Co-ions are as essential in biological systems as they are ignored. The purpose of this letter is to demonstrate the importance of including ionic dispersion potentials acting between ions and interfaces in any realistic theoretical modeling of biological systems. We demonstrate through a well-known biological example that co-ion effects can be understood once these previously ignored forces are included. Experiments have in the past revealed that addition of salt solutions with different co-ions give fundamentally different results for the formation of meta 2 rhodopsin (which is involved in dim light vision). For systems with low salt concentrations, addition of salt favors the formation of meta 1 rhodopsin. Exactly the opposite is observed in high-concentration salt solutions. This is true even after surface pH. effects have been screened out with the addition of 0.5 M sodium acetate buffer. A theoretical explanation for the role of co-ions behind this effect is here given in terms of ionic dispersion potentials and ion specific surface pH.