IA exam #2

Metallic Indicator Electrodes

(1) Electrodes of First Kind (2) Electrodes of Second Kind

Potentiometry pros

- Relatively free from interferences and provide a rapid, and convenient - Nondestructive means of quantitatively determining numerous important ions and cations - Simple and inexpensive

Molecular-Selective Membrane Electrode

1) Gas Sensing Probes (CO2 and NH3) 2) Enzyme Substrate (urease membrane for blood urea)/Biocatalytic Membrane Electrodes:

Ion-selective Membrane electrodes: Non-crystalline membranes

1) Glass: silicate glasses for H+, Na+ 2) Liquid: liquid ion exchanger for Ca2+ 3) Immobilized liquid : liquid/polyvinyl chloride (PVC) matrix for Ca2+ and NO3-

Ion-selective Membrane electrodes: crystalline membranes

1) Single crystal: LaF3 for F 2) Polycrystalline or Mixed crystal: Ag2S for S2- and Ag+

boundary potential, Eb

Eb = E1- E2: varies with the pH of the analyte solution.

Ecell =

Ecathode (right) - Eanode (left)

Potentiometry cell potential equation

Ecell = Eind - Eref + Ej

Nernst equation

Ecell= E°cell - (RT/nF) (logQ)

Ecell = Constant - 0.0592 pH

L' + EAg/AgCl + Easy is constant value

Alkaline Error of pH Electrodes

In strong basic solution, [H+] is extremely small and [Metal+] is large. Glass electrodes respond to the concentration of both hydrogen and alkali metal ions such as Na+. pH values are lower than the true value.

reference electrode

Independent of the concentration of the analyte or any other ions in the solution. Always treated as the left-hand electrode in potentiometric measurement.

Exclusions of Nernst equation

Pure solid, liquid, or solvent, its activity is unity, and no term for A, B, C, or D is included in the equation. • A gas is replaced by PA , the partial pressure

Standard Hydrogen Reference Electrode (SHE)

Relative half-cell potentials can be measured against the same reference half-cell. replace Eleft (anode) with SHE

(Electrodes of Third Kind)

Responsive different cation than the Metal used Example: Hg electrode used to measure Ca+2

Junction potential (Ej)

The potential difference resulting from this charge separation. by ion diffusion across membrane ~ several hundredth of a Volt

in a irreversible battery

changing the direction of current causes entirely different half-reaction to occur at one or both electrodes

F = Faraday constant

charge of a single electron 9.649 x 10^4

In the case of the glass electrode, the [H+] inside the membrane is constant, and the concentration outside is determined by the

concentration or activity of the H+ in the analyte solution. -> The concentration difference produces the potential difference that we measure with a pH meter.

As reaction goes on, the initial cell potential

decreases continuously and approaches zero as the overall reaction approaches equilibrium -> the cell voltage is zero ("dead battery")

cathodes in batteries become electron

deficient due to its tendency to reduce Ag+(aq) + e- -> Ag (s)

Ecell related to free energy formula

deltaG0 = - nFE^0cell

current (I) is the opposite of

electron flow in a battery

how electrons travel in the electrochemical cell

electrons that are oxidized in anode are taken up by electrode traveling through circuit the Cl- from salt bridge travels to anode Ag ions move towards the silver electrode where they are then reduced by the incoming electrons from anode

Anodes in batteries collect

electrons, becoming electron rich due to metal used tendency to oxidize Cu (s) -> 2e- + Cu+2 (aq)

It is reduced when it

gains electrons.

E2 makes up

indicator reference electrode EAg,AgCl

Membrane electrodes are divided into

ion selective and molecular selective electrodes

The activity, or effective concentration, of species X depends on

ionic strength of the medium

SHE electrode is a universal reference, but

is really a hypothetical electrode (not used in practice) - Very sensitive to temperature, pressure, and H+ ion activity

electrolytic cell current flow

is the opposite of a galvanic cell what would be anode and cathode on a galvanic cell switches spots (redox rxns flip) reversible battery

A molecule is said to be oxidized when it

loses electrons.

advantages of electroanalytical methods

low detection fast inexpensive

The pH Glass (Membrane) Electrode

measure the potential that appears across a thin glass membrane that separate two solution with different hydrogen ion concentrations. has two reference electrodes

Types of Indicator Electrodes

metallic and membrane

Salt bridge (sat'd KCl):

minimize the liquid junction potential. The mobilities of K+ and Cl- are nearly equal. Typically a few millivolts

The potential of a hydrogen electrode depends

on temperature the activities of hydrogen ion and molecular hydrogen in the solution At PH2 = 1.0 atm and aH + = 1.00 M (standard state), the potential of the standard hydrogen electrode is 0.00 V at all temperatures.

The Anode is where

oxidation occurs

Electroanalytical methods measure

potential (volts) and/or current (amperes) in an electroanalytical cell containing analyte

Interfacial methods used

potentiometry voltammetry coulometry

Potentiometry salt bridge

prevents the components of the analyte solution from mixing with those of the reference electrode A potential develops across the liquid junctions at each end of the salt bridge. If the mobilities of the cation and the anion in the bridge solution are approximately same, these two potentials tend to cancel one another (KCl, the mobilities of K+ and Cl- ions are nearly equal). Although the junction potential is small enough to be neglected, the junction potential can be factors that limit measurement accuracy and precision.

inside of membrane has a known amount of

protons

q = n · N · F

q = Coulombs n = Unit charges per mole N = moles F = Faraday constant

if a cell potential is negative

reaction is not spontaneous, delta G is positive

if a cell potential is positive

reaction is spontaneous, delta G is neg

The Cathode is where

reduction occurs

E1 makes up

reference 1 ESCE and Ej Ej across the salt bridge separates the calomel electrode (SCE) from the analyte solution.

Electrolytic Cells

require an external applied voltage to drive electrical current. external source of energy for it to work

Ideal indicator electrode:

responds rapidly and reproducibly to changes in the concentration of an analyte ion (or group of analyte ions). Only a few are now available that are remarkably selective. Ecell = Eind - Eref

A pH-sensitive glass membrane involves an ion- exchange reaction between

singly charged cations in the interstices of the glass lattice and protons from the solution.

Galvanic cells operate

spontaneously

bulk methods

study whole analyte of solution conductometry

galvanic cell produces electricity because

the cell reaction is not at equilibrium once is reaches equilibrium electricity stops being produced

If there were no salt bridge,

the left half-cell would soon build up positive charge from excess Zn2+ and the right half-cell would build up negative charges by depletion of Cu2+ In an instant, chemical reaction and the process would cease.

Standard electrode potential (E0):

the measure of individual potential of an electrode at standard ambient conditions (298K, solutes at a concentration of 1 M, and gas pressure at 1 bar).

Although the internal reference electrode is a part of the glass electrode, it is not

the pH-sensing element. Instead, it is the thin glass membrane at the tip of the electrode that responds to pH.

EAg/AgCl

the potential of the internal Ag/AgCl reference electrode.

Current (I)

the quantity of charge (coulombs) flowing each second past a point in an electric circuit.

Ampere (A)

the unit of current = q(C)/s

potentiometry current

there is no current its zero

The thin glass membrane bulb (Indicator) at the tip of the electrode responds

to pH. - A charge imbalance across any material, there is an electrical potential differences across material. The internal reference (Ag/AgCl) is a part of the glass electrode, it is not the pH- sensing element.

The potential of a silver electrode is proportional

to the negative logarithm of the chloride ion activity.

Indicator electrode for ions:

used with reference electrode to measure potential of unknown analytes solution.

Electrodes of First Kind

• A Pure metal electrode is in direct contact with its own cation in the solution. (Cu electrode detects a Cu2+ cation) • A single reaction is involved.

SHE electrode

• E0 is relative to SHE • Assigned 0.000 V • Can be anode or cathode • Pt does not take part in reaction • Pt electrode coated with fine particles (Pt black) to provide large surface area • Cumbersome to operate

Electroanalytical methods can characterize

• Extent of adsorption or chemisorption • Rates and equilibrium constants for chemical reactions • Stoicheometry • Charge transfer

Electrodes of First Kind exhibit

• Low selectivity. e.g.) Cu electrode can not be used for the determination of Cu2+ in the presence of Ag+. • Many metals are easily oxidized (analyte solutions must be deaerated to remove oxygen.)

Ideal Reference Electrode characteristics

• Reversible • Little hysteresis • Follows Nernst equation • Stable potential with time

Glass membrane composition

• Si is bonded to oxygen and each oxygen is shared by two Si • Sufficient cations in the empty spaces balance the negative charge of the silicate groups • Single charged cations (e.g. Na+, Li+) are mobile in the lattice and are responsible for electrical conduction within the membrane.

Liquid Junction Potential

Ions diffuse across the boundary from the more concentrated to the more dilute solution. Driving force for each ion is proportional to the activity difference between the two solutions (H+ more mobile than Cl-). • The more diluted side of the boundary becomes positively charged because of the more rapid diffusion of the hydrogen ions. • The charge developed tends to counteract the differences in diffusion rates of the two ions so that a steady-state condition is attained rapidly.

Short hand electron configuration

1 line for phase boundary 2 line for salt bridge be sure to include all phases that occur in each cell

Oxidizing Agent

Accepts electrons and becomes reduced.

alternative reference electrodes the SHE

Ag/AgCl electrode Calomel electrode (Hg2Cl2)

Coulometry

Current through an electrochemical cell is held constant, at a specified potential, and the duration of the current is measured and related to concentration. Quantity measured = I and/or Time, Controlled = V, I (when measuring Time)

Easy

Differences in strain on the two surface of the membrane imparted during manufacture, mechanical abrasion and chemical etching of the surface during use.

Reducing Agent

Donates electrons and becomes oxidized.

Ideal Reference Electrode

Has a potential that is accurately known, constant and completely insensitive to the composition of the analyte solution

Voltammetry

Measurement of current through an electrolytic cell as a function of applied potential. Cyclic voltammetry and Amperometry are types of Voltammetry. Quantity measured = I (current), Controlled = V

Potentiometry

Measurement of the potential (voltage) of electrochemical cells (a galvanic cell) at zero current flow. Quantity measured = V, Controlled = I = zero measure cell potential and quantify analyte

potentiometry

Measurement of the potential of electrochemical cells without drawing substantial current. The voltage being proportional to the activity (concentration) of the ion being measured. Examples: pH measurements, ion-selective electrodes, titrations Quantity measured = V, Controlled = I = zero I= current

electrodes of the second kind

Metals not only serve as indicator electrodes for their own cations but also respond to the activity of anions. The anions with which its ion forms a precipitate or a stable complex ion

salt bridge

Permit the movement of ions across them to maintain electroneutrality in each vessel but prevent siphoning of liquid from one electrolyte solution to the other. Isolation is necessary to prevent direct reaction between copper ions and the zinc electrode.

Ion-selective Membrane electrodes types

Non-crystalline membranes and crystalline membranes

Acidic Error of pH Electrodes

Opposite in sign to the alkaline error (pH < 0.5). pH values are higher than the true value. The causes of the acidic error are not well understood.

cell potential (Ecell)

The difference in electric potential between the cathode and the anode

The Composition and Structure of Glass Membranes

The effects of glass composition on the sensitivity of membranes to protons and other cations, and a number of formulations are now used for the manufacture of electrodes.

Open Circuit

When a voltmeter of high internal resistance is connected or the electrodes are not connected externally -> no net reaction occurs. battery not connected

Electroactive species

a molecule can donate or accept electrons at an electrode.

Galvanic (Voltaic) Cells

a spontaneous chemical reaction generate electricity. One reagent must be oxidized and another must be reduced. Galvanic cells produce energy by a spontaneous oxidation reduction reaction. A battery is an example of a galvanic cell. no charging!!

Potentiometry- The voltage being proportional to the

activity (concentration) of the ion being measured. Widely used to locate end points in titrations. • Ion concentrations are measured directly from the potential of ion-selective membrane electrodes.

indicator electrode

an electrode placed in a sample in order to measure the concentration of an ion in the sample Develops a potential that depends on the activity of the analyte. responds to analyte

when Ecell is neg and rxn is not spontaneous

an electrolytic cell is being used

Electrolytic Cells Require

an external source of electrical energy for operation. Potential difference greater than galvanic potential difference to drive away from equilibrium.

Interfacial methods

analyze near electrodensor

Salt Bridge flow

anion will always migrate towards anode, cation will always migrate toward cathode in galvanic cell, salt bridge balances out movement of electrons from anode to cathode in electrolytic cell, anion just is attracted to positively-charged anode without a salt bridge, current immediately stops

electrochemical cells flow from

anode to cathode

electron flow is from

anode to cathode

Two reference electrodes (SCE and Ag/AgCl) for pH meter

are the mean of making electrical contact with the two sides of the glass membrane, and their potentials are essentially constant except for the junction potential (depends to a small extent on the analyte solution). Ecell = Eind - Eref + Ej

the more positive the E0

better electron acceptor more readily reduced/ oxidizing agent

the more negative the E0

better electron donor more readily oxidized/ reducing agent