A direct reading hydrogen-ion meter / by C.G. Pope and F.W. Gowlett.

  • Pope, C. G.
Date:
[1927?]
Catalogue details

Licence: In copyright

Credit: A direct reading hydrogen-ion meter / by C.G. Pope and F.W. Gowlett. Source: Wellcome Collection.

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    resistance box. When a given range has been calibrated by applying known potentials to the grid, some method of standardizing the apparatus before and after each pH reading is necessary. For this purpose a known e.m.f. is applied to the grid, and the filament tem¬ perature (which is the only variable) is adjusted by means of the rheostat until a pre¬ determined meter reading is obtained. The known e.m.f. may be obtained from a standard Weston cell as shown in Fig. 3, or more simply by connecting the grid directly to the negative end of the valve filament. Calibration for the Hydrogen Electrode System Using the saturated calomel half cell at a temperature of 18° C., the change in potential from pH 0-12 is from 0*2503 to 0*9427 volt, and the apparatus is arranged to read 0-120 divisions on the scale of the milli-voltmeter when this range of voltages is applied to grid (a). This is done by adjusting the value of the resistance at (9) in the following manner. Set the rotary switch (16) to the H2 electrode terminals and connect a potentiometer to these terminals so that its negative lead goes to the grid of the valve. Apply a potential of 0*2503 volt from the potentiometer to the outer grid. In the position which will finally be occupied by resistance (9) connect the variable resistance box; adjust the resistance box until the reverse current is about equal to the anode current from the valve. Great care must be taken to prevent damage to the meter during these balancing operations. The shunt (12) across the meter must not be removed by depressing the button (13) until it is obvious that the anode current is almost balanced by the reverse current. The meter should be so connected that increasing the reverse current increases the meter reading. Now make adjustments by means of the rheostat (7) (controlling filament temperature and therefore the slope of the anode current curve); at the same time adjusting the resistance at (9) by means of the resistance box until, with a potential of 0*2503 volt on the grid, the meter reads o, and with a potential of 0*9427 volt it reads 120 divisions. As the anode current curve is not an absolute straight line, it is then advisable to apply to the grid the potential for pH 7*0, and by very slight alteration of the rheostat and resistance box adjust until the meter reading is exactly 70 divisions. The lowest and the highest readings may not quite fit the meter scale, but the deviations should be very slight. Table I shows the results Table I. Calibration Table showing results obtained with the present apparatus for the Hydrogen Electrode System. H2 electrode—Sat. Calomel Cell. T = 180 C. e.m.f. applied to grid Corresponding pH Meter reading divisions Error in pH 0-2503 0 0-9 + 0-09 0-3080 1 IO'O 0-00 0-3657 2 20-0 0-00 0-4234 3 30-0 0-00 0-4811 4 40-0 0-00 0-5388 5 50-0 Standard 0-00 0-5965 6 6o-o reading 0-00 0-6542 7 70-0 60 divisions 0-00 0-7119 8 8o-o 0-00 0-7696 9 90-0 0-00 0-8373 10 100-2 + 0-02 0-8550 11 110-9 + 0-09 0-9427 12 122-0 + 0-20 0-2503 0 0 0 Standard 0-00 0-8550 11 1 io-o reading 0-00 0-9427 12 120-2 59 divisions + 0-02 The standard can be adjusted to 59 as shown to correct the 0,11 and 12 7TI readings when these are used.