A: The solution employed for measurement of absorbance in the Assayexhibits a maximum at 310±2nm.

B: Dissolve 5g in 50mLof ether,extract the ether solution with 5mLof 3Nhydrochloric acid,and filter the acid extract.Add 2mLof potassium thiocyanate solution (1in 2)to the filtrate:a crystalline precipitate is formed,and when recrystallized from water and dried at 80for 2hours,it melts between 130and 132(see Melting Range or Temperature á741ñ).

Standard preparation— Transfer about 20mg of USP Tetracaine Hydrochloride RS,accurately weighed,to a 100-mLvolumetric flask,dissolve in water,add water to volume,and mix.Transfer 5.0mLof this solution to a second 100-mLvolumetric flask,add 5mLof dilute hydrochloric acid (1in 240)and 10mLof Buffer No.6,10percent,pH6.0(see Phosphate Buffers á81ñ),dilute with water to volume,and mix.The concentration of USP Tetracaine Hydrochloride RSin the Standard preparationis about 10µg per mL.

Assay preparation— Transfer an accurately weighed portion of Ointment,equivalent to about 9mg of tetracaine,to a separator,and dissolve in 15mLof ether.Extract with one 20-mLportion and two 10-mLportions of dilute hydrochloric acid (1in 240),collecting the acid extracts in a second separator.Render the aqueous solution alkaline by the addition of 5mLof sodium carbonate TS,and extract immediately with two 50-mLportions of ether,collecting the ether extracts in another separator.Wash the ether solution with 20mLof water,discard the washing,and extract the ether solution with two 20-mLportions and one 5-mLportion of dilute hydrochloric acid (1in 240),collecting the acid extracts in a 50-mLvolumetric flask.Dilute with water to volume,and mix.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,add 10mLof Buffer No.6,10percent,pH6.0(see Phosphate Buffers á81ñ),dilute with water to volume,and mix.

Procedure— Concomitantly determine the absorbances of the Assay preparationand the Standard preparationin 1-cm cells at the wavelength of maximum absorbance at about 310nm,with a suitable spectrophotometer,using water as the blank.Calculate the quantity,in mg,of C15H24N2O2in the portion of Ointment taken by the formula: in which 264.36and 300.82are the molecular weights of tetracaine and tetracaine hydrochloride,respectively;Cis the concentration,in µg per mL,of USP Tetracaine Hydrochloride RSin the Standard preparation;and AUand ASare the absorbances of the Assay preparationand the Standard preparation,respectively.