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INDUSTRIAL AND ENGINEERING CHEMISTRY
in the form of the ammonia complex until the first drop in potential indicated the complete precipitation of iodide. The solution was then neutralized with nitric acid and the titration continued until the second drop in potential occurred, corresponding to the complete precipitation of chloride or bromide. Excellent results were obtained. I n following up the possible uses of silver nitrate with the dead-stop end point, potassium cyanide was titrated with interesting results. This salt was found by experiment to be an anodic depolarizer like potassium iodide and sodium nitrite, which makes it unnecessary to add an indicator. In titrating potassium cyanide, two galvanometer deflections are produced which correspond quantitatively to the complete formation of XAg(CN)Z and Ag,(CN)2, respectively. When 0.1 N silver nitrate is used as the titrating agent, the galvanometer deflection produced a t the potassium argenticyanide equivalence point is only a flash, the light spot returning immediately to zero, but the deflection is very pronounced, going the full galvanometer scale, so that results are very readily reproduced. The second deflection corresponds to the complete precipitation of silver cyanide. A tentative explanation of the two end points produced is based on the assumption that the lag between complete formation of the silver cyanide complex and the beginning of precipitation of the silver cyanide is sufficient to furnish a concentration of silver ions which will momentarily depolarize the cathode. Experiments in which 25 ml. of approximately 0.1 N potassium cyanide were titrated with i2.9 ml. of 0.1 N silver nitrate a t the potassium argenticyanide equivalence point
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gave 25.75, 25.85, 25.80, 25.65, 25.80, and 25.80 ml. of total silver nitrate.
Summary This paper describes a simple and accurate method for the electrometric titration of acids, bases, halides, cyanides, and silver ions. Results are as highly reproducible as those obtained by the present accepted methods of electrometric analysis. The method has the advantage of using two simple platinum wire electrodes, and it eliminates the use of a reference electrode. The electrodes are seldom, if ever, poisoned, which removes the difficulty in the use of certain other electrode systems. Adequate warning of the approach of the end point is given by momentary deflections of the galvanometer pointer. Since the end point is reversible, back-titration is possible. Titrations of copper, mercury, and other metallic ions should be possible by the same principle employed in the titrations of silver ions. This opens a new field for the volumetric determination of the metallic ions with a low deposition potential. The titration of zinc with ferrocyanide should be investigated, because the ferrocyanide would serve as an anodic depolarizer. Literature Cited (1) Behrend, R., 2. physik. Chem., 2, 466 (1893). (2) Foulk and Bawden, J. Am. Chem. SOC.,48, 2045 (1926). (3) Willard and Fenwick, E d . , 45, 715 (1923). (4) Wright and Gibson, IND.ENG.CHEM.,19, 749 (1927).
Preparation of Carbon Electrodes for Spectrographic Analysis Two Useful Lathe Tools A. T. MYERS AND B. C. BRUNSTETTER United States Department of Agriculture, Bureau of Plant Industry, Washington, D. C.
T
HE exposure time for complete volatilization of a sample of plant ash in spectrographic analysis is dependent, among other factors, on the depth of the cavity drilled in one end of an electrode to receive the sample, and on the thickness of the crater wall. The diameter of the carbon rods used in this study is 8 mm. (0.3125 inch). A cavity size which has proved suitable in this work has the following dimensions: inside diameter, 6 mm. (0.25 inch); depth, 3.5 mm. This crater will hold 10 to 25 mg. of dried plant material or 0.1 ml. of liquid. A wall thickness of 0.3 mm. is a good compromise. Using electrodes of the above dimensions (with a 15-ampere and 150-volt current) pointed upper electrodes (8-mm. rods pointed in a pencil sharpener) and an arc length of 3.0 mm., the ash in a 10-mg. sample of dried plant tissue is volatilized completely in 60 seconds. If the wall is too thin, liquid will leak through, and saIt will be deposited on the outside of the wall when the solution is evaporated to dryness; if the wall is too thick, the arc will wander and too much time is required to burn the wall down to the.cavity floor, which is essential if the last trace of fused ash is to be burned off. Too thick a wall also unduly increases background on the plate. Since in the course of analytical routine, a large number of electrodes are prepared, it is desirable that facing the end of the electrode, drilling the hole,
and cutting down the outside wall be accomplished in one operation. A tool was made that answers the above specifications. It was designed to produce craters of uniform wall thickness but variable depths by an adjustment of the set screw as is shown in Figure 1. By machining the outside of the cavity wall, uniformity of thickness is ensured, so that irregularities in graphite electrodes as purchased become unimportant; a long outside cut of 9.5 mm. when the cavity depth is only 3.5 mm. has the advantage of the smaller diameter 6-mm. (0.25-inch) electrode. It will be noted from Figure 1 that the angle on the cutting edge of the bit has been reduced, so that the depression in the floor of the cavity is comparatively slight. This ensures a rapid cleanup of traces of the ash residue when the wall has burned down to the floor. I n spectrographic determination of mercury in plant tissue the authors found that a more intense mercury line is produced a t 2536.7 A. by using an unusually deep electrode cavity (about 15 mm.). With the use of 8-mm. electrodes, pointing the wall at the top serves to prevent arc wandering during the short exposure needed to volatilize the mercury present. A tool designed to perform the operations of drilling and pointing, simultaneously, is shown in Figure 2. It
APRIL 15,1939
ANALYTICAL EDITION
consists of a standard 6-mm. twist drill and a steel collar with a set screw on which projects a cutting edge a t a suitable angle (about 20"). The depth of the cavity here again can be varied by merely sliding the collar along the drill and tightening the screw.
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Also, a good drill press combined with an electrode-holding device (chuck, collet, etc.), attached firmly to the bed or base plate, can be substituted where a lathe is not available. For successful production of this thin-walled cavity, both tool and carbon electrode must be held immovable as in a chuck or collet. Performance has demonstrated that the use of these tools makes possible rapid preparation of electrodes with the desired uniformity of dimensions. Electrodes can easily be produced a t the rate of two a minute. The tools should be operated at a minimum speed of 1300 r. p, m.; at slower
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assem bled foe/
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-1
FIGURE 1. CARBON DRILLING AND CUTTING TOOLDRILLBIT Top view
n The tools should be made of tool steel and tempered to keep an edge on the cutting parts. Any good tool steel will be satisfactory, but must be rotected against corrosion under high humidity conditions y storage in a desiccator or by wrapping in an oiled cloth. Suitable alterations in the given dimensions will permit the use of these designs for producing cavities on electrodes of different diameter than the one the authors are using, and will give different wall thicknesses to suit the requirements of any special material. The electrodes are machined in a small bench lathe in the laboratory. The carbons are allowed to revolve held in a collet on the headstock end, while the tool is fastened in a chuck in the tailstock. This lathe is kept clean and is reserved for the production of electrodes. With a suitable lathe and equipment the procedure can be reversed, with a resultant increase in the number of electrodes produced.
1
u DRILLING AND POINTING TOOL FIGURE 2. CARBON speeds breakage of the electrode wall is apt to occur. It was pointed out to the authors that this type of tool is less subject to borrowing than the ordinary drills, therefore the contamination danger is avoided.
Acknowledgment Acknowledgment is hereby made of the help of Leonard Smith of the Bendix Corporation who made the experimental models for this study.
