Water Module: Session 9 Ion Selective Electrodes and ppm Concentrations

How can a potential difference be related to a concentration?

The electrodes: E = E_indicator – E_reference

Reference Electrode – Constant potential half cell

AgCl (s) + e^- à Ag (s) + Cl^- silver chloride ref.

E˚ = 0.222 or saturated ACE = 0.197 V

Hg₂Cl₂ (s) + e^- à 2Hg (s) + 2Cl^- Calomel ref

E˚ = 0.268 or saturated (SCE) = 0.241 V

Use saturated KCl because the diffusion rates of the ions are similar which reduces junction potentials

Indicator Electrode – potential changes with changes in analyte concentration

Logarithmic relationship allows use to very low concentrations

pH electrode good from 0.1 to 10^-12 M

Generally have a 5-6 order of magnitude range in concentration

Must determine by generating a calibration curve

Over this large range, must control ionic strength

Potential develops due to an equilibrium of the ions of interest between the solution and solid membrane.

In the F^- electrode: LaF₃ (s) « LaF₂⁺ (s) + F^- (aq)

How would you convert a potential relative to one reference electrode to another? For example, measure using the Ag⁺ indicator electrode vs. Cu⁺²/Cu reference a potential of 350 mV. What would the potential vs. SHE if the [Cu⁺²] = 0.10 M?

First, what is the potential of the copper reference electrode vs. SHE

E = 0.34 + 0.059/2(log[Cu⁺²])

E = 0.34 – 0.059/2(0.10 M)

E = 0.314 for copper reference vs SHE

Now the Ag⁺ indicator electrode vs. SHE will be 0.664 V

____________|_______________|_________________ SHE

.314 V .664 V

____________|_______________|_________________ Cu⁺²/Cu

0 V .350 V

What does the presence of NO₃^- reveal about a drinking water source?

10 ppm is the maximum allowable concentration

Infants get “blue baby” syndrome from NO₂^- formation

The maximum allowable concentration of F^- is 4 ppm

Too low, tooth decay; too high, mottling of teach and possible but rare skeletal fluorosis