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Methenamine Oral Solution
(Monograph under this new title—to become official June 1,2005)
(Current monograph title is Methenamine Elixir)
»Methenamine Oral Solution contains not less than 90.0percent and not more than 110.0percent of the labeled amount of methenamine (C6H12N4).
Packaging and storage—
Preserve in tight containers.
Identification—
Heat a volume of Oral Solution,equivalent to about 1g of methenamine,with 10mLof 2Nsulfuric acid:formaldehyde is liberated,recognizable by its odor and by its darkening of paper moistened with silver ammonium nitrate TS.On the subsequent addition of an excess of 1Nsodium hydroxide to the solution,ammonia is evolved.
Uniformity of dosage units á905ñ—
FOR ORAL SOLUTION PACKAGED IN SINGLE-UNIT CONTAINERS:
meets the requirements.
Deliverable volume á698ñ—
FOR ORAL SOLUTION PACKAGED IN MULTIPLE-UNIT CONTAINERS:
meets the requirements.
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 for Assay preparation,beginning with “Transfer a 2.0-mLportion of this stock solution to a 100-mLvolumetric flask.”The concentration of USP Methenamine RSin theStandard preparationis about 1µg per mL.
Assay preparation—
Transfer an accurately measured volume of Oral Solution,equivalent to about 1.5g of methenamine,to a 500-mLvolumetric flask,dissolve in 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 (C6H12N4)in each mLof the Oral Solution 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 Oral Solution 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.
(Official June 1,2005)
Auxiliary Information—
Staff Liaison:Behnam Davani,Ph.D.,MBA,Senior Scientist
Expert Committee:(PA7)Pharmaceutical Analysis 7
USP28–NF23Page 1241
Pharmacopeial Forum:Volume No.29(6)Page 1930
Phone Number:1-301-816-8394
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