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8.a-b
1. Potential of electrodes
1.a. Halide and nitrate ion selective electrodes
The main component of the halide ion selective electrode (ISE) is the membrane
from insoluble 𝐴𝐴𝐴𝐴𝐴𝐴 monocrystalline compound, where 𝑋𝑋 is a halide. The
membrane separates two solutions containing the anions 𝑋𝑋−
of the activities 𝑎𝑎1 and
𝑎𝑎𝑋𝑋−.
Inner solution 𝑋𝑋−
(𝑎𝑎1)
membrane
𝐴𝐴𝐴𝐴𝐴𝐴
Outer solution 𝑋𝑋−
(𝑎𝑎𝑋𝑋−)
The membrane potential ME is given by relationship:
𝐸𝐸𝑀𝑀 = 𝐶𝐶 +
𝑅𝑅𝑅𝑅
𝑛𝑛𝑛𝑛
𝑙𝑙𝑙𝑙
𝑎𝑎1
𝑎𝑎 𝑋𝑋−
= 𝐶𝐶 +
2,303 𝑅𝑅𝑅𝑅
𝑛𝑛𝑛𝑛
log 𝑎𝑎1 −
2,303 𝑅𝑅𝑅𝑅
𝑛𝑛𝑛𝑛
log 𝑎𝑎𝑋𝑋− (1.1.)
where 𝑛𝑛 = 1 is the number of transmitted electrons, 𝐶𝐶 is electrode constant. The
symbols 𝑅𝑅, 𝑇𝑇, 𝐹𝐹 have the usual meaning. The halide activity 𝑎𝑎1 of the inner solution is
determined by the electrode filling solution and it is constant in contrary to outer halide
activity 𝑎𝑎𝑋𝑋−. Both activities can be replaced by the molarities of the anions at low
concentrations (<0.01M).
The theoretical dependence of the
electrode membrane potential of is linear
function of log 𝑎𝑎𝑋𝑋− in the outer solution (eqn
(1.1.). The slope of the dependence is:
𝑑𝑑𝐸𝐸 𝑀𝑀
𝑑𝑑(log 𝑎𝑎 𝑋𝑋−)
= −2,303 𝑅𝑅𝑅𝑅
𝐹𝐹� = −0,059𝑉𝑉 (1.2.)
This value is called Nernst's electrode
response. The response is -59mV at
standard (25°C) and ideal conditions. The
real response may deviate for real
electrode. The Figure 1 shows the
dependence of the electromotive voltage
(EMV) of a cell composed of a chloride ISE
and a reference calomel electrode that are
immersed in a solution of KCl.
The constant inner activity of the chlorides
can be achieved by submerging the silver
wire coated with AgCl in water. The
solubility product constant of the AgCl is
P a aAgCl Ag Cl
= ⋅ =+ −
−
10 10
at 25°C. It means
that 𝑎𝑎1 = 𝑎𝑎𝐶𝐶𝐶𝐶− = 𝑎𝑎𝐴𝐴𝐴𝐴+ = 10−5
if the ideal
solution exists and other ions are not
present. The disadvantage of this design is
that the membrane response error occurred if low chloride concentrations (see Figure
1) is measured in outer solution because chloride ions are transported across the

Figure 1: Calibration curve (log 𝑎𝑎𝑋𝑋− =
log 𝑐𝑐𝐶𝐶𝐶𝐶− = log 𝑐𝑐𝐾𝐾𝐾𝐾𝐾𝐾) of chloride ISE in
electrochemical cell with a reference electrode.
L . T r n k o v á ú l o h a
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8.a-b
membrane from the inner to the outer solution. This disadvantage can be removed if the
inner solution is 0.1𝑀𝑀 𝐾𝐾𝐾𝐾𝐾𝐾 thus 𝑎𝑎1 = 𝑎𝑎𝐶𝐶𝐶𝐶− = 10−9
, the value 𝐶𝐶 in eqn (1.1.) changes to 𝐶𝐶`
(but remains constant) and the potential of chloride ISE is given by relationship:
𝐸𝐸𝐶𝐶𝐶𝐶 = 𝐶𝐶`
−
2,303 𝑅𝑅𝑅𝑅
𝐹𝐹
log 𝑎𝑎𝐶𝐶𝐶𝐶− (1.3.)
The presence of the other halogens (iodides, bromides) dissolved in the outer solution
disrupts selectivity of the chloride ISE:
𝐸𝐸𝐶𝐶𝐶𝐶 = 𝐶𝐶`
−
2,303 𝑅𝑅𝑅𝑅
𝐹𝐹
log(𝑎𝑎𝐶𝐶𝐶𝐶− + 𝐾𝐾𝐵𝐵𝐵𝐵 𝑎𝑎𝐵𝐵𝐵𝐵− + 𝐾𝐾𝐼𝐼 𝑎𝑎𝐼𝐼−) (1.4.)
where 𝐾𝐾𝐵𝐵𝐵𝐵 = 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴⁄ (or 𝐾𝐾𝐼𝐼 = 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴⁄ ) is the selectivity ratio of 𝐵𝐵𝐵𝐵−
(or 𝐼𝐼−
) anion
for the chloride ISE.
The chloride ISE will be damaged if the relationship 𝑎𝑎𝐵𝐵𝐵𝐵− > 𝑎𝑎𝐶𝐶𝐶𝐶−� 𝑃𝑃𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑃𝑃𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴⁄ � (or
𝑎𝑎𝐼𝐼− > 𝑎𝑎𝐶𝐶𝐶𝐶−� 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴 𝑃𝑃𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴⁄ �) valid because 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 (or 𝐴𝐴𝐴𝐴𝐴𝐴) deposes on the surface of the
𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 membrane. The electrode behave as bromide (or iodide) ISE after that event.
Therefore, the measurement of the chloride concentration using chloride ISE in iodide
(bromide) solutions is avoided. On the other hand, iodide ISE can be used to measure
chloride and bromide concentrations because the electrode is completely regenerated
after consequent measurement in the iodide solution.
Measurement with halide ISEs and their properties. The halide membrane
electrodes are mainly used for so-called direct potentiometry, whereby the
concentration of halides is obtained from the measured EMN. The cell is made up of a
membrane ISE and a reference electrode. The concentration is determined from the
knowledge of the calibration curve in Figure 1. All silver halide membrane ISEs react to
halide anions as well as loose silver cations with Nernst response, thus the least soluble
Ag2S membrane ISE is preferable for silver determination. The potentiometric
measurements with halide ISEs disrupt both anions forming less soluble silver salt than
membrane material (eg 𝑆𝑆2−
), and anions forming soluble silver complexes (𝐶𝐶𝐶𝐶−
, 𝑆𝑆𝑆𝑆𝑆𝑆−
).
Reducing agents cancel the halide measurement if they reduce AgX to Ag (see principle
of chemical photography).
Halide ISEs have a large internal resistance (similar to a glass electrode), their potential
in the solution stabilizes within 3 to 6 minutes. As a reference, we use a calomel
electrode and we connect the calomel electrode to the measured solution using a salt
bridge filled by 𝐾𝐾𝐾𝐾𝐾𝐾3 solution. The EMV of the cell can be measured with a pH or Ion
meter. Typically, the combined ISEs are used. The combined ISE includes membrane
ISE, salt bridge and reference electrode in one sensor. The halide ISEs used to be
stored in KX electrolyte (chloride in saturated KCl), if we do not measure.
Nitrate ISE. The inner solution of this electrode forms a solution of [tris­(1,10phenanthroline)]nickel(II)
nitrate in 2,4-dinitrophenyl n-octyl ether solvent. The
membrane is made polyvinyl chloride (PVC). Determination of nitrate ions is disrupted
by ions: ClO4−
, ClO3−
and I−
. The reference electrode is the calomel electrode without
the salt bridge with 𝐾𝐾𝐾𝐾𝐾𝐾3. The nitrate ISE used to be stored in 5·10-2
M 𝐾𝐾𝐾𝐾𝐾𝐾3 .
TASK: Measure the calibration curve of the combined nitrate membrane ion
selective electrode. Determine the 𝑁𝑁𝑁𝑁3
−
concentration in an unknown sample
(drinking water, beverages, milk, etc.). Determine the Nernst's response of the used
electrode in linear section of the calibration graph and compare it with the theoretical
response.

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8.a-b
LABORATORY AIDS AND CHEMICALS: combined ISE, Ion meter capable of mV
readings, 2 beakers (100 cm3
), scale pipettes (25, 10 and 5 cm3
), 9 volumetric
flasks (50 cm3
), storage solution, 𝑁𝑁𝑁𝑁3
−
stock solutions: 1M; 0.1M; 0.05M; 0,01M; 10-3
M a
10-4
M. Sample (drinking water).
INSTRUCTIONS: Get acquainted with the use of the pH meter in mode mV reading.
Assemble, if needed, the electrochemical cell: ISE | measured solution | salt
bridge | reference solution | reference electrode. Combined ISE cell is assembled from
the manufacturer so that then can be immersed directly in the measured solution.
1. PREPARATION OF STANDARD SOLUTIONS. Using a stock 𝑁𝑁𝑁𝑁3
−
solutions, prepare
calibration solutions in volumetric flasks with the following concentrations: 1·10-2
;
5·10-3
; 2.5·10-3
; 1·10-3
; 5·10-4
; 2.5·10-4
; 1·10-4
; 5·10-5
; 1·10-5
M.
2. PREPARATION OF COMBINED ELECTRODE. Clean the electrode with distilled water and
place it in a Teflon crucible with distilled water. Insert the stirrer. Switch the
instrument and mix on. Pour out the water after about 2 minutes of mixing. Repeat
the cleaning several times until the EMV ranges in interval ± 10mV.
3. CALIBRATION. Pour the weakest calibration solution into a dry Teflon crucible. Mix
about 3 min, switch the stirrer out, note the ENV of the electrode in mV. Measure
other calibration solutions in the same way, from the weakest to the more
concentrated.
4. SAMPLE MEASUREMENT. Prepare the electrode according to point 2. Measure the EMV
of the cell with the sample. Determine an unknown concentration of 𝑁𝑁𝑁𝑁3
−
using
calibration graph.
REPORT: TABULE 1: for each standard solution: 𝑁𝑁𝑁𝑁3
−
concentration 𝑐𝑐, 𝑙𝑙𝑙𝑙𝑙𝑙(𝑐𝑐), 𝐸𝐸𝐸𝐸𝐸𝐸.
Calibration graph 1: dependence of the EMV on log(c). Next: 𝐸𝐸𝐸𝐸𝐸𝐸 of the
sample and its concentration in (mg of 𝑁𝑁𝑁𝑁3
−
)/dm-3
unit. Experimental Nernst's response
of the used electrode.


