respectively. The emission maxima is 590 mp. The secondary filter should transmit from 560 to 600 mp. Excellent results were obtained in the detection of 10-6 gram of Xl+d per ml. and acgram ceptable results with 27 X of h l + 3 per ml. It was also noted that fluorescence decreases as the amount of ethanol decreases. This decrease as shown in Figure 1 v a s obtained from solutions, each of which contained 2.5 X gram of Al+3ion per ml. and 7 . 5 X lop6 gram of PAN per ml. of solution. Other working curves using a different per cent by volume of ethanol can also
be obtained; however, a 70 to 95’70 ethanol solution gave satisfactory results. ACKNOWLEDGMENT
The authors thank Mrs. Janet Bennett of the P-L Biochemicals, Inc., and Mr. Robert Sullo of the College of Engineering (M.U.) for their helpful assistance. LITERATURE CITED
(1) Cheng, K. L., Bray, R. )I., ANAL. CHEM.27, 782 (1955). ( 2 ) b’laschka. H., Abdine. H.. ChernistAnalyst 45; 2 (1956). ~
(3) Haworth, D. T., Starshak, R. G., Surak, J. G., J . Chem. Ed. 41, 436
(1964). (4)Holzbecher, Z , SSb. Celostatni Pracovni Konj. Anal. Chemiku 1, 166 (1952). C . A . 49, 15610 (1955). ( 5 ) Lm, J. Ch-I, Ph.D. thesis, Cniversity of Illinois, Urbana, Ill. (1951). (6) Weissler, A , , White, C. E., ISD.ENG. CHEM.,ANAL.ED. 18, 530 (1946). J. G . YURAK 11. F. HERM.IS D. T. H.IWORTH
Department of Chemistry Marquette Universit,y Milwaukee, Wis. 53233 WonK supported by an N.S.F. grant
Switching Circuitry for Fast Chronopotentiometry SIR: Recent work involving shorttime chronopotentiometry has indicated hazards in switching circuitry which applies the current to the cell ( 1 , 2, 6). I n addition, it is of some intereqt to carry out chronopotentiometry from an initially biased electrode rather than from the equilibrium potent a1 as is normally done (3, 5 ) . The present correspondence is concerned with a relatively simple switching circuit which may be successfully applied to both problems. It is advantageous to utilize makebefore-break high-pressure mercury relays in chronopotentiometric switching. While having the disadvantage of momentarily shorting the indicator and counter electrodes together, this shorting does prevent fluctuation in the constant current source, regardless of type. However, the shorting may result in a large indicator electrode polarization for the approximately 0.5 msec. involved. A fourth electrode against which the counter electrode may be prebiased represents a ’ solution to this problem ( I ) , as does the use of break-beforemake relays which, however, may cause current source fluctuation ( 2 ) . If it is desired to prebias the indicator electrode, the bias must be removed just prior to application of the constant current, or else a parallel path for current flow may be present. D P D T TOGGLE SWITCH
1NDlCATOR
4 l H ELECTRODE
1
1 POTENTIOMETER
Figure 1 . Switching circuitry
~
The circuit in Figure 1 indicates one solution to the problem. Relay 1 is a Clare HG2,41003 make-before-break high-pressure mercury relay. SW1 is a double pole, double throw toggle switch. The potentiometer serves to bias the indicator and counter electrodes, which are shorted together when SWl is in the off position. When SWl is thrown on, the bias potentiometer is first removed, then the relay coil energized. When Relay 1 is energized, it momentarily shorts all the contacts together, prior to breaking from the “Normally Closed” position. However, because the indicator and counter electrodes have been biased as a shorted pair, no significant polarization takes place; further, because the current source is shorted, no current flows through the cell until the contacts break. Figure 2a shows a potential-time trace using a make-before-break relay but without the use of the fourth electrode and related switching circuitry. The initial equilibrium potential of about -0.4 volt corresponds to a thallium amalgam. The potential excursion observed when the relay was shorted polarized the electrode to mercury dissolution, as the platinum counter electrode had been strongly anodic prior to the chronopotentiogram shown. When the relay breaks, applying the constant current to the cell. mercuric reduction is noted, followed by a rise to thallous reduction. Figure 2b shows the start of two chronopotentiograms with thd switching circuitry of Figure 1. The lower curve is biased a t 0 volt, the upper a t -0.3 volt us. the SCE. There is no potential excursion, a n J the rise to the start of the thallous reduction corresponds to double layer charging. The constant current source employed was a portion of an operational amplifier set-up. similar to that previously described ( 4 ) . However, any
*--io5 MILLISECONDS f b)
Figure 2. electrode
Hanging
drop
mercury
5mM TI(I) in 1 M H C I 0 4 ; horizontal axis, 0.5 msec. per div. (a) Make-before-break relay, Tl(Hg) hanging drop formed from a prior chronopotentiogram. Vertical axis, 0.2 volt per division ( b ) With switching circuitry of Figure 1 ; lower curve prebiased at 0 volt; upper curve prebiased a t -0.3 volt; vertical axis, 0.1 volt per division
constant current rource may be used with this switching circuitry. LITERATURE CITED
(1) .41isoii, F. C., AZIAI..CHEM.36. 520 ( 1964). ( 2 ) Christ,ie, J., Laiier, ( i . , I b i d . , p. 2037. ( 3 ) Gierst, L., “Ciiietiqiie d’spproche
et reactioiis d’elwtrode irreversibles,” These d’aggregatioii, University of Briissels, 1958.
(4) Laiier, G., Schleiii, H., Osteryoiiiig, R., . 4 N A L . CHEM.35, 1780 (106s). ( 5 ) lIiirrayj It., [bid., p. 1i84. (6) Osteryoiuig, II., Aiison, F. C., I h i d . , 36, 975 (1964). 13. .4.OS.rERYOI.S(; Sorth American Aviatioii P(ieiic.e Center 1049 Carniiio dos Rios Thoiisaiid Oaks, Calif. VOL. 37, NO. 3, MARCH 1965
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