Multiple Electrode System for Potent' Titration Studies H. A. FREDIANI AND WM. B. WARREN Fisher Scientific Company, Pittsburgh, Penna. LUG A iece of Pyrex glass tu1i n g usually DLILUUCU IUI roc? was then machined 1to the sham indicated in Figure 1 and
F O R the practical applcation of
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potentiometnc titration mlethods ..,&?:.I to industrial analyses, it is U ~ I J desirable t o select with care the electrodes to he used in following the course of the reaction.
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6 . ~ tubing'(7 s mm. in outsidediameter, 12 cm. long, ends ground) with Sauereisen cement No. 31. The copper wire lead was soldered t o a brais insert, sealed to the o p posite end of the tube with Pyseal cement, a 15-em. length of push-back vire was in turn soldered to the brass insert, and a pin tip was connected. For metals which could not be cast (platinum, tantalum) a short length of pure wire was welded to the copper lead and then sealed into the glass tube as above. If possible, the wire electrode was coiled to expose greater surface. The machined electrodes were highly polished. A 1-cm. length of 0.47 cm. (O.19inch) bore Scimatco rubber tubing was cut and slipped onto the glass tube as shown, so that a standard 11 X 75 mm. test tube could be slipped over the end of the electrode tip and kept in place by the rubber tubing. With the test tube in place an electrode may be shielded from contact with corrosive solutions. Solutions for preservation or cleanin of any electrode may be placed in the test tube and the electrote stored in favorable media when not in use without removing. it from the dectrode support.
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ELECTRODE
A relatively large change in potential with a small Ehange in concentration should occur in the vicinity of the eouivalence mint: un to the %init$ of the eduivd&ncepoint the
coune of the reaction may De foilowea closely; and the electrodes used should not be subject to poisoning nor he attmked bv ~" the solution durine the titration.
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, , . . pracacai appncanons of porennometnc procedures in this laboratory it has been found expedient to make use of a flexible multielectrode system with which it is possible to determine the optimum system t o be used
Electrode Support The electrodesweremounted in a Bakelite bimrd cut and drilled an the periphery as indicated in r'igUre 2. 'rhey were of a circle 7 em. in diameter, so that with ail electrodes and their
shielding test tubes in place a 600-ml. beaker in which titrations
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for anv suecific determination "___ s obtain& The present setup includes ofle reference electrode (calomel or silver chloride electrode as d,esired) and eighteen indicator electrodes. llll.
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Electrodes Used L'
The calomel and glass electrodes used are standard Beckman type electrodes with the leads modified to fit in with the rest of the wiring. The remaining electrodes (Figure 1) were prepared in several WBS-S. determined largely Dhvsical limitations - . by . the . . of the materi& When possible (antimony, bismuth) the pure or purified metal was cast into a cylindrical rod (about 1 cm. in diameter, 5 em. long) with a piece of aopper wire (for lead) imbedded in one end.
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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.
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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
