Potassium chloride stock solution— Prepare a solution of potassium chloride,previously dried at 125for 30minutes and accurately weighed,in water to obtain a solution having a known concentration of about 7.5mg per mL.

Sodium chloride stock solution— Prepare a solution of sodium chloride,previously dried at 125for 30minutes and accurately weighed,in water to obtain a solution having a known concentration of about 7mg per mL.

Lithium diluent solution— Transfer 1.04g of lithium nitrate to a 1000-mLvolumetric flask,add a suitable nonionic surfactant,then add water to volume,and mix.

Standard preparation— Transfer 5.0mLof Potassium chloride stock solutionand 5.0mLof Sodium chloride stock solutionto a 50-mLvolumetric flask,dilute with water to volume,and mix.Each mLof this intermediate solution contains about 0.75mg of potassium chloride and 0.7mg of sodium chloride.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,dilute with Lithium diluent solutionto volume,and mix.

Assay preparation 1— Weigh and finely powder not less than 20Tablets.[NOTE—Tablets and powder are hygroscopic.After removal from the container,grind the Tablets promptly in an atmosphere of low relative humidity,and weigh the powder promptly.]Transfer an accurately weighed portion of the powder,equivalent to about 3000mg of potassium bicarbonate,to a 1000-mLvolumetric flask,dissolve in 500mLof water,swirl until effervescence ceases,dilute with water to volume,and mix.Dilute an accurately measured volume of this stock solution quantitatively with water to obtain a test solution containing about 1mg of potassium bicarbonate per mL,on the basis of the labeled quantity.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,dilute with Lithium diluent solutionto volume,and mix.

Assay preparation 2— Dilute an accurately measured volume of the stock solution used to prepare Assay preparation 1quantitatively with water to obtain a test solution containing about 1mg of sodium bicarbonate per mL,on the basis of the labeled quantity.Transfer 5.0mLof this solution to a 100-mLvolumetric flask,dilute with Lithium diluent solutionto volume,and mix.

Procedure— Using a suitable flame photometer,adjusted to read zero with Lithium diluent solution,concomitantly determine the potassium flame emission readings for the Standard preparationand Assay preparation 1at the wavelength of maximum emission at about 766nm.Calculate the quantity,in mg,of potassium bicarbonate (KHCO3)in each Tablet taken by the formula: in which 100.12and 74.55are the molecular weights of potassium bicarbonate and potassium chloride,respectively,Lis the labeled quantity,in mg,of potassium bicarbonate in each Tablet,Cis the concentration,in mg per mL,of potassium chloride in the intermediate solution used to prepare the Standard preparation,Dis the concentration,in mg per mL,of potassium bicarbonate in the test solution used to prepare Assay preparation 1,on the basis of the labeled quantity of potassium bicarbonate in each Tablet and the extent of dilution,and RU,Kand RS,Kare the potassium emission readings obtained from Assay preparation 1and the Standard preparation,respectively.Similarly determine the sodium flame emission readings for the Standard preparationand Assay preparation 2at the wavelength of maximum emission at about 589nm.Calculate the quantity,in mg,of sodium bicarbonate (NaHCO3)in each Tablet taken by the formula: in which 84.01and 58.44are the molecular weights of sodium bicarbonate and sodium chloride,respectively,Lis the labeled quantity,in mg,of sodium bicarbonate in each Tablet,Cis the concentration,in mg per mL,of sodium chloride in the intermediate solution used to prepare the Standard preparation,Dis the concentration,in mg per mL,of sodium bicarbonate in the test solution used to prepare Assay preparation 2,on the basis of the number of Tablets taken,the labeled quantity of sodium bicarbonate in each Tablet,and the extent of dilution,and RU,Naand RS,Naare the sodium emission readings obtained from Assay preparation 2and the Standard preparation,respectively.

Cation-exchange column— Mix 10g of styrenedivinylbenzene cation-exchange resin with 50mLof water in a suitable beaker.Allow the resin to settle,and decant the supernatant until a slurry of resin remains.Pour the slurry into a 14-mm ×30-cm glass chromatographic tube (having a sealed-in,coarse-porosity porous glass disk and fitted with a stopcock),and allow to settle as a homogeneous bed.Wash the resin bed with about 100mLof water,closing the stopcock when the water level is about 2mm above the resin bed.

Procedure— Transfer an accurately measured volume of the stock solution used to prepare Assay preparation 1in the Assay for potassium bicarbonate and sodium bicarbonate,equivalent to about 40mg of anhydrous citric acid,carefully onto the top of the resin bed in the Cation-exchange column.Place a 250-mLconical flask below the column,open the stopcock,and allow to flow until the solution has entered the resin bed.Elute the column with 60mLof water at a flow rate of about 5mLper minute,collecting about 65mLof the eluate in a suitable flask.Boil the eluate for 1minute,cool,add 5drops of phenolphthalein TS,swirl the flask,and titrate with 0.02Nsodium hydroxide VSto a pink endpoint.Each mLof 0.02Nsodium hydroxide is equivalent to 1.281mg of C6H8O7.