# R/O Water



## Gunner10 (3 Apr 2022)

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.


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## GHNelson (3 Apr 2022)

Probably needs to run in/flush for an hour!


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## john6 (3 Apr 2022)

Whenever i change filters i flush the system for an hour before using it. Never sure if an hour is overkill but but its what ive always done


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## Gunner10 (3 Apr 2022)

Thanks for the replies. It still seem's high for an 80 ltr container !


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## JacksonL (3 Apr 2022)

It seems very high. Have you tested the KH?


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## Gunner10 (3 Apr 2022)

JacksonL said:


> It seems very high. Have you tested the KH?


I will do that this afternoon when I get 5 mins .


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## jolt100 (3 Apr 2022)

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


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## Gunner10 (3 Apr 2022)

Gunner10 said:


> I will do that this afternoon when I get 5 mins .


K.H is under 1 !!


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## Gunner10 (3 Apr 2022)

K.H is below one !!


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## dw1305 (3 Apr 2022)

Hi all, 


Gunner10 said:


> K.H is under 1 !!


Just add a bit of tap water (5 - 10% by volume should be fine), this will supply you with 1:1  dKH and dGH (from dissolved limestone (CaCO3)). 

cheers Darrel


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## MichaelJ (3 Apr 2022)

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


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## dw1305 (3 Apr 2022)

Hi all,


MichaelJ said:


> 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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