New Aliquot and Filter Devices for Analytical Laboratories T. M. SHAW,Bureau of Chemistry a n d Soils, Washington, D. C.
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N THESE laboratories large numbers of mechanical analyses of soils have to be made as a part of the service routine. It is desirable to make these as rapidly as is possible, consistent with accurate results. I n this work there have been developed recently two devices which have materially speeded up the process and diminished errors due to the personal equation. These devices are of general utility wherever processes of similar type are employed in routine procedure, and may therefore be of general interest. USP PENSION ALIQUOTDEVICEWITH DEPTH CONTROL I n the course of a mechanical analysis by the pipet method ( I ) , three aliquots are taken using a Lowy pipet, which is lowered into the suspension to the proper depth. I n ordinary- routine, suspensions of eight soil samples are placed in sedimentation c y l i n d e r s for pipetting. The samples are pipetted with a regular time interval between them. This m e a n s t h e d e p t h a t which the aliquot is taken is the same for the set of eight samples. It was with this idea in mind and in an effort to eliminate errors in lowering the pipet to the correct depth that the following device w a s d e v e l o p e d . It makes possible the lowering of the pipet definitely to any prearranged d e p t h a n d repeating the o p e r a t i o n any number of times without a change in setting. From Figure 1 it will be seen t h a t t h e d e v i c e consists of a f r a m e , A , B, C, supporting two parallel rods, D and E, which serve as rails. Upon t h e m t h r e e FIGURE1. SUSPENSION p a r a l l e l blocks, F , G, H, ALIQUOT DEVICE slide, G is p r o v i d e d with clins. J and K. with which the pipet is secured firmly. F and serve as limits, limiting the travel of G. L , M, and N are thumb wheels used in securing the position of F, G, and H. 0 is a calibrated rod used in adjusting the distance between F and H-i. e., depth of pipetting. For smooth operation, a good mechanical construction is necessary. Blocks F, G, and H should have good bearing surfaces, and if rods D and E are in alignment, no jamming or chattering of F, G, or H should result. Any shaking of G would result in undue disturbance of the suspension. I n operating, the procedure is as follows: The pipet is inserted in clips J and K provided on G. N is loosened and the upper edge of H is set against the scale on 0 a t the desired depth of pipetting. N is tightened and requires no further attention until a new depth is used. The pipet
tip is brought directly over the suspension. M is loosened and G allowed to descend until the tip of the pipet touches the top of the suspension. F has also descended with its lower face in contact with the upper face of G (Figure 2). L is tightened and G allowed to descend again until its lower s u r f a c e is in contact with the upper surface of H . At this point the pipet has r e a c h e d the desired depth and the a l i q u o t i s t a k e n . When this has been done, L is r e l e a s e d and G is raised as far as it will go, carrying with it F and its associ ated parts. When the pipet is clear of the suspension, M is secured and the pipet is removed from J,K. The time consumed in the c o m p l e t e operation, exclusive of filling the pipet, usually does not exceed 10 seconds. Over a period of 15 months the device has proved to be accurate, rapid, trouble-free, and labor-saving. Its cardinal a d v a n t a g e lies in the FIGURE 2. SUSPENSION ALIfeature whereby, once set for QUOT IN PLACE a particular depth, t h e accuracy of this most important factor is insured throughout the series of samples. NEW FILTER RACK In the analysis of a large number of samples by the pipet method ( I ) , a considerable amount of filtering is necessary. The filtering is done by means of Pasteur-Chamberland filter tubes, using the laboratory vacuum, and the filtrates are saved for evaporation and determination of soluble material. To aid in this work, a filter rack was developed. Figure 3 shows this device as used in this laboratory, connected to the filters shown in the foreground. Figure 4 shows the
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FIGURE 3. FILTERRACK
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FIGURE 4. TOP VIEW OF UNIT OF RACK
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A , glass T; B , rubber tubing. C, clamp; D ,stopoook; E , suotiod flask
FIGURE 5. SIDEVIEW OF UNITOF RACK top view of one of the units, and Figure 5 is a side view of A , glass T; B , rubber tubing; C, clamp; D,stopoook: E , suction flask the same unit. As now constructed, the rack is composed of eight such units (Figure 6). The frame is constructed tubing, glass T connections, and flasks. The individual of ordinary 8/4-inch lumber finished with a heavy coat of stopcocks afford several conveniences: The rack may be shellac as protection against used for less than the usual moisture. I n Figure 5 n u m b e r of s a m p l e s b y the i n c l i n e d shelf may be simply t u r n i n g off the unseen. Upon this shelf the used units, or, if a few sammanifold a n d s t o p c o c k s ples filter before the others are mounted by means of the group, they may be of brass clips. T h e shelf c u t o u t a n d a l l of t h e serves a double purpose in vacuum utilized in speeding that it affords a m e a n s of up the remaining ones. securing the parts and I n addition to its present helps to p r e v e n t p o s s i b l e use, it is quite possible that c o n t a m i n a t i o n of the filt h e rack^ m a y b e used i n FIGURE 6. FILTER RACKOF EIGHTUNITS trates should they spray or connection with other filterfoam up into the connecting ing devices such as Biichner tubes. Flasks of 250 ml. capacity are used, and since 200 funnels, conventionally mounted in the mouths of the filter to 700 ml. of filtrate are required, the flasks have to be emptied flasks. Uneven distribution of vacuum may easily be cleared several times. This is accomplished easily, as the filters are up by inserting additional T’s a t convenient places in the disconnected from the flasks and removed in a group, leaving manifold; one connection a t each end is used a t present. the flasks completely accessible from the front and in posi- The number of units in the rack may be varied to suit the intion to be disconnected with an ease comparable to that dividual need of the user. of breaking an electrical connection by means of the conLITERATURE CIT~D ventional attachment plug. After emptying, the flasks are (1) Olmstead, L. B., Alexander, L. T., and Middleton, H. E., U. S . replaced in the same manner. Since the rubber connections Dept. Agr., Tech. Bull. 170 (1930). are securely fastened and in individual units, the analyst is spared the confusion attached to the usual tangle of loose RECEIVED June 27, 1932.
Selenium in Determination of Nitrogen by Kjeldahl Method J. TENNANT, H. L. HARRELL, AND A. STULL, New York Hospital, New York, N. Y. AURO (a), from results on cereal extracts, has suggested the use of selenium in place of copper sulfate as a catalyst to reduce the time of digestion in Kjeldahl nitrogen determinations. I n the aqueous extracts used in the Allergy Clinic of the New York Hospital, standardization is based on the nitrogen content as determined by the conventional Kjeldahl method ( I ) . As the digestion of these extracts required several hours, Lauro’s modification was applied to their standardization. It will be evident from Table I khat these extracts are of wide variety. Analyses of extracts, on which parallel nitrogen determinations were run using copper sulfate and amorphous precipitated selenium, showed checking results with a decrease of one-half to two-thirds the time necessary for digestion where selenium was used. Owing to the variation in burners and the fact that heating in some cases was carried on longer than necessary in order to assure complete digestion, the time factor is not absolute. The results were considered sufficiently conclusive to adopt the modification in these analyses.
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T A B LI.~ ANALYSESOF EXTRACTS EXTRACT
COPPERSWLFATE M g . N/cc.
Date
0.53
Pumpkin Potato flour
0.17
Horse epithelium
Green pepper Guinea p1.g epithelium Pumpernickel Birch pollen Ash pollen Oak pollen Poplar pollen Plantain pollen Ragweed pollen 1 Ragweed pollen 2 Ragweed pollen 32 Timothy pollen 31 Timothv Dollen 32 Cashew-nit Urjne 1 Urine 2
0.07 0.04 0.26 0.88 1.26 0.58 0.43 0.58 0.92 1.00 0.43 0.39 0.98 0.62 0.31 0.40 3.73 7.29 9.85
Min.
IS0 150 300 165 140 140 175 170 215 180 170 166 170 135 180 75
SELENIWM Mg. N / c c . Min. 0.56 0.53 0.18 0.06 120 0.27 0.86 1.26 0.56 0.42 0.60 0.94 1.00 0.46 0.37 0.99 0.63 0.32 0.40 3.72 7.26 9.90
(1) Assoc. Official Agr. Chem., Methods, 2nd ed., 1925. (2) Lauro, M. F., IND.ENO.CHBM.,Anal. Ed., 3,401 (1931). REC~IVED May 3, 1932.
75 150 90 50 75 136 85 135 90 95 90 90 60 150 35
