A. Chemical Functional Groups organized by pKa (y-axis) and acidic atom (x-axis: oxygen, nitrogen, carbon, other.) B. Key for using A given a solution pH. First, mark the position of the solution pH on the pKa axis (dotted horizontal line). all functional groups above it are neutral or positively charged while all functional groups below it are negative or neutral. Second, the pKa axis is useful in further categorizing functional groups by their ability to participate in hydrogen bonds.