Hi all,
What I'm getting here is that measuring ph with an electronic device is much more reliable than using a drop checker for measuring CO2, is that the general consensus? If that's the case then great, I like reliable accuracy and it's one less fiddly glass thingy hanging in my display! I've used a ph controller on my reef system but that was for running a calcium reactor and balancing KH which is quite a different kettle of fish
This bit is quite important, if you have very heavily buffered salt rich water (sea water is carbonate buffered and ~53,000 microS) then any pH meter will give you a fairly accurate measurement. As you move towards pure H2O (an electrical insulator) pH becomes both a less meaningful measurement, and much more difficult to measure.
You can't extrapolate from a buffered system (like Lake Tanganyika) to a much less buffered situation like the Amazon basin black (and clear) water rivers and lakes.
These problems come about because pH is a ratio, and "pH7" just means you have an equivalent number of H+ ion donors ("acids") and H+ ion acceptors ("bases"). It doesn't matter what that number is it could be 1:1, 10:10, 100,000:100,000 as long as it is a balanced equation.
This is also why pH can never be stable in very soft water, very small changes in the relative proportions of weak acids (like H2CO3) and weak bases (like O2) cause large changes in pH.
Provided you haven't contaminated the drop checker fluid it will be right and pH pen wrong.
This is also why "Ian_m" is right, pH meters are high maintenance bits of kit that need to be regularly calibrated (with pH buffers) and have their electrodes stored in a KCl solution etc.
They aren't "plug and play". If you are willing to spend a couple of hundred pounds on a pH meter/electrode combination, and have the time to buffer it before each use, it will give you an accurate and repeatable reading.
Drop checkers are accurate (
<"read this thread">), and use the fact that carbonic acid (H2CO3) and bi-carbonate (HCO3-) are the weak acid and weak base pair in carbonate buffering.
Calcium carbonate is insoluble in water, but in water with carbonates present the small amount of CO2 (that goes into solution as H2CO3) is in equilibrium with the HCO3- to give a stable value of ~pH8 at atmospheric CO2 levels (400ppm CO2) and standard barometric pressure (1013mb).
When we add CO2 above atmospheric levels we drive the H2CO3 ~ HCO3- equilibrium towards H2CO3. We know that pH is a ratio, and that an acid is defined as a H+ ion donor and we've added extra H+ (from H2CO3), so the pH falls. How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH".
If we have a narrow range pH indicator ("bromothymol blue"), a solution made to contain 4dKH solution ("4dKH solution"), a drop checker with an air gap and a chart with experimentally derived CO2 levels for a range of pH values at a known carbonate hardness (below) it is a pretty robust system.
cheers Darrel