Ph Of Ro Water

[AK77]

Hi Guys,

A couple of my corals have been looking a bit ropey for two days. The leather toadstool and leather finger corals were fully extended with their polyps extended about 10mm and 2mm respectively. I shut the lights off a few nights ago and when I put them on again the next day, the leather toadstool didn't respond. At the moment the toadstool head is curled up like a buttercup or something. Other corals seems to be doing fine.

I tested all my water parameters, ammonia, nitrite, nitrate, phosphate and calcium. All are at acceptable levels (0,0,0, 0.25, 530). I took my ph meter and put it in the water and its reading a ph of 7.93 @ 27c/80f. A few days ago it was at 8.3, but I did do some largish top ups with RO water.

I've just recalibrated my PH meter and it came out spot on with the buffer solutions, so I know its accurate down to 0.01 of PH. I was soaking it in some RO water to remove any traces of the PH buffer solution when I noticed that the PH reading from the RO water was 4.65 @ 16c !!

I'm guessing that the largish RO water top ups have dragged the PH of my water down from 8.3 to 7.9.

I do have some PH 8.3 buffer from Waterlife, so I will add a teaspoon of that and gradually bring the ph back up again.

Could this be what's effecting the corals?

Is it normal for RO water to be so acidic?

 

[robbhp]

well i have just done two tests on the big tank for you, my RO is comming out at about 6.7 , and my tank water is comming out at about 8.8-9 colours are very close LOL. i would have thought that your water changes would have bumped up your PH for you(as the salt we all use has buffer added), as i dont think top ups alone would have alowed it to drop by that much. Also i have been told to let the tank run at its own ph as it will stay more stable , thus things will learn to live with it. but when you keep havng to add boosters to the water , the levels are always swinging about thus = not good.

rob

 

[Underwurlde]

Am I having a thick moment here, surely RO water should have a neutral pH of 7.0? If not, then your RO device will need membranes etc. replacing?

Andy

 

[Lynden]

Since RO water lacks any buffering agents the pH can swing quite widely. As soon as it gets into the tank, it will probably stabilize from all the buffers in salt water. So I doubt the RO alone is dropping your pH. A pH of 7.9 isn't unacceptable either.

 

[jeasko]

Have you checked your KH level? should be somewhere between 9 & 13 as this will have a direct effect on your PH level, adding a buffer will do nothing if your KH is to low, also bear in mind if your KH drops under 5-6 you could be heading for a KH crash which could wipe out most live stock in your tank

 

[SkiFletch]

Since RO water lacks any buffering agents the pH can swing quite widely

Ding ding ding, we have a winner . The only affordable test (ie not some lab-grade insanity) that you can reliably perform on RO water is a TDS. All others will give unreliable readings. If your TDS is fine, the RO is fine . Resistivity meters (TDS meters) don't lie

 

[AK77]

Well the TDS meter is reading 0ppm at the output, so I know the RO water is ok in the purity sense. I don't have a KH test kit but will get one. I can't think of what else would be responsible for the drop in ph as nothing else in the tank has changed - well not that I can readily identify anyway.

 

[SkiFletch]

Do yourself a favor and check it before lights on and at lights off just to be sure you don't have a CO2 issue

 

[Musho3210]

carbon dioxide may dissolve in RO since it is pretty "magnetic" (dont know how to say it scientifically) and since carbon dioxide creates an acid, the water takes a huge acidic crash with just a little bit of CO2 due to the lack of buffering. The moment it hits your buffered, high oxygenated tank water, it duplicated the tanks pH conditions.

Dont worry about pH unless it drops below 7.8 during the middle of the day....

 

[steelhealr]

Pure RO water should have a pH of 7. Remember: pH measures the concentration of hydrogen ions, H+, in water. Neutral water contains 50% hydrogen ions and 50% hydroxyl ions, OH-. A common misconception is that adding RO water will change your pH. It does NOT directly change pH. Adding pure water to your system adds equal amounts of those ions and therefore CANNOT alter the pH of your tank DIRECTLY. If your pH is off from your RO water, it is not pure and/or the membrane is failing or one of the filter components is blocked, clogged, etc. A TDS meter is mandatory equipment to measure RO filter maintenance/health.

SH

 

[Underwurlde]

I wasn't having a thick moment then!

Yay!

 

[AK77]

The TDS meter is reading 0 on the output and the filters are only a few months old. Its a six stage unit with a deionizer and carbon too I think. The PH meter had just been calibrated too, so I was at a bit of a loss as to why the PH reading was so low on the RO water.

I am thinking though that the drop in PH is due to running GFO, as it can lead to a drop in calcium, alk and PH as a result of increased calcification by corals and coralline algae, but also as it can remove other elements in the water.

 

[steelhealr]

If you are getting a 0 TDS, that's pretty darn good. The 6th stage filter is usually unnecessary. Most of them contain coconut shell carbon and it is added to 'improve taste'. Mine is disconnected as sometimes it can add to the TDS. SH

 

[andywg]

Pure RO water should have a pH of 7. Remember: pH measures the concentration of hydrogen ions, H+, in water. Neutral water contains 50% hydrogen ions and 50% hydroxyl ions, OH-.

This is accurate for almost all practical purposes (especially ours) but is not precisely correct.

As wiki points out:

Neutral pH at 25 °C is not exactly 7. pH is an experimental value, so it has an associated error. Since the dissociation constant of water is (1.011 ± 0.005) × 10−14, pH of water at 25 °C would be 6.998 ± 0.001. The value is consistent, however, with neutral pH being 7.00 to two significant figures, which is near enough for most people to assume that it is exactly 7.

So it is safe for us to assume it is 7, but remember it is not exactly 7 (though that level of measurement is well beyond the average hobbyist).

 

[AK77]

So would it be safe to assume that the PH reading I was getting was due to CO2 being dissolved in the RO water, which was giving me a PH of under 5 ??