[1] DOUBLE INDICATOR

Carbonates and Carbonate Mixtures

qualitative and quantitative determination of the constituents in a solution containing NaOH, Na₂CO₃ and NaHCO₃, either alone or admixed

Carbonates and Carbonate Mixtures

• no more than two of these constituents can exist in an appreciable amount in any solution because reaction will eliminate the third

Carbonates and Carbonate Mixtures

analysis of such mixture requires two titrations:

• one with an alkaline-range indicator (e.g. Pp)

• the other with an acid-range indicator (e.g. MO)

Quantitative Determination

composition of the solution can then be deduced from the relative volumes of acid needed to titrate equal volumes of the sample

Pure NaOH
Volume Relationship

mL_Pp

mL_MO = 0.00

Pure NaOH

Volume Expression

mL_Pp

Pure Na₂CO₃

Volume Relationship

mL_Pp = mL_MO

Pure Na₂CO₃

Volume Expression

• (mL_Pp + mL_MO) 

• 2(mL_Pp) or 2(mL_MO)

Pure NaHCO₃

Volume Relationship

mL_Pp = 0.00

mL_MO

Pure NaHCO₃

Volume Expression

mL_MO

Mixture: NaOH & Na₂CO₃

Volume Relationship

mL_Pp > mL_MO

Mixture: NaOH & Na₂CO₃

Volume Expression

(mL_Pp - mL_MO) or 2(mL_MO)

Mixture: Na₂CO₃ & NaHCO₃

Volume Relationship

mL_Pp < mL_MO

Mixture: Na₂CO₃ & NaHCO₃

Volume Expression

2(mL_Pp) or (mL_MO - mL_Pp)

NaOH & Na₂CO₃

mL used to neutralize OH^-

(mL_Pp - mL_MO)

NaOH & Na₂CO₃

mL used to neutralize CO₃^-2

2(mL_MO)

Na₂CO₃ & NaHCO₃

mL used to neutralize HCO₃

(mL_MO - mL_Pp)

Na₂CO₃ & NaHCO₃

mL used to neutralize CO₃^-2

2(mL_Pp)

Hydroxides

• complete reaction / neutralization

• titration will only happen in Pp because the solution is already neutral under MO

Carbonates

• equal volumes of the acid titrant were consumed under Pp and MO

• neutralized in 2 elementary steps

Carbonates

• neutralized in 2 elementary steps:

• Pp + acid → salt + base

the reaction is still considered incomplete since a base is still present

• MO + acid → salt + CO₂ + water

expected scenario: yellow upon the addition of MO, then titrated to be salmon pink

Bicarbonates

• opposite of hydroxides

• expected scenario: light pink upon the addition of Pp, but yellow after adding MO

  • since bicarbonate is still present, the solution is not yet neutral

Mixture of Carbonates & Hydroxides

• more Pp than MO

Mixture of Carbonates & Bicarbonates

more MO than Pp

Qualitative Determination

once the composition of the solution has been established, the volume data can be used to determine the concentration of each component in the sample

Pure NaOH

chemical equation

NaOH + HCl → NaCl + H₂O

Pure Na₂CO₃

_{chemical equation}

Na₂CO₃ + HCl → NaCl + NaHCO₃  

NaHCO₃ + HCl→ NaCl + CO₂ + H₂O