A process for reducing TAN of hydrocarbon feed oil, comprising: a. combining alkaline earth metal, in an aqueous solution having a pH of 10 to 14, with the hydrocarbon feed oil to make a mixture; b. vigorously mixing the mixture; c. precipitating a metal-naphthenate salt; and d. removing the metal-naphthenate salt to reduce the TAN by 50 to 100%; wherein a weight ratio of the hydrocarbon feed oil to the aqueous solution is from 0.25:1 to 20:1. Also, a process for reducing TAN, comprising: a. combining an inorganic alkaline earth metal salt having a water solubility at 20° C. from 35 to 150 g/100 g water, in an aqueous solution having a pH of 10 to 14, to make a mixture; b. precipitating a metal-naphthenate salt; c. collecting the metal-naphthenate salt at an interface between an oil phase and an aqueous phase; and d. removing the metal-naphthenate salt.