Low pH - what am I missing?

[Andy V]

So my pH cannot get above 7.96 for whatever reason. I have had the windows in the house open for a few days and it only peaked at 8.10, so it doesn't seem to be an oxygen issue. Let's starts with the parameters, which were all tested 2x each last last using two different brands of kits, i.e. 4x per parameter:

Tank - 60 Gallon cube, 20 Long sump, Reef Octopus XP1000-SSS Skimmer, 2 MP10's and 1 Tunze 6025, 250W Radium in Lumenbright Mini 6 hours. Running BRS Carbon (1/2c) and GFO (1/2c which is half recommended) in a BRS dual reactor.

Sg - 1.026
Temp - 79
Alk - 8.0 - 8.5dkh
Calc - 420ppm
Mag - 1450ppm
Ammonia, Nitrite, Nitrate - All 0ppm
Phosphate 0.08ppm

So here is the confusion. All of the parameters that affect pH (besides oxygen), i.e. alk, cacl, mag, are spot-on. I have been dealing with STN on my SPS, and it is definitely not due to lighting or flow. I hadn't tested phosphates in forever because I have zero algae, so I decided to test that, thinking it may be the cause of STN. Initially, I tested at 0.25ppm on the Hanna Checker, but I figured out I was testing incorrectly, and after more tests, it came in at 0.08ppm, confirmed (as best as could be) with the Red Sea Algae Management tests.

So last night I get home and the pH was at 7.96, and the alk was at 8dkh. Normally that is fine, but the Hanna checker actually reads about 1-1.5dkh higher than actual, so I decided to add 11ml of BRS Two-Part (Soda Ash) to raise the alk by 0.5dkh. I did this at 10pm, but did not check the pH until this morning, when it was reading 7.6! It appears the extra addition (beyond the normal dosing) lowered the pH! And yes, I know the pH drops overnight, but it has typically been reading 7.8-7.9 in the morning.

And by the way, I am using the Digital Aquatics probe and reefkeeper to measure. I calibrated it this past weekend and checked using reference solutions that it was reading correctly.

I really am confused as to what is going on...and this has been a problem for awhile now, but so far only seems to affect the SPS. Is it possible that my system just cannot have alk higher than 8 - 8.5dkh, and the addition of Soda Ash solution (which normally increases pH) is actually lowering the pH?

Does anyone have a different pH probe near Winchester recently calibrate that I could try? I am really at my wits end here, and I am considering Kalkwasser, but that seems like a pain and potential problem, and certain to add complexity while I have been trying to reduce complexity.

One other thing is that my alk can't seem to stabilize. It varies between 8 and 9dkh on the Hanna checker, despite the fact that I have kept the dose the same and haven't added any corals. I am not seeing any calcification on any equipment, so I don't think I am overdosing either.

 

[JohnK]

I have heard of there being calibration solutions on the market that aren't all that accurate. Otherwise it doesn't seem to add up. To more or less quote Randy, "PH in seawater is a function of alk and Co2" which it sounds like you certainly do understand. That said, with your alk level the PH should be higher, so the only thing that seems to make sense would be test error of some sort.

What brand of PH test solution do you have? Is it single use packets or bottles (that could have become contaminated?) Are you calibrating to 7 and 10, or just one point? IIRC Randy wrote something about test solutions somewhere but I don't have time to go looking right now.

 

[Matt W]

I am an Analytical chemist. If you would like I have a handheld pH probe with 3pt calibration and can provide you with fresh calibration solutions. Burlington during business hours and North Reading in the evening.

Have you considered that unstable alkalinity rather than pH is a root cause for the STN? Sometime we worry too much about pH, yours is in the acceptable range.

 

[CaptainAndy]

Buy a CO2 scrubber

 

[Andy V]

John, I am using the Precision Marine single-use pouches and using 7 and 10. I read Randy's study and the Precision Marine ones were some of the good ones. It is possible that temp had something to do with it (meaning during the calibration), but I don't think so. The thing is, if I stick the probe in the reference solutions of 7 or 10 right now, the probe reads correct.

Matt, I really appreciate the offer. I will PM you. As far as the Alk goes, I would generally agree that it is the most important parameter and could be an issue. However, in my experience, and variation of 1 dkh over the course of a week does not cause the problems I am seeing. Some of the SPS that are having issues are ones I have had for awhile and have been through a lot worse! I have never seen my pH drop below 7.7, which is why I became concerned.

I agree that people worry about pH too much, but I thought 7.6 was where you DID have to start worrying?

 

[Andy V]

Buy a CO2 scrubber

Two things on this...I am trying to reduce complexity, and as I stated, even with the windows open in the entire house for three days, the pH only raised slightly. I don't think it's a CO2 issue. i appreciate the help though.

 

[Matt W]

A question about your testing method. How long do you submerge and swirl the probe before reading the value? You will notice that if you stick the probe in a Cal buffer sln, the reading will change rapidly. While in sln's such as NaCl, are much slower to get a accurate reading and will increase slowly with mixing and time. I would recommend swirling the probe for about 30 sec to a minute before taking a reporting a result.

 

[Andy V]

Matt, got your PMs. See you then!

As far as this question goes:

Not sure exactly what you are asking, so I will just answer everything I can think of.

First, in terms of calibration, I follow the manufacturers directions, which are to simply rinse the probe in RO water, then submerge the probe in the reference solution for at least five minutes, or until the values on the display stop changing. I let it go for ten minutes just to be sure. Then I rinse in RO again, and repeat for second solution. There was no swirling discussed anywhere.

Second, after calibration, the probe is constantly submerged in my sump, giving a constant reading.

 

[Andy V]

So thanks to Matt for letting me use his pH meter, which is better than the Digital Aquatics probes.

I learned a couple things here...first, the DA probe is off by .2 even though it is basically new and calibrated in the last week, so the pH is fine. Second, the reason it dropped in the first place is that the skimmer somehow was shut off overnight and I didn't notice it in the first place because the collection cup and neck were so dirty!

Anyway, the lesson learned is that the pH has really been closer to 7.9 overnight and 8.2 during the day, which is completely normal.

Also, the STN I have been experiencing has definitely stopped, so we will see what happens. The frustrating thing right now is that almost all growth seems to have stopped, which is not good.

 

[PSU4ME]

Glad you found the issue! Most times stability in PH is better than the reading from our probes......as a common problem can just be bad equipment. I'm the same way, 7-8 to 8.0 but not much higher.

Maybe the club should get a super awesome PH meter to pass around s well!

 

[Matt W]

The club should. The probe is cheap and fits in a nice case. There are enough chemists around here for the pH cal sln's. Might to get that is 7.XX vs 7.X.