Iodometric Estimation of Acetylacetone in Free Form and in Complexes

MICROCHEMICAL JOURNAL ARTICLE NO.

57, 65–72 (1997)

MJ971491

Iodometric Estimation of Acetylacetone in Free Form and in Complexes La´szlo´ J. Csa´nyi1 and Ka´roly Ja´ky Institute of Inorganic and Analytical Chemistry, A. Jo´zsef University, P.O. Box 440, Szeged, Hungary Received March 3, 1997; accepted March 15, 1997 DEDICATED TO PROFESSOR PETR ZUMAN ON THE OCCASION OF HIS 70TH BIRTHDAY

The iodoform reaction is recommanded for the estimation of acetylacetone. In 20 ml moderately alkaline buffer solution, 0.04–0.40 mmole acetylacetone is oxidized with 15–20 ml 0.2 N KI3 solution at a sufficiently slow delivery rate (0.03 mlrs01 or less) under efficient stirring during addition of the reagent. Subsequently, the solution is acidified with 10 ml 3.3 N hydrochloric acid and the excess iodine is back-titrated with thiosulfate. The method is also suitable for determination of the acetylacetone content of metal complexes (10–70 mg). q 1997 Academic Press

INTRODUCTION

Acetylacetone is a well known and widely used chelating agent. Many of its metal complexes are often applied as efficient oxidation catalysts. It is an appropriate reagent for the spectrophotometric determination of the given metal ions. Nevertheless, no simple titrimetric method is available for the estimation of acetylacetone. Acetylacetone Like other 1,3-dioxo compounds in which a methylene group is situated between two carbonyl groups, acetylacetone is capable of oxo–monoenol tautomerism. Enol formation takes place readily (to ca. 80%) because the double bond formed on Hatom transfer is conjugated with the other carbonyl group: C©CH‹ CH‹©C©CH¤©C©CH‹ O

CH‹©C©CH®C©CH‹ O

O

HC

O

C

H

HO CH‹

O

The monoenol form of the molecule is further stabilized by H-bond formation between the enolic H and the O-atom of the carbonyl group, forming a stable 6-membered ring. (It should be mentioned that the dienol derivative is not formed, because the cumulated double bond system is less favorable thermodynamically.) In the presence

1

To whom correspondence should be addressed. 65 0026-265X/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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of alkali, the H-bond is broken through proton dissociation, but the enolate ion thus formed is nearly as stable as the H-bonded ring. It is known that acetylacetone splits into acetone and acetate in alkaline medium. To confirm this reaction, 0.388 mmole acetylacetone was added to 10 ml borate buffer of pH 12.9 and the acetone formed by hydrolysis was flushed out by bubbling pure N2 through the solution. The receiver was similarly filled with buffer. After 60 min, 0.0023 mmole (0.6%) acetone was found, which means that the extent of hydrolysis of acetylacetone at room temperature in moderately alkaline solution is neglibible. At 1007C, more than 70% acetone formation was observed under similar experimental conditions. Haloform Reactions It has long been known that compounds containing CH3-CO- or CH3-CH-(OH)- groups undergo the haloform reaction with halogens in alkaline medium (1). Accordingly, it is to be expected that acetylacetone will behave similarly and that this will offer a means for its volumetric estimation. The product of halogenation is an unsymmetrically trihalogenated species, which splits quickly into the corresponding haloform compound (CHX3) and a carboxylic acid derivative. These findings are briefly reported in this paper. In alkaline solution, the halogen molecules hydrolyze: X2 / 2 OH0 ` OX0 / X0 / H2O. For these reactions, the equilibrium constants are all favorable, 7.5 1 1015 for chlorine, 2 1 108 for bromine, and 30 for iodine (2), and the reactions are all rapid. The situation, however, is complicated by the tendency of the hypohalite ions to disproportionate in basic solution to produce halate ions: 3 OX0 ` 2X0 / XO30. For these reactions, the equilibrium constants are very favorable, 1027 for ClO0, 1015 for BrO0, and 1020 for IO0. Thus, the actual products obtained on dissolving halogens in base depend on the rates at which the hypohalite ions initially produced undergo disproportionation, and these rates vary from one to the other and with temperature. The disproportionation of ClO0 is slow at room temperature. The rate of bromate formation is moderately high, even at room temperature. The rate of disproportionation of IO0 is very high at all temperatures, so hypoiodite exists in solution only transitorily. Since the haloform reaction occurs only in alkaline medium, the corresponding hypohalite must be the active species in the halogenation reaction. With regard to the data on the hydrolysis of the halogens and the disproportionation of the hypohalites, chlorine appears to be the most promising reagent for the halogenation of diketones. Although the highest concentration of hypohalite can be obtained with chlorine, from an analytical aspect it is not advisable to perform the haloform reaction with chlorine due to its high volatility. The haloform reaction in analysis. The iodoform reaction was first used by Lieben (3) for the determination of acetone. This method was modified by Kra¨mer (4) into a gravimetric procedure by extracting the iodoform formed with diethylether, evaporat-

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ing off the solvent, and weighing the iodoform residue. Somewhat later a volumetric method was recommended by Messinger (5), who used the same reaction and backtitrated the excess iodine with thiosulfate. Years later, many papers were published which criticized the Messinger method with regard to its precision and accuracy (6, 7). Collischonn (6) and Goodwin (7) investigated the Messinger method in detail and found that this method furnished reliable and confident data when the following conditions were fulfilled: (i) the samples should be shaken efficiently during the addition of the iodine solution, (ii) the iodine solution should be added slowly, (iii) a sufficiently long time should be allowed for the quantitative formation of iodoform, and (iv) during the backtitration of the excess iodine, too high a concentration of acid should be avoided. Some 20 years later, it was confirmed by Haughton (8) that the Messinger method is reliable for the estimation of acetone. He observed an iodine overconsumption (more than 6.0 equivalents) when the oxidation of acetone was carried out in 1 M sodium hydroxide solution. He considered that the oxidation by iodine takes place to a smaller extent according to another stoichiometry (with a higher consumption of iodine), CH3-CO-CH3 / 10 I / 5 NaOH r 2 CHI3 / HCOONa / 4 NaOH, and he detected the formation of some formate. MATERIALS AND METHODS

Acetylacetone (Hacac, Merck for chromatography grade) was distilled twice at atmospheric pressure. Acetylacetonato metal complexes of c. p. grade (Aldrich, Fluka, and Koch–Light Lab. Ltd.) were used after recrystallization from chloroform in the presence of some free acetylacetone. A 0.2 N KI3 reagent was prepared by dissolving 25.4 g pure iodine and 66.4 g KI in about 200 ml distilled water and then diluting to 1000 ml. Buffer solution: 40 g Na2B4O7r10 H2O and 23 g NaOH were dissolved in 1000 ml distilled water. Speed of titration. The 0.2 N KI3 reagent was added slowly by using a Metrohm Multidosimat E 415 fitted with a 20 ml syringe buret. The Dosimat was actuated with a microprocessor which allows delivery of the reagent at the desired rate. RESULTS AND DISCUSSION

Estimation of Acetylacetone Haughton (8) observed that the oxidation of acetone with iodine led to the formation of somewhat more than one molecule of iodoform (6.15 meq iodine/mmole acetone) when 20 ml N NaOH was applied in the sample. Accordingly, the stoichiometry of the oxidation of acetylacetone with iodine was investigated primarily as a function of the pH. The relevant data in Table 1 show that at pH õ 9 little iodoform is formed. In a borax solution at pH 9.46, at a delivery rate of 0.095 mlrs01, and after a 20-min waiting period following the addition of iodine, a stoichiometric factor of 3.972 was obtained, but this increased to 4.684 when the waiting time was prolonged to 240

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TABLE 1 Dependence of the Stoichiometry of the Iodoform Reaction of Acetylacetone on pH pH

Base

(mmol)

Na-acetate HaHCO3 Na2HPO4 Na2B4O7 Na2B4O7 Na2B4O7 / NaOH Na2B4O7 / NaOH NH4OH NH4OH NaOH NaOH NaOH NaOH NaOH NaOH

7.5 7.5 7.5 7.5 7.5 7.5 15.0 14.7 14.7 7.5 10.0 20.0 30.0 50.0 70.0

before after addition of KI3 8.34 8.29 9.17 9.49 9.49 12.96 12.95 11.12 11.07 13.60 13.71 13.94 ú14 ú14 ú14

7.88 8.30 9.16 9.46 9.46 11.62 11.52 10.45 10.38 12.81 12.97 12.26 — — —

Consumption of iodine (mequiv)

Stoichiometric factor

Hacac (%)

0.046 0.126 0.259 0.771 1.165 1.164 1.165 1.164 1.163 1.177 1.220 1.220 1.213 1.175 1.037

0.238 0.648 1.332 3.972 4.684a 5.998 6.000 5.994 5.990 6.061 6.180 6.284 6.249 6.052b 5.339c

3.97 10.97 22.20 66.20 78.10 99.96 100.00 99.90 99.83 101.02 103.00 104.73 104.15 100.86 88.98

Note. Conditions: the given quantities of bases were dissolved in 20.0 ml distilled water and 20.0 ml (0.1941 mmole) acetylacetone and 15.00 ml 0.2 N KI3 solution were added in small increments (0.095 ml s01). The flask was next stoppered and, after 20 min standing, the solution was acidified with 10 ml 3.3 N hydrochloric acid and the liberated iodine was titrated with 0.1 N thiosulfate. In measurement denoted by a the waiting time was 240 min. In measurement denoted by b 20 ml 3.3 N HCl was applied. In measurement denoted by c 30 ml 3.3 N HCl was applied.

min. This indicated that at that pH a considerable quantity of free iodine remained (due to the slower disproportionation of hypoiodite to iodate) and therefore the iodoform formation could progress during the waiting period. When the alkalinity of the solution was further increased, iodoform formation was enhanced and the expected value of 6.00 equivalents/Hacac mmole was reached according to the equation CH3-CO-CH2-CO-CH3 / 3 OI0 r CHI3 / 2 CH3COO0 / OH0. However, when the pH of the titrated solution exceeded 13, the stoichiometric factor (consumption of iodine (in mequ)/mmole Hacac) exceeded 6.00. Such an overconsumption of acetylacetone could be observed up to 2 M sodium hydroxide; while at even higher alkalinities, the factor dropped below 6. This means that experimental conditions resulting in the formation of 2 molecules of iodoform during the iodination of one molecule of acetylacetone could not be found. Therefore, the use of a moderate base concentration (borax buffer or ammonia) is indispensable. To simplify the procedure, an attempt was made to omit the waiting time by slowing down the iodine addition. It was found that, when the iodine solution was added at a lower delivery rate, e.g., 0.03 mlrs01, correct results were obtained without any waiting period after the addition of iodine.

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TABLE 2 Iodometric Estimation of Acetylacetone in Free Form and in Chlorobenzene Solution Hacac taken (mmol)

Chlorobenzene (ml)

Delivery rate (ml s01)

Consumption of I2 (meq)

Stoich. factor

Hacac (%)

0.0484 0.0484 0.0969 0.0969 0.1453 0.1938 0.1938 0.2906 0.2906 0.1942 0.1942 0.2912 0.2912 0.3703 0.3703 0.1938 0.1938 0.0969 0.0969 0.1938 0.1938 0.3875 0.3875 0.3875

— — — — — — — — — — — — — — — 1.00 1.00 5.00 5.00 10.00 10.00 20.00 20.00 20.00

0.095 0.095 0.095 0.095 0.095 0.095 0.095 0.095 0.095 0.031 0.031 0.031 0.031 0.031 0.031 0.095 0.095 0.095 0.095 0.095 0.095 0.095 0.095 0.095

0.2905 0.2905 0.5810 0.5819 0.8708 1.1621 1.1628 1.7411 1.7396 1.1651 1.1631 1.7497 1.7467 2.2190 2.2200 1.1638 1.1617 0.5814 0.5809 1.1628 1.1628 2.3054 2.2971 2.3084

5.997 5.997 5.997 6.006 5.992 5.997 6.001 5.990 5.985 6.001 5.990 6.007 5.997 5.991 5.994 6.006 5.995 6.000 5.996 6.001 6.001 5.949 5.928 5.957

99.95 99.95 99.95 100.10 99.87 99.95 100.01 99.84 99.75 100.01 99.84 100.12 99.95 99.86 99.90 100.10 99.93 100.00 99.93 100.01 100.01 99.15 98.82 99.28

Note. After addition of KI3 reagent solution at the higher delivery rates the samples were put aside for 20 min in the dark to complete the iodoform reaction, then back titrated with thiosulfate. At lower delivery rates, the back titration of iodine was carried out immediately after the addition of KI3 solution. In the presence of an immiscible solvent efficient stirring is indispensable.

Recommended procedure. To 20 ml buffer solution is added 0.04–0.40 mmole acetylacetone and a measured quantity of 15–20 ml 0.2 N KI3 reagent solution is introduced at a delivery rate of 0.031 mlrs01 or slower, while the sample is stirred efficiently with a magnetic stirrer. The solution is then acidified by adding 10 ml 3.3 N hydrochloric acid and the excess iodine is backtitrated with 0.1 N thiosulfate, using starch solution as indicator. A quantity of 1.00 ml 0.1 N thiosulfate is equivalent to 1.666 1 1002 mmole acetylacetone. The usefulness of the recommended procedure can be judged from the data in Table 2. The determination of acetylacetone with KBr3 reagent solution was also attempted. These measurements, however, furnished rather scattered data: the stoichiometric factor varied between 5.1 and 8.2. When a higher KBr concentration was used to reduce the volatility of bromine, the variation in the stoichiometric factor was somewhat reduced, but useful analytical data could still not be obtained. Thus, the bromination reaction is not recommended for the volumetric determination of acetylacetone.

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Determination of Ligand Contents of Acetylacetonato Metal Complexes This investigation was initiated by the need for an appropriate method for determination of the ligand contents of several catalytically active transition metal acetylacetonato complexes. Here, the analyses of the vanadyl, nickel(II), copper(II), cobalt(II), iron(III), and cobalt(III) complexes of acetylacetone are described. The data obtained are compiled in Table 3. As an exception, the vanadyl ion is amphoteric enough for dissolution in the recommended buffer solution. Consequently, acetylacetone content can be estimated without any change in the procedure given above. The oxidation state of vanadyl ion does not change during oxidation with iodine. The vanadium content of the complex was determined by a literature method (9). Ni(acac)2 samples were dissolved in 0.5 ml 20% sulfuric acid and, after addition of 20 ml buffer and some sodium hydroxide (equivalent to the applied acid), were oxidized with KI3 reagent. Nickel(II) behaves neutrally from a redox aspect, so no correction is necessary to obtain the acetylacetone content. Cu(acac)2 samples can be added immediately to the buffer solution. It was observed that the copper(II) is readily transformed into CuI and iodine when the KI3 reagent is added to the solution. The quantity of iodine liberated by copper(II) ions should be added to the iodination consumption to obtain a correct acetylacetonate content. The copper(II) content was determined by complexometry (10). Co(acac)2 samples were similarly dissolved in 0.5 ml 20% sulfuric acid and, after alkalization (buffer solution and equivalent sodium hydroxide), the titration were performed with KI3 . Since cobalt(II) is oxidized quantitatively by dissolved oxygen, the iodine liberated by the cobalt(III) formed should be added to the iodine consumption. For this correction the cobalt content was determined by complexometry (11). Fe(acac)3 was analysed analogously to the cobalt(II) complex and the iron(III) was determined by EDTA according to the prescription in (12). Co(acac)3 samples (10–70 mg) were dissolved in 1.0 ml glacial acetic acid, 1.0 ml 20% sulfuric acid was then added and the solution was warmed until the greenishblue color had turned faint pink. Note that without the preliminary reduction of cobalt(III) with water the acetylacetone cannot be iodinated quantitatively, due to the high stability of the cobalt(III) complex. During heating the reaction vessel was fitted with a small reflux condenser cooled with water at 107C to prevent the loss of acetylacetone. After heating, the condenser was washed thoroughly with 20 ml buffer solution and the necessary sodium hydroxide (equivalent to the quantity of acids applied) was added, and oxidation with 15–20 ml KI3 was started at a delivery rate of 0.03 mlrs01. After acidification with 10 ml 3.3 N hydrochloric acid, the excess iodine is backtitrated with 0.1 N thiosulfate. In alkaline medium cobalt(II) is oxidized to cobalt(III) by dissolved dioxygen. Therefore, the total iodine consumption should be corrected by the iodine quantity liberated by cobalt(III). The cobalt(III) content was determined by titration with EDTA (11) in a separate sample dissolved similarly as above. CONCLUDING REMARKS

The experiments convincingly demonstrated that the iodination of acetylacetone furnishes adequate results only when the iodine reagent solution is introduced at

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HALOGENATION AND DETERMINATION OF ACETYLACETONE TABLE 3 Determination of Ligand and Metal Ion Contents of Some Metal Acetylacetonato Complexes Taken (g)

Consumption of I2 (mequiv)

Found [acac] (mmol)

0.0112 0.0200 0.0303 0.0400 0.0522

0.5063 0.8949 1.3620 1.7969 2.3386

0.0844 0.1491 0.2270 0.2995 0.3897

Found [metal] (mmol)

[acac] [metal]

— — — — —

0.0422 0.0754 0.1142 0.1507 0.1960

2.000 1.977 1.988 1.987 1.988

— — — — — —

0.0455 0.0753 0.1066 0.1102 0.1446 0.1791

2.001 1.992 2.005 2.008 1.998 1.996

0.0763 0.1554 0.2256 0.3068 0.3970

0.0381 0.0759 0.1133 0.1534 0.1984

2.002 2.047 1.991 2.000 2.001

0.0791 0.1443 0.2237 0.3022 0.3116 0.3772

0.0397 0.0722 0.1119 0.1513 0.1552 0.1880

1.992 1.998 1.999 1.997 2.008 2.006

0.0913 0.1840 0.2400 0.3493 0.3502 0.4183

0.0304 0.0611 0.0801 0.1163 0.1168 0.1394

3.003 3.011 2.996 3.003 2.998 3.001

0.0991 0.1745 0.2659 0.2658 0.3525 0.4259 0.5642

0.0330 0.0581 0.0886 0.0886 0.1174 0.1420 0.1887

3.003 3.003 3.001 3.000 3.002 2.999 2.990

Corrected [acac] (mmol)

VO(acac)2

Ni (acac)2 0.0128 0.0212 0.0300 0.0310 0.0407 0.0504

0.5465 0.9044 1.283 1.3286 1.7347 2.1458

0.0910 0.1507 0.2137 0.2214 0.2890 0.3575 Cu(acac)2

0.0100 0.0199 0.0297 0.0402 0.0520

0.4198 0.8567 1.2402 1.6872 2.1835

0.0700 0.1428 0.2067 0.2812 0.3639 Co(acac)2

0.0103 0.0187 0.0290 0.0392 0.0402 0.0487

0.4349 0.7933 1.2302 1.6621 1.7144 2.0752

0.0725 0.1322 0.2050 0.2770 0.2857 0.3459 Fe(acac)3

0.0108 0.0217 0.0311 0.0413 0.0415 0.0495

0.5174 1.0428 1.3596 1.9793 1.9843 2.3704

0.0862 0.1738 0.2266 0.3299 0.3307 0.3950 Co(acac)3

0.0118 0.0208 0.0317 0.0317 0.0420 0.0508 0.0675

0.5615 0.9887 1.5070 1.5060 1.9974 2.4135 3.1966

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0.0936 0.1648 0.2512 0.2510 0.3329 0.4022 0.5328

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sufficiently reduced rates. Therefore, it is to be expected that the reagent-generating coulometry will be a suitable method for the determination of acetylacetone on a semimicro scale. The results of our orienting experiments appear to support this expectation. The relevant data will be published in the near future. ACKNOWLEDGMENTS This work was supported by a grant from the Hungarian Research Foundation (OTKA T 16138/1995).

REFERENCES 1. March, J. Advances in Organic Chemistry, 4th ed., pp. 587–590, Wiley, New York, 1992. 2. Cotton, F. A.; Wilkinson, G. Advances in Inorganic Chemistry, 5th ed., pp. 563–568, Wiley–Interscience, New York, 1968. 3. Lieben, H. Ann. Chem. Pharm., Suppl. VII, 1870, 218. 4. Kra¨mer, G. Chem. Ber., 1880, 13, 1000. 5. Messinger, J. Chem. Ber., 1888, 21, 3366. 6. Collischonn, F. Z. Anal. Chem., 1890, 29, 562. 7. Goodwin, L. F. J. Am. Chem. Soc., 1920, 42, 39. 8. Haughton, C. O., Ind. Eng. Chem. Anal. Ed., 1992, 9, 587–590. 9. Budevska, O.; Johnova, L. Talanta, 1965, 12, 291. 10. Cheng, K. L.; Bray, R. Anal. Chem., 1955, 27, 782. 11. Flaschka, H. Mikrochem. Ver. Mikrochim. Acta, 1952, 39, 38. 12. Kinnunen, J.; Merikanto, B. Chem. Anal., 1954, 43, 13. 13. Flaschka, H. Mikrochim. Acta, 1954, 37, 361.

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