Polarographic determination of methyl methacrylate

Talanta.

1967. Vol.

14, pp. 643 to 653.

Pergamon

Press Ltd.

Printed

in Northern

Ireland

POLAROGRAPHIC DETERMINATION METHACRYLATE

OF METHYL

MILOSLAV PANTGEEK Public Health Station, Hradcc Kralove, Czechoslovakia (Received 21 October 1966. Accepted 19 January 1967) Snmmary-The permanganate oxidation of methyl methacrylate in weakly acidic solution yields methyl pyruvate. Hydroxylamine hydrochloride is used for destroying the excess of permanganate. At the same time it is consumed for oximation of the pyruvate, and the resulting oxime is determined polarographically. The reaction scheme of permanganate oxidation of methyl methacrylate is suggested and optimum working conditions are found both for permanganate oxidation and for polarographic determination. The method is used for the determination of vapours of methyl methacrylate in the air, for industrial hygiene purposes.

methods for the determination of methyl methacrylate are based on spectrophotometry in the infrared1s2 or ultraviolet region,1*3 gas chromatography,4s5 polarography6* and on the addition of halogens,lJ’ mercaptan$O or mercury compoundP to the double bond. Indirect photometric methods are based on the photometry of the iron(III) salt of hydroxamic acid.12 Oxidation methods are based either on the complete oxidation of the ester to carbon dioxide and water-l3 with titrimetric determination of the remaining oxidation reagent, or on the fission of the double bond, the end carbon atom of the double bond yielding formaldehyde; either the remaining oxidation reagent14 or the liberated formaldehyde15 is then estimated. This paper deals with the mechanism and analytical application of the permanganate oxidation of methyl methacrylate in a slightly acidic solution. ANALYTICAL

EXPERIMENTAL Apparatus For the polarographic determination Heyrovskfs recording polarograph type V301 and reflecting ealvanometer tvnc Z9 of sensitivitv 1.4 x lo-OA were used. The samples were analvsed under &rogen in shag cells with a needlettype SCE and Smolef’s dropping me&y electrode: The flowrate of mercury was 0.55 mg/sec. The drop-time, measured in a 064 x lo-“M solution of methyl pyruvate oxime, at 1.2 V and at 40.3 cm mercury height was 3.00 sec. For the estimation of pH, a Beckmann compensation pH-meter was used. Reagents For the preparation of hydrochloric acid-potassium chloride buffers, 50 ml of 0.2&f potassium chloride were mixed with different amounts of 02M hydrochloric acid and made up to 200 ml with distilled water. Walpole’s 0*2M acetate buffer solution : different amounts of 0.2M acetic acid were made up to 100 ml with 0*2M sodium acetate. The methyl pyruvate16 and methyl acetate” were prepared by standard methods. The esters and pyruvic acid were freshly distilled before use. Development of method In most experiments, measured amounts of standardized permanganate solution and acid or buffer solution were made up with water to 45 ml. To this oxidation mixture 05 ml of standard ester 643

644

MILOSLAVPANTMEK

solution was added. After a definite time, the oxidation was stopped by addition of 1 ml of acid hydroxylamine hydrochloride solution and the oxime obtained was determined polarographically. The maximum on the polarographic waves was suppressed by addition of 0.05 ml of 05% tylose solution. For determination of the amount of permanganate necessary for the oxidation, a titrimetric determination of unreacted permanganatel*JO was carried out in addition to the polarographic determination of the methyl pyruvate oxime. To a series of solutions of different permanganate content (total volume 125 ml), 25 ml of O.OSN sulphuric acid and 10 ml of aqueous methyl methacrylate solution (25 pmole) were added. Immediately after mixing, 5 ml of each solution were measured into a separate flask. After 24 hr at 20’ these 5-ml portions were analysed polarographically and to the remaining 20 ml of solution 10 ml of 0.5M manganese sulphate, 4 ml of 4N sulphuric acid, 30 ml of a saturated solution of sodium pyrophosphate, 30 ml of distilled water and 3 drops of 1% diphenylamine (in sulphuric acid) were added and they were titrated with O*lN hydroquinone, standardized with dir&ornate. The result was calculated for 5 pmole of ester. “1

l,,b-•

!

60

/O

YP ,

/

I I

/

I

/

E .--

I

I

/

40

:0’

0’

/ I’ ,

zoe-

, ‘-1

--

0*-------

,d' ___-e---e__ Q

z-0

b -200

I

I IO '2 min

I IO0

I I IOOO

FIG. I.-Rates of oxidation of equal amounts (0.82 mmole) of various substances. O-methyl methacrylate (0 mm); O-methyl pyruvate (70.7 mm); C-methyl lactate (0 mm); o-pyruvic acid (74-O mm). The heights of the polarographic waves (galvanometer sensitivity l/20) correspond to the amounts of oxime produced on oxidation at 22’C with @02N pennanganate in 0.005N sulphuric acid. The heights of the polarographic waves of the starting material are given in brackets. RESULTS

AND

DISCUSSION

The mechanism of methyl methacrylate oxidation To explain the oxidation mechanism of methyl methacrylate in slightly acid solution (0405X sulphuric acid), the end product of oxidation was isolated and identified, and the kinetics of oxidation of methyl methacrylate at low temperature, and of the oxidation of pyruvic acid, methyl pyruvate and methyl lactate were followed, for these compounds can be considered as possible products of the oxidation of methyl methacrylate. Hydroxylamine hydrochloride in a solution of 1M sodium hydroxide20 was used for the preparative oximation of methyl pyruvate. The product was extracted with ether, the ether was evaporated and the product was refined by sublimation twice at normal pressure, and dried over silica gel. The half-wave potential and the characteristic polarographic wave of this product are in accord with the properties of the oxime of the product resulting from the oxidation of methyl methacrylate. The melting point of the product is 73.3”. (Literature values : m.p. = 68-69” after crystallization from etherZ1 or m.p. 71” after crystallization from benzeneF2) Elemental

Polarographic

determination

of methyl methacrylate

I I b

I

I

I

FIG. 2.-Rate of oxidation of methyl methacrylate (l-21 mmole). Direct and derivative polarographic curves for the oxime are shown. Galvanometer sensitivity l/30; oxidation as for Fig. 1. u-5 set oxidation at 5’C; b, c, d, e-5,10, 30, 600 min oxidation at 21°C.

analysis: theoretical-C 41.03 %, H 6.03 %, N 11.96 %; found-C 40.8 %, H 6.2 %, N 12.3%. The methyl pyruvate oxime is hence the product which is electro-reduced in the polarographic determination. From the rate of oxidation of pyruvic acid, methyl methacrylate, methyl lactate and methyl pyruvate (Fig. 1) it is evident that neither methyl lactate nor pyruvic acid is formed in considerable amounts in the course of reaction. Pyruvic acid undergoes rapid and complete oxidation, whereas methyl pyruvate resists further oxidation, This difference in reactivity can be explained in the following manner : the methacrylates

646

MILCSLAVPANTWEK

cannot form a complex with manganese(III), the intervention of which is, according to Drummond and Waters,= the necessary condition for fission of a double bond. When methyl lactate undergoes quantitative oxidation the velocity of the reaction is much slower. The oxidation of methyl methacrylate to methyl pyruvate proceeds uia a compound of which the oxime has a half-wave potential O-3V more negative than that of methyl pyruvate oxime (Fig. 2). The sum of the heights of the polarographic waves of this compound and of methyl pyruvate oxime varies during the oxidation only slightly. Hence, both compounds have similar diffusion coefficients, molecular weights and chemical character. It is possible that the methyl ester of 2-hydroxy-2-formylbutyric acid may be this intermediate oxidation product. The suggested reaction scheme is as follows: CH,

CH, C==CH,

I

COOCH,

-=%

HOC-CH,OH%

CJA I

y”+_

I

‘H

HOC-

I

COOCH,

COOCH,

1;’

+ HCOOH --2e

’ COOCHs

i NH&H

I

V

COa

+ H,O

CH,

C==NOH COOCH,

The oxidation of other methacrylates (Fig. 3) and acrylates (Fig. 4) follows a similar scheme. From the oxidation of acrylates, glyoxalates result, which undergo further oxidation much more easily than pyruvates. This fact could be a basis for differentiation between pyruvates and glyoxalates or methacrylates and acrylates.

I

0 FIG. 3.-Rate

of oxidation of butyl methacrylate (0.41 mmole) at 22°C. Conditions as for Fig. 1.

Optimum conditions for oxidation of methyl methacrylate The time dependence of the effect of different permanganate excesses on methyl methacrylate at different pH values, temperatures and ionic strengths, was studied. The optimum pH for the oxidation is 2.70 (Fig. 5) or a mineral acid concentration of 0*005iV(Fig. 6); good results are also given by O.lM acetic acid. The investigation of the rate of reaction (Fig. 7) has shown that in a more acidic solution the reaction is

Polarographic

/ :

-/

it

FIG. 4.-Rate

60,

IO

determination

of methyl methacrylate

647

‘_ 30

of oxidation of methyl acrylate (0.82 mmole) at 22°C. Conditions as for Fig. 2.

1

FIG. 5.-Effect

of pH on degree of oxidation of methyl methacrylate (0.82 mmole) at 23°C. The heights of the polarographic waves (sensitivity l/20) refer to the oximes of the products obtained after 45 (0) and 75 (0) hr oxidation with 0.02N permanganate at various pH values. HCl/KCl buffers were used for the range of pH 1*22-3.05, and acetate buffer for pH 3.73~571.

not stoichiometric and further oxidation occurs. At higher pH the result is similar. Excess of permanganate promotes further oxidation (Fig. 8). The investigation of permanganate consumption and its dependence on the volume of the added permanganate shows that the six-electron oxidation that gives rise to methyl pyruvate is followed by the two-electron oxidation of the liberated formic acid to carbon dioxide and water. Whilst the first step of the oxidation is stoichiometric, the second step needs a great excess of permanganate for its completion (Fig. 9). Increasing the temperature of the reaction mixture accelerates not only the oxidation to the pyruvate but also the further oxidation, which is undesirable analytically (Fig. 10). The relationship of the temperature and the time required to achieve 97-100%

~~ILOSLAVPANTWEK

648

SO-

0.004

Fro.

0.008

0.012 Acidity,

0.016

0.020

N

6.-Effect of acid concentration on degree of oxidation of methyl methacrylate (0.82 mmole) by treatment with 0.02N permanganate for 4.5 hr at 25°C.

1 FIG. 7.-Rates

1

3

I

I

IO 30 1. min

I

I

IO0

ICQO

of oxidation of methyl methacrylate (@82 mmole) at 20°C with 0.02N pennanganate at different acidities.

0

Fro. S.-Effect

Y

I

I loo

200 KMnO,.

I

I

I

300 pmole

400

500

of excess of permanganate on degree of oxidation of methyl methacrylate (5 pmole) for 24 hr at 20°C in @OOSNsulphuric acid.

Polarographic determination of methyl methacrylate

649

FIG. 9.-Stoichiometry of the oxidation of methyl methacrylate (5 pmole) for 24 hr at 20°C with permanganate in O*OOSN sulphuric acid. O-oxime formed (galvanometer sensitivity l/30), epermanganate consumed.

I 3

FIG.

IO

1 30 100 1. mill

I 1000

IO.-Rate of oxidation of methyl methacrylate (0.82 mmole) with 0.02iV permanganate in O*OOSiV sulphuric acid, at different temperatures.

conversion is presented in Fig. 11. Ionic strength within the range 0-05-O-20 has no Higher ionic strength accelerates further oxidation of pyruvate. effect on the reaction.

Polarographic determination of methyl pyruvate oxime The determination of pyruvate can be combined with there moval of excess of oxidant. With the aid of hydroxylamine hydrochloride, the excess of permanganate is destroyed and the resulting methyl pyruvate is converted into oxime, which can be determined polarographically. The polarographic determination of pyruvate in the form of oxime is also advantageous, because it is more sensitive than

650

MILOSLAVPANTCEEK I

I IO

0

t 20

1

30

I

40

50

T. OC FIG. 11.-Inter-relationship of time and temperature needed for different degrees of oxidation of methyl methacrylate (042 mmole) by 0+02N permanganate in 04lO5N sulphuric acid. O-97%; O--99%; t400%

30/

-

0.6

o/O

0

I 2

I

,

,

\l6

3

4

5

PH

FIG. 12.-Effect

of pH on the polarography of the oxime of methy pyruvate (O-79 mmole). @4miting current ; O-half-wave potential. Polarograms were recorded immediately after preparation of the solutions, to avoid errors from hydrolysis of the oxime at pH ~2.8. polarographic determination of the pyruvate alone. While the former compound undergoes a four-electron polarographic reduction, the latter undergoes only a twoelectron reduction. At the dropping mercury electrode, the oxime is easily reduced in the protonated form:2k26 CH, H CH, C===NOH

+H+_

J&&i

I COOCH,

\ OH COOCHIl

I

Polarographic

determination

of methyl methacrylate

651

Generally, oximes are very weak bases and the pK of their positive ion, according to Bronsted’s theory, is less than unity. 25 For this reason, at pH >l, the kinetic current arising from the recombination of the cation contributes considerably to the total height of the polarographic wave. The dependence of the wave height on pH is represented in Fig. 12. At pH 4.4 a more negative polarographic wave appears,

I 0I

4o

20 NHPH,

FIG.

13.-Effect

40

I

I

60

60

m mole

of excess of hydroxylamine on limiting current for methyl pyruvate oxime (0.82 mmole) at various acidities.

caused by the reduction of uncharged molecules of the oxime. It is advantageous to work with a strongly acidic solution, where the excess of hydroxylamine does not interfere% and the oxime yields a simple diffusion current. To prevent the acidcatalysed hydrolysis of oxime,2s an excess of hydroxylamine is maintained in the solution (Fig. 13). The maximum on the polarographic wave may be suppressed by gelatin26 or tylose.25 Practical application

The need for a suitable method of determination of methyl methacrylate vapours in air for the purposes of industrial hygiene was the first stimulus for this work. First, it was necessary to seek a suitable method of absorption of methacrylate vapours from the air. The best absorption is given by permanganate solution, because of the immediate oxidation of absorbed ester. For this reason, the permanganate oxidation has been taken as a basis for the method. Laboratory control of the absorption process was achieved by means of aeration of the ester from a standard solution into two absorption vessels in series. At an aeration velocity of 0.4-1.2 l/min about 3.5 ‘A of the total amount of ester passes into the second absorption vessel. At velocities of 0~08-0~20l/min, an average of 2.8 % of the total amount of ester passes into the second absorption vessel. Interfering substances in this determination include aldehydes and ketones, and alcohols, esters and unsaturated compounds which can produce aldehydes and ketones by oxidation (Table I). In an oxidation lasting 100 min, the presence of acetaldehyde, benzaldehyde, acetone, ethanol, and esters of acetic acid does not interfere. These compounds can only interfere by consuming some of the permanganate used to oxidize the methacrylate. The interference of acrylates and styrene can be prevented by prolonged oxidation. The oxidation of formaldehyde is very slow and hence it interferes in this determination as also does methanol, from which formaldehyde arises by oxidation.

652

MILOSLAVPANTWEK TABLE I.-INTERFERENCE OF VARIOUS COMPOUNDS IN THE DETERMINATION OF METHYL ME'lFIACRYLATE

Compound

Amount, pmole x

Height of interfering polarographic wave, mm, after n min oxidation 0 1 10 100 1000 9.5

8.0

-

0

0

24.5 0 6.0 0 0 0

8.0 0 6.0 0 0 0

0.5 0 60 0 0 0

0

26.0

40.5

31.0

2.0

250 1.13

0 0

10.0 7.0

17.0 7.0

14.0 4.0

5.00

0

24.1

50.2

2.50

0

4.3

19.9

Formaldehyde

6.34

10.0

10.0

Acetaldehyde

5.00

0

0

Benzaldehyde Acetone Methanol Ethanol Ethyl acetate Butyl acetate Methyl acrylate Butyl acrylate Styrene Methyl methacrylate Butyl methacrylate

5.62 5.00 5.36 5.00 503 5.00

30.5 0 0 0 0 0

5.00

Interference after x min oxidation, % 0 100 1000

Evl’a* 0.64

15

12

-

-

0

0

-

0 -

52 0 0 0 0 0

0.9 0 11 0 0 0

0 -

0.89 0.83 1.05 0.52 0.94 0.70 1XMl -

0

60

3.8

0.47

3.0 0

0 0

54 34

11.6 0

0.34 0.52

52.0

49.0

0

100

94

0.49

24.5

23.4

0

94

90

040

The interference is expressed as a fraction of the height of the polarographic wave for an equal molar concentration of methyl pyruvate oxime. The last column records the values of Ells (vs. S.C.E.) of the polarographic waves of the oxidation products, in the presence of excess of hydroxylamine at pH 0.9. Recommendedprocedure for the determination of vapours of methacrylate in air Absorb the vapours of methacrylate in 5-10 ml of absorption solution (100 ml of O*lN potassium permanganate and 25 ml of O*lN sulphuric acid made up to 500 ml with distilled water) in an absorption vessel fitted with a fritted disc. The sampling velocity is about 1 l/mm. If the absorption solution is decolorized during sampling, take a new, smaller, sample. When the sampling is finished, wait 100 min (for temperature 5-28”) and then add 1 ml of reducing solution (2.5 % hydroxylamine hydrochloride in 1M hydrochloric acid) for every 5 ml of absorption solution. Record the polarographic curve, after the addition of 0.05 ml of 0.5 % tylose for every 6 ml of the sample, in an atmosphere of nitrogen, with a saturated calomel electrode, over the range 0.2-1.0 V. For calibration, use an aqueous solution of freshly distilled methacrylate, oxidized for 100 min with an equal volume of absorption solution containing double the designed amount of potassium permanganate and sulphuric acid. Alternatively, the methyl pyruvate oxime alone can be used for calibration, dissolved in a mixture of the absorption and reduction solutions (5 + 1). Acknowledgement-The author wishes to thank Dr. P. Zuman for hi interest in this work and for his critical review of this manuscript. Zusammenfassung-Die Oxidation von Methyhnethacrylat mit Permanganat in schwach saurer Lbsung liefert Methylpyruvat. Zur Zerstiirung tiberschtissigen Permanganats wird Hydroxylaminhydrochlorid verwendet. Gleichzeitig wird es zur Oximierung des Pyruvats verbraucht; das entstandene Oxim wird polarographisch bestimmt. Ein Reaktionsmechanismus ftir die Permanganatoxidation von Methylmethacrylat wird vorgeschlagen und die optimalen Arbeitsbedingungen fti Permanganatoxidation und polarographische Bestimmung angegeben. Die Methode wird zur Bestimmung von Methyhnethacrylatdiimpfen in der Luft zu industriehygienischen Zwecken verwendet.

Polarographic

determination

of methyl methacrylate

653

Resum&L’oxydation permanganique du methacrylate de methyle en solution faiblement acide donne du pyruvate de methyle. On utilise le chlorhydrate d’hydroxylamine pour detruire l’exc&s de permanganate. En meme temps, il est consomme pour l’oximation du pyruvate, et l’on dose polarographiquement I’oxime resultante. On suggere le schema reactionnel de l’oxydation permanganique du m6thacrylate de methyle et l’on a trouve les conditions de travail optimales tant pour l’oxydation permanganique que pour le dosage polarographique. On utilise la methode pour le dosage de vapeurs de methacrylate de methyle dans l’air, a des fins d’hygiene industrielle. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

J. Haslam, Chem. Age, 1954,71, 1297. R. T. Scheddel, Anal. Chem., 1958,30,1441. A. N. SabadaS and L. A. Igonin, Zavodsk. Lab., 1956,22,1324. E. A. Radell and H. C. Strutz, Anal. Chem., 1959,31,1890. J. Strassburger, G. M. Brauer, M. Tryon and A. F. Forziati, ibid., 1960,32,454. V. D. Bezuglii and V. N. Dmitrieva, Zh. Prikl. Wim., 1957, 30, 744. R. J. Lacoste, I. Rosenthal and C. H. Schmittinger, Anal. Chem., 1956,28,983. B. Matyska and K. Klier, Collection Czech. Chem. Commun., 1956,21, 1592. N. L. Nemirovskii and G. I. MeeroviE, Gigiena i Sanit., 1958,23, 83. D. W. Beesing, W. P. Tyler, D. M. Kurtz and S. A. Harrison, Anal. Chem., 1949,21,1073. K. J. Mallii and M. N. Das, Gem. Znd. London, 1959,162. V. Sedivec, Collection Czech. Chem. Commun., 1960,25,897. W. Deichmann, J. Znd. Hyg. Toxicol., 1941,23,343. S. D. Nogare, L. R. Perkins and A. H. Hale, Anal. Chem., 1952,24,512. C. E. Bricker and K. H. Roberts, ibid., 1949,21,1331. Organic Syntheses, Vol. 24, p. 72. Wiley, New York, 1944. Organic Syntheses, Vol. 26, p. 6. Wiley, New York, 1946. A. Berka and S. Hilgard, Mikrochim. Acta, 1966, 164. A. Berka, J. Vulterin and J. Zfka, Massanalytische Oxydations- und Reduktionsmetoden, Geest and Portig, Leipzig, 1964. M. Vei%a and J. GaspariE, Dikaz a identifikace orpnickjch lftek, p. 176. SNTL, Prague, 1963. R. Locquin, Bull. Sot. Chim. France, 1904 [3] 31, 1068. E. Sharratt and W. Wardlaw, J. Chem. Sot., 1936, 563. A. V. Drummond and W. A. Waters, ibid., 1955,497. H. Lund, Acta Chem. Stand., 1959,13,249. P. Souchay and S. Ser, J. Chim. Phys., 1952,49, C172. Z. Vodraika, Chem. Listy, 1952,46,210.