The logarithmic pH scale

The pH scale is logarithmic. For dilute solutions, lowering pH by one corresponds approximately to a tenfold increase in hydrogen-ion concentration. A two-unit decrease corresponds to a hundredfold increase, not a doubling. A solution at pH 3 has about ten times the hydrogen-ion concentration of one at pH 4, and about one hundred times that of one at pH 5. This comparison concerns hydrogen ions, not total acid concentration in every kind of solution.