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Practical No: 06

Practical: Preparation of Buffer Solutions

Date: 07/02/2016
Practical No: 06
Practical: Preparation of Buffer solutions
Objectives: To learn the technique of buffer solutions and to learn the technique of pH
Introduction:
Buffers are the solutions that are capable of resisting the pH changes when small amounts
of water, acid or bases are added
...
The
biological catalysts, enzymes are active only within certain pH ranges
...
A buffer system consists of a weak
acid and its conjugate base or vice versa
...
When acid is added the conjugate base takes up H+
...

Measurements of pH are made with an instrument known as a pH meter
...
One electrode is an H+ sensitive glass electrode
...
These two electrodes can be combined into a single probe, called a
“combination pH electrode”
...
This membrane becomes hydrated in the presence of water
...

This develops a pH dependent electrical potential with respect to the reference electrode,
which is measured by the potentiometer
...
This voltage is measured by the potentiometer and
converted into the scale of pH
...


50mM H3PO3/NaOH

pH 2
...
0

50mM CH3COOH/CH3COONa

pH 5
...
0

50mM Tris/HCl

pH 8
...
0

50mM Glycine/NaOH

pH 10
...
144ml of H3PO3 was measured and the pH of the prepared solution was measured using
pH meter
...


 Amount of Glycine to be measured =
187
...
675g of Glycine was measured and the pH of the prepared solution was measured using
pH meter
...


H+ + H2PO4-

H3PO4

pH

H+ + HPO4-

pKa

1

+

50 mM

Substitute by equation 01
2
...
99 mM

Amount of Na2HPO

519
...
0

50mM Na2HPO4/ NaH2PO4

6
...
0

Discussion

Buffer systems play important roles in nature and in laboratory settings
...
Acidic solution contains high concentrations of hydrogen ions (H+) and
have pH values less than seven
...
Neutral solutions contain equal
concentrations of hydrogen and hydroxide ions and have a pH of 7
...

Examples: Our blood is a buffer system that keeps pH between 7
...
45
...
Seawater is also a buffer; the average pH of ocean water is 8
...
The
buffering ability of ocean water protects sea life from the corrosive effects of acids and
bases
...
A classic
example of a weak acid based buffer is acetic acid (CH3COOH) and sodium acetate
(CH3COONa)
...
Chemists can prepare buffer systems to be almost any pH they want
...

Sensitive tools like pH probes that are exposed to acids and bases are often stored in buffer
solutions
...
Weak acids for example, don't
release large amounts of hydrogen ions in solutions like strong acids, rather the hydrogen
ion prefers to stick to the rest of the compound
...
Here's a figure representing what happens to a generic buffer system when
either acid or base is added
...
At equilibrium, the concentrations of HX and X- are
equal
...
Thus the ratio of HX/X- increases, but not enough to affect pH
significantly
...
This increases the amount of conjugate base present and
decreases the amount of weak acid present
...

Let's

take

that

classic

example of a buffer system,
the

one

with

equal

concentrations of the weak
acetic acid and its sodium
acetate
...
In this case, the
acetate ion is the conjugate
base of acetic acid
...
The key feature of the pHsensitive electrode is
a thin glass membrane whose outside surface contacts the solution to be tested
...
1
M HCl)
...
This electrode carries current through
the half-cell reaction
...
1 M), the electrode potential is also
constant
...
A common choice is to
use another Ag/AgCl electrode as the reference
...
1 M KCl solution which makes contact with the test solution through a porous fiber which
allows a small flow of ions back and forth to conduct the current
...

An accurate pH reading depends on standardization, the degree of static charge, and the
temperature of the solution
...

References:
•

http://www
...
org
...
htm (accessed on 15th February 2016)

•

pH Theory and Practice (accessed on 30th January 2016)



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