Double Indicators

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Datalogging Experiment (4)

Acid-base Titration using Method of Double Indicators
Student Handout Purposes To determine the composition of the following mixture by double indicator method: 1. NaOH(aq) and Na2CO3(aq) 2. NaHCO3(aq) and Na2CO3(aq)

Introduction Consider a mixture of NaOH(aq) and Na2CO3(aq). Reaction between HCl(aq) and Na2CO3(aq) takes place in two stages: HCl(aq) + Na2CO3(aq) ⎯→ NaHCO3(aq) + H2O(l) …………………. (1) HCl(aq) + NaHCO3(aq) ⎯→ NaCl(aq) + CO2(g) + H2O(l) …………. (2) While that between HCl(aq) and NaOH(aq) completes in only one step: HCl(aq) + NaOH(aq) ⎯→ NaCl(aq) + H2O(l) ……………….………. (3) Solution mixture of reaction (1) at the equivalence point is alkaline, that of reaction (2) is acidic and that of reaction (3) is neutral. Thus the whole titration should have three breaks in the pH curve, corresponding to the above three stages. Reactions (1) and (3) can be indicated by phenolphthalein and that of reaction (2) can be indicated by methyl orange. Stoichiometry confines each of the above pH reactions to react according to a mole ratio of 1 : 1. This means, say from equation (2), the number of mole of HCl(aq) determined from the methyl orange titration is equal to the number of mole of NaHCO3(aq). Likewise, total number of moles of NaOH(aq) and Na2CO3(aq) in the solution mixture can be calculated according to the volumes of HCl(aq) added at the end point Vol. of HCl indicated by the colour change of the phenolphthalein indicator. Alternatively, the Fig. 1: Titration curve for a mixture of three break points (see Fig. 1) also indicate NaOH(aq) and Na2CO3(aq) with HCl(aq) the volume of HCl(aq) required for each reaction.


Datalogging Experiment (4)

For the titration of a mixture of NaHCO3(aq) pH and Na2CO3(aq) with HCl(aq), only two break points are expected (see Fig.2). Volume of HCl(aq) added for each break point can be easily obtained by observing either the colour change at the end point or the shape of the titration curve....
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