Cation-exchange column— Mix 10g of styrene-divinylbenzene cation-exchange resin with 50mLof water in a beaker.Allow the resin to settle,and decant and discard the supernatant until a slurry of resin remains.Pour the slurry into a 15-mm ×30-cm glass chromatographic tube having a pledget of glass wool in the bottom and a stopcock,and allow to settle as a homogeneous bed.Place a pledget of glass wool on the top of the bed.Wash the resin bed with about 100mLof water,closing the stopcock when the water level has just entered the glass wool pledget on the surface above the resin bed.

Test solution— Transfer an accurately measured volume of the constituted Oral Solution,equivalent to about 9g of anhydrous citric acid,to a 100-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Transfer 5.0mLof the Test solutioncarefully onto the top of the resin bed in the cation-exchange column.Place a 250-mLvolumetric flask below the column,open the stopcock,and allow to flow until the solution has entered the resin bed.Elute the column with 70mLof water at a rate of about 5mLper minute,collecting the eluate in a beaker.Boil the eluate for 1minute,cool,and add 5drops of phenolphthalein TS.Titrate with 0.1Nsodium hydroxide VSto a pink endpoint.Each mLof 0.1Nsodium hydroxide is equivalent to 6.404mg of anhydrous citric acid (C6H8O7).The content of anhydrous citric acid is between 76.6%and 107.8%of the labeled amount of magnesium citrate.

NOTE—Perform the following tests on the Oral Solution obtained after constitution as directed in the labeling.Accurately measure the volume of the constituted Oral Solution.