V O L U M E 21, NO. 12, D E C E M B E R 1 9 4 9 methods did not give consistent or reliable color data and, consequently, much time and effort were uselessly expended in attempting to interpret color information. The situation has heen completely changed and color results are now accepted without question. This has saved a great deal of time and manjioivPr in plant and laboratory investigations. LITERATURE CITED
(1) Balch, It. T., IND.ESG.CHEM.,. 1 ~ . 4 ~ED., . 3,124 (1931). (2) Bates, F. J., and associates, Natl. Bur. Standards, Circ. C-440, 2G5 (1942). (3) Brewster, J. F., and Phelps, F. P., B u r . Standards J . Research, 10, 365 (1933) : Research P a p e r 536. 14) Hrowne, C. A , , and Zerban, F. W.,“Physical and Chemical llethods of Sugar Analysis,’’ 3rd ed., New York, John TYiley R- Sons, 1941.
1497 ( 5 ) Gillett, T. R., and Holven, 9.L., Inrl. Eng. Cheni., 28, 391
(1936).
(6) I b i d . , 35, 210 (1943). (7) Gillett, T. R., Meads, P. F.,and Holven, -\. L.. U. S.Patent 2,356,288 (Aug. 22, 1944). ( 8 ) Holven, A . L., and Gillett, T. R., Facts about S u g a r , 30, 169
(1935). (9) Peters, H. H., and Phelps, F. P., B u r . Standards Tech. P a p e r , 338 (March 12, 1927). (10) Spencer, G. L., and Meade, G. P., “Cane Sugar Handbook,” 8th ed., p. 474. New York, John Wiley BE Sons, 1945. (11) I b i d . . D. 479.
(12) Zerban, F. W., and Sattler, Louis, ISD.ESG.CHEM.,ANAL.ED.. 8, I68 (1936). (13) I b i d . , 9 , 229 (1937). RECEIVED April 15, 1919. Presented before the Division of Sugar Chemist r y a n d Technology a t the 115th Meeting of t h e .I\IERICAX CHEMICAL SoCIETY, Ran Francisco, Calif.
Rapid Measurement of Cellulose Viscosity by the Nitration Method W. .J. ALEXANDER
AND
K. 1,. JIITCHELL, Rayonier Incorporated, Shelton, IVash.
A siniplified technique is described for rapidly determining the nitrate viscosity of cellulose. Convenience in procedure is achieved by forming the pulp into soft, thin disks which may be easily handled in the various stages of nitration and stabilization. Degradation is kept to a minimum by using a small sample of open structure and further by using an optimum acid composition under favorable conditions of time and temperature. Data are presented to show the effecl of various changes in method on the level of calculated degree of polymerization.
T
HE nitration method for measuring cellulose viscosity has in recent years been widely used in both fundamental and development research dealing with the structure and properties of wlluloeic materials. It has not, however, been accepted by the pulp and paper industry as a control method for mill operations. .Uthough Berl ( 1 ) has suggested such use and has outlined a proi d u r e for obtaining rapid measurement of viscosity, it has usu:~llyh w n thought, and rightly so, that the method was highly coniI>lic,ztcdand n-ould yield precipe data only in the hands of a highly .killed analyst,. In :idapting the measureniei:t of nit,rate viscosity fur more general and practical use, the present method provides a much Yimplified technique n-hich gives highly reproducible data and t~liniinatcsmany of the difficult, tedious, or dangerous steps that h:ive formed the basis for most objections t o previous methods The nitrate viscosity method offers certain advantages which other viscosity methods do not afford: Because the solution is made in a simple organic solvent, a representative sample may be readily dissolved, aliquoted, and diluted t o any suitably ]om- con(.cntr;ition for viscometric determination of degree of polynierization. The degree of polymerization of the carbohydrate portion of unbleached pulps containing large quantities of lignin, or even o f wood itself, may be reliably measured, inasmuch as xvith suitable nitration conditions, the lignin is substantially removed in the nitration step and therefore does not interfere with the viscaosity measurement on the nitrated cellulosic constituents ( 6 ) . The nitration method is based on conversion of the cellulose sample into cellulose nitrate by a nitration step which does not appreciably degrade the cellulose (2, a), measurement of the viscosity of a dilute ethyl acet,ate solution of the nitrate, and calculation of the average chain length (3,4). The method in its present, status is an accurate and fairly rapid
means for nieasuring cellulose intrinsic viscosity and may well replace the very slow cupranimoniuni method or serve as a check and mean3 of calibration for the very fast cupriethylenediamint method. The time requirement is about 2 to 3 hours and the rpproducibility of degree of polymerization values is \Tithin 1%. The values a9 calculated are not presumed t o be absolute. They serve, nevertheless, as a convenient means for expressing the relative significance of measured tliffer~iicrsin solution viscositv IIETHOD
Preparation of Cellulose. The cellulose sample is prepared by remaking it into a thin soft sheet that \vi11 permit ready penetration of the nitrating acid. For wood cellulose this is best accomplished by dispersing the pulp fibers in water and flowing the slurry onto a sheet mold. About, 20 grams of pulp are used for a 13 X 13 inch (32.5 X 32.5 cm.) sheet which is lifted from the sheet mold and, without pressing, dried at 50” C. on a stainless wire screen support. Enough disks, 40 mm. in diameter, are cut from the dried sheet to give about 1.0 gram of pulp (three or four disks). The disks are further softened by flexing between the fingers, placed in a weighing bottle, and redried to a constant weight at 50” C. (about 99% bone dry) in a mechanical convection oven. Preparation of Nitrating Mixture. The nitrating acid mixture is prepared by adding cautiously, Tvith a spatula, 404 grams of‘ phosphorus pentoxide very slowly to 1000 grams of cold 90% nitric acid (907, fuming acid but not red fuming) contained in a 2-liter Erlenmeyer flask. T h e acid is kept ice-cold by imniersion in an ice water bath and is swirled continuously during addition of the phosphorus pentoxide. This produces a mixture with the composition: . nitric acid, 647,; phosphoric acid, 26% : phosphoric pentoxide, 10%. With occasional gentle shaking. solution is complete in a few hours and the acid mixture is then filtered t,hrough glass wool into a glass-stoppered bottle and stored in a cool dark place. Nitration. I n the actual nitration, a 40-gram portion of the prepared nitrating acid mixture is weighed out into a tall weighing bottle of about 100-ml. capacity, and placed in a constant
ANALYTICAL CHEMISTRY
1498 temperature bath at 20" * 0.1" C. TVith the use of stainless tweezers, the cellulose sample comprising several disks of pulp totaling approsimately 1 gram is introduced quickly into the nitrating acid, a single disk at' a time. The nitration is allowed to proceed for 20 minutes, the sample being swirled a t about 5minute intervals, and the disks are then removed with the tweezers and stacked in a sintered-glass Buchner-type filter funnel which is about 40 mm. in diameter. Through an appropriately trapped filter apparatus, suction is applied and a t the same time the disks are gently pressed into the slightly constricted bottom of the funnel with a flattened stirring rod. After a few seconds of suction, rvhich suffices to remove the escess acid, the disks are removed with tweezers and drowned in cold (10" C.) distilled water, stirred for a moment, and neutralized by adding a small amount of powdered sodium carbonate t o the water. The nit,rate is washed with about three changes of water, t,he disks being transferred with tweezers to the fresh water. The &rate is then boiled for 20 minutes in distilled water, drained, soaked for 10 minutes in 50 ml. of methyl alcohol, and drained again, preferably with suction. The disks are placed in small 200-mesh stainless viire baskets, dried for about 1 hour at 50" C. in a mechanical convection oven, weighed, and stored. Measurement of Viscosity. For the measurement of viscosity a 0.250-gram sample of nitrate is weighed out and dissolved in a 50-ml. portion of absolute ethyl acetate, and a n aliquot is diluted t o yield a final solution containing 0.05 gram of cellulose nitrate per 100 ml. of ethyl acetate. The viscosities of the solvent and the solution are o m e m r e d in a suitable Cannon-Fenske (S) type viscometer at 20 * 0.1 O C. Calculation of Degree of Polymerization. The following formula is used:
DP = K[77]
qsp
=
It-1
viscosity of solution viscosity of solvent K = 75 k' = 0.35 C = grams of nitrate per 100 ml. of solvcmt I, =
Effect of Sample Form on Nitrate Degree of Polymerization DP K Using t o Give DP [v I K = 75 of Std. Method Sample Standard Pulp Disks unpressed, flexed, 15.2 1140 75 1 gram, 20 min. Carded fluff, 1 gram, 20 15.7 1180 72.5 min. 5 X 5 mm. pulp square, 11.4 835 100
Table 1.
10 grams, 5 hours
DISCUSSION OF AlYALYTlCAL PROCEDURE
Conditions of this standardized procedure have been selected to provide a nitration technique which, in addition to being rapid, convenient, and easy to follon~,gives a minimum of degradation of the cellulose. Convenience in operation is achieved through the use of pulp disks, and degradation is minimized by the use of a sample of small size and open structure. I n addition, an acid of optimum composition is used under favorable conditions of time and temperature. For these small scale nitrations, fluffed pulp ib, of course, ideal for obtaining immediate and instantaneous netting of each individual fiber by the acid a t the start of nitration and by the water on drowning, but is less convenient than disks in several respects: Fluff is rather difficult to prepare. If a fluffing card is used, one must either laboriously clean it after each sample or have a different card for each type of pulp. The sample may be slurried, dewatered with acetone, and fluffed with a jet of air, but cleaned air must be provided and suitable precautions taken to prevent loss of fines. Fluff is much less easily handled than the disks. The acid slurry containing the fluff must be transferred t o the filter at the end of nitration (danger of acid splash) and filtered at
Table 11. Effect of Moisture Content of Pulp on Nitrate Degree of Polymerization Moisture Content of DP Standard Pulp, % Y-ing K = 73 0 (100% B.D.) 1 (99% B.D.) 6 (94% B.D.) 10 (90% B.D.) 16 (84% B.D.) 18 (82% B.D.)
1150 1150 1140 1120 1110 1085
each change of wash water, whereas the disks may be handled wit'h tweezers in a much simplified operation. In Table I i t is shown that the degree of polymerization values obtained with soft, thin disks are only very slightly lower than with fluff. The standard prowdure specifies that the pulp be dried to about 99% bone d r ~ p, h v t o nitration. Ho\vrver, Table I1 indicates that relatirrly miall variations in the nioihture content have little effect on the measured degree of polymerization. The preparation of the nitrating acid is a rather disagreeable task; the one redeeming feature is that one batch of 1401 grams will suffice for 35 samples of 1 gram of cellulose. If desired, an alternative method of acid make-up may be used; the r e q u i r d amount of phosphoric anhydride may be added to 85% phosphoric acid and the warm mixture poured into cooled 98 to 0970 nitric acid. The acid misturp should be stored in a cool tiark place and preferably used only after i 2 hours and within :I f ~ i v n-eeks after make-up (see Table 111). Deteriorition of t h r acid is indicated by a marked increase in color clue 70 the asides of nitrogen. The particular nitrating mixture chosen gives 5 minimuin of degradation with a high degree of nit,ration (13.95 to 14.10% nitrogen) and is easy to handle because it is not sirupy. The acetic acid and acetic anhydride mixtures also give a high degree of nitration and result, in high viscosities, but are more dangerous and deteriomte more rapidly than do phwphoric and phosphoric anhydride mixtures, and do not effect suffic-kit deligriification of wood or of high T.\PPI I( number ( I O , unbleached pulps to give satisfactoi.y nitrate solubility. The c~onventionalsulfuric acid mixtures give a lower degree of nitration (12 to 13% nitrogen) and an apprecinhly higher degree of degr:idntion !Table IV).
Table 111.
KRect of Age of Nitrating lllixture on Nitrate Degree of Polymerization
Age of Mixture Used
on Standard Pulp 2 hours a t 20' C. Days 1 2
3
1
8 16 13 30 37 44
31 65 73 83 90
100 107
1011,
1085 1120 1150 1140 1160 1160 1160 1150 1150 1160 1120 1130 1115 LO85
1080
960
The nitration temperature of 20' C. has been chosen as the most convenient. .A temperature of 0 O gives slightly less degradation but requires an ice bath, makes it necessary t o use longer nitration times, particularly with unbleached pulps, and results in a more sirupy misture which is slower to penetrate and more difficult to remove from the cellulose sample (Table V). Although 50% acetic acid is preferred by some workers ( 8 ) ,water has been found in these experiments to be a convenient and satisfactory drowning medium.
1499
V O L U M E 21, NO, 12, D E C E M B E R 1 9 4 9 The iiitration tiiiic ol 20 minutes is used for IJeached pulps (Table 1-1). Unbleached pulps should be nitrated long enough to ensure sufficient attack on the lignin so that it may be solubilized and removed in t,he stabilization process. Aiworkable rule appears to be t,o nit,rate for a time in minutes equivalent to about 10 times the TAPPI K number ( I O ) of the unbleached pulp. Samples of properly prepared r a v ~wood shavings usually require about 24 hours' nitration time ( 6 ) . The nitrate from unbleached pulps, and particularly wood, should be thoroughly washed and boiled with many changes of distilled rvster. then carefully leached with methanol. Good lignin removal is essential in obtaining a riitratetl carboh3,drate fraction which will be conipletely soluble i n ethyl acetate or acetone.
Table I V .
Effect of Nitrating Acid Composition on Nitrate Degree of Pol) merization
h i t r a t i n g Mixture Used for Std. Pulp HIPO,, %: HN03, % PsOS, % loa 16 74 loa 2c 64 loa 36 54 10" 46 44 100 56 34 51% HzS04 33 16% H20 10% ( C H s C 0 ) L j 52 38% CHICOOHb Sirupy. h Dangerout tnixi:iri'.
Temp,, Time, C. Mln. 20 20 20 20 20 20 20 20 20 20 35 150 0 60
Table VI.
Effect of Nitration Time on Nitrate Degree of Polymerization DP Using K = 75 1110 1130 1140 1145 1120
Minutes a t 20' C. Used for Standard Pulp 5 10 20 60 300
Table VIT.
Effect of Solvent on Nitrate Degree of Polymerization [VI"
Acetone, highest uurity Ethyl acetate, absolute Butyl acetate, C.P. a
12.1 15.2 15.4
nP
L-sing K = 75 905 1140
1155
K t o Give D P of S t d . hlethod 94
75 74
Value of k' = 0.35 used for each solvent in calculating intrinsic viscosity.
DP
r s l n g K = 75 1095 1140 1135 1105 1085 Son 1260
40
Table V. EEect of Nitration Temperature on Nitrate Degree of Polymerization __ K Temperature Gsed
for Standard Pulp.
0 20 40
' C.
[VI 16.1
15.2
13 9
nP
r s i n g K = ii 1210 1140 1045
t o Give DP of Std. Method
30
71 75 82
20 the case of u~~bleauhed pulps, the yield of the nitrated cellulo.;ic constituents is especially valuable, as it may be used to estimate the lignin content. Knowing the yield of nitrate from a pure crllulose, one may divide the difference i n yield between a ligriified cellulose and a pure cellulose by the yield from the pure cellulose and multiply tjy 100 to ohtain the percentage lignin content. Airayon grade n.ood cellulose usually gives, by this method, :Lnitrate containing 13.9 to 14.0y0 nitrogen n-ith a yield of about 180%. I,-nbleached pulps give loner >-ield:. and. c~~rresponding to lignin content, may range down, for example, to 12070 or so for wood itself. Western hemlock gives a yield of about 1267C,which represents a lignin content in the wood of about 30% on a solventextracted moisture-free basis; the other i O % comprises 50% CYcellulose with average degree of polymerization of about 2500 and 20% hemicellulose 11 i t h average degree of polymerization of about 70. The time needed to obtain complete solution depends on the degree of polymerization and dpgree of subdivision of the sample, usually ranging from a few minutes for rayon or cellophane pulps to several hours for very high viscosity paper pulp or raw cotton. If speed is desirable, it is best to pick the nitrate disk into small pieces before weighing out for the dissolving step. Absolute ethyl acetate gives high intrinsic viscosity values and is the preferred solvent. Butyl acetate, being more viscous, requires too much time in the dissolving step and in viscosity measurements. -4retone is much more susceptible t o pickup of moisture t,han ethyl acetate, gives appreciably lower intrinsic viscosity values, and, furthermore, gives nitrate solutions t,hat show a greater t endenc>- toward viscosit,y decrwse on standing (Table V I I ) . The preferred temperature for measurement of viscosity is arbitrarily 20" C. Solvent temperature has a noticeable effect on measured intrinsic viscosity, as shown in Table VIII. I n making viscosity measurements, it is desirable to vary the 111
IO
1
Figure 1. Typical Curves Used in Evaluation of Huggins k' (4) for Solutions of Cellulose Nitrate in Ethyl Acetate conceritiatioii of the cellulose nitiate to suit the level of degree of polymerization, inasmuch as the flow time and specific viscosity are more constant if one uses, for example, a concentration of 0.10 gram per ml. in the case of rayon or very low viscosity pulps and 0.028 gram per 100 ml. in the case of very high viscosity paper pulps, native cotton, or rayon grade wood cellulose. In line with obtaining a high degree of reproducibility, a good procedure t o follow in making up the 0.05% solution from the 0.50% solution is to pipet 2 ml. of the 0.5% nitrate solution into a 2-ounce bottle; with the same 2-1111. pipet, pipet four 2-ml. portions of ethyl acetate into the bottle; and with a 10-ml. pipet add 10 ml. of ethyl acetate to the bottle. The viscometer now in use in this laboratory, Cannon-Fenske type for nonviscous liquids ( S ) , has a flow time of 72.3 seconds per ml. for ethyl acetate a t 20" C. and requires no kinetic energy correction. The capillary was made t o specification, 140 X 0.4 mm. For any given viscometer a curve may be plotted which gives degree of polymerization directly from seconds of flow time for 0.10, 0.05, and 0.025 gram per 100 nil. of solutions.
ANALYTICAL CHEMISTRY
1500 The value 0.35 for k' was determined according to Huggins ( 4 ) by plotting q.p/C b s . q s p for selected ccllulose nitrates, covering a wide range in q , a t concentrations of 0.025, 0.05, and 0.10 gram per ml. of ethyl acetate. The family of curves was found to be a series of straight lines having a ratio of slope to intercept falling nith no apparent order within the range 0.335 to 0.365 (see Figure 1 for typical curves). Some data obtained on the value of k' for acetone and butyl acetate solutions indicate that 0.35 is a good average for these solvents also.
Table VIII.
Effect of Solvent Temperature on Nitrate Degree of Poll merization
E t h y l Acetate, Solvent Temperature, 10 20 30 40
' C.
[q]
16.3 15.3 14.2 13.15
DP Csing K = 75 1220 1150 1065 986
K
t o Give D P of Std. Method 70.5 75 80.5 87
Table IX. Degree of Polymerization for Cellulosic Materials Nitrated by Standard Method DP
Lint cotton (chlorite hleached) Wood pulp (very mildly cooked) Rayon grade wood pulp Cellophane grade wood pulp Commercial rayon
Using K = 75 3800 2200 1150 780 350
values of the same order of magnitude as values calculated by Staudinger's formula using K , = 11 X from viscosities obtained in acetone solution a t suitably low concentrations. The formula used is, however, much t o be preferred to that of Staudinger, mainly because it eliminates the concentration effect. Some typical nitrate degree of polymerization values for various celluloses listed in Tahle IX range from 3800 for lint cotton to 3.50 for rayon. LITERATURE CITED
With the present nitration procedure utilizing a sample of rellulose of 1 gram or less, and with ethyl acetate as the solvent, the value of 75 for K give- values of degree of polymerization n hich appear to be consistent n i t h the acceptrd level for rayon, mood pulp, and cotton (5, 1-9). In fractionation studies involving larger scale nitrations under slightlv different conditions, a value of' K equal to 100 for calculating the degree of polymerization of vparated fractions has been used ( 6 ) , but the pre-ent technique I imlts in higher viscosities and therefore gives the same level of degree of polymerization with a loaer value of K . The valuc of K for butyl acetate is little different than for ethyl acetate, about 7 1 as compared with 75; the value for acetone is appreciably higher, about 94 (Table VII). This method a i t h K equal to 75 yields degree of polvmerization
(1) Berl, E., I N D . ENG.C H E M . , AhL%I,.ED., 13, 322 (1941). (2) Berl, E., and Rueff, G., Cellulosechem., 14, 115 (1933). (3) Cannon, M . R., and Fenske, S I . P., ISD.ENG.CHEM.,ASAL,ED.. 10, 297 (1938). (4) Huggins, M .L., Ind. Eng. Chem., 35, 980 (1943). ( 5 ) Jullander, Ingvar, Arkia K e m i Jfineral. Geol., 21A, No. 8 . 142 (1945). (6) Mitchell, R. L., Ind. Eng. Chem., 38, 983 (1946). (7) Schieber, W., Papier-Fabr., 37, 245-50 (1939). (8) Staudinger, H., and Mohr, R., Ber., 70B, 2296 (1937). (9) Staudinger, H., and Reinecke, F., Holr Roh u . WerX.sto,f.. 2 , 321-3 (1939). (10) TAPPI Standard T 214 m. RECEIVEDM a y 20, 1949. Presented before the Division of Celluluse C H E i r x c A L SocIErY, San Chemistry a t the 115th Meeting of the AMERICAN Francisco, Calif. Contribution KO.6 from the Central Chemical Laboratory of Rayonier Incorporated.
Estimation of the Color of Tomato Paste Application of a Color Index W. B. DAVIS Fruit and Vegetable C h e m i s t r y Laboratory, U . S . D e p a r t m e n t of Agriculture, Pasadena, Calif.
A simple, objective, and quiclcly prepared color index, obtained by measuring the color of extracts by a photoelectric colorimeter instead of by reflectance measurements, has been applied to tomato juice and to tomato paste prepared in the laboratory and to commercial samples of paste collected from plants in California. The color index method described is not significantly less accurate than existing methods and is more quiclcly and conveniently applied.
T
H E growth of the tomato paste industry in California has been rapid in recent years, but improvements in chemical methods for controlling the quality of the paste have not kept pace with this rapid development. Suitable objective methods for measuring the color of processed food products are needed by the industry, and because the color of tomato products is regarded as a good criterion of quality, a convenient method for measuring the color would aid in setting up and maintaining caontrol measures. This paper reports results of an attempt t o apply such a method of color measurement, referred to as the color index, to commercial samples of paste collected in California, to pastes prepared in the laboratory, before and after storage, and to the juices from a hich the pastes were prepared. The color index is defined as a series of graduated numerical readings obtained by measuring the light absorption of the acetone extracts of tomato pigments in a Klett-Summerson
colorimeter with a No. 44 blue filter. The scale readings of thiq instrument are proportional to light absorption. The importance of color in the production and marketing of tomato products is too well known t o require emphasis. The need for a simple and rapid test for gaging the color not only of tomato products but of other fruits and vegetables has been made clear from previous studies by other workers (9, 10). As early as 1918, efforts were made by Howard ( 6 1to determine the effect of heat on the color of tomato pulp and paste. There is evidence that the Munsell system ( 1 1 ) and the color chart procedure (3),which are not entirely objective, do not alwaia give satisfactory results ( 1 2 ) . Although the estimation of color of tomato products by visud means or by spectral reflectance curves has been developed (9, 15), little attention has been given t o the measurement of the color of extracts from these products and its use as an index of
