Modified Hook-Up for Routine Use of Glass Electrode E. C. GILBERTAND ALANCOBB,Oregon State College, Corvallis, Ore.
T
H E steadily increasing use and popularity of the glass electrode for p H determinations make it a desirable addition to any laboratory. I n many instances, however, the apparatus described in the literature is not commonly available or the cost is prohibitive. It is the purpose of this paper to describe a modified hook-up in which it is possible to use apparatus commonly available in the ordinary laboratory, achieving a t the same time a good degree of accuracy in measurement and simplicity in design, The use of the glass electrode requires the measurement of low potentials through very high resistances, The methods CommonlY used may be divided roughly into three groups: first, the use of a quadrant electrometer; second, the use of a condenser and ballistic galvanometer; and third, various arrangements of the vacuum-tube potentiometer. The first two methods involve the use of expensive apparatus of a type not commonly found in the average laboratory, and are subject to some experimental difficulty. Several apparently successful types of the vacuum-tube potentiometer have been designed differing somewhat in principle. The very simple design of Elder (1, 2’) is based on the use of a high-sensitivity galvanometer as a currentmeasuring device. For some reason, probably not the fault of the method, this method seemed to be unworkable even with the special galvanometer recommended. The method of Stadie (3, 4) is representative of a considerable group and is based on the Wheatstone bridge principle, using the galvanometer as a null-point instrument to indicate a balance in the bridge. Two of the resistance arms are internal (filament to plate) resistances of UX 222 tubes, and two are the external (variable) resistances of the plate circuits. If the tubes possess the same electrical properties, changes in the filament and plate batteries affect the internal resistances of both tubes alike so that the balance is not disturbed. Difficulty was met since the two tubes purchased did not have the same characteristics and the hook-up had to be further complicated to Overcome this, Furthermore, two tubes which are the same a t the beginning do not age equally.
door screening on a frame of heavy steel wire which was hinged to the back of the desk and could be raised and lowered like the lid of a trunk. The tube itself was painted with paraffin, and all sensitive parts of the apparatus were set on paraffin blocks. OPERATION.The apparatus functions in the following manner: 1. The potentiometer, A , is adjusted by disconnecting the clip t o the top of the tube, the standard cell connected at (1) and connection made with the ground at (21, cutting out the glass cell. The switch, Sw, is turned to connect the standard cell t o the galvanometer circuit. When the potentiometer is set to read directly the voltage of the standard, there will be no deflectionof the galvanometer. I
FIGURE 1. SET-UPOF SIMPLIFIED DESIGN 8. standard cell K tapping key S&, D. P. D. T. switch X ,glass cell
Rz 10 000-ohm variable resistance Ra’ 25:ohm rheostat E,: Ra, 10,000-ohm fixed resistance A , student type potentiometer 2 4 2 0 4 Leeds & Northrup galvanometer
2, The tube is next adjusted to operate at its free grid POtential. The standard cell is disconnected a t (1) and Sur is turned to the right. The wire t o the top of the tube is still disconnected. Rs is then adjusted until the tube voltage is 3.0 volts, and after the tube is warmed up, Rz is adjusted until there is no galvanometer deflection. 3. Connection is now made t o the top of the tube and potentiometer A is again adjusted to thenull point of the galvanometer, measuring the grid potential which is designated in this paper as Eo. 4. The glass electrode cell is now introduced by changing wire (2) to (3). When the potentiometer is again adjusted t o the point of balance, the recorded voltage is designated RS El. The voltage of the cell is then E = E1 - EO. In operation it was necessary to check Eofrom time to time. Results could be duplicated within *0.0005 volt.
SIMPLIFIEDDESIGN
It was found that by substituting a variable resistance and a small Potential for one of the tubes, the Same accuracy
was obtainable with a simpler and more compact set-up, the principle being that the potential and resistance caused a current to flow just as if it were produced in the vacuum tube. The use of only one tube obviates the preliminary calibration and comparison of tubes which is necessary if more than one is used. The design is shown in Figure 1. It will be noted that the essential parts are a potentiometer of the student type, one UX 222 tube, and a Leeds and Northrup 2 4 2 0 4 galvanometer, this latter being quite commonly used in hydrogen-electrode set-ups. This galvanometer has the following characteristics: sensitivity, 0.025 microampere per mm. division; coil resistance, 1000 ohms; damping resistance, 15,000 ohms. SHIELDING.It was found necessary to place the apparatus on a grounded metal plate and to shield the potentiometer and the tube. The shielding was accomplished by ordinary
Since the galvanometer is quite sensitive, it must be protected during preliminary balancing of circuits by % high resistance or shunt, as in ordinary practice. The authors found it more convenient to use a low-sensitivity galvanometer for preliminary work, switching to the sensitive galvanometer for final readings. MEASUREMENTS AND CALCULATIONS The electrodes used were of the type designed by MacInnes and Dole (4) using Corning 015 glass. Various arrangements 69
VOl. 5, No. 1
ANALYTICAL EDITION
70
of cells have been used, but the authors found the following type very advantageous: Satd. calomel-unknown Glass-buffer-satd. calomel electrode known pH electrode
Comparing it to a buffer of pH = 5.00, when Ea = 0.1483 and = 0.0502,
E,
I
If E, is the potential recorded for the cell with the unknown and Eb is the potential with a buffer Of known substituted for the unknown, then AE = E, - Eb = 0.0591 ApH at 25” C. To illustrate, using fresh strawberry juice and comparing = 0.0292 and
it to a buffer whose pH is 3.00, when Ea E, = 0.0502, pH = 0.0502 - 0.0292
+
3,00 = 3,37
0.059
pH
=
0.0502 - 0.1483 0.059
+ 5.00
=
3.34
By this means measurements are rendered independent of the concentration of the buffer in the glass cell and also of the “asymmetry” potential of the electrode. LITERATUREI CITED (1) (2) (3) (4)
Elder, L. W., J.Am. Chem. Soc., 51, 3266 (1929). Elder and Wright, Proc. Nat. Acad. Sci., 14, 936 (1929). Stadie, J. Biol. Chem., 83,477 (1929). Stadie, O’Brien, and Lang, Ibid., 91, 243 (1931).
RECEIVED June 24, 1932.
A New Surface Tension Balance RICHARDJ, DEGRAY,Department of Chemistry, Lehigh University, Bethlehem, Pa,
T
HE measurement of surface tension is even older than the term ‘$xmface tension.” Thus Guthrie (6) and Tate (IO) mention that drop weights of liquids seem to bear some relation to the earlier observed phenomenon of capillary rise, but say nothing about the reasons therefor. Guthrie (6) postulates a picture of a film surrounding a bubble; this is advanced as a new idea, and possibly was the original thought on the subject. Since then, of course, work on surface tension by many methods and with many objects in view may be found. An excellent bibliography and comparison of methods then extant may be found in the paper by Dorsey ( 2 ) .
A H
OF TORSION BALANCE FIGURE1. DIAGRAM
Apparently detachment methods were discussed first in 1887 by Timberg ( l l ) ,who compared the results of the measurement of surface tension by capillary rise, dropweight, and pull-on-a-ring. The correlation was satisfactory to him, though he used the simplest of formulas to calculate the surface tension from the pull exerted:
rections to be applied to the measurements. Thus we find plates, rings, spheres, rectangles, cylinders, and even evaporating dishes being used, and the formulas becoming more and more involved (1, I d ) . Finally DuNouy (3) designed a surface tension balance similar in principle to Timberg’s ( I I ) , but by his wide application of this balance, particularly in the physiological field, he populariaed the method. His results were calculated by Timberg’s simple formula (Equation l), probably because of the intricacy of all correction formulas derived up to that time by the many investigators. I n this country detachment methods have been confined to the ring method, possibly because of the popularity of the DuNouy balance, but in Germany the bent-wire method has found favor. I n both methods the general scheme for the attainment of absolute results was to measure a liquid whose surface tension was known from some other, more accurate method, and adjust all other readings accordingly. The bent-wire method was raised to the eminence of an absolute method by the theoretical derivation of corrections by Lenard (9) and measurements by Dallwitz-Wegener and Zachmann (9) checking the theory against practice. A similar step in the development of the ring method did not come, however, until 1930, when accurate measurements of correction factors to be applied to this method by Harkins and Jordan (6) were combined with a derivation of the corrections for the curvature of the column of liquid raised by the ring by Freud and Freud (4). Since the theory yields the same factors as did comparison with values obtained by the capillary-rise or drop-weight methods, the ring method may now stand on its own values as an absolute method of surface tension, of inherent accuracy practically equal to that of other methods, and rapidity and convenience exceeding most others.
THEORY OF where y is the surface tension, W the force necessary to tear the ring from the surface, g the acceleration of gravity, and L the length of the wire forming the ring. No corrections were applied to this formula, which really assumes that the ring raises a cylinder of liquid, though discussions of the shape of liquids raised by solid bodies may be found as far back as that of Wilhelmy (13) in 1863. Further developments along this line are confined to variations in the shapes withdrawn from the liquid and cor-
THE
TORSION BALANCE
The work of Lenard et al. (9) was performed with a torsion balance. The work of Harkins and Jordan employed a modified chainomatic balance. Since the torsion balance had been so successful in the bent-wire method, it was hoped that it could be applied as well to the ring method, employing as many of Harkins and Jordan’s precautions as would be commensurate with practical applications, yet retaining the inherent advantages of great speed with high accuracy of this type of instrument. A direct-reading instru-
