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AC Water Run-Off for orchids?

This is a discussion on AC Water Run-Off for orchids? within the General Orchid Culture forums, part of the Orchid Culture category; Originally Posted by Halloamey I would first check for pH, dissolved solids and concentrations of ...

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  1. #11
    kubruceiii is offline Junior Member
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    Quote Originally Posted by Halloamey View Post
    I would first check for pH, dissolved solids and concentrations of metal ions in it before using it. The heat exchangers are washed with a strong Alkali (NaOH) during regular maintenance, so it might be present in the condensate. Secondly the water that condenses on the heat exchangers is pure water, hence it has a pH of about 4.5 to 5 on formation, therefore it is very acidic, it can corrode the heat exchangers which are mostly made of copper aluminium alloys, so there can be high concentrations of Copper and Almunium in the water which can hazardous for orchids.

    It would be best to test for these parameters before using it. I would recommend what Ron suggested, to mix it up with normal water and then use it, so that it will dilute any excess metal ions.
    You make a good point about the sodium hydroxide, but as for the pH of pure water being 4.5 to 5, that is absolutely incorrect. Pure water, with no dissolved solids or gasses, has a pH of 7. As soon as you put it into air, pH will start to drop to about 6.5 because of the partial pressure of CO2 in the atmosphere will cause CO2 to dissolve into the water creating some carbonic acid. Also, IF there was any NaOH in the water, it would act as a buffer and the pH would be greater than 7.

  2. #12
    Halloamey's Avatar
    Halloamey is offline Senior Member
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    Quote Originally Posted by kubruceiii View Post
    You make a good point about the sodium hydroxide, but as for the pH of pure water being 4.5 to 5, that is absolutely incorrect. Pure water, with no dissolved solids or gasses, has a pH of 7. As soon as you put it into air, pH will start to drop to about 6.5 because of the partial pressure of CO2 in the atmosphere will cause CO2 to dissolve into the water creating some carbonic acid. Also, IF there was any NaOH in the water, it would act as a buffer and the pH would be greater than 7.
    Yeah I didn't explain it completely, what I meant was since the condensate is pure water on formation, it has no buffering capacity and the pH will drop to 4.5 to 5.6 due to carbonic acid as CO2 is omnipresent at least at the heat exchangers.

    Also the NaOH and acidic water were two different scenarios, there could be NaOH or acidic water and metal chelation phenomenon. Also NaOH is a strong base and not a buffer so if it is present in the condensate the dissolving of CO2 from air will not be sufficient to bring back the pH to 7.

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