INDUSTRIAL A N D ENGINEERING CHEMISTRY P U B L I S H E D BY AMERICAN CHEMICAL SOCIETY W A L T E R J. M U R P H Y ,
EDITOR
Molecular Weight of Cellulose Measurement of Average Degree of Polymerization 0. A. BATTISTA, American Viscose Corporation, Marcus Hook, Pa. tion, and a discussion of the precision of viscosity measurement obtainable with this type of capillary viscometer, are given in the foregoing papers.
Viscosity-concentration data are given for five samples of purified cellulose representing the degree of polymerization range from 300 to 3000. On plotting the data on semilogarithmic paper, linear relationships were found to exist, in each case, between (1) the viscosity function
a and concentration,
and (2) the relative
viscosity function measured at 0.5% concentration and the degrees of polymerization corresponding to values calculated from viscosityconcentration data extrapolated to infinite dilution. The data have been used to derive a mathematical expression b y means of which the value of the viscosity function at the standard concentration of 0.5% may b e converted to degree of polymerization data equivalent to values obtained b y extrapolation OF viscosity-concentration data to infinite dilution.
W
I T H the advent of the more rigorous concepts of cellulose as a long-chain molecule of high molecular weight, the deteriorating action of chemicals and heat on cellulose has come to be considered as a depolymerization reaction whereby the monomeric glucose anhydride units linked continuously in thc cellulose chains become severed a t irregular intervals in the chains, giving rise to shorter molecules. The publication of the Staudinger (11, 12, 13) empirical viscosity-molecular weight relationship gave grext impetus to the investigation of methods for the determination of the mightaverage molecular weights of high polymeric compounds. Coppick ( 2 ) has rcrcntly reviewed and discussed the more significant papers that have been published relating the viscosity of solutions of high poiymcrs with the degree of polymerization. The procedure most widely used is to relate viscosity data obtained at relatively high concentrations with the value of the viscosity function a t infinite dilution through the use of mathematical equations (2, 4, 6, 6, 8, 14). In this paper, viscosity data are presented for five samples of purified cellulose representing the practical degree of polymerization range from 300 to 3000. These data illustrate that a linear semilogarithmic relationship exists between the relative viscosity measured a t 0.5y0 concentration and the degree of polymerization calculated from viscosity data extrapolated to infinite dilution. The constants of the equation expressing this experimentally determined relationship have been obtained by a graphical analyjis of the data, and using these constants thc equation has been satisfactorily checked against extrapolated values.
I
C = gm./ IO0 mi. Figure 1. I. 11. 111. IV. V.
vs. c (ordinary graph paper) Typical viscose rayon Low-viscosity rayon wood pulp Normal-viscosity rayon wood pulp Absorbent cotton Rawcotton
The viscometers used in this work were equipped with groundglass connections ($, 9) and glass stopcocks. Outside dissolving tubes ( l o ) ,whereby the viscometers are reserved for the measurement of fluidity, were used. It was found advisable, in determining the viscosity of highfluidity celluloselike rayon, t o use viscometers possessing capillaries of smaller inside diameter than the 0.88-mm. inside diameter capillary recommended for use with cotton solutions. The large kinetic energy correction that would otherwise be necessary for high-fluidity cellulose solutions may be satisfactorily reduced by the use of viscometers whose capillaries have an inside diameter of 0.675 mm. Pure copper gauze (80-mesh) was used in the preparation of the cuprammonium solvent. The copper gauze was wrap ed around an inlet tube equipped with a fritted-glass jet o?D porosity, and maintained below the level of the ammonium hydroxide in the generating chamber. Agitation was provided for by the fritted-glass jet which served to break up the incoming ammonia-laden air into small bubbles. The use of he-mesh copper gauze facilitated the solution of the copper, and obviated any neceesity for filtering the solvent a t any time in the process
EXPERIMENTAL
METHODO F FLCIDrrY MEASUREhfENT. The general procedure used for the measurement of fluidity v a s based on the papers by Clibbens and Geakc ( 1 ) and Mense (9). The viscometer’. debipn, complete dimcnbional specifications, method of calibrn35 1
Vol. 16, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
352
of its preparation. A siphon was used to transfer the solvent from the generating chamber to the stock bottle. The copper content was determined by means of a calibrated photoelectric colorimeter. This method is rapid and was shown to be as accurate as the volumetric method for determining copper. The copper content was adjusted to 15.0 (*O.lO) grams of copper per liter. The ammonia content was determined volumetrically and wa? maintained at 200 ( 1 5 ) grams of ammonia per liter. The nitrous acid content was determined by means of a Lunge nitrometer and was never found to exceed the maximum limit of 0.5 gram per 100 ml. of solvent. The solvent was stored under oxygen-free nitrogen a t 5' C., and its viscosity in centipoises ranged from 1.32 to 1.36 a t 20" C. All samples of cellulose used in this study received a mild alkaline scouring treatment (1% sodium hydroxide a t 40" C. for a t least 40 minutes), and a thorough extraction with water and organic solvents. Samples were conditioned a t 58% relative humidity and 21.11' C. (70" F.) for a t least 24 hours, after which moisture determinations were made in duplicate on each sample. The weight of the sample used for the measurement of fluidity was calculated on a bone-dry basis. sack glazed analytical weighing paper was used for weighing the samples. The samples were put up in a constant-temperature room (18" C.),and left on the rotating wheel at this temperature overnight. An oxygen-free nitrogen atmosphere was maintained above the surface of the solvent as it was discharged into the dissolving tubes.
0
Table 1. %
G./lOO ml.
Rhes at 20' C.
111.
'1sp
T 8P
Viscose Rayon 0.944 23.3 2.14 29.1 1.51 0.708 0.660 1.42 30.8 0.613 33.5 1.18 0.566 34.8 1.10 0.519 37.0 0.97 0.472 40.0 0.86 0.425 40.9 0.78 0.377 44.9 0.64 0.283 49.6 0.47 0.188 55.7 0.31 0.094 63.5 0.15 Low-Visoosity Rayon Wood Pulp 0.944 10.2 6.18 15.3 0.708 3.80 21.7 0.519 2.38 0.472 24.0 2.06 0.426 27.2 1.70 0.377 29.8 1.47 0.330 32.9 1.23 36.3 0.283 1.03 0.236 40.0 0.84 0.141 49.9 0.47 Normal-Viscosity Rayon Wood Pulp 0.944 5.56 12.2 0.566 14.0 4.24 0.472 17.5 3.18 0.425 19.6 2.76 0.395 21.3 2.45 0.377 22.8 2.22 0.358 22.9 2.21 0.330 26.5 1.78 0.283 30.4 1.42 0.188 39.9 0.87 0.094 52.9 0.39 Absorbent Cotton 0.472 5.19 13.3 0.377 8.28 7.88 0.330 10.3 6.14 0.282 13.6 4.43 0.236 3.24 17.4 0.217 19.1 2.91 0.208 21.0 2.55 0.188 22.5 2.33 0.141 28.5 1.62 0.094 38.8 0.925 Raw Cotton
1.00 0.75 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.15 1.00 0.60 0.50 0.45 0.42 0.40 0.38 0.35 0.30 0.20 0.10 0.50 0.40 0.35 0.30 0.25 0.23 0.22 0.20 0.15 0.10 0.50 0.25 0.20 0.18 0.15 0.12 0.11 0.10 0.09 0.08 0.05
I
l.2
c (semilogarithmic graph paper)
Normal-vircosity rayon wood pulp
IV. Aborbent cotton V. Raw cotton
C
TJ&a1
1.00 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.30 0.20 0.10
I
I
.6 .8 1.0 C = gm./ 100 m 1.
1. Typical viscose rayon II. Low-viscosity rayon wood pulp
Viscosity-Concentration Data Average Fluidity
C
I
4
2 VI.
Figure 2.
c'
I
.Z
.
2.26 2.13 2.15 1.90 1.94 1.86 1.82 1.83 1.69 1.66 1.64 1.59 6.54 5.36 4.58 4.36 4.00 3.89 3.72 3.63 3.55 3.33 12.81 7.49 6.73 6.40 6.18 5.88 6.17 5.39 5.02 4.62 4.14
28.17 20.90 18.60 15.75 13.72 13.40 12.25 12.30 11.50 9.78 101.6 38.05 28.61 25.32 21.49 19.72 18.75 18.61 17.65 18.35 15.55
Pure copper agitators in the form of spirals or solid rods, depending on the viscosity of the sample being tested, were used t o minimize the degradative action of oxygen on cellulose iin cuprammonium solution ( 3 ) . Fluidities were measured a t 20" (*O.lO") C., and flow times were determined by means of a split-second electric stop clock, with an average reproducibility to within less than 1%. The average deviation in the fluidities of the duplicate measurements on a given sample never exceeded 5%, and was usually less than 2%. A solvent blank was run in duplicate with each series of determinations. A standard sample of cellulose was run as a check blank periodically. New batches of solvent were prepared every 2 or 3 months and 3 liters were prepared a t a time. RESULTS
In Table I viscosity-concentration data are given for each of the five samples of cellulose studied: a typical viscose rayon, a low-viscosity rayon wood pulp, a normal-viscosity rayon wood pulp, absorbent cotton, and raw cotton. In Table 11, degree of polymerization data calculated from infinite dilution values of the viscosity function and using the Kraemer relationship ( 7 ) , are compared with degree of polymerization data calculated on the basis of the value of the viscosity function a t 0.5% concentration. RELATIONSHIP BETWEEN APPARENT AND BASICDEGREEOF POLYMERIZATION. It is routine practice in many laboratories to determine the viscosity (or fluidity) of a solution of cellulose in cuprammonium solvent a t a standard concentration high enough to make the viscosity measurement as simple as possible. In this way, it is practical to determine relative changes in viscosity and thereby obtain a measure of the degree of depolymerization of cellulose. The arbitrary standard concentrations most widely used are 0.50 and 1.0%, respectively. The data presented in this paper correlate the values of the viscosity function obtained a t the standard concentration of 0.50% (apparent D.P.) for five representative samples of cellulose with the values for the respective viscosity functions obtained a t infinite dilution (basic D.P.): Limit
(y)
c +o
ANALYTICAL EDITION
June, 1944
353
7SP On plotting the us. c data for each sample on ordinary graph C
paper, the curves in Figure 1 are obtained, in which the rate of increase of the slope is dependent upon the degree of polymerization of the sample being studied. Furthermore, as the degree of polymerization increases, the variation in the slope of the curves even a t very low concentrations precludes reliable extrapolation
P FS. c data. However, when the same data are plotted of the WC
on semilogarithmic paper, linear curves are obtained for each sample (Figure 2) and more reliable extrapolation is possible. The data have been wed to draw a conversion graph (Figure 3) in which the calculated “apparent D.P.” obtained for each sample a t 0.5% concentration are plotted against the corresponding “basic D.P.” calculated from the values of the respective viscosity functions a t infinite dilution, using the Xraemer relationship ( 7 ) . A more applicable conversion relationship has been obtained l), determined from the solution by plotting the values of (qr viscosity a t 0.5% concentration, against the corresponding values for the “basic D.P.” on semilogarithmic graph paper. When this is done, a linear relationship is obtained and is expressed by:
+
Basic D.P. = a[log (nr
+ 1) - b ]
+ 1) - 0.2671
(2)
Table 11.
Comparison of Degree of Polymerization Data Obtained b y Two Methods of Calculation Degree of Polymerization From infinite From data a t 0.5.yQ dilution data uaing concentration using Material Equation 3 Equation 2 Raw cotton 3120 3100 1980 965 735 408
I
I
i
I
I
I
I
I
500
1000
I500
ZOO0
2500
3000
BASIC
Df? (FROM
Figure 3.
INFINITE DILUTION)
Conversion Graph
I
I
I
I
IO00
I500
ZOO0
2500
BASIC D.I?=260[?] Figure 4.
I
3000 35
LIMITC-0
Conversion Relationship
tion data from values of the viscosity function a t infinite dilution are given in Equation 3: D.P. = 260
[q]
(3)
where [ q ]is the value for the intrinsic viscosity a t infinite dilution
The Xraemer relationship and constant ( 7 ) for cellulose in cuprammonium solution used to calculate degree of polymeriza-
1001
I
500
(1)
where a and b are constants representing the slope and intercept, respectively, of Figure 4. The values of constants a and b were obtained graphically from Figure 4, and on substituting them in Equation 1 we obtain: Basic D.P. = 2160 [log (qr
0
Limit
(+h)r
c -* o
The calculated basic degree of polymerization values obtained using the conversion relationship of Equation 2 are compared in Table I1 with the values obtained by extrapolation of the viscosity data t o infinite dilution and using Equation 3. CONCLUSIONS
When viscosity-concentration data, obtained for five representative samples of purified cellulose covering the degree of polymerization range from 300 to 3000, are plotted on semilogarithmic paper, linear relationships are obtained in each case. This permits more. accurate extrapolation of the viscosity data to obtain the intercept values of the viscosity function-i.e., values at infinite dilution-from which degree of polymerization data may be calculated. The logarithmic relationship between the values for the viscosity function (qr l), obtained a t 0.5% concentration, and the respective degree of polymerization data calculated from the values of viscosity function at infinite dilution, is also linear and is expressed by Equation 2. The numerical constants of this equation were obtained by a graphical analysis of the data. A conversion graph has been drawn relating the “apparent” degree of polymerization obtained a t 0.5% concentration to the corresponding values for the degree of polymerization obtained by extrapolation t o infinite dilution (Figure 3). Equation 2 may be used for accurately converting values of the viscosity function obtained a t the standard concentration of 0.50yoto basic degree of polymerization data equivalent to values obtained by extrapolation of viscosity-concentration data to infinite dilution and using the Kraemer relationship and constant
+
( 7 )* ACKNOWLEDGMENT
The writer is indebted to S. Coppick, acting professor of forest chemistry, The New York State College of Forestry, Syracuse,
354
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
N. I-., for helpful criticism and suggestions in the preparation of the manuscript for publication. LITERATURE CITED
(1) Clibbens, D . A,, and Geake, A . , J . TextileZnst., 19, T 7 7 (1928). (2) Coppick, S., P a p e r T r a d e J . , 117, KO.7, 25-9 (1943). (3) Doering, H., Papier-Fabr., 38, 80 (1940). (4) Experimenter, Silk J . R a y o n World, 17, 2 3 (1941); 18, 25, 209 (1941). ( 5 ) Farrow, F. D., and Neale, S. M.,Shirley Inst. Memoirs, 3, 67-82 (1924).
Vol. 16, No. 6
(6) Fickentscher, H., C‘elldosechem., 13, 5s (1932). (7) Kraeiner, E. O., IND.ENG.CHEM.,30, 1200 (1938). (8) M a r k , H., “High Polymers”, Vol. 2, p. 279, New York, Interscience Publishers, 1942. (9) &lease,R. T., J . Research .YatZ. Bur. Standards, 22, 371 (1939). (10) I b i d . , 27, 551-3 (1941). (11) Staudinger, H . , “Die hochmolekularen organischen Verbindungen”, Berlin, Julius Springer, 1932. (12) Staudinger, H., and Heuer, W., Ber., 63, 222 (1930). (13) Staudinger, H., and Nodeu, R.. Ibid., 63, 721 (1930). (14) Strauss, F. L., and Levy. R . M.,PaDer Trade J., 114, N o . 18, 33-7 (1942).
‘Colorimetric Determination of Nickel in Bronze HENRY SEAMAN, 1261 Daly Ave., Bethlehem, Pa.
M
AKY bronzes contain up to 1% of nickel. For these relatively small amounts it would appear that a colorimetric method might be satisfactory for routine work. Feigl (2) found that lead dioxide oxidized nickel in alkaline solution to a valence higher than 2, and that addition of dimethylglyoxime to this solution gave a red coloration rather than a precipitate, This procedure was improved by Rollet ( 4 ) , who used bromine water instead of lead dioxide, and this method has found many applications ( I , 3, 6). This reaction has been applied to the determination of nickel in bronze using a filter photometer such as the Cenco photelometer tvith a cell 10 mm. thick, taking about 17 ml. of solution. PROCEDURE
After the tin is removed by filtration as metastannic acid and the copper and lead by electrolysis, the remaining solution is diluted to 150 ml. and mixed. One milliliter is transferred by pipet to a 100-ml. tall-form beaker, 25 ml. of distilled water are added, and the mixture is shaken after addition of one drop of saturated bromine water. Seven drops of an ammoniacal solution of dimethylglyoxime (10 grams of dimethylglyoxime ,dissolved in 650 ml. of ammonium hydroxide and diluted to 1 liter) are added and the mixture is again shaken well. The orange-red color develops in alkaline solution immediately on shaking. The solution is transferred to a photelometer cell and the absorption determined with the use of the Cenco dark blue filter or a Corning blue filter such as No. 556. The maximum absorption occurs a t 475 mM. The per cent nickel is obtained from the usual type of straight-line curve plotted on semilog paper. The calibration data for this curve can be obtained through the use of a solution of a C.P. nickel salt standardized gravimetrically, or preferably by removing an aliquot from the regular sample, obtaining the colorimetric value from this aliquot, and using the remainder for a gravimetric determination. With bronzes containing manganese, iron, or aluminum, 3 to 5 drops of a solution of ammonium citrate (25 grams of ammonium citrate dissolved in 30 ml. of water) are added before addition of bromine water. RESULTS AND DISCUSSION
Some typical single results obtained by this method are shown in Table I. In general, it is believed the results are satisfactory for the usual type of bronze. The use of ammonium citrate does not eliminate the interference of manganese and iron but reduces it considerably. The precision and accuracy of this method in the range indicated are 0.02 to 0.04% nickel. To obtain satisfactory results with this method it is necessary to standardize on a procedure and use it for all determinations. Among the factors which can affect the intensity of the color are time of standing, amount of bromine used, amount of ammonium citrate used, shaking, and temperature. The color intensity increases on standing, the increase being greatest during the first 20 minutes, and tends to level off after 2 hours. -4 typical increase during the first 20 minutes would be from 0.58 to 0.60% nickel. The use of more than one drop of bromine water and the use of ammonium citrate tend to lower the color intensity slightly-for example, standard 52a gave 0.75y0 nickel with 1
drop of bromine water, 0.74% with 2 drops, and 0.71% with 5 drops. An approximately equal reduction in values for nickel is obtained ~ q i t h3 to 5 drops of the ammonium citrate solution, so that 5 drops of bromine water and 5 drops of the ammonium citrate solution give values of 0.67 to 0.68% nickel for this sample. Low results will also be obtained by the use of too small a drop of bromine water, in which case some nickel will be precipitated.
Table 1. Nickel Determinations on Bureau of Standards Sampler Sample Nickel NO. Interfering Elements Gravimetric Colorimetric % 70 % 37C Fe,0.17 0.58 0.59 37C Fe 0.17 0.58 0.575 37B Fe:0.21 0.46 0.46= 37B Fe, 0.21 0.45 0.46“ 52 Fe, 0.12 0.13 0.13 52 Fe 0 12 0.13 0 . 13a 124 Fe:0:38 0.45 0.47 124 Fe,0.38 0.45 0.46’ 52s. Fe, 0.05: Mn, 0.02 0.73 0.75 62 Fe. 1.13: h h . 1.59: Al.1.13 0.64 0.70“ hlanganese bronze Fe, 2.2; Mn, 3.1; dl,3.8 0.00 0.10s a Using 3 drops of ammonium citrate solution, 25 g r a m per 20 ml. b Using 5 drops of ammonium citrate solution, 25 grams per 30 ml. 0 A commercial sample. _
_
_
_
_
~
____
~~
-
Some experiments indicate that fairly vigorous shaking is necessary to develop the maximum color intensity, although the values obtained were indecisive. Temperature has little effect on the color, except that a hot solution will give a precipitate rather than a color. Temperatures somewhat above or below room temperature gave substantially the same values. To reduce the interference of iron and manganese, ammonium citrate may be added. Under these conditions, these elements will give a yellow solution. A proper choice of wave length might serve to eliminate this interference. However, with a blue filter the interference due to iron was found to be about 0.02% nickel for 1% iron, and 0.03% nickel for 1% manganese with the use of 3 drops of an ammonium citrate solution containing 25 grams of the salt in 30 ml. of water. The dark green Corning filter No. 401 reduced the interference somewhat but gave a less satisfactory curve. Copper and zinc in the amounts usually present offer no interference. LITERATURE CITED
Diehl, Harvey, “Applications of Dioximes to Analytical Chemistry”, P. 35, Columbus, Ohio, G. Frederick Smith Chemical Go., 1940. Feigl, F., B e r . , 57, 758 (1924). Murray, W. M.,Jr., and Ashley, S. E. Q., IND.ENG.CHEM., ANAL.ED.,10, 1 (1938). Rollet, A. P., Compt. rend., 183, 212 (1926). Snell and Snell, “Colorimetric Methods of Analysis”, 1’01. I, p. 314, New York, D . Van Sostrand Co., 1936. ~
