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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