Improved Procedure for Determination of Aldehydes SIDNEY SIGGIA AND WII,LIA%f blAXCY General Aniline & F i l m Gorp., Central Research Laboratory, Easton, Pa. This procedure provides a means for determining aldehydes, without difficulties of equilibrium or end point, by forming the corresponding bisulfite addition product in a sodium sulfite-sulfuric acid mixture. The excess acid is titrated potentiometrically. Plotting a graph of pII zs. milliliters of standard
T
H E fact that, aldehydes add bisulfite has been known for many years and has been uscd as the basis for analytical methods for determination of aldehydes. Ripper’s method, in which (6) aldehydes react with bisulfite and cxccss bisulfite is dckrmined by iodine oxidation, has undergone much modification. Parkinson and Wagner ( 4 ) sum u p the iodometric methods for determining excess bisulfite and also give methods for overcoming some difficulties in earlier procedures. The iodometric methods require standardization of the bisulfite solution each day because of the instability of the solution, and difficulties with the equilibrium between aldehyde and bisulfite us. aldehyde bisulfite addition product sometimes affect rcsults. As thc iodine consumes excess bisulfit,e, the addition product dissociates back to free aldehyde and bisulfite. The estent of the error depends on the equilibrium constant for the particular aldehyde, the amount of bisulfite addition product present, the amount of iodine present, and the time it is in the solution. The determination of increase in alkalinity in the systcm wherc aldehydes rcact with sulfite has been used to determine aldchydes. However, the existing methods exhibit difficultics of equilibria and end point. Seyewetz and Bardin ( 7 ) used this type of approach in the determination of acetaldehyde; acetaldehyde reacted with sodium sulfite and the sodium hydroxide formrd was titrated with standard acid.
H OH
+ SaOH \
SO&a
However, there is an equilibrium involved, and the reverse reaction is pronounced. In the case of acetaldehyde, because of its low boiling point, much aldehyde is lost from the solution. Seyen-etz and Bardin tried to overcome this difficulty by keeping the acetaldehyde content in their samples down to 7 to 854. They also chilled the solutions to 4’ to 5 ” C . In attempts to use the Seyewetz and Bardin procedure, the above draivback was vcry evident. Thc equilibrium caused trouble with the low-boiling aldehyde because of consistent loss of aldehyde and with the higher boiling aldchydcs because of the insolubility of the free aldehyde. Thc procedure can be used for determining formaldehyde (Seyen-etz and Gibello, 8) because of its relatively high solubility. A formaldehyde proredure of this tvpe is described by D’Blelio ( 2 ) . Feinberg ( 3 ) found the sulfitc method usable only in the case of formaldchyde and benzaldehyde, Lyith not, too reliable results in the case of benzaldehyde. FIe examined several neutral sulfite procedures and pointed out the difficulties in cach, all due t o rquilibrium and end point. Feinberg suggested adding acid to the sulfite to make the reaction go to completion but made no quantit,ative study along these lines. He noticed how, when acid was added t o aldehyde in sodium sulfite solutions, the pink
alkali yields results, on the average, reproducible to *0.29’& Titrating to the predetermined pH for the end point yields results good to * 0 . 4 ~ 0 . Ketones generally do not interfere if present in amounts less than 10 mole 9’0. Acetals are not attacked by the reagents and do not interfere in the analysis.
phenolphthalein color disappeared and then gradually reappeared as more aldehyde reacted with sulfite. This showed the existence of a d e h i t e equilibrium. Romeo and D’Amico ( 6 ) , realizing the equilibrium difficulties in the previous procedures, used sodium sulfite-potassium bisulfite mixtures to determine cinnamaldehyde and benzaldehyde; they mentioned good results for cinnamaldehyde but poor results for benzaldehyde. They tried the method on some ketones, with generally poor results. The function of the bisulfite would be to minimize the equilibrium which occurs when sulfite alone is used. However, berause of the instability of bisulfite solutions, it was found advisable in the authors’ work to use sulfuric acid instead. An aliquot of standard sulfuric acid is added to a large excess of sodium sulfite solution, so that sodium bisulfite is essentially the active ingredient. The acid is very stable and can stand for long periods of time without significant change in titer. It is added to the sulfite just before the aldehyde sample is introduced. The aldehyde reacts with the bisulfite, and the excess bisulfite is titrated with standard alkali. (The reaction might also be viewed as the aldehyde reacting with sulfite to liberate sodium hydroxide, and the acid present consuming the sodium hydroxide, pulling the reaction to completion.) The large excess of sulfite is to keep the reaction essentially at completion as the excess bisulfite is titrated with alkali. I n this system, the reaction is so near completion that no aldehyde can be detected above the solution, even in the case of low-boiling aldehydes such as acetaldehyde. This reagent dissolves many insoluble, high-boiling aldehydes. All the procedures mentioned above, using either neutral sulfite or sulfite-bisulfite, employed indicators for end point determination. This was unsatisfactory because the buffesystcms involved dulled the end point. A d a m and Adkins (1 I tried color comparisons to minimize errors due to end point. To detect the end point more accurately, it was found advisable to use a pH meter in the titration. A curve is plotted of p H t ~ .milliliters of standard alkali added, and the end point is detprmined from the curve. The end point in the case of each aldehyde comes at a rather definite pH (there is only slight deviation with the size of sample); once this pH is known, each sample can be titrated to it, thus eliminating the necessity of plotting a curve for each sample. This procedure is rather generally applicable t o determining aldehydes, and it has overcome the difficulties of equilibrium such as incomplete reaction, loss of sample, and poor titration, as well as the end-point difficulties encountered in the previous procedurcs, and has eliminated the necessity of using unstable standard solutions. This makes the procedure accurate, precise and very simple to operate, yet not lengthy. APPARATUS
A 500-ml. glass-stoppered Erlenmeyer flask, a 600-ml. beaker, a pH meter, and standard electrodes.
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V O L U M E 19, NO. 1 2 Table I.
Aldehgde Acetaldehvde
Determination of Aldehydes p H Range of Solution a t E n d Point 9.05-9 15
Propionaldehyde
9.30-9.50
Butyraldehyde
9.40-9.50
Cinnamaldehyde (takes on 2 moles of bisulfite Der mole of sample)' Crotonaldehyde (takes on 2 moles of bisulfite per mole of sample) Benzaldehyde
9.50-9.60
Observed Value, Mole 0 02455
Mole of Sample Taken 0 02456 0 , 0 2 118 0.O2862 0.02090 0.02053 0.01811 0.0243 0.0236 0.0280 0.0106
o.oiia 0.0122
9.20-9.40
0.0250 0.0189 0.0235 (pII 9.30j0
0.0253 0.0190 0.0238
8.85-9.05
0.0163 0.0168 0 0147 (pH 8.95)a
0.0164 0,0173 0.0154
a Rapid method used. All samples were distilled in a Podbielniak fractionating column (manufacturer claims 100 plates) and analyzed less t h a n 2 hours after distillation t o minimize air and autoxidation of sample.
SOLUTIOYS
Sodium sulfite (1 M), standard 1 LY sodium hydroxide, and standard 1 S sulfuric acid. PROCEDURE
Fifty millimeters of 1 S sulfuric acid are added t o ~ 2 5 0ml. of 1 Jf sodium sulfite in a 500-ml. glass-stoppered Erlenmeyer flask. T o this solution is added a sample sealed in a glass ampoule containing 0.02 to 0.04 mole of aldehyde. The flask is stoppered, the stopper being greased in the case of low-boiling aldehydes t o prevent any loss, then vigorously shaken to break t,he ampoule. Some glass beads included in the flask will cause the ampoule to break more easily. The flask is shaken for 2 t o 3 minutes (5 minutes in the case of the more insoluble aldehydes) t o ensure complete reaction. The contents are then quarititatively transferred to a beaker. Electrodes from a pH nirtor are inserted in the solution, and the solution is stirred. Thc pII of the solution is noted as standard I .V alkali is added to titrate the excess acid. For accurate results, the pH reading L'S. milliliters of alkali added is noted and plotted. The end point is determined Jrom t,he plot. A more rapid method of determining the end point, though slightly less precise, is t o add alkali until a pH is obtained which corresponds t o the pH a t the end point for the particular aldehyde. This end-point pH can be taken from Table I or predetermined in cases of aldehydes not mentioned. Sodium sulfit,e contains a small amount of free alkali as an impurity, so that 250 ml. of the solution consume some acid. The blank is very small but not negligible; it amounts t o about 0.4 t o 0.5 ml. of 1 AVacid per 250 ml. of sulfite solution. On each carboy of sodium sulfite solution prepared, the free alkali should be accounted for or the aldehyde results will be slightly high. Rather than use a blank on the sulfite solution, it was found more satisfactory t o add enough 1 .If sodium bisulfite t o the sodium sulfite to neutralize the free alkali and bring t,he pH of the sulfite t o 9.1. This eliminates the blank and need be done only once t o each carboy of solution.
Ketones cannot, on the whole, be determined by this method, even though they do form bisulfite addition products. Acetone, methyl ethyl ketone, quinone, naphthoquinone, and eyclohesanone vwre tried; cyclohexanone is the only ketone tried which might be detcrmiried by this method. The titration curves for ketonvs indicate that the ketone does react with bisulfite, but there is no discernible end point (see Figure 1). The pH climbs steadily oil addition of the sodium hydroside. In the case of cyclolicwmme, a poor end point is obtained comparable to t,hat of benzaldehyde. The titration curves in the case of ketones indicate that the equilibrium between ketone and ketonebisulfite addition product leans toward free ketone and sodium bisulfite to a great extent. As sodium hydroxide is added, the pH i,ises gradually, but no break occurs in the curvt's. This c'm tic explained by the consumption of excess sodium bisulfitc by thrb sodium hydroxide, causing some ketone-bisulfite addition product to decompose, thus liberating some sodium bisulfitc, P O t h a t the addition product is consumed along with the excess sodium lisulfite. The c,xplanation can be applied in the case of furfural, which behaws as do the ketoiiee. The poor end point in the caec. of benzaldehyde can be attributed t o thtx same cause, but the equilibrium is enough in favor of the addition product to make this aldehyde determinable by this method. Ketones, in general, will interfere in the determination of aldchydes only if they are present in excess of about 10 mole c;. The aldehyde produces a break in the curve of pH z's. milliliters of sodium hydroxide, while the ketone does not. If ketones are presmt, the entire curve of pH us. milliliters of sodium hydroxide has to be plotted and the break iii the pH curve determined t o obtain the end point, since, as indicated by Figure 1, the presence of ketones can affect the pH a t the end point and, therrforr, will caust' crroneous results if the rapid method is used. Acidic or basic impurities in the sample should be determined separately before the aldehyde procedure is applied, and the titration for the aldehyde should he corrected for their presencr. .\cctals will not interfere in the procedure. These compounds hydrolyze in strong acid solution to yield acetaldehyde. However, the pH of the sodium sulfite-sulfuric acid solution, in proportions as described in the procedure, is about 6.8, and there is no noticeable hydrolysis of the acetals a t this pH. The aldehydc in the sample consumes the bisulfite so rapidly that the pH of the solution is raised to about 7.5 as soon as the sample comes in contact with the sodium sulfite-sulfuric acid, further lessening any possibility of hydrolysis. ACKSOW LEDGMENTS
The authors are indebted to L. T. Hallett, who proposed that an improved aldehyde procedure be developed, read the manu-
DISCUSSION
/ / /
MOST ALDEHYDES
I n t,he case of most of the aldehydes used, the end point was sharp enough so that using the rapid method of simply titrating t o the p H of t,he end point could cause an error in end-point determination of only 1 0 . 2 to 0.3 ml. This error is not too significant and can be nullified by using a rather large sample of about 0.04 mole of aldehyde which consumes 40 ml. of 1 S acid. A plot of p H us. milliliters of sodium hydroxide is not necessary once the p H at the end point of the particular aldehyde is determined. The reproducibility of the procedure is +=0.270if the entire curve is plotted and -0.47, if the rapid method is used. The end point in the case of benzaldehyde is relatively poor (see Figure l),and the rapid method could cause an error of 1 0 . 4 t o 0.5 ml. I n this case a plot is necessary to determine the end point accurately.
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I
I
I
I
IO
20
30
40
ML.I N NAOH
Figure 1. Titration Curves for Carbonyl Compounds
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DECEMBER 1947 script, and made some valuable comments. They appreciate the assistance of John W. Copenhaver, who contributed valuable information that aided in the initiation of the problem. The authors also wish to thank Richard S. Towne and Mrs. Fred Maisch, who purified and distilled the aldehydes used to test the procedure. LITERATURE CITED
(1) Adams, E. W., and Adkins, H., J A m . Chem. SOC.,47, 135s-67
(1925).
(2) D’&ilelio,G. F., “Laboratory Manual of Plastics and Synthetic Resins,” p. 112, New York, John Wiley & Sons, 1943. (3) Feinberg, B. G., Am. Chem. J . , 49, 87-118 (1913). (4) Parkinson, A , , and Wagner, E., IKD. ESG.CHEX.,-4x.a~.ED.,6 ,
433-8 (1934).
M.,Monatsh., 21, 1079 (1900). Romeo, G.. and D’Amico. E.. Ann. chim. applicala, 15, 320-30 (1925). ( 7 ) Seyewetz and Bardin, Bull. SOC. chim., [3],33, 1000-2 (1905). ( 8 ) Seyewete and Gibello, Ibid., 131, 31, 691 (1904).
(j)Ripper,
(6)
RECEIVED January 1 , 1947.
Determination of Vinyl Ethers and Acetals And of Any Alcohol, Acetaldehyde, and Water Contained Therein SIDNEY SIGGI4, General Aniline 6;. Film Corporation, Central Research Laboratory, Easton, Pa. This procedure provides a means for quantitatively determining vinyl ethers and acetals and makes possible determination of acetaldehyde, alcohols, and water contained in the ether and acetal samples from the synthesis, or hydrolysis, of some of the vinyl ether or acetal. The acetals and vinyl ethers are determined by acid hydrolysis to acetaldehyde and determination of the acetaldehyde formed by the sodium sulfitesulfuric acid method. 30 distillation of acetaldehyde is necessary. Where vinyl ether and acetal are present in the same sample, t h e sample is hydrolyzed and total acetaldehyde determined. Then the sample is hydrogenated to reniove the vinyl ether as a n interference, and this hydrogenated sample is hydrolyzed. This yields
C
OhlMERCIAL vinyl ethers may contain alcohol, acetal-
dehyde, water, and acetal. Alcohol may come from the original reactants in the synthesis of the ether or may be formed by hydrolysis of ether. Acetaldehyde originates by hydrolysis of thc vinyl ether. .%cetals are a product of a side reaction in the vinyl ethcr synthesis, and water enters the system in the n-ashing process and sometimes with the alcohol. The procedure developed for assaying vinyl ethers is also adaptable for assaying acetals. Since acetal syntheses involve alcohols, acetaldehydes, and water, as do the vinyl ethers, the procedures used to determine these impurities in vinyl ethers were applied and found to work as w l l in the case of acetals. The scheme of analysis presented belon- makes possible the determination of acetaldehyde, alcohol, and n-ater in the presence of either, or both, vinyl ether and acetal. I t also makes possible t,he dctermination of vinyl ethers and acetals in the presence of both the above-mentioned impurities and of each other. There has been verv little done in the way of analytical procedures for vinyl ethers. Ruigh ( 7 ) attempted to determine divinyl ether in blood via hydrolysis to acetaldehyde. However, he encountered difficulty and had to resort to a more complicated method involving iodine pentoside. Attempts were made to determine vinyl ethers by quantitative hydrogenation, bromination, and titration with Karl Fiacher’s reagent. The hydrogenation method requires a long time to run an analysis and the reproducibility is poor. The reproducibility is poor also in the case of the Karl Fischer reagent. , Bromination yielded no comprehensive results. \Iore work ljas bccn done in acetal determination than in the
.
acetaldehyde from the acetal onlj Subtracting this value from the total acetaldehyde >ields acetaldehyde from the vinyl ether. Free acetaldehjde is determined by extracting the sample with sodium sulfite-sulfuric acid and titrating the excess acid. The vinyl ether and acetal values are corrected for free aldehyde in t h e sample. 4cetylation of the samples and determination of acetic a n h j dride consumed yield the amount of alcohol present. Water is determined by the Karl Fischer reagent. For pure vinjl ethers and pure acetals the method jields results good to *0.370, When samples contain one or more of the components mentioned above, the errors hare to be determined by accumulatiie error methods.
case of vinyl ethers. Many of the analytical methods are connected n i t h measurement of hydroljsis rates of acetals and are unsuitable for absolute acetal determination, since the measurements are of a relative nature. They are perfectly adaptable for determining rate of hydrolysis but cannot readily be used t o determine absolute amounts of acetal. Among such methods are those of Palomaa and Salonen ( 4 ) and of Tong and Olson ( I I ) , who followd the rate of hydrolysis of acetals via dilatometer measurements. There have been methods for acetals where the acetal was hydrolyzed and the acetaldehyde determined. The drawbacks in these procedures are due, in most cases, to the shortcomings of the acetaldehyde procedure used (9). Many of the procedures distill the acetaldehyde over into the medium in which it is to be determined. This step not only greatly increases the time required for an analysis but also can be a source of error. Peynaud ( 5 ) determined acetal by distilling acetaldehyde over into a potassium dihydrogen phosphate-disodium phosphate buffer containing sodium bisulfite and determined aldehyde by the iodometric measurement of excess bisulfite. This method has the disadvantage of the distillation, and the iodometric bisulfite method for aldehyde determination also has shortcomings (9). Orton and McKie (3) hydrolyzed and distilled acetaldehyde into sodium bisulfite using the Ripper (6) method for determining excess bisulfite iodometrically. They also used the Seyeweta and Bardin procedure (8) which determines increase in alkalinity of a sodium sulfite solution as acetaldehyde. Here again the distillation and weaknesses of the acetaldehyde procedures (9) limit the desirability of using this procedure.
