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R/O Water

Gunner10

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11 Jan 2022
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15
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Lincoln
Good morning folks. Could someone explain why the P.H is 9.0 after changing the filters om my 4 stage R/O unit. It's 8.2 at the tap ! I thought it should be in the region on of 7.0 after going through the R/O. Any ideas please ? Thank you.
 

jolt100

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13 Jan 2008
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152
Location
bury,lancs
pH of freshly filtered RO water is not really measurable and not a parameter that needs testing.
Measuring conductivity will tell you if your RO unit is working, the closer to zero the better. pH WILL be close to 7 when its absorbed some atmospheric CO2 even if you can't measure it.
When you remineralise or add to your tank then you can measure pH because the concentration of Hydrogen ions will be much higher.
If you look back through older posts Darrell @dw1305 has better explanations.
Cheers
John
 

MichaelJ

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9 Feb 2021
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Minnesota, USA
I've been really confused about this in the past until I had it explained by @dw1305 and others (and I probably still don't fully understand :) ). In conclusion, you can't really measure pH reliably in deionized water (which I assume your 4 stage RO unit will deliver). You need a bit of pH buffer as Darrel mentions above to make that work..

Cheers,
Michael
 

dw1305

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7 Apr 2008
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nr Bath
Hi all,
In conclusion, you can't really measure pH reliably in deionized water (which I assume your 4 stage RO unit will deliver). You need a bit of pH buffer as Darrel mentions above to make that work..
That is it. As you move towards pure H2O (rather then the dilute solution of salts with H2O as a solvent that we call "water") pH becomes <"both less stable"> and more difficult to measure. The problem is that pH is both a <"log base 10 scale and a ratio">.

The measurement issue is just that pH meters are a <"specific type of conductivity meter"> and in RO water there aren't many ions of any description. You can raise the conductivity with a <"neutral salt">, like KCl or NaCl, but you are still just getting a snap shot at a specific moment of an inherently unstable parameter.

cheers Darrel
 
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