The distance from the electrode where the ratio \[\frac{c\,-\,c_{\text{e}}}{c_{0}\,-\,c_{\text{e}}}\] reaches a given value. (\(c_{\text{e}}\) is the @I03083@, \(c\) is the concentration, \(c_{0}\) is the value in the bulk solution.) If this ratio is selected as, say, 0.99 the corresponding thickness of the @D01716@ layer is denoted by \(\delta_{0.99}\).