I grow things for a living and regularly use pH meters + strips to read and adjust the pH of various nutrient and pesticide solutions. Oftentimes, I have a need for nearly naked water with the pH adjusted down to 5.5-5.75. I'm aware that taking the pH of naked water isn't possible with the meter or strips that I use so normally I add about 5% tap water to reverse osmosis water so I can get an accurate pH reading, then adjust the pH down with phosphoric acid to my desired level. Recently I've been trying to track down the source of a minor fungal contaminant and I have reason to suspect the tap water so I'm trying to eliminate it entirely. My question is this:

If I fill a bucket with pure reverse osmosis water, then add an amount of phosphoric acid to the RO that I think will get me close to my desired pH, will the added phosphoric acid put enough ions in the RO to get an accurate pH reading with a standard glass ball pH meter and/or pH strips?

I've already tried and it seems to work but I really have no way of double checking. Generally if I have reason to believe that my meter needs calibration (I try to stay on top of it but it's a shared tool and not everybody treats it right) then I just pull out my trusty pH strips and check. But pH strips also don't work in naked water so I'm not really able to blindly trust those either.

Thank you very much for your time.

  • 1
    $\begingroup$ What is "nearly naked water" ? Is it water obtained by reverse osmosis ? What is the pH of this original water? Is it not $5.6$ ? Usually demineralized water has a pH not far from $5.6$ when in contact with air. $\endgroup$
    – Maurice
    Dec 24, 2023 at 20:53

1 Answer 1


RO water is usually passed thru a charcoal filter to remove the chlorine from city water. If the filter is not maintained I have found that the effluent water becomes acidic from HCl formed by reaction of the chlorine with carbon. To check the acidity of the water add a small amount of neutral KCl to a sample solution that is protected from the air [parafilm works] Then add Phosphoric acid to give the desired pH and adjust your water appropriately. If the water is originally acidic check the RO system. Also Phosphoric acid is a better buffer than any HCl from a faulty carbon filter. It is possible that addition of the phosphoric acid will be accurate enough. this can be determined by testing the finished water without and with KCl.

Calibration of the pH meter is important. For low acidity use a flowing reference electrode not a gel filled reference electrode.

  • $\begingroup$ Umm,.. "from HCl formed by reaction of the chlorine with carbon?" The activated charcoal might act as a catalyst, but direct reaction with dilute HCl seems improbable. $\endgroup$ Dec 25, 2023 at 2:21
  • $\begingroup$ Reaction With CHLORINE Carbon is oxidized various chlorocarbons. Cl2 is reduced to chloride. I am not sure how the solution becomes acidic possibly a concurrent disproportional of Cl2. I know this from experience with RO systems. Systems on chlorinated water became acidic and bitter tasteing. Problem solved by an in line carbon filter before the RO to remove chlorine. [never checked for redox potential, should have]. RO systems need monitoring and maintenance usually neglected in home systems. $\endgroup$
    – jimchmst
    Dec 26, 2023 at 21:29

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