Lead(II) chromate is the inorganic compound with the formula (PbCrO4). It has a vivid yellow color and is generally insoluble. Two polymorphs of lead chromate are known, orthorhombic and the more stable monoclinic form. Monoclinic lead chromate is used in paints under the name chrome yellow, and many other names. It occurs also as the mineral crocoite.
Structure
Lead chromate adopts the monazite structure, meaning that the connectivity of the atoms is very similar to other compounds of the type MM'O4. Pb(II) has a distorted coordination sphere being surrounded by eight oxides with Pb-O distances ranging from 2.53 to 2.80 Å. The chromate anion is tetrahedral, as usual. Unstable polymorphs of lead chromate are the greenish yellow orthorhombic form and a red-orange tetragonal form.
Applications
Approximately 37,000 tons were produced in 1996. The main applications are as a pigment in paints, under the name chrome yellow.
Preparation
Lead(II) chromate can be produced by treating sodium chromate with lead salts such as lead(II) nitrate or by combining lead(II) oxide with chromic acid.
Related lead sulfochromate pigments are produced by the replacement of some chromate by sulfate, resulting in a mixed lead-chromate-sulfate compositions Pb(CrO4)1-x(SO4)x. This replacement is possible because sulfate and chromate are isostructural. Since sulfate is colorless, sulfochromates with high values of x are less intensely colored than lead chromate. In some cases, chromate is replaced by molybdate.
Reactions
Heating in hydroxide solution produces chrome red, a red or orange powder made by PbO and CrO3. Also, in hydroxide solution lead chromate slowly dissolves forming plumbite complex.
PbCrO4 + 4 OH− → [Pb(OH)4]2− + CrO42−
Safety hazards
Despite containing both lead and hexavalent chromium, lead chromate is not particularly toxic because of its very low solubility. The LD50 for rats is only 5,000 mg/kg. Lead chromate is treated with great care in its manufacture, the main concerns being dust of the chromate precursor. "Extensive epidemiological investigations have given no indication that the practically insoluble lead chromate pigments have any carcinogenic properties". Despite its low acute toxicity and lack of broad evidence for carcinogenicity, lead chromate is highly regulated in advanced countries. One of the greatest threats comes from inhalation of particles, so much effort has been devoted to production of low-dust forms of the pigment.
In the 1800s, the product was used to impart a bright yellow color to some types of candy. It is used (illegally) to enhance the color of certain spices, particularly turmeric, particularly in Bangladesh.
Unlike other lead-based paint pigments, lead chromate is still widely used.