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 :
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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
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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.
