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Should be: CO2 = 2 times 10 to the a-b power. If the pH drop due to adding CO2 is 1.0 you have ten times the amount of CO2 that you had before adding the CO2. The only problem is that the amount you had before adding the CO2 is unknown.

Shaking a sample, or letting it sit in the open for a couple of days, or however you want to try to reduce the amount of CO2 to what it is in equilibrium with the atmosphere, doesn't give you the same result every time you try it. It can give from about 0.4 to about 4 ppm of CO2. Obviously ten times that is 4 to 40, not an accurate measure of CO2.

You can do some experimenting with this: start with distilled or deionized water. Add a tiny amount of baking soda to it to get a KH you can measure. Then stick a piece of airline tubing in it and blow bubbles for ten seconds or so. Now, start monitoring the pH of that water, measuring it every couple of hours or so. When I have tried this, the pH continues to rise for more than two days. Try it by shaking the solution, without sloshing it out. A problem is that the water starts evaporating as soon as you set it out in the open. That increases the KH. So, you have to monitor KH too.
 

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Thank you hoppycalif!
I'm aware to the fact that this method of measuring CO2 is not accurate, it is meant mostly for someone who doesn't know his KH level.
It isn't that it is inaccurate, it just doesn't give a usable result. If the probable error range gives you a value of 4 to 40 for the ppm of CO2 you haven't learned anything by making this measurement. At least the drop checker method gets you to a range of around 20 to 40, and is a very cheap method to use.

I apologize for raining on this idea, but I think it is important enough to know about how much CO2 you have in the water to make the effort to measure it with a usable method. I have expressed my opinion now, so I will stop belaboring the point.
 
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