A drilling-fluid test to measure pH of muds and mud filtrates, usually performed according to API specifications. The pH test uses a pH meter equipped with a glass-membrane measuring electrode and reference electrode, which read from 0 to 14. The preferred pH meter automatically compensates for temperature. Buffer solutions of pH = 4, 7 and 10 are specified for calibration of the meter. Color-matching pH paper and sticks are not recommended except for simple muds.