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What happens to ph when you mix ro water with tap water ?

anewbie

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We have 7-7.1 ph out of the tap; I want to setup an ro unit to keep low ph fishes and my understanding if the ro unit is 'good' the tds will be between 0-10; and there is a good chance the ph will crash. Lets assume it crashes and is around 5; if i mix it will tap water (for non low ph tank) does ph automagically rise but up to 7 from the ions in the tap water or does it end up somewhere in the middle? Tap is around gh 7 kh 3 tds 120; lets presume the mix is 1:1 which would put the gh/kh/tds in a good range for most non-blackwater cichlid (domestic angels and such).
 

MacZ

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Ok, the basics:

pH, KH and CO2 from the air are constantly trying to reach an equilibrium in water. You might know these lists that planted tank people use? Exactly that thing. It is by that equilibrium that KH predetermines pH. Unadulterated RO has a pH of vaguely neutral, when you let it sit, it balances out at roughly 5.5-6. Here you find more about the chemical reactions: https://en.wikipedia.org/wiki/Carbonic_acid

A pH-crash happens only in one of the following (rare) scenarios.
- NO3 overload. NO3 (Nitrates) are dissolved in water als Nitric acid, hence with rising NO3 levels the pH rises and with it KH is being neutralized. If somebody doesn't do waterchanges in a long time and has no consumers (plants) and no de-nitrifying bacteria (deep sand beds, e.g.) the KH is slowly consumed as per the reaction above, until there is no buffer anymore and then a pH crash occurs.
- You have unadulterated RO / DI / distilled Water and add any acid in overdose.

So in short: A crash only occurs when you not only have water without a buffer, but when als H+ ions (which are measured in pH) are added. No H+ added, no pH crash. Just FYI: It takes 10-fold the amount of H+ to get a pH of 6 down to a pH of 5. And another 10x for 5 to 4.

There are two buffer systems. One is KH (as seen above), the other is humic substances, which buffer in a low range of 4.5 - 6.5.

TDS you can't use as measurement. TDS encircles everything: GH, KH, NO2, NO3, humic substances, DOC, metallic ions, salts... simply everything. So you will have to calculate with your KH.

The amount of carbonates drops and rises linear to dilution, so a 1:1 mix of RO and tap ends you up with 50% of the tap-KH. Let's say your tap has a KH of 5, you do a mix 1:1, results in KH of 2.5. Every time you cut KH in half pH drops by 0.3 from the original value.

So your calculation is missing a value. What's your tap's pH?
 
Last edited:

MacZ

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Oh, sorry. Too many numbers. :D
GH and TDS don't interest for the pH, but I'll just add them here: GH will go down to around 3.5°, TDS to about half = 60mg/l.

So if your pH is 7 (let's ommit the 0.1, because that's just within error margin*) and we add no additional H+:
KH is reduced to 1.5°, pH will lower by 0.3 so you get it down to 6.6 - 6.8 without addition of any acidity. Which is, frankly still in neutral territory to slightly acidic.

I measure my RO with 6.8 right out of the unit, roughly 6 after a day of letting it sit in a closed container. Roughly 5.5 when letting it sit for a day open. That's due to exposure to atmospheric CO2 and the equilibrium of KH, pH and CO2. With no KH this is in a constant flux, but will not drop endlessly.

Hope that helps?

*Almost all measuring methods for pH (except a well calibrated higher quality pH meter) are only accurate in 0.2-0.5-steps. Be also aware, that the lower the EC the less accurate pH meters become and the lower KH the less accurate are chemical pH tests.
 

anewbie

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735
Oh, sorry. Too many numbers. :D
GH and TDS don't interest for the pH, but I'll just add them here: GH will go down to around 3.5°, TDS to about half = 60mg/l.

So if your pH is 7 (let's ommit the 0.1, because that's just within error margin*) and we add no additional H+:
KH is reduced to 1.5°, pH will lower by 0.3 so you get it down to 6.6 - 6.8 without addition of any acidity. Which is, frankly still in neutral territory to slightly acidic.

I measure my RO with 6.8 right out of the unit, roughly 6 after a day of letting it sit in a closed container. Roughly 5.5 when letting it sit for a day open. That's due to exposure to atmospheric CO2 and the equilibrium of KH, pH and CO2. With no KH this is in a constant flux, but will not drop endlessly.

Hope that helps?

*Almost all measuring methods for pH (except a well calibrated higher quality pH meter) are only accurate in 0.2-0.5-steps. Be also aware, that the lower the EC the less accurate pH meters become and the lower KH the less accurate are chemical pH tests.
So if i understand correctly if it sits in a container for a week (the storage tank for ro output is 500 gallon); the ph will be roughly 6 but when i mix it with tap it will go back to 6.7-6.8 (which is fine for sa fishes); but then if i let it sit in an open container another day before adding to the low ph tank it will be around 5.5 (i'm talking about the pure stuff i will use for cherry gourami) which is good. So i don't actually have to use peat to lower the ph. Maybe.
 

MacZ

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but then if i let it sit in an open container another day before adding to the low ph tank it will be around 5.5
Until here: Yes.
But when you mix that with tap, it will just lower KH again by dilution and neutralize it partially so you would end up with (estimate!) around 1°KH instead of 1.5°. pH will then likely be around 6.5.
But be aware: With leaf litter and wood you can now achieve further lowering of the pH. Possibly down to 6 or even 5.5.
 

MacZ

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(the storage tank for ro output is 500 gallon)
I should mention: Closed container means no or very little air inside aswell. If that 500 gallon is not filled to the top it will still acquire quite a lot acidity from CO2 in it.
 

anewbie

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Messages
735
I should mention: Closed container means no or very little air inside aswell. If that 500 gallon is not filled to the top it will still acquire quite a lot acidity from CO2 in it.
Ok. But for the low ph tank I can do something like a 1:10 ratio (tap:ro); to add a tiny bit of mineral for plants but achieve a suitable ph for apisto and chocolate gourami as well as dicrossus filamentosus. I won't call it a blackwater tank but i hope the ph is below 6 and closer to 5.5. I can transfer the ro water to a 20 gallon pail and let it sit overnight before mixing with some tap and adding it to the tank (water changes). I guess i can add a little peat behind the matten filter to stabalize things.
 

MacZ

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to add a tiny bit of mineral for plants
Honestly, go full RO and add a macro&micro fertilizer. It's not just minerals you need for the plants. Much easier.

But for the low ph tank I can do something like a 1:10 ratio (tap:ro); to add a tiny bit of mineral for plants but achieve a suitable ph for apisto and chocolate gourami as well as dicrossus filamentosus. I won't call it a blackwater tank but i hope the ph is below 6 and closer to 5.5.
With enough botanicals and leaf litter, yes.

I guess i can add a little peat behind the matten filter to stabalize things.
You should. A low pH tank won't work without humic substances. You could try and remove some of the tint with activated carbon if necessary. That's how you get to clearwater. And clearwater is closer to what you are aming for than blackwater.
 

dw1305

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Hi all,
Lets assume it crashes and is around 5; if i mix it will tap water (for non low ph tank) does ph automagically rise but up to 7 from the ions in the tap water or does it end up somewhere in the middle?
Basically what @MacZ says. Assuming you have some dKH in your tap water? The pH goes to ~pH8 because that is the CO2 ~ dKH~ pH equilibrium point.

cheers Darrel
 

MacZ

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8 is very likely a typo. I'm sure he meant "towards 7", as that is indeed the equilibrium point.
For 8 you'd need a KH of 7-9 at least.
 

dw1305

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Hi all,
Tap is only 3kh; not sure why you say the ph will go to 8.
It will.
8 is very likely a typo. I'm sure he meant "towards 7", as that is indeed the equilibrium point.
For 8 you'd need a KH of 7-9 at least.
It is a bit strange, but isn't a linear relationship. I've got much better with the chemistry of Total Inorganic Carbon (TIC) since I've been a <"member of UKAPS">, where many of the members are aquascapers who inject CO2.

Theoretically when you have DI water (H2O), (rather than the solution of soluble salts with H2O as a solvent that we call water), any addition of dKH (HCO3-) ions will take the pH to the equilibrium point of ~pH8. This is controlled purely by the level of atmospheric CO2.

The only acid (proton donor) you have to buffer the pH rise is the small proportion of dissolved CO2 that becomes carbonic acid (H2CO3) and which disassociates into H+ and HCO3-. In practice the pH only goes to pH8 when you have more than ~3 dKH.

cheers Darrel
 

anewbie

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Messages
735
Hi all,

It will.

It is a bit strange, but isn't a linear relationship. I've got much better with the chemistry of Total Inorganic Carbon (TIC) since I've been a <"member of UKAPS">, where many of the members are aquascapers who inject CO2.

Theoretically when you have DI water (H2O), (rather than the solution of soluble salts with H2O as a solvent that we call water), any addition of dKH (HCO3-) ions will take the pH to the equilibrium point of ~pH8. This is controlled purely by the level of atmospheric CO2.

The only acid (proton donor) you have to buffer the pH rise is the small proportion of dissolved CO2 that becomes carbonic acid (H2CO3) and which disassociates into H+ and HCO3-. In practice the pH only goes to pH8 when you have more than ~3 dKH.

cheers Darrel

But then why is the tap water stable at 7 and not moving towards 8. I don't test the ph in my tanks constantly but i have a few times and they are fairly stable at 7 (I'm currently not using ro water this entire thread is around my future plans for an ro system).
 

MacZ

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any addition of dKH (HCO3-) ions will take the pH to the equilibrium point of ~pH8.
In practice the pH only goes to pH8 when you have more than ~3 dKH.
If the equilibrium point is indeed 8, which almost doesn't interest anymore (but I neither doubt it, as it does make sense under certain circumstances), then saying "any" addition of KH is a bit too vague. As is saying it takes only about 3°KH for a pH of 8, as this is, in practice, not the case.
Problem I see: Your example is given with
Theoretically when you have DI water (H2O), (rather than the solution of soluble salts with H2O as a solvent that we call water)
so, this doesn't apply to the actual application here.
 

dw1305

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Hi all,
But then why is the tap water stable at 7 and not moving towards 8. I don't test the ph in my tanks constantly but i have a few times and they are fairly stable at 7
If you have <"planted tanks?"> The pH will <"go up and down during photosynthesis">. You may have other compounds in your water supply (pH is a ratio), or it maybe the pH meter, they are <"high maintenance bits of kit">.
so, this doesn't apply to the actual application here.
@MacZ I just don't measure pH in soft water any more. I have access to laboratory pH meters and lab. grade <"neutral salts">, but there <"just isn't any point">, all it does is give me a proxy value <"for the ratio of CO2 : O2 in the tanks">.

I use a conductivity value (and <"the state of the snail shells)"> as an indication of when my tanks are in the "Goldilocks zone". Conductivity isn't ideal as a measurement, but the meters are "plug and play".

I didn't choose the conductivity datum for any reason other than that is about the highest conductivity value that I ever find in the rain-water I use for the tanks. If I had had softer rain-water? I would have used a lower conductivity datum and it would open up the possibility of keeping (and even breeding) more extreme black water fish.

cheers Darrel
 

MacZ

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I'm getting the feeling we're in a misunderstanding here.

@MacZ I just don't measure pH in soft water any more. I have access to laboratory pH meters and lab. grade <"neutral salts">, but there <"just isn't any point">, all it does is give me a proxy value <"for the ratio of CO2 : O2 in the tanks">.
I use a conductivity value (and <"the state of the snail shells)"> as an indication of when my tanks are in the "Goldilocks zone". Conductivity isn't ideal as a measurement, but the meters are "plug and play".
I don't really measure pH myself, especially not with a meter, as I know the conductivity is too low. Exceptions can happen (as with the tank upgrade I did lately, where I probably should for once have measured pH).
If any I use a drip test which is quite inaccurate under said conditions, as we all know.
So I also use an EC/TDS meter for most applications.

No, that one definitely is right in practice. You can however easily "burn through" that dKH and then pH will drop.
Ok, but then, according to you, it is still only correct in DI water, which indeed makes sense. Did I get that right? Then what about non-purified water? Or purified but not fully de-ionized water (RO or rainwater)?

Because here in the thread we're not talking DI. That's where I see our respective trains of thought separating and where I find the info you give, while really interesting, rather overcomplicating things and somewhat off the practical application. And also confusing for people that haven't got half the understanding of water chemistry you have (and I happily admit my understanding of chemistry is relatively solid, but still far behind yours.).

I didn't choose the conductivity datum for any reason other than that is about the highest conductivity value that I ever find in the rain-water I use for the tanks. If I had had softer rain-water? I would have used a lower conductivity datum and it would open up the possibility of keeping (and even breeding) more extreme black water fish.
I have no idea how you get to that all of a sudden. Also, at no point you give the datum you're talking about.

This is how a <"drop checker works"> to measure CO2.
To be frank: I have never injected CO2 to a tank because I find high-tech planted tanks extremely off-putting.
 

dw1305

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Hi all,
I have no idea how you get to that all of a sudden. Also, at no point you give the datum you're talking about.
Somewhere in the links, but in this thread, <"Water Chemistry">.
Or purified but not fully de-ionized water (RO or rainwater)?
Rain water is a perfect, I'll use mine as an example. So it is somewhere between 30 and 120 microS conductivity throughout the year and the majority of those ions will be Ca++ and 2HCO3-.

At the moment it is about 50 microS, but still with a pH value around pH 8. (I'll get some photographs tomorrow when the light is better). If I add some Oak leaves the pH will fall and if I add an aquatic plant and a light? The pH will rise.
To be frank: I have never injected CO2 to a tank because I find high-tech planted tanks extremely off-putting.
Same for me, I know about <"Estimative Index (EI)"> and ,<"added CO2"> because of my time on UKAPS, neither are routes <"I'm ever going down">.

cheers Darrel
 

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