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Titration of Sodium Carbonate

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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

22 2 2 3( ) ( ) ( )

H OCO g CO aq H CO aq

Air/Water Equilibrium

Ionization in Water

6.342 3 3 1

2 10.363 3 2

10

10a

a

H CO H HCO K

HCO H CO K

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Phenolphthalein

Titration of Na2CO3 with HClStep 1:

Step 2:

Na2CO3 + HCl NaHCO3 + NaCl

NaHCO3 + HCl H2CO3 + NaCl

Overall: Na2CO3 + 2HCl H2CO3 + 2NaCl

Equivalents of HCl added

pH

At Ve1:

At Ve2:

2 3

2 3

2 32

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12

Na CO HCl

e HCl

mol Na COn nmol HCl

mol Na COV M

mol HCl

2 3 1Na CO HCl e HCln n V M

Ve1 Ve2

BCG

Derivative Plots

SpreadsheetFirst and Second Derivative Plots

Click on “Derivative Plots” tab

Raw data

2nd derivative

Raw data: pH vs V1st derivative: d(pH)/dV2nd derivative: d2(pH)/(dV)2

Derivative plots can be used for accurate location of the equivalence point

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Determination of Carbonate in a Sample

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A 1.2040-g sample containing sodium carbonate and inert material was dissolved in water and titrated to the bromcresol green end point, requiring 32.50 mL of 0.1020 M HCl.

Calculate the % Na2CO3 in the sample.

For the BCG endpoint, the reaction is

Na2CO3 + 2HCl H2CO3 + 2NaCl

2 3

2 312Na CO HClNa CO

n nHCl

2 3

2 3

1 0.105989 1002% 100

1.2040

HCl HClgM mL

Wt Na CO mmolNa COWt sample g

% Na2CO3 = 14.59%

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Effects of Carbonate Equilibria on Titration of Carbonate

Shift in Phenolphthalein EndpointBoiling to Enhance Second Endpoint

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Effect of Absorbed CO2 on Titration of Na2CO3 with HCl

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Volume of HCl added, mL

pH

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Volume of HCl added, mL

pH

CO2(g) + H2O(l) + CO32(aq) 2HCO3

(aq)

Ve1 Ve2

After

Before

• Ve1 decreases

• Ve2 is not affected

• Use Ve2 for calculations

2 3

2 3

2

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Na CO HCl

HCl e

Na COn n

HCl

M V

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Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl

Na2CO3 + 2HCl H2CO3 + 2NaCl

NaHCO3+H2CO3

NaHCO3

Boiling removes H2CO3 as gaseous CO2

H2CO3(aq) CO2(g) + H2O

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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.