(dye meth' i kone).
» Dimethicone is a mixture of fully methylated linear siloxane polymers containing repeating units of the formula:
stabilized with trimethylsiloxy end-blocking units of the formula:
wherein n has an average value such that the corresponding nominal viscosity is in a discrete range between 20 and 30,000 centistokes. It contains not less than 97.0 percent and not more than 103.0 percent of polydimethylsiloxane ([(CH3)2SiO]n).
The requirements for viscosity, specific gravity, refractive index, and loss on heating differ for the several types of Dimethicone, as set forth in the accompanying table.
Packaging and storage Preserve in tight containers.
Labeling Label it to indicate its nominal viscosity value. Dimethicone intended for use in coating containers that come in contact with articles for parenteral use is so labeled.
USP Reference standards 11
USP Endotoxin RS
Identification Its IR absorption spectrum, obtained as directed in the Assay, exhibits maxima only at the same wavelengths as that of USP Polydimethylsiloxane RS, obtained as directed in the Assay.
Specific gravity 841: within the limits specified in the accompanying table.
Viscosity 911 Determine the viscosity of Dimethicone having a nominal viscosity of less than 1000 centistokes at 25 ± 0.1, using a capillary viscosimeter. Determine the viscosity of Dimethicone having a nominal viscosity of 1000 centistokes or greater at 25 ± 0.1, using a rotational viscosimeter. The viscosity is within the limits specified in the accompanying table.
Refractive index 831: within the limits specified in the accompanying table.
Acidity Dissolve 15.0 g in a mixture of 15 mL of toluene and 15 mL of butyl alcohol, previously neutralized to bromophenol blue TS, and titrate with 0.050 N alcoholic potassium hydroxide to a bromophenol blue endpoint: not more than 0.10 mL is required.
Loss on heating Preheat an open aluminum vessel (60 mm in diameter and 10 mm high) at 150 for 30 minutes, and allow to cool to room temperature. Tare the vessel, transfer to it about 1 g of Dimethicone, accurately weighed, heat at 150 in a circulating air oven for 2 hours, and allow to come to room temperature in a desiccator before weighing: it loses not more than the maximum percentage of its weight specified in the accompanying table.
Heavy metals Mix 1.0 g of Dimethicone with 10 mL of chloroform, and dilute with the same solvent to 20 mL. Add 1.0 mL of a freshly prepared 0.002% solution of dithizone in chloroform, 0.5 mL of water, and 0.5 mL of a mixture of 1 volume of ammonia TS and 9 volumes of a 0.2% solution of hydroxylamine hydrochloride. Concomitantly prepare a Standard solution as follows: to 20 mL of chloroform add 1.0 mL of a freshly prepared 0.002% solution of dithizone in chloroform, 0.5 mL of Standard Lead Solution (see Heavy Metals 231), and 0.5 mL of a mixture of 1 volume of ammonia TS and 9 volumes of a 0.2% solution of hydroxylamine hydrochloride. Immediately shake both solutions vigorously for 1 minute. Any red color in the test solution is not more intense than that in the Standard solution (5 µg per g).
Bacterial endotoxins 85 (where it is intended for use in coating containers that come in contact with articles for parenteral use) Mix 1.0 mL of Dimethicone with 4.0 mL of polydimethylsiloxane having a viscosity of 0.65 centistokes, previously tested and shown to be negative for bacterial endotoxins, by mixing on a vortex mixer for 1 minute in an extraction tube. Add 10 mL of water, and mix on a vortex mixer for not less than 60 minutes. Allow the layers to separate, and use the lower aqueous layer as the test specimen. It contains not more than 1.0 USP Endotoxin Unit per mL, equivalent to not more than 10 Endotoxin Units per mL of the Dimethicone taken.
Assay Use an IR spectrophotometer with a resolution of 4 cm1 and fitted with an accessory for attenuated total reflectance (see Spectrophotometry and Light-Scattering 851) and a germanium (Ge) sample trough (45 or 60). Fill the trough of the accessory with Dimethicone, and record the spectrum between 4000 cm1 and 700 cm1. Clean the trough, fill it with USP Polydimethylsiloxane RS, and record the spectrum as above. Clean the trough, and record the spectrum as above to obtain a background spectrum. Examine the spectra in the range between 1300 cm1 and 1200 cm1, and calculate the absorbance of the peak in each spectrum at about 1259 cm1. Calculate the percentage of [(CH3)2SiO]n in the Dimethicone taken by the formula:
100(AU / AS)(DS / DU)in which AU is the absorbance of the Dimethicone; AS is the absorbance of USP Polydimethylsiloxane RS; and DS and DU are the specific gravities of USP Polydimethylsiloxane RS and Dimethicone, respectively.
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USP35NF30 Page 1787Pharmacopeial Forum: Volume No. 29(1) Page 142