Science, Tech, Math › Science Chemistry Terminology: Definition of pOH How pOH values relate to acidity and basicity Share Flipboard Email Print Nicola Tree / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated October 25, 2019 pOH is a measure of hydroxide ion (OH-) concentration. It is used to express the alkalinity of a solution. Aqueous solutions at 25 degrees Celcius with pOH less than 7 are alkaline, pOH greater than 7 are acidic and pOH equal to 7 are neutral. How to Calculate pOH pOH is calculated based on pH or hydrogen ion concentration ([H+]). Hydroxide ion concentration and hydrogen ion concentration are related: [OH-] = Kw / [H+] Kw is the self-ionization constant of water. Taking the logarithm of both sides of the equation: pOH = pKw - pH An approximation is that: pOH = 14 - pH While the approximation works well in many settings, there are exceptions for which the pKw value should be used instead.