pH meter input impedance and its measurement method

1. Input Impedance When a pH meter is used to measure the acidity of a solution, a glass electrode and a calomel electrode constitute a chemical primary battery in a solution. It has an electromotive force E and an internal resistance r. Therefore, the input impedance of the pH meter and the internal resistance of the primary battery can be equivalent.

E represents the electromotive force of the primary battery, and its value is related to the pH value of the measured solution; r represents the internal resistance of the primary battery, which consists of 3 parts, mainly determined by the internal resistance of the pH meter glass electrode (about 108Ω); The internal resistance of mercury electrode is about 104Ω; the internal resistance of the tested solution is (103～105)Ω. R represents the input impedance of the pH meter, which is the parallel value of the resistance of each part of the input of the pH meter.

The internal battery resistance r is in series with the input impedance R. According to the principle of series resistance voltage division, when the internal resistance of the primary battery is 109Ω, the input impedance of the pH meter should be more than 1000 times larger than the internal resistance of the primary battery, which is more than 1012Ω. Therefore, the influence of the voltage drop on the primary battery r can be ignored, so that the voltage entering the input terminal of the pH meter is close to the electromotive force of the primary battery.

When the input impedance of the pH meter is not large enough, if the input impedance is equal to the internal resistance of the primary battery, there will be half the pressure drop in the primary battery. The value displayed on the pH meter is only half of the electromotive force of the primary battery. Even if the input impedance of the pH meter is one to two orders of magnitude larger than the internal resistance of the primary battery, instability will occur during measurement. This is because the input impedance of the pH meter and the internal resistance of the galvanic cell are not all completely stable constants; they vary with the ambient temperature and humidity.

The input impedance of the so-called pH meter is the impedance seen from the two inputs of the pH meter. The input impedance of the meter is not only related to the high-impedance tube at its input, but also related to the reading of the input switch, the glass electrode jack, the insulation resistance of the input shielded wire, and the filter capacitor leakage resistance at the input. Because, in principle, their insulation resistance is in parallel with the input high-impedance tube. If the reading switch and the glass electrode socket are installed in the pH meter casing, there is a leakage resistance between them and the casing. The voltage signal on the electrode enters the input of the high-impedance tube through the shielded line, and the input filter capacitor is also grounded. therefore. Their insulation resistance or leakage resistance is at least two orders of magnitude greater than the impedance of a high-impedance tube. If one of the above components has an insulation resistance that does not meet the requirements, it will affect the input impedance of the pH meter. If the above components are contaminated, they must be cleaned. The cleaning solvent uses diethyl ether instead of ethanol. After cleaning, it is dried with a hair dryer. The insulation resistance of the components after cleaning can generally meet the requirements.

Second, the pH meter input impedance measurement method The input impedance of the pH meter can not be measured directly, can be obtained by indirect measurement.

R Take 1000MΩ Input voltage E0 from potentiometer to pH meter When switch K is on (R is short-circuited), millivolt value measured with pH meter is E0; switch K is then turned off and R is turned on The millivolt value obtained is Ei, the following formula can be obtained:

Ei=(Ri×E0)/(R Ri)(1)

Where: Ri - the input resistance of the pH meter.

From equation (1), Ri = (R × Ei) / (E0 - Ei) (2)

Third, the calculation example Select a 0.01 pH meter, according to Figure 2 wiring. R is 1000MΩ, the input voltage is 300mV from the potentiometer to the pH meter, and the value of the pH meter is 300.0mV when the switch K is turned on (R is short-circuited); then the switch K is turned off (R is turned on), and the pH meter is turned off. The indicated value is 299.8mV. Substituting the measured data into equation (2) yields Ri=(R×Ei)/(E0-Ei), Ri=1.50×1012Ω