The electronic pH tester is a precise voltmeter with very high input
resistance. A general-purpose digital voltmeter with, for example, 20
MOhm of input resistance will load down a pH electrode or in other
words, send a current through the electrode. Any appreciable current
flow through the electrode will give a reading that is lower than it
should be and could even damage the pH sensor.
To explain why high input resistance is important for a pH tester a short repetition of Ohm's law will be a good start.
Ohm's law is a very simple and useful tool for analyzing electric circuits.
E = RI
Voltage (E) is equal to resistance (R) multiplied by current (I).
If you know the values of any two of the three quantities (voltage, current, and resistance), you can use Ohm's law to determine the third.
If we draw a simple circuit of a glass electrode:
Eg = Ei + Et => Et = Eg - Ei
• Eg is the electrical potential of the glass electrode and a function of the pH value.
• Ri is the internal resistance of the electrode and Ei is the internal potential drop of the glass electrode.
• Rt is the internal resistance of the electronic pH tester and Et is the potential read by the pH tester (voltmeter).
You want Et to be equal to Eg or at least as close as possibly. To achieve that you need the internal potential drop Ei to be very small (almost zero) and that happen if the current I is very low.
The equation for the current in the pH electrode loop can be written as:
Et = Rt ·I
According to the equation: For a given voltage Et, the internal resistance Rt of the pH tester needs to be very high to keep the current I very low.
To construct a high-resistance
voltmeter or a pH tester you need to use an operational amplifier of
very high gain and very high input resistance (1000 000 MOhm) as an
input circuit, see the simplified diagram below.
As already mention the input resistance of an electronic pH tester
should be at least 10000 times larger than the resistance of the pH
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