In the retail trade a form of soda is found which is adulterated with very large quantities of Glauber’s salt. This is recognised by the different form of the crystals. Manufacturers sell soda stating its strength. The colour maker should only buy with this guarantee.
Sodium Hydroxide (Sodium Hydrate, Caustic Soda), NaOH = 40, comes into commerce in the form of hard masses, the use of which would be very convenient for the colour maker if it were not often very impure. Thus it is better to prepare a solution oneself, which is accomplished in the manner given above for caustic potash.
Caustic soda and caustic potash have similar properties; they have a corrosive action on the skin, readily unite with carbonic acid from the air, and separate the heavy metals from their solutions in the form of hydrated oxides.
The strength of caustic soda solutions is given in the following table:—
| Specific Gravity | Caustic Soda per cent. | Specific Gravity | Caustic Soda per cent. |
|---|---|---|---|
| 2·00 | 77·8 | 1·40 | 29·0 |
| 1·85 | 63·6 | 1·36 | 26·0 |
| 1·72 | 53·8 | 1·32 | 23·0 |
| 1·63 | 46·6 | 1·29 | 19·0 |
| 1·56 | 41·2 | 1·23 | 16·0 |
| 1·50 | 36·8 | 1·18 | 13·0 |
| 1·47 | 34·0 | 1·12 | 9·0 |
| 1·44 | 31·0 | 1·06 | 4·7 |
Besides soda and caustic soda, few soda salts are used in colour making. However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently used instead of ordinary saltpetre, from which it differs in being deliquescent.
Sodium Thiosulphate (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a common article of commerce, being much used by photographers; it is used in a few cases in colour making. It forms large crystals with a somewhat bitter taste, permanent in air and readily soluble in water.
Sodium Chloride (Common Salt), NaCl = 58·5, which has a little application in colour making, is sufficiently pure in the form in which it is generally used for household purposes.
Salts of the Alkaline Earth Metals.
The metals which are known as the alkali earth metals have much similarity with the alkali metals in their compounds, with the difference that their alkalinity is much less, and that their salts are much less soluble in water. For colour making the compounds of three of these metals—calcium, barium and magnesium—are used.