Electrochemistry Part 1 Flashcards
(33 cards)
What are the 6 steps for analytical process?
1.Problem definition:
- Selection of analytical method:
- Sampling and sample preparation:
- Validation of analytical method:
- Data collection and interpretation:
- Reporting
Ion Chromatography
sensitive, precise, separates F- from other anions, in-lab, $$
Colorimetric method based on zirconium
SPADNS reagent; decrease in color is measured spectrophotometrically, other anions may interfere
Distillation and colorimetric method
less interference from other anions, time consuming
Ion selective electrode
widely used, simple, portable, fast, inexpensive, wide applicability
2Fe3+(aq) + Cu(s) ⇌ 2Fe2+(aq) + Cu2+(aq) Redox reaction (Anode and Cathode equations and how to calculate the Ecell)
2Fe3+(aq) + Cu(s) ⇌ 2Fe2+(aq) + Cu2+(aq) Redox reaction, copper transfers electrons to iron(III)
Galvanic cell: Cu(s)|Cu2+(aq)||Fe2+(aq)|Fe(s)
e- flow in external circuit → → →
Anode half-reaction: Cu(s) → Cu2+(aq) + 2e-s Eox = -Ered =
Written as reduction: Cu2+(aq) + 2e- → Cu(s) Ered =
Cathode half-reaction: Fe3+(aq) + e- → Fe2+(aq) Ered =
Standard reduction potentials Eo Ecell = E+ - E- = Eright – Eleft = ………Ecell = Ered + Eox
When does the Nernst equation apply?
At non-standard conditions
Silver / Silver Chloride Reference Electrode
The half reaction below has a well known and stable potential at constant temperature and
concentration of chloride:
AgCl(s) + e- → Ag(s) + Cl-(aq) Eo = 0.222 V vs SHE for 𝒜𝐶𝑙− = 1
AgCl(s) + e- → Ag(s) + Cl-(aq) E = 0.197 V (saturated KCl)
In a saturated solution of KCl at 25oC
Glass pH Electrode
The glass electrode used to measure pH is the most common ion-selective electrode.
Glass pH combination electrode
A typical pH combination electrode consists of the following three components:
-inner Ag|AgCl electrode (buffered KCl solution saturated with AgCl)
-H+ selective glass membrane outer Ag|AgCl electrode (saturated with both KCl and AgCl) connected by a porous plug (salt bridge) to the analyte solution
Glass pH combination electrode line diagram
-A line diagram for a representative pH electrode as it measures pH
-all concentrations are constant except those in the analyte solution
-the only analyte that selectively binds to the glass membrane is H+
-the two reference electrodes measure the electric potential across the glass membrane
Salt Bridge in the Glass pH Combination Electrode
-The outer electrode solution is connected to the analyte solution by a porous salt bridge.
-Examples of salt bridges in pH combination electrodes:
- ceramic plug (common)
- platinum diaphragm (expensive, less likely to clog)
Cross-section of the pH sensing glass membrane
-The two surfaces of the pH sensing glass membraneabsorb water to form hydrated gel layers (Figure 15-17)
-Metal ions in these hydrated gel regions of themembrane diffuse out of the glass and into solution.
-H+ can diffuse into the membrane to replace metal ions.The reaction in which H+ replaces cations in the glassis an ion-exchange equilibrium (Figure 15-18).
-A pH electrode responds selectively to H+ because H+ isthe ion that diffuses most effectively into the hydrated gel layer.
Response of Glass electrode
- Potential difference across the glass membrane measured by the inner and outer silver-silver chloride reference electrodes
- Depends on chloride concentration in each electrode compartment and potential difference across the glass membrane
- [Cl−] is fixed in each compartment and [H+] is fixed on the inside of the glass membrane
- Only variable is the pH of the analyte solution outside the membrane
Formula for glass electrode response
See pic
What was the first selective-ion electrode
Glass electrode
Electrochemical detection of fluoride or other anions and cations is not practical with galvanic cell why?
- Would not be sensitive
- Nor selective
- Nor practical
- Nor possible in some cases
What was the fluoride selective electrode based on and belongs to what?
- Based on the lanthanum fluoride crystal
- Belongs to class of solid state electrodes
Selectivity Coefficient
gives the relative response to different species with the same charge
Ion Selective Electrodes shortfalls
not perfectly selective. Interferents can contribute to the overall ISE cell voltage
Smaller selectivity coefficient means what?
The smaller the selectivity coefficient, the less interference by X when measuring A. The response of an ion-selective electrode to its primary ion (A) and to interfering ions (X)of the same charge is
Solid-State Electrodes – The Fluoride Electrode
-Fluoride ISEs are used to monitor the fluoride content of municipal water supplies.
-In a fluoride ISE, a LaF3 crystal doped with a small amount ofEu2+ serves as the ion selective membrane.
-The Eu2+ creates fluoride vacancies in the crystal that enable a tiny current of fluoride ions to move through the LaF3 crystal.
Solid-State Electrodes – The Fluoride Electrode (2)
- At low pH, F− is converted to HF to which the electrode is insensitive
- Measure F− in a high ionic strength buffer containing acetic acid, sodium citrate,NaCl, and NaOH at pH 5.5
Determination of Flouride in Drinking Water Analytical method (steps 1-3)
- Problem definition: Measure fluoride levels in drinking water
- Selection of analytical method: Fluoride ion selective electrode (ISE)
- Sampling and sample preparation:
-Collect water samples in clean polyethylene bottles to prevent contamination.
-Rinse the bottles with the sample before collection.
