Originally Posted by Maryland Guppy
dKH for carbonate hardness/alkalinity.
This is correct.
Alkalinity and PH obviously have a direct relationship. However, as you've noticed there are other buffers such as phosphates. The KH measure hobbyists use is more of an approximation of buffer capacity. It's not an actual measure of buffering capacity.
KH is a measure of calcium carbonate (typically CaCO equivalent) per liter of water, mg/l CaCO.
Buffer capacity is a measure of how much strong acid or base it takes to change PH one unit (up or down), moles/L.
Clearly two entirely separate measures. However, the predominant buffer found in water are carbonates. When we have a measure of carbonates we have a relatively close approximation to the minimum
When I check the TDS of a strong buffer solution (calibration solution for example) it is over 1000, whereas untreated water is under 100 (4.4 pH well water). Based on this data, one might surmise that there is a correlation between TDS reading and buffering capacity. This is logical because buffers are composed of conjugate acid/base pairs, which are made up of anions which register on the TDS meter? Does that sound right?
There is a correlation between TDS and buffering capacity just like there's a correlation between the planet's gravity and coffee cups. If we added more cups (mass) the gravity increases which would indicate we could calculate the number of coffee cups based on current gravity right? Of course not!
In fact, PH is actually more of a theoretical measure anyway. Your PH pen does not exactly equal PH, [-log(H+)]. There is an effective hydrogen concentration to consider. Therefore, many present PH calculation as, PH = -log(H+ activity). This will correct any issues in calculations, right?. Unfortunately, principles of thermodynamics state that the activity of one kind of ion (e.g., H+) is unknowable. This invalidates the second formula leaving the original flawed based on ion activity.
I'm not trying to confuse you by pointing out the problems with PH. Instead, I am trying to illustrate that our aquariums are not theoretical models. Far too many people get tied up in the fine details and never look up to see the horizon ahead. Hobbyists should not have to be concerned with thermodynamics, chemistry, physics and god knows what else.