Assay—
Chromotropic acid solution—
Mix 100mg of chromotropic acid with 50mLof water in a 100-mLvolumetric flask.Cool in an ice bath and,while cooling,cautiously and slowly add 50mLof sulfuric acid,and mix.Allow the solution to reach room temperature,and add dilute sulfuric acid (1in 2)to volume.[NOTE—If excessive heat generated during mixing causes a violet color to appear in the solution,discard the solution,and prepare another,taking precautions to avoid excessive heat.]
Standard preparation—
Transfer about 50mg of
USP Methenamine RS,accurately weighed,to a 1000-mLvolumetric flask,dissolve in and dilute with water to volume,and mix.Proceed as directed under
Assay preparation,beginning with “Transfer a 2.0-mLportion of this stock solution to a 100-mLvolumetric flask.”The concentration of
USP Methenamine RSin the
Standard preparationis about 1µg per mL.
Assay preparation—
Transfer an accurately measured volume of Elixir,equivalent to about 1.5g of methenamine,to a 500-mLvolumetric flask,dissolve and dilute with water to volume,and mix.Transfer 2.0mLof this solution to a 100-mLvolumetric flask,dilute with water to volume,and mix to obtain the stock solution.Transfer a 2.0-mLportion of this stock solution to a 100-mLvolumetric flask,add 25mLof Chromotropic acid solutionand 50mLof dilute sulfuric acid (1in 2),and mix.Transfer another 2.0-mLportion of the stock solution to a second 100-mLvolumetric flask to provide a blank,add 75mLof dilute sulfuric acid (1in 2),and mix.Place the two 100-mLflasks in a boiling water bath for 30minutes,accurately timed,then remove them from the bath,cool immediately to room temperature,add dilute sulfuric acid (1in 2)to volume,and mix.
Procedure—
Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 570nm,with a suitable spectrophotometer,using dilute sulfuric acid (1in 2)to set the instrument.Calculate the quantity,in mg,of methenamine (C
6H
12N
4)in each mLof the Elixir taken by the formula:
(1250C/V)[(AU-BU)/(AS-BS)],
in which
Cis the concentration,in µg per mL,of
USP Methenamine RSin the
Standard preparation;Vis the volume,in mL,of Elixir taken;
AUand
ASare the absorbances of the
Assay preparationand the
Standard preparation,respectively;and
BUand
BSare the absorbances of the blanks from the
Assay preparationand the
Standard preparation,respectively.