In the laboratory there are several routes to measure pH, the simplest is using litmus paper, a piece of paper impregnated with chemicals which colour above or below a known pH value. However, more accurately, a calibrated pH glass electrode is used in conjunction with a standard reference electrode. The pH electrode contains a thin glass membrane containing Na+ ions which, when placed in a liquid solution, exchange protons on the surface. H+(aq) + Na+(glass) = H+(glass) + Na+(aq) The exchange of protons and sodium means a charge is built up on the surface, which is dependent on the proton concentration. This charge is measured by an electric potential with respect to a known reference electrode potential by a laboratory pH meter. By routinely calibrating the pH meter, using suitable solutions of known composition, reliable measurements with an accuracy of 0.01 pH units can be obtained.