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Summary of all the lectures of instrumental analysis
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Summary of all the lectures of instrumental analysis
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Herkansing Instrumental Analysis Video lecturesElectrochemistry
1a: Introduction
Quantitative information you want to know the concentration of a substance in solution.
Quality control how many active compounds are there.
Pharmaceutical availability is the release of active substance.
Bioavailability is the distribution of drugs and metabolites in the body. If the sample is from biological
origin bioanalysis.
Qualitative information is information on the identity or electrochemical behaviour of a substance.
Quantitative information is about the concentration of a substance.
Two types of analytes that can be measured:
- Electroactive analyte: molecules that can be oxidized or reduced. They are electrochemically
active. A current or potential can be measured.
- Measurement where the current i = 0. Whole system is in equilibrium. This is in case
of potentiometry (to measure a potential).
- Current is not 0.
- Complete conversion of the analyte coulometry. Measure the charge that
is converted. Entire oxidation into another molecule.
- Partial conversion (reaction) of analyte voltammetry/biamperometry.
Voltammetry: measure the current as a function of the analyte and as a
function of the potential.
Biamperometry: measure the current as a function of the analyte.
- Non-electroactive: only the conductivity (or resistance) can be measured. The molecule does
not want to lose electrons.
1b: Electroanalysis
Solution with different molecules which are redox active. There is an oxidation and a reduction.
Oxidation: loss of electrons. The compound at the start of the reaction (which is oxidized in the
process) is the reducing agent and gives electrons to another substance.
Reduction: gain of electrons. The compound at the start of the reaction (which is reduced in the
process) is the oxidizing agent and takes electrons form other molecules.
A redox reaction can only happen when the electrons that are lost can be consumed. So oxidation
and reduction always happen together.
Electric charge q in coulombs (C). The minimum amount of charge is the charge of 1 electron.
Faraday constant (F) in C/mol is the charge per mol of electrons.
Current (in A) is the flow of a charge per second in an electric circuit. 1 A = 1 C/s.
1
,Charge and current are related Ohm’s law. The resistance is the difficulty to pass current (any flow
of charge) through a material.
Electrochemical cell:
Two electrodes connected by a conductor for charge transport. The two electrodes are put in a
beaker containing electrolyte (ionic solution). The electrodes act as reservoirs for electrons. Charge
transport/electrochemical reactions happen at the interface of the electrode.
Oxidation=anode=left
Reduction=cathode=right
How is charge transported through the cell?
Electron current through the wire: electrons move from the anode to the wire to the potentiometer
to the cathode which takes up the electrons again.
Ion current in the solution: ions migrate through the solution due to the electric field that is
established between the two electrodes.
The charges are exchanged by oxidation and reduction meaning the charge is transferred from ionic
current to electrical current.
Charge builds up in the potentiometer where a large resistance is located. Then a potential difference
can be measured.
The electric double layer of the electrode:
Apply a charge at the electrode many negative charges. Positive ions from the solution are
attracted to this. This is to counterbalance the charge. Cations are absorbed at the electrode surface.
The double layer consists of two parts:
1. First inner layer: adsorbed layer of the cations. These do not move and are attached to the
electrode by vanderwaals and electrostatic forces. This layer is not enough to counterbalance
the negative charge on the electrode.
2. Second diffuse layer: region where higher concentrations of cations are located to
completely counterbalance the negative charge. These cations can move in the solution.
There are two ways how charge can be transported:
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, 1. Faradaic process direct transfer of electrons from the analyte molecules to the electrode.
The current that is flowing is directly proportional to the amount of oxidations and
reductions of the species.
A potential determines the energy of electrons within the electrode. When you
change/measure the electrode potential then the energy of the electrons is
changed/measured. The electrode potential is always applied compared to the reference
potential. The reference potential is the potential of the solution.
More negative potential more negative charge to the electrode. The electrons then have
higher energy, because many electrons are there which want to get away from each other.
More positive potential lower energy at the electrode. The electrons then have a lower
energy.
When the energy of an electron is higher or as high as the unoccupied electronic shell, the
charge can hop over from the electrode to the substance in the electrolyte solution. This is a
reduction electron flow from electrode to solution.
When the energy of an electron becomes so low that the energy of the occupied electronic
shell is higher. This is oxidation electrons hop over form the solution to the electrode.
2. Non-faradaic process metal or membrane electrodes. No electrons are directly
transferred. You can change the charge in the electrode by applying a voltage/potential. You
can also change the charge in the double layer by changing the electrolyte. In both cases the
charge needs to be counterbalanced again causing a current flow. Once the charge is stable
again, the current stops.
2a: Potentiometry pH electrode
There always is a potential difference between two electrodes in the solution Ecell.
The E anode and E cathode depend on the concentration of the analyte.
Potentiometry:
When the current flows the potential difference is established. The electrodes together with the
solution act like a battery establishing the potential. How much current flows depends on the
resistance in the potentiometer. It is aimed to measure without any current use a potentiometer
with an infinite resistance. The potential difference that now is established at the electrodes is now
also established at both sides of the potentiometer making it able to measure.
Some other potentials can also be established so you do not exactly measure Ecell.
Indicator electrode: an electrode placed in the electrolyte of which we want to know the
concentrations/activities. This electrode directly react with the analytes.
Reference electrode: fixed composition, the material inside never changes. The potential for this
electrode always stays constant with respect to the solution.
So whenever a change in Ecell happens this is always the change in potential of the indicator
electrode.
There are two indicator electrodes:
1. Metal electrode: faradaic processes happen redox reactions happen and electrons are
exchanged. Such reaction can be described by the Nernst equation, which relates the
potential and the concentration with each other.
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, 2. Membrane electrode: potential is established by non-faradaic processes non-Nernstian
relation between the potential and concentration, because no redox reaction happens and
no electrons are exchanged.
Membrane electrode often is an ion-selective electrode (ISE). It consists out of a thin
membrane and only analyte ions in the solution are soluble in this membrane.
Measuring the concentration of the hydrogen ion is important for determining the pH.
pH measurement with the modern glass electrode:
The reference electrode is a silver wire coated with
silver chloride wrapped around the inner glass
tube. The outer glass tube, in which the reference
electrode is located, is filled with aqueous solution
saturated with AgCl and KCl. This is not connected
to the indicator electrode but only connected to
the electrolyte via the porous plug. Ions can be
exchanged here making sure that the potential
between the silver wire and the electrolyte always
stays the same.
The indicator electrode is again a silver wire
coated with silver chloride. There is a solution of
KCl and a known and fixed concentration of
hydrogen ions by HCl giving it a low pH. This
electrode is connected to the electrolyte by a glass
membrane.
The glass membrane:
Glass has some cations inside the silicon dioxide. The oxygen atoms with only one bond are
negatively charged. To balance this charge there are additional cations in the solution which then can
interact with the oxygen. The metal cations can contribute a very small current, so does the glass
membrane, because they can migrate. So a very very small current can develop in the glass if a
potential is applied.
There is dry glass on the inside, and on both the internal solution and external solution site a small
part is a hydrated gel layer. In this hydrated layer there is a combination of hydrogen and sodium
ions, while in the dry glass of the membrane there only are sodium ions. The amount of hydrogen
ions is determined by an exchange with the external solution. Sodium can diffuse out of hydrogen
can diffuse in.
An equilibrium is established between the amount of hydrogen atoms outside and inside the glass
membrane. Therefore, in the internal solution there is a fixed concentration of HCl.
In the external solution the amount of hydrogen ions can be changed as the solution changes.
pH in the internal solution is constant, at the external solution the pH can change.
When hydrogen ions diffuse out, a negative surface can be obtained at the external solution which
will result in a potential difference across the membrane. The sodium ions in the dry glass will
migrate towards the extracellular solution to counterbalance the negative charge.
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