A unit to measure the degree of acidity or alkalinity of a liquid. A neutral solution, such as pure water, has a pH of 7. Acidic solutions have a pH less than 7. Alkaline solutions have a pH greater than 7.
Source: API BULLETIN E3, Environmental Guidance Document: Well Abandonment and Inactive Well Practices for U.S. Exploration and Production Operations, First Edition, January 1993 (Reaffirmed June 2000). Global Standards
The term “pH” denotes the negative logarithm of the hydrogen ion, H+, activity in aqueous solutions (activity and concentration are equal only in dilute solutions): pH = log [H+]. For pure water at 24 °C (75 °F) the hydrogen ion activity [H+] is 10-7 mol/l and pH 7. This system is termed “neutral” because the hydroxyl ion activity [OH ] is also 10-7 mol/l. In aqueous systems at 24 °C (75 °F) the ion product, [H+] [OH–], is 10-14 (a constant). Consequently, an increase in H+denotes a like decrease in [OH–]. A change in pH of one unit indicates a ten-fold change in both [H+] and [OH–]. Solutions with pH less than 7 are termed “acidic” and those with pH greater than 7 are termed “basic” or “alkaline”.
Source: API RP 13B-1, Recommended Practice for Field Testing Water-based Drilling Fluids, Fourth Edition, March 2009. Global Standards
Source: API SPEC 17F, Specification for Subsea Production Control Systems, Second Edition, December 2006 (Reaffirmed April 2011). Global Standards