CHEM2302

A buffer is a mixture of

an acid and its conjugate base, whose ratios are max 1:10

A reaction is fastest near a pH of

8

For buffers, HA and A- are almost unchanged in concentration, except when

dilute solutions, extremes of pH

When ____ a buffer most effectively resist pH change

pH = pKa

If we have 10 diff acids and bases in the solution, the 10 forms of the Henderson-Hasselbach must give the same pH, why?

There can only be one concentration of H+ in a solution

Denote tris and tris hydrochloride

B, BH+

What is the pKa for the conjugate acid of tris

8.072

Why do buffers resist pH change

Buffers consume the added base or acid (B or BH+ consume the strong acid or base)

If you add strong acid to tris?

B is converted to BH+_

If you add strong base to tris?

BH+ is converted to B

What is buffer capacity a measure of? + what does a greater capacity mean

how well as solution resists change in pH when strong acids/bases are added

Greater capacity means the solution is more resistant to pH change

A buffer is most effective in resisting changes in pH when?

pH = pKa ([HA] = [A-]), most useful within +- 1 pH unit

Buffer capcity can be increased by?

increasing buffer concentration

What is an isoionic pH/isoionic point

pH obtained when the pure, neutral polyprotic acid HA is dissolved in water

The concentrations of H2A+ and A- are ___ is an isoionic pH

not equal to each other

The concentrations of H2A+ and A- are ___ is an isoelectric pH

equal to each other (both species in equilibrium with HA), most molecules in the uncharged form HA

What is an isoelectric pH/isoelectric point

pH at which the average charge of the polyprotic acid is 0

For a protein: the isoelectric pH is the pH when

the protein has no net charge

For a protein: the isoionic pH is the pH for

a solution of pure protein, containing no other ions except H+ and OH-

In an electric field, a protein at its isoelectric point will..

not migrate

What is isoelectric focusing + how it works

technique for protein separating

• each protein migrates in a medium designed to have a pH gradient

  • Migration stops when the protein has reached the pH point that equals its isoelectric point

A monodentate ligand binds to a metal ion through…

one atom

A multidentate/chelating ligand binds to a metal ion through…

more than one atom

Metal ions are ____ that accept electron pairs from electron-donating ligands that are __

lewis acids

lewis bases

What is the chelate effect

the ability of a multidentate ligand to form a more stable complex than a similar monodentate ligand

Complexometric titration is

titration based on complex formation

the stoichiometry is 1:1 regardless of the charge of the ion

_____ is the most widely used chelator, why?

EDTA

• every element on the periodic table can be measured with EDTA

• EDTA is a hexaprotic system

What is electrolysis

chemical reaction is forced to occur at an electrode by an imposed voltage

Whta is potentiometry

measuring voltage in the absence of significant current

in electrolysis, electrons flow to the ______ electrode

working

in electrolysis, the cathode is connected to the _______ terminal of the power supply

negative

in electrolysis, the anode is connected to the _______ terminal of the power supply

positive

In electrolysis, what three things can change the voltage required to drive the reaction. Does it make electrolysis more difficult or easier, and do they drive the cell voltage more positive or negative

overpotential, ohmic potential, concentration polarization\

more difficult

negative, requires more voltage from the supply

What is overpotential and how does it change the voltage

• Overpotential = voltage required to overcome the Ea

• Greater overpotential must be applied to have a faster reaction

• A greater overpotential means a higher current density

What is ohmic potential

• Voltage required to overcome electrical resistance of the solution in the cell when a current is flowing

What is concentration polarization

• Occurs when the concentration of reactants or products are not the same at the surface of the electrode as they are in bulk solution

What is an electroactive species

species that can be oxidized or reduced at an electrode

What is a polarizable electrode

an lectrode whose potential is easily changed when small currents flow

What is an example of a polarizable electrode

metal electrodes

What is a non-polarizable electrode

an electrode whose potential does not easily change, requiring high current

what is an example of a nonpolarizable electrode

reference electrodes

How does electrogravimetric analysis work

an electrode is weighed and an analyte is deposited on it by electrolysis. the weight of the electrode is also taken after the procedure to determine how much analyte was deposited

How do we know when electrolysis is complete

• solution colour

• complete deposition

• qualitative analysis of analyte

What is a depolarizer + ex

a species that is more easily reduced than H+

ex: NO3-

what is coulometry + ex

analysis based on counting the electrons used in a reaction

2BR- → Br2 + 2e

How does coulometry work using the Br example

Br2 is generated at the Pt electrode reacts with cyclohexene. When cyclohexene is consumed, the concentration of Br2 rises, signaling the end of the reaction. The rise in Br2 is detected by measuring the current between the two detector electrodes.

What is amperometry

measuring the current between a pair of electrodes that are driving an electrolysis reaction. one reactant is the intended analyte and the measured current is proportional to the concentration of analyte

What is an example of amperometry

biosensors

Explain how a blood glucose monitor works

• Uses amperometry: biosensor

• Measures the electronic current between a pair of electrodes that are driving an electrolysis reaction

• Uses an Ag|AgCl reference electrode

• In the coating above working electrode 1: glucose + oxygen → gluconolactone + H2O2

  • Carbon working electrode 1 coated with glucose oxidase and mediator

    • Mediator = 1,1’-Dimethyllferricinium cation

      • Regenerated at the working electrode

• The current at the working electrode is proportional to the concentration of ferrocene, which in turn is proportional to the concentration of glucose in the blood

• Vitamin C, tylenol, and uric acid interfere with this analysis (oxidize at the same potential as the mediator), so the device has a second working electrode coated with mediator but not glucose oxidase

  • These are reduced at electrode 1 and 2

  • Glucose only reduced at electrode 1

    • Overall current = current 1 - current 2

What does a voltammogram plot

relationship between current and working electrode potential

In a voltammogram, current is _______ when an analyte is oxidized at the working electrode

positive

Limiting current/diffusion current is proportional and governed by the rate at which ______ diffuses to the electrode

analyte

What is polarography

voltammetry conducted with a dropping mercury electrode (hanging mercury drop)

Most reactions using a ___ electrode are reductions

Hg

How does polarography work

• Dispenser suspends one drop of mercury from the bottom of the capillary

• After current and voltage are measured the drop is dislodged

• A new drop is suspended for the next measurement

  • Fresh Hg yields reproducible current-potential behaviour

Why is reduction of a metal into a mercury amalgam (anything dissolved in Hg) done

its more favourable than reduction into the solide state

How do we conduct a polarography measurement

Sampled current polarograph

• staircase voltage ramp

What is the half wave potential in polarography

value for when half the max current is reached

used for qualitative analysis

how do to quantitative analysis using polarography

the diffusion current is proportional to the concentration of analyte

• diffusion current = difference between traces a and b (blank)

• controlled by the rate at which analyte can diffuse to the electrode (minimize convection and migration)

what is residual current in polarography

the reduction of impurities in solution and on the surface of the electrodes

Why should oxygen be avoided in polarography, how do you avoid it?

it gives two polarographic waves when reduced to H2O2 and H2O

• bubble N2 through the analyte solution to remove O2 and the flow N2 in the chamber while analyzing to keep O2 out

What is faradaic current

current due to reduction or oxidation of an analyte at a working electrode

• current we seek to measure in voltammetry

What is charging current

interferes with every measurement by obscuring faradaic current

• flow of ions and electrons in the analyte solution and not from redox reactions

why does faradaic current decay

analyte cannot diffuse to the electrode fast enough to sustain the high reaction rate

why does charging current decay faster

ions near the electrode redistribute themselves faster

1 second after each potential step in polarography, _____ current is still significant while the _____ current is very small

faradaic

charging

What is the most efficient voltage profile for voltammetry

square-wave voltammetry

How does square wave voltammetry work

During each cathodic pulse there is a rush of analyte to be reduced at the electrode surface

During each anodic pulse, analyte that was just reduced is reoxidized

Advantages of square wave voltammetry

• LOD: 10^-7 M

• Resolves species whose half wave potentials differ by 0.05V

• Faster than other techniques

Ho does cyclic voltammetry at a stationary, planar electrode work

• triangular waveform applied to the working electrode of an unstirred solution, flushed with inert gas

• analyte reaches the electrode by diffusion/migration (supporting electrolyte)

• steps:

  • at t0, the potential is made more negative until reduction of analyte happens. the potential where there is a peak cathodic current is Epc

  • After this peak, the current decays as the analyte is depleted near the electrode until t1, this is the cathodic peak

  • After t1, the potential is raised in magnitude to its original value. the reduced analyte is re-oxidized off of the electrode causing an anodic current to flow. the potential of the peak of the anodic wave is Epa

  • As the analyte is depleted from the electrode, the current decays back towards its initial value at t2

what does a reversible reaction mean in cyclic voltammetry

the equilibrium is fast enough to maintain concentrations of reactants and product at the electrode surface

Epc = Epa

half wave potential E1/2 lies midway between both peaks

what does an irreversible reaction mean in cyclic voltammetry

the reactants, once reduced, are not re-oxidized when the voltage is raised at t2

the reaction is too slow to maintain equilibrium at the electrode surface

anodic peak may or may not be observed

advantages of cyclic voltammetry?

• quanlitative (E1/2) and quantitative (Ipc) analysis technique

What are the steps of stripping voltammetry

1. Concentrate the dilute analyte into a thin film of Hg by reduction

2. Reoxidize the analyte by making the potential more positive

3. Measure polarographic signal during oxidation

Which voltametric technique is the most sensitive, why

stripping voltammetry

• analyte is concentrated from a dilute solution

  • longer concentration period = more sensitive analysis

a smaller Ksp value means…

less soluble the solid is in solution

what is autoprotolysis

can act as both an acid and a base

strong ______ completely dissociate in water

acids or bases

rank these from strongest to weakest acids: divalent ions, monovalent metal ions, trivalent ions

trivalent

divalent

monovalent

what do Ka1 and Kb1 refer to

Ka1 = acidic species with the most protons

Kb1 = basic species with the least protons

what is ionic strength

measure of the total concentration of ions in solution. ions of higher charge contribute to ionic strength to a greater degree

what is raw data

individual values of a measured quantity

what is treated data

found by applying a calibration procedure to raw data

what are use objectives

states the purpose for which the results will be used

what are specifications

how good the numbers need to be and what precautions are required in the analytical procedure

what are these categorized under: sampling requirements, acceptable blank values, precision and accuracy, false positives and negatives, selectivity and sensitivity, recovery of fortification, calibration checks, quality control samples

specifications

what does sampling requirements entail

• representative samples must be collected and preserved

• samples not representative/loss of analyte = meaningless analysis

what is a method blank

sample containing everything but the analyte, brough through all steps of analysis

subtracted from the analytical response of real sample prior to calculating the quantity of the analyte

what is a reagent blank

same as the method but not subjected to all preparations

what is a field blank

same as method blank, but it is exposed to the site of sampling

precise vs accurate

precise = reproducible and repeatable

accurate = near truth

what is sensitivity

capability of an analytical technique to respond reliably and measurably to changes in analyte concentration. method must have a LOD lower than the measured concentration

what is selectivity/specificity

ability to distinguish analyte from other species in the sample (to avoid interference)

what is spike/fortification

a known quantity of analyte added to a sample to test whether the response to sample is the same as that expected from a calibration curve

what is a calibration check

solutions of known conc are analyzed to ensure proper function of instrumentation and valid calibration

• should use solution different from the calibration curve to ensure solutions were made correctly

what is drift + cause

minor changes in instrument response over time

• result of temp and standard/reagent age

what is quality control samples (blind samples)

helps eliminate bias introduced by the analyst knowing the concentration of the calibration check sample

• known concentration/composition but analyst doesnt know

what specifications gauge accuracy

raw data, results from calibration checks, spike recoveries, quality control samples, blanks

what specifications gauge precision

replicate portions of the same sample, analytical performance on replicate samples

what is a standard operation procedure

states what steps are taken and how they will be done