AIDS FOR THE ANALYST A n All-Glass Rotary Film Evaporator Murray E. Volk, Nuclear Instrument and Chemical Corp., Chicago 10, Ill. 7EYERAL
devices have k e n described which utilize the prin-
S ciple ' of evaporat,ion from a rotating film under vacuum for concentrating solutions of heat-sensitive materials. A simple all-glass apparatus which employs this principle has been developed. I n this device connection is made from a rotating round-bottomed flask to a commercially available lyophilizing unit through a lubricated ball joint. Since the axis of rotation lies :~pprosimately 15 dpgrees from the horizontal, somewhat more than one half of the volume of the round-bottomed flask may be occupied by the solution when the evaporation is started without clanger of mechanical transfer. The turbulence induced in the liquid by the rotatory motion of the flask effectively prevents humping due to local superheatirg. Aqueous and ethanolic solutions in flasks of 2 liters in capacity have heen evaporated to dryness in 1 hour with this device. The figure illustrates the apparatus. The lyophilizing w i t , 11: 18 X 50 cm. in outside dimensions is firmly mounted on a tablc. Into one of its six outer 29/42 standard taper joints is fitted R short adapter, B , carrying the ball portion of a 35/20 ball joint. The hollow axle, C: is fabricated of 22-mm. outside dimension tubing and carries the socket part of the ball joint a t its ited end, and an inner 24/10 standard taper joint at its lower end, to which the round-bottomed evaporation flask may. be attached. The Kjeldahl trap, D , while included in the design, IILZS not proved essential to satisfactory operation. The entire hollow shaft is 29 cm. long. The bearing, E, consists of four rubher wheels mounted in pairs and serves to support the weight of the rotating aysembly and to absorb any vibration. The drive is a metal pulley, F , with a center hole bored somewhat larger than the glass shaft. The pulley is fastened to the shaft hy four -411en screws which make contact with a split-brass cylinder. The two halves of the brass cylinder are cushioned from the glass tubing by a layer of sponge rubber. A V-belt of suitable motor, G, dimensions drives the pulley. A '/l&orsepower equipped with a speed reducer turning at approximately 40 r.p.m., rotates the assembly. h variable source of power may he employed to control the speed of rotation, but its use has not been necessary. Only dpiezon ?; has proved suitable as lubricant for ,the rotating ball joint. A vacuum pump capable of maintaining prespure below 0.5 cm. of mercury is adequate for operating the apparatus. During operation, the pressure differential is sufficient to hold the evaporation flask to the rotating d e , although spring clips may be used. This design for a device for the concentration of heat-sensitive materials using evaporation from a rotating film under vacuum possesses several points of superiority over others previously described. The apparatus of Craig, Gregory, and Hausmann [ANAL.CHEX.,22, 1462 (1950)l requires the rotation of both the evaporation flask and the condenser bulb. The inherent high
DRY
ICE
inertia of this system makes accidental breakage of the apparatus during operation very possible. Partridge's device [Partridge, S. &I., J . Sci. Instr., 28, 28 (1951)] rot,ates a longnecked flask and employs only a single hearing, but the vacuum take-off is complicated, requiring special machining and packing of the vacuum gland. Only the single hollow shaft and the evaporating flask rotate in the device presented here. Use of a lubricated ball joint as the rotating vacuum connector makes construction simple and straightforward. The entire apparatu? can easily be fabricated Lvithout outside nssiPtance using available standard components. -4condenser of spherical shape such as employed in the apparatus of Craig and coworkers performs its function in a relatively inefficient manner. The ratio of condensing surface area to total volume of the condenser is minimal. The work of this laboratory involves the concentration of solutions containing considerable quantities of radioactivity. The lyophilizer used in the authors' apparatus has been found to act as an efficient condenser which ensures that any radioactivity in solution, which might be carried mechanically into the condenser, is trapped and does not escape into the pumping system and 1)ossibly contaminate the laboratory. Khile a commercial lyophilizing unit such as this device employs is a relatively expensive piece of equipment, it is in use in many laboratories working with biological extracts and other heat-sensitive materials. The alternative function suggested hew, as a condenser for a liquid concentrating device, which use in no way interferw with its original purpose, serves t o increase its usefulnesp. ACKNOWLEDGMENT
The autlior's appreciation is extended to F. E. Icelsey 01 thc University of South Dakota and to R. H. Delgado of this company, who contributed to the design and construction of ihe apparatus.
Matched Test Tubes in the Beckman DU Spectrophotometer Bernard E. Saltzman, U. S. Department of Health, Education, and Welfare, Division o f Special Heath Services, Cincinnati, Ohio
sets of matched test tubes are a great convenience for L colorimetric analysis. Tedious rinsing of photometer cells is no longer required when a dry tube is available for each sample ARGE
or standard color. The light path of 2-cm. length which may be obtained with 10 ml. of solution is more convenient in many cases than that of the usual I-cm. cells. Little time or expense is required t o prepare such tubes, and the accuracy of analysis is not noticeably affected by the slight variation of 2 or 3 parts per thousand in the diameters of the tubes of any one set. Only a single day's time was required t o produce four matched sets comprising 200 tubes. The installation of the tube holder does not require the removal of the regular cell holder and does not impair the use of the instrument with the regular cells. I n this latter case, a blackened cork is used to seal the test tube opening. This report describes the adaptation of the Beckman D U spectrophotometer for this purpose, and a simplified method of matching the tubes. The tube holder was prepared from a hardwood board by boring holes as shown in Figure 1. Care was taken t o maintain true alignment and accuracy, and the two large surfaces were made plane and parallel. The size of the hole may be adjusted t o fit the tubes at hand snugly and yet freely. The lower part of the tube was centered by its round bottom resting on the ledge a t the bottom of the hole. The lower 5/8-inch hole provided 1207
1208 drainage in case of accidental spillage, and vias closed with a blackened cork inserted from underneath. A C X 1.5 inch cardboard mailing tube was used as a tube cover. The holder and cover were finished with a dull black paint. The holder may be inserted between the cell holdei provided with the instrument and the photocell compartment, or in place of the original cell holder if the latter is no longer required. In either case, a nen set of longer screws is required to reassemble the apparatus, the proper length being 5.75 or 5 inches, respectively. These four screws may be improvised by threading a short distance a t both ends of rods with an 8-32-SC die, and screwing a knurled battery post nut tightly on one end. .ifter assembly, the holder and cover may be tested for light leaks by turning the instrument switch on and setting the density scale t o infinity, keeping the lamp off. K h e n the photocell shutter switch is opened and closed, there should be no discernible motion of the meter pointer. Two gross of 22 X 175 mm. test tulws with lip, Kimble Catalog 45050, were procured for matching. The tubes n-ere examined carefully, and a #mall number rejected for being off-color or too large to enter the hole in the holder freely. The remainder were cleaned with dichromate-sulfuric acid and dried. r\ convenient solution for matching the tubes a t a nave length of 373 mp was made by diluting 8 mi. of lo-\- sodium hydroxide and 6.38 ml. of 0.1S potassium dichromate to 2 liters. This solution was adjusted PO that it gave an absorbance (optical density) of exactly 0.500 when compared with distilled water in a 1-em. cell. The strength of the solution as prepared \vas 2.3y0 greater than the theoretical value [Haupt, G. W., J . Research S a t / . BUT.Standards, 48, 414 (1952)l t o allow for this adjustment and for possible losses due t o reduction of the chromate by impurities. Ten milliliters of the colored solution were then placed in each test tube, and the absorbances were measured as described below. These figures when multiplied by 2 gave the effective diameters of the tubes in centimeters. The measurements revealed the fact that less than 10% of the tubes were perfectl2; round. Hence, each tube was rotated and the major and minor axes and density readings were marked with a crayon. T o facilitate the handling of the large numbers of tubes, an open-top shield was used in a darkened room, with a shielded light for the meter scale. This left the protruding tops of the tubes free for turning and marking. I n order to use the most accurate portion of the instrument scale (the low density end), all densities were measured relatively agninst one selected round tube as a reference, which was always I ~ Fd in one marked position. This selected tube, containing 10 n- I . of the adjusted chromate solution, was placed in the holder, r id the instrument was balanced against the scale set a t zero .~bsorbanceor 10070transmittance (sxvitch in 1 position). Various settings of the sensitivity control were used and the balance was made by adjusting the slit width t o bring the meter needle to zero, until the sensitivity setting was found where deflecting the absorbance scale 0.005 from the balanced position resulted in a meter needle deflection of 5 units. The relative densities of large numbers of tubes could then be rapidly and accurately measured within 0.001 by using any fixed convenient density scale setting and correcting this value by the meter needle deflections.
ANALYTICAL CHEMISTRY
Figure 1. Test tube holder for Beckman DU spectrophotometer
I n order to obtain the absolute absorbances, the density of the standard reference tube \vas then measured in the instrument against a tube of like diameter containing distilled water. This value was later used to compute the effective diameter range of each matched set. The value was added to the readings to give the absolute absorbances of the tubes. The latter figure when multiplied by 2 gave the effective diameters in centimeters. The marked tubes were then sorted into groups according to their minor diameters (which were considered less affected l)y flaws in the glass than the major diameters) and their degree of ellipticity. The relative density markings on the tubes gave the differences in parts per thousand directly, a$ the total absolute absorbances were very close to 1. The groups vere chosen so that each set was matched in minor diameters within +2 to 3 parts per thousand (relative densities matched within f0.002 to 0.003). The tubes with an ellipticity exceeding 11 parts per thousand (density difference between major and minor axes esc*eeding0.011)were rejected.
A tabulation of the results (Table I ) shows the four groups finally selected, comprising a total of 200 tubes out of the 288 tested. Each of the selected tubes \+-aspermanently marked with the letter of the group to which i t belonged, just under the lip in the exact position of the minor axis. Khen the tubes were used for analytical work, care was taken to position the let,ter so that the light went through the minor axis. Inmarking Table 1. .\latched Groups Obtained from 288 Test Tubes accurate positioning was not likely to result in an error of more (Figures are number of tubes) 2l to 3 parts per thousand with t'he most elliptical tubes of than b1inor .kXis Ellipticity, Parts per Thousand ~ ~in t ~ each set. It was thus concluded that the sets should match within Group Diameter, C m . 0-3 4 5 G 7 8-9 10-11 Crroiip 0.5%) which is well within the accuracy of usual colorimetric 8 4 5 2 1 30 A 2.031-2.040 .5 5 10 8 23 9 17 21 0 97 B 2.020-2.030 procedures. 1 ; 9 3 9 8 46 c 2 010-2 019 3 3 n 2 000-2 009 3 4 . j l i 4 27 I n order to know the useful wave-length range in which these tubes could be used, the transmittance characteristics of the glass were also determined. These measurements, given in Table 11: Table 11. Transmittance Characteristics of Kimble Glass Test Tubes are the absorbances of a tube filled Lvith distilled water compared [Absorbance of test tube filled with distilled water C S . air (no tubc) as to air (no tube) as a reference. The tubes were found suitable reference] for use over a wide wave-length range. Ware Wave Wave The tubes have been used for several years, with excellent Length. Lengtli, Length, Yip Ijensity M p Density bfp Density results. Cleaning with strong chemicals such as dichromate350 0.077 550 0.033 i50 0 . OIj8 sulfuric acid or alcoholic potassium hydroxide is kept to a mini375 0,050 575 0,030 775 0.058 mum to avoid etching the glass. Generally the tubes are rinsed 4on 0.042 600 0.033 son 0 055 425 0.046 025 0.037 825 n. 055 with distilled water, the outsides wiped dry with a towel, and the 450 0.043 e50 0.035 850 0.071 475 0.040 675 0.036 875 0.078 tubes dried upright in an oven a t 110" C. The tubes have been ,500 0.03~ ion O.03G 900 0.087 525 0.034 725 0.044 found to be accurately matched for all the colorimetric procedum used; uniformly straight plots have been obtained.
