INDUSTRIAL AND ENGINEERING CHEMISTRY
400
With 0.5 ml. of reagent and 0.5 ml. of alcohol per ml. of solution the presence of 10 mg. of potassium increases the sensitiveness of the test to sodium slightly; thus 0.02 mg. of sodium produces a precipitate under these conditions. There is little danger of confusing potassium with sodium even if the quantity of potassium present slightly exceeds the amount permitted b y the sensitivity of the test towards potassium. This arises from the fact that the potassium compound with the reagent crystallizes in long needles which may easily be distinguished from the granular precipitate of sodium magnesium uranyl acetate.
Check Valve for Wash Bottles F. H. SMITH North Carolina Agricultural Experiment Station, Raleigh, N. C.
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HE check valve shown in Figure 1 has been found useful in wash bottles. With its use in delivery tube G, the water from the wash bottle neither spatters nor drips when pressure is applied or released. It also keeps the delivery tube full of water which may be siphoned from the wash bottle dropwise. This type of valve seats promptly and is held in position by the action of both pressure and gravity. FORORGANICSOLVENTS.The valve placed in tube F, through which the air enters the wash bottle, prevents the vapors of organic solvents from entering the mouth. This arrangement is most conveniently used with solvents such as ethyl and petroleum ethers, which do not condense in the valve chamber. For preventing back pressure, assembly F may be used in wash bottles containing either hot water or ammonia. The valves a t F and G may be used separately or in combination. Construction Expand a small bulb 2 to 3 mm. larger than glass tubin which has an internal diameter of 4 mm. Thicken the walls of t%etube at one side of the bulb to form the seat of the valve, as shown in A. Valve B is made from a glass rod with a diameter of 2.5 mm. Ex and the end forming the valve by heating the upper tip of the ro$(cut off squarely) while rotating it in a vertical position until
/ 4 rn m.
A
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A 2.5 mm.
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m
UD
C
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FI
E
FIGURE 1. DIAGRAM
VOL. 8 , NO. 5
Summary Data are presented showing how the iensitivity of magnesium uranyl acetate reagent towards and potassium varies with the proportion of the reagent used in the test and with the presence of alcohol, Literature Cited (1) Noyes and Bray, “Qualitative Analysis for the Rare Elements,” p. 485, New York, Macmillan Go., 1927. R E C ~ I V EJune D 2, 1936.
it will just pass into tube A . Grind with coarse and fine emery until a satisfactory seat is obtained. Cut valve rod B about 23 mm. long, heat the u per end, and press to the shape shown at C (end view) and D (sije view) with a pair of tweezers to permit the air to pass readily. Insert the valve and, after heating tube A with a pointed flame, press in with a sharp pointed instrument to retain the valve as shown in E. Valve assembly E is bent to the shape shown at F,so that it will pass into the neck of the wash bottle, or it may be passed through the stopper and bent as in delivery tube G. With valve F a vent tube is used which is closed with the thumb when the wash bottle is in use. To the upper part of tube F a piece of rubber tubing 25 t o 30 cm. (10 to 12 inches) long is attached. RECEITED May 11, 1936.
Spectrographic Microdetermination of Zinc
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N AN ARTICLE entitled “Spectrographic Microdetermination of Zinc” [IND.ENQ.CFXEM., Anal. Ed., 8, 240 (1936)], the authors made the following statement: “Inasmuch as the concave grating spectrograph used by the authors is not so sensitive as the quartz prism instruments, the zinc present in the plant ash must be separated from the main constituents of the ash and concentrated before spectrographic methods can be applied.” It has been called to the authors’ attention that this statement may be misinterpreted t o reflect adversely upon the use of grating instruments for analytical work; thus they would like to point out that what was actually meant was that spectrographs of greater photographic speed exist than the grating spectrograph used, which make possible the arcking of a smaller amount of sample and hence possess greater absolute sensitivity. On the other hand, the higher the resolution and dispersion of a spectrograph the slower the photographic speed, owing to the fact that the size of the diffracting medium is limited. These properties of high resolution and dispersion, however, tend t o make for higher relative sensitivity, since the background is reduced by obtaining sharper lines and spacing them at greater distances. Thus the grating instrument employed is certainly on a par with all prism instruments as far as relative sensitivity is concerned, and in fact has been proved so by numerous experiments. Since for most work the size of the original sample is not the limiting factor, a high relative sensitivity is the most important attribute of the spectrograph for general analyses. As a matter of fact, the sensitivity of the grating spectrograph used by the authors is sufficient for the positive detection of less than 0.001 mg. of zinc, all three of the persistent lines (3282.32, 3302.8,,and 3345.0) being present when this amount of zinc is arcked. ALBERTP.VANEELOW
