pH = -log[H
3O
+]
[H
3O
+] = 10
–3 pH = 3
[H
3O
+] = 10
–5 pH = 5
[H
3O
+] = 10
–pH is a number that
is a property of a solution.
pH is related to the concentration of H3O+ (hydronium ion) in the solution.
pH is measured with a pH meter or pH paper.
pH depends on which acids and which bases are present, and on their concentrations.
Water is the reference compound for defining a solution as either acidic or basic.
[H
3O
+] in water is 10
–7 . Thus the pH of pure water is 7. Water is sometimes said to be pH neutral.
The numbers shown highlight why pH is used to describe [H
3O
+]. Typically, [H
3O
+] is very small
It is easier to talk about and compare numbers like 7 and 8.6 than the corresponding [H
3O
+] (10
–7 and
have a higher [H
3O
+] than water.
The negative sign in the definition of pH means that acidic solutions have a lower pH than water.
The pH of acidic solutions is less than 7.
Solutions classed as basic have a lower [H
3O
+] than water.
The negative sign in the definition of pH means that basic solutions have a higher pH than water.
The pH of basic solutions is greater than 7.