White’s Tar Paste
DEFINITION
Zinc and Coal Tar Paste contains 6% w/w each of Zinc Oxide and Coal Tar with 38% w/w of Starch in a suitable hydrophobic basis.
Extemporaneous preparation
The following formula and directions apply.
| Emulsifying Wax | 50 g |
| Coal Tar | 60 g |
| Zinc Oxide, finely sifted | 60 g |
| Starch | 380 g |
| Yellow Soft Paraffin | 450 g |
Melt the Emulsifying Wax at 70°, add the Coal Tar and 225 g of the Yellow Soft Paraffin, stir at 70° until completely melted, add the remainder of the Yellow Soft Paraffin, cool to 30°, add the Zinc Oxide and the Starch, stirring constantly, and stir until cold.
The paste complies with the requirements stated under Topical Semi-solid Preparations and with the following requirements.
Content of zinc oxide, ZnO
5.7 to 6.3% w/w.
IDENTIFICATION
The residue obtained in the Assay is yellow when hot and white when cool.
ASSAY
Heat 1.0 g of the paste gently in a porcelain dish over a small flame until the basis is completely volatilised or charred.
Increase the heat until all the carbon is removed. Dissolve the residue in 10 mL of 2M acetic acid and add sufficient water to produce 50 mL. To the resulting solution add 50 mg of xylenol orange triturate and sufficient hexamine to produce a violet-pink colour. Add a further 2 g of hexamine and titrate with 0.05M disodium edetate VS until the solution becomes yellow. Each mL of 0.05M disodium edetate VS is equivalent to 4.069 mg of ZnO.
