A: It responds to the tests for Sodium á191ñand for Magnesium á191ñ.

B: To 10mLof the Irrigation add 1mLof mercuric sulfate TS,heat to boiling,and add a few drops of potassium permanganate TS:a white precipitate is formed.

Mobile phase— Add 2.0mLof sulfuric acid to 800mLof water,mix,and dilute with water to 1000mL.Filter through a membrane filter,heat to 40,and degas.Maintain the temperature of the Mobile phaseat 40throughout the analysis.

Standard preparation— Dissolve a suitable quantity of anhydrous citric acid,accurately weighed,in water to obtain a solution having a known concentration of about 1.2mg per mL.

Resolution solution— Prepare a solution in water containing about 1mg of citric acid and about 2mg of boric acid per mL.

Assay preparation— Transfer an accurately measured volume of Irrigation,equivalent to about 130mg of citric acid monohydrate,to a 100-mLvolumetric flask,dilute with water to volume,and mix.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a refractive index detector and a 7.8-mm ×30-cm column that contains packing L17.The column temperature is maintained at 40.The flow rate is about 0.6mLper minute.Chromatograph the Standard preparationand the Resolution solution,and record the peak responses as directed under Procedure:the relative standard deviation for replicate injections of the Standard preparationis not more than 1.5%,and the resolution,R,between the citric acid and boric acid peaks is not less than 4.0.

Procedure— Separately inject equal volumes (about 50µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C6H8O7·H2Oin each mLof the Irrigation taken by the formula: in which 210.14and 192.12are the molecular weights of citric acid monohydrate and anhydrous citric acid,respectively,Cis the concentration,in mg per mL,of anhydrous citric acid in the Standard preparation,Vis the volume,in mL,of Irrigation taken,and rUand rSare the peak responses for citric acid obtained from the Assay preparationand the Standard preparation,respectively.

Sodium chloride stock solution— Transfer 475mg of sodium chloride,previously dried at 105for 2hours and accurately weighed,to a 100-mLvolumetric flask.Dissolve in water,dilute with water to volume,and mix.

Internal standard solution— Transfer 636mg of lithium chloride to a 1000-mLvolumetric flask,dissolve in water,dilute with water to volume,and mix.

Standard preparation— Quantitatively prepare a mixture of Internal standard solutionand Sodium chloride stock solution(99:1).

Assay preparation— Dilute an accurately measured volume of Irrigation quantitatively with water to obtain a stock solution containing about 4.4mg of sodium carbonate per mL.Quantitatively prepare a mixture of Internal standard solutionand this stock solution (99:1).

Procedure— Concomitantly determine the emittances of the Standard preparationand the Assay preparationat 591nm and 671nm with a suitable flame photometer,adjusting the instrument to zero emittance with Internal standard solution.Calculate the quantity,in mg,of Na2CO3in each mLof the Irrigation taken by the formula: in which 105.99is the molecular weight of sodium carbonate,116.88is two times the molecular weight of sodium chloride,Cis the concentration,in mg per mL,of sodium chloride in the Sodium chloride stock solution,Lis the labeled quantity,in mg per mL,of sodium carbonate in the Irrigation,Dis the concentration,in mg per mL,of sodium carbonate in the stock solution used to prepare the Assay preparation,on the basis of the labeled quantity in each mL,and the extent of dilution,RU,591and RU,671are the emittance readings obtained from the Assay preparationat the wavelengths indicated by the subscripts,and RS,671and RS,591are the emittance readings obtained from the Standard preparationat the wavelengths indicated by the subscripts.