 111. Determination of Trace Amounts of Acetaldehyde in Vinyl Ethyl Ether sample Acetaldehyde, % by Rt. Size, bll. Taken Found 5 ... 0,0074 ... 0.0064 ... 0.0080 ... 0,0074 0.0069 10 , . . 0.0068 20 0,0879" 10 0.0892 0.0892 0 087ga a 98.5y0 recovery.
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ANALYTICAL CHEMISTRY
the other flask, introduce a n amount of sample containing no more than 0.3 meq. of reactive carbonyl compound. The volume of sample added should not exceed 10 ml. Allow blank and sample to stand a t room temperature for the length of time specified in Table 11. (In some media, an increased reaction time may be required.) Add 3 drops of 0.3% thymol blue indicator to each flask. Titrate the blank n-ith standard 0.02iY nitric acid in carbonyl-free methanol until the color changes from yellow to a definite orange. Titrate the sample until the color matches that of the blank, approaching the end point dropwise. The difference between blank and sample titrations is a measure of the carbonyl content.
A
cC
DISCUSSION
A reagent concentration of 0.01N and a titrant strength of 0.02N were selected to give the optimum balance between osimation rate and sensitivity. Using this combination, the recovery data shown in Table I1 were obtained, indicating good recovery and reproducibility, independent of sample size. Ketones in general react quickly with hydrosylammonium formate. In the macro method, all of the ketones tested react within 15 minutes. Because there are a minimum of hydrogen bonding effects, this same high rate of osimation is maintained in dilute solution and acetone requires only 15 minutes reaction. As little as 10 p.p.ni. of acetone can be determined by this method using 0.02N reagent and 0.01.Y titrant. Although the oxime of acetone is slightly basic, no interference is encountered in titrating the excess hydroxylammonium ion. Titration curves for 0.02.V and 0.5N nitric acid are depicted in Figure l. These curves are equally well defined, probably because the usual loss of sharpness m-ith dilute titrant is offset by less buffering of the end point because of the decreased acetoxime content. Because of hydrogen bonding effects, formaldehyde requires a long reaction time. In dilute solution, the maximum sample size requires 4 hours for complete reaction. In spite of the !ong reaction time, the data in Table I1 show good recovery. The steric effects of the ethyl group of 2-ethylbutyraldehyde on the oximation reaction are evident. 2-Ethylbutyraldehyde requires 90 minutes, whereas butyraldehyde reacts completely in 30 minutes. Although glyoxal is a small, highly polar molecule, it reacts quickly (30 minutes) and is typical of aldehydes in general rather than of formaldehyde. The semimicro method n-as applicable to the determination of small amounts of carbonyl compounds in the presence of large aniountq of materials
i
15
- I k o p ~ o p y IOICohol
a-
Malhmol-Methyl
--
.
Clllololve
30 45 E E L C T l O h TIME, MINUTES
60
Figure 2. Oximation rate of butyraldehyde as function of reagent concentration (top) and solvent medium (bottom, 0.02N reagent) ( c a . 3 mg. of butyraldehyde reacted)
n hich readily acid-hydrolyze to produce aldehydes or ketones. Such materials include acetals, ketals, and vinyl ethers. The hydrosylammonium formate reagent is essentially nonaqueous and only weakly acidic; therefore, this reagent is ideally suited for determining low concentrations of carbonyl in compounds which readily hydrolyze. Vinyl ethyl ether readily hydrolyzes to form equimolar quantities of acetaldehyde and ethyl alcohol. Because of the high volatilities of both vinyl ethyl ether and acetaldehyde, the hydroxylammonium formate method must be applied a t - 10" C. to avoid sample loss. Using 5 nil. of a O . l N hydroxylammonium formate reagent, complete reaction of a maximum amount of acetaldehyde (0.1%) in a large sample size (20 ml.) of vinyl ethyl ether is obtained within 30 minutes a t -10" C. These reaction conditions n ere employed for subsequent studies of sample size and for recovery data. Analyses of a sample of vinyl ethyl ether were made using 5, 10, and 20 ml. of sample. The data in Table I11 show good precision (0.0064 to 0.0080% acetaldehyde) and indicate no variation with sample size. Of 0.0892% acetaldehyde added to a sample of vinyl ethyl ether, 0.08i9% was found by analysis for a recovery of 98.5%. Interferences. The interferences encountered with the semimicro method are the same as those found with the macro procedure; viz., basic substances and mineral acids. Up to 5% water can be tolerated in the titration medium, thus relatively large quantities of water can be present in the sample (providing the material is not sensitive t o hydrolysis) if a
sufficient quantity of carbonyl-free methanol is added prior to titration. Effects of Reagent Concentration and Solvent on Oximation Rate.
The reaction rates of butyraldehyde with various concentrations of hydroxylamnionium formate in a niethanol-methyl Cellosolve medium are illustrated in the top portion of Figure 2 . Slow, uncertain reactions are obtained with 0.02N and 0.05.\\- reagents, wvhcrcas normalities of 0.1 and 0.25 provide quantitat,ive rpsulti within 30 minutes. To conipare the effect of various sulvent media on the rate of osimation, the reaction rates of butyraldehyde with 3.02.11 reagent' were studied in several media. Figure 2 (bottom) shows reaction curves for butyraldehyde in the conventional methanol-methyl Cello-
solve mistuw, in isopropyl alcohol, and in benzene. The differences in reactivity are caused by hydrogen bonding effect
