Quote:
Originally Posted by ALToronto
And the pH is 8.5. Unfiltered water pH Is 7.3, and when I was using a jug water filter (ion exchange), filtered water was clocking in at 6.7.
I checked my pH meter with store bought distilled water - 7.0 on the dot. What gives with the RO? The 8.5 reading was after the initial tank flush, and the TDS reading was 12 ppm (down from 136 for cold tap water)
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The TDS value indicates that the RO system is working correctly. The pH of RO water anywhere between about 5 to 9 has very little meaning precisely because the water is pure. To change the pH of pure water from 7 to 8.5 would require the addition of 3 micromoles of strong base per liter of water; for sodium hydroxide that would be 180 micrograms per liter which would be 0.18 ppm. So as I believe other people have mentioned you just have minute amount of base getting into the water from the membrane or apparatus or even your pH measuring device.
To bring the pH of your RO water back to 7 you could add about 3 milligrams of vinegar to each liter of RO water. This assumes that the pH above 7 comes from a strong base, it could be that there is a larger amount of some weak base that is acting as a buffer at pH 8.5 in which case you would need to add a larger amount of acid but by the TDS you have at most 12 ppm of buffer so at most you would need to add 200 milligrams of vinegar (assuming that the buffer has the same equivalent weight as acetic acid).
Edit:
Thinking about it; sodium bicarbonate solution has a pH of about 8.5 so your 12 ppm TDS could conceivably be sodium bicarbonate. So then you would need to add about 200 milligrams of vinegar per liter of RO water to bring the pH down to 7.
ALToronto 
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