Simply put alkalinity is seen as a measure of a solutions' ability to neutralize added acid without a change in pH. This is quantified in terms of how much alkaline substance is dissolved in the solution. This is also known as "buffering capacity" or ability to resist becoming acidic.
Alkalinity does not refer to pH or basicity, though closely related as above, as a highly alkaline solution need not have a high pH. Pure water has a pH close to 7. Adding alkali to it will move it toward basicity or higher pH. However, the alkalinity of the water at that point, can remain unchanged even when the pH drops, for example from the formation of carbonic acid from co2.
Alkalinity is sort of weird to grasp since it seems to refer to a higher than 7 pH, however the easiest way to think of it is as the amount of carbonates, bicarbonates or other alkali substances dissolved in the water.
The resulting pH really is independent from those dissolved substances, though they do in fact raise the pH increasing basicity.
So, the pH shows the acidity or alkinity of a solution, but the measure of alkalinity is a separate quantity, the amount of dissolved alkali. Clear? As if...
More Alkalinity from Sheila Murphy...
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