Would you trust years old pH calibration solution?

[Brett S]

So I got my Trident yesterday and as I was looking at the readings from the Trident it occured to me that I haven’t calibrated my pH and salinity probes since I installed them when I upgraded my Apex. The probes have a May 2017 date on them and I think they were probably installed at about that time. When I installed them I used the single use calibration solution that came with the Apex.

I remembered that I had calibration solution on my shelf, but when I went to get it I realized that it’s probably seen better days...

It was on it’s side and it’s clear that it leaked at one point in time (although it was dry when I got it of the shelf just now). I remember I got it before I moved and set up my current tank, so that makes it around 4 years old.

Just for fun I poured out some of the solution into a bag and stuck my probe in it and it came back at 9.76, but I’m not sure if that’s because my probe hasn’t been calibrated recently or because the solution hasn’t aged well (or a combination of the two). But it was at least a bit encouraging that the number was reasonably close to 10.

Would you trust this solution to calibrate my probe or am I better off just ordering new solution? I also have a pH 4.00 solution that was purchased at the same time, but it looks to be in a bit better shape. It was upright and doesn’t appear to have leaked, at least. My probe read that as 4.23.

I’m thinking that given the infrequency of my calibrations that I might be better off just ordering single use packets each time I need to calibrate.

 

[Crabs McJones]

For 99 cents each, it's probably better to buy a fresh calibration solution for ph

 

[Brett S]

For 99 cents each, it's probably better to buy a fresh calibration solution for ph

Yeah, you’re probably right. But that means that I need to go out and buy it or order some and wait for it to arrive and patience was never my strong suit.

Then again, I’m really in the wrong hobby for someone lacking patience

 

[ludnix]

I wouldn't trust the old solution any more than your probe's current settings. It's promising that it reads close to 10, but I would get a new pouch to verify and if they both read 9.76 you'll know it's still good to use for the future.

 

[Dkeller_nc]

Don't trust it. Calibration solutions require a certain concentration of buffer salts to maintain a particular pH. When that concentration changes because of evaporation, the pH will change. And although plastic bottles do provide some degree of a water vapor barrier, there's still an appreciable amount of transfer over time. Precisely how much would be dependent on the exact plastic used, its thickness and the temperature and humidity of the atmosphere that the solution bottle is stored in.

This is partly why Milwaukee, Neptune and others sell their solutions in mylar pouches. Mylar is a multi-layer composite film that contains a layer of aluminum that blocks water vapor transmission, which helps the stability of the solution inside.

 

[AdamNC]

If it were the Milwaukee or Neptune packets I would say sure but I wouldn’t trust an opened bottle.

 

[Randy Holmes-Farley]

I've tested expired standard for an old article that I wrote, and some can be seriously off due to CO2 entering them, driving down the pH. Not so much pH 4-7, but those at higher pH than 7 suffer from that.

 

[Randy Holmes-Farley]

Here's the article, although the site seems down that the moment:

A Comparison of pH Calibration Buffers by Randy Holmes-Farley - Reefkeeping.com

 

[Brett S]

Thanks guys. I ordered a set of Neptune packets, but it’ll take a little while for them to get here. I’ll probably hang on to this solution to test it again just for fun after I calibrate the probe. But I think the single use packets are probably a better idea for me going forward.

 

[Randy Holmes-Farley]

Thanks guys. I ordered a set of Neptune packets, but it’ll take a little while for them to get here. I’ll probably hang on to this solution to test it again just for fun after I calibrate the probe. But I think the single use packets are probably a better idea for me going forward.

After you recalibrate with new fluid, measure the old solutions and you can then use those for a while, assuming your meter allows you to enter a value such as 9.8 for a pH 10, rather than itself selecting exactly 10. I used bottles of fluid for many years this way. All the pH 10's ( I had a bunch) dropped from 0.05 to 0.3 pH units over time.

 

[Brett S]

After you recalibrate with new fluid, measure the old solutions and you can then use those for a while, assuming your meter allows you to enter a value such as 9.8 for a pH 10, rather than itself selecting exactly 10. I used bottles of fluid for many years this way. All the pH 10's ( I had a bunch) dropped from 0.05 to 0.3 pH units over time.

That’s a great idea. And I’m sure it would work if I calibrated my probe more often than once every two years two years or so. At this rate by the time I get around to calibrating the solution will just have further degraded. I probably need to get on a better schedule for calibration really.

 

[Dkeller_nc]

After you recalibrate with new fluid, measure the old solutions and you can then use those for a while, assuming your meter allows you to enter a value such as 9.8 for a pH 10, rather than itself selecting exactly 10. I used bottles of fluid for many years this way. All the pH 10's ( I had a bunch) dropped from 0.05 to 0.3 pH units over time.

Yep. I actually observed this effect with pH 10 buffers at a medical manufacturer in the mid 1990's. The manufacturer (Fisher) claimed a 6-month shelf life after opening. The lab testing I did clearly indicated this was not the case - I was getting drops of about 0.1 pH unit over the course of a month. For our purposes, that was the limit that we'd accept, so we put our own expiration of 1 month from opening them, and 6 months regardless.

 

[Brett S]

Well, just to follow up on this I got my new (single use) calibration standards today and calibrated the probe, then I measured the old solution. The old 7 standard fared pretty well and measured at 7.04, but the old 10 standard didn’t do so well and that was at 9.5.

But given how infrequently I calibrate my probe I think I’ll just stick with single use packets ordered shortly before I calibrate from now on.