ANALYTICAL EDITION
September 15, 1941
may be conducted could be raised to surround them. Provision is also made in the Bakelite board for an all-glass stirrer and modified buret tip (Figure 3). I n mounting the electrodes in the Bakelite board a 0.62-em. (0.25-inch) hole was drilled through, then beveled to 0.94 cm. (0.375inch) on the upper side. The electrode was inserted so that the top of the brass insert projected through the board and sealed in place by filling the beveled region around the glass with Pyseal. The electrodes are thus held firmly but may easily be removed by warming the cement with a hot file or small soldering iron.
Electrode Assembly Two Centralab 23 position selector switches were mounted on the front panel and the common terminal of each was connected to individual insulated jacks (one red, one black, Figure 3) through a double-pole double-throw toggle switch. A series of jacks (taking phone tips) was then mounted on the electrode support (20 in this particular instance) and each jack was connected to the same position on both switches. Thus when the antimony electrode was installed the tip from the antimony lead was inserted in jack No. 2 and the latter in turn connected to position 2 on switches A and B (Figure 3). The calomel electrode was plugged to jack No. 5 and the latter connected to position 5 on switches A and B. The electrode leads from the titrimeter used were plugged . _. into the red and black jacks on the front panel. Thus, with the electrodes immersed in the solution to be studied. bv settine: switch A to Dosition 2 and B to 5 . with the toggle Switch in txe up position;the antimony electrode is con-
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nected to the red jack and the calomel to the black jack (and in turn to the titrimeter). Flipping the toggle to the down position immediately reverses the polarity, connecting the antimony electrode to the black and the calomel to the red plug. Reversal of polarity could be obtained, of course, by leaving the toggle in the up osition and turning switch A to 5 and B to 2. With this flexibfe arrangement any pair of electrodes could be used and the polarity with respect to the titrimeter reversed at will. By including a standard reference electrode (calomel half-cell) comparative studies are simplified. A glass electrode was included for ease in adjustment of solution pH. For example, the effect of acid concentration on a reaction may be investigated by connecting the glass and calomel electrodes to the titrimeter, adding acid from a buret until the desired pH is attained, connecting the desired indicator electrode (or electrodes) to the titrimeter by readjustment of switches A and B, and titrating with the standard reagent solution. At any point during the titration the initial electrodes may be reconnected to the titrimeter by adjustment of A and B and the pH checked rapidly.
A list of the electrodes used, with their position on selectors A and B , is mounted on the front panel for convenience. The nature of the electrodes t o be used for any one study depends upon the reaction to be investigated. With this setup the best electrode pair for titration of any solution, following the five considerations outlined above, may be readily and quickly determined.
A Shaping Lathe for Graphite Electrodes Used in Spectrochemical Analysis K . R. 3fAJORS AND T. H. HOPPER C . S. Regional Soybean Industrial Products Laboratory, Urbana, Ill.
T
HE need for a rapid and convenient means of shaping graphite electrodes for use in spectrochemical analysis
of plant ash led to the design and construction of a small portable shaping lathe.
This consists of t\vo essential units,
a lathe head and an electrode feeding assembly, securely held in proper relative position on a base plate.
Lathe Head A motor-driven shaping bit mounted horizontally, and an enclosure for guarding the cutting end of the bit and collecting the graphite trimmings, are the main Of the lathe head (Figures 1 and 2). The outer section of the shaping bit (T, Figure 2) forms the shaft of the rotatin part and turns in two snugly fitting dustproof ball bearings fBB, Figure 2). A pulley wheel locked to this shaft between and in contact with the two bearings prevents thrust play in the shaping bit. The support for bearings and shaft is fastened securely to the main base plate. A shallow cylindrical housing encloses the cutting end of the bit. A section of the underside cut away allows heavier graphite trimmin s to drop into a removable bin (B, Figure 15. Lighter graphite dust is removed by suction throueh a tube fastened in the cylinder top. If the type of shaping or trimming operation does not produce large graphite chips or shells, the bin and bin enclosure can be omitted and the suction port placed on the underside of the cylindrical housing. The vacuum system will keep graphite dust from collecting. A removable flanged disk (D,Figure l ) , with a central opening, fits into the open end of the cylindrical housing. The central opening, while large enough to allow the entrance of the largest diameter electrode used, permits neither the operator’s fingers nor the electrode carrier t o come in contact with the rotating bit.
Electrode Feeding Assembly The slide trough ( S T , Figure 1) for carrying the electrode, the guide trough (GT, Fi ure 1) fastened to a vertical plate, and a slottej metal block(BL, Figure 1) make up the electrode feeding
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Certain fixed depths of craters are obtained by using gages to set the position of the shaft. The gages are short pieces of 0.375-inch brass rod, each of which has a hole of appropriate depth drilled in one end. The holes permit passing the gages over the blunt end of the inner drill, but not over the end of the outer trimmer. The inner drill, released by turning setscrew SS (Figure 2) which extends through the pulley collar and outer trimmer, is seated into the outer trimmer as far as the specific gage permits. To point the upper electrodes, a special bit (PB, Figure 2 ) is used which is interchanged with the inner drill of the shaping tool used for cupping electrodes. It is set into correct position by the use of a gage - - similar to ones just described. The Chemistry Department of the University of Illinois has made further variations in shaping tools used with a copy of this lathe. For spectrochemical work done there, a facing tool and two small-diameter drills are required. Types of electrodes produced are described in a n &count of the spectrochemical technique used by Keirs and Englis ( 1 ) .
ES
-
Figure 2
I
I Inch
Operation
PB
T
assembly. The sliding trough has the end away from the lathe head closed by a metal block. By means of knurled screws ( K S , Figure 1) the vertical plate of the guide trough is clamped against an inner wall of the slotted block in such a manner that the line formed by the angle of the trough lies parallel and directly below an extension of the shaping bit axis. This forms a rigid guide for the slide trough. The contacting surfaces of the two troughs are machined true to prevent rocking or side play. The flat plate holding the slotted block is fastened to the main base plate at each corner by steel screws. The screw holes in the upper plate are slotted a t right angles to the shaping bit axis to allow sufficient lateral alignment of the guide trough. This lateral adjustment should be required only when the lathe is first assembled. The vertical adjustment of the guide trough, necessary whenever the diameter of the electrode material is changed, is made by movement of the steel elevator screws (ES,Figures 1 and 2) located in each end of the vertical plate. Accidental alterations in screw settings are prevented by brass setscrews (LS,Figure 1). The length of the elevator screws determines the maximum and minimum electrode diameters which can be centered on the sha ing bit axis. $he completed lathe assembly, a 0.05-horsepower motor which drives the shaping bit at about 1700 r. p. m., and a control switch are mounted on a wooden base. A water aspirator supplies sufficient vacuum to collect the fine raphite dust. To prevent graphite from clogging the vacuum !ne or aspirator, a trap located on the wooden base near the lathe head is provided.
Shaping Tools
A variety of sizes and designs of shaping tools can be interchanged readily in the lathe described. Size and design are limited only by inside diameters of the pulley wheel and bearings, and dimensions of the cylindrical housing enclosing the bit end. Shaping bits and drills with diameters smaller than the maximum fixed by pulley and bearings can be accommodated by using sleeves or holders as adapters.
For the shaping operation, the electrode is placed in the carrier with one end extending I about 0.75 inch beyond the end of the sliding trough. The other electrode end lies against Li the closed end of the trough. While the electrode is being held down firmly in the trou h by manual pressure, the carrier is moved slow!y into the shaping bit by means of the knurled knob. The slide is drawn directly back to remove the shaped electrode from the cylindrical housing. Electrodes of shorter length can be accommodated by using spacers placed between the electrode and the closed end of the trough. Electrode material cut into appropriate lengths is suitable for this purpose. Frequent rim breakage of the cupped electrodes during the shaping operation can be traced to several possible causes, such as the guide trou h not in proper alignment, shaping bits which are not machinef true, dull cutting edges, and low bit speeds. According to Myers and Brunstetter, bit speeds below 1300 r. p. m. cause excessive breakage. The principal features of the lathe are the speed, accuracy, and convenience with which electrodes may be shaped. Eight or twelve cupped electrodes may be prepared per minute. Speed has been increased by omitting the operations of clamping and unclamping electrodes in a chuck or other similar device. Accidental contamination of cutting surfaces of the shaping tools by dust or indiscriminate handling is prevented b y t h e cylindrical housing. It is possible, therefore, to keep the shaping lathe on the bench where samples for spectrochemical analysis are prepared. Since the tool is not adapted to other sundry uses, contamination from miscellaneous usage is avoided. With the lathe immediately accessible, and because its manipulation is not difficult, there is no necessity for preparing a large number of electrodes at one time and storing them for future use. The danger of rim breakage or contamination which often accompanies storage is eliminated b y shaping only enough electrodes for immediate use.
Literature Cited (1) Keirs, R. J., and Englis, D. T., IND. ENG.CHEM., ANAL.ED.,12, 27.5-6- I\ -1- 940). --I
The shaping bit used in this laboratory was made according t o the design and dimensions described by Myers and Brunstetter (g) for forming shallow, thin-walled craters in 0.31-inch graphite electrodes. I n using this shaping bit in the shaping lathe, the blunt end of the inner drill (CB, Figure 2) is made to extend beyond the blunt end of the outer trimmer (T, Figure 2). By changing the length of the shaft which protrudes, craters of varying depths can be made.
(2) Myers, A. T., and Brunstetter, B. C., Ibid., 11, 218-19 (1939). THElaboratory ia a cooperative organization participated in by the Bureaus of Agricultural Chemistry and Engineering and Plant Industry of the U. 8. Department of Agriculture, and the Agricultural Experiment Stations of the North Central States of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.
