Originally Posted by Bunsen Honeydew
Are we talking about the same chart?
I am talking about this one.
kH and pH are the Y and X axes, respectively, so I don't understand how they would need to match up perfectly. I also don't understand why the other components would be meaningful, since this is governed by the carbonic acid dissociation equilibrium with respect to pH. LeChatelier's Principle is pretty straightforward (for chemistry
I think you fundamentally don't understand that chart, I was in your shoes 3 months ago so I'll try one more time to explain it. In my opinion(and many others) more people have been misled by that chart than it helped.
First as an approximation the chart assumes your starting pH degassed of your tank water has 3ppm CO2 in it and nothing else. Your water must nothing in it Carbonates for hardness which is difficult to know unless you use RODI. (This is the approximation that fails and makes the chart unuseable for most)
To give you some typical theoretical values for what your degassed pH (from your tap/tank before CO2 injection) should be in order for the chart to be accurate here are some:
Now lets look on that chart at each kH value for what your final pH(after CO2 injection) should be for exactly 30ppm of CO2.
Exactly a one point drop gives you 30ppm at all Kh values in the chart(which isn't accurate at low Kh but I won't get into that now).
The chart is calculated using the following formula 3*kh*10^(7-ph)=[Co2]ppm
I will reiterate again that the chart does not take into consideration your starting pH it assumes it based on 3ppm CO2 dissolved in water and no other hardness but Carbonates.
Now I will ask you a question based on the chart:
1) If you have kh=5 water and your pH is 7.2(not 7.7) and your pH drop is to 6.7 how much CO2 can you estimate you injected using the chart? (Do you think it is the same as a tank with starting ph of 7.7?).
2) How about my tank which has a degassed pH of 8.0 and a kh=5 and my pH drop is to 6.5 how much CO2 should I assume I injected into my tank from the chart?
If you can answer those two questions you have gone far beyond the simple calculations used to construct that chart.
You don't have to take my word for it perhaps @plantbrain
(Tom Barr here) might explain or you can go on barrreport.com and he might reply.