What is diffusion pressure?why dp of pure solvent is higher than its solution?

Diffusion pressure

It is a hypothetical term coined by Meyer (1938) to denote the potential ability of the molecules or ions of any substance to diffuse from an area of their higher concentration to that of their lower concentration. Alternatively, it may also be defined as the force with which the diffusing molecules move along the concentration gradient.

Diffusion Pressure Deficit (DPD) or Suction Pressure (SP)

• The term diffusion pressure (DP) and diffusion pressure deficit (DPD) were putforth by B.S. Meyer in 1938. Originally, the DPD was described by the term suction force (Saugkraft) or suction pressure (SP) by Renner (1915). Now a days, the term water potential (y) is used which is equal to DPD, but negative in value.

• Each liquid has a specific diffusion pressure. Pure water or a pure solvent has the maximum diffusion pressure.

• If some solute dissolved in it, the water or solvent in the resulting solution comes to attain less diffusion pressure than that of the pure water or pure solvent. In other words, diffusion pressure of a solvent, in a solution is always lower than that in the pure solvent.

• 'The amount by which the diffusion pressure of water or solvent in a solution is lower than that of pure water or solvent is known as diffusion pressure deficit (DPD)'.

• Because of the presence of diffusion pressure deficit, a solution will always tend to make up the deficit by absorbing water. Hence, diffusion pressure deficit is the water absorbing capacity of a solution. Therefore, DPD can also be called suction pressure (SP).