2 Carbonate Chemistry CO2 in atmosphere and dissolved in water Major global buffering systemIndustrial sourceslimestone: CaCO3(s) + heat CaO(s) + CO2(g)trona (Na2CO3) depositsAir/Water EquilibriumIonization in Water
3 Sodium Carbonate As a Base Commercially importantsource or base or carbonate for industrial processeswashing soda (automatic dishwashers)Carbonate is a moderately strong baseSoluble SaltIonization as a baseKa2 for H2CO3
5 Titration of Sodium Carbonate CO32HCO3H2CO3pH[CO32] = [HCO3]pOH = pKb,CO3pH = pKa1pH = pKa2,H2CO3Equivalents of HCl added
6 Derivative Plots Raw data Spreadsheet Derivative plots can be used for accurate location of the equivalence pointRaw dataSpreadsheetFirst and Second Derivative PlotsClick on “Derivative Plots” tabRaw data: pH vs V1st derivative: d(pH)/dV2nd derivative: d2(pH)/(dV)22nd derivative
7 Determination of Carbonate in a Sample A g sample containing sodium carbonate and inert material was dissolved in water and titrated to the bromcresol green end point, requiring mL of M HCl.Calculate the % Na2CO3 in the sample.For the BCG endpoint, the reaction isNa2CO3 + 2HCl H2CO3 + 2NaCl% Na2CO3 = 14.59%
8 Effects of Carbonate Equilibria on Titration of Carbonate Shift in Phenolphthalein EndpointBoiling to Enhance Second Endpoint
9 Effect of CO2 Absorption on Phenophthalein Endpoint Reaction of CO2 with carbonate solutionsCO2(g) + H2O(l) + CO32(aq) 2HCO3(aq)Let 1 mmol CO2 react with 1 mmol CO32For titration with HCl to BCG endpoint to form H2CO3Initial 1 mmol CO32 would require 2 mmol HClResultant 2 mmol HCO3 requires 2 mmol HClNet effectsVolume of HCl require to reach phenolphthalein end decreasesThis effect may be avoided by excluding air from the titration system.Volume required to reach bromcresol green endpoint is not affected.RecommendationUse BCG endpoint to calculate amount of carbonate in your sample
10 Effect of Absorbed CO2 on Titration of Na2CO3 with HCl Ve1Ve2Ve1 decreasesVe2 is not affectedUse Ve2 for calculationsBeforeAfterCO2(g) + H2O(l) + CO32(aq) 2HCO3(aq)
11 Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl Na2CO3 + 2HCl H2CO3 + 2NaClNaHCO3Boiling removes H2CO3 as gaseous CO2NaHCO3+H2CO3H2CO3(aq) CO2(g) + H2O
12 Summary: Titration of Sodium Carbonate Titration of Na2CO3 with strong acid yields two equivalence pointsAt phenolphthalein end pointNa2CO3 NaHCO3At bromcresol green (or methyl red) end pointAll carbonate is converted to H2CO3pH of carbonate solutions may be unstable due to exchange of CO2 with atmosphere.
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