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having covered much the theory behind
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electrochemistry we now turn our
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attention to looking at the more
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practical aspects in this case we'll be
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exploring the principles behind
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voltammetry voltammetry looks at the
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effect of currents as we vary the cell
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potential so we apply a cell potential
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and we measure the current which comes
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through so far we've really only
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considered small deviations from an
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equilibrium cell potential so are very
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small over potentials in the region
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where we can do measurements of the
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kinetics using butter Vollmer
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relationships these small deviations are
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requirement for butler-volmer kinetics
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to be analyzed and a key assumption made
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is that the concentrations don't vary
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significantly however as we go to higher
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over potentials these can change the
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concentration of the electrode surface
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so looking at our cations and our
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reduced species we get a fundamental
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change in the concentration and
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diffusion effects start to change the
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behavior of what's going on at that
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electrode these processes cause a change
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in concentration at the electrode
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surface in a process called
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concentration polarization we start to
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get a buildup of particular species at
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electrodes which changes the current
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that we can draw from them and from this
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we can infer a great deal about the
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kinetics of the electrode but also
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insight into the processes of the
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reactions going on the principles of
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voltammetry are fairly simple potential
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is swept through a predefined range so
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we set a range for our potential and
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we'd simply ramp the potential through
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those values we maintain a constant rate
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of sweep so if we look at the variation
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of our electrode potential with time we
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have a constant rate with linear
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voltammetry we increase the potential at
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a constant rate so we allow it to
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equilibrate for a time period and then
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we ramp up the potential at a constant
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rate and look at what happens to the
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current
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in this case increasing the potential
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increases oxidation at the working
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electrode by increasing the oxidation we
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increase the current of that electrode
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so let's now look at what happens to the
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current as we vary the electrode
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potential the current we would expect to
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increase rapidly past the reduction
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potential once we get past that
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potential we would expect the current to
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ramp up severely because we now start
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oxidizing the
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the species at that electorate what we
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see is it reaches a maximum value and
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this is a key observation in voltammetry
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experiments the reason for this rise is
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because we find ourselves limited by the
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supply of reactant to the electrode as
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material reaches the electrode it's
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immediately oxidized or reduced and
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starts to hinder the process of material
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getting to that electrode thereafter the
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peak of this maximum is proportional to
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the concentration of reactant so as we
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increase the concentration we would
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expect to see a greater maximum current
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let's explore this process in a bit more
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detail so let's look at a cathode
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process so where we have a reduction of
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a cation so we have a process which we
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detail cation picks up an electron at
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the cathode and is reduced to an
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uncharged species if you remember from
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what we did earlier in other sessions
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the current is related to the rate of
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reaction and if we have a higher
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concentration that means we have a
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faster rate so basic kinetics tells us
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that the rate of change of our cation is
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proportional to its concentration if we
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have a faster rate there are more
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electrons being supplied or removed from
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the electrode which gives us a higher
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current through the external cell what's
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the effect of increasing the potential
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then if we increase the potential we
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increase the electric field by
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increasing the electric field that
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increases the electro-motive force on
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ions in solution so if we remember our
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drift speed our ion drift speed which is
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proportional to the electric field
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strength if you remember that's the you
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term here is the ion mobility which
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considers things like solvent viscosity
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ionic charge hydrodynamic radius and so
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on by increasing the drift speeds we
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increase the rate at which ions arrive
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at the electrode that allows us to more
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rapidly deliver more currents so it
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seems no surprise that increasing the
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field should increase the current so if
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we look at our voltammogram we see as we
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increase the potential we see a faster
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and faster rate of reaction which is
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manifest as a greater and greater
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increase in the current provided but we
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see it start to level off and it's worth
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considering what's happening as that
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levels off well let's look at what's
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happening at the electrode surface as we
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increase the magnitude of the potential
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we get more and more ions attract
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surface which are then immediately
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reduced to this uncharged species but
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what happens when we increase the
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magnitude of the potential still further
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well as we increase it still further we
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see that the cations are immediately
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reduced at the electrode and these
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uncharged species start to collect at
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the electrode surface and they block
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active sites to new cations so it
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inhibits the ability for a cation to get
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to the surface the diffusion force is
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simply a driving force against a
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concentration gradient so this diffusion
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force is not affected at all by the
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potential so no matter how much of this
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uncharged species we have they will
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still be diffusing at the same rate
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related to their concentration are these
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high magnitudes of potential these
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uncharged species will hinder the
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reductive process at this electrode and
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this causes the current to tail off from
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its maximum value it could it reduces
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the rate at which the reaction can take
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place and this is a key feature in
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voltammetry linear voltammetry however
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is just one aspect of voltammetry that
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we do and more often we tend to look at
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cyclic voltammetry cyclic voltammetry is
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a situation where potential is swept up
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to a maximum value and then back down
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again we maintain a constant rate of
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sweep up and down which has a profile
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that looks a little bit like this we
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have a constant rate increase and then
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we suddenly turn and ramp it back down
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again at the same inverse rate and
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notice again we allow the cell to
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equilibria at beginning and the end with
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us fixed potential okay let's now look
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at the current curve what happens to the
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current as we change that electrode
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potential well we expect a similar
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current profile initially to linear
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volts Hammond tree but we then see a
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rapid change in current when we reverse
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it so as we increase the potential the
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potatoes but then as we come back we see
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a considerably different shape from
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these curves we can identify cell
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potentials and the cell potential is
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identified as the point equidistant
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between the oxidation peak and the
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reduction peak here so to understand
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what's going on let's look at the anode
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solution interphase at five points
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remember the anode is where we have
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oxidation taking place the points we're
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going to look at is point a where we
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have a low potential on the oxidation
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sweep then we're going to look at the
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peak at high potential of the oxidation
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sweep and then we're going to look at
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Point C which is the turning point
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then we're going to go back down at high
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potential in the reduction sweep and
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then we're going to look at a low
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potential on the reduction sweep so
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let's look at each step in turn
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considering the anode process so we're
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looking at the low potential on the
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oxidation sweep we have a relatively low
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potential on our anode and as we
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increase the potential we see the
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current rising as the anions are
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oxidized this rate is limited by the
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migration of anions so the rate at which
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these anions can move through the
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solution carried on the electric field
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affects the rate of our reaction that is
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what's limiting the rate of reaction the
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diffusion of the oxidized species these
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uncharged species here the diffusion of
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these through solution is considerably
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faster than iron migration so is not
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providing a significant inhibitor so we
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see an increase in the current with
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increased potential when we get to point
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B where we have a high potential on the
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oxidation sweep we see that our
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electrode surface is starting to become
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crowded so you have an exchange between
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the migration of the anions and a
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diffusion of the oxidized species at
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this point the oxidation current is in
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the absolute maximum there's no more
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increase that can be obtained from
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oxidation and as we say the migration
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rate of anions is equal to the diffusion
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rate of the oxidized species so if we
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look at this equilibrium here where we
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have this analytic potential in the
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reductive potential we have established
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an equilibrium let's now look at the
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turning point at a very high potential
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at this point our surface is completely
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saturated with the oxidized species and
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this hinders the ability for an anion to
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get to the surface and causes a
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reduction in the current at this point
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the rate is limited by the diffusion of
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the oxidized species from the electrode
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as a turning point we start to lower the
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potential and see what happens as we go
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backwards so on this reduction sweep
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remember that the surface is still
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saturated with oxidized species so
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there's a rapidly reduced back to the
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anion and the current starts to go in
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the opposite direction so the current
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starts to flow the reduction current
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starts to become significant and starts
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to counteract the oxidation current so
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the cathodic reduction current starts to
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contribute in a significant way to
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counteracting the oxidation current
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remember as we say this is a constant
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process happening at any one electrode
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and it's just whether the applied
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potential drives it forward or backwards
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once we get down to a particular value
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at a low potential on the reduction
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sweep the cathodic current is at an
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absolute maximum and it dominates the
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current profile meaning that the overall
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current is at a minimum at this point
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the diffusion of the oxidized species is
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considerably faster than ionic migration
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so these oxidized species are leaving
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the electrode faster than the anions can
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get to the surface so this summarizes
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what's going on at each stage in a
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cyclic voltammogram so let's now explore
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the features of this voltammogram the
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peaks of the current light either side
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of this standard electrode potential for
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the cell and this allows us to work out
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what the standard electrode potential
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might be for a particular species the
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electrode potential can be identified
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quite simply by being equidistant
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between the reduction peak at the bottom
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and the oxidation peak at the top we can
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also tell something about the kinetics
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of the process by analyzing the shape of
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the curve so depending on what shape the
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curve might be we always expect to see
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this sort of shape for a reversible
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process so for a fully reversible
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process we see an oxidation as we ramp
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the potential up and a reduction as we
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ramp it back down again so this would be
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for a fully reversible process and
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nothing really surprising there but the
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timescale of the sweep also affects the
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results if we sweep our potential too
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quickly
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we might miss the subtleties of reaction
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processes diffusion or migration
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kinetics so they may not occur on the
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timescale the sweep so we can infer
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extra information by either slowing the
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speed down or speeding it up let's not
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consider asymmetric processes so we
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define our standard cell potential
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reaiiy identify particular process where
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we've identified a cell potential so we
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now do our sweep we ramp the potential
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up we see our oxidation as expected but
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then on the way back down we end up with
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an asymmetric curve we don't see a
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reduction peak so this tells us
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something about the process that we're
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seeing are we getting an irreversible
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oxidation serve for example in the
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electrolysis of sodium chloride we see
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the oxidation of chloride two chlorine
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gas which then escapes from solution so
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there is no
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there's no way that that can come
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backwards so we don't see a reduction
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peak happening another possibility is
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that there are slower reduction kinetics
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so or uncharged oxidants which are not
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carried to the electrode we may not be
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driving a sufficiently high
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overpotential it may be that our
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reduction peak actually appears at
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considerably lower potentials and we
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need to drive past to try and find it so
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all of this tells us a little bit about
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what's going on if we have a process
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with a very low exchange current density
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we'd expect to have a high over
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potential required and that might be
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causing us to miss the reduction another
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form of asymmetric process is where we
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see two oxidation Peaks for example so
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we might see a shape that looks a bit
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like this so in this case we're seeing
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two oxidations present with different
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equilibrium cell potentials so an it for
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an example that we might use here if we
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have chloride being oxidized to chlorine
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and bromine being oxidized to bromine in
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the same solution we would expect to see
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two different oxidation Peaks but we
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don't see a reduction peak in the
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reverse trace for cell two we see a
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reduction peak for cell one but not for
00:12:08
sale - so is this causing an
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irreversible oxidation so are reforming
00:12:14
gas could it be flow reduction kinetics
00:12:17
again we go through the same processes
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whether they require a very high over
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potential but we do see the reduction
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peak for e1 which allows you to
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determine a cell potential for e1 and we
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can identify what's happening in that
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cell so this is once again a reversible
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process so to summarize our discussion
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on voltammetric processes we can
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determine information on cells from
00:12:38
voltammetry from electrode potentials to
00:12:41
reaction kinetics we can determine a
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great deal about it we can identify
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reversible processes versus irreversible
00:12:47
processes and we can determine kinetic
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information on our cells the signals
00:12:53
that we get are affected by the
00:12:54
concentration of our reactants and the
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kinetics of those cell processes so if
00:12:57
you're careful control of concentrations
00:12:59
we can unravel information on the
00:13:01
kinetics of cell processes fundamentally
00:13:03
in voltammetry we are looking at
00:13:04
processes at a single electrode we're
00:13:07
ramping the potential of that electrode
00:13:08
up and down and monitoring the current
00:13:10
that comes out of it we're looking at a
00:13:12
working electrode compared to a standard
00:13:14
reference electrode
00:13:15
and depending on the potential that
00:13:17
we're offering it behaves is either an
00:13:18
anode or cathode depending on the
00:13:20
relative potential and which way through
00:13:22
this equilibrium we're working and we
00:13:24
use counter electrode to complete the
00:13:26
circuit in our voltammetric measurements
