What are you trying to say? Is that where you want to stop the reaction? I thought I saw 10 ppm somewhere and this should be a do not exceed amount.
An example problem: Suppose electrodes are 2 x 3 cm and the distance is 1 cm, then R = rL/A; R = 3 M ohms. At 100 C, this would be about 150 % less or about 20K.
So, temperature and separation distance is a BIG factor.
Thus, I am proposing a (in complianace) LED for the circuit. This would allow you to set the spacing. Using 1 mA and 40 volts, this means 40K would be the minimum resistance. As the reaction proceeds, you could move the electrodes apart.
A way to achieve this is to be able to differentially amplify the voltage across the electrodes. I bet we can do it using a discrete FET differential amplifier followed by say and AMP02 from analog devices. I'm nor sure how to design the animal.
One he had separation distance, the area of the electrodes, the current, the temperature he can then determine when to terminate the reaction. A DVM cannot be used to do this.
Alternatively, he can buy one of those meters and measure the conductivity. Apply the temperature equation (possibly) and then set the initial separation of the plates start. With 1 mA, this means he has to see 40K resistance (possibly temperature compensated)
I suppose, that using the meter is a better choice than incorporating a voltage measuring device for the electrodes.
If the current source is well behaved, you could look at the voltage difference across the source and compare it to the supply, Suppose the source had 2 diode drops. You could say, that if 3 drops or more was seen across the source, then it's working. I think that mod should be essential.
Use the same OP amp as in your current source. Put say three diodes to the + supply (assuming it normally drops at least 2) and a resistor to ground. Set the comparitor to look at the output and compare it the the supply less 3 diode drops.
Now, he has a way to set the initial position of the electrodes. He could then at various times, move the electrodes farther away.
The process is useless UNLESS the circuit sees 40 K minimum resistance.