Electrolytic Reductions at Constant Cathode Potentials Electronically Controlled Apparatus MILTON J. ALLEN Endocrinology Section, National Cancer Institute, National I n r t i t u t e s of Health, Bethesda, Md.
An electronically mntroUed a p p a r a t u s is described for electmlytie reduction at constant cathode potentials. T h i s i n s t r u m e n t is extremely sensitive a n d stable with negligible "hunt" characteristics. It is applicable to Currents as high as 20 amperes a n d 75 volts applied voltage. With t h e plot of c u r r e n t against t i m e :* is possible to determine t h e total curnnt mnsumed in the electrolytic reduom. T h e i n s t r u m e n t does not require preliminary calibration a n d oan he uee give a plot of cathode potential against ourrent of solutions too concentrate h e polamgraphed with the s t a n d a r d i n s t r u m e n t s . c o m e of research i t became necessary to develop INanTHE instrument for the electrolytic reduction of organic comat constant cathode potentisls. I n addition, the in-
pounds strument had to be extremely responsive to changes in the amount of reducible substance in the wththolyte and be capahle of delivering the relatively high currents or applied voltages called for by high cathode potentials. It also hnd to be sensitive and responsive enough to give voltamme& ric curves (9) of solutions of such concentrations of reducible substance S E C T LIS19 CELL which could not be handled bv the polmgraph. Hnber in his classical work on the reduction of nitrobensene a t constant cathode potential (6, 7) used an instrument in which the total applied AMPLIFIER voltage was manually adjusted so as AIMP OINE to maintain a constant catbode POOTNmdTOR tential. Hickling (8) developed an all-electmnic unit which unfortunately Figure 1 could deliver only smsll currents. AIthough the apparatus designed by Csldwell, Parker, and Diehl(6) could deliver higher currents, the total applied voltage obtainable was in the neighborhood of 10 voltg. Lingane's apparatus (lo), although automatic, had certain disadvantages for the author's work. This unit as described could deliver only 10 to 15 volts and a msltimum current of %bout 4 amperes with good conductors. Although his instrument bad a sensitivity of *0.02 volt, its response was somewhat slow, 8 8 would be expected in such n mechanical device. The instrument described here utiliaes an electronic amplidyne as a control and eurrent source. It gives the relatively high currents or applied voltages required for the reduction of many organic compounds a t cathode potentials as high 88 6 volts and will control in both directions. Its sensitivity is *O.W5 volt. The parts wed in the construction of this instrument me readily available m d easily aasembled and can be built for approximately s1m. Essentially this instrument will give automatically controlled impremed potential to secure constant cathode potential. The desired cathcde potential is obtained by opposing the e.m.f. of the standard calomel electrode with an e.m.f. from potentiometer R, as shown in Figure 1. During an electrolysis experiment as the amount of reducible substance diminishes, the e.m.f. between the calomel cell and the opposing e.m.f. from the potentiometer will become unbalanced. This condition will transmit itself to a bank of tubes in the amplilier, cawing the amplidyne generator to change the total
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applied voltage to the electrolysis cell ylu-i nmuuy balanced condition again between the standard calomel electrode e.m.f. and the potentiometer e.m.f. Aa the electrolysis proceeds and the amount of reducible substance diminishes, t h e current will automatically drop and reaeh a plateau when the electrolysis is completed. Aa can be seen from Figure 2, which is a typical curve of current "8. time obtained as a result of B bimaleculrtr reduction of p-hydroxybensaldehyde to 4,4'-dihydrosyhydrobenzoin(I), the degree of hunt is negligible snd as the reduction proceeds the current drops until a plateau is reached, indicsting completion of the reaction. Each vertical space on the chart represents 1.93 minutes and each horizontal space represents 0.05 ampere. Figure 3 gives the schematic details of the assembly.
The total applied voltageand current delivered to theelertmly-
cell arc indicated by voltmeter 1'2 (Woston Model W3 D.C. oanel voltmeter. 0- to 100-vult scale) and ammeter A f\Vestan Model 931 muldple-range ammeter hdicating 0 to 2.5, 0 to IO, and 0 to 25 amperes). A three-point rotary switrh, S5, controls the nmmeter range. si8
Figun 2. C u r r e n t
"(I.Time
V O L U M E 22, N O . 6, J U N E 1 9 5 0
c
t
Figure 3
A plot of current "8. time is obtained with a Speedomax recorder ( k e d s & Narthrup No. 601014 Model S Speedomax potentiometewtype indicating recorder) which has a scale of 0 to 100 divisions and B range of 0 to 50 mv. The external circuit consists of five 5&mv. shunts (Westan portable testing shunts) of values 1. 2. 5. 10. and 20 amperes, which gives the record& varisbk r&hs i f o io I . o to 2, o to 5, o to IO. and o to 20 amperes. The variability of he Sper$omsx rangp is cantrolled hv S3.B five-~ointrotarv selector switch ( k e d s & Northm p .\Idel 8240.. The cylEr can be,pherd in or out a( !hr circuit hy SI, LL three-pmmon tug& s w t r h , the center of which DIPW~W the current frum Bowma.through t r % : trolysis E G C u i t . The desired cathode potential is obtained by placing the standard calomel electrode aeainst the surface of the cathode. This e.m.f. is oppmicd by an
