Removal of Air from Powders in Density Determination J
ERNEST L. GOODEN, U. S. Department of Agriculture, Agricultural Research Administration, Bureau of Entomology and Plant Quarantine, Beltsville, Md.
I
N THE determination of the density or specific gravity of
'
given sample. The ideal is to maintain continuous but not violent bubbling, until not enough air is left to affect sensibly the measurement of the sample volume. A test for completeness of air removal in this sense is to observe whether the height of the liquid in the pycnometer changes on application or removal of vacuum. The test is most sensitive when the full vacuum is suddenly released, as b y jerking the finger away from E, and when the bottom of the meniscus is nearly in line of sight with some letter or other marking on the flask. K h e n the last significant traces of air are persistent, two means are available for hastening their removal. One is to stir the sample with a stiff wire rod, R, extending through a flexible rubber stopper in the top of the bell jar. Another may, which is more convenient and may prove adequate, is to jerk the flask repeatedly b y jiggling the electric plug that connects the transformer to the house line. The current should not be left on longer than necessary, lest in its vacuum-jacketed condition the vibrator should become overheated. The time required for the whole operation of air removal is usually only a few minutes. The density values obtained by this method are equivalent to those by the more common process, for in both cases the air removal is carried to the same end point as indicated by the expansion-contraction test. The saving of time by the method here described may be negligible or large, depending on the difficulty of manipulation of the given sample by the older method. The principal advantages, evident particularly with the more troublesome samples, are convenience and relative safety from loss of sample by spurting. The term
fine powders by the method of immersion in a liquid in a pycnometer, it is often difficult to remove the air from the spaces between particles, even though the sample be of a material that in coarser form is easily wetted by the immersion liquid. The air is commonly removed by keeping the open pycnometer, containing the sample and enough liquid to cover it well, in a n evacuated space while the air comes off in rarefied bubbles. This process seldom goes through smoothly; the bubbling is spasmodic, sometimes coming in unanticipated bursts that throw part of the sample out of the bottle. The last bubbles of air hang on tenaciously, and considerable agitation is required to shake them out from the interior of the powder bed. This part of the work is commonly handled by alternate stirring and evacuation, which often proves to be a tedious operation. Previous workers have introduced between the pycnometer and the pump a tube designed to catch any particles unavoidably thrown out from the pycnometer; the material thus caught was weighed and allowance made for it in the calculation (1). The advantages of using a suitable wetting liquid have been cited by Nutting ( 2 ) . Where water is used to immerse organic crystalline compounds, it is often necessary to add a wetting agent. With the setup described below the air can be removed smoothly by controlled agitation while the sample is under vacuum. The method has been used in this laboratory for several months. The apparatus, shown in Figure 1,is simple and may be assembled from readily available elements. The source of agitation is a vibrator, V , of the type sold for massaging the face and scalp. In place of the original vibrator head is a special head. H , which holds the pycnometer flask, F; this head consists of a one-hole rubber stopper in which three nails are inserted a t a slight angle to grasp the body of the flask. The vibrator is mounted head upward on a heavy base, B, which rests on a thick rubber sheet, G, covering the top of a wooden block, W . The rubber sheet serves as B gasket between the block and a vacuum bell jar, J, which covers that part of the assembly so far described. One or more air holes, A , are cut in the gasket to prevent the central portion from being lifted when the jar is evacuated. A round-bodied electric lamp cord (Type 85) is run from the vibrator down through the gasket into the block and out at one side, the passage through the wood being bushed airtight with a rubber stopper, S. For convenience in handling there should be a coupling, C, in the line between the vibrator and the gasket. Adjustment of the vibration intensity and of the pressure reduction is provided by the following arrangement: The apparatus is plugged into a continuously variable transformer, T,with voltage range from 0 to 115 or slightly above. The rubber tube from the bell jar to the house vacuum line has a branch connected to a manometer, M . and another branch left open at the end, E. With one hand on the adjusting knob of the transformer and the index finger of the other hand resting lightly over tube end E , the operator has fingertip control over vibration and vacuum at all times. If it is desirable t o maintain a particular adjustment for some time without attention, the transformer may be set for the necessary voltage and the vacuum kept fairly constant by leakage through a screw pinchcock, P; or if the maximum vacuum is desired, the end of a cork stopper may be left resting against E.
M
X little practice will enable one to judge readily the optimum adjustments of agitation and vacuum, both of which may need considerable variation during the treatment of a
FIGURE1
578
ANALYTICAL EDITION
September 15, 1943
"relative safety" is used advisedly, for the method is not foolproof. T h e chief function of the vibration is to loosen the bubbles while they are expanding, thereby permitting them to come off in a comparatively steady stream rather than build UP t o a condition that has to be a concentrated outburst. T h e vibration, judiciously applied, is
579
thus a means of safeguarding against accidents that otherwise are difficult to avoid.
Literature Cited (1) Hillebrand, W. F., U. S.Geol. Survey, Bull. 700, p. 55 (2) Nutting, P. G., J. Wash. Acad. &i., 26, 1 (1936).
(1919).
Determination of Sugars in Apple Tissue Some Modifications of the Usual Procedures R. H. LEONARD, R. C. MEADE, AND R. B. DUSTMAN West Virginia Agricultural Experiment Station, Morgantown, W. Va.
N
UMEROUS methods are available for extracting and determining the sugars found in plant tissues. Many of these have been reviewed recently by Browne and Zerban (6) in their excellent treatise on sugar analysis. The Association of Official Agricultural Chemists recommends 50 per cent alcohol for the extraction of sugars from grain and stock feeds (2) and 80 per cent alcohol for the removal of sugars from plants (1). Jackson and McDonald (8) used the volumetric
dichromate-ferrous ammonium sulfate-phenanthroline method for cuprous oxide and later reported (9) a study of hlunson and Walker's tables for reducing sugars. Erb and Zerban ( 7 ) combined the Munson and Walker procedure for reducing sugars with the Jackson and Mathews (10) method for levulose, in a procedure for the individual determination of dextrose and levulose in a mixture containing sucrose. Erb and Zerban used a table of modified Munson and Walker values and calculated the variable reducing factors for different ratios of dextrose and levulose. Stegeman and Englis (14) compared several volumetric procedures for determining cuprous oxide with the gravimetric method and discussed briefly the possible use of diphenylamine with dichromate in sugar work. The rupture of cell structure before extracting is advantageous. Kneen and Blish (11) heated wheat plants for 15 minutes in an oven held at 140" C. before extracting the sugars. The use of the Waring Blendor for sample preparation has already been described (6) and its application is rapidly being extended.
Experimental RECOVERY OF REDUCED COPPER. I n a n investigation of the sugars in apple tissue preliminary trials with several methods led the authors t o consider the possibility of using diphenylamine with dichromate, for the recovery of reduced copper, as suggested by Taran ( I S ) . Table I shows the values obtained for the recovery of copper from varying weights of cuprous oxide by this procedure, the details of a hich are described later. OF REDUCED COPPER TABLE I. RECOVERY
(From cuprous oxide dissolved in ferric sulfate solution by titration with potassium dichromate and diphenylamine indicator) Copper Copper Added Recovered Recovery
MQ. 42.4 64.5 67.7 82.2 87.2 89.7 91.4 98.8
1m.1
107.9 109.8 121.9 130.7 135.5
Mg
.
42.6 64.8 67.7 83.1 87.7 89.4 91.9 98.6 99.5 107.1 110.8 121.9 133.1 136.1
% 100.4 100.5 100.0 101.1 100.5 99.7 100.6 99.8 99.4 99.3 100.9 100.0 101.9 100.4
Meen 100.32
* 0.18
I n another series of trials varying quantities of cuprous oxide obtained by the Munson-Walker procedure were weighed, dissolved, and titrated directly b y the dichromateferrous ammonium sulfate-o-phenanthrolineferrous sulfate method of Jackson and McDonald and by the dichromateferric sulfate-diphenylamine procedure herein described. Table I1 gives the results of these trials. TABLE11. cO?tIPARISOX O F J.4CKSON AND MCDONALD AND DICHROMATE-FERRIC SULFATE-DIPHENYLAMINE ~ ~ E T H O DFOR S RECOVERY OF REDCCED COPPER Method of Jackson and McDonald Copper Copper RecovAdded Recovered ery Mg. 29.7 50.0 55.1 74.8 91.7 96.0 135.3 179.3 182.3 232.0 254.6 266.5
MQ.
30.2 49.5 55.0 75.3 91.3 96.1 134.8 178.8 181.0 230.8 254.6 268.1
%
101.7 99.0 99.8 100.7 99.6 100.1 99.6 99.7 99.3 99.5 100.0 100.6 Mean 99.97 * 0 . 2 1
Dichromate-Ferric SulfateDiphenylamine Method Copper Copper RecovAdded Recovered ery MQ. MO. % .-
..
39: 7 45.4 49.2 91.9 128.7 136.8 140.0 184.3 225.2 318.2
..
..
39: 5 99: 5 45.8 100.9 48.6 98.8 91.2 99.2 127.8 99.3 136.8 100.0 140.7 100.5 185.9 100.9 227.0 100.8 100 4 319.5 Mean 100.03 f 0.25
EXTRACTIOIU. Several procedures were tried for convenience and thoroughness of ext'raction. Quadruplicate samples were analyzed raw and after being heated for 15 minutes in an ordinary convection oven set for 140" C. Although the differences xere small, the heated samples averaged 7.06 per cent for reducing sugars and 1.66 for sucrose as compared with 6.71 and 1.63, respectively, for the unheated samples. The slight gain in reducing sugars was attributed to a more thorough extraction of the heated sample. However, additional trials weie run to ensure absence of pronounced changes in the distribution of the sugars present. For this purpose triplicate samples of 70 to 80 grams each, of Rome Beauty tissue, in 10-cm. (+inch) aluminum dishes Fith tight coven, were heated for periods of 5, 10, and 15 minutes in an ordinary convection oven set a t 110", 120°, and 140" C. A similar series was run with duplicate samples from a different stock sample of Rome Beauty tissue, but the heating was accomplished in a forced draft oven set for the same range of temperatures and 15-minute heating periods only. Unheated samples were analyzed at the same time for comparison. The average values for both series are shoyn in Table 111.
The original procedure used to extract the apple tissue with alcohol consisted in two hand grindings in a glass mortar, but trials showed t h a t the operation could be accomplished
