Another Peat Filtration Question

[gerald]

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.

 

[dw1305]

Hi all,

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.

No, Gerald inevitably you are correct and I'm wrong, I think it is to do with the self-ionization of water, but the total of OH-/H+ is always 10 power -14.

I've learned quite a lot about acids, that I'd failed to fully understand before in my last couple of posts, so thank you to Apistogramma forums. I hope I'm getting there slowly and eventually maybe I really will understand it all.

cheers Darrel

 

[Jacco]

Hi Gerald,
I agree.... after reading wikipedia I came to the same conclusion.
log[H] + log [OH] = 14
so the pH is a certain concentration of H or OH in water.
at very high concentrations this doesn't work... because in that case we are not talking about an acid solved in water, but water solved in acid...
Darrel does have a point that pH measurement at low conductivities is 'complicated'. That is because a pH measurement with an electrode is also a (special) kind of conductivity measurent (electro chemical reaction).
Two electrodes are involved. One for reference and one to measure H+ 'activity' in the water. The difference in potential between the two electrodes is used to calculate pH. (A pH meter does this for you.)
The tricky part is that when you calibrate your pH meter, you should use a calibration solution (buffer) with the same range of conductivity as the solution you want to take a pH reading from. At (extreme) low conductivity this is by far not the case, meaning that you reading is of.
When you use a different method for measuring your pH conductivity doesn't matter.

 

[dw1305]

Hi all,
Jacco I'm sure you are right about the reasons for the instability in the pH reading as you approach pure water using calomel or silver chloride reference electrodes. By the look of it this doesn't effect the solid state "Ion-Sensitive Field Effect Transistor" meters that Gerald refers to as much, although I also think they are generally more expensive to buy.

The bromothymol blue indicator is the indicator that planted aquarists use in their drop checkers, they aim for 4dKH in the checker and tank water, and then use the colour of the indicator (yellow below pH6 to blue above pH7.6, green in the middle) and the CO2/carbonate equilibrium to give them ~25ppm CO2 (even though to me it sounds like a sure recipe for gassing all your fish sooner or later). This is a change in pH without a change in alkalinity, so presumably indicator papers would also give you an accurate reading of whatever the pH value was when you tested the water.

The pH in my planted tanks (about 100 microS conductivity and some dKH) varies from pH6 - pH7.8 dependent upon where we are in the photocycle (lowest immediately before lights on and highest during active photosynthesis), but I don't know what value I'd get if I allowed the water to equilibrate with the atmosphere before testing.

cheers Darrel