A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid. This test is a well-known water-analysis procedure to estimate hydroxyl, carbonate ion and bicarbonate ion concentrations. There are two pH endpoints, P and M, in this titration, corresponding to phenolphthalein and methyl orange indicators. The "P" endpoint is at pH 8. 3 and the "M" endpoint is at pH 4. 3. Each is reported in units of cm³ acid/cm³ sample. For water samples and very simple mud filtrates, P and M data indicate OH^-, HCO₃^- and CO₃^-2 concentrations, but an alkalinity test is unreliable for analyzing complex mud filtrates. The API has established standards for conducting alkalinity tests.