# Q & A: pH

Q:
What is pH in simple form?
- Emma (age 13)
Aberdour , London,England
A:

pH is a logarithmic scale that measures how concentrated the hydrogen ions (H+) are in solutions. (There are more general definitions, but they probably aren’t the ’simple form’ you’re looking for.)

pH is defined as the -log [H+], where [H+] is the molar concentration of H+, meaning the number of moles of H+ per liter of solution. In case you are unfamiliar with logarithms, Log of 10^x = log(10^x) = x, e.g. log(100)= 2, log(0.001)= -3.)

Even in pure water, some of the water molecules (H2O) fall apart into H+ and OH-. At room temperature in pure water [H+]= [OH-] = 10^-7 M, so pH=7.

Acids have more H+ (and less OH-) so they have pH <7. For example, 10^-3 M of HCl will give almost 10^-3 M of H+, so it will have pH of 3. Bases have more OH- and less H+, so they have pH >7.

You might wonder why a solution can’t be both acidic and basic- have lots of H+ and OH- ions. The reason is that if there were, most of them would just combine to form neutral water molecules. The product [H+] [OH-] stays at 10^-14 M^2 because of this chemical reaction.

Some pH values for common substances can be found here: http://www.aqion.de/site/191

(On terminology, by "strong hydrofluoric acid" this page must mean what we would usually call "concentrated hydrofluoric acid".)

Jason (w. Mike W)

(published on 10/22/2007)