1. Titration of Sodium Carbonate

2. 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

3. 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

4. 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

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

6. 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

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 is
Na2CO3 + 2HCl  H2CO3 + 2NaCl
% Na2CO3 = 14.59%

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

9. 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

10. 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)

11. 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

12. 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.