Environmental Engineering Reference
In-Depth Information
6.3 Phosphate MEA Sensor
Cobalt (Co) metal was used as the electrode material for the phosphate mea-
surements [59]. This direct measurement approach is based on the formation of
Co
3
(PO
4
)
2
precipitate on Co surface [71-74]. The sensing mechanism involves
dissolution of cobalt on the electrode surface and formation of oxide film [72]:
4H
+
+
4e
−
2Co
+
2H
2
O
↔
2CoO
+
4H
+
+
4e
−
↔
O
2
+
2H
2
O
2CoO
Phosphate present in solution leads to the formation of cobalt phosphate on the
electrode surface, depending on the solution pH [72]:
2Co
+
O
2
↔
2H
2
PO
4
2H
+
↔
3CoO
+
+
Co
3
(PO
4
)
2
+
3H
2
O
(at acidic pH)
2HPO
2
4
4OH
−
3CoO
+
+
H
2
O
↔
Co
3
(PO
4
)
2
+
(at neutral pH)
2PO
3
4
6OH
−
3CoO
+
+
3H
2
O
↔
Co
3
(PO
4
)
2
+
(at basic pH)
These coupled reactions show the shift in the equilibrium potential that is
dependent upon oxidation of cobalt, reduction of oxygen, and Co
3
(PO
4
)
2
pre-
cipitate forming on the electrode surface. This leads to a shift of the mixed
potential to more negative, while keeping other factors constant. The shift is
related to the phosphate concentration since equilibrium potentials are governed
by the Nernst equation [75]. Thus, a linear potential response may be expected
with exponential change in phosphate ion concentration (at constant levels of
dissolved oxygen) [76]. Such Co-based phosphate sensors are highly selective,
stable, and inexpensive, and can detect both the inorganic and organic phosphate
[72-74].
The cobalt-coated phosphate MEA was characterized with six different concen-
trations of standard solution ranging from 10
-5.1
to 10
-3
MKH
2
PO
4
at ambient
temperature [59, 60]. Based on the pH value in which typical biological nutrient
removal systems are operated, the pH of each standard solution was adjusted to pH
7.5 by adding potassium hydroxide. At pH 7.5 standard solutions have dihydrogen
phosphate ion and hydrogen phosphate ion. The phosphate MEA was oxidized by
immersing in DI water along with a Ag/AgCl reference electrode for 30 min, fol-
lowed by 30 min in 10
-4
MKH
2
PO
4
solution. The data acquisition system included
the Denver Instrument pH/mV meter (model 225) and BalanceTalk Software
(Labtronics Inc.). The long-term stability, stirring effect, oxygen sensitivity, and
interference tests were conducted to characterize sensor performance at room
temperature.
