pH and redox troubleshooting

[panou]

I have install a system for the combined measurement of pH and ORP(redox) in a starch slurry tank. The electrode is Mettler Toledo 3250SG and it is cleaned periodically. Unlikely i have noticed a permanene deviation of pH measurement (sensor 2,95, laboratory measurement 3,3). Which are the things that i should notice about this installation ?

 

[m777182]

Hello Panou,
concept of pH was introduced by definition for aqueous solutions without solids and later for nonaqueous as well. Manufacturers suggested measurements in pastes, gels, tissues even in solids provided there is enough moisture (paper, food, soil etc). But the answer to a simple question like "Do I really measure activity (or concentration in a first approximation)of hydrogen ions" is no. In slurries, solutions of coloids or solutions of surface active agents a pH meter will certainly show you something but it is not the log(activity(H-ions)).
Secondly, you will get an offset with an instrument that has not a symmetric high impedance input for both electrodes if you measure in a grounded metal vessel compared to a measurement in a lab on a higly insulated desk in a glass or PE (or similar )highly non conductive beaker.
Maybe it would be a good idea to ask Mettler to solve your problem- if they sold you the instrument they should know the drawbacks of its use in your system.
m777182

 

[djs]

Hello;
Most process instruments need periodic calibration. This is not just a cleaning process, but rather (usually) inserting the probe into know buffer solutions and inputting the corrections into the instrument. Also pH probes have a limited life. How old is the probe?