Joined
·
4,334 Posts
I'm also becoming a little suspect of the KH/CO2 relationship. There are two factors that may influence this. Not all of the acid in the tank is coming from carbonic acid. In non-planted tanks the pH gradually moves downward over time as organic acids accumulate. Water that started at pH 7.0 with no added CO2 would read 6.7 or 6.8 after a while. I'm not sure that simply adding plants 'soaks up' all this organic acid.
The second factor (being discussed above) is the buffering capacity of the water. As I understand the test kits that most of us use, the actual chemical reaction is a simple titration. An acid is added drop by drop until the pH in the tube is low enough to overcome the buffer that was there and the indicator turns colors. The test kit doesn't 'know' if the buffer is CO3, PO4 or, or something else. The reported value is expressed as an 'equivalent carbonate' alkalinity.
As you can clearly see, I know just enough about the chemistry of this to be entirely dangerous. I look at it this way: If you look at the chart for KH = 2 it takes a pH drop of 7.1 to 6.3 (0.8 units) to go from 5 ppm to 30 ppm. At KH = 4 it takes a drop of 7.4 to 6.6 (0.8 units). At KH = 8 it takes a drop of 7.7 to 6.9 (0.8 units).
I let some water degas, read the pH, and turn up the bubbles until I've achieved a pH 0.8 units below what I read. I assume (correctly or incorrectly) that the resultant drop is entirely due to carbonic acid. I freely admit that the chart is only accurate if carbonate buffering is controlling.
If you have a KH of 8 and your degassed pH isn't around 7.7 then something else is living in the water. My water tends to read lower than it should assuming a degassed CO2 of 2 or 3 ppm and a given KH. I attribute this to other organic acids and/or inaccuracies in my KH test kit.
.... just a few random thoughts on the subject. This thread is interesting and needs more input from the true 'chemists'.
The second factor (being discussed above) is the buffering capacity of the water. As I understand the test kits that most of us use, the actual chemical reaction is a simple titration. An acid is added drop by drop until the pH in the tube is low enough to overcome the buffer that was there and the indicator turns colors. The test kit doesn't 'know' if the buffer is CO3, PO4 or, or something else. The reported value is expressed as an 'equivalent carbonate' alkalinity.
As you can clearly see, I know just enough about the chemistry of this to be entirely dangerous. I look at it this way: If you look at the chart for KH = 2 it takes a pH drop of 7.1 to 6.3 (0.8 units) to go from 5 ppm to 30 ppm. At KH = 4 it takes a drop of 7.4 to 6.6 (0.8 units). At KH = 8 it takes a drop of 7.7 to 6.9 (0.8 units).
I let some water degas, read the pH, and turn up the bubbles until I've achieved a pH 0.8 units below what I read. I assume (correctly or incorrectly) that the resultant drop is entirely due to carbonic acid. I freely admit that the chart is only accurate if carbonate buffering is controlling.
If you have a KH of 8 and your degassed pH isn't around 7.7 then something else is living in the water. My water tends to read lower than it should assuming a degassed CO2 of 2 or 3 ppm and a given KH. I attribute this to other organic acids and/or inaccuracies in my KH test kit.
.... just a few random thoughts on the subject. This thread is interesting and needs more input from the true 'chemists'.