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
- Eo = 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
- Co = 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 = Eo –S • log(C + Co)
where is the ideal slope of the ISE.
The following table describes ideal behavior:
|Potassium (K+), Ammonium (NH4+)
|Nitrate (NO3-), Chloride (Cl-)
Assuming C0 is near zero, the equation can be rewritten as:
C = 10˄[(E – Eo) / 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 – Eo) / Sm]
C = concentration of ion to be measured (mg/L or ppm)
E = measured potential of sample (mV)
Eo = measured potential (mV) at a C = 1 mg/L Ca2+ concentration
Sm = measured electrode slope in mV/decade
The value of Sm, the measured electrode slope, is determined by measuring the potential of two standard solutions, and solving the equation below:
Sm = – [(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:
1 mg/L NH4+ standard
100 mg/L NH4+ standard
C = 10^[(88 mV – 0 mV) / 58 mV/decade] = 33 mg/L NH4+-N
Ammonium in the Environment
The Ammonium Ion-Selective Electrode (ISE) can be used to determine concentrations of NH4+ ions in aqueous solutions, in units of mg/L, ppm, or mol/L. Concentrations of aqueous ammonium ions should not be mistaken for concentration of aqueous ammonia, or NH3(aq). The concentrations of these two species, though different, are often involved in the same equilibrium reaction:
Reaction 1: NH3(aq) + H+(aq) ↔ NH4+ (aq)
In a more acidic environment, higher concentrations of H+ ions will cause this reaction to shift toward the right, resulting in higher concentrations of NH4+. In a more basic (alkaline) environment, the concentration of NH4+ will be lower, causing the reaction to shift toward the reactants, producing higher concentrations of NH3. At pH values greater than 10, most of the ammonium ions will be converted to ammonia. At pH values less than 7.5, most of the aqueous ammonia will be converted to ammonium ions.
Freshwater Samples for Ammonium Concentration
While permissible levels of ammonium in drinking water should not exceed 0.5 mg/L, streams or ponds near heavily fertilized fields may have higher concentrations of this ion. Fertilizers containing ammonium sulfate, (NH4)2SO4, or ammonium nitrate, NH4NH3, may result in runoff from fields containing higher levels of the ammonium ion, NH4+. Monitoring ammonium levels on a stream that borders fertilized fields may show significant seasonal differences in NH4+ concentrations. In this kind of study, you may also take pH measurements in your water samples; as indicated in the previous paragraph, higher or lower pH values can greatly affect the ratio of NH4+ / NH3 in a sample. Since the Ammonium ISE measures only NH4+ levels, you may want to adjust your samples to the same pH value each time you make measurements; this may not be necessary if you have relatively “hard” water. Hard water is naturally buffered against changes in pH.
Expressing Ammonium Concentration
Concentrations of ammonium are often expressed in units of mg/L NH4+ as N. Here is a calculation for a 100 mg/L NH4+ as N standard solution that is prepared by adding solid NH4Cl to distilled water: