Antimony Microelectrode for Facile Measurements
of the Near‐Surface pH
Identification of Electrode Processes Involving Protons
I. Flis‐Kabulska1, J. Flis1, T. Zakroczymski1, Y. Sun2
1
Introduction
Miniaturized sensors for pH measurements have found a wide
application especially in medicine. They are also of importance for
examination of electrochemical processes on corroding metals
and inside of occluded cells, as e.g. pits.
Many reactions are reversible to protons and can be used to
measure pH. The basic is the hydrogen electrode, whereas the
most popular is glass electrode. The latter is not suitable for
miniaturization because it is fragile and has high electrical
resistance.
Easily miniaturized are metal/metal oxide electrodes. Most
popular is antimony electrode Sb/Sb2O3.
Sb/Sb2O3 electrode:
2 Sb + 3 H2O = Sb2O3 + 6 H+ + 6e‐
ESb = 0.152 – 0.059 pH
(VSHE)
Institute of Physical Chemistry PAS, Warszawa, Poland
2 De Montfort University, Leicester LE1 9BH, UK
Potential ESb of the
antimony electrode
Schematic presentations of
antimony microelectrode
Molten antimony was sucked into
thick‐wall tubes of boron glass and
then capillaries were made by
stretching.
To avoid the distortion of the measuring systems, the potential
of the antimony electrode (ESb) was measured vs. SCE in a
circuit being separate from that for controlling the working
electrode.
The edge of the capillary tip can
be lent on the horizontal or the
sample at a desired angle. In
vertical surface of this way, a
distance of the measurement of
pH from the electrode depends
only on the diameter of the tip
and on the inclination angle.
The slope 59 mV/pH is theoretical. Each antimony electrode
should be calibrated against buffer solutions.
of ‐40 mV/pH.
Glass
Φ 1.5 mm
80 o
d = 0.13 mm
Sample
Response of Sb electrode to voltammetric cycles
in 0.1 M Na2SO4 + 0.4 M NaCl, bulk pH=2.8 (SO4/Cl)
Material: AISI 316L ss
‐ untreated and nitrided
Plasma nitriding: 80 N2 + 20% H2, 415 oC, 15 h
Untreated 316L steel
j / mA cm
SO4/Cl, pH 2.8
0.4
0.2
ESb vs. SCE / V
0.0
pH
Eb
-0.2
-0.24
3.0
Decrease of pH at the rising
anodic current (at Eb) gives
evidence for the occurrence
of reactions which generate
protons H+:
3.2
-0.26
pH drop
-0.28
3.4
3.6
2 Cr3+ + 3 H2O = Cr2O3 + 6 H+
3.8
-0.8
-0.6
-0.4
-0.2
0.0
0.6
-2
-2
0.6
316L
Nitrided 316L steel
j / mA cm
A simple antimony microelectrode was
prepared for facile measurements of pH at a
fixed small distance from the electrode
surface.
For the capillary used, the
antimony tip was at a distance of
0.13 mm from the sample surface.
316L + N
0.4
AR
II
0.2
IIR
-0.2
pH
0.0
-0.2
-0.4
0.2
E vs. MSE / V
I
0.0
ESb vs. SCE / V
This work
In the pH region of 3 to 10, the relationship
between ESb and pH was linear with the slope
Sb
Φ 0.1 mm
pH drop
pH rise
-0.8
Mo3+ + 2 H2O = Mo2O + 4 H+ + e‐
2.0
3.0
6.0
-0.6
-0.4
-0.2
0.0
0.2
E vs. MSE / V
Peak I:
Increase of pH at peak I
can be explained by
binding of protons H+ into
NH4+:
[N] (from steel) + 4 H+ (in solution) + 3e‐ = NH4+
Peak II:
Potential between peak II and IIR fits to the equilibrium
potential for the couple Fe2+/Fe(III) oxide.
Conclusions
Possible reactions:
2 Fe2+ + 3 H2O = Fe2O3 + 6 H+ + 2e‐
Fe2+ + 2 H2O = γ‐FeOOH + 3 H+ + e‐
1. Advantages of the Sb microelectrode for the near‐surface pH
measurements:
-2
0.8
j / mA cm
‐ measurement is made at a small distance (about 0.1 mm) from the
examined surface;
I
0.4
II
0.2
AR
ESb vs. SCE / V
pH
0.1
0.0
-0.1
pH drop
pH rise
Decrease of pH
indicates the
occurrence of H+
producing reactions.
2
3
4
-0.2
-0.3
-0.8
‐ the electrode is easy to install, because it is freely leant on the examined
surface;
-0.4
0.0
0.4
E vs. MSE / V
0.8
In transpassive region:
Cr2O3 + 5 H2O = 2 HCrO4‐ + 8 H+ + 6e‐
For water oxidation:
2 H2O = O2 + 4 H+ + 4e‐
‐ distortion of electrode processes on sample is minimised; because only
an edge of the tip is in contact with the examined surface;
2. The pH measurements at the sample surface provide valuable
information on electrode processes.
III
0.0
‐ distance of the Sb electrode tip to the examined surface is the same for
each insertion; it depends only on diameter of the tip and on the
inclination angle;
‐ the electrode does not contain mercury, is relatively simple in
preparation and operation.
0.6
316L+N
Acknowledgement
Financial support from European Union 7.FP under grant
REGPOT‐CT‐2011‐ 285949‐NOBLESSE is gratefully acknowledged
NanOtechnology, Biomaterials and
aLternative Energy Source for ERA integration
FP7–REGPOT–CT–2011–285949‐NOBLESSE