Hydrogen ion concentration results from the addition of an acid to a substance. Since we know Marco's pH is 6.1, we can easily calculate the hydrogen ion concentration. The formula for pH is: pH = -log [H+]. Therefore, at pH = 5.5, [H+] = 10-5.5. This results in 3.16 x 10-6 M or [H+] = 0.0000032 M.