(b) Preparation of the standard homosalate sunscreen.

1. (1) The standard homosalate sunscreen is prepared from two different preparations (preparation A and preparation B) with the following compositions:

   Composition of Preparation A and Preparation B of the Standard Sunscreen

   Ingredients	Percent by weight
   Preparation A
   Lanolin	5.00
   Homosalate	8.00
   White petrolatum	2.50
   Stearic acid	4.00
   Propylparaben	0.05
   Preparation B
   Methylparaben	0.10
   Edetate disodium	0.05
   Propylene glycol	5.00
   Triethanolamine	1.00
   Purified water U.S.P	74.30

2. (2) Preparation A and preparation B are heated separately to 77 to 82 °C, with constant stirring, until the contents of each part are solubilized. Add preparation A slowly to preparation B while stirring. Continue stirring until the emulsion formed is cooled to room temperature (15 to 30 °C). Add sufficient purified water to obtain 100 grams of standard sunscreen preparation.

(c) Assay of the standard homosalate sunscreen. Assay the standard homosalate sunscreen preparation by the following method to ensure proper concentration:

1. (1) Preparation of the assay solvent. The solvent consists of 1 percent glacial acetic acid (V/V) in denatured ethanol. The denatured ethanol should not contain a UV radiation absorbing denaturant.
2. (2) Preparation of a 1-percent solution of the standard homosalate sunscreen preparation. Accurately weigh 1 gram of the standard homosalate sunscreen preparation into a 100-milliliter volumetric flask. Add 50 milliliters of the assay solvent. Heat on a steam bath and mix well. Cool the solution to room temperature (15 to 30 °C). Then dilute the solution to volume with the assay solvent and mix well to make a 1-percent solution.
3. (3) Preparation of the test solution (1:50 dilution of the 1-percent solution). Filter a portion of the 1-percent solution through number 1 filter paper. Discard the first 10 to 15 milliliters of the filtrate. Collect the next 20 milliliters of the filtrate (second collection). Add 1 milliliter of the second collection of the filtrate to a 50-milliliter volumetric flask. Dilute this solution to volume with assay solvent and mix well. This is the test solution (1:50 dilution of the 1-percent solution).
4. (4) Spectrophotometric determination. The absorbance of the test solution is measured in a suitable double beam spectrophotometer with the assay solvent and reference beam at a wavelength near 306 nanometers.

5. (5) Calculation of the concentration of homosalate. The concentration of homosalate is determined by the following formula which takes into consideration the absorbance of the sample of the test solution, the dilution of the 1-percent solution (1:50), the weight of the sample of the standard homosalate sunscreen preparation (1 gram), and the standard absorbance value (172) of homosalate as determined by averaging the absorbance of a large number of batches of raw homosalate:

   Concentration of homosalate = absorbance × 50 × 100 × 172 = percent concentration by weight.