You have a 5 mL sample of a protein in 0.5 M NaCl. You place...

You have a $5 \mathrm{mL}$ sample of a protein in $0.5 \mathrm{M}$ NaCl. You place the protein/salt sample inside dialysis tubing (see Fig. $2-14$ ) and place the bag in a large beaker of distilled water. If your goal is to remove as much $\mathrm{NaCl}$ from the sample as possible, which would be more effective: (1) placing the dialysis bag in 4 L of distilled water for $12 \mathrm{h},$ or (2) placing the bag in 1 L of distilled water for $6 \mathrm{h}$ and then in another 1 L of fresh distilled water for another $6 \mathrm{h} ?$

You have a $5 \mathrm{mL}$ sample of a protein in $0.5 \mathrm{M}$ NaCl. You place the protein/salt sample inside dialysis tubing (see Fig. $2-14$ ) and place the bag in a large beaker of distilled water. If your goal is to remove as much $\mathrm{NaCl}$ from the sample as possible, which would be more effective:
(1) placing the dialysis bag in 4 L of distilled water for $12 \mathrm{h},$ or (2) placing the bag in 1 L of distilled water for $6 \mathrm{h}$ and then in another 1 L of fresh distilled water for another $6 \mathrm{h} ?$

Key Concepts

Equilibrium and Dialysate Replacement

Equilibrium is reached when the concentration of solute becomes equal on both sides of the semi-permeable membrane, which can limit further removal of the unwanted molecule. Replacing the dialysate with fresh water prevents the establishment of equilibrium by continuously restoring the concentration gradient, thereby enhancing the net removal of the solute.

Effect of External Solution Volume

The volume of the external solution (dialysate) plays a crucial role in dialysis efficiency because a larger volume or the replacement of the dialysate dilutes the solute being removed. This dilution sustains the concentration gradient over time, driving more complete removal of the solute from the inside of the dialysis tubing.

Dialysis

Dialysis is a separation technique that relies on a semi-permeable membrane to allow small molecules to pass while retaining larger molecules. It is commonly used to remove unwanted small solutes or impurities from a mixture, such as removing salt from a protein solution, by taking advantage of the differential diffusion rates across the membrane.

Diffusion and Concentration Gradient

Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. In dialysis, the driving force behind the movement of small solutes like NaCl is the concentration gradient established between the solution inside the dialysis tubing and the external distilled water, facilitating continuous efflux of salt until equilibrium is reached.

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