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Accuracy of ph/kh relation for CO2...

6K views 28 replies 10 participants last post by  JerseyScape 
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'.
 
Dennis,

I've been thinking 'bout the PO4 effect on the relationship. Once upon a time I might have had a better grasp of the chemistry involved. PO4 has a pK (maximum buffering capacity) at 7.21. CO3 has its pK at 6.37. The range of most people's tanks is somewhere in this vicinity so one buffering system shouldn't be that much more powerful than the other (in an aquarium at pH 7.21 the phosphate would have its maximal effect).

Assuming they have nearly equal capacity to buffer, the impact of each should be proportional to its concentration. KH 4 (if from carbonate) corresponds to 71 ppm CO3. If you have a PO4 concentration of 5 ppm it's still roughly 14 times weaker than the CO3 buffer. I'm guessing you'd need truly colossal PO4 levels (some people do) to really mess with the relationship.

I kind of think the bigger error is the organic acid in most aquariums. This would lead people to think they have more CO2 than they really do. The 'chart' only works if the resultant carbonic acid is the only acid present.
 
For comparison I set out some water and tested it. My aquarium is usually at 6.60 depending on the exact bubble rate (it fluctuates a little for some reason). My 'degassed' water tested at pH 7.15. With a KH of 4.5 it should have been 7.6 or so. I'm assuming 'other' acids are accounting for the difference.

My total pH drop due to CO2 is 7.15-6.60 = 0.55. From the chart at KH 4.5 this means I probably only have about 24 or 25 ppm CO2 right now. I get this at 120 to 130 bpm injected into the intake of an Eheim filter in a 46 gal aquarium.

I gotta go turn up my CO2, bye.

- GB
 
MatPat, it's possible that your tap water comes directly from a city well without ever being exposed to the atmosphere, therefore having only 1ppm CO2. An increase to 4 ppm isn't that unusual since the atmosphere contains about that amount. I will agree, though, that the large pH shift is a little hard to understand. 3ppm CO2 shouldn't make a big difference with your KH. Water companies add all sorts of things though....

Acid in a tank comes from more sources than just driftwood. Huge quantities come from the fish as they metabolize. Uneaten food also breaks down in processes that produce organic acids. I strongly suspect that most people have less CO2 than they think they do, wrongly assuming that the only acid in their aquarium is carbonic acid. I also suspect that you could go up to 100 ppm CO2 or so without harming the fish provided the pH itself doesn't get too low. Am I willing to try it in my tank? No, but others report huge CO2 levels and no apparent effects. The kidneys & gills of the fish do eventually get overwhelmed at a certain acidity depending on the species.

Now, if I could just make the algae go away.............
 
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