1688
ANALYTICAL CHEMISTRY
Unitized Rotating Platinum Electrode Assembly for Amperometric Titration. Eligio J . Agaesi, Thomas D. Parks’, Halbert A. LeFebre, and Louis Lykken, Shell Development Co., Emeryville, Calif. titrations a t the rotating platinum electrode A (3,7) are becoming increasingly useful in determining the MPEROMETR~C
end point in volumetric titrations because they are rapid and permit the determination of small concentrations of certain anions and cations without indicator corrections or interference from colored solutions. While the amperometric technique has become significantly important in analytical chemistry (8, 4, 8), no convenient, compact cell assembly is available for making amperometric titrations, espeoially under service conditions. This need is especially noticeable in titrations utilizing the rotating platinum electrode. A compact unitised assembly has been designpd whioh houses a rotating platinum electrode, a salbbridge and calomel reference electrode, and their accessory parts. I n amperometric titrations, it is used in conjunotion with a suitable polarograph or, if i t is unnecessary to impose a potential upon the cell, with a suitable currenbmeitsuring instrument. APPARATUS
The assembly, based on the apparatus of Laitinitinen, Jennings, and Parks (6), consists of: (1) a platinum eleotrode rotated at a speed of approximately 800 r.p.m., (2) a saturated calomel refer-*,-- d - r t m i l o a n d (2)a salt bridge to provide electrical cannecce electrode and the solution being ti-
mercury and contact i s made between the electrode and the terminal on the back panel through a stiff copper wire that dips into the mercury but does not touch the glass tube. The electrode is rotated through a pulley and belt arrangement by means of a synchronous motor which operates at a speed of approximately 1150 r.p.m. A saturated-calomel reference electrode of large area is mounted below the motor. The sheet of Transite upon which the motor rests protects the reference electrode from the heat of the motor and a perforated housing provides for ventilation and efficient dissipation ofheat. The electrode is constructed from a 50-ml. glass-stoppered Erlenmeyer flask and is filled in the usual manner. A wire leading from the back terminal of the neeted to a‘ ilO-mm. length of 6-mm. outside diameter &ss tubing (having a right-angle bend) by means of a 300-mm. length of 6-mm. inside diameteF Tygon tubing. The entire bridge is filled with a wqrm, Z,’% agar-agar solution which contains 20 erams of ootassium nitrate or ootassium chloride Der 100 ml.. &ninatin’p all air bubbles. T6e ends of the bridge are closed ~
bridae is inserted intdsn outer sleeve w h h is fiited with a side
rated potksium nitrate or ehlo;ide, placed in the tube to provide contact with the solution being titrated) requires about 30 minutes to drain from the side a n n to the sintered glass when the bridge is suspended in air. A unitized amoerametrio titration unit, based on that described. is being manufktured for commercid sale by the Precisictn Scientific Co. DISCUSSION
Figure 1. Unitized Rotating P l a t i n u m Eleotrode Assembly with Protective Housings and Burets Removed
The general arrangement of the parts in the electrode assembly is depicted in Figure 1, which shows the assembly with protective housings removed from the motor and referenceelectrode compartment, and the rotating-electrode mount. The base, front, and back panel are made of Bakelite. Two terminals (not shown), located on the rear panel, connect the electrodes to the separate current-measuring unit. The rotating platinum electrode is of the usual type (6). The electrode is inserted through a brass sleeve (Figure 1)and is held in position by two short sections of Tygon tubing located a t either end of the brass sleeve. The glass tube is filled with I
Present address. Stanford Research Institute. Stanford, Calif.
Titrations are generally carried out in 50- or 100-ml. beakers, but the sleeve of the salt bridge and the rotating platinum eleotrode are designed to accommodate 20-ml. beakers for reducedscale titration. Shifting of the beakers by vibration is prevented by means of a circular section of rubber embedded in the adjustable swinging table. This is particularly desirable when small beakers are used, as slight shifting of the beaker is sufficient to cause the rotating electrode to strike the beaker. The apparatus is designed to isolate the two electrodes electrically 8.6 completely as possible. In many amperometric cell assemblies, there is danger of short-circuiting the system bcoiluse the solution in the Sleeve tube creeps, oontacts the clamp holding it, and (through the supporting ring stand) shorts out the rotating electrode. In the present tLppamtus, this possibility is eliminated by mounting the clamp on the nonoonducting Bakelite panel. In order t o provide some criterion for the porosity of the sintered-glass section on the salt bridge sleeve tube, B drainage time has been recommended. Rapid drainage is objeotionable because the solution is diluted and may diffuse into the sleeve tube owing t o loss of liquid head within the tube. Thecell assembly embodies the general features found convenient in potentiometric titration apparatus (9); between titrations, i t is only necessary to swing aside the supporting table, rinse down the salt bridge and rotating platinum electrode, and exchange the beaker containing the titrated solution for a fresh one. LITERATURE CITED
(1) Hume. D. N.. and Harris, W. E., IND.ENR.CaEM., ANAL.ED., 15,465(1943). (2) Kolthoff. I. M., and Leussing, D. Z., Z. anwg. Chem., 262, 161 (1950). (3) Kolthoff. I. M., and Lingme, J. J., “Polarography,” Chap. XXXIII, New York, Intersoienoe Publishers, 1941. (4)Laitinen, H.A,, ANAL.CHEM.. 21, 66 (1949). (5) Laitinen, H.A,, IND.ENG. CHEM.,ANAL.Eo.. 13,393 (1941). (6) Laitinen, H.A,, Jennings, W. P.. and Parks, T. D., Ibid., 18,355 (1946). (7) Laitinen. H. A,, and Kolthoff, I. M., J . Phm. Chem., 45, 1079 (1941). . (8) Lykken, L., ANAL. C a m . . 22, 396 (1950). (9) Lykken. L.,and Rolfson, F. B., IND.ENG.CnEM., ANAL.ED.,13. 653 (1941).
