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Oilfield Glossary
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methyl orange alkalinity

1. n. [Drilling Fluids]
A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid and methyl orange indicator. The alkalinity test is a well-known water-analysis procedure to estimate hydroxyl ion, 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. Mf therefore represents the amount of acid required, per unit volume of the sample, to reduce the pH to 4.3. 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.