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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
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 Ve2 Ve1 Equivalents of HCl added
Derivative Plots 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 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.