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cc. in excess. If any cloudiness develops due t o cuprous oxide, add sufficient hydrogen peroxide to clear the solution. Cool the solution t o room temperature, transfer to a 500-cc. beaker, and electrolyze for copper at 2.5 amperes.
Precautions Interruption of the stirring during the early stages of the silver electrolysis will cause precipitation of considerable amounts of silver which remain undissolved on the bottom of the beaker. lnsufficient stirring near the end of the electrolysis will cause deposition of copper. With these precautions in mind, the analyst may even double the rate of deposition and obtain accurate results by using a sufficient stirring rate. TABLEI. PERCENTAGE OF SILVER IN REPRESENTATIVE SILVER SOLDERS Sample No.
1 2 3 4 5 6
7 8 9
10
Electrolytic Method 15.10 15.19 20.26 20.08 45.10 46.04 64.99 64.98 50.22 50.21
Silver Chloride Method
Lead and metals that are more noble than copper will deposit with the silver. Lead deposits partly on the anode and partly on the cathode. Nickel present in amounts over 5 per cent in the sample causes poor adherence of silver and loss by dusting. Cathodes should be carefully stripped of all silver before heating to high temperatures or silver will alloy with the platinum.
Discussion Table I shows results obtained by electrolysis as compared with the A. S. T. M. silver chloride precipitation method. The figures given in the silver chloride column were obtained by making several determinations until absolute checks were obtained. Five different types were selected as representative
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of the more common silver solders. In addition to the silver, the compositions of the samples were as follows: 1 and 2 contained 80 per cent of copper and 5 per cent of phosphorus; 3 and 4 contained 45 per cent of copper and 35 per cent of zinc; 5 and 6 contained 30 per cent of copper and 25 per cent of zinc; 7 and 8 contained 20 per cent of copper and 15 per cent of zinc; 9 and 10 contained 15 per cent of copper, 17 per cent of zinc, and 18 per cent of cadmium. TABLE11. WEIGHTOF SYNTHETIC SAMPLES Sample No.
11 12 13 14 15 16 17 18 19 20
Silver Added Grams 0,9998 1.9996 0,9998 0,9998 0,9998 0.3862 0.2536 0.3506 0.3057 0,3029
Silver Deposited Grams 0,9998 1.9995 0.9999
0,9997 0.9997 0.3861 0.2636 0.3606 0.3067 0.3028
Co er Ac&d Grams Nil Nil 0.1
0.5 1.0
1.0 1.0 0.8169 0.5075 1,0122
Copper Deposited Grams
.... .... .... .... .... .... ....
0.8171 0.5076 1.0124
Table I1 shows the wide range of applicability of the electrolytic method. The silver and copper used in preparing the samples were carefully assayed by A. S. T. M. methods. Both metals were 99.98 per cent pure and the weights used in the tables are based on this metallic content. Both tables indicate that the error for silver may be due only t o weighing. Slightly high results for copper may be due to air oxidation. The procedure apparently causes no abnormal error for copper.
Conclusion Electrodeposition of silver from an ammoniacal nitrate solution is rapid and accurate. Copper may be determined rapidly by electrolysis from the acidified electrolyte. The method may be used for silver solders and various other silver alloys.
Literature Cited (1) Am. SOC.Testing Materials, “A. S. T. M. Standarda,” Part I, pp. 831-3 (1933). RECEIVED June 16, 1936.
Determining the Evaporation Rate of Solvents at High Temperatures F. C. THORN AND C. BOWMAN, Garlock Packing Co., Palmyra, N. Y.
T
HE recent technical literature has shown evidence of a considerable revival of interest in the subject of evaporation rate of solvents (1, 2, 5, 6, 7, 9). Most of the methods, however, have been designed to duplicate the conditions prevailing during the evaporation of solvents from varnish and lacquer films-i. e., the evaporation of thin layers of solvent into large volumes of air a t approximately room temperatures. There is an extensive field for solvents in factory processing wherein the solvent is expelled with the aid of heat, and for which the foregoing methods of solvent evaluation do not appear to be adequate. The authors thought that it might be of interest a t this time to report a method which has been used in this laboratory in substantially its present form for the past ten years for the purpose of evaluating petroleum solvents employed in the manufacture of rubber cements and compressed asbestos sheet doughs,
from which they are subsequently expelled with the aid of heat and air. The features that distinguish this method from any of the methods above referred to are: 1. Approximately complete saturation of the air stream, thereby eliminating the time element and making results available directly as liters (or cubic feet) of air per cubic centimeter (or gallon) of liquid solvent. 2. Provision for maintaining sample a t any desired temperature by vapor heating. Vapor heating is preferred to an air or water thermostat because of its high rate of heat input. 3. Provision for reading volume of sample a t all stages of evaporation, without moving any part of the equipment. 4. Use of a large sample, permitting accurate determination of the dry point.
NOVEMBER 15, 1936
ANALYTICAL EDITION
The apparatus is shown in Figures I and IV arranged for heating by steam. The 100-cc. sample is inserted in the conical centrifuge tube, A , which is similar to the container used by Wetlaufer and Gregor (7) except that it has more graduations and is corrected for the volume of the inlet tube, The sample expands on heating to a new volume (about 110 cc.), which figure is noted and used as a divisor in the subse uent percentage calculations. If the sample boils partly below 1003 C., this method of correcting for volume cannot be employed; it is necessary in such cases to employ an arbitrary correction for volume based on extrapolation of the expansion rate of the solvent as determined at lower temperatures. After the sample has reached the temperature of the steam and any low-boiling fractions have boiled away the air stream is started through the wet test meter, B, preheater coil C, and 2-mm. inlet tube D. An air-drying tube was formerly employed but was found unnecessary and is omitted. The rate of air flow is not critical, provided the withdrawal of heat by evaporation is not rapid enough to lower the temperature of the sample appreciably. In practice the authors test gasolines at 300 cc. per minute, having found that there is practically no difference between curves obtained at this and at lower speeds. After 25 per cent has been evaporated they generally increase the rate to 500 cc. per minute. At intervals the air flow is shut off long enough to take simultaneous readings on the tube and meter. Results are plotted as liters of air, corrected to 21" C., as a function of per cent evaporated based on an initial 100-cc. sample. 90 80
60
40
20
0
20
In Figure I1 is shown the same apparatus arranged for heating by the vapor of any constantb o i l i n g s o l v e n t , the ones the authors use being principally ethyl ether (b. p. 35" C.) and methanol (b. p. 65" C.). I n Figure 111is shown a curve o b t a i n e d o n c. P. toluene a t 100" C., together with the curve calculated from the vapor press u r e a s g i v e n i n International Critical Tables (3). The close adherence to theoretical saturation will be noted. I n Figure V will be noted the evaporation curves a t 100" C. of two petroleum solvents, one light, A, as received from the refinery, and one heavy, B, as obtained from a recovery system. The corresponding distillation curves by the A. S. T. M. method are shown in Figure VI. Experience has shown that a product to be satisfactory for factory processing should be completely volatile in
FIGURE I1
FIGUREI
n "
433
40
60
PERCENT EVAPORATED
80
100
STEAM INPUT MET€REDAIR
I '
C
DD FUF ATCD
.STEAM CONDENSATE. FIGURE IV. DIAGRAM OF APPARATUS 120 liters of air. As the passage of 120 liters through heavy solvent B leaves about 3 per cent residue, it might be expected that a somewhat similar residue would be found in factory processing, which was the case. On the other hand, solvent A used alone is unnecessarily volatile, resulting in heavy handling losses. Examination of evaporation curves for A and B suggests that a suitable solvent could be obtained by a blend of 6 parts of B and 4 parts of A, as indicated by the line D, the ordinates of which are interpolated between the ordinates of curves A and B on a uniform 60 to 40 ratio. Curve C is the evaporation curve actually obtained from such a blend, indicating that a t least for the type of solvent employed the air volumes required for evaporation are approximately additive. In Figure VI1 are shown the evaporation curves of solvent C a t three temperatures. If the air volumes a t 100 per cent evaporation of these curves are replotted (Figure VIII) as a function of temperature, and compared with similar curves for
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
20
40
60
80
100
0
100
a
PERCENT EVAPORATED
20
40 60 PERCENT DlSTl LLED
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80
100
500 55 400 Q
LI.
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zIJ
300 200 100
0 0
20
40
60
80
PERCENT EVAPORATED
200 400 600 800 LITERS OF AIR PER 100 CC.0F SOLVENT
1000
pure straight-chain paraffin hydrocarbons, calculated from vapor-tension data given by Wilson (8),shown in dotted lines, the curve lies close to that of pure octane. It may be said, then, that in respect to the air volume required for complete evaporation, solvent C is equivalent to octane. It would be a fair assumption therefore that its "equivalent molecular weight," and consequently vapor volume, explosive limits, etc., a t complete evaporation would correspond approximately to those of octane. This conception is very useful in designing fume systems, recovery equipment, etc., for handling the solvent, the necessary data being difficult to secure by other methods. The formula used for computing the curves for the pure hydrocarbons is: T 273 A P 273 cc. evaporated = V X -X 294 A - P x E O x m -Mx - =1 5.45 x 10-6 x V X A x P X h!l 22.4 D (A - P) D where V = liters of ingoing air, measured at 21" C. and prevailing atmosphericpressure A T = temperature of evaporator tube (" C.) A = atmospheric pressure in mm. of mercury (assumed as 760 in computing curves above) P = vapor pressure of liquid at temperature T in mm. of mercury M = molecular weight 22.4 = Avogadro's constant D = density of liquid at 21' C. (grams per cc., 3)
methods, such as the one described above, which measure the volume of incoming air rather than volume of outgoing vapors, are not independent of barometric pressure, and the magnitude of the correction for barometric variation increases with the vapor pressure of the solvent-i. e., with the discrepancy between incoming and outgoing vapor volumes. A further correction should theoretically be applied to the authors' indicated meter readings, based upon the varying hydrostatic head created by the height of liquid in the sample tube. I n practice such corrections for the type of solvent which the authors employ, and for the magnitude of barometric and hydrostatic pressure variations encountered, do not total more than 5 per cent, so that they can be ignored. A more rigid application of this method, especially to more volatile solvents, would necessitate taking these corrections into account. It is also realized that the method is defective as a measure of the evaporation rates a t incomplete saturation, such as occur in industrial applications, because, as pointed out by Lewis, Squires, and Sanders (4), it takes no account of the factor of variable diffusion rates of the various components. It would not be safe, therefore, to extend the method to mixtures of components of widely different molecular weights. For the particular type of solvents referred to, however, it has been found to furnish a good index to factory performance.
The formula differs from that used by Hofmann (2) in the presence of the terms A in the numerator and A P in the denominator. It seems probable that evaporation test
The writers wish t o acknowledge the assistance of the Will Corporation, Rochester, N. Y., in developing the improved equipment and the permission granted by the officials of
+
-1
Acknowledgment
NOVEMBER 15, 1936
ANALYTICAL EDITION
the Garlock Packing Company to publish the above experimental data.
Literature Cited (1) Bent and Wik, IND.ENQ.CHEM.,28, 312 (1936). (2) Hofmann, Ibid., 24, 135 (1932). (3) International Critical Tables, Vol. 3.
435
(4) Lewis, Squires, and Sanders, IND.ENG.CHEM.,27, 1395 (1935). (5) Lowell, Ibid., Anal. Ed., 7, 278 (1935). (6) Rubek and Dahl, Ibid., 6, 421 (1934). (7) Wetlaufer and Gregor, Ibid., 7, 290 (1935). (8) Wilson, IND.ENG.CHEM.,20, 1363 (1928). (9) Wilson and Worster, Ibid., 21, 592 (1929). RECEIVED April 30, 1936.
A New Colorimetric Procedure Adapted to Selenium Determination KURT W. FRANKE, ROBERT BURRIS,
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I
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AND
ROBERT S. HUTTON, South Dakota State College, Brookings, S . D.
barium sulfate mat. When the filtration is completed the rubber sleeve is slid onto the lower tube while the upper one is held in place. The filter paper disk and mat are removed and allowed to dry for a few minutes. Two drops of glycerol are placed on a microscope slide, and the slide is inverted and pressed gently and uniformly in contact with the colloidally covered mat surface. Colorimetric comparisons are made with standards prepared in the same way from standard solutions.
A m m.
-2 0-
1
When the 20-mm. tube is used in the case of colloidal selenium, comparisons are best made in a range from 0.005 to
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--k--Coarse
r-
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E
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1
1'4
m m.
porous disc
hut outside must he the Some as cylinder above, 45 Q sleeve of rubber tubinp holds them together
FIGURE1. FILTER TUBEUNIT
THE
instability of many colloidal sols necessitates the daily preparation of a new set of standards for colorimetric comparison. The following procedure provides for more permanent standards, eliminates turbidity difficulties, and also makes feasible the filing of the unknowns. A diagram of the apparatus is given in Figure 1. A disk of filter paper (S. & S. 589 blue ribbon) is moistened and placed on top of the porous plate tube which has been attached to a suction filter flask. The cylinder is placed on the filter paper and held in place by a sleeve of rubber tubing. A mat of barium sulfate, freshly precipitated, is filtered onto this filter paper. For a disk 20 mm. in diameter a mixture of 20 cc. of 1 per cent barium chloride and 10 cc. of 1per cent sulfuric acid provides a satisfactory mat for collecting colloidal selenium. This retains colloidal selenium as a thin surface layer against a standard white background. Little suction is applied when filtration of barium sulfate is first started, but a vacuum of about 30 cm. should be reached at the end of the mat formation and should be maintained during the filtration of the colloidal precipitate. The mat having been formed, the solution containing the colloidal precipitate is shaken and poured into the inclined filter
is of the Bame magnitude. Above this range gradations are not so sharp, unless a unit of larger diameter is used. It was
were retained by a barium sulfate mat prepared as for selenium. Preliminary experiments indicate that for white or other light-colored sols black or other colored mats can be used.
.m
.Cl
.02
.M
a .08
FIGURE2. SELENIUNI STANDARDS One-half size, panchromatic, process film, F 7.7, Wratten B filter, 2.5 minutes' exposure
It is obvious that this method can be used in micro ranges by substitution of mats of small diameter. For example, 0.001 mg. of iron converted to Prussian blue gives a very distinct color on a 14.1-mm. mat; a 5-mm. mat would give the same intensity with an eighth this amount of Prussian blue:
Conclusion A method is described by which colored precipitates of colloidal fineness can be filtered onto a mat of barium sulfate. Permanent standards are produced and turbidity difficulties are removed. The procedure has been used for the estimation of 0.005 to 0.15 mg. of selenium with an accuracy of 0.001 to 0.01 mg. REOBIWED July 18, 1936. With the permission of the Director of the South Dakota Agrioultural Experiment Station as communication No. 23 from the Department of Experiment Station Chemistry.
