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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