4.15
Dialysis
Dialysis is a diffusion-based purification process that separates analyte molecules from a complex matrix. This is accomplished by allowing molecules…
Dialysis separates desired analyte ions or molecules from unwanted molecules, or 'interferents', by allowing the sample solution to diffuse through a semipermeable membrane inserted between two miscible liquids.
The membrane is made of cellulose acetate or cellulose nitrate, with pores ranging from 1 to 5 nanometers in diameter. As a result, only interferents smaller than the cut-off size of the membrane diffuse through it.
In this technique, the sample is placed in a dialysis bag, which is then sealed and immersed in a container of a solution, known as the dialysate, that contains a different concentration of the same interferent.
The difference in concentration creates a concentration gradient, allowing smaller inorganic ions and organic molecules to selectively permeate from high- to low-concentration areas until equilibrium is reached, while larger analyte molecules remain in the original solution.
Although dialysis is usually a slow process, the rate of molecular diffusion can be increased by heating the solution or reducing the membrane thickness.
View the full transcript and gain access to JoVE Core videos
Q1: How does dialysis separate desired molecules from unwanted interferents?
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.
Q2: What materials are used to make dialysis membranes and why?
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.
Q3: What role does concentration gradient play in the dialysis process?
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.
Q4: Why is dialysis considered a slow separation technique and how can it be accelerated?
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.
Q5: What types of molecules can pass through a dialysis membrane?
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.
Q6: How does dialysis compare to other equilibrium-based separation methods?
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.
Q7: What are the practical applications of dialysis 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.