Pharmaceutical Analytical Chemistry

213 PHC

Acid-Base TitrationsIndicators

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.

By the end of the lecture you should be able to:

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

How to detect the end point of a reaction?

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

Indicators

HIn H+ + In-

unionized ionized form form

pH = pKIn + log [In-] / [HIn](Henderson-Hasselbalch equation)

Weak acid indicators

[In-] / [HIn] = 1/10 (only color of unionized form is seen)

pH = pKa + log 1/10 = pKa – 1

[In-] / [HIn] = 10/1 (only color of ionized form is seen)

pH = pKa + log 10/1 = pKa + 1

The pH in going from one color to the other has changed from pKa - 1 to pKa + 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 = pKa

The pKa of the indicator should be close to the pH of the equivalence point

At midway of transition range, the pOH = pKb and the pH= 14 – pKb

A weak base indicator should be selected such that pH = 14 - pKb

Weak base indicators

pH transition ranges and colors of some common indicators

Acid-Base Titration

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 + H2O

Strong acid versus strong base

Titration curve for 100 ml 0.1M HCl versus 0.1 M NaOH

The analyte (weak acid) is partially ionizede.g. titration of HOAc with NaOH

HOAc + NaOH NaOAc + H2O

Weak acid versus strong base

Titration curve for 100 ml 0.1 M HOAc versus 0.1 M NaOH

The analyte (weak base) is partially ionizede.g. titration of NH3 with HCl

NH3 + HCl NH4Cl

Weak base versus strong acid

Titration curve for 100 ml 0.1 M NH3 versus 0.1 M HCl

Questions?

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

Summary

P. 247 18

Homework

Thank you