In either case a solution is prepared containing bisulphite of lime, together with a certain proportion of free sulphurous acid, the object of the pulp manufacturer being to obtain a solution containing as large a proportion of free sulphurous acid as possible. The composition of a solution will vary on this account, and the following may be quoted as being an example of such a liquor:-- Free sulphurous acid 3·23 per cent. Combined sulphurous acid 0·77 " " ---- 4·00 " " ---- For experimental purposes the bisulphite of lime solution may be prepared by passing sulphurous acid gas into a mixture of water and sulphite of lime. The latter compound is insoluble in water, but gradually dissolves when the gas is absorbed. A known weight of sulphite of lime is added to a measured volume of water, and the sulphurous acid gas discharged into the mixture from a siphon of compressed sulphurous acid. The amount of gas absorbed is determined by weighing the siphon before and after use, the loss of weight representing the gas discharged. The following figures may be quoted as an example:-- Quantities used. Calcium sulphite 536 grammes. Water 7100 c.c. Gas absorbed 534 grammes. Density of solution 18° Twaddell. The composition of the solution prepared is-- Combined sulphurous acid 3·50 Free sulphurous acid 6·54 Lime 3·06 Water 86·90 ------ 100·00 ------ _Analysis._--The examination of sulphite liquors for free and combined sulphurous acid is made by means of standard iodine solution and normal caustic soda solution. A known volume of the sulphite liquor is first titrated with standard iodine solution, the number of cubic centimetres required being a measure of the total sulphurous acid. Each cubic centimetre standard iodine solution = ·0032 grammes SO_{2}. The titrated liquor is then treated with standard caustic soda in quantity sufficient to exactly neutralise the acid. The volume of caustic soda solution used minus the number of cubic centimetres of iodine first added is a measure of the free sulphurous acid.