Electroanalytical Chemistry Chapter 3 Electroanalytical Chemistry

Best seen broken into four categories Part 1: Ion Selective Electrodes Part 2: Amperometric Sensors Part 3: Step Voltammetry Part 4: Cyclic Voltammetry

(1) Ion Selective Electrodes Electroanalytical devices that measure concentration of a selected ion in a sample by virtue of electric potential difference due to concentration of that ion. There are many applications (cba to list) And many analytical advantages! (again cba to list) Naming is Trivial!!!!!! ISE to detect F- Fluoride selective electrodes ISE to detect Ca2+ Calcium selective electrodes etc Note ISE to detect H+ pH electrodes

General Outline of an ISE Morgan. T. 2014, Summary of Electrodes, www.che-revision.weebly.com

The different types of ISE Click button below to visit the ‘Summary of Electrodes’ page on the revision website. Just re-open the presentation afterwards. If you are confident enough on this topic (AS IF!!!!!) then alternatively continue through the presentation. Summary of Electrodes at CHE-Revision Highly Recommended! Documentary on The Crazy Nastyass Honey Badger Not Recommended! May result in procrastination

How do ISEs Work? Nernst Equation 𝐸 𝐼𝑆𝐸 = 𝐶𝑒𝑙𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 + 2.303𝑅𝑇 𝑗𝐹 log 𝑋 𝑠𝑎𝑚𝑝𝑙𝑒 R Gas Constant 8.314 J K-1 mol-1 T Absolute Temperature K j Charge on ion being detected F Faraday Constant 96484 C mol-1 When T = Room Temperature (25°C) 𝐸 𝐼𝑆𝐸 = 𝐶𝑒𝑙𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 + 0.059 𝑗 log 𝑋 𝑠𝑎𝑚𝑝𝑙𝑒

Prediction of Nernstian Responses EISE Log10[X] For a Uni-Cation (ie Na+) at 25’C For a Di-Anion (ie O2-) at 25’C EISE Log10[X] Gradient = 0.059 V per log10[X] Gradient = - (0.059/2) V per log10[X]

Example Questions A Ca2+ selective electrode reads an EISE value of 0.473 V in a sample containing 3 x 10-3 M Ca2+ at 25’C. Calculate Cellconstant for this electrode.

Example Questions Calculate EISE for this electrode in a sample containing 30 x 10-2 M Ca2+ at 25’C. Note Cellconstant was calculated to be 0.547 V

Example Questions What is the Ca2+ concentration when EISE reads 0.347 V at 25’C? Note Cellconstant was calculated to be 0.547 V

Best go back and revise it again then! What do you need to know Can you describe what an ISE is to someone who knows nothing about chemistry? How do you name them? Can you illustrate a simple design of an ISE? Can you recall all the different types of ISE with illustrations? Do you remember and can you use the Nernst Equation? Can you predict graphical responses of Nernst relationships? Best go back and revise it again then!

Best seen broken into four categories Part 1: Ion Selective Electrodes Part 2: Amperometric Sensors Part 3: Step Voltammetry Part 4: Cyclic Voltammetry

(2) Amperometric Sensors Electroanalytical devices that measure the concentration of a selected redox active species in a sample. Concentration determined by virtue of flow of electrical current due to oxidation or reduction. There are many applications of Amperometric Sensors Dissolved Oxygen Glucose Neurotransmitters They also have many analytical advantages (same as ISE) BUT MORE EFFECTED BY SURFACE DETERMINATION (revisit later)

Oxidation and Reduction Really? If you want me to go through this, I don’t even think you should be here :L But ill explain briefly: Remember GCSE: OILRIG Lets try some questions! Is species A reduced or oxidised? A- + B  A + B- Its OXIDISED AS ITS LOST AN ELECTRON! That is it!

Oxidation / Reduction in Solution and at electrode surface Can you spot the mistake? Solution No net direction of electron transfer Electrode All electron transfer in same direction to give a flow of electrical current! TADA!

The different types of Amperometric Sensors Click button below to visit the ‘Summary of Electrodes’ page on the revision website. Just re-open the presentation afterwards. If you are confident enough on this topic (AS IF!!!!!) then alternatively continue through the presentation. Summary of Electrodes at CHE-Revision Highly Recommended! Documentary on The Crazy Nastyass Honey Badger Not Recommended! May result in procrastination

Direction of Current

Equation on Amperometrics 𝐸= 𝐸 0 + 𝑅𝑇 𝑛𝐹 𝑙𝑛 [𝑂] [𝑅] R Gas Constant T Temperature N Number of electrons transferred in half cell F Faraday Constant [O]/[R] Concentration of Oxidised and Reduced Species

Questions on Amperometrics Dopamine is oxidised in a two-electron process that has a reduction potential E0 of 0.2 V vs SHE. Which of the potentials below would be best applied to a sample to induce dopamine oxidation at 25’C?

Questions on Amperometrics Calculate the percentage of dopamine at the electrode surface when it has equilibrated at the potential given to your answer above.

Relationships between Current and Concentration Current is proportional to Area of Electrode Concentration is proportional to Current 𝒊 =𝒏𝑭𝑨 𝒌 𝒉𝒆𝒕 𝑪 𝒂𝒏𝒂𝒍𝒚𝒕𝒆 𝒂𝒕 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒅𝒆 𝒔𝒖𝒓𝒇𝒂𝒄𝒆 |i| Current Magnitude A Area of electrode khet Heterogeneous electron transfer rate consatant

Questions on Amperometrics A clark electrode with a sensor electrode area of 0.2 cm2 measured a current magnitude of 10 x 10-8 Amps in a stirred solution containing 10x10-6 M dissolved oxygen. What is the dissolved oxygen concentration in a stirred solution for which the same clark electrode measured a current magnitude of 3 x 10-8 Amps?

Questions on Amperometrics A clark electrode with a sensor electrode area of 0.2 cm2 measured a current magnitude of 10 x 10-8 Amps in a stirred solution containing 10x10-6 M dissolved oxygen. What current magnitude is predicted to be measured in a stirred solution containing 10x10-6 M dissolved oxygen with a clark electrode having sensor electrode area of 0.05 cm2?

Questions on Amperometrics A clark electrode with a sensor electrode area of 0.2 cm2 measured a current magnitude of 10 x 10-8 Amps in a stirred solution containing 10x10-6 M dissolved oxygen. What is the sign (positive or negative) of the current measured during the detection of oxygen with a clark electrode?

Best go back and revise it again then! What do you need to know? Can you describe an amperometric sensor to someone who doesn’t do chemistry? Do you know the principals of oxidation and reduction? Do you know the direction of current and their signs? Can you illustrate the basic design of an amperometric sensor and recall different types? Can you recall and use equations of relationships between sensor response and concentrations? Best go back and revise it again then!