Quant exam 4

what are potentiometric methods based on?

potentiometric methods are based on measuring the potential of electrochemical cells without drawing appreciable current

what is the analyte in potentiometry?

the analyte is an electroactive species that participates in redox by donating or accepting an electron at an electrode within the galvanic cell

what is an indicator/working electrode?

electrons are transferred to or from the analyte through an electrode (i.e. a Pt wire) which is referred to as an indicator or working electrode

what is a reference electrode

the second half-cell is fixed (i.e., no unknown/analyte) providing constant potential, and thus called the reference electrode

what reference electrodes did we talk about in lecture

silver/silver chloride

saturated calomel (SCE)

standard hydrogen electrode (SHE)

what type of cell is this?

galvanic cell

which is the anode?

the Ag/AgCl half cell

which is the working electrode?

the cell measuring electroactive species, the Pt/Fe²⁺, Fe³⁺ half cell

what is true at the reference electrode?

[Cl^-] should be constant because it is in a saturated KCl solution. this means that the potential from this half cell is constant

write the half reactions

right: Fe³⁺ + e^- → Fe^2-

left: AgCl + e^- → Ag + Cl^-

write the reactions in cell notation

Ag (s) | AgCl (s) | Cl^- (aq) || Fe²⁺, Fe³⁺ (aq) | Pb (s)

what are the benefits of using double-junction reference electrodes?

they minimize interaction between reference KCl solution and sample solution

• longer lifetime

• best for samples with heavy metals, proteins etc. that can diminish performance over time

label the components of the silver/silver chloride reference electrode

1. Ag wire

2. Sat KCl and AgCl

3. AgCl paste

4. solid KCl (some AgCl)

5. porous plug/frit-salt bridge to external solution

write the equation for the saturated calomel electrode

Hg₂Cl₂ + 2e^- → 2Hg + 2Cl^-

label the SCE

1. Hg (l)

2. Hg, Hg₂Cl₂, + KCl

3. sat. KCl

4. solid KCl

5. porous plug/frit-salt bridge to external solution

compare Ag | AgCl and SCE reference electrodes to SHE

these are the three common reference electrodes. SHE cannot be made into compact pen-sized devices, but it still serves as the zero point for comparing reference electrodes

when converting from SHE to Ag/AgCl or SCE, do you add or subtract?

subtract

what are the two types of indicator electrodes?

metal and ion selective

what are metal electrodes used for?

conventionally used for redox analyte detection

what are ion-selective electrodes used for?

conventionally used for ion-specific interaction (i.e., ion specifically absorbs/binds to the surface of the electrode membrane)

what are the common metal electrodes? what do you want in general for a metal electrode?

• Pt is most common- very inert, easy to clean

• Au is inert too

• conductive carbon materials are also common

in general, you want a pristine electrode with a large surface area

the nernst equation describes the net driving force for a redox. what do the red and blue components represent?

red: represents the driving force under standard conditions (i.e. activities = 1)

blue: shows the dependence on reagent concentrations

how do you find E for the entire cell?

what is a potentiometric titration

Ag⁺ + Cl^- → AgCl

• analogous to an acid-base titration curve

• Ag⁺ is like H⁺ and Cl^- acts like a base being titrated

• effectively, the E_cell expression can be rearranged as a halide electrode

  • E_cell = 0.558V + 0.05916V log (K_sp/[Cl^-])

• in general, metal electrodes can be indicators for their given aqueous ions

  • though, metal-metal ion equilibrium can be difficult to establish for proper analysis

what is junction potential

a junction potential is a voltage difference occurring at the interface between dissimilar electrolyte solutions (i.e., at salt bridge termini)

what are the effects of junction potential

limits the accuracy of direct potentiometric measurements, as these contributions are difficult to measure precisely

what is a junction potential difference a result of? provide an example

a result of ion mobility difference

• ex. Cl moves faster than Na, creating a defined charge interface

why is saturated KCl preferred for salt bridges?

K + Cl ions have similar ion mobility, this reduces junction potential

what is the response (mV) of a pH electrode per pH unit?

59.16 mV per 1 pH unit

what are the benefits of studying ISEs

many ions measured in blood chemistry are/can be analyzed by ISEs

• chem 7 test constitute up to 70% of hospital lab tests. 4 of the 7 species are quantified by ISEs

what are glass membrane ISEs used for?

H⁺ and other monovalent cations

how do ISEs work?

• diffusion of analyte ions from the membrane creates a slight electrical potential difference at the interface between the membrane and analyte solution

• this potential difference changes variably in relation to the concentration of analyte ion in solution and can be calibrated for quantitation following a nernstian model

• ISEs only measure the activity of free ions!! so complexed ions cannot be detected by ISE

can complexed ions be calculated by ISEs?

No!

what is an ionophore

the component providing selectivity in an ISE

how does a H⁺ ISE work?

Na⁺ within the glass network exchanges for H⁺

what is 𝛽

electromotive efficiency

how are 𝛽 and “constant” determined?

determined via calibration with at least 2 solutions of known pH. 𝛽 usually very close to 1

what is acc

describe the errors in pH measurement

1. the porous plug of the electrode leads to a junction potential, scales with the difference in ionic activity of standard vs unknown

2. junction potential drift- compensate by recalibrating every few hours

3. at high pH, [Na⁺] is high, the electrode responds to Na⁺ measuring a lower pH than true value. this is called alkaline error

4. at low pH, measured pH is often higher than true values because of complete protonation of the glass. this is called acid error

5. solutions must be stirred and allowed to equilibrate, generally for 30s to few minutes

6. electrode should always remain hydrated

7. keep clean, etc.

what is alkaline error

at high pH, [Na⁺] is high, the electrode responds to Na⁺ measuring a lower pH than true value.

what is acid error

at low pH, measured pH is often higher than true values because of complete protonation of the glass.

why is a selectivity coefficient necessary

• ISEs are not completely exclusive, they are merely selective

• ISEs have strong affinities for some ions, but interfering ions (of the same valence and comparable size) are common

what is selectivity coefficient

K ^pot_A,X = response to X/response to A

what is chromatography? what are the two types of chromatography discussed in class?

the separation of a mixture by passing through a medium in which components of differing properties travel at different rates

what is the mobile phase

the solvent moving through the column (can be liquid or gas)

what is the stationary phase?

often a viscous liquid, gel, or solid particle packed into the column

what gives rise to separation in chromatography

equilibration of solutes between mobile and stationary phases give rise to separation

what is the eluent

fluid entering the column

what is the eluate

fluid emerging from the column

in chromatography, column can be _______ or _______________

packed or open tubular

what kind of column is used for HPLC

packed column

what kind of column is used for GC

open tubular (most of the time

what is adsorption chromatography

exploits surface interactions to separate chemical mixtures

the more a solute interacts with the surface, the slower it adsorbs and desorbs from surface, the longer it is retained

• ex. silica (polar molecules bind to silica gel and elute slower through column)

what is partitioning chromatography for a gas chromatography column

open tubular column, solute dissolved in liquid stationary phase

retention time is related to rate of partitioning in and out of stationary phase— GC capillary column

surface functionalized with C8

what is partitioning chromatography on a liquid chromatography column

retention time is related to rate of partitioning in and out of stationary phase - true for reversed-phase HPLC (NP column, polar mobile phase)

surface functionalized with C18

describe ion exchange chromatography

anion-exchange resin, anions are attracted to the positive resin

retention and retention tiem is related to the electrostatic interactions

describe size exclusion chromatography

retention time is related to the distance traveled. since large particles cannot penetrate the maze of pores in stationary phase, they elute first

describe affinity chromatography

one kind of molecule in a complex mixture becomes attached to a molecule that is covalently bound to stationary phase

all other molecules simply wash through

what is a chromatogram

plot of signal vs time

what is a chromatograph

instrument used to perform chromatography

requires an injector, a column and detector

what is the retention factor and what does it tell you

k = t_r - t_m / t_m

time solute spends in the stationary phase/time solute spends in mobile phase

tells you equilibrium information

what is partition coefficient

K = c_s / c_m

what is band broadening-diffusion

as time increases or column lengthens, species separate but also bands naturally broaden

good chromatographic peaks are gaussian and thus, diffusion is limited

what is gas chromatography

in GC, gas analyte is transported through the column by a gaseous mobile phase, called the carrier gas

describe sample injection for GC

• typical liquid injection volumes = ~1 uL

• typical gas injection volumes = 10uL to 5 mL

• leaving some headspace in micro-syringe prevents sample from immediately evaporating

• carrier gas sweeps vaporized sample into the column

• liners get changed every 20 to 100 injections

how can you control separation for GC? how do you improve resolution

you can buy columns with different properties

• ex. high throughput: want a more robust column

narrower column, longer column, different stationary phase

describe retention time in GC

retention time in GC can be viewed as a thermodynamic process that depends on the enthalpy of vaporization and enthalpy of mixing with stationary phase

heat of vaporization scales linearly with boiling point

therefore, retention increases with boiling point for a given class of molecules

describe elution order in GC

a compound’s vapor pressure inside of a GC is a function of two opposing forces

1. the first opposing force is the vaporization of the compound. higher temperatures will increase a compound’s vapor pressure

2. the second opposing force is the strength of the intermolecular interactions between the compound and the stationary phase. stronger IMFs with the phase will decrease a compound’s vapor pressure

the easiest way to control a compound’s vapor pressure in a GC is through the GC oven temperature control