Problems with the use of ph sensors
In actual use, pH In actual use, the following problems may exist in pH sensors: decreased sensitivity/slope, sluggish response, noisy signals, and chemical damage.
(1) Sensitivity/rate slope: There is a certain theoretical relationship between pH and the electrode potential of the probe (see the aforementioned Nernst equation). New pH probes can approach their theoretical slope (i.e., 59 mv of electrode potential per pH unit at 25°C), but sensitivity will continue to decline as the probe ages or is destroyed. Most pH meters or amplifiers are able to control and vary their sensitivity (often labelled slope or sensitivity) in converting a voltage signal into a pH reading, which can be calibrated by mv or temperature (since temperature is the only factor that theoretically affects slope). It is important to note that this is not the same as "buffer setting" or "zero control". The system is calibrated to some pH (controlled by buffer setting) and then tested with one or more buffers. Unlike the expected result, the pH meter reading will systematically deviate from the pH of the known buffer. If the resulting line is steep, the slope is set too low; if the resulting line is gentle, the slope is set too high.
(2) Cleaning: when the pH probe exhibits a delayed response or a decrease in sensitivity, it needs to be cleaned. the main reason for deterioration of the pH probe is contamination of the porous plug by substances in the fermentation broth. the porous plug will turn from white to brown or black if contaminated. To prevent contamination, the pH probe can be immersed in a 10 mmol/L HCl solution, which will not damage the pH sensor (this can also be used to routinely store the pH probe between runs). Sometimes the addition of pepsin helps to remove protein deposits. If the HCl treatment does not work, try the following two methods, which are somewhat effective, although they carry some risk of damaging the pH probe. Immersion of the pH probe in a solution of about 1% H2O2 for about 1 to 2 h; or gentle mechanical cleaning of the porous plug, i.e., using a sharp blade to scrape away deposits from the outer surface.
(3) Electrical interference: The high impedance and amplifier circuits of the pH meter may create problems which make the pH probe more sensitive to noise brought about by induced voltages from stray field entrances of other electrical equipment, and to false responses caused by trace current leakage between the two terminals containing the pH probe signal. For this reason manufacturers of pH sensors or pH meters provide special shielded leads and terminals. If excessive noise is present, the pH probe lead can be moved away from the other wires to reduce the noise. The stirrer motor may be a source of interference, this can be checked by switching the motor off for a few seconds. spikes on the pH trace correspond to the opening or closing of the heater circuit (the switching can be observed by a lamp or by the sound of the relay switching in the heater control unit). Noise or inaccurate readings after autoclaving can reflect contamination of the terminals and wires caused by steam condensation during the sterilisation process.
(4) Prevent mechanical damage: pH probes are quite fragile and can be easily broken during installation and cleaning of the fermenter. For these reasons it is recommended that the pH probe be inserted at a later stage in the preparation of the fermenter (where it needs to be calibrated), and that the pH probe be removed first when disassembling it after use (down the tank). In many cases sensor breakage occurs when the top of the fermenter is lifted without removing the sensor. An easy way to avoid breakage of the probe during storage between runs is to place the sensor in a plastic measuring cylinder containing a special solution. Choose a suitable size of cylinder so that the wider part of the probe fits in and the spherical detection site is suspended above the bottom (e.g. a cotton plug can be inserted into the bottom of the cylinder), and it is advisable to secure the cylinder with a clamp.
