Patent GB-626962-A (GB)
Title: Apparatus for measurement of electrochemical potentials
IPC: G01N27/416
Filed: 1946-08-08
Abstract: 626,962. Thermionic valve voltmeters; determining physical qualities of materials. PAULLY, F. G. Aug. 8, 1946, No. 23558. Convention date, Aug. 25, 1945. [Class 37] In apparatus for determining electro-chemical potentials and comprising an electrode assembly for generating a voltage, an electronic amplifier, and a direct reading measuring instrument, unitary adjusting means are provided by which the output voltage may be adjusted with respect to a balancing potential and the effect of the asymmetry potential of the electrode assembly simultaneously obviated, the measuring instrument being provided with a manually adjustable index pointer for registering any variation due to asymmetry potential. A glass electrode 1, Fig. 2, is connected through a switch 2 to the control grid 3<SP>1</SP> of an amplifier valve 4<SP>1</SP> whilst a calomel reference electrode 5<SP>1</SP> is connected to the control grid 6<SP>1</SP> of a second amplifier valve 7<SP>1</SP>. The cathodes 8<SP>1</SP>, 9<SP>1</SP> are connected to a degenerative resistor 12<SP>1</SP> which is connected to the negative terminal of a power supply 13<SP>1</SP>. The positive terminal of the power supply 13<SP>1</SP> is connected to the anodes 15, 16 through an adjustable potentiometer 14<SP>1</SP> and resistors 17, 18. Any unbalance between these resistors is indicated by a microammeter 19, the sensitivity of which is adjustable by a variable resistor 20. The adjustable grid bias resistor comprises resistors 32, 35 connected across the power supply and shunted by a condenser 36. For calibration the switch 2 is depressed to connect the grids 3<SP>1</SP>, 6<SP>1</SP> and the potentiometer 14<SP>1</SP> is adjusted until the meter 19 reads at the centre of the scale. The grid 3<SP>1</SP> is then floated by disconnecting the grid cap on the valve 41 and the grid bias resistor 35 is adjusted to zero grid current. The grid cap is replaced and a calibrated voltage source is connected to the grids 3<SP>1</SP>, 6<SP>1</SP>. A voltage equivalent to the desired range in pH units is employed and the resistor 20 is adjusted until the meter reads full scale. The power switch 37 is then closed and the glass and reference electrodes are immersed in a standard buffer solution which may be 0.05 m. potassium acid phthalate having a pH value of 4 or a solution of 0.05 m. phosphate buffer having a pH value of 7. The potentiometer 14<SP>1</SP> is then adjusted until the meter reads the corresponding value. The electrodes are removed from the buffer solution, rinsed and immersed in the solution to be tested, the pH value of which is read on the meter scale. The meter 19 is provided with a manually adjustable index pointer which is set in alignment with the indicating pointer when the amplifier circuit is shorted by the switch 2 immediately after standardisation with a known buffer solution. If the pointers are not in alignment when the switch 2 is operated for transfer of the electrode to a new solution, the potentiometer 14<SP>1</SP> is adjusted to correct the alignment. Suitable electrodes are shown in Fig. 1. The glass electrode consists of a length of soda-lime glass tubing 1 to the lower end of which is sealed a glass membrane. Electrical connection to the inside of the glass membrane is effected by means of a glass tube 3 in which is sealed a platinum wire 4. The projecting portion of this is coated with a silver-silver chloride composition and dips into a solution 5 such as 0.1 N hydrochloric acid or 0.05 n. potassium acid phthalate with decinormal potassium chloride. The glass measuring electrode is immersed in the solution 5a in a beaker 6 into which also dips the reference electrode 7. This is a half-cell constructed from glass tubing 8 in which is mounted a platinum wire 9, a layer of mercury 10, and a paste of mercurous chloride 11, which are held in place by a cotton plug 12. The half-cell is immersed in a saturated solution of potassium chloride 13, connection between the reference electrode and the solution under test being made through a minute hole 14 or by a ground glass joint or a porous material. The glass and reference electrodes may be enclosed in an inert plastic case for protection. The unit may be so arranged that it can be placed directly into a beaker without a support.