Photometric Determination of Acetone-Insoluble Material in Soybean Oil J
Relation of Acetone-Insoluble Material to Break CHAS. ANDREW 3ZIURRAY'
AND
E. B. OBERG, Central Soya Company, Inc., Decatur, Ind.
T
HE quality of soybean oil commonly has been determined b y one of two analytical procedures-namely, the modified Gardner break test (1) and the official A. 0. A. C. foots test ( 2 ) . The American Oil Chemists' Society adopted the modified Gardner break test as a n official procedure in 1940, and this test is in general use throughout the soybean industry. The test consists essentially in heating oil in the presence of hydrochloric acid to 288" C., and separating and weighing the precipitate which appears in the oil at this temperature. The test yields highly reproducible results when run with careful technique, but requires considerable time and measures a decomposition product of constituents of the oil rather than the constituents themselves. Prior to the introduction of the modified Gardner break test, the foots test ( 2 ) was used to determine the quality of soybean oil. The foots test measures the acetone-insoluble material in soybean oil by a volumetric procedure. This test was not entirely satisfactory because of the difficulty of obtaining reproducible results and the time required. Gravimetric methods of determining acetone-insoluble material by actual separation and weighing of the material are time-consuming and are complicated by instability of the acetone-insoluble material in oxygen and heat after separation from the oil phase. This makes the obtaining of reproducible results very difficult. I n this investigation a photometric procedure has been developed for the measurement of acetoneinsoluble material in solvent-extracted soybean oil without exposing the material to air or heat. The method yields highly reproducible results and is simple and rapid in operation. The photometric procedure is standardized against gravimetrically determined values and depends upon the preparation of stable colloid sols of acetone-insoluble material from solvent-extracted soybean oil in acetone. These sols remain stable for several hours when prepared in proper concentrations and in the absence of electrolytes. The turbidities of the sols thus prepared were examined in a filter photometer and were found to conform t o Beer's law over the concentration range investigated. A correction for the interference of natural color in the oil was made by determining the light extinction coefficient of a solution of oil in hexane. Subtracting this value from the light extinction coefficient of the acetone sol yielded a corrected extinction coefficient which was a measurement of the turbidity of the acetone-insoluble material.
test tubes were used as absorption cells. Acetone and hexane used in the investigation were dried over anhydrous sodium sulfate and carefully redistilled. PHOTOMETRIC PROCEDURE, For preparing acetone sols of acetone-insoluble material and determining light extinction coefficients, a sample of solvent-extracted soybean oil was prepared by heating to 70" C., until all of the "foots" had been brought into solution. The sample was then cooled quickly to 25' C. in the water bath, 50 ml. of redistilled acetone were pipetted into a clean 125-ml. Erlenmeyer flask and quickly stoppered, and 50 ml. of redistilled hexane were pipetted into a second 125-m1. Erlenmeyer flask in a similar manner. Exactly 1 ml. of the previously prepared solvent-extracted soybean oil was pipetted into each of the solvents, with removal of the stoppers for only sufficient time to allow for the transfer. The addition to the acetone was made with as little agitation as possible. The acetoneinsoluble sol was then formed with a single vigorous swirl of the flask. Four additional swirls were used to mix the sol thoroughly. A sample of the sol was removed for photometric analysis 20 seconds after formation. The hexane solution was mixed thoroughly and a sample taken for photometric analysis. The photoelectric instrument was set to zero with pure solvent in the absorption cells before each series of determinations and the setting was checked frequently during operation. The zero setting was found to change very little after the instrument had warmed up. GRAVIMETRIC DETERMINATION OF ACETONE-INSOLUBLE MATERIAL. Acetone-insoluble material was determined gravimetrically by the following procedure which has been used in this laboratory for a number of years: A 10-gram sample of solvent-extracted soybean oil was weighed into a 250-ml. beaker and 100 ml. of acetone saturated with potassium iodide were added. The solution was stirred vi orously until the colloidal precipitate a t first formed coagulatef and upon settling left the supernatant liquid crystal clear. Approximately 1.5 grams of filter aid were added and the precipitate was transferred to the asbestos filter mat of a Gooch crucible with a vacuum of about 2.5 cm. (1 inph) of mercury. The precipitate was then washed with six 20-ml. portions of pure acetone. After washing, the precipitate was dissolved and transferred into a tared Soxhlet extraction flask with 100 ml. of pure hexane added in 20-ml. portions. The hexane was evaporated from the acetone-insoluble material on the steam bath and final traces were removed in the vacuum oven a t 105" C. The flask was then cooled in a desiccator and weighed back to determine the acetone-insoluble material. GRAVIMETRIC DETERMINATION OF BREAK. Break was determined by the modified Gardner break test (1). TABLEI. RELATION OF CORRECTED LIGHTEXTINCTION COEFFICIENTS AVD ACETONE-INSOLUBLE MATERIAL AND BREAK Oil No.
1 2 3 4 5 6
Experimental APPARATUS AND REAGENTS.Light absorption was measured with a Klett-Summerson KO.800-3 photoelectric colorimeter. This instrument is a filter photometer of the two-cell type. The dial is marked with a logarithmic scale from 0 to 900, and reads in terms proportional to the light extinction coefficient (log 1/10). A blue filter was used having the rather narrow transmission range of 380 to 430 mi*. Calibrated and carefully matched 1 Present
7
8 9 10 11
12 13 14 15 16 17 18
address, Reiohhold Chemicals, Ino., Detroit, Mieh
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Colorimeter Readings Acetone Hexane Corrected Sol Solution Reading
125 135 111 122 126 140 169 176 198 187 218 236 235 260 285 266 375 370
113 90 79 81 76 81 83 85 92 77 82 84 94 80 100 83 135 93
12 45 32 41 50 59 86 90 106 110
136 152 141 180 185 183 240 277
A. I.
Break
%
5%
0.41 0.68 0.69 0.72 0.81 0.87 1.09 1.08 1.25 1.39 1.54
0.123 0.207 0.182 0.204 0.219 0.243 0,292 0.314 0.337 0.357 0.404 0.457 0.437 0.506 0.533 0.513 0.683 0.757
1.60
1.66 1.93 2.00
2.04 2.45 2.84
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 14, No. 10
Relative extinction of light by the acetone sols of acetoneinsoluble material was found to be directly proportional to the acetone-insoluble content of the oil. When relative light extinction coefficients were plotted on the abscissa and per cent acetone-insoluble material on the ordinate, a straightline curve resulted which indicated conformity with Beer's law over the concentration range investigated and served as a calibration curve. The system was shown, therefore, to be adapted to photometric analysis. The calibration curve is shown in Figure 1. Relative light extinction of the acetone-insoluble sols was found likewise to be directly proportional to the break content of the oils. The calibration curve relating per cent break to relative light extinction is shown in Figure 2. Average deviation of the points from the calibration curve was =t0.006 break per cent, while the maximum deviation of any point from the curve was 0.016 break per cent. Average precision S C A L E R E A D I N G ICORR
I
FIGURE 1. PHOTOMETRIC DETERMINATION OF ACETONE-INSOLUBLE MATERIAL IN SOLWNT-EXTRACPED
-
SOYBEAN OIL Calibration curve
Discussion I n this investigation data were obtained showing (1) the relative light extinction coefficients of stable sols of acetone-insoluble material in acetone, (2) the gravimetrically determined acetone-insoluble contents, and (3) the break contents of 18 different samples of solvent-extracted soybean oil. The samples were selected so that a wide range of acetone-insoluble contents were covered. The collected data are shown in Table I. Each value is the average of two or more checking determinations.
o
0.10
0.20
0.30
040
0.50
0.60
a70 0.80
PER C E N T " B R E A K "
FIGURE 3. RELATION BETWEEN ACETONE-INSOLUBLE MATERIAL AND BREAKIN SOLVENT-EXTRACTED SOYBEAN OIL
SCALE READING (CORR.)
DETERMINATION OF BREAK FIGURE 2. PHOTOMETRIC IN
SOLVENT-EXTRACTED SOYBEAN OIL Calibration curve
Column 2 shows relative light extinction coefficients for the acetone sols, column 3 relative extinction coefficients for hexane solutions. Corrected relative light extinction coefficients are listed in column 4. Acetone-insoluble and break contents in per cent are recorded in columns 5 and 6, respectively. Acetoneinsoluble contents ranged from 0.41 to 2.84 per cent, corresponding to 0.123 and 0.757 per cent break.
of the photometric method was found to be approximately 2 scale divisions, corresponding to 0.005 break per cent. This compares favorably with the precision obtained with the modified Gardner break test. Although it is possible to obtain a high degree of precision with this latter test when duplicate analyses are run by the same analyst in parallel and with careful technique, different analysts will generally find larger differences in results. The photometric procedure is not influenced to so great an extent by differences in technique. Approximately 5 minutes are required for the photometric determination as compared to between 1 and 1.5 hours for the modified Gardner break determination. Examination of the data in columns 5 and 6 of Table I revealed that a direct empirical relation existed between acetone-insoluble and break. When per cent acetoneinsoluble material is plotted as ordinates and per cent break as abscissas, a straight-line curve results which passes through the origin and has a slope of 3.8 (Figure 3). This would indicate that break amounts to approximately one quarter of the acetone-insoluble material. Although the exact chemical composition of neither the acetone-insoluble material nor the break is known, this empirical relation would seem to indicate that the break is formed from the acetone-insoluble material and that this occurs through some stoichiometric reaction, The immediate value of the relation lies in the fact that upon the basis of the relation break can be calculated directly from acetone-insoluble material content.
ANALYTICAL EDITION
October 15, 1942
Summary A photometric procedure has been described for the determination of acetone-insoluble material and break in solventextracted soybean oil. The photometric method is rapid, possesses a n average precision of approximately 0.005 break per cent, and is relatively free of errors introduced by differences in technique. A direct empirical relation has been
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found between acetone-insoluble material and break in solyent-eutracted soybean oils.
LiLerature Cited (1) - h i . Oil Chemists’ SOC., Official and Tentative Methods, p. 16e. ( 2 ) A ~ ~ O COfficial . ~ ~Chem., r . Official and Tentative Methods of Analysis, 5th ed., p. 114 (1940).
Determination of Oxide Copper R. S. YOUNG AND D. G. 31. GRAHARI Nchanga Consolidated Copper Mines, Ltd., Chingola, Northern Rhodesia
I
N RECENT editions of widely used reference books on the
chemical analysis of metallurgical products (1, 2 ) , i t is stated that cuprite, tenorite, malachite, chrysocolla, and other oxidized forms of copper can be determined by leaching with 3 per cent sulfur dioxide. This method depends on the fact that oxidized copper is soluble in dilute sulfurous acid, whereas copper sulfides are unattacked under these conditions. It is recommended that 1 to 2 grams of 100- to 150-mesh ore be agitated with approximately 100 ml. of 3 per cent sulfur dioxide for 0.5 to 2 hours, filtered, and washed, and copper determined on the filtrate or oxide portion. I n many parts of the world oxide copper is determined by leaching with 5 per cent sulfuric acid saturated with sulfur dioxide, instead of the 3 per cent sulfur dioxide recommended in the literature. During a study of local oxidized copper ores a large number of comparative tests were carried out on mine and concentrator products, using these two solutions. Slightly higher and more consistent results were secured when using 5 per cent sulfuric acid saturated with sulfur dioxide, but in these complex products it is difficult to obtain an absolute or calculated value for the oxide copper present. The authors then turned to pure copper minerals, whose composition they could determine by careful chemical and mineralogical analysis, in their study of the determination of oxide copper. The material was ground to
