Question

What is phenolphthalein alkalinity? How is it related to the hydroxide and carbonate levels in water?

Answer 1

See below.

Explanation:

Alkalinity

Alkalinity is a measure of the capacity of water to neutralize acids.

Alkalinity in water is caused primarily to the presence of carbonate, hydrogen carbonate, and hydroxide ions.

Phenolphthalein alkalinity

Phenolphthalein alkalinity is a specific category of alkalinity.

It is determined by titrating to pH 8.3 (where phenolphthalein changes colour).

If you are titrating `"OH"^"-"` with a strong acid, the titration curve looks like this.

Thus, all of the `"OH"^"-"` is neutralized at the phenolphthalein endpoint·

If you are titrating `"CO"_3^"2-"` with a strong acid, the titration curve looks like this.

Since `"CO"_3^"2-"` is dibasic, only half of it is neutralized at the phenolphthalein endpoint.

Thus, phenolphthalein alkalinity gives the total hydroxide and half the carbonate level.