~~
Table 111.
Analysis of Gases Liberated from Refinery Caustic Sample
Weight yo 1
H?S CHsSH CzHsSH
48.0, 42.5, 9.5,
45.6, 44.5, 9.9,
2 47.3, 42.1, 10.6,
10 minutes past the retention time of the last mercaptan thought to be presmt (retention times of mercaptans, hydrogen sulfide, and carbon dioxide being determined previously on highpurity samples). Identify the peaks observed on the basis of their retention times (Table I). From their areas, calculate the weight percentage of each component by internal normalization. Convert to the original caustic sample basis by determining mercaptan sulfur potentiometrically (7') on the original sample and integrating the two analyses. RESULTS
Table I1 s h o m the results from four determinations on a synthetic caustic sample corrected for trace amounts of carbon dioxide, air, and/or carbonyl sulfide. The average of these deter-
47.6 42.2 10.2
44.8, 44.4, 10.8,
45.7, 43.8, 10.5,
47.8, 42.0, 10.2,
47.3 41.8 10.9
minations agrees with the blended values within about =!=lo%; for methyl and n-propyl mercaptans, within about 3=25%. The reproducibility of the gas chromatographic analysis itself is much better, as shon-n by the check analyses for No. 1. This is confirmed by neutralizations of a plant caustic solution from a regenerative-type mercaptan removal unit (Table 111). The first two analyses were made the day of neutralization of the caustic and the second two were made one day later. This shows that mercaptans when obtained by this method are stable. Table I1 also compares the gas chromatographic analyses of the gas evolved on neutralization of the synthetic blend with potentiometric titration analyses on the original caustic sample, showing the distribution of total
sulfur content betneen H2S and total mercaptans. Seven refinery cauqtic samples from a mercaptan remoT a1 unit 11 ere analyzed. Only methyl and ethyl mercaptans n ere found, nith hydrogen sulfide: 25 to 50, 8 to 20, and 30 to 65%, respectively. By passing refinery gaseous streams through 30Oj, cawtic colutions, trace amounts of H2S and mercaptans could probably be concentrated sufficiently to be determined by thi. method. LITERATURE CITED
(1) Amberg, C. H., Can. J . Chem. 3 6 , 590-2 (1958). (2) Kfrchmer, J. H., ANAL. C H E x 31, 1371-9 (1959). (3). Liberti, A,, Cartoni, G. P., Chirn. e 2nd. ( M i l a n ) 8 , 821-4 (1957). (4) Ryce, S. A., Bryce, B. A, AN.4L. CHEM.29, 925-8 (1957). (5) Spencer, C. F., Baumann, F., Johnson, J. F., Zbid., 30, 1473 (1958). (6) Sunner, S., Karrman, K. J., Sunden, V., Mikrochim. Acta 1956, 1144-51. (7) Tamale, hf. IT., Ryland, L. B., McCoy, R. N., ANAL.CHEM.32, 1007 (1960).
16th South&-estern Regional Meeting, ACS, Oklahoma City, Okla., December 1, 1960.
low-Temperature Crystallization Apparatus for Semimicro Quantitative Work John P. Friedrich, Northern Regional Research Laboratory, Peoria, 111. HE
need often arises for a means of
Tpurifying small samples of lowmelting compounds by low-temperature crystallization. Carrying out such a crystallization in the laboratory is usually tedious. If a jacketed filter is used a t low temperatures, there is a moisture problem, and accurate temperature control is difficult. Also, in precise work there is the problem of transferring material quantitatively from a vessel to a filter. To eliminate some of these difficulties, such devices as sealed cold boxes and the technique of inverse filtration have been used. The limitations of these are obvious. Various types of crystallization equipment are described in the literature (1-5). Some tend t o be specific rather than general in their applications, while others incorporate one or more of the previously mentioned shortcomings. A crystallization apparatus that reduces the various problems to a minimum and that possesses the versatility required for general use is described in this paper. EXPERIMENTAL
Apparatus. The crystallization apparatus can be constructed of readily available materials and is small 974
ANALYTICAL CHEMISTRY
enough to lend itself conveniently to bench use. A schematic diagram (Figure I ) illustrates the components and their functions in more detail. The circulating pump, L, Model D-6 with graphited asbestos packing around the impeller shaft, is manufactured by Eastern Industries (Hamden, Conn.). The Dewar flask, I (manufactured by Virtis, Gardiner, N. Y.), is constructed of stainless steel and insulated lvith a rigid closed-cell plastic foam. The inner container is 8 inches wide, 11 inches deep, and 8 liters in capacity. The standard cone drive stirring motor, P , is manufactured by E. H. Sargent and Co.; the jacketed micro extraction flask, 0, flask head. stirring assembly, and receiver, -11, by Scientific Glass Apparatus. All exposed copper tubing and the pump impeller housing were insulated with a plastic adhesive tape, Presstite, made by the Virginia Smelting Co. (West K'orfolk, Va.). Reservoir E can be made from any suitable container with a !arge enough volume to allow for expansion and contraction of the circulating liquid due to temperature change. All other components, except the specially built pump stand, are either standard laboratory equipment or are readily available locally. Procedure. The material t o be purified and a suitable solvent are introduced t o t h e jacketed filtering flask of the apparatus, 0, by using a n addition funnel. The stirring motor,
P , is started and adjusted to the desired speed. (The stirring speed and rate of cooling may have a great effect on t h e type of crystal produced and must be controlled carefully.) If the crude material is insoluble in the solvent a t room temperature, valve G is closed and valve F opened. The circulating pump, L, is turned on and steam introduced through the steamjacketed section of the circulating system, D. The circulating liquid (95% ethyl alcohol) is thereby warmed, in turn warming the mixture in the flask. To cool the material, the steam is turned off, G is opened slightly, and F closed until the flow through the cooling bath (isopropyl alcohol-solid COz) is large enough to give the desired cooling rate. Stirring is continued 15 to 30 minutes after the desired temperature has been reached. Stirring is then stopped and the crystals are allon-ed to settle. The stopcock is then opened, and a vacuum applied to receiver. M , removes the solvent and impurities and leaves a relatively dry deposit of crystals on the sintered-glass disk, N . The necessity of the drying tube in this step is evident. It may be desirable to run several recrystallizations. This can be done readily by adding more solvent, closing G, opening F, and warming 0 slonly with a heat lamp to about 0" C. Steam can then be introduced, D, and the entire procedure repeated.
---
In certain applications the stirring mechanism can be eliminated by introducing nitrogen a t low pressure into the vacuum flask, M , to achieve a stirring act,ion by ebulation through the sintered disk, N . SUGGESTED MODIFICATIONS
Several possible modifications have become apparent with continued use of this apparatus. Where very precise temperature control is necessary, a suitable control, preferably a thermistor, could be inserted in the flask, 0,and used to control the circulating pump. An electric heating tape might be preferable to the steam jacket, D,for some applications where reactions or crystallizations are to be carried out a t controlled temperatures above room temperature. Most organic solvents, except ethyl alcohol, attack the graphited asbestos packing in the pump. If it were necessary to circulate a higher boiling liquid, a rotary mechanical seal instead of the standard stuffing box would serve the purpose. Valve G could be moved to a position between the pinch clamp, B, and the bypass line. In this position it would serve the additional purpose of allowing one to use a n external jacketed filtering vessel through A and K . Tubing, K , is connected to a tee in the inlet side of the pump. LITERATURE CITED
Craig, L. C., IND.ENG.CHEM.,ANAL. ED. 12, 773 (1940).
(1)
Figure 1.
Diagram of low temperature crystallization apparatus
(2) Piper, J. D., Kerstein, N. A,, Fleiger, A. G., Ibid., 12, 738 (1942). (3) Quakenbush, F. W., Steenbock, H., Ibid., 14, 736 (1942). (4) Scheraga, H. A., Manes, M., ANAL. CHEM.21, 1581 (1949).
(5) Wendland, R., Zbid., 28, 282 (1956). The mention of firm names or trade-products does not imply that they are endorsed or recommended by the Department of Agriculture over other firms or similar
products not mentioned.
Use of Submicron Silica to Prevent Count Loss by Wall Adsorption in Liquid Scintillation Counting
F. A.
Blanchard and 1. T. Takahashi, Radiochemistry Laboratory, The Dow Chemical Co., Midland, Mich.
ERIOUS ERRORS
in liquid scintillation
S radioassays of part per million level
solutions can be caused by adsorption of the radioactive compound to the wall of the counting vial. Such adsorption was reported by Hayes (2) with dilute solutions of benzoic acid-C14 in toluene scintillator. At 0.1 p.p.m., the counting efficiency was only half that a t concentrations of 2 to 1000 p.p.m. He attributed this to benzoic acid molecules being adsorbed on the walls of the glass container and thereby removed from 4 to 2 T geometry. A number of tracer compounds have shown this effect in our laboratory. A counting sample containing polya~rylamide-c'~,after 1, 10, 100, and 1000 hours in the counter, gave 291,
246, 225, and 175 c.p.m. The scintillation solution, when transferred into a clean vial, gave only a background count rate. However, the sample vial itself, filled with fresh scintillator, gave the full equilibrium count rate of 175 c.p.m. Thus, the compound seemed to be completely adsorbed on the wall of the counting vial. An internal standard used with such a sample would give an entirely erroneous counting efficiency. Of course many compounds do not show such adsorption-for example, the monomer from which the polyacrylamide was prepared. The adsorption might be mitigated by varying the solvent or by adding a carrier. Another approach is to make use of the adsorption by controlling its
effects. Thus, if the adsorption occurs primarily on material suspended in the midst of the solvent, the 4 T geometry is preserved, provided the suspension particles are small compared to the beta-particle range. Such adsorbing material should have an active surface area, for the compound, much greater than that of the glass wall of the counting vial. It should remain dispersed and be relatively transparent. Cab-0-Si1 M5 silica (99.0 to 99.770 SiOt, Cabot Corp., Boston, Mass.) is such a material. The particles are reported to range in size from 0.015 to 0.020 micron, giving rise to a surface area of 175 to 200 square meters of surface per gram. The absorption properties of several polymers on such VOL. 33, NO. 7, JUNE 1961
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