Dialysis works on the principle of diffusion and ultrafiltration of solutes and fluid across a semipermeable membrane. It is based on the random, thermal movement of molecules in a solution and leads to a net movement from areas of higher concentration to areas of lower concentration until an equilibrium is reached. 

During a typical dialysis procedure, undesired molecules within a sample chamber diffuse through a semipermeable membrane into a separate chamber containing dialysate (buffer solution). While large-sized molecules that are unable to pass through the membrane pores, remain within the sample change, smaller molecules easily diffuse across the membrane, reaching equilibrium throughout the solution volume. As a result, the concentration of these small molecules within the sample is effectively reduced.