oxidation is not necessary for the full recovery of arsenic from animal tissue (6). However, when hydrochloric acid digestion of animal tissue was carried out in an open vessel, the recovery of arsenic vias always low. The modified digestion apparatus (9) permits tissue solution while maintaining a n upper internal temperature below the volatilization point of arsenic trioxide ( 2 ) and arsenic trichloride (IO). Kingsley and Schaffert (6) found that when reflux was used during the tissue digestion, less than 50% of the added arsenic was recovered because of the retention of hydrogen sulfide and other mercapto (-SH) groups in tissue 1% hich combined n i t h arsenic and prevented its distillation. The concentration of mercapto groups in 1 gram of tissue is too low in relation to the amounts of arsenic detectable by this method to produce any significant loss of the metal. Following digestion, the arsenic is
evolved as arsine and collected on cellulose containing mercuric iodide overlayered with mercuric iodide. The arsenic is eluted with iodine solution from the resulting mercury-arsenic compound, probably as arsenic acid. Alginate is incorporated in the eluent to act as a stabilizer for the colloidal molybdenum blue which is ultimately developed; without it the concentration curve is discontinuous above 25 y. The reaction between arsenate, niolybdate, and hydrazine sulfate has been reported by several workers (5, 7 , 8). The color is stable for a t least 1 hour and obeys Beer’s law for the limits investigated in this work.
ACKNOWLEDGMENT
The authors acknowledge the technical assistance of Gerrie Smeenk and E. J. Dillistone.
LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official Methods of Analysis,” 7th ed., p. 369,
1950.
( 2 ) Baster, G. P., Bezzenberger, F. K., Kilson, C. H., J . Ana. Chem. SOC.42,
1386 (1920). (3) Berkhout, H. K.,Jongen, G. H., Chenzst Analyst 43 (3)) 60 (1954). (4) Cassil, C. C., Wichmann, H. J., J. Sssoc. Ofic. Agr. Chemzsts 22,436 (1939). (5) Chaney, A. L., Magnuson, H. J., IVD. ESG. CHEJI., h x a ~ . ED 12, 691 (1940). (6) Kingsley, G. R., Schaffert, R. R., -4x.4~.CHEJL 23, 914 (1951). ( 7 ) Magnuson, H. J., Watson, E. B., IND. ENG. CHEM., ASAL. ED 16, 339 (1944). (8) llorris, H. J., Calvery, H. D., Ibid., 9, 447 (1937). (9) Oliver, W. T., Funnell, H. S., Am. J . Vet. Research, in press. (10) Taylor, F. S., “Inorganic and Theoretical Chemistry,” 3rd ed., p. 570, William Heinemann, London, 1935. RECEIVEDfor revien- Apiil i , 1958. Accepted September 8, 1958.
Determination of Carbonyl Compounds JAMES S. FRITZ, STANLEY S. YAMAMURA,I and EVELIN CARLSTON BRADFORD lnsfifute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa
b Some principles of quantitative oximation with hydroxylammonium salts are discussed, and a simple, accurate method is proposed for determining aldehydes and ketones. The carbonyl compound is oximated in methanol-2-propanol solution, and the excess hydroxylamine is titrated with standard perchloric acid. Unlike most oximation methods, the end point in this titration (determined either visually or potentiometrically) is very sharp. All reagents are stable on storage.
D
of aldehydes and ketones b y osimation is probably the most satisfactory general method. Early work by Walther (14), Bennett ( I ) , Bennett and Donovan (2), Stillman and Reed ( I @ , Schultes ( I I ) , Bryant and Smith (S), Montes and Grandolini (9), Trozzolo and Lieber ( I S ) , and Knight and Swern (7) is significant. The Bryant and Smith method has been widely used for many years, although Higuchi and Barnstein (6) have recently pointed out that the end point is very poor. Metcalfe and Schmitz (8) used a ETERMINATION
1 Present address, Atomic Energy Division, Phillip8 Petroleum Co , Idaho Falls, Idaho.
260
ANALYTICAL CHEMISTRY
hydroxylamine solution, prepared in situ by mixing approximately equal molar proportions of hydroxylammonium chloride and octadecenylamine, for the analysis of high molecular weight ketones. Fon ler, Kline, and Mtchell ( 4 ) determined vanillin in the presence of acetovanillone by controlling the reaction time. Higuchi and Barnstein ( 5 ) investigated hydroxylammonium acetate as a reagent for aldehydes and ketones. The oximation vias performed in glacial acetic acid and the excess reagent n as titrated potentiometrically with perchloric acid. Although a n excellent break a t the equivalence point was observed for samples of aromatic ketones or aldehj des, the potentiometric curves for the lower aliphatic carbonyl compounds were considerably poorer, owing to the basicity of the oximes. Pesez (IO) recently proposed the use of a methanolic solution of hydroxylammonium formate for quantitative oximation. The excess reagent is titrated to a thymol blue end point with perchloric acid in dioxane. The carbonyl procedure described is simple, convenient, and accurate, and eliminates or minimizes the difficulties of existing methods.
Dissolve approximately 22.5 grams of freshly distilled 2-dimethylaminoethanol (Eastman Chemical Products, Inc., white label or equivalent) in 2-propanol to make 1 liter of solution. Hydroxylammonium Chloride, 0.4M. Dissolve 27.8 grams of the pure salt in 300 ml. of absolute methanol and dilute to 1 liter with 2-propanol. ?-Propanol. Reagent grade. absolute. Martius Yellow. Diwolve 0.0667
REAGENTS
Figure 1. Titration of blank showing indicator transition ranges
2-dimethy laminoethanol ,
0.2531.
pC0
51
2’ I, zi
YPRTIUS
YELLOW
and
MET-YL
j
V’OLET
~
“E..OW’
ml
of 0 2 Y
red,
,
I-.,
I
,
,
, , , , , , , , . ,
7 -
3001 ANHYDROUS SOLVENTS
Figure 3. Effect of b a t e r on titration curves for blank c $200 YI
2
8 % WATER PRESENT
-
ml OF 0 2 M ACID
Figure 2. Potentiometric curves for sample titrations 1. 2. 3. 4. 5. 6.
Cyclopentanone Cyclohexanone Methyl isobutyl ketone n-Butyraldehyde p-Nitrobenzaldehyde Vanillin
gram of Nartius yellow (Harleco, Hartman-Teddon Co.) and 0.004 gram of methyl violet in ethanol and dilute to 50 ml. with ethanol. Methyl Cellosolve. Merck 8: Co., Inc., reagent grade or Union Carbide Chemicals Co. Perchloric ilcid, 0.2-If. Pipet 17.0 nil. of $07, perchloric acid and dilute to 1 liter Kith methyl Cellosolve. Standardize by titration of tris(hydroxyniethy1)aniinomethane. Tris(hydroxymeth1 1)aminoniet liane. Primary standard grade. Carbonyl Samples. The compounds :inalyzed n-ere mostly Eastman white label chemicals n-ith an estimated purity of 98 to 100%. Pome were purified bJdistillation or crJ-stallization prior to analysis. PROCEDURE
Keigh the sample containing 1.5 to 2.5 inmoles of reactive carbonyl into a 150-ml. glass-stoppered flask. b d d eyactly 20 ml. of 0.25X 2-dimethylaminoethanol. then add exactly 25 ml. of 0.WI hvdrouylamnionium chloride. Stopper the flask, srrirl gently to mix. and let stand the required length of time. Ten minutes at room kmperature is sufficient for most aldehydes and siniple aliphatic ketones. Check doubtful compounds. using a longer reaction time. Arvl ketones. hindered aliphatic compound.. and dicarbonyl compounds require an oximation period of 45 minutes or longer a t 70' C. Add 6 drops of Martius yellow indicator anti titrate n ith 0 2 J I perchloric acid. Take the change from vellow to colorlesi or blue-gray as the end point. Determine the blank by titrating a similar mixture of 2-dimethylaminoethanol and hydroxylammonium chloride that has stood for the same period of time as the sample. Use the difference b e t w e n the blank, V b , and the sample titration, V,, to calculate the percentage of the carbonyl compound in the qaniple.
yc carbon? 1 compound (TT0 -
=
(moles of HC104) (mol. wt ) lO(samp1e Ivt., grams)
L6
8
DEVELOPMENT OF METHOD
I n developing a quantitative method involving oximation with a hydroxylammonium salt, several principles should be considered. The solution should be buffered a t a neutral or slightly acidic p H during oximation. If this is not done, the acid liberated may reduce the ratio of to a low value (KH,OH) to (",OH] and result in incomplete or very slow osimation. B basic solution is generally avoided because of the instability of hydroxylamine in basic solution and condensations or other side reactions of carbonyl compounds. Because oximation is subject to general acid catalysis, a high concentration of protonated species in the buffer will increase the rate of oximation. Reaction in a solvent containing little or no water should favor the oxiniation equilibrium. The buffer and solvent must be such that the excess hydroxylamine (or some other appropriate reactant or product) can be titrated accurately after the oximat'ion is complete. The reactions involved in the present met,hod are as follo\vs: ( S H 3 0 H i +C1-
SH:OH
+
R BH+ C1- f SH2OH (1)
+ >C=O
-f
e :C=SOH
(Escess)SH,OH
I
+ H,O
(2)
H ++ (SH3OH)- (3)
A solution of hl-droxylammoniurn chloride in methanol-2-propanol is approximately half neutralized by a measured aliquot of a strong base, B, in 2-propanol. After the oximation is complete, the excess base (now present as -I;H20H) is titrated with standard acid. The difference betn-een this titration and a blank permits calculation of the amount of carbonyl present. Choice of Organic Base. T h e organic base used to neutralize t h e hydroxylammonium chloride should be
10
IZ
14
I6 IB m l OF 0 2 M
20
22
24
25
WCL3p
sufficiently basic t o force the equilibrium in Reaction 1 t o the right. h tertiary amine should be used to avoid Schiff base formation or other side reactions with carbonyl. The hydrochloride of the base should be soluble in 2-propanol, because extensive precipitation niay lead to occlusion or coprecipitation of the sample. Finally the base should be stable over extended storage. Triethanolamine was sufficiently stable and basic, but copious precipitation of triethanolanimoniuii chloride resulted. 2-Dimethylaniinoethanol arid 2-diethylaminoethanol fulfilled the requirements and a solution of 2-dimethylaminoethanol in 2-propanol \vas adopted. Choice of Solvent. Alcohols are good solvents for t h e reagents involved, and permit a much sharper final titration of t h e hydroxylamine t h a n is possible in water or in n a t e i alcohol mixtures. Even 5 or 10% water in the solvent mixture significantly reduces the sharpness of the end point. 2-Propanol permits a sharper end point than does methanol. Some methanol is necessary to dissolve the hydroxylammonium chloride. Order of Adding Reagents. Recause acetal or ketal formation is a n acid-catalyzed process, it is minimized by adding t h e base t o t h e sample prior t o the addition of hydrouylammonium chloride. Metcalfe nnd Schmitz (8) suggested a similar sequence. The ease of formation of acetals or ketals is also dependent upon the nature of the alcoliol. Primary alcohols form acetals and ketals the most readily, secondary alcohols less readily, and tertiary alcohols the least of all. Johnston (6) proposed 2methyl-2-propanol as the soh ent to minimize these interferences. 2-Propanol is satisfactory. Choice of Titrant. Both 0.2M hydrochloric acid and perchloric acid in 2-propanol give sharp end points and accurate results if the final VOL. 31, NO. 2, FEBRUARY 1959
261
titration is performed very rapidly. However, d h titrations carried out a t a slower rate, results several per cent low were invariably obtained for t h e analysis of carbonyl compounds. The reason was finally traced to the reaction of carbonyl impurity in the reagent grade 2-propanol during the titration. Although carbonyl in the solvents present during the oximation reaction is accurately accounted for by the blank, it is difficult to correct exactly for any carbonyl in the solvent used for the acid titrant, because more titrant is used to titrate the blank than the actual sample. This introduces an error that can be avoided only by titrating so rapidly that none of the titrant carbonyl has time to react with hydroxylamine, or by making up the titrant in a solvent free of carbonyl. Methyl Cellosolve, unlike 2-propanol, exhibited no carbonyl band in the infrared spectra, and was satisfactory in actual practice. A methyl Cellosolve solution of either hydrochloric or perchloric acid is a satisfactory titrant, but perchloric acid keeps its titer on storage much better than hydrochloric acid. Detection of End Point. T h e end point in t h e titration of a sample or blank may be detected either potentiometrically or by a visual indicator. Martius yellow mixed with a little methyl violet gives an excellent color change (yelloiv to colorless) that corresponds exactly with the potentiometric end point (Figure 1). Thymol blue can also bc ust.d, but is less satisfactory because the color change Gccurs a little late. Bromophenol blue changes too early and is unsatisfactory. Reaction Time and Temperature. T h e choice of reaction time and temperature depends on the ieactivity of t h e carbonyl compound with hydroxylamine. Simple aldehydes and ketones oximate completely in 5 to 30 minutes a t room temperature. Aryl ketones, hindered ketones, etc., require a longer reaction period and may also require an elevated temperature for quantitative oximation. Hydroxylamine should not decompose excessively during the oximation period. The data in Table I show that the reagent blank is perfectly constant for a t least 2 hours a t room temperature. For blanks immersed in a 70" C. circulating water bath there is some initial decrease, but the blank is constant between 15 minutes and 2 hours. The initial decrease in the blank may be caused by oxidation of hydroxylamine by dissolved oxygen. RESULTS
Quantitative results for the analysis of several aldehydes and ketones are given in Table 11. Most of the results 262
ANALYTICAL CHEMISTRY
obtained are in the range 98 to loo%, which is the estimated purity of the samples taken. As a check, a vanillin sample was found to be 99.5 5 0.2% pure by oximation and 99.5 f 0.1% by acid-base titration. Although a 20or 30-minute reaction time was used
Table I.
Time, Min. 0
15
Variation of Reagent Blank with Time 0.2M HClOc, hll. 25" C. 70" 24,04 23.93
30
60 120 240
c.
24.05 24.04
24,05 24.04
24.00
23.65 23.67 23.65 23.69
nater. Figure 3, for example, compares a blank titration with approximately 8% water present [as used by Trozzolo ( I S ) and others] with the titration curve in anhydrous solution. The reagent used shows little or no decomposition during oximation, even a t i o " C. Higuchi and Barnstein (6) found that hydroxylammoniuni acetate in acetic acid is very stable a t room temperature but decomposes fairly rapidly a t 60" or 80" C. Most other authors have failed to give definite information on the stability of their blank a t elevated temperatures. LITERATURE CITED
(1) Bennett, A. H., Analyst 34, 14 (1909). (2) Bennett, -4. H., Donovan, F. K., Zbid.,
...
47, 146 (1922). ~~~
Table 11.
Compound 2-Acetonaphthone
Determination of Aldehydes and Ketones
Reaction Time, hlin. 120 60 90
Reaction Temp., C. Room
30 45
ilcetophenone
60 ..
Benzaldehyde Benzoin
20a
120 180
n-But yraldehyde 20a Cyclohexanone 30a Cyclopentanone 30" Dibenzylketone 3Oa Furfural 20a p-Hvdroxybenzaldehyde 5 to 15 Methyl isobutyl ketone 3OU p-Xtrobenzaldehyde 20" Salicylaldehyde 5 to 15 5 to 15 Tridecanone 5 to 15 5'ani11in a This reaction time is probably excessive. mean. Technical grade sample used. for many of these compounds, it was later found that many simple aldehydes and ketones react completely in 10 minutes or less. Representative titration curves are plotted in Figure 2 . One striking feature of these curves is the sharpness of the break a t the equivalence point. Also the equivalence point potential is x-irtually the same for the various carbonyl compounds. Samples of vanillin 17 ere analyzed in the presence of carboxylic acids and esters (Table 111). Although an equal molar amount of benzoic acid caused no interferencc, a 5 to 1 benzoic acid to vanillin ratio resulted in a less sharp potentiometric break and slightly lon results. DISCUSSION
Unless the acid titrant is free of carbonyl impurities, there is a possibility of error, especially if the titration is performed slon 1y. Better results are obtained if the solvents used are essentially free from
O
S o . of
Detns.
TO TO
2 3 3 2 5 2
TO
2
TO TO
TO
Room 70 Room
7
I
3
-
7
Room Room
2 2
Room
3
Room Room Room Room Room Rooni
2 G 2 2 4
s
Found. % b 97.9 10 . 2 99.4 f 0 . 1 100 3 5 0 3 95 8 z!= 0 . 0 999501 99 9 1 0 1 996k01 9 8 6 f 0 1 9 9 3 f 0 4 985501 985501 9 8 4 f 0 1 05 8 5 0 2 c O C I ~ + O ?
99?+02 08 2 5 0 1c 99 4 2c 0 0 99 6 5 0 3 9 9 9 5 0 2
995502 Room b Prrcision index is average deviation f r'om
Table 111. Determination of Vanillin in Presence of Equimolar Amounts of Potential Interferences
Compound Taken, Added AIoles
Found, Moles
yo
99 4=
Soiie
Benzoic acid
Found,
2.102
1.950
"088
99.3b 99.8b 99.5 99 4 99.0' 98 QC
2 161 1 932 2 193
1.946 2.238 2 147 1.913 2 169
1.782 1.879
1.781 1.881
100 1
2.041 1.993
2.031 1 986
99 G
Methyl salicylate 1.969 2.183
1,960 2.141
99.5 99.4
2.249
Phell\-l:ice t ic wid
Ethyl acetate
99 9
09 5
a .iveraye of 6 determination
