4
Ion Examples
n (valence of
ion)
S (at 25 °C),
mV/decade
Calcium (Ca
2+
)
+2
+29.58
Potassium (K
+
), Ammonium (NH
4
+
)
+1
+59.16
Nitrate (NO
3
-
), Chloride (Cl
-
)
–1
–59.16
Assuming
C
0
is near zero, the equation can be rewritten as:
C
= 10
˄
[(
E – E
o
) /
S
]
allowing for the calculation of the ion concentration.
It is very important to note that this table reflects ideal behavior. Ion-selective
electrodes have slopes that are typically lower than ideal. It is generally
accepted that an ISE slope from 88–101% of ideal is allowable. The slope (S) is
an indicator of ISE performance. If the slope changes significantly over time, it
may indicate that it is necessary to replace the ISE sensor tip.
Potential
vs.
Concentration
To measure the mV readings from an aqueous sample, calibration is not
required. To convert mV readings to concentration (mg/L or ppm), the software
uses a modified version of the Nernst Equation:
C
= 10
˄
[(
E – E
o
) /
S
m
]
C
= concentration of ion to be measured (mg/L or ppm)
E
= measured potential of sample (mV)
E
o
= measured potential (mV) at a C = 1 mg/L Ca
2+
concentration
S
m
= measured electrode slope in mV/decade
The value of
S
m
, the measured electrode slope, is determined by measuring the
potential of two standard solutions, and solving the equation below:
S
m
= – [(Low Standard – High Standard) / # of decades*]
Example Calculation, converting mV to mg/L
For this example, the measured quantities are shown in the chart below:
Solution
Measured Potential
1 mg/L NH
4
+
standard
0 mV
100 mg/L NH
4
+
standard
116 mV
unknown sample
88 mV
C = 10^[(88 mV – 0 mV) / 58 mV/decade] = 33 mg/L NH
4
+
-N
Battery Information
The Go Direct Ammonium Ion-Selective Electrode contains a small lithium-ion
battery in the handle. The system is designed to consume very little power and
not put heavy demands on the battery. Although the battery is warranted for one
year, the expected battery life should be several years. Replacement batteries are
available from Vernier (order code: GDX-BAT-300).
Water Resistance
The Go Direct Ammonium Ion-Selective Electrode is not water resistant and
should never be immersed in water above the BNC junction.
If water gets into the device, immediately power the unit down (press and hold
the power button for more than three seconds). Disconnect the sensor and
charging cable, and remove the battery. Allow the device to dry thoroughly
before attempting to use the device again. Do not attempt to dry using an
external heat source.
How the Sensor Works
Combination Ion-Selective Electrodes consist of an ion-specific (sensing) half-
cell and a reference half-cell. The ion-specific half-cell produces a potential that
is measured against the reference half-cell depending on the activity of the
target ion in the measured sample. The ion activity and the potential reading
change as the target ion concentration of the sample changes. The relationship
between the potential measured with the ISE and the ion activity, and thereby
the ion concentration in the sample, is described by the Nernst equation:
E
= measured potential (mV) between the ion-selective and the reference
electrode
E
o
= standard potential (mV) between the ion-selective and reference electrodes
R = universal gas constant (R = 8.314 J mol
-1
K
-1
)
T
= temperature in K (Kelvin), with T (K) = 273.15 + t °C where
t
is the
temperature of the measured solution in °C.
F = Faraday constant (96485 C mol
-1
)
n
= valence of the ion
C
= concentration of ion to be measured
C
o
= detection limit
Since R and F are constant, they will not change. The electrical charge of the
ion (valence) to be measured is also known. Therefore, this equation can be
simplified as:
E = E
o
–S
•
log(C + C
o
)
where
is the ideal slope of the ISE.
The following table describes ideal behavior:
