69
V O L U M E 2 8 , NO. 1, J A N U A R Y 1 9 5 6 shows t h a t whereas - A H , and - AH2 both increase in the order: henzene, carbon tetrachlorlde < nitrobenzene < chloroform, the vhange in -AHn is much greater than that in -AHl, with the iesult t h a t -AHS increases in the reverse order. I n choosing a .olvent, therefore, consideration must be given t o the heat of interaction between Lewis acid and solvent, - AH2, as well as t o the heat of interaction between base and holvent, -AH4; the greater the values of -AH2 and --Ha, the smaller nil1 be the heat developed during the titration. For the bases used ~n this investigation -AH4 was negligible, although it ma! not be so for other haw\ \c:Klon LE:I)(:\lEVT
T h c authors n i d i to :irl;nowlctige support of tliia n.orli by a
Cottrell Grant from the Research Corp. and by funds from the Rutgers University Research Council. LITERATURE CITED
(1) Becker, J. A, Green, C. B., and Pearson, G. L., BdI System Tech. J . 2 6 , 1 7 0 (1947). ( 2 ) La Mer, V. K., and Doivneq. H. C . , J . B m . Cheni. SOC.5 3 , 888 (1931). (3) Linde, H. W., Iiogerr. L. E., and Huiiie, D. S . , =Ixar.. CHEM. 2 5 , 4 0 4 (1953). (4) Lingane, J. J.,I b i d . , 20, 28.5 (1948). (5) Rice, R.V., Zuffaiiti, S., and Luder, IV. F..Ibid., 2 4 , 1022 (1952). (6) Somiya, T., J . Soe. Cheni. Inid.. .Japan&51, 135T (1932)). (7) Trambouze, Y., C o m p f . rend. 233, 648 (1951). (8) Vold. K.D., J . A m . Chenz. S o c . 59, 1515 (1937). R E C E I ~ - Efor I J reviea. .Jiinc. 2 5 , 1933.
Accepted October 19, 1833
Determination of Aldehydes by Mercurimetric Oxidation JAMES E. RUCH
and
JAMES B. JOHNSON
Chemical and Physical Methods Laboratory, Carbide and Carbon Chemicals Co., Division o f Union Carbide and Carbon Corp., South Charleston, W. V a .
imethod o f atialjsis has been deielopect for the determination of aldehjdes, which is hased on the oxidation of alrlehjde to acid b> niercuric ion which, in turn, is reduced to free rnercur?. The analjsis i4 concluded h\ an iodometric measurement of the mercurj. The niethod is applicable to the determination of lirtuall? an) concentration of aldehj d e in the presence of most alcohols, acids, esters, acetals, ketones, ethers, organic chlorides, and epoxides. Reaction conditions and puritj data are presented for 12 aldehjdes which can he determined b! the method.
A
COJ2MO9 prohleni iii organic malysis is the quantitative
analytical resolution of mixtures containing both the aldehyde and ketone functions. Various hydroxylamine hydrochloride proredures have been developed t o determine the total c:trbonyl value of such a mixture, but they are not specific for :tldehydeP. Several methods have been proposed for the determination of aldehydes by oxidation with silver compounds, such as the procedures of Alitchell and Smith ( I O ) and Siggia ( 1 2 ) . A n excellent review of the field of carbonyl analysis has t)c,en prepared by llitchell (9). tteriipted the determination Previous investigators who ha\ o f aldehydes by some mercurimetric procedure have recomniended the method primarily for the estimtttion of formaldehyde (1, 2, 12). I n addition, Ihiigault :nid Gros ( 3 ) have reported the determination of furfur;il, benzaldehyde, and piperonal, and Gosrrsnii, Das-Guptx, aiid Ray (j),Goswami and Das-Purkaystha ( 6 ) , and Goswami and Shaha ( 7 ) have estimated sugars with various degrees of success using empirical factors. These investigators all employed an alkaline solution of potassium mercuric iodide, Ii?Hg14, as an oxidizing agent, In the reaction, aldehyde is oxidized t o the corresponding acid whereas mercuric ion is reduced to free mercury:
ItCHO
+ K,FIgI, + 2IiOH+RCOOH
+ H g ” + 4KI + HZO
Both isolation ant1 nonisolation methods have been proposed for tmhedetermination of the free mercury. I n the authors’ opinion it is best t o acidify the reaction mixture and react the free mercury with a measured excess of iodine. Thc amount of iodine consumed is a stoichiometric function of the free mercury
which, in turnt is a measure of the aldehyde originally present. .4gar is employed as a protective colloid t o maintain the free mercury in a finrly divided state, thus promoting its reaction Ivith iodine. The name, ”n~ercuralreagent,” has becn coined to differeiitiate the reagent from other potassium mercuric iodide preparations such as Nessler’s reagent. “Ylercural” signifies x mercuric oxidation of aldehydcs. 1tE:iGESTS
Mercural Reagent. To 1830 nil. of distilled water contxi~ied in a 1-gallon jug add 150 grams of reagent grade potassium chloride, 240 grams of U. S. Pharmacopeia grade mercuric chloride (mercury bichloride), 642 grams of reagent grade potassium iodide, and 1000 nll. of an aqueous 40% by weight, potassium hydroxide solution. Shake the contents after each addition t o ensure complete solution. This reagent is stable and does iiot deteriorate on standing. The slight amount of yellow or brown precipitate which may foim is assumed t o be due t o ammonium ion in the reagent?, h o w v e r , it is not detrimental t o the dfectiveiiess of the reagent, Agar Solution, 0.1%. Add 3.0 grams of Difco Barto-Agar to 300 nil. of boiling distilled water. Continue heating with ne(::+ sional swirling until the solid has dissolved and the resulting solution is essentiall. clear. Cool and dilute t o 3 liters with additional distilled water. .4dd 0.1 gram of mercuric iodide as a preservative arid shake vigorously for a few seconds. Acetic Acid, analytical reagent grade Iodine, approximately 0 . l S Starch Indicator, 1 .OY0solution Standard 0.1NSodium Thiosulfate Methanol, commercial grade, Carbide and Carbon Chemicals co. SAMPLING
Unless direct satnplc addition is specified, introduce t h r sample into a tared 50-ml. volumetric flask containing 30 nil. of the required solvent (methanol which has been neutralized t o hromothymol blue iritlirator, or distilled water) using a hypodermic syringe fitted with a 3-inch needle and chilled if necessary t o facilitate tranpfer. Stopper the flask and swirl t o effect solution. An acetaldehyde dilution must he allowed to stand for approximately 15 minutes, with occasional venting t o the atmosphere t o reach equilibrium before recording the gross weight. The gross weight of dilutions of other aldehydes may be determined immediately. Dilute to t,he mark with additional solvent and mix thoroughly. .i 5-mL aliquot of this dilution should contain not more than 3.0 meq. of aldehyde. Fill the pipet by pressure t o avoid loss of aldehyde. If the sample is weighed directly into the reagent, care must
ANALYTICAL CHEMISTRY
70 be exercised t o shake the flask vigorously a t once t o intimately mix the contents and prevent localized side reactions. PROCEDURE
The determination is best performed in 500-ml., Erlenmeyer glass-stoppered flasks which are fitted with 24/40 ground-glass joints. Prepare sample and blank flasks by adding 50 ml. of mercural reagent to each. Consult Table I for the proper reaction temperature and, if necessary, cool each of the flasks in a wet-ice bath for 10 minutes. With constant swirling during the addition, introduce an amount of sample containing not more than 3.0 meq. of aldehT.de using the procedure specified in Table I. If a dilution is used, add a similar amount of solvent t o the hlank. Allow the flasks to stand together at the temperature and for the length of time specified in Table I. Add 50 ml. of agar .;ohtion to each flask and swirl vigorously for approximately 1 minute t o disperse the mercury precipitate, then add 25 ml. of glacial acetic acid with constant agitation during the addition.
Table I. Sampling Procedure and Reaction Conditions for Determination of Aldehydes by Mercural Procedure Maximum Sample Reaction Time, Size for Pure Min. b Material, G.a Compound 0.66 5 t o 60 Acetaldehyde 1.3 5 t o 606 Acetaldol 0.84C 180 t o 240d Acrolein 1.6C 15 t o 60d Benzaldehyde 1.1 30 t o 60 Butyraldehyde 0.15' 15 t o 60/ 2-E thylbu tyraldehyde 0.45 1 t o 60 Formaldehyde 0.75 15 t o 60 Glutaraldehyde 0.15e 30 t o 60f Hexaldehyde 1.1c 5 t o 60d Isobutyraldehyde 0.9oc 15 t o 6 O d Methacrolein 0.87 15 t o 60 Propionaldehyde a Use. distilled water as a solvent in t h e sample dilution unless otherwise specihed. specified. b Minutes a t room temperature unless otherwise specified. C Use methanol, which has been neutralized t o bromothymol blue indicator, as t h e dilution solvent. d Minutes in a wet-ice b a t h (0' t o ' 3 C.). e Add t h e sample directly t o t h e sample flask, stopper, and immediately rigorously. b . diake the contents rinorouslv vy hand for 1 minute prior t o the mechanical s ha kinp. I Miniites on a mechanical shaker.
hydrouide, and ratio of potassium iodide to mercuric chloride. I n each case a sample of acetaldehyde was reacted for 1 hour a t room temperature with approximately 50 ml. (70 grams) of reagent. Results showed 10 to 20% by weight of the potassium mercuric iodide complex in solution gave quantitative results. Likewise, a potassium hydroxide content of 10 to 20% by m i g h t afforded a quantitative oxidation of acetaldehyde. Higher percentages of either component caused solubility difficulties, whereas lesser amounts resulted in incomplete reaction. Variation of the potassium iodide-mercuric chloride ratio indicates best results were obtained when the ratio of iodide t o mercuric ions was slightly higher than the 4 t o 1 of the potassium mercuric iodide, K2Hg14, complex. $ny ratio less than 4 to 1 tended to produce a n undesirable precipitate of mercuric iodide, vhereas a ratio significantly higher than 5 to 1 not only gave loa results, but also impaired the effectiveness of the agar used as a protective colloid, yielding a mercury precipitate which n as less reactive n-ith iodine. On the basis of these experiments a reagent was formulated to contain 16% by weight potassium mercuric iodide, 13% by weight potassium hydroxide, and approximately 1 gram of excess potassium iodide per 50 ml. of reagent. Csing this reagent, experiments were undertaken to establish the necessary reaction conditions for pure aldehydes. Watersoluble aldehydes were sampled in the form of aqueous dilutions and oxidized a t room temperature. As a mutual solvent for higher molecular weight aldehydes, methanol has proved satisfactory. Its exact use depends on the particular aldehyde, but the usual procedure is to employ neutralized methanol as a dilution solvent and conduct the reaction a t the temperature of a wet-ice bath ( O o t o 3" C.) to prevent any oxidation of the methanol. I n some instances direct addition of sample t o reagent, accompanied by shaking, is the best procedure. The most suitable method of sampling, reaction conditions, and sample size for a number of aldehydes for which this procedure has been found satisfactory, are given in Table I. RESULTS
If the sample contains acetaldehyde, allow the flasks t o stand a t Foom temperature for approximately 15 minutes before proceeding. The standing period is not required for samples of other aldehydes. Pipet exactly 50 ml. of approximately 0.LV iodine into each flask, using presmre t o fill the pipet. Stopper each flask and shake vigorously until all of the gray mercury precipit a t e goes into solution. If necessary, place on a mechanical shaker for 5 minutes. Carefully remove each stopper, rinse any adhering liquid into the flask, and rinse down the inside walls of the flask with distilled water. Titrate with standard 0.1s sodium thiosulfate until the brown iodine color begins to fade. Add a few milliliters of starch indicator solution and continue the titration just t o the disappearance of the blue color, approaching the end point dropwise while swirling constantly. From the difference between blank and sample titrations the percentage of aldehyde present in the sample can be calculated; one aldehyde group conwmes two equivalents of iodine: -CHO~Hgo~12~S10~-Hence for nionoaldeh\-des the equivalent weight is one half of the molecular weight. DISCUSSION
The reagent originally investigated was of the composition usually specified as Sessler's reagent although, generally speaking, no two authors use the same formulation. Hoxever, i t was found a t this point that Sessler's reagent would not quantitatively oxidize most aldehydes. A study of the reagent was therefore initiated to determine its optimum composition Experiments were conducted t o determine the effect of the following variables: concentration of potassium mercuric iodide complex, concentration of potassium
Comparable data on the puritj- of a number of aldehydes were obtained by the mercural procedure and a hydroxylamine hydrochloride-triethanolamine method ( 4 ) . The average result, the precision attained, and the number of determinations for each sample are given in Table 11.
Table 11. Purity Determinations on Aldehydes by Mercural and Hydroxylamine Procedures Purity by Mercural Procedurea,
Purity by Hydroxylamine Procedureb,
Compound % % Acetaldehyde 9 8 . 9 zt 0 . 3 ( 5 ) 9 8 . 9 zt 0 . 3 (4) .4cetaldol 101.5 zt 0 . 2 (2) 101.6 0.3 (3) Acrolein 98.8 + 0.3 (5) 99.0 0.0 (2) Benzaldehyde 9 5 . 3 zt 0 . 2 (8) 9 5 , 3 i. 0 . 2 ( 5 ) Butyraldehyde 9s 0 I O . 5 (11) 9 7 . 7 =t 0.5 (7) 96.9 0 . 1 (2) 2-Ethylbutyraldehyde 9 6 . 5 zt 0 . 3 (3) Formaldehyde 3 5 . 9 It 0.1 (A) 35.7 0.1 (2) Glutaraldehyde 2 8 . 3 zt 0 . 0 5 (4) Hexaldehyde 95 4 0 2 (4) 9 4 . 7 ' 10.3 (2) Isobutyraldehyde 9 7 . 7 zt 0 . 3 (9) 9 7 . 1 I O . 1 (2) 9 0 . 6 I 0 . 1 (2) hlethacrolein 9 0 . 7 + 0.1 (3) Propionaldehyde 9 7 . 1 =t 0 . 0 (4) 9G.8 i 0 . 4 ( 5 ) a Fiarires in parentheses represent number of determinations 3 H ~ d r o x y l a m i n ehydrochloride-triethanolamine ( 4 ) .
*
*
*
*
Sufficient purity determinations for statistical treatment of data were conducted on a given sample of acetaldehyde by the mercural method. The standard deviation for the determination of acetaldehyde using aqueous dilutions was 0.39% for 13 degrees of freedom on a sample whose average purity was 97.5%. The sampling error was not significant.
71
V O L U M E 28, NO. 1, J A N U A R Y 1 9 5 6 T h e procedure has been modified and found suitable for the determination of trace amounts of aldehydes in organic conipounds. For example, determinations of acetaldehyde in ethylene oxide and propionaldehyde in propylene oxide have been performed successfully. The method used is similar to the one previously described. Transfer 25 ml. of mercural reagent to each flask, add 25 ml. of distilled 11-ater, and chill the flasks t o 0" t o 3" C. in a wet-ice bath. Add 20 ml. of the chilled epoxide sample from a graduate, swirl the flasks, and return to the ice bath for 60 minutes. Add 50 ml. of agar solution and 150 ml. of distilled water t o each flask and swirl vigorously. Pipet exactly 25 ml. of approximately 0 . 1 S iodine into each flask and swirl until the gray mercury precipitate has completely reacted. Titrate the excess iodine x i t h standard 0.1S sodium thiosulfate until the brown color begins t o fade. Add a few milliliters of starch indicator solution ant1 continue t o titrate t o the disappearance of the blue color.
A synthetic sample prepared t o contain 0.037% propioiialdehyde in propylene oxide analyzed 0.036% by this procedure, and a synthetic containing 0.064% acetaldehyde in ethylene oxide gave a result of 0.061%. I n addition, comparative analyses of acetaldehyde in ethylene oxide were performed on tn.0 synthetic samples by the mercural procedure and a sodium bisulfiteiodine method (4). One sample contained 10 & 4 p.p.ni. acetaldehyde by the mercural procedure, whereas the sodium bisulfite method gave 5 5 -1 p.p.m. A second sample nas 61 =t6 p.p.m. acetaldehyde by the mercural procedure and 60 f 10 p p . m . using bisulfite. INTERFERENCE STUDIES
Many organic compounds do not interfere w-ith this procedure, permitting the determination of aldehyde in the presence of most acids, ketones, esters, acetals, ethers, alcohols, epoxides, and organic chlorides. Oxidation studies were conducted on methanol, ethyl alcohol, isopropyl alcohol, and butyl alcohol, both a t room temperature and a t the wet-ice bath temperature. Methanol is sloivly attacked by the reagent a t room temperature, but is completely resistant to oxidation a t 0" t o 3" C. and is, therefore, a preferred nonaqueous solvent for some aldehydes, as indicated in Table I. Isopropyl alcohol is the worst offender, not only because it is oxidized even a t 0" to 3" C., but also because its oxidation product, acetone, complexes the mercuric ion. Ethyl and butyl alcohols are only slightly oxidized a t 0" to 3' C. Studies indicate that, the oxidation of alcohols by mercural reagent follows the mass action law, enabling one t o compensate for this deleterious reaction by using a reagent diluted 50 to 50 with distilled water, adding a similar amount of alcohol t o the blank, and performing the oxidation in a wet-ice bath, allowing a suitably longer reaction time t o offset the dilution of reagent and reduction in temperature. Errors introduced by this procedure are not serious when alcoholic samples containing only a few per cent aldehyde are involved. Samples containing esters require the same conditions, because they are saponified to alcohols by the potassium hydroxide in the reagent. Some vinyl compounds are known to interfere with this procedure by adding iodine, thus yielding a high result; in the case of vinyl ethers, the addition is essentially quantitative. This method has been found applicable t o the determination of acrolein (acrj-laldehyde) and methacrolein (methacrylaldehyde) (see Table II), whereas crotonaldehyde has been analyzed with an accuracy of within f20j,. However, no satisfactory results have been obtained on unsaturated aldehydes containing more than four carbon atoms-e.g., 2,4-hexadienal (sorbaldehyde), 2-ethylcrotonaldehyde, and 2-ethyl-3-propylacrolein. Therefore, the determination of unsaturated aldehydes or of aldehyde in any mixture containing an unsaturated compound must be rhecked for interference.
Acetone reacts with mercuric ion in the following manner (8): Hg+-
+ 2CHa-C-CHz It
5 Hg(CH3-C=CH,),
0
+ 2fI-
0 I
I n the presence of the alkaline reagent and excess mercuric ions this equilibrium reaction is displaced t o the right, depositing the mercuric ion-acetone complex as a yellon solid. On acidificstion the reaction is reversed, proceeding to the left. This reversal must be complete, as indicated by the absence of the, rellow precipitate, or else iodine is consumed, presumably through iodination of the double bonds. Lower temperatures induce precipitation or even resinificxtion of the mercuric ion-acetone complex, hence greater solubility difficultim are experienced a t 0" to 3" C. than a t room temperature. I n order to illustrate the effect of the presence of acetone, a series of blank determinations was made as specified in the method, using reaction conditions of 30 minutes a t 0" to 3" C. From 0 t o 3.0 grams of acetone were added to each flask. With the addition of up t o 0.3 gram of acetone, a yellon- precipitate was formed which easily dissolved on acidification. More than 0.3 gram of acetone caused deposition of a resin, requiring additional potassium iodide t o effect solution. Hence, the acetone tolerance of this method is approximately 0.3 gram for determinations performed in a wet-ice bath. Because a portion of t h e mercuric ion contained in 50 ml. of reagent is complexed by 0.3 gram of acetone, it was then necebsary to prove that a sufficient amount of reagent was still available for the quantitative deterniination of aldehyde. Results on the determination of propionaldehyde in the presence of 0.3 gram of acetone show quantitative oxidation is attained even when the maximum sample size of propionaldehyde is taken. Methyl ethyl ketone complexes mercuric ion t o a much smaller degree than acetone, whereas methyl isopropyl ketone and ethyl butyl ketone are practically inert. Hydroxy ketones constitute a positive interference, as do other easily oxidized substances or anything which consumes iodine. Conversely, oxidizing agents such as peroxides are likely to produce low results, either by competing with mercuric ion in the oxidation of aldehyde or by oxidizing iodide t o iodine. As a rule the amount of acid or ester which can be tolerated must not be so great as t o neutralize more than one third of the potassium hydroxide in the reagent, whereas no more than one half of the mercuric ion content should be reduced and/or complexed. LITERATURE CITED
(1) Alexander, E. R., and Underhill, E. J., J . A m . Chem SOC.71, 4014-19 (1949). (2) Bolle, J., Jean, J., and Jullig, T., MBm. sertices chim. &at (Paris) 34, 317-20 (1948). (3) Bougault, J., and Gros, R., J . pharm. chim. 26, 5-11 (1922). (4) Carbide and Carbon Chemicals Co., South Charleston, IT.I'a., unnublished method. (5) Goswami, M., Das-Gupta, H. S.. and Ray, K. L., J . I n d i a n Chem. Soc:12, 714-18 (1935). (6) Goswami, AI., and Das-Purkaystha, B. C., Ibid., 13, 315-22 (1 936). ~. -_,_
(7) Goswami, A l . , and Shaha. .L, Ibid., 14, 208-13 (1937). (8) Fernandes, J. B., Snider, L. T., and Riets, E. G., A x ~ L C . m x 23, 899-900 (1951). (9) Mitchell, J., Jr., "Deterinination of Carbonyl Compounds" in "Organic Analysis," vol. 1, p. 243, Interscience, Ken. York, 1953. (10) Mitchell, J., J r . , and Smith, D. A f . , ANAL. CHEM.22, 746-50 (1950). (11) Siggia, S., "Quantitative Organic Analysis via Functional Groups," 2nd ed., pp. 32-6, Wiley, Xew York, 1954. (12) Stuve, W., Arch. Pharna. 244, 540 (1906). RECEIVED for review June 17, 1955. Accepted September
24, 1955
