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I rarely disagree with Darrel and know I'm on shaky ground when I do, but ...
If I remember correctly, the concentration of H+ multiplied by the concentration of OH- in any aqueous solution is always 10 to the power -14. If you add more H+ or OH-, the other ion must decrease so that their mathematical product remains 10 exp-14. If you add both H+ and OH-, they combine to make water. If pH is 7, then H+ and OH- concentrations are both 10 exp-7. If pH is 6, then H+ concentration is 10 exp-6 and OH- concentration is 10 exp-8. So pH does tell you exactly how much H+ and OH- are present. It's the quantities of all those OTHER pH-changing ions (H+ donors and acceptors, measured as conductivity) that cause pH to rise or fall and to be relatively stable or unstable.
I'm not a fan of paper test strips, but liquid tests like Bromothymol blue (useless below ph 6.0) do just fine in very low conductivity water. All meters rely to some extent on the water's conductivity; color indicator dyes don't. Among meters, the ISFET type electrodes seem to do better in very soft water than the regular glass bulb electrodes. See http://www.hach.com/minilabmeters.
If I remember correctly, the concentration of H+ multiplied by the concentration of OH- in any aqueous solution is always 10 to the power -14. If you add more H+ or OH-, the other ion must decrease so that their mathematical product remains 10 exp-14. If you add both H+ and OH-, they combine to make water. If pH is 7, then H+ and OH- concentrations are both 10 exp-7. If pH is 6, then H+ concentration is 10 exp-6 and OH- concentration is 10 exp-8. So pH does tell you exactly how much H+ and OH- are present. It's the quantities of all those OTHER pH-changing ions (H+ donors and acceptors, measured as conductivity) that cause pH to rise or fall and to be relatively stable or unstable.
I'm not a fan of paper test strips, but liquid tests like Bromothymol blue (useless below ph 6.0) do just fine in very low conductivity water. All meters rely to some extent on the water's conductivity; color indicator dyes don't. Among meters, the ISFET type electrodes seem to do better in very soft water than the regular glass bulb electrodes. See http://www.hach.com/minilabmeters.