Question: Why is zinc Chromate bright yellow?...

Question

Why is zinc Chromate bright yellow?

Answer 1

Solution

We need to know that zinc Chromate has the chemical formula of $ZnCr{O_4}$ . It consists of Chromate ion and appears as a odourless yellow powder or yellow-green crystals. The colour of zinc yellow varies depending on the concentration of the
components.

Complete answer:
We need to remember that the zinc chromate is produced by Cronak Process in the industries. This process includes putting zinc or zinc plated metal in Sodium dichromate solution and sulphuric acid for a few seconds. Zinc chromate can also be synthesized by using potassium chromate and zinc sulphate which forms a precipitate. The reaction can be given as:
${K_2}Cr{O_4} + ZnS{O_4} \to ZnCr{O_4} + {K_2}S{O_4}$
Zinc chromate is soluble in dilute acids like acetic acid but Insoluble in water. Zinc Yellow’s made earlier were soluble in water and had effloresce in humid conditions. It appears microscopically as round particles with strong birefringence.
Charge Transfer (CT) transitions occur similar to d-d transitions and also involve metal d orbitals. These CT bands are observed if the energies of empty and filled ligand-metal centered orbitals are similar. Ligand to metal charge transfer (LMCT) occurs in $Mn{O_4}^ - ,CrO_4^{2 - },$ etc. In compounds like $KMn{O_4}$ the color is observed not because of d-d transition but due to charge transfer between $O_2^ - {\text{and}}Mn(VII)$ , called LMCT.
In Zinc Chromate also there is no possibility of d-d transition since $Zn$ has ${d^{10}}$ electronic configuration. Here too the colour is observed due to Charge Transfer between $Z{n^{ + 2}}{\text{and}}CrO_4^{2 - }$ .
Hence the bright yellow colour of Zinc Chromate is due to LMCT (ligand to Metal Charge Transfer)

Note: We need to remember that the zinc Chromate is mainly used in industrial painting as a coating. It is also used in Spray Paints, pigments in varnishes and in making linoleum. Zinc Chromate is highly toxic, and it's also a carcinogen. Prolonged exposure could lead to causing tissue ulceration and cancer.