Page 43 - PC 101 practical notes 24-25..

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL I PHARM. ANAL. CHEM. I (PC 101)

Exp.: Determination of NaOH & Na2CO3 mixture

P Pr ri in nc ci ip pl le e : :

 According to the two-indicators method:

a) First step: (E.P ph.ph.)

Titration of 10 ml of the mixture ≠ standard HCl using ph.ph. indicator.

- 1 --
E.P ph.ph. ≡ OH + CO 3
2

b) Second step: (E.P M.O.)

Titration of another 10 ml of the mixture ≠ standard HCl using M.O. ind.

- --
E.P M.O. ≡ OH + CO 3
 Reaction equations:

For NaOH:

NaOH + HCl → NaCl + H 2O

For Na2CO3:

Na 2CO 3 + HCl → NaCl + NaHCO 3 ...................Half neutralization

(pH>10) (pH ≈ 8.3)

NaHCO 3 + HCl → NaCl + CO 2 + H 2O ...............Complete neutralization

(pH ≈ 3.8) .

Na 2CO 3 + 2HCl → 2NaCl + CO2 + H 2O

N.B.:

The reaction between HCl & Na 2CO 3 is an example of displacement titration

where HCl (strong acid) displaces carbonic acid (very weak acid) in its salt.

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