the brief of electrodes

1. VISHNUPRIYA.C
I MSc BIOCHEMISTRY

2.  An ion-selective electrode (ISE), also known
as a specific ion electrode (SIE), is a
transducer (or sensor) that converts the
activity of a specific ion dissolved in a
solution into an electrical potential
 Ion-selective electrodes are used in
analytical chemistry and
biochemical/biophysical research, where
measurements of ionic concentration in an
aqueous solution are required

3.  The voltage is theoretically dependent on
the logarithm of the ionic activity, according
to the Nernst equation
 It responds directly to analyte
 Used for direct potentiometric studies
 Selectively binds and measures the activity
of ions.

4.  I.S.E are few electrodes which can measure
both positive and negative ions
Examples:
 PH electrode
 Glass electrode
 Calcium (ca2+ ) electrode
 Chloride (cl-) electrode

7.  It works on the basic principle of galvanic cell
 I.S.E consists of a thin membrane across which
only the intended ion can be transported
 The transport of ions from higher to low one
through selective binding with some sites within
the membrane creates a potential difference

9.  Ion selective electrode:
with the membrane at the end allows
ions of interest to pass through but excludes
the passage of other ions.
 Internal reference electrode :
 present within the ion selective electrode
which is made up of silver wire coated with
solid silver chloride embedded in
concentrated potassium chloride solution

10.  This solution also contains the same ions as
that to be measured.
 Reference electrode:
 similar to that of ion selective electrode,
provide stable reference voltage
 commonly used-calomel electrode
 The lower end of the reference electrode is
sealed with a porous ceramic frit which allows
the slow passage of the internal filling solution
and external test solution

11.  Dipping into the filling solution is a silver
wire coated with a layer silver chloride
which is joined to a low- noise cable which
connects to the measuring system
 It is necessary as the potential between an
electrode and solution cannot be directly
measured.
 The reference electrode should have a
known or at least constant potential value
under prevailing experimental conditions.

12.  Problem with reference electrode is that in
order to ensure a steady voltage, it is
necessary to maintain a steady flow of
electrolyte through the porous frit
 Thus it results in contamination of test
solution with electrolyte ions
 This can cause problems when trying to
measure low levels of K , Cl , Ag

13.  Overcome this by double junction electrode
 In this Ag/Ag Cl is inserted into outer tube
containing a different electrolyte which is
then in contact with outer test solution
through second porous frit.
 The outer filling solutions form “salt bridge”
between the inner reference system and the
test solution so that it does not contaminate
the test solution

14.  Commonly used outer filling solutions
1. Potassium nitrate- for Pb, Hg, br ,Cl
2. Sodium chloride- for K
3. Ammonium sulphate-for NO3
 It must be noted E0 factor in Nernst equation
is the sum of all the liquid junction
potentials present in the system

15.  The two solutions in a double junction
reference electrode are connected by liquid
junction
 It is a semi-permeable membrane separating
the two solutions which prevents wholesale
mixing but allows the passage of ions by
diffusion

16.  Circuit
 on the both ends of electrode is fitted with
low noise cable or gold plated pin for
connection to the millivolt measuring device
 Working:
It consists of reference electrode ,ion
selective membrane and voltmeter. The
transport of ions from high to low
concentration through selective binding of ions
with the specific sites of the membrane
creates a potential difference.

18.  It is used in many diverse branches of
biology
 Different anode- cathode combinations for
oxygen are available , the platinum with
silver/ silver chloride is the most commonly
used anode- cathode electrode.
 Arrangement of these electrodes are annular
with the anode enclosing the platinum
electrode

19.  Electrodes dipped into electrolyte solution
(usually buffered KCl solution ) which is held
inside an electrode by oxygen permeable
membrane
 Membrane might be very thin polypropylene
 Polarization of electrodes at 0.6v is achieved
with the help of a mercury cell

26.  It contains pH sensitive glass electrode and a
reference electrode immersed in a
bicarbonate buffer system
 This is separated from the solution under the
test mainly blood by a plastic membrane
permeable to gaseous co2, but not
permeable to dissolved ions.

27. Contd..
 The co2 in the blood diffuses through the
plastic membrane and reacts with buffer
system to change the pH

31.  It contains pH sensitive glass electrode and a
reference electrode immersed in bicarbonate
solution
 Separated from test solution under test
mainly blood by a plastic membrane
permeable to gaseous co2
 Co2 in blood diffuses through the plastic
membrane and reacts with buffer and results
in change in pH

32.  The H+ ions concentration change in
dissolution of co2 is detected by pH sensitive
and reference electrode and it is measured
 The meter scale is usually calibrated for
pCO2 in semi logarthmic fashion since pH is
inversely proportional to the log of the pCO2