 # 213 PHC. Indicators  Describe the indicator theory.  Select a suitable indicator for a particular reaction.  Explain the different stages of strong.

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213 PHC

Indicators

 Describe the indicator theory.  Select a suitable indicator for a particular reaction.  Explain the different stages of strong acid- base titration curves.  Calculate the pH at these stages.

The point at which the reaction is observed to be complete is called the end point The end point can be detected by adding an indicator to the solution and visually detect a color change

 It is a weak acid or weak base that is highly colored  The color of the ionized form is markedly different from that of the unionized form

HIn  H + + In -  unionized ionized form form pH = pK In + log [In - ] / [HIn] (Henderson-Hasselbalch equation)

[In - ] / [HIn] = 1/10 (only color of unionized form is seen) pH = pK a + log 1/10 = pK a – 1 [In - ] / [HIn] = 10/1 (only color of ionized form is seen) pH = pK a + log 10/1 = pK a + 1

The pH in going from one color to the other has changed from pK a - 1 to pK a + 1 (a pH change of 2 units) Most indicators require a transition range of about 2 pH units

At midway of transition range, the concentration of the two forms are equal, and the pH = pK a The pK a of the indicator should be close to the pH of the equivalence point

At midway of transition range, the pOH = pK b and the pH= 14 – pK b A weak base indicator should be selected such that pH = 14 - pK b

Titration curves

 An acid-base titration involves a neutralization reaction ( an acid is reacted with equivalent amount of base)  A titration curve is constructed by plotting the pH of the solutuion as a function of the volume of the titrant added  The titrant is always a strong acid or base  The analyte may be either a strong acid or base or a weak acid or base

Both titrant and analyte are completely ionized e.g. titration of HCl with NaOH HCl + NaOH  NaCl + H 2 O

The analyte (weak acid) is partially ionized e.g. titration of HOAc with NaOH HOAc + NaOH  NaOAc + H 2 O

The analyte (weak base) is partially ionized e.g. titration of NH 3 with HCl NH 3 + HCl  NH 4 Cl

 The indicator theory.  Strong acid-base titration curves.  pH calculations.

 P. 247  18

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