I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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Vol. 20, No. 11
Copper and Silver Numbers as Factors for the Evaluation of Cellulose Products’ J. Rinse BVREAV OF T ~ S T INTERNATIONAL S, PAPERCo.,GLENSFALLS, N. Y.
Copper Number
NE way to determine the value of cellulose products is
0
to find the copper number-i. e., the number of grams of copper which can be precipitated (in the form of cuprous oxide) by 100 grams of the material by treatment with copper solutions according to certain prescribed methods. The copper figure can be considered as a measure for those impurities of the cellulose which have reducing propertiesoxycellulose, lignin, and sugars. A low copper figure indicates a high pure cellulose content (a-cellulose). However, only by thorough standardization of all manipulations the results will be reproducible and make possible a comparison between different cellulose samples. One of the principal causes for unsatisfactory determinations is that the sample is boiled with alkaline solutions, which convert the cellulose into products that have reducing properties. The concentration of the alkaline copper solution, the time of heating, the temperature, and the absorption power of the sample all have marked influence on the results. The last named property is dependent chiefly upon the size of the particles of the sample.
to be a great improvement over the o!d method. When after prolonged use filtering becomes slower, the filter can be restored to its initial condition by clean-
Figure 1-Copper Number Determinations a-Fehling’s method 1 per cent alkali concentration b-Fehling’s method: 2.6 per cent alkali concentration c-Braidy’s method
FEHLING’S SOLUTION h‘IETHoD-of the various methods, the Fehling’s solution procedure is most objectionable. As recommended by several investigators,* a boiling Fehling’s solution containing 1.25 per cent sodium hydroxide is used for 15 minutes. The influence of the time of heating with a Fehling’s solution containing 1 per cent sodium hydroxide is shown in curve a of Figure 1. Except for the time of boiling all the manipulations for this series of determinations are the same. The copper figure rises rapidly during the first 10 minutes, then more slowly, and after 30 minutes another rapid rise occurs, due chiefly to decomposition of the liquid. At no time is there a constant value. The cellulose sample used was a rayon-grade bleached sulfite pulp. 1 Received
M a y 14, 1928. Paper and Pulp,” Vol. 111, Sec. 8, p. 48 (1927).
* “Manufacture of
ing. If instead of this low concentration a Fehling liquor is used of 25 cc. copper sulfate solution, 25 cc. Rochelle salt-sodium hydroxide solution, and 50 cc. water, in which the NaOH concentration is 2.5 per cent, the change of the copper figure with time of boiling is represented in curve b of Figure 1. It is obvious that the decomposition of the
November, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
flask (Figure 2) with a boiling solution made up of 5 cc. 10 per cent copper sulfate (anhydride) solution and 95 cc. of a solution of 13 per cent sodium carbonate (anhydride) and 5 per cent sodium bicarbonate. The manipulations when heating is completed are similar to those with the Fehling copper number. The results are represented in Figure 1, curve c. A rise is shown here also, as the liquid is considerably alkaline. There is, however, no indication of decomposition of the complex copper solution itself, as is the case with the Fehling method. Accordingly, the second method is more reliable and may be preferred to that of Fehling. Braidy determines the amount of reduced copper by dissolving the cuprous oxide in a solution of ferric alum and sulfuric acid. The ferric sulfate is then reduced to ferrous sulfate by the cuprous oxide and the ferrous sulfate can be titrated with potassium permanganate. As the solution before titration is exposed to the air and suction, there is a considerable chance for oxidation of the ferrous sulfate. For this reason the determinations of copper in the experiments for curve c were also done electrolytically. It was not possible to continue boiling longer than about 90 minutes as the liquid starts bumping after a short time of boiling, resulting sometimes in an explosion. CoNcLusroN-These determinations of copper number do not give conclusive values, which show the relation to the absolute amount of impurities of the cellulose protlucts. The Braidy method can give relative values, but the determinations have no high degree of accuracy and they require a rather long time and complicated apparatus for the analysis.
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cellulose slightly. Accordingly, for practical purposes a boiling of 24 hours are sufficient and will give a value for the reducing properties which cannot be far from the absolute value. The accuracy of the silver number determinations can be illustrated by the following examples: PULP
SAMPLE
1
2
3
TIMEOF
Ag NUMBERS
BOILING Hours 18 19 19
4.53, 4 . 6 0 4 . 0 4 , 3.95 2.92, 2.93, 3.06
These are much better than copper values, where only an accuracy of 5 to 8 per cent is accepted. Though the time of the reaction is much longer, the method of analysis for the silver number is easier and more accurate. The analytical part of a copper value determination requires a t least 20 minutes and for the silver number, 7 minutes. No danger from bumping occurred with the silver solutions even after several days of boiling.
Silver Number
A recent paper by Gotze4 deals with the determinations of silver numbers of cotton and of rayon silk by using dilute solutions of silver nitrate and sodium acetate. As the author states, the determinations were of importance for the evaluation of rayon silk. Gotze, however, did not reach an end value for the silver number, as repeated treatment with the silver solution gave always new silver precipitation. The probable cause of this will be discussed below. I n the following the principle of Gotze’s method is applied to bleached sulfite pulps: The pulp is shredded as described above and 0.5 gram of the bone-dry sample is brought in 50 cc. of a boiling liquid containing 1 per cent silver nitrate and 0.7 per cent sodium acetate (hydrated salt) in wcter. The flasks shown in Figure 2 were used and care was taken to boil sufficiently, but not so violently that steam escaped from the condenser. This could be regulated with asbestos paper between the plate and the flask. After a certain time, cold water was poured into the flask and the contents were filtered through a porous glass filter. The pulp, which had a black color due t o the precipitated silver, was washed thoroughly with sodium nitrate solution t o prevent colloidal soluticm of the silver and then, after cleaning the suction flask, hot dilute nitric acid (25 per cent) was used to dissolve the silver. Three times washing with distilled water was sufficient to bring all silver nitrate in the flask. The amount of silver was then titrated with potassium thiocyanate (0.02 to 0.05 normal) and 2 cc. of a saturated ferric alum solution as indicator.
By this method values were obtained for the curves in Figure 3, which shows the variation of the silver number with time of boiling for three different kinds of bleached sulfite pulp and for cotton wool (lowest curve). After 24 hours the reaction is nearly completed and a continuation of boiling for another 24 hours gives an increase from 5 to 10 per cent of the silver value. This increase, however, will be due chiefly to a conversion of the cellulose itself in reducing substances, as boiling water has a tendency to attack 4 Mclliands’ Tcrfilbcr.,1927, 624, 696; cf. C. A., 2.2, 163, read 10 grams AgNOa instead of 30 grams.
Figure 3-Silver
Number Determinations
Three upper curves for bleached sulfite pulp, lowest curve for cotton wool
Other silver concentrations were tried, but they did not give results differing much from these mentioned. Gotze found new silver formation by repeated boiling with fresh silver solution. This seems contradictory to the experiments described in this paper, but can be explained by assuming that the treatment of the pulp with nitric acid to dissolve the silver attacks the cellulose also, which results in renewed reducing properties. Only boiling for a long time will give correct values of the silver number. Comparison of Various Methods
To compare the results of the different methods on different kinds of pulp, values are given for three bleached sulfite pulps, as follows: cu (Braidy, 30 min.) 0.5 2.1 2.2
cu (Fehlin 15 min.$’ 1.0 1.9 2.2
Ag 24 hours 3.10 4.20 4.74
Ag X 0.59 1.82 2.47 2.79
In the fourth column the factor 0.59 (from 63.6/107.9) is used to convert silver numbers in copper values. As it is clear from Figure 3 that the silver values are near to the true reducing value, the copper values (columns 1 and 2) are all too low. With purer pulps the differences are larger. Summary
The determination of copper values is reviewed and some changes in the methods are advised to improve the accuracy
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and to diminish the time required for the analysis. The method of Braidy is preferred to t,he Fehling's solution procedure. In both methods the analytical part of the determination requires a long time and complicated apparatus. Moreover, the copper values may differ largely from the true reducing figures, as the cellulose itself is attacked by the caustic solution and the time of boiling is too short for complete oxidation of the original impurities.
Vol. 20, No. 11
Giitze's method to determine silver numbers is reviewed and applied to bleached sulfite pulps. A suitable apparatus and some changes are advised. The results show that after 24 hours the reaction is nearly completed and that the values will be much nearer to the true value than the copper numbers. Moreover the manipulations of the analysis are much simpler, standardization is less necessary, and the results are more accurate.
Determination of Solubility of Sucrose in BeetHouse Sirup' R. J. Brown, J. E. S h a r p , a n d H. W. Dahlberg GREATWESTERNSUGARCOMPANY, DENVER, COLO.
A m e t h o d for obtaining t h e solubility of sucrose in L A S S E N 2 has shown A low-purity molasses of beet-sugar sirups has been described, a n d results a n d that the solubility of known composition was taken comments using t h i s m e t h o d have been given. sugar in impure soluas a starting point for the The basing of t h e solubility d e t e r m i n a t i o n s on an tions depends on the nature work, and from it various original molasses analysis a n d on d r y substance deterof the impurities present. It solutions of known impuritym i n a t i o n s on the s a t u r a t e d s i r u p s has been shown to is therefore to be expected w a t e r ratio were prepared. decrease t h e labor a t t e n d a n t to s u c h determinations that the solubility of sugar These solutions were satua n d to offer a simple m e a n s for e s t i m a t i n g the accuracy in siruns from different terrir a t e d w i t h sucrose a t the tories will vary. In addition of the original results. proper temperature and withto the effect of climate and out the loss of moisture. It is a t once evident that the purity and sucrose content soil, important changes in re1at)ive composition of impurities take place during the refining process. Certain classes of sub- of these saturated solutions can be calculrtted from their dry stances which are present in the defecated juices are constantly substance, thus greatly reducing the amount of analytical being eliminated, and others concentrated, as the low-purity wo-k required. The results thus obtained may readily be sirups pass through the Steffen or other molasses desugarizing plotted in such form that regularity of the curves gives a cycles. This change is roughly measured by the change in measure of the accuracy of the data, provided the original raffinose content, stated as per cent raffinoseon total impurities, analysis of the molasses and dry-substance determinations which varies from 5 per cent to 50 per cent in sirups handled on individual samples are known to be accurate. by the processes employed in the Rocky Mountain region. PROCEDURE-sirups of predetermined water-impurity ratio Practically all the published data on the solubility of sugar were prepared from a given molasses and placed in brass drums, in beet sirups are based an determinations made under Euro- fitted with small brass caps. Sufficient sucrose was added to pean conditions and cannot be applied to sirups encountered almost saturate the solution at 40' C. The drums were then closed, completely submerged in a thermostatically controlled in this country. Since a knowledge of the solubility of sugar water bath, and rotated slowly until all the sugar had dissolved. in the sirups is necessary for the intelligent and efficient con- The caps of the drums were then removed and sufficient screened trol of the crystallization processes, it has been deemed ad- "Confectioners' A" sugar was added to insure the presence of visable to determine these data for the sirups encountered in excess crystalline sugar under all conditions. The bath was adjusted to the proper temperature and the this region. drums were rotated long enough to saturate the solutions. It is proposed to determine the solubilities over a temper- Samples were then removed and the temperature was raised ature range of 40" to 80" C. on sirups of varying content and slightly for 24 hours and then lowered to the original point and held there until equilibrium had again been reached. The composition of impurities. sirups were again sampled, the drums closed, and the temperature At the present time a method of procedure has been devised raised IO" C. for the next determination. In this manner and that part of the field has been covered which is repre- solubility results were obtained approaching equilibrium from sented by sirups containing a low ratio of raffinose to im- undersaturation and suoersaturation. The temperature was controlled by means of a mercurypurities, such as are encountered in non-Steffen factories. thermostat operating a dual electrical heating system. The data given in this paper will be supplemented from time column Owing to the large fluctuations in room temperature and the to time as the work progresses. wide range of temperatures which were used in the solubility
C
Method
The most difficult problem facing t h e investigator in this field is the accurate determination of sucrose in the saturated solutions. The accurate determination of sucrose by direct analysis is a tedious procedure, and since in any event a drysubstance determination on the sirup examined is required in order to determine the purity, a method was adapted by which the purity of the saturated solutions could be calculated from the dry-substance determination alone. 1 Presented before the Division of Sugar Chemistry at the 75th Meeting of the American Chemical Society, St. Louis, Mo., April 18 to 19, 1928.
$2.Vn. dcut. Zuckmind., 64, 807 (1914).
determinations, a large heater furnishing automatic rough temperature adjustment and a smaller heater for the purpose of fine adjustment were used. The thermostat was able to keep the temperature of the bath constant within 0.02" C. over unlimited periods of time. In sampling the sirups the drum cap was replaced by a sampler consisting of a cap pierced by two tubes. One tube was of copper, and t o the end extending into the drum was attached a disk made of two sheets of monel-metal iilter cloth. This disk was submerged below the level of the liquor in the drum. Air pressure was applied at the second opening in the cap and crystalfree, saturated sirup was forced out of the drum into a flask surrounded by ice, t o reduce loss of moisture by evaporation t o a minimum.
The samples of sirup were analyzed for dry substance.
