I have to find the effective pH range of a buffer. I read that this optimal buffering occurs when the pH is within 1 pH unit from the pK value for the buffering system. But can anyone explain why its is optimal within 1 pH unit? Thanks.
The pH of a acid/base pair equals the pKa value of the acid when base/acid ratio is 1.0 (therefore log 1.0 = 0 and pH = pKa). As acid or base is added to the buffer the ratio changes slightly. If one plots the buffer capacity of a buffer on the y axis vs the cncn base/acid ratio, one gets a bell shaped curve with minimum deviation at ratio of 1.0 but declines on both sides of the ratio of 1.0. Therefore, the maximum buffering capacity can be obtained when the pKa = desired pH. You can read more about it here.
http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity