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1 Titration of Sodium Carbonate. 2 Carbonate Chemistry CO 2 in atmosphere and dissolved in water Major global buffering system Industrial sources –limestone:

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Presentation on theme: "1 Titration of Sodium Carbonate. 2 Carbonate Chemistry CO 2 in atmosphere and dissolved in water Major global buffering system Industrial sources –limestone:"— Presentation transcript:

1 1 Titration of Sodium Carbonate

2 2 Carbonate Chemistry CO 2 in atmosphere and dissolved in water Major global buffering system Industrial sources –limestone: CaCO 3 (s) + heat  CaO(s) + CO 2 (g) –trona (Na 2 CO 3 ) deposits Air/Water Equilibrium Ionization in Water

3 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 K a2 for H 2 CO 3

4 4 Phenolphthalein Titration of Na 2 CO 3 with HCl Step 1: Step 2: Na 2 CO 3 + HCl  NaHCO 3 + NaCl NaHCO 3 + HCl  H 2 CO 3 + NaCl Overall:Na 2 CO 3 + 2HCl  H 2 CO 3 + 2NaCl Equivalents of HCl added pH At V e1 : At V e2 : V e1 V e2 BCG

5 5 Titration of Sodium Carbonate Equivalents of HCl added pH CO 3 2  HCO 3  H 2 CO 3 [CO 3 2  ] = [HCO 3  ] pOH = pK b,CO3 pH = pK a2,H2CO3 pH = pK a1

6 Derivative Plots Spreadsheet First and Second Derivative Plots Click on “Derivative Plots” tab Raw data 2 nd derivative Raw data: pH vs V 1 st derivative: d(pH)/dV 2 nd derivative: d 2 (pH)/(dV) 2 Derivative plots can be used for accurate location of the equivalence point

7 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 % Na 2 CO 3 in the sample. For the BCG endpoint, the reaction is Na 2 CO 3 + 2HCl  H 2 CO 3 + 2NaCl % Na 2 CO 3 = 14.59%

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

9 9 Effect of CO 2 Absorption on Phenophthalein Endpoint Reaction of CO 2 with carbonate solutions CO 2 (g) + H 2 O(l) + CO 3 2  (aq)  2HCO 3  (aq) –Let 1 mmol CO 2 react with 1 mmol CO 3 2  –For titration with HCl to BCG endpoint to form H 2 CO 3 Initial 1 mmol CO 3 2  would require 2 mmol HCl Resultant 2 mmol HCO 3  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 10 Effect of Absorbed CO 2 on Titration of Na 2 CO 3 with HCl CO 2 (g) + H 2 O(l) + CO 3 2  (aq)  2HCO 3  (aq) V e1 V e2 After Before V e1 decreases V e2 is not affected Use V e2 for calculations

11 11 Boiling to Enhance Visual Endpoint in Titration of Na 2 CO 3 with HCl Na 2 CO 3 + 2HCl  H 2 CO 3 + 2NaCl NaHCO 3 + H 2 CO 3 NaHCO 3 Boiling removes H 2 CO 3 as gaseous CO 2 H 2 CO 3 (aq)  CO2(g) + H 2 O

12 12 Summary: Titration of Sodium Carbonate Titration of Na 2 CO 3 with strong acid yields two equivalence points At phenolphthalein end point –Na 2 CO 3  NaHCO 3 At bromcresol green (or methyl red) end point –All carbonate is converted to H 2 CO 3 pH of carbonate solutions may be unstable due to exchange of CO 2 with atmosphere.


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