The strength of a solution of acetic acid cannot be found by a simple estimation of specific gravity, since the density does not increase with the percentage of acetic acid. If an accurate estimation of the strength of acetic acid is required, it must be obtained by neutralising the acid with an alkali by a process of volumetric analysis.
For practical purposes, where it is generally known whether a very strong or a more dilute acetic acid is under consideration, the following table, showing the connection between specific gravity and percentage strength, is sufficient.
| Specific Gravity | Acetic Acid per cent. | Specific Gravity | Acetic Acid per cent. |
|---|---|---|---|
| 1·0635 | 100 | 1·058 | 48 |
| 1·0670 | 98 | 1·055 | 46 |
| 1·0690 | 96 | 1·054 | 44 |
| 1·0706 | 94 | 1·052 | 42 |
| 1·0716 | 92 | 1·051 | 40 |
| 1·0730 | 90 | 1·049 | 38 |
| 1·0730 | 88 | 1·047 | 36 |
| 1·0730 | 86 | 1·045 | 34 |
| 1·0730 | 84 | 1·042 | 32 |
| 1·0730 | 82 | 1·040 | 30 |
| 1·0735 | 80 | 1·038 | 28 |
| 1·0732 | 78 | 1·035 | 26 |
| 1·0720 | 74 | 1·031 | 22 |
| 1·0710 | 72 | 1·027 | 20 |
| 1·0700 | 70 | 1·025 | 18 |
| 1·0700 | 68 | 1·023 | 16 |
| 1·0690 | 66 | 1·020 | 14 |
| 1·0680 | 64 | 1·017 | 12 |
| 1·0670 | 62 | 1·015 | 10 |
| 1·0670 | 60 | 1·012 | 8 |
| 1·0660 | 58 | 1·008 | 6 |
| 1·0640 | 56 | 1·005 | 4 |
| 1·0630 | 54 | 1·002 | 2 |
| 1·0620 | 52 | 1·001 | 1 |
| 1·0600 | 50 | —— | —— |
Oxalic Acid, C₂H₂O₄.2H₂O = 126, has but a limited use in colour making. It comes into commerce in the form of more or less pure white crystals which readily dissolve in water, and are almost pure oxalic acid, containing only small quantities of oxalate of lime, the presence of which is without importance for the purposes to which the acid is put in colour making. Frequently, instead of oxalic acid, the acid potassium oxalate (salt of sorrel) is used.
Tartaric Acid, C₄H₆O₆ = 150, occurs as white or yellowish crystals, with a slightly burnt smell, which dissolve readily in water, and have a strong acid taste. The pure acid, which is white and without smell, is considerably dearer than the yellow variety. The impurities of the latter, which are small in quantity, are without influence on the colours prepared by its help, so that this form is generally used.
CHAPTER V.
METALLIC COMPOUNDS.
Alkalis.
The compounds of the alkali metals, potassium and sodium, play a considerable part in colour making. Formerly the potassium compounds were in general use, but the sodium compounds are at present obtainable at a much lower price, and in most cases they can be used equally well. Thus, in colour making, sodium compounds are chiefly employed. The cyanogen compounds are an exception; their potassium compounds are used exclusively.
Potassium Compounds.—The potassium compounds which are chiefly used in colour making are potassium carbonate (potashes, pearl-ash), potassium hydroxide (caustic potash), potassium nitrate (saltpetre), potassium tartrate (tartar), and potassium ferrocyanide and ferricyanide (yellow and red prussiate of potash). The cyanogen compounds have peculiar properties. We shall describe them separately after the potassium and sodium compounds.
Potassium Carbonate (carbonate of potash), K₂CO₃ = 138, is known commercially as potashes, a name derived from its former method of preparation by heating the ashes of plants in pots. At present potashes are prepared in large quantities from other sources.