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283
ANALYTICAL EDITION
If a straight line is drawn from a particular excess of iodine and working temperature of the experiment, it will intersect the correction scale. When the correction indicated on this scale is applied to the iodine value obtained experimentally under laboratory conditions, one gets directly the iodine value at 250 cg. excess of iodine and 20" C. It is understood that the time of contact is to be exactly 1 hour.
Working temperature, 32" C. Time of contact in the dark, 1 hour When 223.5 on the excess scale and 32 on the temperature scale in the nomograph are joined by a straight line, the line will intersect the correction scale at -2.36. The iodine value at the proposed standard conditions is then equal to 170.1 - 2.36, or 167.7, which checks very well with the value calculated by the formula.
Example
The authors have carefully checked the data contained in their previous article (1). The iodine value calculated with their formula and those read off from this nomograph are tabulated together in Table I for comparison.
Oil taken, 0.1505 gram Sodium thiosulfate in blank titration, 46.86 CC. Sodium thiosulfate used for the test, 26.61 cc. Difference, 20.25 cc. Strength of sodium thiosulfate solution, 1.264 cg. of iodine per cc. Experimental iodine value, 20'25 X 1.264 = 170.1 0.1505 26 61 Excess of iodine, 170.1 X -= 20.25 223.5 cg. of iodine per gram of oil
Literature Cited (1) Ho, K., Wan, C. S., and Wen, S.H., IND.ENG.CHEW,Anal. Ed., 7, 96-101 (1935). RJCEIVED February 20,1936. Published with the permission of C. Y. Wang, Commissioner of Government Testing Bureau, Ministry of Industry, Hankow, China.
Quantitative Determination of 5-Methyl Furfural HAROLD A. IDDLES AND KENDRICK S. FRENCH, University of New Hampshire, Durham, N. H.
I
N T H E production of furfural by the distillation of various
pentose- and pentosan-containing natural products with mineral acids, i t would be expected that any methyl pentoses and methyl pentosans would yield 5-methyl furfural in an analogous manner and the polysaccharides which hydrolyze to hexoses would give rise to small amounts of p-hydroxy &methyl furfural. The effect of these compounds, particularly 5-methyl furfural, on the quantitative determination of furfural was first studied by Votocek (18), who prepared a pure sample of 5-methyl furfural from rhamnose, with which precipitations with phloroglucinol were carried on to determine the ratio of product t o aldehyde employed. Later Ellet and Tollens (5) studied the relation between the quantity of phloroglucinol precipitate and the amount of rhamnose employed as a test sample in hydrochloric acid distillation. Fromherz (6) determined the furfural and methyl furfural in samples of wood by the production of the phloroglucides with a subsequent attempted separation of the mixed precipitate by using the alcohol solubility of the phloroglucides of 5-methyl furfural. Dox and Plaisance (3) questioned the reliability of this alcohol separation of the phloroglucides and recorded in their work the qualitative reaction of thiobarbituric acid with 5-methyl furfural but gave no quantitative data because of the limited amount of material on hand. Since it is now possible to prepare pure 5-methyl furfural in quantity according to the directions of Rinkes (Id), it seemed desirable to make a comparative study of the various gravimetric and volumetric methods for the determination of furfural when applied to pure 5-methyl furfural itself. In this direct study it is possible to eliminate the variables introduced when the calculations refer back to an original methyl pentose or methyl pentosan sample which has undergone an acid distillation. The methods selected for study were the phloroglucinol or A. 0. A. C. method (I), the thiobarbituric acid method (3,10, 17, 19), the 2, 4-dinitrophenylhydrazine method (8, IS), the volumetric bromidebromate titration method ($ 4,10-13,15), and the volumetric bromide-bromate titration a t 0" C. ( 7 ) .
Preparation of 5-Methyl Furfural The 5-methyl furfural was prepared according to the method of Rinkes (14)in which levulose, produced by acid hydrolysis of sucrose, is dehydrated to produce pchloro5-methyl furfural and the chlorine is replaced by hydrogen
by means of stannous chloride reduction. The resulting product was vacuum-distilled at 75" to 76" C. and 13 mm. pressure, yielding clear samples for analysis which showed a I L D (Pulfrich) of 1.53049 a t 20" C., and 1.52643 a t 25" C. G r a v i m e t r i c Methods PHLOROGLUCINOL METHODAND METHODOF CALCULATION. In the procedure as finally developed, a weighed sample of the pure redistilled 5-methyl furfural was diluted to 1 liter with distilled water and 5-, lo-, or 15-ml. aliquot portions were drawn from a buret into 200 ml. of 12 per cent hydrochloric acid solution. To this was added 0.33 gram of phloroglucinol in 50 ml. of 12 per cent hydrochloric acid solution, a quantity which is in excess of the amount necessary for the 5-methyl furfural present. After waiting 8 to 10 minutes for precipitation to begin, the solution was diluted to 400 ml. using 12 per cent hydrochloric acid and allowed to stand 16 t o 20 hours in the dark. Finally the precipitate was collected on a tared Gooch crucible, washed with 150 ml. of cold water, and then dried in a vacuum desiccator over a dehydrating agent to prevent the darkening and decomposition caused by drying in the oven at 100' C., ohich is the practice with furfural. I n the work of Fromherz (6) the ratio of the weight of precipitate to the weight of sample increased with the size of sample used. To substantiate these results, a series of determinations was made, using samples varying from 0.018 to 0.132 gram to determine the effect of sample size on the ratio of precipitate to sample. TABLEI. EFFECTOF SAMPLE SIZEON RATIOOF PRECIPITATE TO SAMPLE Weight of Sample Oram
0.0180 0.0226 0.0265 0,0451 0.0529 0.0541 0.0677 0.0794 0.0902 0.1058
Weight of Precipitate
Weight of Precipitate Weight of Sample
Cram
0,0182 0.0270 0.0345 0.0745 0.0897 0.0917 0.1201 0.1468 0.1686 0.2019
1.012 1.194 1.302 1.652 1.696 1.686 1.774 1.849 1.869 1.906
It can be seen from Table I that the ratio tends to increase regularly with the size of sample from 1.012 to 1.905. This increase may be explained by considering the solubility of precipitate to remain constant in the constant volume of
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284
TABLE11. GRAVIMETRIC DATA Methyl Furfural Used
Precipitant Used
Gram
Gam
Precipitate Gram
Phloroglucinol 0,0182 0.0184 0,0179 0,0267 0.0272 0.0270 0.0750 0.0741 0.0746 0.0897 0,0902 0.0893 0.1194 0.1203 0,1210 0,1475 0.1462 0.1424 0.1684 0,1688 0.1686 0.2024 0,2017 0,2016 0.2642 0,2564 0.2547
Methyl Furfural Found Gram
Method 0.0182s 0 0183 0,0181 0.0223 0.0225 0.0224 0.0454 0.0449 0.0462 0.0524 0.0526 0.0522
0,0180 0.0180 0.0180 0.0226 0.0226 0.0226 0.0451 0.0451 0.0451 0.0529 0.0529 0.0529 0.0677 0.0677 0.0677 0.0794 0.0794 0.0794 0.0902 0.0902 0.0902 0.1058 0.1058 0.1058 0.1322 0.1322 0.1322
0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33
0.0206 0.0206 0.0206 0.0237 0.0237 0.0237 0.0411 0.0411 0.0411 0.0474 0.0474 0.0616 0.0616 0.0616 0.0711 0.0711 0.0711
Av. Thiobarbituric Acid Method 0.0202 0.0434 0.15 0.0201 0.0431 0.15 0.0207 0.15 0.0444 0.0237 0.0508 0.15 0.0507 0.0236 0.15 0.0503 0.0234 0.15 0.0408 0.0875 0.15 0.0404 0,0866 0.15 0.0413 0.0886 0.15 0.0473 0.15 0.1015 0.0471 0.15 0.1011 0.0616 0.15 0.1321 0.1327 0.0619 0.15 0.1331 0.0621 0.15 0.1511 0.0704 0.15 0.1502 0.0700 0.15 0.1534 0.0715 0.15
0.0666 0.0670 0,0674 0.0800 0.0794 0.0776 0.0900 0,0902 0.0901 0.1063 0,1059 0,1059 0.1310 0,1321 0.1314
Methyl Furfural Recovered 15 2b 5% 5% 82.2 83.0 81.3 85.2 86.4 86.0 99.2 98.2 98.6 99.1 99.6 98.8 100.5 101.2 101.6 104.5 103.9 101.0 104.0 104.1 104.1 105.9 105.2 105.2 105.4 105.9 105.5 98.0
Av.
101.2 101.7 100.4 98.6 99.6 99.2 100.8 99.6 100.2 99.0 99.5 98.7 98.4 99.0 99.5 100.8 100.0 97.7 99.8 100.0 99.9 100.4 100.1 100.1 99.0 99.8 99.3 99.7
98.1 97.6 100.5 100.0 99.6 98.7 99.3 98.3 100.5 99.8 99.4 100.0 100.6 100.8 99.0 98.5 100* 8 99.5
2,4-DinitrophenylhydrazineMethod
M1. 101.7 101.7 100.9 100.6 100.6 101.2 100.0 100.9 100.9 100.6 100.0 100.0 Av. 100.8 Calculated by formula W = ~/I.o ( P h n X 0.000018). b Calculated by formula from curve W = 0.4780 ( P h 0.0199). 0 Methyl furfural found by phloroglucinol method calculated as in b .
0.0110 0.0110 0.0110 0.0162 0.0162 0.0162 0.0221 0.0221 0.0324 0.0324 0.0486 0.0486
50 50 50 50 50 50 50 50 50 50 50 50
0.0294 0.0294 0.0293 0.0430 0.0430 0.0432 0.0582 0.0589 0.0861 0.0860 0.1282 0.1282
0.0112 0.0112 0.0111 0.0163 0.0163 0.0164 0.0221 0.0223 0.0327 0.0326 0.0486 0.0486
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solution, so that the effect would be proportionately greater in the cases of small samples. To allow for this solubility factor a formula was devised which could be used to calculate the weight of 5-methyl furfural from the weight of phloroglucide precipitate. The weight of precipitate was plotted against the weight of sample used (Figure l), giving a curve which is a straight line. The curve does not pass through the origin but would cross the abscissa on which the phloroglucide was plotted a t a point corresponding to -0.0199 gram. If the weight of precipitate is increased by 0.0199 gram for each sample, the resulting curve would be parallel to the first, and would pass through the origin. By taking the slope of the curve in Figure 1 the following formula was obtained: W = 0.4780 (Ph 0.0199)
+
where W is the weight of sample used, Ph is the weight of phloroglucide precipitate, and 0.0199 is the correction factor
to account for the solubility of precipitate, together with any other factors which may tend to lower the yield of precipitate when working under the specified conditions in a constant volume of 400 ml. Using this formula to calculate the results in Table 11, satisfactory agreement is obtained between the values of sample taken and found, the average error being 0.2 mg. and the maximum, 2 mg. in thirty analyses. The earlier formula of n X 0.000018), where n = Fromherz (8), W =- l / 1 9 (Ph volume of solution in ml., gives satisfactory calculated values only in the region of 0.06-gram samples. For smaller samples the calculated values are low and for larger ones the results are high, the amounts computed by the Fromherz formula rising steadily from about 82 to 106 per cent of the 5-methyl furfural actually used in the authors' experiments. THIOBARBITURIC ACID METHOD.To a sample of pure methyl furfural, diluted to 200 ml. with 12 per cent hydrochloric acid, was added the precipitant consisting of 0.15 gram of thiobarbituric acid, slightly in excess of the equivalent amount, also dissolved in 12 per cent hydrochloric acid. After precipitation had commenced, the final volume was made up to 400 ml, with 12 per cent hydrochloric acid and allowed to stand for 2 days t o ensure complete precipitation. The methyl furfural thiobarbiturate precipitated very slowly but was more dense and filtered more easily than the corresponding furfural precipitate. The weight of methyl furfural was calculated on the basis of a reaction ratio of one molecule of 5-methyl furfural to one of thiobarbituric acid, using the conversion factor 0.4660. The results which are given in Table I1 show a very close agreement between the actual weight of sample taken and the amount calculated from the precipitate by use of the above theoretical conversion factor. The average error is about 0.2 mg. and the maximum about 1 mg. 2,4-DINITROPHENYLHYDRAZINE METHOD.A measured Sample of the aqueous solution containing a weighed quantity of 5methyl furfural was added dropwise to 50 ml. of a saturated 2 N hydrochloric acid solution of 2,4-dinitrophenylhydrazine. The hydrazone which formed as a granular red precipitate was allowed to stand for 1 hour at 0" C. in an ice bath, and was then filtered, washed with 2 N hydrochloric acid and water, and finally dried over phosphorus pentoxide in a partial vacuum. From the weight of the dried hydrazone, the weight of 5-methyl furfural may be calculated, using the factor 0.3793. The results recorded in Table I1 show that the error of the method is generally less than 0.2 mg.
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Volumetric Methods VOLUMETRIC POTASSIUM BROMATE-BROMIDE METHOD.Following the method suggested by Powell and Whittaker (IS) for furfural, 25-ml. portions of approximately 0.1 N potassium bromate-bromide (0.132 N actually used), were pipetted into four glass-stoppered flasks. To two of these the samples of 5-methyl furfural made up to 200 ml. with 12 per cent hydrochloric acid were added, and t o the other two, 200 ml. of 12 per cent hydrochloric acid were added for blank runs. After standing for 1 hour in the dark, 10 ml. of 10 per cent potassium iodide were added and the liberated iodine was titrated with 0.10038 N sodium thiosulfate. The number of milliliters of standard sodium thiosulfate required by the sample subtracted from the number required for the blank was a measure of the bromine consumed by the 5-methyl furfural. Whereas the addition of bromine to furfural takes place to the extent of 4.05 atoms per molecule of furfural, which is an empirical value under carefully controlled conditions, the results in Table I11 indicate that 5-methyl furfural was more active in the addition of bromine than furfural, so that approximately 5 atoms of bromine were added for each molecule of 5-methyl furfural. Using varying sized samples with the same quantity of reagent, the addition of bromine proceeded further in the cases of smaller samples during the reaction period of 1 hour. Since this effect may be caused by the proportionally larger concentration of bromine, as stated by Hughes and Acree (7), three samples were run
ANALYTICAL EDITION
JULY 15, 1936
285
containing 0.0275, 0.0551, and 0.0826 gram to which 24, 34, and 43 ml. of potassium bromate-bromide mixture were added, respectively, so that the excess of bromine would be nearly the same in each case. The results obtained correspond t o 5.02,4.99, and 4.97 atoms of bromine and were more nearly constant a t 5 atoms for the varying sized samples of 5-methyl furfural. The increase in bromine consumption with 5-methyl furfural to approximately 5 atoms, whereas furfural used 4.05 atoms, may be due to an increase in chemical reactivity toward bromine, as well as possible temperature effects in these experiments, for Magistad (1.2) and Hughes and Acree (7) have observed large temperature coefficients for these brominations in the case of furfural. TABLE111. ADDITIONOF BROMINE TO FURFURAL Weight of Sample Gram 0.0303 0.0303 0.0303 0.0448 0.0448 0.0448 0.0448 0.0605 0.0605 0.0605 0.0605 0.0672 0.0672 0.0672 0.0672
M1. of 0.10038 for Blank Less
i dforNazSsOs Sample
Atoms of Br per Mole of Methyl Furfural
13.75 13.95 13.80 20.53 20.48 20.53 20.48 26.70 26.70 26.75 26.80 29.23 29.12 29.30 29.41
5.017 5.090 5.036 5.017 5.054 5.017 5.054 4.872 4.872 4,881 4.890 4.808 4.788 4,820 4,838
VOLUMETRIC POTASSIUM BROMATE-BROMIDE TITRATION The method of Hughes and Acree (7‘) differs from the foregoing in that 3 per cent hydrochloric acid is used and the temperature is maintained a t 0” C. which reduces the addition of bromine to furfural to exactly 2 atoms per molecule of furfural. In the case of furfural the present authors obtained very satisfactory results but, in using exactly the same procedure, the greater reactivity of &methyl furfural made i t impossible to hold the addition to exactly two atomic proportions of bromine but instead much higher and erratic addition values were obtained.
for the determination of varying sized samples of unknown, since the amount of potassium bromate-bromide solution to be used would depend on the size of sample to be titrated. I n the work on the determination of furfural in woods by one of the authors (9), higher results for furfural in the distillate were reported for the volumetric method than in the phloroglucinol gravimetric method. Since some 5-methyl furfural is undoubtedIy produced from hard woods, it seems that the slightly higher results may be due to the increased reaction of bromine with the 5-methyl furfural present.
Conclusions
AT 0” C.
Comparison of Methods In this study quantitative precipitation of &methyl furfural was obtained by using each of the three reagents phloroglucinol, thiobarbituric acid, and 2,4-dinitrophenylhydrazine, under conditions which are adjusted carefully while the volumetric potassium bromate-bromide methods did not yield satisfactory results, employing conditions outlined in this work. In the case of the phloroglucinol method, it was necessary to carry on the precipitation in a definite volume; to prevent decomposition of the precipitate during the drying process; and to weigh the precipitate rapidly, since it was hygroscopic. When a solubility correction factor was applied, recovery values showed a maximum error of less than 2 mg. with an average error of 0.2 mg. for the complete series of determinations. The thiobarbituric acid method gave a more stable precipitate which offered no trouble in drying and weighing. The results showed a maximum error of 1 mg. with an average error for the seventeen recorded samples of 0.2 mg. The use of 2,4-dinitrophenylhydrazine as a precipitant gave the most rapid precipitation and the most consistent results, showing a maximum error of 0.3 mg. and an average error of 0.13 mg. In applying the volumetric methods, the reaction with bromine has been shown to take place much more readily with 5-methyl furfural than with furfural and seemed to vary with the concentration of bromine and the quantity of sample to be determined. Consequently this method is not suitable
1. Phloroglucinol has been used to determine 5-methyl furfural by introducing an empirical correction factor for solubility of the phloroglucide precipitate. In calculating the 5-methyl furfural from the phloroglucide the formula W = 0.4780 (Ph 0.0199) was found to be more satisfactory than the formula used by Fromherz, W = 1/1.9 (Ph n X 0.000018). 2. 5-Methyl furfural precipitates quantitatively with either thiobarbituric acid or more satisfactorily with 2,4dinitrophenylhydrazine. 3. The use of the potassium bromate-bromide titration procedure a t room temperature or at 0” C. does not yield quantitative results under the conditions used in analogous furfural determinations.
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Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods, p. 120 (1925). (2) Deshpande, J. I n d i a n Inst. Sci., 13A, 110 (1930). (3) Dox and Plaisance. J. Am. Chem. Soc.. 38, 2156 (1916). (4) Eck, Van, Versl. v. d. verricht. v. h. Centr. Lab. v. d. Volksgerondh., 1918; Chem. Weekblad, 16, 1395 (1919). (5) Ellet and Tollens, Ber., 38, 492 (1905). (6) Fromherz, Z. physiol. Chem., 50, 209, 241 (1906). (7) Hughes and Acree, IND.ENQ.CHEM.,Anal. Ed., 6, 123, 292 (1934). (8) Iddles and Jackson, Ibid., 6, 454 (1934). (9) Iddles and Robbins, Ibid., 5, 55 (1933). (10) Kline and Acree, B u r . Standards J. Research, 8 , 25 (1932). (11) Kullgren and Tyden, Ing. Vetenskaps A k a d . H a n d . , 94, 3 (1929). (12) Magistad, IND. ENQ.CHEM.,Anal. Ed., 5, 253 (1933). (13) Powell and Whittaker, J. SOC.Chem. Ind., 43, 35T (1924). (14) Rinkes, “Organic Syntheses,” Vol. XIV, p. 62, New York, John Wiley & Sons, 1934. (15) Sasaki, J. Agr. Chem. SOC.J a p a n , 6 , 535 (1930). (16) Simon, Biochem. Z.,247, 171 (1932). (17) Sweeney, Iowa Eng. Expt. Sta., Bull. 73 (1924). (15) VotoEek, Ber., 30, 1195 (1597). ENG.CHEM.,22, 362 (1930). (19) Wise and Peterson, IND. RECEIVED December 11,1935.
