V O L U M E 2 8 , NO. 1, J A N U A R Y 1 9 5 6

shows tha 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 tha t -AHS increases in the reverse order. In choosing a .olvent, therefore, consideration must be given to 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\

69

\c:Klon LE:I)(:\lEVT

Thc authors n i d i to :irl;nowlctige support of t l i i a 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 Sys tem Tech.

( 2 ) La Mer, V. K., and Doivneq. H. C. , J . B m . Cheni. SOC. 53, 888

(3) Linde, H. W., Iiogerr. L. E., and Huiiie, D. S., =Ixar.. CHEM.

J . 26,170 (1947).

(1931).

2 5 , 4 0 4 (1953). (4) Lingane, J. J., Ib id . , 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., Compf . rend . 233, 648 (1951). (8) Vold. K. D., J . A m . Chenz. Soc . 59, 1515 (1937).

RECEI~-EIJ for 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. Va.

i method o f atialjsis has been deielopect for the deter- mination of aldehjdes, which is hased on the oxidation of alrlehj de 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 de 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.

COJ2MO9 prohleni iii organic malysis is the quantitative A analytical resolution of mixtures containing both the alde- hyde and ketone functions. Various hydroxylamine hydro- chloride proredures have been developed to determine the total c:trbonyl value of such a mixture, but they are not specific for :tldehydeP. Several methods have been proposed for the de- termination 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 o f aldehydes by some mercurimetric procedure have recomniended the method primarily for the estimtttion of formaldehyde (1, 2, 12). In addition, Ihiigault :nid Gros ( 3 ) have reported the de- termination of furfur;il, benzaldehyde, and piperonal, and Gos- rrsnii, 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 to the corresponding acid whereas mercuric ion is reduced to free mercury:

Previous investigators who ha\

ItCHO + K,FIgI, + 2IiOH+RCOOH + Hg” + 4KI + HZO

Both isolation ant1 nonisolation methods have been proposed for tmhe determination of the free mercury. In the authors’ opinion it is best to 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 to maintain the free mercury in a finrly divided state, thus promoting its reaction Ivith iodine.

The name, ”n~ercural reagent,” has becn coined to differeii- tiate the reagent from other potassium mercuric iodide prepara- tions 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 chlo- ride (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 to 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 to be due to ammonium ion in the reagent?, howver , it is not detrimental to the dfective- iiess 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 to 3 liters with addi- tional distilled water. .4dd 0.1 gram of mercuric iodide as a pre- servative arid shake vigorously for a few seconds.

Acetic Acid, analytical reagent grade Iodine, approximately 0 . l S Starch Indicator, 1 .OY0 solution Standard 0.1N Sodium Thiosulfate Methanol, commercial grade, Carbide and Carbon Chemicals

co . SAMPLING

Unless direct satnplc addition is specified, introduce thr sample into a tared 50-ml. volumetric flask containing 30 nil. of the re- quired solvent (methanol which has been neutralized to hromo- thymol blue iritlirator, or distilled water) using a hypodermic syringe fitted with a 3-inch needle and chilled if necessary to facilitate tranpfer. Stopper the flask and swirl t o effect solution. An acetaldehyde dilution must he allowed to stand for approxi- mately 15 minutes, with occasional venting to the atmosphere t o reach equilibrium before recording the gross weight. The gross weight of dilutions of other aldehydes may be determined im- mediately. 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

70

be exercised to shake the flask vigorously a t once to intimately mix the contents and prevent localized side reactions.

A N A L Y T I C A L C H E M I S T R Y

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 reac- tion 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 to 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 to 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

Compound Acetaldehyde Acetaldol Acrolein Benzaldehyde Butyraldehyde 2-E thylbu tyraldehyde Formaldehyde Glutaraldehyde Hexaldehyde Isobutyraldehyde Methacrolein Propionaldehyde

Maximum Sample Reaction Size for Pure Time, Material, G.a Min. b

0.66 1.3 0.84C 1.6C 1 . 1 0.15' 0.45 0.75 0.15e 1. 1 c 0.9oc 0.87

5 to 60 5 to 606

180 t o 240d 15 to 60d 30 to 60 15 to 60/ 1 t o 60 15 to 60 30 t o 60f 5 to 60d 15 to 6Od 15 to 60

a Use. distilled water as a solvent in the sample dilution unless otherwise specihed.

b Minutes a t room temperature unless otherwise specified. C Use methanol, which has been neutralized to bromothymol blue indicator,

as the dilution solvent. d Minutes in a wet-ice ba th (0' to 3' C . ) . e Add the sample directly t o the sample flask, stopper, and immediately

diake the contents rigorously b y hand for 1 minute prior to the mechanical s ha kinp.

I Miniites on a mechanical shaker.

specified. b Minutes a t room temperature unless otherwise specified. C Use methanol, which has been neutralized to bromothymol blue indicator,

as the dilution solvent. d Minutes in a wet-ice ba th (0' to 3' C . ) . e Add the sample directly t o the sample flask, stopper, and immediately

diake the contents rinorouslv b v hand for 1 minute prior to the mechanical . . s ha kinp.

I Miniites on a mechanical shaker.

If the sample contains acetaldehyde, allow the flasks to stand a t Foom temperature for approximately 15 minutes before proceed- ing. 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 to fill the pipet. Stopper each flask and shake vigorously until all of the gray mercury precipi- ta te 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 to the disappearance of the blue color, approach- ing 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:

- C H O ~ H g o ~ 1 2 ~ S 1 0 ~ - -

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 speak- ing, no two authors use the same formulation. Hoxever, i t was found a t this point that Sessler's reagent would not quanti- tatively oxidize most aldehydes.

A study of the reagent was therefore initiated to determine its optimum composition Experiments were conducted to determine the effect of the following variables: concentration of potassium mercuric iodide complex, concentration of potassium

hydrouide, and ratio of potassium iodide to mercuric chloride. In 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 might afforded a quantitative oxidation of acetaldehyde. Higher per- centages 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 to 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 an 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. Water- soluble 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 satis- factory. Its exact use depends on the particular aldehyde, but the usual procedure is to employ neutralized methanol as a dilu- tion solvent and conduct the reaction a t the temperature of a wet-ice bath ( O o to 3" C.) to prevent any oxidation of the meth- anol. In some instances direct addition of sample to reagent, accompanied by shaking, is the best procedure. The most suit- able 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

Comparable data on the puritj- of a number of aldehydes were obtained by the mercural procedure and a hydroxylamine hydro- chloride-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 Purity by Mercural Hydroxylamine

Procedurea, Procedureb, Compound % %

Acetaldehyde 9 8 . 9 zt 0 .3 ( 5 ) 98 .9 zt 0 . 3 (4) .4cetaldol 101.5 zt 0 . 2 (2) 101.6 * 0.3 (3)

99.0 * 0 .0 ( 2 ) Acrolein 9 8 . 8 + 0 . 3 ( 5 ) Benzaldehyde 9 5 . 3 zt 0 . 2 (8) 9 5 , 3 i. 0 . 2 ( 5 ) Butyraldehyde 9s 0 I O . 5 (11) 97 .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) 3 5 . 7 * 0.1 (2) Glutaraldehyde 28 .3 zt 0 . 0 5 (4)

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 97 .1 =t 0 .0 (4) 9G.8 i 0 . 4 ( 5 )

Hexaldehyde 95 4 * 0 2 (4) 9 4 . 7 ' 1 0 . 3 ( 2 )

a Fiarires in parentheses represent number of determinations 3 H~droxylamine hydrochloride-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.

V O L U M E 28, NO. 1, J A N U A R Y 1 9 5 6

The procedure has been modified and found suitable for the determination of trace amounts of aldehydes in organic coni- pounds. For example, determinations of acetaldehyde in ethyl- ene 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 pre- cipitate has completely reacted. Titrate the excess iodine x i th standard 0.1S sodium thiosulfate until the brown color begins t o fade. Add a few milliliters of starch indicator solution ant1 con- tinue to titrate t o the disappearance of the blue color.

A synthetic sample prepared to contain 0.037% propioiialde- hyde 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%. In addition, comparative analyses of acetaldehyde in ethylene oxide were performed on tn.0 synthetic samples by the mercural procedure and a sodium bisulfite- iodine method (4). One sample contained 10 & 4 p.p.ni. acetal- dehyde by the mercural procedure, whereas the sodium bisulfite method gave 5 5 -1 p.p.m. A second sample nas 61 =t 6 p.p.m. acetaldehyde by the mercural procedure and 60 f 10 pp .m. using bisulfite.

71

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" to 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 i t 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 in- dicate that, the oxidation of alcohols by mercural reagent follows the mass action law, enabling one to compensate for this delete- rious reaction by using a reagent diluted 50 to 50 with distilled water, adding a similar amount of alcohol t o the blank, and per- forming the oxidation in a wet-ice bath, allowing a suitably longer reaction time to 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 pro- cedure 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 to 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. There- fore, 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):

H g + - + 2CHa-C-CHz 5 Hg(CH3-C=CH,), + 2fI- It 0 0

I

I n the presence of the alkaline reagent and excess mercuric ions this equilibrium reaction is displaced to the right, depositing the mercuric ion-acetone complex as a yellon solid. On acidifics- tion the reaction is reversed, proceeding to the left. This re- versal 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 resinificx- tion of the mercuric ion-acetone complex, hence greater solubility difficultim are experienced at 0" to 3" C. than a t room tempera- ture.

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 to 3.0 grams of acetone were added to each flask. With the addition of up to 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 to effect solution. Hence, the acetone tol- erance of this method is approximately 0.3 gram for determina- tions performed in a wet-ice bath.

Because a portion of the mercuric ion contained in 50 ml. of reagent is complexed by 0.3 gram of acetone, i t was then neceb- sary to prove that a sufficient amount of reagent was still avail- able 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 to 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 to 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 com- plexed.

LITERATURE CITED

(1) Alexander, E. R., and Underhill, E. J., J . Am. Chem SOC. 71,

(2) Bolle, J., Jean, J., and Jullig, T., MBm. sertices chim. &at (Paris)

(3) Bougault, J., and Gros, R., J . pharm. chim. 26, 5-11 (1922). (4) Carbide and Carbon Chemicals Co., South Charleston, IT. I'a.,

4014-19 (1949).

34, 317-20 (1948).

unnublished method. (5) Goswami, M., Das-Gupta, H. S.. and Ray, K. L., J . Ind ian

(6) Goswami, AI., and Das-Purkaystha, B. C., Ibid. , 13, 315-22 Chem. Soc:12, 714-18 (1935).

(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., Jr . , 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