1. n. 
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.