Solution: 1 in 25.

Medium: carbon disulfide.

Standard thymol solution— Prepare a standard solution of thymol in 0.25 N sodium hydroxide containing 0.1 mg of thymol per mL.

Buffer solution— Use pH 8.0 alkaline borate buffer (see under Solutions in the section Reagents, Indicators, and Solutions).

Chlorimide solution— Dissolve 100 mg of 2,6-dibromoquinonechlorimide in 25 mL of dehydrated alcohol. Prepare a fresh solution for each assay.

Standard thymol curve— Pipet into three 100-mL volumetric flasks 1 mL, 3 mL, and 5 mL, respectively, of Standard thymol solution, and add 0.25 N sodium hydroxide to make the final volume 5.0 mL. Add 5.0 mL of 0.25 N sodium hydroxide to a fourth flask in which the blank is to be prepared. To each flask add 10 mL of Buffer solution, mix by gentle swirling, and add 1 mL of Chlorimide solution. Allow to stand for 15 minutes, accurately timed, add 3 mL of 0.25 N sodium hydroxide to each flask, and add water to volume. With a suitable spectrophotometer, measure the absorbances of the thymol-containing solutions relative to the blank at 590 nm. Plot the readings and draw the curve of best fit.

Procedure— Place about 2 mL of Halothane, accurately weighed, in a 100-mL volumetric flask containing 5 mL of 0.25 N sodium hydroxide, and mix by gentle swirling. Evaporate the halothane under a stream of nitrogen, and add 10 mL of Buffer solution and 1 mL of Chlorimide solution. Swirl gently, allow to stand for 15 minutes, accurately timed, add 3 mL of 0.25 N sodium hydroxide, and add water to volume. Read the absorbance of the resulting solution, and by reference to the Standard thymol curve, calculate the percentage of thymol in the weight of Halothane taken.

Standard preparation— Add 1.0 µL of 1,1,2-trichloro-1,2,2-trifluoroethane to 20.0 mL of the test specimen.

Chromatographic system— Under typical conditions, the gas chromatograph is equipped with a flame-ionization detector, and contains a 3-m × 2-mm stainless steel column packed with 20% G24 on support S1AB. The column is maintained at 60, the injection port and detector block are maintained at 200, and nitrogen is used as the carrier gas at a flow rate of about 15 mL per minute. Typical retention times are about 5 minutes for 1,1,2-trichloro-1,2,2-trifluoroethane and about 13 minutes for halothane.

Procedure— Inject separately equal volumes (about 2 µL) of the Standard preparation and Halothane into a suitable gas chromatograph, and record the chromatograms. The total area of all peaks (except that of halothane) recorded for the test specimen does not exceed that due to the added 1,1,2-trichloro-1,2,2-trifluoroethane in the Standard preparation (0.005%).