Measuring co2 - I'm confused

[Carissa]

I've been thinking for two days about why I'm confused about this and how I can ask the questions that are unanswered in my mind about the whole thing. I think I'll start by asking this:

From what I understand, the co2 formula
CO2 = 3 * KH * 10(7-pH)

is reliable only if 1. you don't have any phosphate based buffers in the water and 2. you don't have anything else in the water that will affect pH.

So lets just say for the sake of argument, that somebody's water does NOT have phosphate based buffers (I don't understand how this works well enough to know how to get around it), but DOES have something to skew pH. Couldn't they just measure their offgassed sample of tank water (KH and pH), then assume 2 ppm of co2, then find the place on the chart where their particular pH lines up with 2 - 3ppm of co2, and use THAT KH instead to find their co2 values? Wouldn't that be close enough? I mean, they would probably only be off by 1ppm if that, with that assumption.

 

[Carissa]

Well, I just answered my own question. The values between 2 - 3ppm vary widely. So even if it would work, you would get quite a range of values. For instance, at a drop of 1.0, you get results anywhere from 19 - 50 based on what you are assuming as a starting point.

I have another question though. With a drop checker, everyone uses a 4KH reference solution. So why doesn't someone just post what the pH is on a 4KH sample without co2 injection and calibrate the chart to your own water that way? I suppose theoretically the pH should be different depending on how much co2 is in the atmosphere to start with in each location...but why not find out?

Even so...if one could make 4KH reference solution, without the drop checker, and test the pH; couldn't one figure out the co2 concentration in their location and calibrate the whole chart that way?

 

[VaughnH]

You can take extra care to accurately judge the green drop checker color (pH 6.6), which would say you have about 30 ppm of CO2 in the tank. Then measure the tank pH, and use that pH as a target to get back to 30 ppm of CO2. But, you would have to feel sure that the water remained the same, as far as anything affecting pH were concerned. So, you couldn't do this right after a water change, if you have wood in the tank - the tannins wouldn't have had time to stabilize in the water. You couldn't do this a couple of months without re-establishing the target pH, in case the water company changes the amount of phosphates they add to the water. And, about all it would help with is using a pH controller. I prefer just glancing at the drop checker to see if it is still green.

 

[Carissa]

Yes that's true, if the water changes obviously your whole calibration is out.

But how about this...couldn't one make 4KH reference solution or even any KH, doesn't matter; and then test it to find out the exact atmospheric equilibrium in your location, and use that method to determine the amount of co2 after offgassing a sample of tank water? You would need a pH test kit that is fairly accurate, to .1 probably, for it to be useful. But if you use a KH that's relatively easy to see the difference on the chart, you could be pretty accurate right? At least probably within 5 - 10ppm of the real amount of co2 in your tank? I'm not saying that this supercedes drop checkers, but for people who don't have one.

Or how about this as an alternative to figure out if your water "works" for the chart... take distilled water and add enough KH to bring it up to your tank's KH, then test pH on both your tank and this water. It should be very simple to compare two colors and see if there is any difference at all one way or another. If you can't tell the difference that way, you won't be able to tell on the chart either.

 

[VaughnH]

The second method works, but you already have the drop checker to be able to get both the tank and the 4 dKH at the same ppm of CO2, so why not just use the drop checker?

The first doesn't work, because it takes a long time for water to reach CO2 equillibrium with the room air - at least two days, unless you are stirring up the water to get surface turbulence. Then, the amount of CO2 in the air in a room of a house has to vary considerably due to human activity, cooking, smoking, etc. So, I don't see that working for that reason either.

I did some testing using very low KH distilled water in the drop checker, and in a small container, where I stuck the drop checker. So, both the drop checker and the container had the same dKH of distilled water in them. It works for measuring low ppm of CO2 in the container of water. I was blowing thru a straw into the container to add CO2 to reach a higher level to check how long it took for the drop checker to reach equillibrium with the container water. You can do several interesting things that way.

 

[Carissa]

Good points. I was just wondering if those methods would work. Knowing whether it would work helps me see if my understanding of the whole thing is correct.


I can get my tank water to offgas in about 5 minutes if I repeatedly shake it up though. But the point about varying levels of co2 is interesting.