INDUSTRI.4L A S D ENGINEERISG CHEMISTRY
326
sidered negligible in a substance containing 70 to 80 per cent moisture. Even though the fresh material is thoroughly mixed, a large sample should be used for the moisture determination in order to obtain a representative share of the larger particles. After this material has been dried, ground, and thoroughly mixed, it is homogeneous and small samples can be used safely for other analyses. Ether-extract determinations were made on samples stored in tightly stoppered bottles at room temperature for 12 weeks. No appreciable difference was noted between these values and those obtained from the freshly prepared samples. As the other constituents may be considered stable, it appears that these samples may be stored for a t least 12 weeks without appreciable change in the values obtained by proximate chemical analyses. These samples, however, are extremely hygroscopic and should not be unduly exposed to the air. The Dog Food Division of the Institute of American X e a t Packers (2) has recommended a method for the preparation of samples and determination of moisture content of canned dog food in which the food is first dried sufficiently to permit grinding and the moisture content is calculated The food is then giounrl, mixed, and whsampled to determine the residual
VOL. 10, NO. 6
moisture content. This method is open to the criticism that dog foods which have been dried sufficiently to grind easily are often hygroscopic enough to take up considerable amounts of moisture from the air during the grinding, rendering the subsampling inaccurate. Thus the accuracy of the method is dependent upon the humidity of the air and the moisture content of the food during the grinding process I n the method proposed in this paper, this possible source of error is eliminated by determining total moi-ture before the food is ground. The use of the composite dry sample, obtained from the moisture determination for the other proximate analyses, eliminates t o a great extent the errors which usually accumulate in the value obtained for nitrogen-free extract.
Literature Cited (1) I s s o c . Official .Igr. Chem., Official and Tentative Methods of Analysis, 4th ed., p. 353 (1935). (2) Dog Food Division, Institute of American !vIeat Packers, "Methods of Chemical .inalysis for Canned Dog Foods."
Syringe Attachment for Accurate Volumetric Work ALBERT L. CHAIVEY Los Angeles County Hospital, L o s .4ngeles, Calif.
B
Y PROVIDIKG suitable stops for measuring linear displacement, the hypodermic syringe is capable of ex-
ceptional accuracy in volumetric measurement, thus replacing the volumetric pipet, as has been shown by Krogh and Keys (1, 2 ) . -4modified arrangement as shown in Figure 1 has the ad-
C
FIGCRE1
\ antage ut ieadily elinlinating any air bubbles produced duiing filling and is also more convenient to handle in use. It is simple in construction and mag be attached to the ordinary syringe. The shape of the flange, D , a t the top of the ordinary syringe makes it very convenient to use as a stop. The rod, (Figure is threaded into a metal cap, A , which fits over the end of the plunger and is held in position after calibration by a lock nut. The top of the plunger and the top flange of the syringe should be ground on a flat glas5 plate with fine silicon carbide before assembly. Saturally this grinding should he as nearly at right angles to the axis of the syringe as possible. The cap, A , is held o n the plunger by a threaded collar, B, and a suitable washer of fiber or soft metal. In use the syringe is slowly filled until by a slight rotation the end of the rod may slip over the projecting flange. The excess liquid is then discharged. The exact volume ithen ready for discharge and by a slight reverse rotation the rod slips over the side of the flange.
The syringe is excellently adapted to blood chemical analyses, where the exact volume of blood may be measured a t t'he time of taking the sample and added directly to the tungstic acid or other precipitating fluid. I n drawing blood it is almost impossible to avoid the forniation of some gas bubbles or foam. By taking slightly more than the desired sample, and then inverting the syringe, the bubbles together with the excess sample can be dipcharged into absorbent. cotton.
FIGCRE 2
JUNE 15, 1938
.kNALYTICIL EDITION
A cleaner delivery results from using a needle with the syringe, and because of the small size droplets produced accuracy is more readily obtained. A 16- or 18-gage needle gives drops small enough for ordinary work where 5-cc. samples or larger are used. Calibration is readily made with distilled water by weighing the discharged sample and adjusting the threaded rod which may then be locked in position by the locking nut. By providing two rods of different lengths as shown in Figure 2, the syringe may be made to deliver three different volumes, one for each rod and one for their difference in length-for example, with rods for 2 and 5 cc., a 3-cc. volume may be obtained by going from the 5-cc. position to the 2-cc. position. Such a syringe appears to have the following advantages over a pipet, many of which have been already listed by Krogh :
327
1. It is much faster to use, since there is no drainage time. 2 . Volatile or poisonous liquids may be handled without danger. 3. Considerably greater accuracy is attainable. Using well-constructed 10-cc. syringes, calibration is readily carried out to 1 mg. or 0.001 cc. of water. For samples smaller than 2 cc., greater accuracy can be obtained by using smaller syringes. 4. Fluids of increased viscosity or poor drainage are measured without increased error.
Literature Cited (11 Krogh, d..ISD. ESG. CHEM.,Anal. Ed., 7, 130 (1835) (‘71 Krogh and Keys, J . Chern. Soc., 1931, 2436. RECEIVED J a n u a r y 31, 1938.
Apparatus for Evaporating Solutions on Electrodes HARLEY A. WILHEL3I Iowa State College. Ames. Iowa
S
OME methods of quantitative spectiographic analysis involve the evaporation of definite amounts of solutions directly on electrodes. S e e d for an apparatus by means of which several evaporations could be carried out simultaneously and conveniently by one operator developed in the author’s laboratory n-hen a number of samples of drinking n-aters were t o be analyzed spectrographically for fluorine. The apparatus, which was built for that purpose and is described below, i5 useful also in other work involving the analysis of solution^ by cpectrographic means.
electrodes firmly in place. A piece of heavy copper sheet, B , bent to form a flat-bottomed trough that is open a t the two ends, is fastened in an inverted position on the bottom of the copper bar. The bar is about 2.8 cm. (1.125 inches) square by 37.5 em. (15 inches) in length.
FIGURE 2 . EVAPORATOR
FIGURE1. GOOSESECK PIPET Figure 1 shows the gooseneck form of the pipets, made from 12-mm. glass tubing, that are used in the apparatus. Each pipet will hold at least 2 cc. of solution without leaking from the tip. However, the solution can be forced out at the tip by placing a finger over the large end of the pipet and pressing down. The slight piston action of the finger in the pipet then forces the solution out. A strip of adhesive tape around the body of each pipet keeps them from slipping when placed in a wooden clamp. The arrangement of the pipets, electrodes, and other parts of the evaporator is shovn in Figure 2, where twenty electrodes and a unit of tR-enty pipets are mounted in position for the evaporation of solutions. Tventy other pipets removed from the apparatus show more clearly the arrangement of the parts. The electrodes fit in holes in the copper bar, A , and setscrews hold the
I n using the apparatus, the electrodes and pipets are put in place, then measured representative portions of the solutions to be analyzed are introduced into the gooseneck pipets. The bar is then brought to the proper temperature by regulation of the microburner, and the solutions are forced dropwise from the pipets and onto the electrodes where the evaporations take place. At the finish of the process, about 2 drops of pure solvent are placed in each pipet in order to transfer the last part of each solution to its electrode. The rate of evaporation depends principally on the bar temperature, which is usually maintained high enough for rapid evaporation without boiling or spattering of the solutions from the electrodes. RECEIJED Februarq 18 1938.
