3g mixture of naoh,nahco3,na2co3 was titrated with1n hcl solution at 34.4 ml of volume when hph as indicator
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Double indicator acid-base titration:
In the acid-base titration the equivalence point is known with the help of indicator which changes its color at the end point. In the titration of polyacidic base or polybasic acid there are more than one end point. One indicator is not able to give color change at every end point. So to find out each end point we have to use more than one indicator. For example in the titration of Na2CO3 against HCl there are two end points.
Na2CO3 + HCl → NaHCO3 + NaCl
NaHCO3 + HCl → H2CO3 + NaCl
When we use phenolphthalein in the above titration it changes its color at first end point when NaHCO3 is formed and with it we cannot know second end point. Similarly with methyl orange it changes its color at second end point only and we cannot know first end point. It is because all indicator changes color on the basis of pH of medium. So in titration of against acid phenolphthalein cannot be used.
A sample contains NaOH, Na2CO3& NaHCO3. This solution is titrated with HCl or H2SO4
(i) When phenolphthalein (HPh) is used as indicator the reaction are given below.
Na2CO3 + HCl → NaHCO3 + NaCl
This is the half neutralization of Na2CO3
NaHCO3 + HCl → X
NaOH + HCl → NaCl + H2O
Hence
Meq. Of HCl = Meq. Of NaOH + (1/2) Meq. Of Na2CO3
(ii) When methyl orange (MeOH) is used as indicator
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
This is the complete neutralization
NaHCO3 + HCl → NaCl + H2O + CO2
NaOH + HCl → NaCl + H2O
in this case
Meq. Of HCl = Meq. Of NaOH + Meq. Of Na2CO3 + Meq. Of NaHCO3
(iii) If MeOH is added after the first end point obtained from HPh
NaHCO3 + NaHCO3 + HCl → NaCl + H2O + CO2
Produced original
Meq. Of HCl = Meq. Of NaHCO3(produced) + Meq. Of NaHCO3(original)
Note:
When we carry out dilution of solution, Meq eq. Milli mole or mole of substance does not change because they represent amount of substance, however molar concentration may change.
In the acid-base titration the equivalence point is known with the help of indicator which changes its color at the end point. In the titration of polyacidic base or polybasic acid there are more than one end point. One indicator is not able to give color change at every end point. So to find out each end point we have to use more than one indicator. For example in the titration of Na2CO3 against HCl there are two end points.
Na2CO3 + HCl → NaHCO3 + NaCl
NaHCO3 + HCl → H2CO3 + NaCl
When we use phenolphthalein in the above titration it changes its color at first end point when NaHCO3 is formed and with it we cannot know second end point. Similarly with methyl orange it changes its color at second end point only and we cannot know first end point. It is because all indicator changes color on the basis of pH of medium. So in titration of against acid phenolphthalein cannot be used.
A sample contains NaOH, Na2CO3& NaHCO3. This solution is titrated with HCl or H2SO4
(i) When phenolphthalein (HPh) is used as indicator the reaction are given below.
Na2CO3 + HCl → NaHCO3 + NaCl
This is the half neutralization of Na2CO3
NaHCO3 + HCl → X
NaOH + HCl → NaCl + H2O
Hence
Meq. Of HCl = Meq. Of NaOH + (1/2) Meq. Of Na2CO3
(ii) When methyl orange (MeOH) is used as indicator
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
This is the complete neutralization
NaHCO3 + HCl → NaCl + H2O + CO2
NaOH + HCl → NaCl + H2O
in this case
Meq. Of HCl = Meq. Of NaOH + Meq. Of Na2CO3 + Meq. Of NaHCO3
(iii) If MeOH is added after the first end point obtained from HPh
NaHCO3 + NaHCO3 + HCl → NaCl + H2O + CO2
Produced original
Meq. Of HCl = Meq. Of NaHCO3(produced) + Meq. Of NaHCO3(original)
Note:
When we carry out dilution of solution, Meq eq. Milli mole or mole of substance does not change because they represent amount of substance, however molar concentration may change.
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