A solution of pH = 8.1 will have an [H+] concentration of 10−8.1 mol/L, or about 6.31 Ã? 10−9 mol/L. Thus, its hydrogen activity αH+ is around 6.31 Ã? 10−9. A solution with an [H+] concentration of 4.5 Ã? 10−4 mol/L will have a pH value of 3.35.

In solution at 25 °C, a pH of 7 indicates neutrality, because water naturally dissociates into H+ and OH− ions with equal concentrations of 1Ã?10−7 mol/L. A lower pH value (for example pH 3) indicates increasing strength of acidity, and a higher pH value (for example pH 11) indicates increasing strength of basicity. Note, however, that pure water, when exposed to the atmosphere, will take in carbon dioxide, some of which reacts with water to form carbonic acid and H+, thereby lowering the pH to about 5.7.