SATURATION, a term applied in chemistry to denote two widely different phenomena : namely, first to the solution of the greatest possible quantity of any substance in a liquid medium ; and, secondly, to the neutralisation of a base by an acid or of an acid by a base.
When common salt, and indeed most other saline and many vegetable Iodises, are added to water until it ceases to dissolve them, the solution se obtained is termed a saturated solution of the substance dissolved.
Saturation of this kind may exist with regard to one body and not to another : thus water saturated with common salt will still dissolve sulphate of soda and rice rersd ; so also a saturated solution of com mon salt will dissolve sugar. The saturating power of bodies is in many cases greatly influenced by heat, while in others variations of temperature produce but little effect : thus cold water will take up nearly or much common malt as hot water ; but sulphate of soda is more soluble In hot water than in cold, and hence it is that a saturated hot solution of this and many other salts deposits crystals on cooling. Cold water, on the contrary, dissolves more lime than hot, and a saturated solution prepared with water at about 32° holds nearly twice as much lime in solution as one prepared at 212°, and when the cold prepared solution is heated lime Is deposited. This, however, is a case of touch rarer occurrence than the contrary one.
As instances of the second kind of saturation, the following may bo ailduced :—If to a solution of carbonate of potash any strong acid, such as the sulphuric, be added until effervescence ceases, the potash is said to be saturated. In like manner, if a solution of caustic soda be added to nitric acid until the latter be exactly neutralised, the acid is said to be saturated. In these cases the point of saturation is determined by the use of papers stained with different vegetable colours ; if, for example, too much carbonate of potash should have been added to the nitric acid to saturate it, its presence will be indicated by turning paper coloured yellow with turmeric, brown ; while, on the other hand, excess of acids is in general ascertained by paper stained blue with litmus, which is rendered red by the action of acids.
By these moans a very important process in the manufacture of soap and glass is conducted ; it is termed alkatimetry, and employed for ascertaining the strength of different samples of the carbonates of potash and soda, so largely used in glass- and soap-making. Sulphtule acid diluted to a known extent is added to the alkaline solutions, and when they affect neither blue nor yellow paper, the saturation is perfect, and the purity and strength of the alkalies are determined.
LALKALTMIZTli