Titration of Sodium Carbonate

Carbonate Chemistry CO2 in atmosphere and dissolved in water Major global buffering system Industrial sources limestone: CaCO3(s) + heat  CaO(s) + CO2(g) trona (Na2CO3) deposits Air/Water Equilibrium Ionization in Water

Sodium Carbonate As a Base Commercially important source or base or carbonate for industrial processes washing soda (automatic dishwashers) Carbonate is a moderately strong base Soluble Salt Ionization as a base Ka2 for H2CO3

Titration of Na2CO3 with HCl Step 1: Na2CO3 + HCl  NaHCO3 + NaCl Step 2: NaHCO3 + HCl  H2CO3 + NaCl Overall: Na2CO3 + 2HCl  H2CO3 + 2NaCl At Ve1: Phenolphthalein pH At Ve2: BCG Ve1 Ve2 Equivalents of HCl added

Titration of Sodium Carbonate CO32 HCO3 H2CO3 pH [CO32] = [HCO3] pOH = pKb,CO3 pH = pKa1 pH = pKa2,H2CO3 Equivalents of HCl added

Derivative Plots Raw data Spreadsheet Derivative plots can be used for accurate location of the equivalence point Raw data Spreadsheet First and Second Derivative Plots Click on “Derivative Plots” tab Raw data: pH vs V 1st derivative: d(pH)/dV 2nd derivative: d2(pH)/(dV)2 2nd derivative

Determination of Carbonate in a Sample A 1.2040-g sample containing sodium carbonate and inert material was dissolved in water and titrated to the bromcresol green end point, requiring 32.50 mL of 0.1020 M HCl. Calculate the % Na2CO3 in the sample. For the BCG endpoint, the reaction is Na2CO3 + 2HCl  H2CO3 + 2NaCl % Na2CO3 = 14.59%

Effects of Carbonate Equilibria on Titration of Carbonate Shift in Phenolphthalein Endpoint Boiling to Enhance Second Endpoint

Effect of CO2 Absorption on Phenophthalein Endpoint Reaction of CO2 with carbonate solutions CO2(g) + H2O(l) + CO32(aq)  2HCO3(aq) Let 1 mmol CO2 react with 1 mmol CO32 For titration with HCl to BCG endpoint to form H2CO3 Initial 1 mmol CO32 would require 2 mmol HCl Resultant 2 mmol HCO3 requires 2 mmol HCl Net effects Volume of HCl require to reach phenolphthalein end decreases This effect may be avoided by excluding air from the titration system. Volume required to reach bromcresol green endpoint is not affected. Recommendation Use BCG endpoint to calculate amount of carbonate in your sample

Effect of Absorbed CO2 on Titration of Na2CO3 with HCl Ve1 Ve2 Ve1 decreases Ve2 is not affected Use Ve2 for calculations Before After CO2(g) + H2O(l) + CO32(aq) 2HCO3(aq)

Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl Na2CO3 + 2HCl  H2CO3 + 2NaCl NaHCO3 Boiling removes H2CO3 as gaseous CO2 NaHCO3 + H2CO3 H2CO3(aq)  CO2(g) + H2O

Summary: Titration of Sodium Carbonate Titration of Na2CO3 with strong acid yields two equivalence points At phenolphthalein end point Na2CO3  NaHCO3 At bromcresol green (or methyl red) end point All carbonate is converted to H2CO3 pH of carbonate solutions may be unstable due to exchange of CO2 with atmosphere.