Lead sulfochromate

Both lead chromate and lead sulfochromate (the latter is a mixed-phase pigment) can be orthorhombic or monoclinic the monoclinic structure is the more stable [3.132], The greenish yellow orthorhombic modification of lead chromate is metastable at room temperature, and is readily transformed to the monoclinic modification under certain conditions (e.g., concentration, pH, temperature). The latter modification occurs naturally as crocoite. 

Production. In large-scale production, lead or lead oxide is reacted with nitric acid to give lead nitrate solutions, which are then mixed with sodium dichromate solution. If the precipitation solutions contain sulfate, lead sulfochromate is formed as a mixed-phase pigment. After stabilization the pigment is filtered off, washed until free of electrolyte, dried, and ground. 

Ten Horn describes a process for the production of lead sulfochromate containing at least 50 % lead chromate [3.139]. This has a low acid-soluble lead content (< 5 % expressed as PbO, by BS 3900, Part B3, 1965). 

Both lead chromate and lead sulfochromate (the latter is a mixed-phase pigment) can be orthorhombic or monoclinic the monoclinic structure is the more stable... 

Primrose chrome (greenish-yellow lead sulfochromate)... 

---