PH Probe vs. Salifort PH test

[Treefer32]

So, I recently got a Salifort PH test because my PH probe is showing consistently low PH despite calibration, despite dosing 250 ml of Baked baking soda per day and dosing some kalkwasser per day.

Salinity is slightly above 1.026 (around 1.0265)
Mg is always around 1500-1540
Ca - around 500-540
Alk- 8 - 8.5 dkh

PH Probe at night shows 7.78 and during the day sometimes gets up to 8 if I have doors open. However, there's only 3 people and no pets in a 3200 square foot house (other than fish). We have lots of house plants as well.

I changed my PH probe out about a year and a half ago. When I switched from old probe to the brand new one my ph dropped significantly. I was routinely reading 8.1 to 8.3. The new probe was between 7.6 and 7.8. (Until I increased my dosing).

I have calibrated the probe multiple times with PH 7 and 10 solution. I timed it and the probe was still settling after 25 minutes with the calibration solution. Well, I've tested twice around 6 days a part with the salifort ph test and compared the results to my probe.

After the 6 drops, the tank water always turns a light green. Which is between the 8.0 and 8.15. My ph probe today showed 7.82.

I have a neptune classic Apex. I use the double lab grade ph probes only. I feel like I've been trying to solve the wrong problem with my ph probe no longer a trustworthy device. It's not been right since I got it new. I'm not saying the salifort test is right either. Anyone else have experience with this?

 

[tzabor10]

Most reefers use multiple tests and ICP. It’s frustrating but worth it.

 

[Formulator]

ICP has nothing to do with pH… Anyways, to answer the OP question, pH probes can be very accurate if appropriately calibrated. So can the salifert titration test. But at the end of the day, the real question is, why are you chasing a specific pH? Stability is more important than the actual number as long as you are in the range of about 7.8 to 8.6. And a daily pH swing is normal and for the most part unavoidable. You can run a refugium on opposing light schedule to reduce the daily swing, but photosynthesis is always going to give you a regular oscillation. Many successful tanks have daily swings of +/- 0.3-0.5.

You shouldn’t be dosing your tank to maintain a specific pH. You dose to maintain a specific alkalinity value, which in turn helps maintain your pH.

Regarding the pH probe calibration problems and taking 25 minutes to settle, get some pH storage solution (KCl) and stick the probe in the solution for a few hours. Rinse and then try to recalibrate. You may also want to get some new calibration solution if it is old. Also make sure there are no bubbles getting into the probe junction during measurement.