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HARRISON
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Photocolorimetric Determination of Furfural R . A . STILLINGS AND B. L. BROWSIXG The Institute of Paper Chemistry, .Appleton, TI is.
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Reaction of Furfural with Aniline
HE red color produced with aniline or xylidine salts has been used frequently for the determination of furfural (I, 6,9, IO). The colorimetric method possesses distinct advantages in analysis of the furfural distillates from plant tissues because of the presence of methyl- and hydroxymethylfurfural. These react as furfural with precipitants used in gravimetric methods or with reagents used in volumetric procedures, but they do not interfere in the colorimetric method. The work described in this paper abtempts to define, more exactly than has been ,done previously, the conditions necessary for precise determination of furfural by the colorimetric method.
The reaction has been applied with aniline hydrochloride alcohol solution (5,9),with aniline acetate in aqueous acetic acid solution (IO), and with aniline acetate in alcoholic acetic acid solution (1). The determination of furfural by direct application of the aniline color reaction to the 12 per cent hydrochloric acid tlistillate obtained in analysis of plant tissues would be most convenient. This is not possible because of the low color intensity produced under thebe conditions. The reaction applied in alcoholic solution provides satisfactory sensitivity, h i t the relatively large effect of acid concentration on color intensity require. itandardization of t h k factor. iii
Apparatus and Materials Spectral transmission measurements were made with a General Electric recording spectrophotometer. Photometric nieasurements were made with an absorption meter which employs a Weston photronic cell connected through a potentiometer to a microammeter. A Jena BG18 glass filter was fitted to the opening of the photronic cell. Glass absorption cells providing a liquid thickness of 5 mm. were used. The aniline and xylidines were purified by redistillation shortly before use. Furfural was purified by redistillation. hlethylfurfural was prepared by the method of Rinkes ( 7 ) and purified by steam distillation, and the concentration in the distillate was determined by precipitation wit'h 2,4-dinitrophenylhydrazine according t o Iddles and French (3). Hydroxymethylfurfural was prepared by the method of Erdmann ( 2 ) from chloromethylfurfural ( 7 ) and a standard solution was prepared by the met,hod used for methylfurfural.
Reaction of Furfural with Xylidine It has been stated that xylidine and furfural produce a color which is 1.8 times stronger than the aniline-furfural color and
at the same time more stable ( 8 ) . Spectrophotometric studies under the conditions specified (except that a temperature of 20" instead of 1.5' C. was used) showed that neither technical xylidine nor any of the four xylidine isomers tested with furfural gave as low a minimum traiismittance as did aniline (Figure 1). The spectral transmittances were corrected for the color of the reagent if present. The color produced b y the xylidine isomers developed more rapidly than that produced by aniline. I n each case; spectrophotometric transmittances were determined at intervals until the minimum t'ransmittance value had been passed. The typical curves given in Figure 1 represent the minimum transmittance observed. I n view of the difficulty of obtaining pure isomers or definite mixtures of them in "xylidine", and of the widely divergent color reactions of the isomers with furfural, the xylidine method appears less satisfactory than the aniline method for phot,ometric work.
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F u r f u r a l P mg. per liter, acetic acid 52.5 grams per liter, sodium chloride 20 grams per liter ( 1 ) Xylidine (technical, ; ( 2 ) 2-amino-1 3-dimethyibenzene. (3) .+amino1 3-dimethylbenzene. (4) 2-amino-1 I-dimethylbenzene: a i d ( 5 ) I-amino1:3-dimethylbenzene,' 10 per rent by b l u m e ; (6) aniline, 50 grams per liter, Time? of itanding. minutes: ( 1 ' 3 8 , i?120. (3) 9, (4) 18, ( 5 ) 30, (6) 6 5
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FIGURE2. SPECTRAL
TRASSMITTANCE O F METHYLFURFURAL, HYDROXYRIETHYLFURFUR,4Lz S S D FURFUR AT. WITH ASILIKE
Aniline 5 0 , acetic acid 525, and sodium chloride 20 grams per liter; time 59 minutes (1) Methylfurfural, 9.8, (2) hydroxymethylfurfufal, 11.1,and (3) furfural, approximately 2 mg. per liter
The use of aniline acetate in aqueous acetic acid solutions has been found most satisfactory. Adequate sensitivity is obtained without the presence of alcohol, while the effective acid concentration is readily controlled by neutralization and addition of measured quantities of glacial acetic acid. PROCEDCRE. The conditions of test used for study of the method were chosen to meet the requirements of dealing with hydrochloric acid distillates by the aniline acetate-aqueous acetic acid procedure. These include a final furfural concentration of 1 to 5 mg. per liter and the presence of sodium chloride. The furfural solution, water, sodiuni chloride solution if used, and anilineacetic acid were successively transferred to a volumetric flask. Nearly all of the dilution water was added before the addition of acetic acid and aniline to prevent exposure of the furfural to abnormally high concentrations of these reagents while preparing the solut'ion. Dilution t o the mark changed the concentrations only slightly. Except inothe study of temperature effect', the solutions were st,ored at 20 * 1' C. for development of t'he color. Because of statements that the intensity of the color is affected by exposure to light (especially sunlight), the various tests were allowed t o stand in total darkness until the transmission measurement was made. Later tests confirmed the necessity for this procedure. The percentage error due to standing in the diffused daylight of the laboratory until minimum transmittance was attained varied from zero for a furfural concentration of 1.77 mg. per liter to 1.8for a concentration of 4.42 mg. per liter. SPECTRALTRANSMITTANCE OF FURFURAL-AXILINE. Characteristic spectrophotometric transmittance curves for furfural, methylfurfural, and hydroxymethylfurfural are shown in Figure 2. I n the case of furfural, the wave length of minimum transmittance lies between 500 and 518 millimicrons for a wide variety of experimental conditions. For photometric work, Wratten filter 65, the maximum transmittance of which is at 500 mmu, is entirely suitable and was used in this work. The maximum transmittance of this filter in combination with the Jena BG18 filter used is also at 500 mmu. EFFECTOF ACETICACID CONCEKTRATION. Increase of acetic acid concentration markedly increases the stability of t h e color (Figure 3). The minimum transmittance a t first
FIGURE 3. EFFECTOF ACETICACIDCOI~CESTRATION O N TRAKSMITTANCE Concentration, grams per liter: (1) 21, (2) 6 2 . 5 , (3) 105, (4) 262, ( 6 ) 525, (6) 787
decreases rapidly and then reaches a region of constancy (Figures 3 and 4). These data are derived from spectrophotometric transmittance curves, using 20 grams of aniline per liter. From these curves also i t is observed t h a t the wave length of minimum transmittance increases from 506 mmu a t 21 grams of acetic acid per liter to 512 mmu a t 787 grams of acetic acid per liter. The use of acetic acid concentrations of 200 to 500 grams per liter is satisfactory in sensitivity and color stability and is desirable because small changes do not affect t h e m i n i m u m t r a n s m i t t a n c e . The use of 40 per cent acetic acid ( I O ) falls w i t h i n the optimum range. EFFECTOF ANILINE CONCENTRATION. The effect of aniline conCONCENTRATION OF ACETIC ACID - G / L centration on the intensity and stability of FIGURE4. EFFECT OF ACETIC color is shown in FigACID COiYCENTRATION ON MINIure 5. The data are MUM TRANSMITTANCE ,
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FIGERE7 . EFFECTO F SODIUM CHLORIDE TRANSMITTANCE
Furfural 2 mg. per liter, acetic acid 523 gram8 per liter Concentration, grams per liter: (1) 20, (2) 30, (3) 40, (4) 50 ( 5 ) 60, ( 6 ) 80, (7) 110
Furfural 1.86 mg. per liter, aniline 50 grams per liter, acetic acid 325 warns per liter Concentration of sodium chlzride. grams per lit,er: (1) 0, ( 2 ) 30, (3) 25, (4) 30
based upon photometric observations, using a furfural concentration of 2 mg. per liter. The stability of the color is markedly decreased and the intensity increased with increasing concentration of aniline, while, as shown in Figure 6, the intensity of color increases rapidly up to an aniline concentration of 50 grams per liter and the change with further increase in concentration is much less marked. A concentration of 50 grams per liter was selected as most suitable since i t provides high sensitivity and reasonable stability of color, and in this region the effect of change in aniline concentration is relatively slight. EFFECT OF SODIUM CHLORIDE.The analysis of furfural in distillates containing approximately 12 per cent hydrochloric acid requires neutralization of the acid before treatment with aniline acetate. After neutralization with sodium hydroxide, the solution may contain u p to 200 grams per liter of sodium chloride. The final c o n c e n t r a t i o n after addition of other rea g e n t s i s a b o u t 20 grams per liter. The p r e s e n c e of s o d i u m chloride markedly increases the stability of the color and slightly increases the time required to reach minimum t r a n s m i t t a n c e (Figure 7 ) . The minimum transmittance is slightly decreased, but no appreciable error ANILINE CONCENTRATION - G./L. is introduced by slight changes in concentraFIGURE6. EFFECTOF ANILINE CONCENTRATION ON MINIMUM tion. The maximum deviation in transmitTRANSMITTAXCE
tance between 20 and 30 per cent sodium chloride is 0.6 per cent. Because of the favorable effect of sodium chloride in increasing stability of the color, and because in any case it must be present in analysis of hydrochloric acid distillates, i t is recommended that a concentration of 20 grams per liter be maintained in photometric measurements. EFFECT OF FURFURAL CONCENTRBTION. The statement of other in__ -1.4vestigators (1,6,9)that Beer's law is valid for the f u r f u r a l - a n i l i n e color mas found to be true for furfural concentrations of 0.5 to 4.5 mg. per liter (Figu r e 8). D e v i a t i o n s occur a t higher and lower concentrations under the conditions of analysis used. Measurements were made photometrically on solutions containing 20 grams of sodium chloride per liter. EFFECTOF TEMPERATURE. Increasing temperature decreases the stability of the color and the time required to reach minimum transmittance as shown in Figure 9. The transmittance a t the minimum is not affected over the range from 15" to 30" C. For precise work the temperature should be controlled, preferably a t 20" * 0.5" C., at which temperature minimum transmittance is reached in 55 minutes. ~
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FIGURE10. PERCENTAGE OF ERROR IN FURFURAL DETERMINATION CAUSEDBY METHYL- AXD HYDROXYMETHYLFUR-
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FIGURE 9. EFFECT OF TEMPERATURE ON TRANSMITTANCE Furfural 2.3 mg. per liter, aniline 50, acetic acid 525, and sodium chloride 20 grams er liter (1) 15' C . , (2) 20". (3) 2 5 O , (4) 30p. ( 5 ) Relation of temperature t o time required t o reach minimum transmittance
Methylfurfural, Hydroxymethylfurfural, and Formaldehyde blethylfurfural produces a yellow and hydroxymethylfuriura1 a n orange color with aniline acetate, spectral transmittances of which are given in Figure 2. The error introduced by these substances in the photometric determination of furfural is shown in Figure 10. Any error introduced by methylor hydroxymethylfurfural is less than 1 per cent if the concentration is less than that of the furfural. In the usual pentosan distillate, their concentrations are relatively much less. Formaldehyde in a concentration of 10 mg. per liter (the only concentration a t which a test was macle) has no effect.
Method of Analysis Pipet into a 100-ml. volumetric flask an amount of the furfural solution which contains 0.05 to 0.45 mg. of furfural. Add one drop of phenolphthalein indicator and exactly neutralize with 10 per cent sodium hydroxide solution. Dilute to approximately 40 ml. with water, including, if necessary, sufficient sodium chloride solution (200 grams per liter) to produce a final concentration of 20 grams per liter. To 50 cc. of glacial acetic acid add from a pipet (specially calibrated) 5 grams of freshly redistilled aniline, and after adjustment of temperatures to 20' add to the vflumetric flask and dilute to the mark. Store in the dark at 20 * 0.5' C. and measure the transmittance photometrically with suitable filter after 55 minutes. The transmittance should be measured in comparieon to that of the same reagents without the
FURAL Furfural 2.2 mg. per liter, aniline 50, acetic acid 525. and sodium chloride 20 grama per liter, time 55 minutes (1) Methylfurfural, (2) hydroxymethylfurfural
presence of furfural as 100 per cent. Determine the quantity offurfural present by reference to a calibration chart obtained by treatment of pure furfural under the same conditions of treatment.
Accuracy The conditions of the proposed method are so chosen that 1,easonable variations produce minimum error. If the conditions are exactly maintained, the accuracy of the method is limited by the accuracy of the calibration chart and hence b y the reproducibility of the photometric measurement. This is in the neighborhood of 1 per cent for the instrument used. I n the analysis of pentosan distillates, the accuracy exceeds that of the production of furfural by the dist'illation procedure since, according to cust,omary methods, a correction factor must be applied to compensate for the incomplete and somen.hat yariable yield of furfural.
Literature Cited (1) B a r t a , Ladislaus, Biochern. Z., 274, 212-19 (1934). ( 2 ) E r d m a n n , E., Ber., 45, 2391-8 (1910). (3) I d d l e s , H. A., and F r e n c h , K. S..ISD.ESG.CHEY.,Anal. E d . , 8, 283-5 (1936). (4) L a m p i t t , L . H., Hughes, E . B., a n d Trace, L. H., Analyst, 52, 260-4 (1927). ( 5 ) M o h l e r , H., a n d Benz, H., Mitt. Lebensm. Hyy., 25, 161-5 (1934). (6) Riffart, H.. a n d Keller, H., 2. L'ntersuch. Lebensm., 68, 113-3s (1934). (7) Rinkes, I. J., "5-Methylfurfural", in H a r t m a n , W. W., "Organic Syntheses", Vol. 14, pp. 62-4, S e w York, John Wiley & Sons, 1934. (8) Suminokura, K., a n d K a k a h a r a , Z., Trans. Totfori. S O C . B g r . S C ~ 1. . . 158-9 (1928). (9) T o l m a n , ' L . M., 'and T r e s c o t t , T. C . , J . A m . Chern. S O C . ,28, 1619-30 (1906). (10) 1-oungburg, G. E., a n d P u c h e r , G. W., J . Bid. Chem.. 61, T41-6 (1924).
