High ph of RO water

[ludlumjohn]

Hello,

New to this site but have been keeping fish and plants for more than 50 years, yes I’m old 🤪

Currently running 2 planted tanks with C02 and doing PPS auto dosing. Tanks are only about a month old. The startup went very well, absolutely no algae issues other than some minor diatom issues after the 1st couple weeks.

I have available both RO and well water available and I trying to get the right mix of both to end up with about a 7.5 ph.

I set aside two glasses of water from the well and RO and had the following readings straight from the tap.

RO 6.2 ph and a tds of 10
Well 7.07 ph and tds of 220

After airating for a few hours I end up with the following
RO 7.95
Well 8.64
Any ideas on why my RO water would go so high. I want to reach equilibrium but curious if the airation is throwing it off and I should just let it settle.

The 2 tanks that are running now I did a 75 RO and 25 well. Tanks read about 7.0 in the morning and down to about 6.1 after c02. I would prefer to start with 7.5 and go down to 6.5 after c02. When I set these two tanks up I just did my normal mix of the 75/25 but was surprised how low is is. This is after a water change (2-3 days. I was thinking I would end up with a 50/50 mix but confused by my RO reading and my tanks actual reading. Hoping someone can make some sense of this for me.

Thanks in advance

 

[dw1305]

Hi all,

Any ideas on why my RO water would go so high

After airating for a few hours I end up with the following
RO 7.95
Well 8.64

The problem is <"with the lack of solutes"> in the RO <"water">, it just means that pH is a moveable feast.

I want to reach equilibrium but curious if the airation is throwing it off and I should just let it settle.

It shouldn't be, you could only get a higher pH value if you weren't at equilibrium with atmospheric CO2 levels, e.g. CO2 levels were below the equilibrium point.

The 2 tanks that are running now I did a 75 RO and 25 well.

That sounds about right as a mix to me, you should have about 60 ppm TDS? ((0.75 x 10) + (0.25 x 220) = 63 ish)

would prefer to start with 7.5 and go down to 6.5 after c02.

That is trickier, because we don't know which ions are in the well water and pH isn't a linear scale like conductivity (ppm TDS). Why do you want to start at pH 7.5?

I was thinking I would end up with a 50/50 mix

That should also be OK. Are you near the sea? If you are you might have sodium (Na+) ions in the well water, which wouldn't be great.

If you are away from the sea? And particularly if the geography is limestone (CaCO3)? The ions will be overwhelmingly calcium (Ca++) and bicarbonate (2HCO3-).

cheers Darrel

 

[ludlumjohn]

Thanks for the awesome reply.

One at a time per your reply.
I’ve heard measuring ph of ro water can be trickle because it devoid of almost everything. Should I assume my RO water is the ph at which it comes from the tap? It also sits in holding tanks before I tap it and that’s has to be a day or 2.

As for the TDS based on what I start with that should be about right but Bcz of dosing and bio load I’m about 125.

As for why 7.5, I guess having lived with high ph is previous locations guess I’m just nervous with a ph in the low 6’s. I guess also if I could use a little less RO that would also be good, so a 50/50 mix would help there. I live in northern Thailand. No ocean in sight.

Curious if you have done any pps dosing. It recommends in lieu of weekly w/c to do a tds test. When your TDS in the tank is greater than 300 plus original water in use for the change then do a change. Any thoughts of experience with that?
Thanks again!
My god I love this hobby

 

[dw1305]

Hi all,

I’ve heard measuring ph of ro water can be trickle because it devoid of almost everything.

Yes that is it, pH isn't a useful measurement in RO because of the lack of solutes.

Should I assume my RO water is the ph at which it comes from the tap?

You can just ignore the pH of the RO, it is a blank slate.

As for why 7.5, I guess having lived with high ph is previous locations guess I’m just nervous with a ph in the low 6’s.

No need to be, the pH reduction due to CO2 addition doesn't reduce the alkalinity, you just have more Total Inorganic Carbon (TIC) <"dissolved in the water">. That is one of the problems with pH, you <"can't really extrapolate"> from hard to soft water.

I guess also if I could use a little less RO that would also be good, so a 50/50 mix would help there. I live in northern Thailand. No ocean in sight.

I'd go for that. I use rainwater in my tanks, but if I didn't have rainwater? I would use our (very hard) tap water.

Curious if you have done any pps dosing. It recommends in lieu of weekly w/c to do a tds test. When your TDS in the tank is greater than 300 plus original water in use for the change then do a change. Any thoughts of experience with that?

I change a small volume of water most days, but I use a combination of the <"Duckweed Index"> and <"tank conductivity"> for <"tank management">.

cheers Darrel

 

[Andy Pierce]

I asked ChatGPT to make a table that would show "the calculated equilibrium pH values for water at 20°C with varying alkalinity (measured in dKH) and CO2 levels. The CO2 levels are presented as atmospheric concentration (ppm) and the resulting dissolved concentration in water (ppm)." and below is what it came up with.

To @ludlumjohn original request, for a low tech tank and if the numbers are to believed, with what might be typical indoor CO2 at 600 ppm in air, you'd need a dKH of 6 to get a pH of 7.5.
For CO2 injected tanks, having a final water CO2 at 30 ppm is equivalent to not injecting CO2 if there were 20000 ppm CO2 in atmosphere.

Down: Alkalinity (dKH) Across: CO2 (atm ppm / water ppm)	400 / 0.6	500 / 0.8	600 / 0.9	700 / 1.1	800 / 1.2	900 / 1.4	1000 / 1.5	2000 / 3.0	20000 / 30.3
1	6.7	6.6	6.5	6.4	6.3	6.2	6.1	5.8	4.7
2	7.0	6.9	6.8	6.7	6.6	6.5	6.4	6.1	5.0
3	7.2	7.1	7.0	6.9	6.8	6.7	6.6	6.3	5.2
4	7.4	7.3	7.2	7.1	7.0	6.9	6.8	6.5	5.4
5	7.6	7.5	7.4	7.3	7.2	7.1	7.0	6.7	5.6
6	7.7	7.6	7.5	7.4	7.3	7.2	7.1	6.8	5.7
7	7.8	7.7	7.6	7.5	7.4	7.3	7.2	6.9	5.8
8	7.9	7.8	7.7	7.6	7.5	7.4	7.3	7.0	5.9
9	8.0	7.9	7.8	7.7	7.6	7.5	7.4	7.1	6.0
10	8.1	8.0	7.9	7.8	7.7	7.6	7.5	7.2	6.1
11	8.2	8.1	8.0	7.9	7.8	7.7	7.6	7.3	6.2
12	8.3	8.2	8.1	8.0	7.9	7.8	7.7	7.4	6.3
13	8.4	8.3	8.2	8.1	8.0	7.9	7.8	7.5	6.4
14	8.5	8.4	8.3	8.2	8.1	8.0	7.9	7.6	6.5
15	8.6	8.5	8.4	8.3	8.2	8.1	8.0	7.7	6.6
16	8.7	8.6	8.5	8.4	8.3	8.2	8.1	7.8	6.7
17	8.8	8.7	8.6	8.5	8.4	8.3	8.2	7.9	6.8
18	8.9	8.8	8.7	8.6	8.5	8.4	8.3	8.0	6.9

 

[Andy Pierce]

Worthy effort by ChatGPT, but came up a bit short. I redid the query using the Dust - Accelerate your entire organization with custom AI agents LLM and after negotiating with it for a considerable length of time got what I think is the correct answer.

CO2 concentrations going across the top, first listed dissolved in water and then the equilibrium value for CO2 found in air in the environment of the tank; KH values (titratable alkalinity) going down the left side listed in ppm CaCO3 equivalents and dKH.

pH values at 20°C:

KH⬇️ CO2➡️	0.6 ppm (400 in air)	1 ppm (670 in air)	2 ppm (1300 in air)	3 ppm (2000 in air)	5 ppm (3300 in air)	10 ppm (6600 in air)	20 ppm (13000 in air)	30 ppm (20000 in air)
0 ppm (0 dKH)	5.62	5.51	5.36	5.27	5.16	5.01	4.86	4.77
5 ppm (0.3 dKH)	6.94	6.72	6.42	6.25	6.03	5.74	5.45	5.29
15 ppm (0.8 dKH)	7.42	7.20	6.90	6.72	6.50	6.20	5.90	5.73
50 ppm (2.8 dKH)	7.94	7.72	7.42	7.25	7.02	6.72	6.42	6.25
100 ppm (5.6 dKH)	8.24	8.02	7.72	7.55	7.32	7.02	6.72	6.55
150 ppm (8.4 dKH)	8.41	8.19	7.90	7.72	7.50	7.20	6.90	6.72
200 ppm (11 dKH)	8.53	8.32	8.02	7.85	7.62	7.32	7.02	6.85
300 ppm (17 dKH)	8.70	8.49	8.19	8.02	7.80	7.50	7.20	7.02

A prettier version as well as the first-principles derivation and some python code you can run to do your own calculations are posted here: Aquarium pH, CO2 and alkalinity - Fireplace aquarium

 

[Ahnungsloser]

.

 

[Ahnungsloser]

Auf welchen KH-Wert in der Tabelle beziehen Sie sich? Auf den KH-Wert vor der CO₂-Lösung aus einer externen Quelle oder auf den KH-Wert danach ?

Ihr Wasser hat zunächst einen bestimmten KH -Wert , einen bestimmten pH -Wert und einen bestimmten DIC -Wert (gelöster anorganischer Kohlenstoff) . Wenn CO₂ hinzugefügt wird – beispielsweise über einen Ausströmer –, nimmt das Wasser nur dann CO₂ auf, wenn der Partialdruck in der Atmosphäre höher ist als im Wasser. Nehmen wir an, das ist der Fall.

Wenn sich CO₂ auflöst, steigt die DIC , aber auch pH- und KH-Wert ändern sich .

 

[Ahnungsloser]

Da Sie Werte von 0=KH haben, nehme ich an, dass Sie das Wasser VOR CO2 meinen, denn es gibt kein Wasser mit KH=0 und ohne CO2. Ich habe Ihre Tabelle also so gelesen und Zeile 5 verwendet: Ein Wasser (vor CO2) hat 5,6 KH und mit 6 ppm CO2 hat es einen pH-Wert von 8,24. Aber das ist falsch. Ein Wasser mit einem pH-Wert von 8,24 und CO2=6 ppm hat einen KH-Wert von 2,88. Wenn ich ein Wasser mit KH 5,6 und CO2=0 nehme, bräuchte es einen pH-Wert von 10,75. Da es kein Wasser mit CO2=0 gibt, nehme ich CO2=0,01. Wenn ich es nach CO2 (Gleichgewicht) lese, ist es auch falsch: Ein Wasser mit 5,6 dH und einem pH-Wert von 8,24 hat 1,17 ppm, nicht 0,6. Bitte helfen Sie. We develop a device for automatic water change and correct water chemical calculations are important for us. My calculators are online, if you want to compare my and your data I would send you a link. Maybe you used PH calculation with a formula. Since the equations are unsolvable by formulas we used iteration very precisly for calculation PHs. But in this case the PH is gven, so if you apply Henderson equations you can calculate it as I did.

 

[PeerUnk]

pH values at 20°C:

KH⬇️ CO2➡️	0.6 ppm (400 in air)	1 ppm (670 in air)	2 ppm (1300 in air)	3 ppm (2000 in air)	5 ppm (3300 in air)	10 ppm (6600 in air)	20 ppm (13000 in air)	30 ppm (20000 in air)
0 ppm (0 dKH)	5.62	5.51	5.36	5.27	5.16	5.01	4.86	4.77
5 ppm (0.3 dKH)	6.94	6.72	6.42	6.25	6.03	5.74	5.45	5.29

@Andy Pierce You might be aware, at low alkaline (bicarbonates / dKH), the phosphate buffer system comes into play, and pH would be affected / different.

Cheers!

 

[Andy Pierce]

@Andy Pierce You might be aware, at low alkaline (bicarbonates / dKH), the phosphate buffer system comes into play, and pH would be affected / different.

It's a fair point and people are generally adding phosphate fertiliser in the low single digit ppm range so there is probably some type of effect (not sure what though). The literature says that in seawater borate is another buffering ion of relevance - more at higher pH though. I didn't ask the AI to include a first-principles derivation for phosphate in the calculations. 😜 The KH2PO4 most people add will be acidic and lower pH when first added - the AI thinks a 5 ppm solution of KH2PO4 will have a pH of 4.8 by itself. In the lab when we made phosphate buffers they were a defined ratio of KH2PO4 and K2HPO4 to get both the specific pH you wanted and the specific phosphate concentration you wanted but I haven't seen that level of detail used in estimative index dosing.

Wenn sich CO₂ auflöst, steigt die DIC , aber auch pH- und KH-Wert ändern sich . [When CO₂ dissolves, DIC increases, but pH and KH values also change.]

DIC increases and pH changes (decreases) yes, but KH does not change when CO2 is dissolved in water.

"TA [total alkalinity] does not change with CO2 introduction to, or removal from, a waterbody (CO2 invasion or evasion) and is a conservative parameter with respect to temperature and salinity (Wolf-Gladrow et al., 2007). As TA does not change with ambient CO2, it is easier to sample for and store compared to other carbonate system parameters such as pH and DIC. As TA can be measured to a good degree of accuracy, within 1–2 μmol.kg−1, it is a preferred tracer variable in numerical models of the oceanic carbon cycle and is the predominantly measured parameter alongside DIC to describe the carbonate system." from The influence of organic alkalinity on the carbonate system in coastal waters

So, it's perfectly possible to have 0 dKH water and have significant levels of CO2 dissolved in it. That might be counter-intuitive, but the experiment is easy enough to do with an alkalinity titration kit and CO2-injected vs. degassed water.

Here's another way of looking at it: TA = 2[CO3-2] + [HCO3-] + [H2BO3-] + 2[HBO3-2] + 3[BO3-3] + [OH-] + [organic/inorganic H+ acceptors] - [H+]. When you add CO2 gas it converts to HCO3- and H+ in equal amounts since CO2(g) + H2O ⇌ H2CO3 ⇌ H⁺ + HCO3⁻ so the overall TA doesn't change because the amount of bicarbonate produced from the CO2 is exactly balanced by the amount of H+ produced from the CO2.

 

[RO-FR]

Worthy effort by ChatGPT, but came up a bit short. I redid the query using the Dust - Accelerate your entire organization with custom AI agents LLM and after negotiating with it for a considerable length of time got what I think is the correct answer.

CO2 concentrations going across the top, first listed dissolved in water and then the equilibrium value for CO2 found in air in the environment of the tank; KH values (titratable alkalinity) going down the left side listed in ppm CaCO3 equivalents and dKH.

pH values at 20°C:

KH⬇️ CO2➡️	0.6 ppm (400 in air)	1 ppm (670 in air)	2 ppm (1300 in air)	3 ppm (2000 in air)	5 ppm (3300 in air)	10 ppm (6600 in air)	20 ppm (13000 in air)	30 ppm (20000 in air)
0 ppm (0 dKH)	5.62	5.51	5.36	5.27	5.16	5.01	4.86	4.77
5 ppm (0.3 dKH)	6.94	6.72	6.42	6.25	6.03	5.74	5.45	5.29
15 ppm (0.8 dKH)	7.42	7.20	6.90	6.72	6.50	6.20	5.90	5.73
50 ppm (2.8 dKH)	7.94	7.72	7.42	7.25	7.02	6.72	6.42	6.25
100 ppm (5.6 dKH)	8.24	8.02	7.72	7.55	7.32	7.02	6.72	6.55
150 ppm (8.4 dKH)	8.41	8.19	7.90	7.72	7.50	7.20	6.90	6.72
200 ppm (11 dKH)	8.53	8.32	8.02	7.85	7.62	7.32	7.02	6.85
300 ppm (17 dKH)	8.70	8.49	8.19	8.02	7.80	7.50	7.20	7.02

A prettier version as well as the first-principles derivation and some python code you can run to do your own calculations are posted here: Aquarium pH, CO2 and alkalinity - Fireplace aquarium

THE TABLE IS WRONG !!!

 

[Marcel G]

THE TABLE IS WRONG !!!

Good point! This is a nice example of the fact that many claims are considered true not on the basis of evidence, but often on the basis of credibility (blind faith).

But even when you present the correct content (and for that I thank you!), the form you choose to do so is terribly aggressive and unnecessarily offensive. @Andy Pierce certainly did not intend to mislead anyone here, and anyone can be misled. Just point it out politely.

 

[Andy Pierce]

I don't mind the theatrics actually. 😉 The topic is complicated... I measure KH using the Hanna Alkalinity test kit which measures KH in ppm CaCO3, not ppm HCO3. I'm still trying to wrap my head around which is the correct conversion factor to go from ppm CaCO3 to molarity. I also worried a little that the the molecular weight of composite carbonic acid is not the same as the molecular weight of CO2, but these are actually super close since CO2 is by far the dominant species.

 

[Marcel G]

Here is an interactive chart where anyone can find the corresponding relationship between [CO₂:HCO₃:CO₃]-pH: golias.net/akvaristika/co2.php

PS: The graph was created by @hax47.
PPS: I checked hax47's graph with my calculator [I made years ago] and it agrees => Here are my results for KH = 5.5 and pH = 7.81 at 25°C:

 

[Oldguy]

I thought that carbon dioxide dissolved into water mainly by solvation and very little was ionised. It is easy enough to gas your fish regardless of water parameters if you inject.

 

[Andy Pierce]

Ok, sorted it out and @RO-FR caught a legit mistake. The confusion arises (for me) from the proton balance equation (definition of total alkalinity) which goes like:

[TA] = [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] – [H⁺]​

This equation superficially resembles a chemical mass balance equation, but is fundamentally different because here we are describing only protons and the capacity of the various species to accept or donate them. That means that when total alkalinity (TA) is given in ppm CaCO₃, the conversion to molarity (mol/L) for use in the proton balance equation must also account for the stoichiometry of proton acceptance. So, if TA is given in ppm CaCO₃ you first convert ppm to g/L (divide by 1000) then divide by the molecular weight of CaCO₃ (pretty close to 100) and then you need to multiply by 2 because the carbonate ion (CO₃²⁻), which is derived from CaCO₃, can accept two protons, a factor that is already reflected in the "2[CO₃²⁻]" term on the right side of the proton balance equation.

As a result, the pH for non-zero alkalinity values increases a little. Here's the corrected table:

 

[Andy Pierce]

Probably this is a bad idea but since the topic came up, I asked Claude 3.7 about the effect of phosphate. Interestingly in the very first go, Claude made the exact same mistake as me about converting ppm CaCO3 to eq/L. We fixed that and I inquired about effects of phosphate at some typical aquarium concentrations. An interesting point that came up relates to whether you measure the TA of your water and then add KH2PO4, or whether you have first added the KH2PO4 and then measured the TA of your water. Here's what Claude came up with (absolutely no guarantees on any of it although it seems plausible). These are all done with dissolved CO2 at 3 ppm, but the magnitude of the effect of phosphate is pretty similar when CO2 is 30 ppm.

Alkalinity Level	Scenario	KH₂PO₄ (3 ppm PO₄³⁻)	Measured TA	Carbonate Alkalinity	pH	Change
Zero TA	Baseline	No	0 ppm	0 ppm	5.27	-
Zero TA	Scenario 1	Yes	0 ppm (includes KH₂PO₄)	-1.58 ppm	4.50	-0.77
Zero TA	Scenario 2	Yes	1.58 ppm	0 ppm	6.13	+0.86
Low TA	Baseline	No	3 ppm	3 ppm	6.33	-
Low TA	Scenario 1	Yes	3 ppm (includes KH₂PO₄)	1.42 ppm	6.07	-0.26
Low TA	Scenario 2	Yes	4.58 ppm	3 ppm	6.51	+0.18
Moderate TA	Baseline	No	30 ppm	30 ppm	7.33	-
Moderate TA	Scenario 1	Yes	30 ppm (includes KH₂PO₄)	28.42 ppm	7.29	-0.04
Moderate TA	Scenario 2	Yes	31.58 ppm	30 ppm	7.35	+0.02
High TA	Baseline	No	300 ppm	300 ppm	8.32	-
High TA	Scenario 1	Yes	300 ppm (includes KH₂PO₄)	298.42 ppm	8.32	0.00
High TA	Scenario 2	Yes	301.58 ppm	300 ppm	8.33	+0.01

 

[Yugang]

How would one measure pH in RO water? Can the theory be experimentally verified?

 

[dw1305]

Hi all,

How would one measure pH in RO water?

Theoretically you can measure the pH of even ultra-pure DI water <"https://www.researchgate.net/public..._and_Ultrapure_Water_PPCHEM_JOURNAL_23_202101"> , but it is an entirely pointless exercise for us, a bit like calculating the nth digit of Pi.

The pH of pure water (<1μS·cm–1) is of absolutely no practical importance and is neither needed, nor is it required by any of the well appreciated and accepted worldwide standards. It is not a measure of the purity of the water and varies considerably when the water is used. Obtaining exact pH values for ultrapure water samples is particularly difficult due to its sensitivity to contamination from trace elements in the air, sample containers and test equipment, as well as measurement errors. Conventional pH probes do not give reliable results.

cheers Darrel