ANALYTICAL CHEMISTRY ACS
QUESTIONS & SOLUTIONS
Potentiometry - ANSWERan analytical method in which an electric potential difference
(voltage) is measured
1. measurement of ion activity from the electric potential of an ion at an electrode
surface
2. Msmt of potential difference between two electrodes of a galvanic cell at zero current
Redox titrations and electrode potential - ANSWER-Reduction (adding e-) occurs at
cathode (E+)
-Oxidation (removing e-) occurs at anode (E-)
Cell voltage = E+ - E-
Single line denotes phase boundary; double lines denote salt bridge.
Silver-Silver Chloride Reference Electrode (Ag|AgCl electrode) - ANSWERhalf-reaction
(written as reduction):
AgCl(s) + e- <--> Ag(s) + Cl-
-potential measured at anode (E-); Ag(s) | AgCl(s) | Cl-(aq) plus salt bridge
-problem: commonly, porous plugs become clogged causing a slow and unstable
electrical response.
S.C.E - ANSWERsaturated calomel electrode (calomel electrode saturated with KCl) is
a reference electrode, based on half reaction:
Hg2Cl2(s) + 2e- <--> 2Hg (l) + 2Cl-
and has potential denoted by E-
Indicator electrode - ANSWER2 types:
1. Metal electrodes: develop elec pot in response to redox reaction at metal surface. Pt
is most common metal indicator electrode (inert).
-Example is Ag indicator electrode, which is just a solid Ag wire dipped into the solution.
The half-reaction is:
Ag+ + e- <--> Ag(s) V= E+ denoted by Ag | Ag+
E+ = E0(+) - .05916log(1/[Ag+]) = E0(+) + .05916log[Ag+]
**a silver electrode is also a halide electrode if solid silver halide is present, and [Ag+] =
Ksp/[X-]
2. ion-selective electrodes (ISEs) are indicator electrode that do not involve redox
processes, and have a membrane that specifically binds ion of interest. usually liquid-
based. electric potential difference (voltage) is measured across the membrane with 2
(inner and outer) reference electrodes. E = Eouter - Einner = constant + .05916/n(loga0)
where n is charge of analyte ion and a0 is its activity in the outer unknown solution.
, ** the glass electrode (pH electrode) most common ISE.
pH combination electrode - ANSWERCell line diagram:
Ag(s) | AgCl(s) | Cl- (aq) || H+ (aq, outside) ..|..H+ inside, Cl-(aq) | AgCl(s) | Ag(s)
a glass membrane separates H inner/outer and pH is unknown in outer.
Response of real glass electrodes:
E = constant + (beta)(.05916)*pH(outside)
because pH = A(H+) outside (activity)
detection limit - ANSWERconcentration of an element that gives a signal equal to two
times the peak-to-peak noise level of the baselin
interference - ANSWERany effect that changes the signal while analyte concentration
remains unchanged.
Types:
spectral = unwanted signals overlap analyte signal
chemical - chem rxns decreasing the concentration of analyte atoms
ionization - (of analyt atoms) decreasing the concentration of neutral atoms.
Can be corrected by removing source of interference of by preparing standards that
exhibit the same interference (method of standard addition)
matrix effect - ANSWERa change in analytical signal caused by anything in the sample
other than analyte
Indeterminate/Random error: type of error that can be either positive or negative and
cannot be eliminated; based on the ultimate limitations on a physical measurement
Atomic Absorption Spectroscopy - ANSWERAn analytical technique that uses light
absorption (in plasma, flame, or furnace) to measure the concentration of a metal in a
sample. (also: emission and fluorescence spectroscopy)
buffer - ANSWERsolution of weak acid with conjugate base that resists changes in pH
upon adding acid or base. Buffer capacity (beta) measures ability of buffer to resist
changes in pH (larger = better ability). Ideally, the ratio of weak acid: conj base is 1:1
and pH = pKa
chemical ionization - ANSWERA gentle method of producing ions for a mass
spectrometer without extensive fragmentation of the analyte molecule, M. A reagent gas
such as CH4 is bombarded with electrons to make CH5+, which transfers H+ to M
giving MH+.
cold trapping - ANSWERSplitless gas chromatography injection technique in which
solute is condensed far below its boiling point in a narrow band at the start of the
column.
confidence interval - ANSWERrange of values within which there is a specified
probability that the true value lies
QUESTIONS & SOLUTIONS
Potentiometry - ANSWERan analytical method in which an electric potential difference
(voltage) is measured
1. measurement of ion activity from the electric potential of an ion at an electrode
surface
2. Msmt of potential difference between two electrodes of a galvanic cell at zero current
Redox titrations and electrode potential - ANSWER-Reduction (adding e-) occurs at
cathode (E+)
-Oxidation (removing e-) occurs at anode (E-)
Cell voltage = E+ - E-
Single line denotes phase boundary; double lines denote salt bridge.
Silver-Silver Chloride Reference Electrode (Ag|AgCl electrode) - ANSWERhalf-reaction
(written as reduction):
AgCl(s) + e- <--> Ag(s) + Cl-
-potential measured at anode (E-); Ag(s) | AgCl(s) | Cl-(aq) plus salt bridge
-problem: commonly, porous plugs become clogged causing a slow and unstable
electrical response.
S.C.E - ANSWERsaturated calomel electrode (calomel electrode saturated with KCl) is
a reference electrode, based on half reaction:
Hg2Cl2(s) + 2e- <--> 2Hg (l) + 2Cl-
and has potential denoted by E-
Indicator electrode - ANSWER2 types:
1. Metal electrodes: develop elec pot in response to redox reaction at metal surface. Pt
is most common metal indicator electrode (inert).
-Example is Ag indicator electrode, which is just a solid Ag wire dipped into the solution.
The half-reaction is:
Ag+ + e- <--> Ag(s) V= E+ denoted by Ag | Ag+
E+ = E0(+) - .05916log(1/[Ag+]) = E0(+) + .05916log[Ag+]
**a silver electrode is also a halide electrode if solid silver halide is present, and [Ag+] =
Ksp/[X-]
2. ion-selective electrodes (ISEs) are indicator electrode that do not involve redox
processes, and have a membrane that specifically binds ion of interest. usually liquid-
based. electric potential difference (voltage) is measured across the membrane with 2
(inner and outer) reference electrodes. E = Eouter - Einner = constant + .05916/n(loga0)
where n is charge of analyte ion and a0 is its activity in the outer unknown solution.
, ** the glass electrode (pH electrode) most common ISE.
pH combination electrode - ANSWERCell line diagram:
Ag(s) | AgCl(s) | Cl- (aq) || H+ (aq, outside) ..|..H+ inside, Cl-(aq) | AgCl(s) | Ag(s)
a glass membrane separates H inner/outer and pH is unknown in outer.
Response of real glass electrodes:
E = constant + (beta)(.05916)*pH(outside)
because pH = A(H+) outside (activity)
detection limit - ANSWERconcentration of an element that gives a signal equal to two
times the peak-to-peak noise level of the baselin
interference - ANSWERany effect that changes the signal while analyte concentration
remains unchanged.
Types:
spectral = unwanted signals overlap analyte signal
chemical - chem rxns decreasing the concentration of analyte atoms
ionization - (of analyt atoms) decreasing the concentration of neutral atoms.
Can be corrected by removing source of interference of by preparing standards that
exhibit the same interference (method of standard addition)
matrix effect - ANSWERa change in analytical signal caused by anything in the sample
other than analyte
Indeterminate/Random error: type of error that can be either positive or negative and
cannot be eliminated; based on the ultimate limitations on a physical measurement
Atomic Absorption Spectroscopy - ANSWERAn analytical technique that uses light
absorption (in plasma, flame, or furnace) to measure the concentration of a metal in a
sample. (also: emission and fluorescence spectroscopy)
buffer - ANSWERsolution of weak acid with conjugate base that resists changes in pH
upon adding acid or base. Buffer capacity (beta) measures ability of buffer to resist
changes in pH (larger = better ability). Ideally, the ratio of weak acid: conj base is 1:1
and pH = pKa
chemical ionization - ANSWERA gentle method of producing ions for a mass
spectrometer without extensive fragmentation of the analyte molecule, M. A reagent gas
such as CH4 is bombarded with electrons to make CH5+, which transfers H+ to M
giving MH+.
cold trapping - ANSWERSplitless gas chromatography injection technique in which
solute is condensed far below its boiling point in a narrow band at the start of the
column.
confidence interval - ANSWERrange of values within which there is a specified
probability that the true value lies
