• Carbonate Chemistry

• CO2 in atmosphere and dissolved in water

• Major global buffering system

• Industrial sources

–limestone:  CaCO3(s) + heat à CaO(s) + CO2(g)

–trona (Na2CO3) deposits

• 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

• Titration of Na2CO3 with HCl

• Titration of Sodium Carbonate

• Derivative Plots

• Determination of Carbonate in a Sample

• 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) à 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

• Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl

• Summary: Titration of Sodium Carbonate

• Titration of Na2CO3 with strong acid yields two equivalence points

• At phenolphthalein end point

–Na2CO3 à 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.

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