| 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 SO2. 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.