Dialysis

Dialysis uses a semipermeable membrane to separate molecules based on size. The sample is placed in a dialysis bag immersed in a dialysate solution with different concentration. Smaller interferent molecules diffuse through membrane pores (1-5 nanometers) from high to low concentration, while larger analyte molecules remain in the original solution until equilibrium is reached.

Dialysis membranes are typically made from cellulose acetate or cellulose nitrate. These materials create pores ranging from 1 to 5 nanometers in diameter, allowing selective permeation of small ions and organic molecules while retaining larger proteins, hormones, and enzymes based on molecular size.

The concentration gradient, created by differences in analyte concentration across the dialysis membrane, provides the driving force for molecular diffusion. Smaller molecules migrate from areas of high concentration to low concentration until equilibrium is reached, while larger molecules cannot cross the membrane pores.

Dialysis is slow because it relies on equilibrium diffusion across the membrane. The rate of molecular diffusion can be increased by raising the temperature or decreasing membrane thickness. Multiple dialysis cycles are often needed to reduce unwanted small molecule concentrations to acceptable levels.

Small inorganic ions such as chloride and sodium, and organic molecules like glucose can pass through dialysis membrane pores. Larger molecules including proteins, hormones, and enzymes with diameters significantly greater than the pore diameter are retained in the original solution.

Like recrystallization solid ndash solution equilibria, dialysis relies on equilibrium principles to separate components. However, dialysis uses membrane permeability and concentration gradients rather than solubility differences. Both techniques may require repetition to achieve desired purity levels and are commonly used in analytical chemistry.

Dialysis is an inexpensive technique commonly used to purify proteins, hormones, and enzymes from complex matrices. It effectively removes small interfering ions and molecules while preserving larger analytes, making it valuable for sample preparation in analytical and biochemical work. Dialysis can be performed overnight or over several days.