GB556649A - Improvements in apparatus for and methods of measuring or controlling the value of ph, or of the conductivity of solutions or of oxidation reduction potentials and the like - Google Patents

Abstract

556,649. Voltage and resistance measurements ; determining physical qualities of materials. MARCONI INSTRUMENTS, Ltd., BARTLEY, W. B., and BROCKLESBY, C. F. April 13, 1942, No. 4850. [Class 37] In an arrangement for measuring pH or rH values or conductivity of solutions, a liquid X under test in container C1 is compared with a standard solution S in a container C2 in heatexchanging relation with C1, and electrodes E1, E2 are connected to the grids of valves V1, V2 forming part of a known symmetrical differential bridge comprising anode resistances, with an amplifier A, if desired, connected to +, and with a high-value cathode impedance Rk connected to -, the terminals being shunted by resistors R3, R4, the centre point of which is connected to a half-cell Ch and conventional salt bridge having electrodes B1, B2 in the receptive solutions. A resistance thermometer T in the tested solution makes a correction for temperature-changes thereof. The indicator is shown at 1. In Fig. 5, the half-cell Ch is connected by a movable slider P to a tapping on one, R5, of a pair of resistors connecting the cathodes together, the electrical mid-point of the resistors being connected to Rk. For balancing the bridge. the tapping is adjustable by a reversible motor M driven through a control system CS from the amplifier output, the tapping position indicating the quantity to be measured. In Fig. 4, the reference half-cell Ch2 is connected through ammeter A1 to resistances R3, R4, as in Fig. 3, but the half-cell Ch1 in solution X is connected to a point in a further shunt circuit acting as a feedback amplifier comprising a fixed impedance R7, a bias battery, and a variable impedance comprising a triode V3 having its grid connected to the amplifier A. A resistance-thermometer T is connected across the half-cells Ch1, Ch2. When the potential-changer at E1 upsets the normal balance, current flows through the ammeter A1 and thermometer T, and the reading of A1, compensated for temperature, is a measure of the potential-change. In measuring conductivity, a pair of electrodes are immersed in each of the liquids, as in the pH applications, a suitable standard solution being one of potassium chloride, which has substantially the same temperature coefficient as most of the unknown liquids met with in practice.