»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 nhas an average value such that the corresponding nominal viscosity is in a discrete range between 20and 30,000centistokes.It contains not less than 97.0percent and not more than 103.0percent 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.
Identification Its IRabsorption 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 á841ñ: within the limits specified in the accompanying table.
Viscosity á911ñ Determine the viscosity of Dimethicone having a nominal viscosity of less than 1000centistokes at 25±0.1,using a capillary viscosimeter.Determine the viscosity of Dimethicone having a nominal viscosity of 1000centistokes or greater at 25±0.1,using a rotational viscosimeter.The viscosity is within the limits specified in the accompanying table.
Refractive index á831ñ: within the limits specified in the accompanying table.
Acidity Dissolve 15.0g in a mixture of 15mLof toluene and 15mLof butyl alcohol,previously neutralized to bromophenol blue TS,and titrate with 0.050Nalcoholic potassium hydroxide to a bromophenol blue endpoint:not more than 0.10mLis required.
Loss on heating Preheat an open aluminum vessel (60mm in diameter and 10mm high)at 150for 30minutes,and allow to cool to room temperature.Tare the vessel,transfer to it about 1g of Dimethicone,accurately weighed,heat at 150in a circulating air oven for 2hours,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 1.0g of Dimethicone with 10mLof chloroform,and dilute with the same solvent to 20mL.Add 1.0mLof a freshly prepared 0.002%solution of dithizone in chloroform,0.5mLof water,and 0.5mLof a mixture of 1volume of ammonia TSand 9volumes of a 0.2%solution of hydroxylamine hydrochloride.Concomitantly prepare a Standard solution as follows:to 20mLof chloroform add 1.0mLof a freshly prepared 0.002%solution of dithizone in chloroform,0.5mLof Standard Lead Solution(see Heavy Metals á231ñ),and 0.5mLof a mixture of 1volume of ammonia TSand 9volumes of a 0.2%solution of hydroxylamine hydrochloride.Immediately shake both solutions vigorously for 1minute.Any red color in the test solution is not more intense than that in the Standard solution (5µg per g).
Bacterial endotoxins á85ñ(where it is intended for use in coating containers that come in contact with articles for parenteral use) Mix 1.0mLof Dimethicone with 4.0mLof polydimethylsiloxane having a viscosity of 0.65centistokes,previously tested and shown to be negative for bacterial endotoxins,by mixing on a vortex mixer for 1minute in an extraction tube.Add 10mLof water,and mix on a vortex mixer for not less than 60minutes.Allow the layers to separate,and use the lower aqueous layer as the test specimen.It contains not more than 1.0USP Endotoxin Unit per mL,equivalent to not more than 10Endotoxin Units per mLof the Dimethicone taken.
Assay Use an IRspectrophotometer with a resolution of 4cm-1and fitted with an accessory for attenuated total reflectance (see Spectrophotometry and Light-Scattering á851ñ)and a germanium (Ge)sample trough (45or 60).Fill the trough of the accessory with Dimethicone,and record the spectrum between 4000cm-1and 700cm-1.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 1300cm-1and 1200cm-1,and calculate the absorbance of the peak in each spectrum at about 1259cm-1.Calculate the percentage of [(CH3)2SiO]nin the Dimethicone taken by the formula:
100(AU/AS)(DS/DU),in which AUis the absorbance of the Dimethicone;ASis the absorbance of USP Polydimethylsiloxane RS;and DSand DUare the specific gravities of USP Polydimethylsiloxane RSand Dimethicone,respectively.
Auxiliary Information Staff Liaison:Elena Gonikberg,Ph.D.,Scientist
Expert Committee:(PA4)Pharmaceutical Analysis 4
USP28NF23Page 3001Pharmacopeial Forum:Volume No.29(1)Page 142