ANALYTICAL EDITION
October 15, 1943
or calomel electrode is used as anode. This is shown in Figure 2 by comparison of curve 4 with curves 1,2, and 3. Stirring greatly improves the performance of the quinhydrone electrode as anode, as is shown by Figure 3, where the unstirred electrode as anode gives a polarogram more drawn out than the stirred. The concentration of buf€er necessary to make a hydrogen electrode a sufficiently stable anode is suggested by the results of dilution of buffer shown in Figure 4. Here phosphate b d e r of pH 6.8 is satisfactory when log M = -1 but allows the polarogram to be distorted where log M = -3 and even more so when logM = - 5 . Since the hydrogen electrode is perfectly reversible, its merits as an anode also apply to its use as a cathode when the dropping mercury electrode is anodic.
Summary The hydrogen electrode may be used as anode (or cathode) in polarographic work, if it is sufficiently well buffered to remain unpolarized. The quinhydrone electrode, even when rapidly stirred, is less Satisfactory for this purpose. The relatively low potentials obtainable with the hydrogen electrode permit the use of low bridge voltages and consequently the spreading of polarographic waves.
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The hydrogen electrode potential can be readily adjusted by changing the pH of the solution in which it is immersed; this makes it a convenient half-cell for polarography. Several changes have been made which add to the usefulness of the Sargent type of Heyrovskf polarograph.
Literature Cited (1) Baumberger, J. P.,Cold Spring Harbor Symposia Quant. BbZ., 7, 195-215 (1939). (2) BrdiEka, R., Acta internationalen Vsreinigung Krebabeklimpfung, 3, 13-30 (1938). (3) . . Clark, W. M.. “Determination of Hydrogen Ions”. 3rd ed.. Baltimore, Williams and Wilkins, 1928. (4) Heyrovskg, J., Polarographie, in Bottger’s “Physikalischs
Methoden”, Band 11, Leipzig, Akademische Verlagagemellschaft, 1936. (5) Kolthoff, I. M.,and Lingane, J. J., “Polarography”, New York, Interscience Publishers, 1941. (6) MacCardle, R.C., Baumberger, J. P., and Herold, W. C., Arch. Dsrmatol. Syphylia, 47, 517-46 (1943). (7) Miiller, 0. H., “Polarographic Method of Analysis”, Earton, Penna., J. Chem. Education, 1942. (8) Miiller, 0.H., private communication (May 26, 1941). (9) Miiller, 0.H.,and Baumberger, J. P., TTana. Electrochem. Qoc., 71, 169-80, 181-94 (1937).
Automatic Pressure-Regulating Manometer ROGER GILMONT AND DONALD F. OTHMER, Polytechnic Institute of Brooklyn, Brooklyn, N. Y.
T
HE device shown in Figures
1and 2 was developed in the experimental study of the vaporliquid equilibria of the binary system, water-acetic acid, a t subatmospheric pressures ( I ) , using the equilibrium still previously described (S). It has also proved of use in controlling pressurea in other operations, such as vacuum distillations. This pressure regulator contains no moving parts. It can be constructed without g h blowing and includes a direct-reading manometer.
FIGURE 1. GENERAL ARRANQEMENT OF M A N O M ETER AND CONSTANT
PRESSURE DEVICE
Upper open line goei t o source of vacuum lower open line goei to ‘ayatem. both preferably by way of trap3 to catch any mercury pulled through L i e d .
A meter stick measures the height of the mercury column in the closed-tube manometer above the mercury level in the trap, A movable indicator with a vernier reads the mercury level in the manometer tube when the meter stick is adjusted so that a fixed indicator reads the level in the lower trap. Two slots in the meter stick fitting over screws with thumb nuts enable vertical adjustment. With the manometer tube filled with mercury and the level of mercury in the lower trap below the openin of the ta ered tip of the ri&thand ut,! suction is applied. The level in the lower trap is adjusted by means of the mercury reservoir-a standard separatory funnel. When the level of the mercury in the manometer falls to the desired point, the level of the mercury in the lower trap is adjusted so thatasealis just madewiththe
H
U I
FIQURE 2. DETAILS OF MANOMETER AND CONSTANT-PRESSURE DEvxcm
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 15, No. 10
opening of the tapered tip, thereupon the StoPcock ofthe reservoir is closed. The lower trap then cuts off suction of gas through the right-hand tube. If the system were erfectly gas-tight at constant temperature, the pressure woulBremain constant and the pressure regulator would be unaffected. If air does leak in, mercury rises in the manometer and the two other tubes to de ress the level in the lower trap, so that the higher pressure 0% the system causes gas to rise through the right-hand tube, carrying the mercury with it. The mercury returns to the lower trap by way of the left-hand tube, Gas is withdrawn by the evacuator until the pressure and mercury drop and the level is restored in the lower trap to its original position, at which further evacuation is prevented by the seal.
ing atmospheric. By using vertical tubes of smaller bore, the regulator can be made more sensitive. The manometer can be read to approximately 0.1 mm.9 SO that in distillations the Corresponding temperature should be read simultaneously with the pressure, when the pressure drifts to the desired value. Since vapor-fiquid equilibria are insensitive to changes in as large rts 10 mm., the control of the regulator is sufficiently precise for this work as well as for most vacuum distillations.
The regulator maintains constant pressure within about 2 mm. a t lower pressures and within about 5 mm. a t pressures approach-
(1) Gilmont, R,, and Othmer, D, F.,to be published, (2) O t h e r , D. F.,IND.ENQ.CHEM.,35, 614 (1943).
Literature Cited
Apparatus for Photoelectric Titrations Application to Dark-Colored Resins ROBERT H. OSBORN, JOHN H. ELLIOTT, AND ARTHUR F. MARTIN Hercules Experiment Station, Hercules Powder Company, Wilmington, Del.
A photoelectric instrument, generally applicable to acid-base and oxidation-reduction titrations employing colored or fluorescent indicators, is described. It is especially useful for titrations which cannot be carried out satisfactorily by the usual visual or electrometric means. The application of this instrument to the determination of acid and
T
H E first objective measurement of the color change of an indicator as a means of determining the end point of a titration was made in 1926 by Field and Baas-Becking (7), who applied it to the starch-iodine reaction. These workers used a radiomicrometer with a receiver consisting of a silver-bismuth thermocouple. In 1928, Miiller and Partridge (33) described a photoelectric apparatus for automatic titrations employing a single vacuum phototube, an amplifier, a relay, and an electromagnetic buret release. This equipment wrts used for acidimetry and alkalimetry, permanganate, dichromate, and iodometric titrations. Furthermore, it was found that certain precipitation reactions, such as the determination of chloride ion by silver ion in the presence of chromate ion, could be followed. The precision in every case was greater than that of visual estimation. During the past decade, several papers have been published by Hirano (f3-24), Somiya (44-47), Kasai (96), and their coworkers in Japan describing the application of photoelectric titration to various quantitative procedures. During the same period, Ringbom and Sundman (40) and Miiller (99) in Germany, del Campo and co-workers (3, 4) and Gonzales (9) in Spain, and Lur’e and Tal (88)in Russia have made contributions to the subject. In this country, Alyea (f), Boyer ( d ) , Goodhue (IO), Hickman and Sanford (fz), Miiller and his co-workers (30-34, 97-39), Rowland (41), Russell and Latham (4,9), and Styer (@) have described improvements in photoelectric titration technique. In the authors’ laboratories electrometric titrations have been used in the past where visual end points with colored indicators are difficult or impossible to obtain. However, in certain crtses, electrode equilibrium is slow and titrations are time-consuming. For example, in applicatioris involving two-phase systems, or in those carried out in certain organic solvent media, it is sometimes
saponification numbers of dark-colored resins is discussed in detail. A precision of *l per cent is easily obtained. Preliminary work has shown that precipitation reactions and reaction rates can be studied, and, in addition, the apparatus may be used as a chemical colorimeter.
necessary to wait for 10 to 20 minutes after each addition of titrant before equilibrium is established. Furthermore, in these cases, the electrometric end point is often not very sharp, whereas indicator color changes sometimes remain sharp, although they are often obscured by background color or turbidity.
MICRO BURET
I
d FILTERS
LENS \
LAMP
PHOTOCELLS
+w SOLUTION-/-’ TITRATED
GLASS PLATE
(4
PHOTOCELL NO.1
PHOTOCELL N0.2
GA
CIRCUIT
(b) DIAGRAM OF PHOTOELECTRIC TITRATION FIGURE 1. SCHEMATIC APPARATUS
