Zinc and Salicylic Acid Paste

General Notices

Lassar’s Paste

Definition

Zinc Oxide, finely sifted

240 g

Salicylic Acid, finely sifted

20 g

Starch, finely sifted

240 g

White Soft Paraffin

500 g

Extemporaneous preparation

The following directions apply.

Melt the White Soft Paraffin, incorporate the Zinc Oxide, the Salicylic Acid and the Starch 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

22.5 to 25.5% w/w.

Content of salicylic acid, C7H6O3

1.9 to 2.1% w/w.

Identification

A. Shake 1 g with 10 mL of water, filter and add iron(iii) chloride solution R1 to the filtrate. An intense reddish violet colour is produced which remains on the addition of 5m acetic acid but disappears on the addition of 2m hydrochloric acid with the separation of a white, crystalline precipitate.
B. Heat 0.5 g gently in a porcelain dish over a very small flame until the basis is completely volatilised or charred. Increase the heat until all the carbon is removed. The residue is yellow when hot and white when cool.

Tests

Assay

For salicylic acid

Shake 0.5 g with 10 mL of 1m hydrochloric acid and 10 mL of ether until fully dispersed. Decant and reserve the aqueous layer. Extract the ether layer with two further 10 mL quantities of 1m hydrochloric acid, combine the aqueous extracts with the reserved aqueous layer, wash with 10 mL of ether and reserve for the Assay for zinc oxide. Combine the ether extracts, add 15 mL of petroleum spirit (boiling range, 40° to 60°) and extract with successive quantities of 20 mL, 10 mL and 10 mL of a mixture of equal volumes of ethanol (90%) and 1m sodium hydroxide. Dilute the combined extracts to 100 mL with 2m hydrochloric acid, further dilute 15 mL of the resulting solution to 50 mL with the same solvent and measure the absorbance of the final solution at the maximum at 302 nm, Appendix II B. Calculate the content of C7H6O3 taking 260 as the value of A(1%, 1 cm) at the maximum at 302 nm.

For zinc oxide

To the combined aqueous extracts obtained in the Assay for salicylic acid add 20 mL of 1m sodium hydroxide and 50 mg of xylenol orange triturate. To the resulting solution add sufficient hexamine to change the colour of the solution to red and then a further 3 g of hexamine and titrate with 0.1m disodium edetate VS. Each mL of 0.1m disodium edetate VS is equivalent to 8.139 mg of ZnO.