Week 4: Electrochemistry

chemical reactions occur due to flow or presence of ___________

electrons

for current to flow, _______ > _________

potential, resistance

electrochemical cell

1. 2 electrodes (half-cells) immersed in electrolyte solution

2. cathode electrode = reduction

3. anode electrode = oxidation

general principle of a pH meter

measures the voltage difference between a reference electrode and a measuring glass electrode submerged in a solution

most pH probes contain two electrodes inside the body: a ___________ electrode and a ___________ electrode

glass/measuring, reference

potentiometry measures cell _________ difference between the electrodes

potential

amperometry, conductimetry, and voltammetry measure cell _________

current

what is measured in potentiometry

the electrical potential (voltage) difference between the reference and indicator electrodes when NO CURRENT is passing through the cell

the reference electrode produces a…

stable potential

the indicator (measuring) electrode produces a ___________ potential that is proportional to the sample’s ion ____________

variable, concentration

salt bridge (liquid junction)

completes the circuit between the half cells and the voltmeter

a millivoltmeter uses the __________ equation, where the __________ produced by the electrochemical cells is proportional to the ion _____________

Nernst, potential, concentration

components of the reference electrode

1. metal in contact with metal salt

2. internal reference electrolyte solution

3. salt bridge (liquid junction)

what is the purpose of the internal reference electrolyte solutions, and what is it made of?

1. ensures a stable potential is produced

2. high concentration, with the same anion as metal-salt

types of reference electrodes

1. Ag/AgCl in saturated HCl or KCl (most popular)

2. calomel Hg/Hg2Cl2 in saturated KCl

calomel electrodes are mainly ______ electrodes

pH

indicator electrodes must be _________, and measures a specific analyte of interest

selective

the potential of an indicator electrode varies _____________ to sample ion activity

proportionally

indicator electrode components

1. metal in contact with metal salt

2. membrane

3. internal electrolyte solution

function of indicator electrode membrane

separates internal electrolyte solution from sample

purpose of different membrane types

determines analyte measured

quality of internal electrolyte solution of indicator electrode

1. low concentration, with the same anion as metal-salt

2. allows variable potential produced

what are indicator electrodes also called?

ion sensitive electrodes

what equation is used to calculate ion activity?

Nernst equation

how does the membrane work to determine ion activity?

analyte of interest selectively interacts/incorporates into the membrane to develop a membrane potential

types of membranes

1. glass membrane

2. liquid and polymer membranes

3. solid-state membrane

4. gas-sensing electrode

glass membrane

1. thin layer of glass selective for a monovalent cation

2. acts an ion exchanger

3. H+ and Na+

what type of membrane does the indicator electrode of a pH meter use?

H+ sensitive glass

liquid and polymer membrane

1. liquid ion exchanger or neutral carrier ionophore reacts with the ion of interest

2. polymer is inert matrix

3. Na+, K+, Cl-, Ca++

K+ electrode membrane is composed of ____________

valinomycin

solid state membrane

1. solid crystal or salt pellet of ion of interest

2. the analyte ion selectively fits into crystal’s surface

3. Cl-

4. less specific because other anions can react

gas-sensing electrode

1. gas-permeable membrane separates sample from an indicator electrode

2. only the sample gas is exposed to internal solution

3. pCO2, pO2

in a pCO2 electrode, a _____________ membrane (teflon or silicon) is combined with a pH indicator electrode

gas-permeable

pCO2 is indirectly proportional to __________

buffer

general principle of amperometry

a fixed potential is applied to electrochemical cells to excite specific sample compounds, and the current produced is measured

in amperometry, analytes are __________ or _________ producing electrons, measured as current

oxidized, reduced

in amperometry, the __________ produced is proportional to the concentration of analyte

current

analytes measured in amperometry

pO2

what is a gas-permeable membrane in amperometry made of?

polypropylene

in an oxygen electrode (amperometry), oxygen is reduced at a ___________________ electrode

platinum cathode

what type of anode electrode is used in amperometry?

Ag/AgCl

general principle of voltammetry

it varies the potential applied to the electrochemical cell to measure the resulting current

voltammetry is very __________

sensitive

clinical application of voltammetry

1. anodic stripping

2. trace metals like lead

general principle of conductometry

a solution’s conductivity is proportional to the total number of ions in solution and potential applied

conductometry is highly ___________, but ___________

sensitive, nonspecific

clinical applications of conductometry

1. deionized water purity

2. chromatography detector

biosensor components

1. immobilized biological/biochemical reagent

2. transducer

function of the transducer in biosensors

measures the biochemical reaction

purpose of ISE calibration

corrects for small differences in indicator and reference electrode potentials

pH meter calibrations

1. 1 point: setting pH 7, set zero point

2. 2 points: setting 2 pH points

glass/liquid and polymer membrane rejuvenation

wipe carefully with moistened soft cloth

solid-state membrane rejuvenation

resurface with fine sandpaper

gas-permeable and enzyme membrane rejuvenation

replace

henderson-hasselbalch equation

pH = pKa + log( [A-] / [HA] )