Cou Iostatic Ana lysis Direct-Reading a n d Recording Instruments PAUL DELAHAY and YASUSHI IDE Coafes Chemical Labclrafory, Louisianu Sfafe Universify, Baton Rouge, l a .
b A direct-reading instrument is described for coulostotic analysis in which the cell voltage during decay of potential is sampled, stored in capacitors, and reaa with a vacuum tube electrometer-e.g., a pH meter for glass electrode. Adaptation to recording of complete potential-time curves with a pen-cind-ink recorder is also described. P1:rformance data are given.
P
CURVES in previous work (1, 3) on coulostatic analysis were recorded with a cathode-ray oscilloscope because the decay of potential occurred in less than l second. Two suggestions were made to simplify measurements: the rate of potential decay, after coulostatic charging, is decreased by means of a capacitor in parallel with the cell, and potentialtime curves are determined with a penand-ink recorder ( 3 ) ; the cell voltage during decay of poteritial is sampled,
OTENTIAL-TIME
stored in capacitors, and read directly with a vacuum tube electrometer ( 2 ) . Instrumentation for application of these two methods is described and tested. EXPERIMENTAL
Direct-Reading Instrument. The circuit of the direct-reading instrument of Figure 1 is composed of the coulostatic charging circuit, which is the same as in previous work (S), and a circuit providing sampling, storing, and reading of the cell voltage. The only change in the coulostatic charging circuit is the introduction of switch s1 with which the cell circuit can be open when measurements are not taken. The cell voltage is stored in highquality capacitors cz and c8 a t two times tl and tz during decay of potential. More than two time intervals could be selected by use of additional capacitors. The vacuum tube electrometer El was connected between the cell CL and capacitors c t and CI because the charges on c2 and c g were not negligible in
n '
RL,
comparison with the charge on the double layer of the working electrode e,. The sampling times were adjusted by means of the timer of Figure 2. Relays RLz and RL3 remained closed for only 7 mseconds, since the sampling time had to be short in comparison with time tz. Simpler timers could be used, and the circuit of Figure 2 was selected only because components were on hand. The difference of voltages across cp and c3 which is equal t o the variation of the cell voltage during the interval t 2 - tl, &as read with the vacuum tube electrometer Ez-i.e., a Beckman pH-meter, Model G in this work. Capacitors c2 and c g were short-circuited by means of switch s2 before measurements, and switch sa was open as soon as voltages were stored in c2 and c3 to minimize leakage. Readings on E1 were taken rapidly (10 to 20 seconds) to minimize leakage in cz and c3. Recording Instrument. Potentialtime curves were recorded with a Sargent pen-and-ink recorder over a 10-second interval with the previously described instrument (3) which had been modified as follows: a 3-pf. capacitor was connected across the cell and the oscilloscope was replaced by a Keithley electrometer, Model 600A connected to the recorder. This instrument and the one of Figure 1 could easily be combined in a single, versatile unit. Cells and Solution. The technique was the same as in previous work (3).
C3
s3
Figure 1. Instrument for direct reading of cell voltage during decay of potential CI.
0.03 pf.
c2, c3.
0.06-pf. General RadiJ flxed capacitors
C l . Cell e,. Reference electrode (sotuioted calomel electrode) Working electrode (hang ng mercury drop) El. Keithley vacuum tube electrometer, Model 600A, operating at a gain of 1 E?. Beckman p H meter, model G P1. 10,000-ohm Helipot potentiometer with 0- to 22.5-volt adjustment of span voltage Pz. 100-ohm Helipot potentiometer with 0- to 2-volt adjustment of span voltage R l 1 , R12, Rls. Relays 51, 12, IS. Switches (SL and 13 with high insulation)
e,
Figure 2.
Delay circuit
6. 45-volt battery C. 0.01 -pf. capacitor R I . 4 7 0 0 ohms R2. 10,000 ohms Ra. 100,000 ohms RL1, R l z , R h . Relays of Figure 1 TK1, TK?. Tektronix waveform generator, Type 1 6 2 TK3, TKc Tektronix pulse generator, Type 161
VOL. 35,
NO. 9,
AUGUST 1963
11 19
Table 1.
Analysis of Data in Figure
3
Ah'. mv. Exptl.
extrap-
b.E/Co for olated Calcd. 1 second volts/ to 1 for 1 CQ, mole/liter second second mole/liter 5 . 8 X lo-' 23 20 3 . 9 7 x 10' 2 . 0 X lo-& 61 68 3 . 0 3 10' ~ 5 . 1 X 1 0 - 8 154 173 3.01 XIO'
cn
c
0 > ..-
100-
E
u
DESCRIPTION AND DISCUSSION OF RESULTS
4I5O-
Direct-Reading Instrument. Experimental results are summarized in Figure 3 and Table I. The measured cell voltage variation A V from ti to & was converted to the change of potential AE from t = 0 to tz by
This equation holds for the plane electrode but a similar equation could be written for sphe.rica1 or cylindrical electrodes (3). Correction for sphericity was quite negligible here. It is seen that AE us. tZ1'* plots are linear except when AE is quite small and tz < 0.2 second. Further, experimental AE's in Table I, except for the lowest concentration, are smaller than the calculated values as in previous work (3) on this system with oscilloscopic recording. Leakage in capacitors cz and Ca probably also accounted in part for the discrepancy. Results were quite reproducible since the standard deviation for AE after correction for the bIank was 3.0 mv. for the followingconditions: 138 mv., 20 determinations, Co = 5.1 X 10-eM Zn+2, t 2 = 0.36 second. The influence of the cell resistance was greater than in previous work (3),
a=
0
blank
h
Zn" 5 . 8 O-7M ~
200 -
.
2.0x O - ~ MZn"
5.I
:
