GB631459A - Improvements in or relating to apparatus for polarographic analysis - Google Patents

Abstract

631,459. Determining physical qualities and dimensions of materials; cathode-ray oscillographs. RANDLES, J. E. B., and AIREY, L. Sept. 5, 1946, No. 26729. [Class 37] The invention relates to apparatus for displaying polarograms on a cathode-ray tube. In Fig. 1 the opening of switch K to charge a large condenser C to effect a voltage sweep across a polarographic cell A occurs automatically at a selected time in the formation of a mercury drop constituting the cathode in the polarographic cell A. The closing of K to effect cessation of the voltage sweep occurs automatically as the mercury drop falls. The cell A in series with a variable resistance R<1> and adjustable potentiometer resistance Q, forms a connection across the condenser C. A bias voltage from a battery B<1> via the tapping on the potentiometer resistance Q adjusts the starting P.D. across the cell A. In Fig. 1 the varying voltage across the resistance R<1> as the current changes due to reaction in the test solution occupying the cell A means a non-linear voltage sweep across the cell A as K opens to charge the condenser C. In the practical form of the equipment a cathode follower circuit, Fig. 3 (not shown), is included to linearize the voltage sweep across the cell A. The X and Y outlets connected to R<1> (R<1> voltage proportional to current through the cell A), and the cell A, are directed to a D.C. amplifier, Fig. 5 (not shown). The amplified versions of the cell A and resistance R<1> potentials as successive voltage sweeps take place are applied to the X and Y plates respectively of a cathode-ray tube having a long afterglow. Thus a continuous display is provided of the polarogram for the test solution in the cell A. The switch K is controlled by a relay in a flip-flop synchronizing circuit, Fig. 4 (not shown). There is a sudden change in cell current coincident with fall of a mercury drop and thus a sharp change in the resistance R<1> voltage and the generation of a pulse in the Y deflection amplifier. This pulse " triggers off " the " flip-flop " synchronizing circuit which then actuates the relay to close the switch K for an interval of time which depends on the delay in the flip-flop circuit. On expiration of this time the flip-flop reverts to open the switch K and start the voltage sweep across the cell A. This delay in the flip-flop circuit may be such that the switch K is not opened until the next drop is near full size to eliminate from the polarogram the effect of current changes due to growth of the drop. A variable resistance R<2> is inserted in the battery B feed to condenser C, so that the amplitude of the voltage sweep may be increased to permit the display of fragmentary polarograms when the test solution in the cell A contains a mixture of substances. For calibration purposes a standard solution occupies the cell A and resistances R<1>, R<2> and Q are appropriately set with the D.C. amplifier controls, Fig. 5, adjusted to give a satisfactory deflection. The extent of re-adjustment necessary to produce the same deflection affords a measure of the reacting substance in the test solution. The resistance R<1> may be calibrated in terms of peak current which corresponds to concentration of the reacting substance. For high concentrations it may be necessary to use a D.C. amplifier incorporating resistancecapacity coupling to ensure a pulse adequate to trigger the " flip-flop." A condenser of small capacity may be connected across the resistance R<1> to remove parasitic oscillations from the trace on the cathode-ray tube. The " flip-flop circuit, Fig. 4, is convertible by a switching operation into a multivibrator which supplies a recurrent potential sweep for standardizing the dropping time of the mercury cathode. U.S.A. Specification 2,246,981, which relates to similar apparatus utilizing a cathoderay tube, is referred to.