e
TRANSFERENCE NUMBERS BY THE MOVING BOUNDARY METHOD PAUL BENDER and DONALD R. LEWIS University of Wisconsin, Madison, Wiscpnsin
SEVERAL YEARS ago Longsworthl pointed out
-
in THIS JOUHN.\I. rh3r the rrratn~cnt of t r : ~ ~ l ; f ~ wI ~I ~I vI r ~ I ~ rnrns~~n,mrnt* in the ~~ndrrgrndunre physicnl rhenlistry I~borntoryroulil rn~dilyl ~i,xtrndrd r to inih(lt~ rhe rnovingboundnry method i n ilddition to thr clu-.;i,.:~l1Iittorf : rnttthotl. It is our purp~.(~ ro ilwvribr n curnpnct rnodi'. ficnrion of the urrrsiory :~pp.~l.iltus rcqnirrd whkh 1 ~ m bines adequate accuracy of results with marked convenience in operation. The earlier experimental arrangement employed a constant-voltage supply (B-battery bank) with the current through the moving boundary tube held constant by co~lfinurdm:lrlunl ndjustrnent of a series rrsistnncr. The nev rnrrhorl introduces un rlectronicnllv controllrd constant-current supply, with which only occasional manual adjustment is required to compensate for the resistance change occurring in the moving boundary tube. A calibrated milliammeter isincorporated in the instrument for convenient measurement of the current Figure 1. C o n s t - t - C m n t Supply Unit in routine work; the potentiometric method previously recommended1can be substituted if desired. The com- are shown the plate characteristics for the 6AC7 videopleted unit is shown in Figure 1, and the circuit diagram amplifier pentode which gave us the best results in the in Figure 2. present application. It is seen that a t the comparaThe operation of the constant-current supply depends tively low current level required here the characteristics uuon making the current flowing through the moving are almost flat in the higher plate voltage range. bbundary tube the plate currentif a pentode tube, be: The electrolytic solution in the moving boundary tube cause it is characteristic of the pentode that under acts as a variable load resistance R, in the pentodeproper conditions the plate current will be practically plate circuit. If the voltage supplied to the 6AC7plate independent of changes in plate voltage. In Figure 3 circuit is E,, and the plate current passing through the moving boundary tube is i, there will be a potential LONCISWORTH, L. G., J. CHEH.EDUC.,11,420 (1934). F~~u.. a. circuit ~ i . ~ . .for ~ Constant-C-nt 8upplg Unit
SW-l
Sw-2
S.P.S.T.toggle mitch
D.P.S.T. saitohback on R-3 PI, 0-watt 110-volt pilot light Thordarson T13Rll power TI transformer Ch-1,2 Filter ehokea, 8 henry or larger C*, C,. C. 8-mf.. 460-volt dry' eleh tr01ytic oondensers 26.000-ohm, 60-watt RI 15.000-ohm, 20-watt RI 100.000-ohm potentiometer Ra 10.OM)-ohm potentiometer RI 0-3-ma.d-c. Weston model MA 301 meter VT-I SF3 Rectifier VT-2 6AC7 Vldeo-amolifier entode YR VR-105 Voltage regulator Bou. 7lIr-volt "C" battery, Burxeaa tvoe . .. 6540 Fuas 1-amp. Little fuse Cobinsl
Bud No. 1746
SEPTEMEER, 1947
drop i,R, across R, so that the plate voltage will be given by (E,, - i&). The plate voltage will thus be a t its maximum value a t the start of the experiment and will gradually decrease as the boundary moves up the tube. This change in plate voltage is quite appreciable. In a typical determination of the transference number of the hydrogen ion in 0.1 N HC1 solution the initial resistance of the moving boundary apparatus was 5300 ohms, and the final resistance approximately 50,000 ohms. For a current of three milliamperes the corresponding decrease in plate voltage is nearly 135 volts. If, however, we have provided for operation under conditions such as specified by the point 0 in Figure 3 it is obvious that there would have been very little change in current in spite of the large change in plate voltage. The efficiency of current regulation is illustrated by the data of Table 1, wbich were obtained with a regular student unit. Improved control a t a higher current level can be obtained by using a higher voltage plate supply. The magnitude of the plate current is, of course, determined by the control grid bias, of which coarse and h e adjustment is provided by Rg and R4, respectively. The unit should be allowed a 20-minute preliminary warm-up period for best results. The full-wave rectifier is followed by a two-section filter which reduces the alternating-current ripple voltage superimposed on the 350-volt direct-current output voltage to below 0.1 per cent. Condenser input to the filter is employed because the higher voltage so obtained permits more efficient operation of the pentode curreutcontrol section. There is no need for regulation of the plate voltage supply since such variations as occur in it are equivalent to small resistance changes in the moving boundary tube for which the control circuit automatically compensates. It has been found, however, that the current level a t which regulation takes place is markedly affected by the filament temperature of the 6AC7 as well as its grid bias, so that less satisfactorj. control will be obtained if the line voltage is subject to drift or marked fluctuations. In such a case the use of a constant-voltage transformer or other voltage regulation of the power transformer primary voltage will produce a decided improvement in performance.
455
50
100
200
300
Plate Voltage Figure3. Plate Cha~ecteristicsfor 6AC7 Pentods
the VR-75, 90, or 150. Because of the high trapsconductance of the 6AC7 it was found absolutely necessary to use a battery source to insure constancy of control grid bias. Cathode bias as employed in the circuit given by Reich2 is not recommended in this application because the control obtained is not good enough. Standard banana plugs and jacks are used to connect the moving boundary tube electrodes to the current supply. Since the negative side of !he power supply is grounded to the chassis and hence to the metal cabinet as well, insulated plugs and jacks are mandatory, because each of these connections is a t a potential several hundred volts above the chassis. The currentlimiting property of the circuit minimizes shock hazard in any case, since no more than a few milliamperes can flow, in spite of the high voltage present. In addition to the main power switch a separate high-voltage cutoff is provided so that the tube filaments can be kept hot while the output circuit is dead; in order to protect the plate milliammeter this switch is located on the back of the coarse-current control so that the 6AC7 is biased past cut-off when the high voltage is turned on. Further protection for the meter is provided by the TABLE 1' grid-biasing arrangement used, with which the grid bias can never increase above - 11/2 volts. Plate resistam, Hydrochloric acid solutions (0.1 to 0.05 N) have been ohms Plate euwat, ma. the regular subject of student work in our laboratory, because the use of an indicator such as methyl violet eliminates difficulty in locating the boundary. The amount of indicator used should be held to a minimum. The current should be turned on as soon as possible after the hydrochloric acid solution is placed in the tube to minimize complications arising from the direct 'Current measurement by the potentiometrio method. Priattack of the acid on the cadmium electrode. The mary voltage of power transformer regulated. moving boundary tube assembly suggested by LongsConstancy of screen voltage for the 6AC7 is obtained 2 REICA, H. J., "Theory and Applications of Electron Tubes," by use of a voltage-regulator tube. The results ob- Second Edition, McGraw-Hill Book Company, Inc., New York, tained with the VR-105 were superior to those given by 1944.
456
worth' is very satisfactory and can be conveniently constructed from pyrex glass by use of the graduated pyrex pipets manufactured by the Ace Glass Company. A silver-silver chloride electrode is used as the cathode. Measurements on KC1 solutions are recommended for additional work only, since they are more difficult to carry out and it is first desirable to insure experimental results adequate to illustrate the principles involved. This equipment has been successfully employed for the past several semesters. The results observed in this period indicate that an accuracy of the order of one per cent is readily obtained. Customarily a current of 2.95 ma. is recommended in order to obtain maximum accuracy in setting the current level on the Weston
JOURNAL OF CHEMICAL EDUCATION
Model 301 meter employed. A deviation of one-fifth of the smallest meter division is easily detected, corresponding in this case to a maximum deviation of 0.3 per cent. The meter calibration is checked periodically by the potentiometric method; under these conditions a level of performance considerably above that guarantekd by the manufacturer is achieved. The major source of difficulty emphasized by the students lies in the timing of the motion of the boundary. This observation provides additional justification for use of the milliammeter for current measurement instead of the potentiometric method, whose inherent accuracy cannot be fully utilized in student work.
