Every aqueous solution is either acidic, basic or neutral.
There is a quantitative relationship between the concentration of hydronium and hydroxide ions in the solution. The brackets as usual denote molar concentrations. The pH scale is a numerical scale which, for most applications extends from 0 through to 14. The numbers on the scale represent the relative acidity of solutions and can be converted into actual hydronium ion concentrations. The pH scale is based on the self-ionization of pure water.