Use of a Condenser to Reduce Galvanometer Oscillations in Polarographic Measurements With Particular Application to Compensation Method of Measuring Small Diffusion Currents J411ES J . LING4KE AND HERBERT KERLINGER Universitj o f California, Berkeley, Calif.
T
HE polarographic method of analysis is based on the
interpretation of the current-voltage curves that are obtained when solutions of electroreducible or electrooxidizable substances are electrolyzed with the aid of a dropping mercury electrode ( 1 ,2 , 3). As a result of the periodic change in area as each mercury drop grows and falls a t the dropping electrode, the current varies from virtually zero at the very beginning of the formation of a drop to a maximum value a t the instant the drop falls.
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1
noted that the condenser greatly reduced the galvanometer oscillations, without changing the value of the diffusion current. The condenser damps the galvanometer oscillations by decreasing the variation in e. m. f . across the shunts of the galvanometer, that otherwise occurs as a result of the periodic change in current through the cell. When a mercury drop falls the condenser partially discharges, and more or less completely maintains constant current through the galvanom eter until the next drop has grown sufficiently so that the current through the cell is restored to its average value, 2. During the later groJTth of the drop, when the current through the cell increases above its average value, the condenser absorbs an increment of charge equal to that lost by discharge when the preceding drop fell. Since the internal resistance of the electrolytic condenser is very large compared to that of the galvanometer and shunts, there is no net flow of current through it, and hence the average deflection of the galvanometer is the same with and without the condenher.
1
8 -
To Polarograph FIGURE1. NETHOD OF CONXECTISG .i COND E M E R , C, INTO CIRCUIT FOR Danmsc GaLVAXOLIETER OSCILLATIOSS
6 -
m
i
a 4 -
0
The average current is generally less than 100 microamperes, and i t is usually measured by means of a sensitive mirror galvanometer of relatively long period (20 seconds or greater). K i t h such a n instrument the observed oscillations are much smaller than the true change in current during the life of each drop; they usually amount to about 5 to 10 per cent of the average current. The oscillations are very uniform, and ordinarily there is no great difficulty in measuring their average value, which corresponds very closely to the true average current (3, 4). However, in certain cases the oscillations are large enough to be troublesome and i t is therefore very desirable to have a method of reducing their magnitude. This is particularly true in the “compensation method” of measuring small waves which is described below. The authors have found that the most satisfactory method of damping the oscillations without affecting the value of the diffusion current is to connect an electrolytic condenser of high capacitance across the galvanometer shunts as indicated by C in Figure 1. They employed a Heyrovsky-Shikata type of polarograph M hose principle has been described elsen here in detail ( I , 2, 3 ) . The effect of the condenser is demonstrated by the typical current-voltage curves in Figure 2 . Curve 1 in this polarogram was recorded in the usual manner ( 1 , 2, 3) with an airfree solution of 0.001 cadmium sulfate in 1 A- potassium chloride, without a condenser and with a drop time of 4.2 seconds. Curve 2 was recorded with the condenser (2000 microfarads) in the circuit as shown in Figure 1. It will be
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= 2 -
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0.3
0.5
I I
05
0.3
E,
07
09
Volts
FIGURE 2. DAMPIXG EFFECTOF CONDEXSER OK GALVAXOMETER O~CILLATIOX3
1 . 0.001 M cadmium sulfate in 1 .V potas-
sium chloride without condenser
2 . Repeated with 2000-microfarad Condenser
in parallel with galvanometer shunts
The essential condition to be fulfilled for effective damping is that the Capacitance, C, of the condenser shall be large enough so that the average charge, C;R,, which it acquires due to the ohmic potential drop, iR,, across the combined parallel resistance, R,, of the galvanometer and shunts, shall be much larger than the increment of charge that flows into and out of it during the formation of each drop. The authors have found empirically that n-ith the usual type of galvanometer with a natural period four to five times larger than the drop time, t d , the damping will be effective if the average charge stored in the condenser, CiR,, is equal to or greater than the arerage charge, it,, associated with each drop. T h a t is,
DECEMBER 15, 1940
ASALYTICAL EDITION
TABLEI. DAMPINGEFFICIESCYOF COSDENSER.IT V.~RIOUS SETTISGSOF AYRTOXSHUNT (Various concentrations of CdSOd in 1 S KCl plus 0.005% gelatin, with a 2000-microfarad electrolytic condenser in parallel with t h e galvanometer a n d shunts a s shown in Figure 1. Drop time = 3 seconds: Ro = 38 ohms; Rt 7.7 ohms: R A = 10,000 ohms; osclllations without and with the condenser compared a t E d . = 1.0 volt B S . S.C. E. Shunt settingfadjusted t o give a total deflection of 90 t o 110 mm. a t each concentration of CdSO4 )
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CdSOi .\I illimolar
R,
0.95
2.0 3.0 5.0 10.0
0.15
1.0
25.0 50.0
105 475 lSi5 2100 2400 1600 1275
0.99
0 4 0.5 0.6 0.7
0.75 0.70 0.40 0.20
820 430
0.09 0.045
200 100
0,020 0.010
Oscillations Without With condenser condenser 4.5 6.0 6.0 6.0 4.5
4.0 7.0 7.0 8.0 7.0 7.5
4.0 2.5 0.7
