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Analytical Chemistry > Exam 2 > Flashcards

Exam 2 Flashcards

1
Q

What is potentiometry?

A

the use of electrodes to measure voltages that provide chemical information

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2
Q

What are the two electrodes in a potentiometry experiment?

A

indicator/working electrode - responds to analyte activity
reference electrode - maintains a fixed reference potential

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3
Q

How is a classic galvanic cell constructed?

A
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4
Q

How do you write two half reactions for an electrode?

A

as reductions (electrons on left of equation)

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5
Q

What is E0 for a half cell?

A

the standard potential when the activities of the products and reactants are in unity

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6
Q

How do you calculate the potential for a complete galvanic cell?

A

Ecell = E+ - E-
Calculate E+ and E- by applying the Nerst equation to both half cells

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7
Q

What are three reference electrodes?

A

silver-silver chloride, calomel, and SHE

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8
Q

What is the purpose of a double junction electrode?

A

it minimizes contact between analyte solution and KCl from the electrode

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9
Q

What reference electrodes are normally used and why?

A

Silver-Silver Chloride and calomel are usually used because they are convenient. SHE requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions

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10
Q

How is a silver-silver chloride reference electrode constructed?

A
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11
Q

How is a double junction reference electrode constructed?

A
  • Allows you to fill the outer compartment with a solution that is compatible with the analyte solution
  • the inner and outer solutions slowly mix, so the outer compartment must be refilled periodically with fresh KNO3 solution
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12
Q

What is a problem with Ag/AgCl2 and SCE electrodes and how can it be fixed?

A
  • these electrodes leak chloride ions into the solution through their built in salt bridges, and they must if they are going to work
  • using a double junction electrode can help avoid contaminating the solution with Cl- and other ions
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13
Q

How do you calculate the pH at the equivalence point of a weak base and a strong acid?

A
  • calculate the formal potential of A- based on volume (v/2v+v)*F
  • then use the Kb equation to solve for [OH-]
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14
Q

What is buffer capacity?

A
  • ability to resist pH changes
  • # of moles of strong acid or base per liter required to change the pH one unit
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15
Q

How do you ensure high buffer capacity?

A
  • make the buffer concentrated not too dilute
  • use a weak acid that has a pka within one unit of the desired pH so that 0.1 < ([A-]/[HA]) < 10
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16
Q

How do you prepare a buffer experimentally from scratch?

A
  • choose a weak acid with pka ~ desired pH, make sure it is compatible with other solution components
  • For high buffer capacity choose log (Ca-/Cha) < 1
  • From the log(Ca-/Cha) term calculate the required concentration of the other component
  • Prepare with those concentrations then adjust the final pH by adding strong acid or base and checking with a pH meter
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17
Q

A buffer solution can be diluted with water, but its pH remains constant. Why?

A
  • Dilution affects the concentrations of the weak acid and its conjugate base almost equally so the ratio hardly changes
  • When dilution does cause a small pH change it is likely due to a change in the solutions ionic strength which effects activity
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18
Q

Why does a buffer made from scratch virtually always need to be adjusted?

A

we prepare buffers based on concentration not activity but pH meters measure activity

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19
Q

What is the Kjeldahl method?

A
  1. Place a nitrogen-containing organic sample inside the flask and digest in boiling conc. sulfuric acid to form NH4+, CO2, H2O, etc.
  2. Add excess base to convert NH4+  NH3.
  3. Distill the NH3 from the flask into a solution containing known moles of HCl. NH3 neutralizes some of the HCl.
  4. Titrate the excess HCl with standard NaOH. The difference in moles of HCl equals the moles consumed by NH3, which equals the original moles of organic nitrogen.
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20
Q

What are the faults of the Kjeldahl method?

A

melamine - used industrially to produce plastics, heavy in nitrogen, baby formula

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21
Q

What is an equivalence point?

A

when exactly enough moles of titrant have been added to neutralize the analyte

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22
Q

What is an endpoint?

A

the point in which an indicator changes color

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23
Q

What can be done if an endpoint may not match the equivalence point?

A
  • If significant titrant is required to change the color of the indicator alone perform a blank titration with indicator only (without any analyte)
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24
Q

What does H-H predict about the titration curve?

A

at half the eq point volume pH = pKa of the weak acid

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25
Q

What is the leveling effect?

A
  • all strong acids and bases are not equally strong but they appear to be in water - the leveling effect
  • A dilute solution of a strong acid in water contains essentially none of the intact acid. The acid is converted quantitatively to H3O+ instead. Since all strong acids produce the same product when dissolved in water they all appear to be equally acidic in the solvent
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26
Q

What is the strongest acid that can exist in an aqueous solution?

A

H3O+

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27
Q

What solvents are typically used in non-aqueous titrations?

A
  • need a solvent polar enough to dissolve the reactants and the products of the titration, possibly engage in hydrogen bonding
  • methanol, acetone, methyl ethyl ketone, methyl isobutyl ketone
  • no leveling effect
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28
Q

When looking at a titration curve of a base with multiple acids the _____ acid to react is the _____ acid

A

first, strongest

29
Q

What is a galvanic (voltaic) cell?

A
  • we insert electrodes into the cell and read the voltage that nature produces
30
Q

What is an electrolytic cell?

A
  • we insert electrodes into the cell and control the voltage between them. Since our voltage may differ from natures, electrons may need to flow into or out of the cell causing a chemical reaction
31
Q

Describe the usual set-up of a galvanic cell.

A
  • oxidation connected to the negative terminal (anode)
  • reduction connected to the positive terminal (cathode)
  • then the potentiometer always displays a positive value for the voltage
32
Q

What does the potentiometer display (+ or -) when a reference electrode is used?

A
  • electrode is always connected to the - terminal, the reading can be + or -
33
Q

What is the purpose of a salt bridge?

A
  • ox and red reactions must be physically isolated from each other or else the electrons would be passed directly from one redox species to another without being forced through the potentiometer
  • salt bridge conducts electric current easily
34
Q

How is a salt bridge made?

A

dissolve 30g KCL and 3g agar in 100ml h2o and pour into a u tube to cool

35
Q

When do you use the nernst equation?

A

only applies to reduction reactions

36
Q

What occurs when a half reaction is multiplied by a constant?

A

E is unchanged

37
Q

Describe how E0 value predicts oxidation/reduction.

A

The more positive the E the greater tendency of the oxidized form to get reduced

38
Q

How can you tell if a reaction is spontaneous from E?

A

A reaction is spontaneous if the sum of the E (not E0) values for the two half reactions comprising the balanced equation is >0

39
Q

SHE
- construction
- half rxn
- conditions/ definition

A
  • if 25°C, H+ A = 1, and P = 1 bar, then E° = 0 V by definition.
  • Half reaction: 2H+ + 2e- -> H2
  • Construction of an SHE
    —Make electrode from platinum metal coated with platinum black to increase its surface area
    —Immerse the electrode in H+(aq) with activity = 1 M.
    —Bubble H2(g) over the electrode surface at 1 bar (i.e., AH2 = 1).
    —Maintain the temperature at 25°C.
    —Always connect the S.H.E. to the – terminal of the potentiometer.
40
Q

How do Ag/AgCl and calomel electrodes work?

A

Leak ions through their salt bridges

41
Q

What are four common indicator electrodes?

A

 inert (e.g., platinum, gold)
 Ag, Fe, Cu, etc., if that metal is involved in the redox
 glass electrode (for pH measurements)
 ion-selective electrodes

42
Q

What is the liquid-liquid junction potential?

A
  • the potential difference that develops at liquid interfaces (e.g., the ends of a salt bridge).
  • It causes an error but is typically small (a few mV)
  • Results from differing ion mobilities
43
Q

How do you minimize junction potentials?

A
  • Make the cell with the fewest possible liquid junctions
  • Make salt bridges using salts having ions with similar mobilities (KCl, KNO3)
44
Q

What can be used to solve incompatibility problems?

A

KNO3 (aq)

45
Q

What is a glass pH electrode?

A
  • indicator electrode
  • mobile sodium ions transport the charge through the glass membrane making it slightly conductive
46
Q

What is line notation for a glass combination electrode?

A

Ag(s) | AgCl(s) | Cl–(aq) || H+(aq, unkn. conc., outside) | H+(aq, 0.1 M, inside), Cl–(aq, 0.1 M) | AgCl(s) | Ag(s)

47
Q

What are two types of electrodes and advantages/disadvantages?

A
  • The advantage and disadvantages of gel-filled electrodes: don’t need refilling; if the gel gets contaminated, it can’t be replaced; also the gel can dry out and cannot be rehydrated, often only last 1-2 years, substitutes a KCl/AgCl gel for the filling solution
  • The advantages and disadvantaged of refillable combination electrodes: If the filling solution gets contaminated, just replace it, electrodes cost $80-$170, can last for years, can include a double junction
48
Q

What are the ways that electrodes fail?

A

the filling solution gets contaminated (usually easy to fix by replacing it), the salt bridge gets plugged, or something coats the surface of the glass.

49
Q

What are the main sources of error in pH measurements?

A
  • Standard buffers
  • Temperature drift
  • Junction potentials (Most cancel out when calibrated with buffers)
  • Acid error
  • Sodium error
50
Q

What is the acid error?

A
  • As pH decreases, [H+] increases.
  • More H+ ions bind to the glass electrode outer surface, making it more positive.
  • If too many H+ ions try to bind, the surface begins to run out of binding sites.
  • So, the actual potential on the glass surface is not as positive as predicted based on the [H+].
  • The acid error causes the glass electrode to report a pH that is too high.
51
Q

What is the sodium error?

A
  • As pH increases, [H+] decreases.
  • Fewer H+ ions bind to the glass electrode outer surface, leaving many open binding sites.
  • If other (non-H+) cations are present in the solution, they can bind weakly to the electrode.
  • So, the actual potential on the glass surface is more positive than predicted based on the [H+].
  • The sodium error causes the glass electrode to report a pH that is too low. Na+ is especially problematic, but other cations can also cause this error.
52
Q

What are ion selective electrodes?

A
  • An electrode specifically designed to produce voltage changes in response to variations in the concentration of only one kind of ion.
  • An ISE is made with a membrane having a special affinity for the ion.
  • A glass electrode is an ISE for H+.
53
Q

What is the selectivity coefficient?

A

Ka,x = response to X / response to A
- small is desireable
- for ion selective electrodes

54
Q

How does a redox indicator work?

A
  • A redox indicator changes color over an approximate range of ±(59/n) mV, centered at E° for the indicator. n is the number of electrons in the indicator half-reaction.
  • If the difference in formal potentials is > 0.4 then a redox indicator usually gives a satisfactory visual endpoint
55
Q

How does methylene blue work?

A

For methylene blue a conjugation change from oxidized (conjugated) to colorless (reduced protonated form) causes the change in color during when the compound is reduced

56
Q

What is coulometry?

A

Measure the current and time needed to reach the end point

57
Q

KF - coulometric

A
58
Q

KF - volumetric

A
59
Q

What is electrogravimetric analysis?

A

Quantitation by plating metal cations onto a weighed cathode and measuring the weight gain - Uses an electrolytic cell

60
Q

What is overpotential?

A
  • relates to electrogravimetric analysis
  • Overpotential (needed to overcome activation energy): If a potential difference of exactly 0.890 V is applied to the electrodes, nothing happens. Considerably greater voltage is required (up to 1-2 volts more!). This is the “overpotential” and it overcomes the activation energy of the reaction.
61
Q

What is a voltammogram?

A

Voltammogram = (plot of current vs. potential).

62
Q

What is CV?

A

Cyclic voltammetry
We control the electrode potential in CV, unlike in potentiometry where we just measure it.

63
Q

What does it mean when a voltammogram is symmetrical? unsymmetrical? How to measure the reduction potential for a species from the cyclic voltammogram? How to measure the number of electrons transferred in the half-reaction from CV? How to quantify the concentration of an analyte by using CV?

A

What does it mean when a voltammogram is symmetrical? unsymmetrical?
* The symmetry indicates the reversibility of the reaction
How to measure the reduction potential for a species from the cyclic voltammogram
* Reduction potential E = (Epc + Epa) / 2
How to measure the number of electrons transferred in the half-reaction from CV
* Number of electrons transferred ≈ 57.0 mV / DE
How to quantify the concentration of an analyte by using CV
* Current amplitude is directly proportional to species conc. – the basis for quantitation –

64
Q

What is ASV?

A
  • Before running the voltammogram, concentrate the analyte metal from the sample into a tiny drop of liquid mercury (the working electrode), forming an amalgam.
  • When the voltammogram is run, the concentrated analyte is stripped out of the mercury drop.
  • Since the local analyte concentration in the drop far exceeds that in the bulk sample, the current is much greater than it would be otherwise.
65
Q

What are the major steps in ASV? What is a safe alternative?

A
66
Q

How does ASV differ from CV?

A
  • Differs from cyclic voltammetry in that:
  • the analyte is first pre-concentrated by reducing (or oxidizing) it and depositing it into / onto the cathode (or anode).
  • During the later oxidation (or reduction) step, that produces a much bigger signal when the deposited analyte is stripped from the electrode.
67
Q

What technique does ASV resemble?

A

This technique resembles running one half of a cyclic voltammogram, recording I versus E.

68
Q

How does ASV compare to CV?

A

Gives very high sensitivity and low detection limit (nM). (Has much better sensitivity and detection limits compared to cyclic voltammetry.)

69
Q

The Nernstian behavior of the glass electrode.

A

The Nernstian behavior of the glass electrode. (A cell potential vs. pH plot is linear with a slope of
0.05916 X b., in volts per pH unit)

Analytical Chemistry (4 decks)

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