bent wrote:
....
... But if it is acidic (low number on the pH
scale), by definiton it CANNOT be alkaline at the same time, or vice
versa: it cannot be alkaline and be acidic too.
....

Overall, yes, but regardless of the pH there are still both types of
ions present; simply in differing ratios.

The other poster's link gives a reasonable explanation of why the
testing is done for both in a pool altho under the Alkalinity section
there's some real voodoo "chemistry"...

"... High levels of alkalinity are lowered by the addition of an acid
(similar to pH). Experts recommend "pooling" the acid in a small area
of low current for a greater effect on alkalinity. That is, adding an
acid will lower both pH and alkalinity. Walking the acid around the
pool in a highly distributed manner is said to have a greater effect
lowering the pH than the alkalinity. Pooling the acid has the opposite
effect. ..."

That's a load of hooey for sure--pooling acid (or any other additive)
will have an disproportionate effect locally, but can have no effect on
the bulk properties--the same amount of acid or base will have the same
effect on the total average properties once the water is thoroughly
mixed as if it were distributed initially.

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