Tonicity of Solutions

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Cell walls in living organisms are seldom perfectly semipermeable. Usually they permit passage to some of the simpler substances and then these substances fail to register an osmotic pressure contribution on the cell walls. The reduced osmotic pressure of a solution operating on a particular cell membrane is called the tone of the solution with respect to such membrane.

Tone of a solution is not a purely colligative property.

Red blood cells are covered by a wall, permeable to some of the simple molecular species present in blood though not to the complex ones.

The solutions, in which blood cells retain their normal form, are said to be isotonic with blood e.g., normal saline used by physicians has about 0.9% NaCl and has a tone of 7.65 atmospheres against red blood corpuscles at the body temperature.

Solutions in which blood cells shrink have higher tone than blood and are termed hypertonic solutions. Solutions from which blood cells can extract water are said to be hypotonic.