Hi there Avis,
Thankyou for your inquiry with regard to the ADC-24. Looking through your application, I suspect its to do with impedeance matching and you will need to buffer the input.
The ADC-24 is just 1 Megohm in single ended mode (2 Megohm in differential mode).
The only other alternative, using the ADC-24, is to make your own high impedance buffer amplifiers and use a scaling file to get the correct readings:-http://www.picotech.com/experiments/ph-measurements/index.html
(The above article refers to the ADC-16 logger but the principle is the same for the ADC-24).
Please contact us again if you have any other questions.
The following manuals will also be of helphttp://www.picotech.com/document/brochures.html?id=44
You are more than welcome to contact me at support at picotech.com