pH meter in water with salt added

[claude saunders]

I have a system that I had added a small amount of salt to a while back in order to help with potential nitrite poisoning of my fish. I don't recall the exact amount, but it was less than the recommended safe amount that has been batted around. It didn't really affect my fish or plants.

Using the API test kit, my pH is about 7.0.

I just bought a nice industrial quality submersible pH probe, and it pegged at 14.0. I was puzzled and figured I must have got a bad signal amplifier or way bad calibration. Until I dropped it in tap-water, at which time it read a nice 7.6. Back in my system water and it's back to 14.0.

So, has anyone else experienced this? Looks like the salt in the water greatly affects the electrical potential that a pH probe uses to measure pH.

I guess I'll have to do major league water changing over time to get the salt out so I can use a meter for pH.

Any other way to get the salt out??

  

[Jonathan Kadish NYC AA Chair]

Did you check the PH meter against an API result?

[Vlad Jovanovic]

I don't know what probe you have, but I'd think it highly unlikely that it would be the salt. Take some of that same tap water, measure pH, then put a bit of salt in the glass, measure again see what happens. IDK, it may be the specific probe you have (again, seems weird for it to be that though)?

Water change seems pretty drastic at this point.

How/why did you figure it was the salt?

[claude saunders]

You're right about the water changing. I'm not going to do that until I'm certain what is happening.

My API test kit reads 7.0, so this is something particular to this probe. It's a standard kind of probe, though, with the blue glass bubble filled with electrolyte (sealed) and a little wire sensor suspended in it. Industry standard item off of Omega Engineering web site.

I figured it was the salt as that effects conductivity of water. I suppose it could be other things present in my system that aren't in tap water. Most of those things are in ppm, though, and salinity is probably closer to ppt levels (I think). I also know that pH measurements are different for saltwater systems, so I figure that might be in play.

Your idea to add the same salt to my tapwater and measure again is perfect. I'll try that!

[Vlad Jovanovic]

I don't think any kind of conductivity from mineral salts should affect the electrode. How would hydro folks ever measure their pH then? I have a similar 'electrolyte filled, glass bubble-wire probe, and just for giggles tried it out a few minutes ago in a glass of water, then with a little salt, then a bunch more salt...Mine is as I figured it'd be...A OK...

[claude saunders]

Hmm. This is getting interesting. You really got me thinking now.

I wonder if some of my electrical equipment has some leakage current into the water? Like the water heater, pump, or even the temperature probe. Could that raise the potential off ground and affect the pH sensor?

I know I can test this as soon as I get home today. I'll just shut off each one-at-a-time.

I can't imagine what else it might be...  I'll follow up as soon as I get home.

[claude saunders]

Well, it's the RTD Pt100 temperature sensor I have in the tank water (it's connected to an analog input in the PLC). Take that out, and pH reads fine. Put it in, and I get 14.0. Odd thing is, I don't read any voltage or current between the temp sensor and ground. So maybe there is some kind of current loop between the temp sensor and some other piece of equipment in the water (pump, heater). That's going to take some thinking and tests. It could be the temp sensor is grounding current off something else.

Thanks, Vlad, for doing your salt test, and encouraging me to think this through!

[Vlad Jovanovic]

[David R. Russell]

I work with multiple saltwater recirculating systems at commercial hatchery.  This to me definitely seems like a stray voltage issue (doesn't take much).  Two ways to try and solve issues with stray voltage -1) install a grounding wire from water to ground or 2) use a beaker to remove sample water for testing (stray voltage no longer an issue).