kh=7 co2=30ppm ph=6.94 - how?

[lmvt]

I am sure this is common type of question but I need a personalized answer.

I have started putting CO2 with a DIY system.
I have just connected the bottle to an airstone and put in the bottom of the aquarium. I have some surface agitation and did not expect it to be much effective.

At the end of the day I measured the CO2 concentration with a tetra kit - it gave me 32ppm. (I have now put the airstone halfway the aquarium height).

I measured teh Ph with an electronic Phmeter (in the lab where I work) and gave 6.94.

The water kH is 6 to 7. This, the pH and the Co2 content don't match according to tables circulating around.

I then measured the Ph of water from the aquarium that was aerated - 8.37. I measured the CO2 content and gave 6ppm.

I read somewhere that [CO2]=10^(pH-pHaerated-0.3) and this would match with the results obtained.

How is it possible to get this kH/pH relationship?
Do electonic pHmeter have some sort of problems with aquarium water (lots of buffers)?

How do you control CO2 concentration - through the kh/ph/co2 method or directly testing the CO2 concentration?

The other parameters are:
NO3 - 20mg/L
NO2 and NH4 - undetectable.

Thanks in advance.

Luis

[happychem]

It's been a long time since I've studied pH meters, but I'm pretty sure that they're made with a glass membrane made sensitive to changes in H+ concentration, an electochem. cell. Like I said, a long time, but I can't think of anything in aquarium water that would affect your readings.

That said, do you have a basic pH test kit in your home? How long was your water sample out of the tank for before analysis? Was it kept cool and dark?

I'm interested in your power-10 relationship, if you can remember where you read it, please post.

As for the rest, there are far more experienced people better equiped to answer your question. I don't have a CO2 test kit, so I don't know what it really measures and therefore I don't know what may be interfering.

[lmvt]

happychem:

I have some pH-indicator strips - they give me the values 6.8 and 8.1. I suppose the pH meter is working properly (it better be).

The sample was 6 hours out of the tank with an air pump aerating it. It was not particular cold/hot dark/bright.

As for the the power 10 relationship you can find it at:

http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

Luis

[happychem]

Thanks for the link.

I must have misread your initial post, I thought that you meant that the aquarium was aerated not the water.

I did look up pH electrode intereferences and under alkali metal (Na+, K+) concentrations 1M and up you could be measuring a lower pH than is really present. But I don't think that this is relevant to fw aquarium water. You'd need about 23g/L Na to start getting into trouble.

You shouldn't be surprised that the aerated water had a higher pH, when you aerated it, you drove CO2 concentrations down to atmospheric concentrations, bringing pH up.

As for water samples, if they're going to be isolated from the bulk for later analysis (like at your lab), they should be kept in the dark and in cool/cold storage. You want your samples to remain as representative as possible. This means preventing/minimizing biological activity. But it looks like your strips are doing as good a job as the pH meter, so why bother? Depending on the size of your sample, you don't need 6hrs to aerate. Probably only 10 minutes.