Transfer an accurately measured volume of Oral Solution, equivalent to about 1 g of sodium citrate dihydrate, to a 100-mL volumetric flask; dilute with water to volume; and mix. Pipet 5 mL of this solution carefully onto the top of the resin bed in the Cation-exchange column.
Place a 250-mL conical flask below the column, open the stopcock, and allow to flow until the solution has entered the resin bed. Elute the column with 60 mL of water at a flow rate of about 5 mL per minute, collecting about 65 mL of the eluate. Add 5 drops of phenolphthalein TS to the eluate, swirl the flask, and titrate with 0.02 N sodium hydroxide VS. Record the buret reading, and calculate the volume (B)
of 0.02 N sodium hydroxide consumed. Calculate the quantity, in mg, of sodium citrate dihydrate (C6
O) in each mL of the Oral Solution taken by the formula:
in which 1.961 is the equivalent, in mg, of C6
O, of each mL of 0.02 N sodium hydroxide; V
is the volume, in mL, of Oral Solution taken; 294.10 and 210.14 are the molecular weights of sodium citrate dihydrate and citric acid monohydrate, respectively; and C
is the concentration, in mg per mL, of citric acid monohydrate in the Oral Suspension, as obtained in the Assay for citric acid.