ANALYTICAL EDITION
July, 1944
cision of duplicate analysas beyond the ordinary tolerance of sptrophotometric or colorimetric andyses. In Table 11 are results obtained by both methods on some unknown experimental Bamples. SUMMARY
A spectrophotometnc method and a colorimetric method have been developed for the quantitative determination of gallic acid. suppression of the ionization of gallic acid in the former method is necessary* The latter method may be applied to Other polyphenolic compounds.
429 LITERATURE CITED
K.,J . Indian Inst. Sei.. 21A,417-41 (1938). (2) Glasstone, 8 , A d y s t , 50 49-53 (1925) (3) Hertz6wna, G , and Marchlewski, L , Bull intern. aced. polonaiae closaesd. math. mi.,Seriea A , 45-63 (1934 35). (4) Mitchell, C.A.,Analyst, 48 2-15 (1923) (1) Abichandani, C T., and Jatkar, 8. K
THE work deacriberl in this paper we.0 done under
YI contract, reoomrneiided by the Committee on Mediral Research between the O 5 c e of Scientific Reaearch and Development and the University of Pittsburgh. Contribution No. 621 from the Department of Chemistry, University of Pittsburgh.
Photometric Method For Determination of Hemicellulose CHARLES J. BARTON
AND
ARTHUR J. PRUTTONI, Industrial Rayon
THE
first step in the production of viscose rayon i8 the steeping of the pulp in 18% sodium hydroxide solution to convert the cellulose to soda cellulose and to remove the greater part of the alkali-soluble material contained in the pulp. This alkali-soluble material is usually referred to as hemicellulose. According Eo Heuser ( 1 ) for sulfite pulp, it consists principally of degraded cellulose, with some xylan and mannan. A constant and relatively low hemicellulose concentration in the steeping solution is required, necessitating the use of a recovery system for economical plant operations. I n order to assure the neceamry close control of hemicellulose concentration, a number of determinations must be made daily in a viscose plant. Previous methods used in the Industrial Rayon laboratories involved oxidation of the hemicellulose with chromic acid-sulfuric acid mixtures by boiling for different lengths of time, either with or ~ i t h o u at reflux condenser, and determining the excess of dichromate iodometrically. These methods give reproducible results but are time-consuming and require the use of a rather expensive chemical, potaasium iodide. The reduction of orange chromic acid to green chromic sulfate results in R mixture of colors not suitable for accurate colorimetric determinations, but the extent of the reduction can be easily determined with a spectrophotometer or filter photometer.
EXPFRIMENTAL
According to Jiiger (2) it is necessary to maintain a kmperature of 125" to 135' C. for at least 5 minutes in order to complete the oxidation of hemicellulose and glucose wit.h a chromic acid-sulfuric acid mixture. However, one of the authors found that in the temperature range 130" to 140' C. the reaction is e% sentially complete in one minute or less-further heating changes the results so little that, for control work, the additional time required is considered unjustified. The authors' method w u m e s that the composition of the dissolved hemicellulose is the same at all times, since a change in composition would result in a different reducing power per unit weight of hemicellulose. UnL formity in the composition of the hemicellulose is aasured by ilie high degree of uniformity of the pulp supplied to rayon manufacturers.
A wide range of hemicellulose concentrations is encountered in these laboratories. In order to ohtain high sensitivity and still keep the method simple as possible, the same amount of N potassium dirhromate solution, 5.0 ml. is mixed with varying timounts of hemicellulose solution, accorhing to the concentration -present. The following tsble shows the amount of sample used or the different hemicellulose concent,rations:
I Preaent address, Conti-Glow Division, Continental Lithograph Corp., Cleveland, Ohio.
Figure 1.
Corp., Cleveland, O h i o
Transmittance Curves for Chromic Acid-Hemicellulose Solutions
Concentration Range. 9i 0.00-0.12 0.12-0.30 0.30-1.20 1 . 2 -2.4
M1. of Sample 26.0 10.0 2.0 1.0
For samples less than 25.0 ml.,'uffivient C.P. sodium hydroxide solution or distilled water is added to bring the volume to 25.0 ml. Small variations in the sodium hydroxide concentration of the solutions do not affect the resulta appreciably, but the sodium hydroxide concentration should be approximately the same as in the hemicellulose solutions used in preparing the calibration curves. Finally, 25 ml. of concentrated sulfuric acid are added cautiously to the mixture of potassium dichromate and sodium hydroxide solutions in a 260-ml. Erlenmeyer flask. The solution is boiled for about 30 seconds and then cooled. The final volume is 49 * 1 mI. The solutions thus prepared are stable, showing no change in transmittance a t 600 millimicrons after standing for months in glass bottles. All transmittance measurements were made with a Coleman Universal spectrophotometer. The solution cuvettes have a thickness of 13 111111. The wave band passing through the exit slit is 35 millimirmns wide. Distilled watm is
430
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 16, No. 7
hemicellulose concentrations, 25 ml. of N potassium dichromate are mixed with 5.0 ml. of hemicellulose solution in a 250-ml. Pyrex volumetric flask, and 25 ml. of concentrated sulfuric acid are added cautiously. The solution is heated to boiling, cooled, and diluted to the mark with distilled water, and the transmittance of the well-mixed solution a t 600 millimicrons is measured. This method, in addition to the advantage of the larger sample, also eliminates the volume error. Dilution to a definite volume can also be used with the first method described. However, a rather slow change in the color of the solution takes place after dilution. Figure 3 shows the change in transmittance of a solution that contained an excess of hemicellulose. The change is very nearly complete in 30 minutes after dilution. It seems to be due to the reduction in concentration of sulfuric acid, since little change occurs when the solution is diluted with 1 to 1 sulfuric acid instead of with water. SUMMARY
used in the reference cell a t all times and the transmitt.anrr i, measured t o the nearest 0.25%. Figure 1 shows transmittance measurements for solutions prepared as above, using 25.0-ml. samples of sodium hydroxide solutions containing varying amounts of hemicellulose. The curves show that the maximum spread between the transmittance of the chromic acid and chromic sulfate occurs a t 600 and 625 millimicrons. The former wave length was chosen for transmittance measurements because an error in the wave-length setting of the spectrophotometer will result in a minimum error in the transmittance of the solution. If a photoelectric filter photometer is employed for measurements with these solutions, a red filter should be used. Calibration curves were prepared, following the procedure outlined above, measuring the transmittance of the cooled solution a t 600 millimicrons, and then determining the hemicellulose in the same solution iodometrically. The hemicellulose concentration is calculated using the factor 1 ml. of N potassium dichromate = 0.00675 gram of hemicellulose (as glucose). The conditions assumed in the calculation of this factor-Le., only pure cellulose present and complete oxidation to carbon dioxide and water-are not realized in the authors’ work, so no claim is made concerning the accuracy of the determinations. The results are shown in Figure 2, plotted on a semilogarithmic scale. The concentration of hemicellulose is on a weight basis, although the solutions were measured by means of pipets. The curves show that Beer’s law holds up to a hemicellulose concentration at which nearly all the chromic acid is reduced. This simplifies the preparation of calibration curves. For routine use, charta were prepared from the calibration curves. The precision under routine conditions is approximately *1.5%, but this can be improved by carefully reproducing experimental conditions. The principal source of error in the method aa described is in the evaporation loss. It is necessary to control the boiling time closely. In a modification developed to permit the use of a larger ssrnple for the higher
A quick photometric method for the determination of the organic material diMolved in sodium hydroxide solutions from wood pulp is presented. A Coleman Universal spectrophotometer waa used in developing the method, but it has also been used with two different photoelectric filter photometers. The method is well suited to routine use and determinations can be completed in 5 minutes. The authors believe that the method can be adapted for the quick determination of a variety of organic materials dissolved or suspended in an aqueous medium. ACKNOWLEDGMENTS
The authors wish to acknowledge the cooperation and suggestions of C. R. Smith and P. J. Whitesell, chief chemists, respectively, of the company’s Covington and Painesville laboratories. LITERATURE CITED
(1) (2)
Heuser, Emil, Papimfak., 25, Tech.-Wiss. Teil, 238 (1927) Jiiger, A., Chcm.-Ztg., 56, 670 (1932).
PBEEXNTBD before the Division of Analytical and Micro Chemistry at the 107th Meeting of the AMEBICAN CHEMICAL Socxmr. Cleveland, Ohio.
Figure
3.
Change in Transmittance with Time
