Electrochem 2 Flashcards
Slide 1 calc
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What is amperometry
The current (in amps) is measured throughout the oxidation reduction reaction
We measure the current at a specific time mutiple times since the current changes with time
In anoerometry what is the current proportional to
Proportional to the analyte concentration
As concentration increases, Amps incresse
What does it mean is the current becomes zero
The analyte concentration is zero meaning it fully reacted
Give an example of how a amperometry experiment can work
There are two working electrodes in a glucose monitor
The first working electrode is coated in an enzyme glucose oxidase which makes the reaction of glucose with O2 to h2o2 go forward
Then the h202 reacted at the working electrode and the current was measured throughout he reaction to see the amount of h202 in the solution
If current is proportional to h202 then it’s proportional to the glucose since the h202 came from glucose’s
This is how we can tell the amount of glucose
Why is a mediator used in the glucose amperometry analysis
The reaction of glucose to h202 depends on the amount of o2 in the environment
So if the o2 is low then the reaction to h202 doesn’t work and it makes it seem like there is no glucose when there actually is
To stop this dependence on o2, you substitute o2 with a mediator
What is a mediator
A thing that moves electrons from the analyte to the working electrode
Replaces the thing that the reaction is dependent on
In amperometry does all of the spangle need to be consumed to get the graph
No because the current is measured at each time interval, don’t need entire reaction do finish
Also because the current isn’t stochimetricslly related to the analyte (doesn’t need to do a full reaction to get proper values)
What are things that effect the measured current in amperometry
The analyte concentration (larger concentration, larger current)
But even if same concentration of ions, the current is diff for each because of
mobility of the ions
E standard vs E of working electrode
How do amperometric detectors work for hplc
You hold the working electrode at a specific potential that matches your target analyte that you think will elute
Then you monitor the current as the eluate passes over the electrode
The current will tell you what type of thing passed through and its concentration
What is volametry
You’re changing the voltage at the working electrode then measuring the current as the reaction progresses
What does the current in volametry depend on
The faradaic contributions
The capacitive contributions
What are faradaic contributions to current
current is flowing through the electrode because of electron transfer from a reaction happening at its surface
What are capacitive contributions to current in voltametry
because the potentiostat is changing the voltage
The ions in the solution and at the double layer (near the electrode) move and change the current
Where would the reactants that are consumed in a reaction be
More of it is far away from the electrode and comes close to the electrode from the bulk of the solution
Where would the products that are made in a reaction be
They would be close to the electrode when their concentration is high
Then far away as their concentration decreases and time passes and they diffuse away from the electrode
What is the mass transport limit
When the reactant is consumed faster than it can move toward the surface of the electrode
This changes the maximum current that we observe since some didn’t react at the electrode
What is a rotating disk electrode
What is it made of
A disk shaped electrode that rotates vary fast
It’s inert so it’s made of pt au or C
How does a rotating disk electrode work
It’s rotations stir the solution near the electrode surface
This brings fresh solution to the electrode but not completely
The analyte needs to move last few distance from the electrode by diffusion
What is the limiting current of a rotating disk electrode
When does the value of max current change
It’s the maximum current that is observed at an electrode, also called the diffusion limit
If a concentration of analyte increases, limiting current increase
If the rotation of the electrode increases , limiting current increase
What is linear scan voltametry
The potential (E) at the working electrode is scanned overtime
This gives a linear graph
When the reaction begins, the current increases on the current vs E (or time) graph and we can see at what E the reaction can start
What does the current (I) vs E graph look like with a rotation disk electrode
First around zero straight, then increase at a certain E, then reaches a max (diffusion limit)
The max current is proportional to the concentration of analyte
What does the current (I) vs E graph look like with a planar electrode
Reaches a max then goes a bit lower,
Doesn’t stay at the max because there’s still some reactant drifting in toward the electrode
The max current is proportional to the concentration of analyte
With a planar electrode what is special
It has no stirring
Eventually all the reactant near the surface of the electrode is used up and more can only come in through diffusion or convection (stirring)
