For the lighter chrome yellows, lead sulphate is precipitated simultaneously with the chromate by adding sulphuric acid or a soluble sulphate to the solution of the bichromate; these yellows have less tendency to change colour than the pure chromate of lead, if the above precautions are observed. Chrome yellow, precipitated from an excess of lead acetate solution, by means of potassium bichromate, corresponds to the formula PbCrO4. When dried it forms light pieces, which show a conchoidal fracture. A still more voluminous product corresponds to the formula PbCrO4, PbSO4. The yellow having the composition PbCrO4, 2PbSO4, is very heavy and shows a smooth fracture. Lightness is often imparted to chrome yellow by the addition of magnesium carbonate.
Nitrate of lead offers no advantages over the acetate, and is generally more expensive to use. Free nitric acid is more objectionable than free acetic acid, because it may act as a solvent on chrome yellow and liberate free chromic acid, which is liable to “turn” the yellow. When using lead nitrate it is preferable to neutralise potassium bichromate with soda, to avoid the presence of free nitric acid. The yellows mostly in demand are the inferior qualities, prepared by mixing pure chrome yellows with white mineral matters, generally barytes, gypsum, and kaolin, usually stirred in with the bichromate solution before adding the lead salt. Barytes injures the colour least, but kaolin has the advantage that it does not increase the weight of the colour so much. Gypsum occupies an intermediate position; it is much more voluminous than barytes, and does not injure the colour so much as kaolin, but it is generally used together with barytes. It is not advisable to use this combination, for the reason that the colour on drying forms very hard pieces, which offer difficulties in grinding. Gypsum, too, from being more easily acted on by reagents than barytes or kaolin, tends to take part in the reaction by decomposing the potassium bichromate before the addition of the lead salt, and this is objectionable. (Weber.)
A writer in the Chemical Trade Journal gives the following formula for the production of various yellows:—
(1) For soluble lead salts:—
| Kilos. | |
| Lead acetate | 100 |
| Potassium bichromate | 18 |
| Sulphuric acid (66° B.) | 12 |
This mixture yields a yellow of the formula PbCrO4, PbSO4. To obtain good shades, the amount of water employed should not be less than 1000 litres, or double as much with the nitrate. In the latter case, it is better to neutralise the lead nitrate and to replace the acid by the sulphate of an alkali or of magnesium, or preferably of aluminium; for the neutralisation of the bichromate, the best substance is magnesite. The formula thus becomes—Lead acetate and bichromate as before; magnesite, 6 kilos.; aluminium sulphate, 27 kilos.
(2) The Basic Acetate Method.—Litharge, 76 kilos.; acetic acid (30 per cent.), 42; bichromate, 21·5; sulphuric acid, 21·5; water, 2000 to 3000 litres. To obtain a denser chrome, 10 kilos. of soda should be added to the acetate, and 5 of the sulphuric acid replaced by 10 of aluminium sulphate. For the production of an orange chrome, the following formula is given: Litharge, 76 kilos.; acetic acid (30 per cent.), 42; bichromate, 24; Solvay’s soda, 15; and caustic soda (100 per cent.), 5. Care must be taken that the temperature does not rise sufficiently high to spoil the shade.
(3) The White Lead Method.—In this case the white lead must be in the finest state of subdivision possible, and suspended in the water: White lead, 100 kilos.; nitric acid (36° B.), 12; bichromate, 13; and aluminium sulphate, 10: or nitric acid (40° B.), 44; bichromate, 24; sulphate, 20, the latter giving the more fiery shade. For the production of an orange: white lead, 100 kilos.; nitric acid (36° B.) 18; bichromate, 28; and caustic soda, 8, the latter being best added to the bichromate before precipitation, and the temperature kept between 150° and 165° F.
(4) The Basic Chloride Method.—The same proportions and temperature are suitable here as in the case of the white lead.
(5) The Sulphate Method.—Lead sulphate, 100 kilos.; bichromate, 24 to 25; Solvay’s soda, 8·75 to 16; ammonia (24 per cent.), 1 to 2; and acetic acid (30 per cent.), 5 to 10. The sulphate, in the form of a cream, is gradually added to the other ingredients after solution.