1103170801

The acidity of a substance is measured by its $pH$ value. In practice, $pH$ is often assumed to be the negative logarithm of the hydrogen ion concentration $H^{+}$ (i.e. the number of moles of hydrogen ions per liter of solution): $$pH=-\log (H^{+}).$$ In one of the following pictures is a part of the $pH-H^{+}$ graph. Choose the picture.