Pharmaceutical Analytical Chemistry

213 PHCDr. Mona AlShehri

4th Lecture

Acid-Base Titrations (1)

(1 )Gary D. Christian, Analytical Chemistry, 6th edition.

Define buffer and buffer ratio. Calculate the pH of acidic and basic

buffers. Describe the indicator theory. Select a suitable indicator for a particular

reaction.

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

Buffers

A solution that resists change in pH when a small amount of an acid or base is added

A mixture of a weak acid and its salt or a weak base and its salt at predetermined concentrations or ratios

Definition

Why do we use buffer?

◦Maintaining the pH for a reaction at an optimum value

◦Keeping the pH constant

HA H+ + A-

(Weak acid) (Salt)

Apply Henderson-Hasselbalch equation

pH = pKa + log [A-] / [HA]

pH = pKa + log [salt] / [weak acid]

Buffer of weak acid

Calculate the pH of a buffer solution containing 0.1M acetic acid and 0.1M sodium acetate (Ka = 1.75 x 10-5)

Problem 1:

B + H+ BH+

Weak base salt

Applying Henderson-Hasselbalch equation

pOH = pKb + log [BH+]/ [B]or

pH = pKa + log [B] / [BH+]

Buffer of weak base

Calculate the pH of a solution containing 0.07M NH3 and 0.08M NH4Cl (pKb = 4.76)

Problem 2:

The ratio of salt / acid

[salt] / [acid] = 1pH = pKa

By changing the buffer ratio, buffers

of different pH values can be prepared

Buffer Ratio

Baking soda

Gastric fluid

Vinegar

Lemon juice

Orange juice

Coffee

Soft drinks

Milk

Saliva

Urine

Blood

Pure water

Sea water

Soapy water

Bleach

Household ammonia

HomeworkWhat is the pH of the following substances?

Indicator

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 indicator

[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

A weak base indicator should be selected such that pOH = pKb

Weak base indicator

A good indicator must have the following properties:

The color change must be easily detected. The color change must be rapid. The indicator molecule must not react with

the substance being titrated. To minimize errors, the indicator should

have a pKin that is within one pH unit of the expected pH at the equivalence point of the titration.

pH ranges and colors of some common indicators

Natural IndicatorRed cabbage juice contains a mixture of substances

whose color depends on the pH value. Each test tube contains a solution of red cabbage juice in

water, but the pH of the solutions varies from pH = 2 - 11

You can make a very effective pH indicator in a few steps with hot water and red cabbage leaves. Follow the steps below:

Record your reu

results

Homework:Make your own indicator

Buffer Definition. buffer ratio. pH Calculation of weak acid and weak base buffers.

The indicator theory. Weak acid and weak base indicators.

Summary

Thank you