INTRODUCTION
Electrochemistry is a breach of
chemistry that studies chemical reactions which take place in a solution at the
interface of an electron conductor (the electrode: a metal or a semi-conductor)
and an ionic conductor (the electrolyte) and which involves electron transfer
between the electrode and the electrolyte or species in solution.
If a chemical reaction is driven by
an external applied voltage, as in electrolysis or if a voltage is created by a
chemical reaction as in a battery, it is an electrochemical reaction. In
contrast, chemical reactions where electrons are transferred between molecules
are called oxidation /reduction (redox) reactions. In general, electrochemistry
deals with situations where oxidation and reduction are separated in space or
time, connected by on external electric circuit
ELECTROCHEMICAL METHOD OF WATER ANALYSIS
BASED ON OXIDATION-REDUCTION (REDOX) REACTION
Electrochemical reactions in water
are better understood by balancing redox reactions using the ion- electron
method where H+, OH-
ions, H20 and electrons (to compensate the oxidation changes) are added to cell’s half reactions for
oxidation and reduction.
ACIDIC
MEDIUM
In acidic medium, H+ ions
and water are added to half- reactions to balance the overall reactions. For
example when manganese reacts with sodium bismuthate.
Unbalanced Reaction:-
Mn2+(aq)
+ NaBi03(S) ----- Bi3t(aq) +MnO4(aq)
Oxidation:-
4H20(1) + Mn 2+(aq) ------ MnO4 + 8H+(aq)+ 5e
Reduction:-
2e - + 6H+ +
BiO3 ----- Bi3+(aq) + 3H20(1)
Finally, the reaction is balanced by
multiplying the number of electrons from the reduction half reaction and vice
versa and adding both half reactions, thus solving the equation
8H2O(1)
+ 2Mn2+(aq) -------
2MnO4 (aq) + 16H+(aq) + 10e -
10e
- + 30H+ (aq) + 5BiO3(s)
- ------- 5Bi3t
(aq) + 15 H2 O(l)
BASIC MEDIUM
In basic medium, OH- ions
and water are added to half reactions to balance the overall reaction. For
example, on reaction between potassium permanganete and sodium sulphate.
Unbalanced Reaction:
KMnO4
+ Na2SO3 + H2O ----- MnO2
+ Na2SO4 + KOH
Reduction:
3e - + 2H20 + Mn4 - ----- MnO2
+ 4OH -
Oxidation:
2OH - + SO32 - ----- SO42 – + H2O + 2e -
The same procedure as followed on
acid medium by multiplying electrons to opposite half reactions solve the
equation thus balancing the overall reaction.
6e
- + 4H2 O + 2MnO4- -----
2MnO2 + 80H –
6OH
- + 3SO32 - ----- 3SO42
- + 3H2O + 6e -
Balanced Equation.
2KMnO4
+ 3Na2SO4 + H2O ----- 2MnO2 + 3Na2SO4
+ 2KOH
NEUTRAL MEDIUM:
The same procedure as used on acid medium is applied. For
example on balancing using electron ion method to complete combustion of
propane.
Unbalanced Reaction
C3
H8 + O2 ----- CO2 + H2O
Reduction:-
4H+ + O2 + 4e - ----- 2H2O
Oxidation:- 6H2O + C3H8 -----
3 CO2 + 20e - + 20H+
As in acid and basic medium, electrons
which were used to compensate oxidation change are multiplied to opposite half
reaction, thus solving the equation.
20H+
+ 5O2 + 20e - -----
10H2O
6H2O
+ C3H8 ----- 3CO2 + 20e - + 20H+
Balanced
equation
C3H8
+ 5O2 ----- 3CO2
+ 4H2O
MEASUREMENT TECHNIQUE IN WATER ANALYSIS
This involves using Meters with electrodes. The outer
portions of all atoms and molecules consist of shells of electrons and all
chemical reactions involve interaction with these outer electrons; sharing or
transfer, or something in between. It
is not surprising that electricity and chemistry are interrelated (just think
of batteries) and that electrical measurements can be used to detect and determine
some substances of interest. The procedure involves placing electrodes in a
water sample and measuring either an electrical potential (voltage), in
millivolts or a current in milliamperes which is related to the concentration
of analyte. Depending on what they are designed to measure, electrodes can be
simple pieces of metals such as gold, silver, platinum, copper etc; or they may
be elaborate system with semi-permiable membranes and several internal
electrodes and filling solutions. The instrumentation may be capable of reading
out directly in concentration units. Usually, some sort of calibration
procedure is necessary, using one or more standard solutions of known
concentration.
ELECTROLYSIS OF WATER
Water can be converted to its
component elemental gases, H2
and 02 through the application of an external voltage. Water
doesn’t decompose into hydrogen and oxygen spontaneously as the Gibbs free
energy for the process at standard conditions is about 474.4kj. The decomposition
of water into hydrogen and oxygen can be performed in an electrolytic cell. In it,
a pair of inert electrodes usually made of platinum immersed in water acts as
anode and cathode in the electrolytic process. The electrolysis starts with the
application of an external voltage between the electrodes. This process will
not occur except at extremely high voltage without an electrolyte such as
sodium chloride or sulphuric acid (most wed 0.1.M). Bubbles from the gases will
be seen near both electrodes. The following half reaction describe the process
mentioned above
Anode
(oxidation):
2H2O(l) ----- O2(g), + 4H(ag)+
+ 4e -
Cathode (reduction)
2H2O(g)
+ 2e ------ H2(g)
+ 2OH - (aq)
Overall Reaction
2H2O(1)
----- 2H2(g) + O2
(g)
Although strong acids may be used in
the apparatus, the reaction will not net consume the acid. While this reaction
will work at any conductive electrode at a sufficiently large potential,
platinum catalyzes both hydrogen and oxygen formation, allowing for relatively
mild voltages (~2V depending on the pH)
Electrochemical method for the measurement of water
analysis that make use of meters with electrodes which is an instrument that
can detect the temperature and pH of the water.
APPLICATION OF ELECTRODE WITH METER FOR
WATER QUALITY TESTING
For analyzing water in pools and spas, lithographic processes,
boiler and cooling tower water analysis and all types of quality assurance and
water quality testing.
In environmental / agricultural
sector it is used in ecology studies, aquarium, aquaculture and nutrient levels
in hydroponics.
FACTORY OF AGRICULTURE AND NAT RESOURCE MANAGEMENT
DEPARTMENT OF SOIL SCIENCES AND ENVIROMENT MANAGEMENT
WATER ANALYSIS BASED ON
OXIDATION-REDUCTION (REDOX) REACTION