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PH remains the same when diluting tap with RO- Why?

W

Whagwan

Member
Joined
15 Feb 2022
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53
Location
Bristol
Hey, my tap water is pH 7.4, kh 8, gh 15.

I have one tank I've recently started which I use 50/50 RO/Tap.

The kH and gH halve as expected to 4 and 7-8 respectively, but the pH stays at 7.4

What could be causing this buffering effect? Surely diluting to 50/50 with RO water should have an effect on this?

Any water chemistry experts who can explain what is going on here? Techy explanations are welcome, I was an analytical chemist many many moons ago so should be able to follow.
 
The RO water is highly reactive and will pull CO2 out of the air thus driving down the pH to somewhere around 6 - 6.5 ... which is hard to actually measure as @hypnogogia points out, but your alkalinity (KH) or buffering capacity, is too high for the dilution with the RO water to make any meaningful impact to your pH. In addition, the pH scale is logarithmic (log 10) so the math for mixing is similarly log 10 (the difference between pH 7 and pH 6 is a 10 fold difference in H+ ion concentration)

Cheers,
Michael
 
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Hi all,
Whagwan said:
The kH and gH halve as expected to 4 and 7-8 respectively, but the pH stays at 7.4. What could be causing this buffering effect? Surely diluting to 50/50 with RO water should have an effect on this?
Click to expand...
The pH doesn't actually start to change until <"you get below about 2 dKH">. It all relates to the CO2 ~ carbonate ~ pH equilibrium point and this is dependent upon the <"amount of atmospheric CO2">. Conductivity (<"ppm TDS">) is linear and should <"reduce in direct proportion to the RO added">. The problem is with pH, it isn't a linear scale but a ratio and log base10 values*.
*edit what @MichaelJ says

These curves are for strong acid and bases (H2CO3 and CO3 are weak bases), but you still have the same curves, just a lot flatter.

ck12_screenshot_21-19-1-png.png

Whagwan said:
What could be causing this buffering effect?
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The remaining dKH (<"alkalinity">) of the tap water, you've reduced this, but not to below the level where the pH will change. Have a look at <"Some handy facts about water">.
hypnogogia said:
Can’t really be measured reliably.
Click to expand...
True, pH is not a useful (or easy) measurement in pure water.

cheers Darrel
 
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Just in case someone is dying to know how this logarithmic math business is working: (Note: lets dispense with our knowledge about annoying things such as alkalinity (KH) for a bit....)

The resulting pH pHr by mixing two liquids of pHa and pHb where the fraction of the liquid with pHa is x (percentage/100) :

pHr = -log10 ( 1/10^pHa * x + 1/10^pHb * (1-x) )

Say we mix waters 50/50 Container A is pH 6.5 and Container B is pH 7.4.

a = pH 6.5 = 1 / 10^6.5 = 0.0000003162278
b = pH 7.4 = 1 / 10^7.4 = 0.0000000398107

a * 0.5 + b * 0.5 = 0.000000178

- log 0.000000178 = 6.75

So the resulting pH would be 6.75

This is of course just an approximation of the real world, when actually mixing bases and acids it depends on what they are made up of etc. It can get fairly more involved when mixing say a strong acid with a weak base and is best done empirically a famous lab rat once told me.... but for practical purposes the above math should work fairly well.

Cheers,
Michael
 
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Andy Pierce said:
Is this the pH of the water as it comes straight out of the tap, or after you've let it equilibrate with the external environment? I ask because pH 7.4 with a KH of 8 indicates 10 ppm CO2 where we know the equilibrium CO2 value is 0.5 ppm. If the KH is really 8 then at equilibrium CO2 the pH would be more like 8.7. You can't trust either the pH in the water report or the pH right out of the tap.
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Hey, yes the pH is 7.4 out of the tap after standing for 24hrs.

Thanks @dw1305 makes sense (and also I haven't seen a titration curve for over 20 years :D)
 
Hi all,
Whagwan said:
Hey, yes the pH is 7.4 out of the tap after standing for 24hrs.
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Interesting, it should be higher than that, just because of the <"CO2 ~ HCO2- ~ pH equilibrium value">. It is worth noting that the present atmospheric CO2 value is 424 ppm <"Global Monitoring Laboratory - Carbon Cycle Greenhouse Gases">.
Whagwan said:
Thanks @dw1305 makes sense (and also I haven't seen a titration curve for over 20 years :D)
Click to expand...
<"I really like graphics">, and unfortunately you have to have a <"bit of chemistry"> when you are talking about pH and buffering. I've never really <"come up with a good analogy"> where you can compare it to every day situations.

Same for <"discussion of cycling and nitrification">, you have to have <"some microbiology"> before you can start to join up the dots.

cheers Darrel
 
What could cause the ph to be lower than expected like in this case?

I'm in Bristol btw, tap water report for my postcode below.

1686436103171.png

1686436136419.png
 
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