[Borek]

Check this Keeping Fish page for an interesting idea of non-contact pH measurements.

My take is that it is all about CO₂ equilibrium, that the answer time is slow, and that it will work only if both solutions have exactly the same composition.

Still, well planned experiment could be interesting - something like both solutions with the same indicator - both have the same color. Now we saturate external solution with CO₂ (perhaps in a closed tank, so that CO₂ won't run away), external solution changes color first, then inner tube changes color in an hour or few. I can't do that by myself here, but if anybody is looking for some fresh experimental idea - here it is

[Borek]

Looks like the same idea is used also by others:

http://www.aquariumadvice.com/viewtopic.php?p=798659

and it looks like it is all about CO₂ equilibrium, as I expected. Now the most interesting thing is the response time.

[billnotgatez]

Fish excrement effects pH. So I assume you are doing the CO₂ experiment to tease out its contribution.

[Borek]

pH measurements is a way of keeping correct CO₂ level in the aquarium. It can't be too high as fish suffocate, it can't be too low as plants need it.

[billnotgatez]

The point I was making was that the pH is not solely determined by Carbon Dioxide concentration. I would think that other factors influence it more. Therefor measuring pH to determine Carbon Dioxide concentration would be inaccurate. In a system that was just water and Carbon Dioxide with no plant or animal life the concentration of Carbon Dioxide should be proportional to pH. If your goal is to detect Carbon Dioxide in a living aquatic system maybe we have to figure out another method of indication. For instance an electrode in the aquarium and an electrode in a pure Carbon Dioxide environment.

It also may be that I am misunderstanding your ideas.

[Borek]

Read again posts at both sites linked to. Trick is to get both solution as similar as possible, so that they both differ JUST by CO₂ concentration. Thta's why you should either use solution prepared especially for the measurements, or just the sample of aquarium water that was taken at most few days ago.

This a slow response system, but it is much cheaper than pH meter based one and could be pretty accurate. That's why I think this system is interesting to investigate.

[billnotgatez]

I reread your link and still I am not getting why other chemicals in the system would not effect a pH meter. On the other hand if you are proposing an alternate method that homes in on carbon dioxide then maybe I get your point. It is my fault I do not comprehend your posts. Let me think about it and maybe the light will shine.

[Borek]

Other substances can change solution pH. However, if you have two otherwise identical solutions that differ only by CO₂ concentration, and if both solutions are allowed to reach equilibrium (ie they are connected through air) you will end with both solutions having the same pH. That's the basic idea here - that the only parameter that is changing is CO₂ concentration.