PH probe questions...

[derek_s]

Ok, so I have been having some questions on PH probes burning in my mind, so I wanted to see if some of you experts can answer them for me.

1 - I believe you are supposed to replace them yearly, but if you calibrate them regularly, why is this? Seems that if it is calibrated, it should work assuming the calibration is/can be done with the reader, and is not outside of it's mechanical/digital adjustment limits.

2 - Why cant they dry out after use? If you re-submerge them then calibrate, what happens? I have a neptune that was used and is now dry.

I ask because of the high price of these things, and I have 2 that probably need replacing. Best I can figure in both cases is that the probes can have a tendency to 'walk' if wore out, fluctuating readings too much despite proper calibration.

 

[mojo]

corvettecris;194142 wrote: 1 - I believe you are supposed to replace them yearly, but if you calibrate them regularly, why is this? Seems that if it is calibrated, it should work assuming the calibration is/can be done with the reader, and is not outside of it's mechanical/digital adjustment limits.

2 - Why cant they dry out after use? If you re-submerge them then calibrate, what happens? I have a neptune that was used and is now dry.

Both reasons stem from the fact that a pH probe (and most ion-specific problems) use inner fluids that create what amount to a battery that is interacted by the sodium and hydrogen ions in the water. It's that interaction that we read as pH.

In the case of a pH probe, there's both a KCl + HCl (potassium and hydrogen chlorides) that interact with a AgCl (silver chrolide) tip, which all together form the 'battery'. Over time, the KCl evaporates and otherwise gets used up, just like a normal battery. Because the KCl evaporates, pH probes have a limited life. There are expensive probes that allow refilling, but at $200-250, it's cheaper to just re-buy the inexpensive ones.

You can also read this information, in a little more depth,

 

[derek_s]

Thanks!

 

[bobz]

I know this is old but I was searching the forum for info on when to replace pH monitor probes. This was really good! So, is the prevailing wisdom to replace the probes in about one year?

Thanks!
Bob

 

[derek_s]

I guess so. And I found really cheap ones at bulkreefsupply. I have been using them for about 6 months, and just recalibrated tehm the other day. They only had varied by .1, which I thought was good.

 

[bobz]

Thanks Derek! I'll check them out.
Z