Lead Nitrate, Pb(NO₃)₂.—This very important compound may be bought, but it is advisable to prepare it in the works. Water is placed in a wooden tub, then half the volume of nitric acid is added, and finely powdered litharge gradually stirred in, the liquid being kept in constant movement. When it is seen that the litharge is only slowly dissolved, the liquid is well stirred after each addition of litharge and then tested by litmus paper. When this is no longer reddened, the nitric acid is completely saturated, and the liquid contains only lead nitrate in solution. It is allowed to stand until the insoluble portions have settled, and then drawn off into another vessel where crystals of lead nitrate separate in a few days. If the salt be required in solid form, the solution may be evaporated in earthenware dishes; generally the solution is used as it is obtained.
Pure lead nitrate forms white crystals which are not particularly soluble in water, 1 part requiring 2 parts of water at the ordinary temperature. Lead nitrate is decomposed on heating, like all nitrates, and litharge remains. The solution of this salt is used in the preparation of those lead pigments which are obtained by precipitation, for example, chrome yellow.
Lead Acetate, Pb(C₂H₃O₂)₂.3H₂O.—The compounds of lead with acetic acid are of great importance. Two of these are to be considered: neutral lead acetate, commonly known as sugar of lead, and basic lead acetate. It may be advisable to manufacture both these compounds, the latter always. Neutral lead acetate comes into commerce in the form of colourless heavy crystals, which are often covered by a white powder of the basic acetate; they dissolve readily in water, and the solution has a sweetish taste, hence the name “sugar of lead”. Frequently the solution is very turbid; this is caused by the carbonates contained in the water. The turbidity may be removed by the addition of a little acetic acid. It is only economical for the colour maker to prepare sugar of lead when he can obtain cheap raw materials, lead or litharge and vinegar. Pyroligneous acid may also be used if it is colourless, its odour being without importance for this purpose.
The best method for preparing lead acetate from litharge is to place the vinegar in a tub and hang in it a strong linen bag filled with finely ground litharge. The tub is kept covered for a few days and its contents then tested with red litmus paper. When this is turned blue, the liquid is drawn off and vinegar gradually added whilst stirring, until blue litmus paper is just turned red. In this process, after neutral lead acetate has been formed, more lead oxide is dissolved, and the liquid thus acquires an alkaline reaction. The further addition of acetic acid reconverts the basic salt into neutral lead acetate.
Lead acetate solution may, with advantage, be prepared directly from metallic lead. For this purpose, lead is granulated by melting and pouring in a thin stream into cold water, where it solidifies in irregular pieces. This is done in order to give the lead as large a surface as possible. Three high narrow tubs placed one above the other so that liquid may flow from the highest to the middle, and from this into the lowest, are filled with granulated lead. Vinegar is placed in the uppermost vessel to cover the lead; after twenty-four hours it is allowed to flow into the middle, and after a further twenty-four hours into the lowest tub. In this way, a solution of basic lead acetate is formed, to which the necessary quantity of acetic acid is added to bring it into the neutral condition. If crystalline lead acetate is required, the liquid is evaporated down and quickly cooled with stirring, so that small crystals are formed. Generally, however, evaporation is unnecessary, since lead acetate is always used in solution in preparing colours.
If the lead acetate solution be not colourless, which is generally the case when coloured acetic acid is used, the defect may be removed by stirring a little bone black into the liquid and filtering after twenty-four hours, when a completely colourless solution is obtained.
It is always necessary to know exactly how much lead acetate is contained in the solutions prepared by these processes. The lead or the litharge is therefore weighed and the volume of the lead acetate solution measured. One hundred parts by weight of crystallised lead acetate are obtained from 62·54 parts of lead.
Lead acetate solutions must be kept in well-covered vessels. The carbonic acid of the air will turn the liquid turbid. The turbidity may be removed by the addition of acetic acid.
In the following table is given the percentage of crystallised lead acetate contained in solutions of different specific gravities:—
| Specific Gravity | Crystallised Lead Acetate per cent. | Specific Gravity | Crystallised Lead Acetate per cent. |
|---|---|---|---|
| 1·0000 | 0 | 1·1869 | 26 |
| 1·0064 | 1 | 1·1955 | 27 |
| 1·0127 | 2 | 1·2040 | 28 |
| 1·0191 | 3 | 1·2126 | 29 |
| 1·0255 | 4 | 1·2211 | 30 |
| 1·0319 | 5 | 1·2303 | 31 |
| 1·0386 | 6 | 1·2395 | 32 |
| 1·0453 | 7 | 1·2486 | 33 |
| 1·0520 | 8 | 1·2578 | 34 |
| 1·0587 | 9 | 1·2669 | 35 |
| 1·0654 | 10 | 1·2768 | 36 |
| 1·0725 | 11 | 1·2867 | 37 |
| 1·0796 | 12 | 1·2966 | 38 |
| 1·0867 | 13 | 1·3064 | 39 |
| 1·0939 | 14 | 1·3163 | 40 |
| 1·1010 | 15 | 1·3269 | 41 |
| 1·1084 | 16 | 1·3376 | 42 |
| 1·1159 | 17 | 1·3482 | 43 |
| 1·1234 | 18 | 1·3588 | 44 |
| 1·1309 | 19 | 1·3695 | 45 |
| 1·1384 | 20 | 1·3810 | 46 |
| 1·1464 | 21 | 1·3925 | 47 |
| 1·1544 | 22 | 1·4041 | 48 |
| 1·1624 | 23 | 1·4156 | 49 |
| 1·1704 | 24 | 1·4271 | 50 |
| 1·1784 | 25 | —— | —— |