A term applied to solutions which have the same power of diffusion as one another. An isotonic solution used in medicine is one which can be mixed with body fluids without causing any disturbance. An isotonic saline solution for injection into the blood, so that it may possess the same osmotic pressure (see OSMOSIS) as the blood SERUM, is one of 0.9 per cent strength – that is, containing 9 grams of sodium chloride to 1 litre of water. This is also known as normal or physiological salt solution. An isotonic solution of bicarbonate of soda for injection into the blood is one of 1.35 per cent strength in water. An isotonic solution of glucose for injection into the blood is one of 5 per cent strength in water.

Solutions which are weaker, or stronger, than the fluids of the body with which they are intended to be mixed are known as hypotonic and hypertonic, respectively.