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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 20, No. 8
The Dipping Refractometer for Determining the Concentration of Dilute Glue Liquors’ A. C. Hart ARMOUR GLUEWORKS, CHICAGO, ILL.
LUE manufacturers have long needed a simple, rapid, yet fairly accurate method for determining the percentage of glue in dilute liquors. At present the “Glueometer,” which is merely a specially calibrated specific gravity spindle, probably is the most commonly used instrument for this purpose. At best its accuracy is about *0.5 per cent. As the temperature correction increases this figure increases accordingly. The pycnometer offers a more accurate method, but one that requires considerable time and very good technic. Since a variation of 1 per cent in the concentration of a glue liquor changes the specific gravity only about 0.0030, the accuracy of this method is limited to about *0.10 per cent and for hurried routine work this percentage is probably doubled. This error is not too great for most practical work, but the need for a more rapid method led to an investigation of the possibilities of the dipping refractometer. A careful review of the literature revealed that surprisingly little work has been done on this subject* considering its possible wide application.
G
Procedure
The Zeiss dipping refractometer, chosen for its accuracy and ease of operation, was used throughout the experiments. The instrument was checked against distilled water and found to be properly adjusted. All readings were checked by two or more individuals, and it was found that they always agreed within d0.2 of one scale division, which is the equivalent of *0.00008 in terms of refractive index or *0.05 per cent as glue. I n routine work various factors, such as inadequate temperature control, cloudiness of samples, etc., would probably limit the accuracy to *0.10 per cent which is still entirely satisfactory for any routine work. Best results were obtained when the samples were clear, the instrument absolutely clean, and a daylight, microscopic lamp used for illumination. Even very cloudy samples were read with little difficulty if the light was properly adjusted. Samples were prepared by accurately weighing the glue on a n analytical balance, adding the requisite amount of water, cold, and allowing to soak for one hour. The glue was then put into solution in the regular way by warming to 60” C. All glue percentages given refer to the commercial product containing 12 per cent moisture, correction for same being made when necessary. All readings were taken at 20” c. Data Table I-Refractometer Readings SCALE REFRACTIVE KINDOF GLUE READING INDEX GLUE Per cent Ashless gelatin 15.5 1.33339 0.25 (Eastman Kodak Co.) 16.7 1.33385 0.50 19.1 1.33478 1.00 1.33651 2.00 23.6 3.00 27.7 1.33798 32.0 1.33972 4.00 5.00 35.9 1.34121 Edible gelatin 15.5 1.33339 0.25 1.33378 0.50 16.5 1.00 18.6 1.33458
ASH Per cent 0.00
0.70
Received March 12,1928. Walpole, Kolloid-Z., 13, 241 (1918-19); Robertson, ”The Physical Chemistry of the Proteins,” pp. 60 and 359; Bogue, “Chemistry and Technology of Gelatin and Glue,’’p. 119. 1 2
Table I-Refractometer Readings-Continued SCALE REFRACTIVE KINDOF GLUE READING INDEX GLUS ASH Per cent Per cent Edible gelatin (cont’d) 23.1 1.33632 2.00 27.2 1.33789 3.00 31.6 1.33956 4.00 35.6 1,34090 5.00 High-grade hide 15.5 1.33339 0.25 2.89 16.5 1.33378 0.50 18 8 1.33466 1.00 20.5 1.33532 1.50 24.7 1.33694 2.50 28.7 1.33847 3.50 33.1 1.34014 4.50 Medium-grade hide 15.3 1.33332 0.25 2.43 16.4 0.50 1.33375 18.8 1.33466 1.00 1.50 20.1 1.33517 2.50 24.7 1.33694 3.50 28.7 1.33847 4.50 32.9 1.34006 High-grade bone 18.6 1.3345s 1.00 2.44 23.0 1.33628 2.00 26.6 1.33766 3.00 30.7 1.33923 4.00 34.5 1.34067 5.00 Medium-grade bone 18.6 1,33458 1.00 3.73 23.0 1.33628 2.00 26.4 1.33758 3.00 30.6 1.33919 4.00 5.00 34.3 1.34059
The specific refractivity of glue may be computed from the formula: n-nl
= a X c
ora=
- nl -
12
where n = refractive index of solution nl = refractive index of solvent (water) c = per cent of glue in solution a = specific refractivity of glue or gelatin
Taking the refractive index of water a t 20” C. as 1.33298 and using values given in Table I, we obtain the results in Table 11. Table 11-Specific Refractivities of Glue Liquors KINDOF GLTJE GLUE n-nl a Per ten6 Ashless gelatin 1.00 0.00180 0.00180 (Eastman Kodak Co.) 2.00 0 00353 0.00177 3.00 0.00500 0.00167 4.00 0.00674 0.00169 5.00 0.00823 0.00165 Av. 0.00172 Edible gelatin 1.00 0.00160 0.00160 2.00 0.00334 0.00167 3.00 0.00491 0.00164 0.00165 0,00658 4.00 5.00 0.00792 0.00158 Av. 0.00163 High-grade hide 1.00 0.00168 0.00168 1.50 0.00234 0,00156 2.50 0.00396 0.00158 3.50 0.00549 0.00157 4.50 0.00716 0,00159 Av. 0.00160 0.00168 1.00 0,00168 Medium-grade hide 0.00146 0.00219 1.50 0.00158 0.00396 2.50 0.00189 0,00549 3.50 0.00157 4.50 0.00708 0.00158 Av. High-grade bone 1.00 0.00160 0.00160 ‘ 2.00 0.00330 0.00165 3.00 0,00468 0.00156 4.00 0.00625 0.00156 5.00 0.00769 0.00154 Av. 0.00158 Medium-grade lione 1.00 0.00160 0.00160 2.00 0,00330 0.00165 3.00 0.00460 0.00153 4.00 0.00621 0.00153 5.00 0.00761 0.00152 Av. 0.00167 Grand average 0.00161
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1928
Converting this grand average of specific refractivity for
all commercial glues studied to a 100 per cent dry glue basis 0.00161 X 100 - o,oo183
88
we find that this work agrees very well with that of Walpole. Discussion These experiments are too few to permit one to draw absolute conclusions, but they do present a number of very interesting possibilities from both practical and theoretical standpoints. The point that first attracted attention was the fact that the specific refractivities of the glues examined were not equal. The variation was too great to be attributed to experimental error alone, so other possible factors were considered in order that the variation might he either eliminated or corrected. To find out if variation in ash content was causing these differences an ash determination was made on each sample, with results as noted in Table I. The extreme range was found to be 3.73 per cent. When we consider that only dilute solutions (none over 5 per cent) were studied, the possible effect due to ash becomes almost negligible. For example, if two glues differing in ash content by 2 per cent are made up to a 5 per cent solution, the resulting difference in ash would be only 0.10 per cent. The difference between t h e specific refractivity of this ash and that of the oorrespond-
A Continuous-Extraction Apparatus' H. L. Maxwell SCHOOL OF CHEMICAL ENGINEERING, PURDUE UNIVERSITY, LAPAYETTE, IND.
871
ing amount of glue is so slight that 0.10 per cent could not be detected. Furthermore, the ash variation did not correlate with specific refractivity. For example, the difference in ash between the high-grade hide and the edible gelatin is more than four times that between edible gelatin and ashless gelatin, yet the difference in specific refractivity is only approximately half as great. It is a well-established fact that in a homologous series, the power of refractivity is dependent upon the size of the molecule. The protein molecule in gelatin is exceedingly large, but breaks down readily into molecules of diminishing size until finally the simple amino acids are reached. Naturally, the specific refractivity of any of these hydrolytic products is less than the original. The grade of any glue is dependent upon this same decomposition. Consequently, we might reasonably except a high-grade glue or gelatin to have greater refractive powers than a low-grade glue when made up in equal concentrations-which is in perfect accord with data given above. As a result the approximate grade of any glue upon which readings are taken should be known before expressing results in terms of percentages, if extreme accuracy is essential. Acknowledgment The writer wishes t o acknowledge the cooperation of Mr. R. S. Chalfont of Carl Zeiss, Inc., Chicago, Ill.
porting and depositing another portion of oil. The tips of the siphon are bent up to prevent the admission of bubbles. If a few bubbles do by chance get into the siphon, they will be caught in the vertical tube immediately before stopcock F and will not interfere with the operation of the siphon.
I
N ORDER to make a rapid and accurate determination of oil in several varieties of kernels, from which the oil proved to be only sparingly soluble in the solvent, it was found advisab!e to construct a continuous-extraction apparatus in which the oil-laden solvent was continuously drawn off and the hot, clean solvent continuously returned to the extraction flask. The first extractor of this type2 designed by the writer was one in which two flasks were connected by a siphon to transfer the oil-laden solvent and also a short glass tube to return the purified solvent as vapor. Although the apparatus has been used extensively, it had a t least two disadvantages-the difficulty in filling the siphon a t the beginning of the extraction, and the tendency of bubbles to collect in the upper part of the siphon during extraction. Recently an improved design, as shown in the accompanying diagram, has overcome the two difficulties with the siphon. The material to be extracted is placed in flask A . Both flasks are than half filled with solvent, ether, alcohol, or water, as the case may be. The siphon C is filled by applying suction through stopcock F . The flasks with conderlser and attachments are placed in a water bath or on a sand bath if the solvent used is water. When heated, the vapor produced in flask 13 forces its way through the short tube D into flask A , where it is condensed by the condenser E. The vapor from flask A condenses in the same way, returning to the same flask. These processes lower the level of the liquid in flask B and raise the level in flask A , thus starting siphon which carries the oil-laden solvent from flask A to B. The solvent vaporizes, leaving its burden of oil in flask B and returning in the form of vapor through D to flask A tu he condensed and begin anew its cycle of extraction, trans1 2
Received Aprd 27, 1928 Chem 3 e m , 116, 122 11918)
This continuous extractor has been tested by placing a solution of alcohol and iodine in flask A and pure alcohol in flask B and noting the time necessary to transport all the iodine to flask B. The instrument is economical in both time and solvent. The advantages in this improved type of extractor are: (1) The material being extracted is treated with clean, hot solvent. (2) Siphon C is readily filled by applying suction through stopcock F. (3) Small amounts of, air accidentally getting into the siphon will be entrapped below F and will not interfere with the operation of the siphon.
