A pH scale is used as an indicator of the hydronium ion (H₃O^₊) concentration in an aqueous solution. And a pOH scale is used as an indicator of the hydroxide ion (OH^-) concentration in an aqueous solution.

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Water has a weak tendency to dissociate into ions - the hydronium ion (H₃O^₊) and the hydroxide ion (OH^-). The tendency of water to form these ions is so weak that the concentrations of ions in solution is very small.  A pH scale is used as an indicator of the hydronium ion concentration. It is a logarithmic scale that is commonly used in science and math to express the magnitude of very large and very small numbers. Because the pH scale is a logarithmic scale, a low pH value is an indicator of a higher hydronium ion concentration. The equation that expresses the relationship between pH and hydronium ion (H₃O^₊) is ...
 

pH = - log ([H₃O^₊])

To determine the hydronium ion concentration ([H₃O^₊]) from a pH value requires some algebra performed on the above equation. The result of the algebra manipulation is the following equation.

[H₃O^₊] = 10^-pH

A pOH scale is also used to indicate the concentration of the other ion - the hydroxide ion (OH^-).  The equations relating the pOH and the [OH^-] are similar to those for the pH scale. They are  ....

pOH = - log ([OH^-])


 [OH^-]= 10^-pOH

At a temperature of 25°C, the product of these two ion concentrations is approximately 1.0 x 10^-14. This fact is the basis of the pH-pOH relationship, at least for temperatures of 25°C. The pH value and the pOH value add up to 14. 

pH + pOH = 14 (at 25°C)

A neutral solution has the same pH value as its pOH value. So a pH of 7 corresponds to a neutral solution. Any pH value less than 7 corresponds to an acidic solution and any pH value greater than 7 corresponds to a basic solution.