A Needle Valve for the Micro-Dumas Determination of Nitrogen E. B. HERSHBERG AND LYON SOUTHWORTH Converse Memorial Laboratory, Harvard University, Cambridge, Mass,
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NE of the most important manipulative details of Pregl’s
bustion tube a t the beginning of the analysis is best performed before connecting it to the azotometer and valve. Besides controlling the flow of gas under the very slight pressures existing during the nitrogen analysis, this valve may be used in other applications under much more severe conditions and in instances where its transparency is an asset. Tests have shown that no leakage occurs a t pressures up to 25 pounds per square inch and that, while care must be used to avoid excessive force in closing the valve, it is rugged and will give long service.
micro-Dumas determination of nitrogen is the adjustment of the stopcock between the combustion tube and the azotometer. Pregl emphasizes this by the statement ($, “This adjustment of the stopcock is probably the only feature of the manipulation in this determination which requires some practice, as it must be so carried out that the above-mentioned velocity of the gas current is not exceeded for even a few seconds.” I
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Construction
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The taper of the glass seat is approximated by eye during the glass working. The needle is ground in with-500- t o 700-mesh silicon carbide suspended in water, finishing with “flour” of the same material. During the grinding operation the needle may be centered in the glass body by means of the bushing, C, which is allowed to turn freely with it. A *? c small stirring motor equipped with a chuck is convenient as the source of power. While grinding, the needle should be lifted frequently from the seat, and heating of the glass due t o excessive pressure must be avoided. If fine cracks develop it is an indication that the needle is being pressed in too hard, or that the speed of rotation is too high. The final lapping-in with “flour” should be done by hand. The stem, E, is of 18-8 stainless steel and the knurled handle, F,is of brass in two sections which are threaded and tightened against each other. The U. S. Standard of 48 threads per inch gives satisfactory regulation, but considerable variation in itch is allowable. Bushing is made from 0.156-inch (3.97-mm.) soft steel rod, counterbored 0.375 inch (9 mm.) with a 0.116-inch (2.95-mm.)
FIGURE1. ASSEMBLED VALVE
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Despite this emphasis no attempt has been made to improve this gas-control device other than to file grooves in the stopcock barrel, or to use a screw adjustment on the elongated handle (1). It ha;s been the authors’ experience that the stopcock is unsuited for the close metering of a gas stream and the needle valve described below has proved decidedly superior, both in this and in other laboratories.
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To determine the performance and reliability of this valve in other hands before publication, samples of an earlier type were sent to L. F. Small of the University of Virginia, to N. L. Drake of the University of Maryland, and t o H. K. Alber of the Biochemical Research Foundation of the Franklin Institute in Philadelphia. All these valves worked well, but in two cases mercury leaked past the threaded portion of the stem. A method for overcoming this difficulty was worked out and an improved valve of the type here described was sent t o Professor Drake, who subsequently reported very satisfactory results, even in the hands of students. The authors wish to express their thanks to these men for their kind and valued cooperation.
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The valve, shown complete in Figure 1 and in detail in Figure 2, is composed of a Pyrex glass body, A , and a stainlesssteel needle, E. A threaded steel bushing, C, contains a packing gland, D, consisting of a short piece of paraffinimpregnated rubber tubing which effectively lubricates and seals the stem against leakage of mercury. The glass side tubes can be bent to fit the requirements of any apparatus, so long as the part containing the mercury is kept nearly vertical. Since this valve has one less function than the three-way stopcock which it replaces, the operation of sweeping out the com-
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FIGURE2. DIAGRAM OF VALVE 404
JULY 15, 1939
ANALYTICAL EDITION
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Sealing wax (Dennison’s No. 391) is used to cement bushing C into the glass body, A . Mercury is placed in the portion marked Hg to act as a positive liquid seal, and to eliminate dead space.
hole. Rubber tubing 0.0625 inch (1.6 mm.) in diameter (spectacle tubing) is used for packing D,and is pretreated by immersing it in hot paraffin for 10 to 15 minutes and allowing it to swell to fit the hole counterbored in C. Because of variations in size of this type of tubing, the diameter of the hole in the bushing and the degree of swelling cannot be,stated very exactly, and some experimentation is necessary with each piece of tubing. No adhesive is necessary to hold the rubber in place, since the comparatively rough inner surface of the hole offers considerably more resistance to rotation than does the polished valve stem.
Literature Cited (1) Rangaswami, S., Proc. I n d i a n Acad. Sci., 8A,220-2 (1938). (2) Roth, H., and Daw, E. B., “Quantitative Organic Microanalysis of Pregl”, p. 83, Philadelphia, P. Blakiston’s Son & Co.,1927.
Handling of Hygroscopic Substances In the Microchemical Determination of Carbon and Hydrogen CLEMENT J. RODDEN, National Bureau of Standards, Washington, D. C.
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HE various drying devices which have been described in
the literature for use in determining the amount of water in the microchemical analysis of a substance of type C (1) are oftentimes unsatisfactory when the anhydrous material is to be analyzed. The apparatus described below consists of a jacketed drying tube, so arranged t h a t i t may be kept at constant temperature, and a weighing bottle of special design. The sampie is dried, weighed, and htroduced into the carbon and hydrogen combustion tube without coming in contact with moisture at any stage of the operation. Figure 1 shows the apparatus.
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each weighing. The results have been corrected for zero variations and are the weights in milligrams in excess of the tare: 9.958, 9.980, 9.966, 9.964, and 9.965. The maximum variation from the mean is 5 micrograms. I n routine analysis of hygroscopic substances the variation in weight of the weighing bottle plus boat plus dry sample was seldom over 5 micrograms.
I n the course of an analysis, the weighing bottle with the empty boat is connected to F by means of a short length of rubber tubing which has been wiped out with glycerol and then with cotton. Air is passed through C, with D open, for a few seconds. The core, K , is turned by means of the handle, J , until closed and, after disconnecting, the bottle is wiped and weighed in the usual manner. n The sample is placed in the boat and an a p proximate weight determined. The boat is now transferred to the dryer and placed midway in chamber G . The liquid in the flask, I, is boiled and refluxed in B. The tube, E, containing the same desiccating agent as in C , is connected , ........I ......... to F, and a slow stream of dry air is sucked tL.........; ......... ........, through the apparatus by connecting H , which J also contains the same desiccating agent, to the K L M u vacuum line. D is closed during this operation. A “bleeder” may be used to obtain the desired vacuum. After the sample is dried, air is applied to C, stopcock D is opened, and air is allowed to escape through E. E is now disconnected and, while air is flowing, weighing bottle M is connected to F by means of the rubber tubing used before. The boat is removed to the N center portion of the weighing bottle by means FIGURE 1. DIAGRAM OF APPARA TU8 of a glass rod having a platinum hook fused into the end. Core K is now turned until closed off and the weight obtained as before. After the weight of the dry sample is obtained, A is essentially a Pregl microdesiccator, modified b y the addiit is transferred to the carbon and hydrogen apparatus as follows: tion of the jacket, G, and the side-arm drying tube, C, which is The stopper a t N is removed and with dry oxy en streaming from N the weighing bottle is attached by means ofthe rubber tubing filled with anhydrous calcium sulfate (indicating Drierite). By using an appropriate liquid, it is possible to keep the temperature previously used. Core K is turned until open and the boat is of the drying chamber constant over long periods of time without pushed into the combustion tube by means of a glass rod. The supervision. The adjustment of the flame is a source of trouble weighing bottle is removed and the stopper replaced a t N . in the original Pregl design in which a copper block is used to B y this method i t is possible to dry and analyze a substance, maintain a constant temperature around the tube. M is a weighing bottle of new design consisting of the shell, L, such as chrysanthemin chloride, which takes u p its water of the core, R,and the wooden detachable handle, J . L has two hydration almost instantaneously. short side arms of glass tubing, the same diameter as the dryer The apparatus may also be used for other purposes in and the combustion tube, N . The weighing bottle is constructed microchemical analysis, when i t is desired to isolate the subof soft glass. The core is greased with a light high-vacuum stopcock grease and the excess removed by wiping. Before use, the stance from moisture and carbon dioxide of the atmospherecore is rotated several times and the inner portion of the weighing for example, the determination of loss on ignition of limestone bottle wiped out with cotton. This is repeated several times to where the substance is weighed in the weighing bottle after ensure a clean inner surface. ignition in a quartz tube. The weighing bottle was tared with glass and constant Literature Cited weight was usually reached in 15 minutes. The weight could be decreased considerably by using a thinner rim. The (1) Alber, H.K.,Mikrochemie, 25,47 (1938). following values were obtained on the empty weighing bottle, PR~S~NT before E D the Division of Microchemistry at the 97th Meeting of the when the stopper was rotated and the bottle wiped between American Chemical Society, Baltimore, Md.
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