air, water and land pollution chapter 11: electrochemical methods for environmental analysis...

Air, Water and Land Pollution

Chapter 11:Electrochemical Methods for

Environmental Analysis

Copyright © 2010 by DBS

Contents

• Introduction to Electrochemical Theories• Potentiometric Applications in Environmental Analysis• Voltammetric Applications in Environmental Analysis

Electrochemical Methods

• Based on measurement of current, potential, and resistance

• Electroanalytical techniques measure:– Gases– Metals– Inorganic non-metallics (pH, anions)– Physical and aggregate properties (alkalinity, conductivity)

• Many of these measurements can be made by wet chemical, spectroscopic or chromatographic techniques

• Electroanalytical methods offer lower cost, faster response and in-situ techniquese.g. Winkler method vs. DO probe

Electrochemical MethodsIntroduction to Electrochemical Theories

Review of Redox Chemistry and Electrochemical Cells

• Oxidation-Reduction (Redox) reactions involve the transfer of electrons (e-)• Results in electric current (amps) and potential (volts) generated in an

electrochemical cell

• OIL RIG – Oxidation Is Loss e-, Reduction Is Gain e-

• Oxidation occurs at the anode (-ve terminal)• Electrons flow from anode to cathode (+ve terminal)



e.g. lead-acid battery used in cars:

Pb(s) + SO42-(aq) PbSO4(s) + 2e-

[OX]

PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- PbSO4(s) + 2H2O(l)

[RED]

Net: Pb(s) + PbO2(s) + 2H2SO4 2PbSO⇌ 4(aq) + 2H2O(l)

• Chemical energy is converted into electrical energy, each cell produces 2 V, six cells will give familiar 12 V battery



What are the oxidation numbers of lead in the net reaction?

Pb(s) + SO42-(aq) PbSO4(s) + 2e-

[OX]

PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- PbSO4(s) + 2H2O(l)

[RED]

Pb(s) + PbO2(s) + 2H2SO4 2PbSO⇌ 4(aq) + 2H2O(l)



e.g the Daniell cell

Zn(s) Zn⇌ 2+(aq) + 2e- (anode)

Cu2+(aq) + 2e- Cu(s)⇌ (cathode)

Cu2+(aq) + Zn(s) Cu(s) + Zn⇌ 2+(aq)

2 half cells:

Zinc electrode in ZnSO4 solution

Copper electrode in CuSO4 solution

Generates potential of 1.1 V



• Circuit must be completed with external metal wire and a salt bridge• e- flow through the metal wire from one electrode to the other (anode to cathode)• Salt bridge allows charge (ions) to transfer through the solutions (completes circuit),

prevents the complete mixing of the solutions, maintains electro-neutrality• Sodium sulfate, sodium chloride, potassium nitrate or similar



• As the oxidation-reduction reaction occurs, the anode becomes more positive as Zn2+ is produced whilst the cathode becomes more negative as Cu2+ is removed from solution

• Salt bridge allows for the migration of ions in both directions to maintain neutrality



Spontaneous(used in potentiometric analysis)

Non-spontaneousUsed in voltammetric analysis

→ →



• Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V)

• emf depends on concentration of REDOX species, described by the Nerst equation:

E = Eo – RT x ln [OX] substituting for R, T, F and

nF [RED] converting to log10 (x 2.303)

E = Eo – 0.059 log [OX]

n [RED]

• Where E = electrode potential (V), E0 = standard potential for the reaction under standard conditions, R = gas constant (8.3145 J/K.mol), T = temperature (K), n = no. moles of charge transferred, F = Faraday constant (96,485 C/mol)



• For the Daniell cell:

E = Eo – 0.059 log ([Zn2+]/ [Cu2+])

E0 can be calculated from tables

e.g the Daniell cell

Zn(s) Zn⇌ 2+(aq) + 2e- E0 = 0.763 V

Cu2+(aq) + 2e- Cu(s)⇌ E0 = 0.337 V

Cu2+(aq) + Zn(s) Cu(s) + Zn⇌ 2+(aq) E0overall = E0

OX + E0RED

= +1.10 V

• +ve E value indicates a spontaneous reaction

Tro, Chemistry: A Molecular Approach

13



• Potential energy between two electrodes is called an electrode potential or electromotive force (emf) measured in volts (V)

E = Eo – 0.059 log [OX]

n [RED]

• What is the emf of a Daniel cell when [OX] = [RED]?

log (1) = 0

E = Eo – 0.059 x log (1) = E0 - 0

2

E = E0 = 1.10 V



• Notation: shorthand description of Voltaic cell

• electrode | electrolyte || electrolyte | electrode

• oxidation half-cell on left, reduction half-cell on the right

• single | = phase barrier– if multiple electrolytes in same phase, a comma is used rather than |

• double line || = salt bridge



• What would be the notation for the following cell?

Zn | Zn2+ || Cu | Cu2+



Daniell Cell Demo


General Principles of Electroanalytical Methods

• Potentiometry• Coulometry• Voltammetry• Conductometry



Potentiometry• Two types:

– Indirect potentiometry - titration– Direct potentiometry

Indirect:• In a potentiometric titration an abrupt change in potential (E) is used for the end-point• Potentiometric titrations can be automated and used where color-changing indicators are

difficult to see

Direct:• Measurement of analyte concentration according to Nernst equation using potential (E)• Potential of indicator electrode w.r.t. a reference electrode

e.g. pH meter



Coulometry• Measures quantity of electricity or the charge generated by a reaction at an electrode• Voltage is applied to drive the nonspontaneous redox reaction (electrolytic cell)• Charge (Q) is related to current (I) according to:

Q = It (constant current)

Q = ∫ I dt (controlled potential)

• Where I = current (A) and t = time (s).



Coulometry

Constant current techniques:

• ‘Colulometric titrations’

• Reagent is generated an one electrode, reacts stoichiometrically with analyte (titrant)

Controlled potential techniques:

• Current resulting from reaction of analyte is monitored w.r.t. time

• Equation relating Q to quantity of analyte (m) is Faraday’s law of electrolysis:

m = Q M

F n

• Where F = Faraday constant (96,485 C/mol e-), M = molec. mass, and n = no. moles e- per mole analyte



Coulometry

Constant current techniques:

Controlled potential techniques:

e.g. 0.5 A was used to deposit Cu2+ in 10 mins in the Daniell Cell, the amount of copper in solution

m = Q M

F n

m = Q M = (0.5 A x 10 mins x 60s/min) x 63.5 g/mol

F n 96,485 C/mol e- x 2 mol e-/mol Cu

m = = 9.87 x 10-2 g C



Voltammetry

• Current (I) is measured as a function of changing potential (E)• Electrolytic cell consisting of 3 electrodes:

– Micro indicator electrode– Reference electrode– Auxillary counter electrode



Voltammetry

• Changing potential (E) is applied on the indicator electrode (working electrode) to drive a nonspontaneous redox reaction

• Counter electrode serves to conduct electricity between the two electrodes• Reference electrode has a constant potential throughout



Voltammetry

• Various voltametric techniques, defined based on potential function applied to the indicator electrode

• General principles:– As potential is applied, electrolysis of analye begins and current rises until it

reaches limiting current– Plot of I vs. E is called a voltammogram– Limiting current = difference between background I and limiting I

– E1/2 = half wave potential (characteristic of every redox reaction)

IL = kC

Where IL = limiting current, k = constant, C = concentration of analyte



Conductometry• Measures conductivity – ability of a solution to carry electric current• Conductivity cell has two electrode plates placed in a solution• Ohm’s law:

R = V/I• Where R = resistance (ohms), V = voltage (volts), I = current (amps)

e.g. Chloride determination using conductivity meter

Mohr method uses potassium chromate as indicator with a difficult to see end-point, conductometric method is faster and more accurate



Conductometry

Conductance (G), ohms-1:

G = 1/R (units ohms-1 or mho or Siemens)

Conductivity (k):

k = GK = GL/A = L/AR (units ohms-1 cm-1, or S cm-1)

• Where K (cm-1) = cell constant (L/A)


Types of Electrodes and Notations for Electrochemical Cells

• Electrodes are important for electrochemical analysis!

– Reference electrodes– Indicator electrodes



• Reference electrodes– Provide constant potential – not affected by sample composition– Follows Nernst equation– Two common reference electrodes:

1. Saturated calomel electrode (SCE)Mercury in contact with a solution saturated with mercury chloride

|| KCl (saturated), Hg2Cl2(s)|Hg(s) (E0 = +0.214 vs. SHE at 25 ºC)

2. Silver wire coated with silver chloride in a saturated KCl solution

|| KCl (saturated), AgCl(s)|Ag(s) (E0 = +0.197 vs. SHE at 25 ºC)

E0 above is calculated based on reference to the standard hydrogen electrode (SHE)



• Indicator electrodes:– Respond to changing analyte concentrations– Two types of indicator electrodes:

1. Metallic electrodes – metal/metal ion, metal/metal salt/anion no longer used

2. Membrane electrodes (AKA ion-selective electrodes)– glass electrodes, polymer membrane electrodes (liquid-phase), crystalline membrane electrodes (solid-state)

ISE has both reference and indicator electrodes

Electrochemical MethodsPotentiometric Applications in Environmental Analysis

• Potentiometric techniques are the most widely used electroanalytical method– Direct potentiometry – pH and ions– Potentiometric titration – titrations


Concentration and Activity

• Activity, (ai): measures the effective concentration of an ion (i) taking into account interactions with ions that may mask it

ai = i [i]

Where = activity coefficient (a function of ionic strength) and [i] = the molar concentration

is a correction factor =1… ai = [i]

for dilute solutions Ca2+ SO42-

H2O

H2ONO3

-

H2O

Surrounding ions balance charge and make central ion less mobile, activity accounts for this canceling out of concentration


pH = -Log10[H+]


Measurement of pH

• Combination electrodes (indicator+reference) for convenience (tube within a tube)

Inner tube: pH indicator electrode (pH sensing membrane, Ag/AgCl reference electrode and HCl

Outer tube: reference electrode (Ag/AgCl) and salt bridge (KCl)


Measurement of pH

• pH sensing component of the indicator electrode is the glass bulb, which is a thin glass membrane ~ 0.03 – 0.1 mm thick

• When immersed, H+ ions from the solution enter the Si-O lattice structure of the glass membrane in exchange for Na+

• Creates an electric ‘boundary’ potential across the membrane w.r.t. internal Ag/AgCl reference electrode

Ecell = constant + RT lnaH+ = constant – 2.303RT pHunknown

F F

• Where aH+ = activity of H+ (= concentration in very dilute solutions)

• Slope factor (2.303RT/F) is temperature dependent, pH meter must be adjusted for changes in temperature


Measurement of pH

• All modern pH meters record potential (mV) and transform the voltage caused by H+ into pH units

• Standard buffers (4.0, 7.0, 10.0) are used for calibration• Automatically recognize standard buffers and adjust for temperature


Measurement of pH

• Exercise: standardize a pH meter using pH units and mV

Ecell = K + 0.0592 pHunknown

Where pH = -Log10[H+]

Ecell = K - 0.0592 Log10H+

Where K = constant


Measurement of pH

• Practical tips:– pH probe should be fully hydrated before use (soak for 24-48hr)– Calibrate frequently– Handle glass (and plastic) electrodes with care– pH reading of dilute or unbuffered solution may be sluggish, solution should be

stirred and allowed to stabilize– Store pH electrode in its wetting cap containing electrode fill solution (3 M KCl) –

do not store in DI water


Measurement of Ions by Ion Selective Electrodes (ISEs)

• Uses direct potentiometry to measure ion concentration – Membrane responds selectively to a given ion– mV reading between sensing and reference electrode

• Samples must be aqueous to avoid damage to membrane– Pros: Much cheaper than IC or AA instruments– Cons: Limited life-span



• Two ISEs commonly used are F- and NO32-

• ISE methods for these ions offer advantages over other methods due to low detection levels and no sample preparation required

Fluoride electrode:• Solid-state membrane, measures to 1 μg/L (ppb)• Single lanthanum trifluoride crystal (mobile fluroide ions within lattice)

Ecell = constant + RT lnaF- = constant – 2.303RT pFunknown

F F

• Where aF- is the activity of F-

• Selective for F- ions, interfered by OH-



• Wiki



• Common problem with ISEs– Interference by other ions– Effect of ionic strengths– Formation of complex compounds not responsive to ISE

• Resolved by adding Ionic –Strength Adjuster (ISA)– e.g. for F- ISA contains acetate buffer, 1 M NaCl and a complexing agent to

remove potential interferences


• ISE electrodes follow the Nernst equation:

Ecell = E0 + 2.303 RT log [M] nF

Where E = voltage (V), E0 = constant, 59.2 = slope, n = charge, and [M] = concentration of ion

2.303 x RT / F = 0.0592

Where R = 8.316 J mol-1 K-1, T = temperature (K) and F = 96,487 C /mol e- E

log[M]

Question

Write down the Ecell expression for a fluoride ISE in mV

Ecell = E0 + 0.0592 log [F-] -1

1 V = 1000 mV,

For Ecell (mV)

Ecell = K - 59.2 log [F-]

Question

Write down the Ecell expression for a Ca2+ ISE in mV, how does it differ from the F- electrode?

Ecell = E0 + 0.0592 log [Ca2+] +2

1 V = 1000 mV,

For Ecell (mV)

Ecell = K + 29.6 log [Ca2+]

Ca2+ has +ve slope vs. –ve for F-

Question

Write down the Ecell expression for a Ca2+ ISE in mV, how does it differ from the F-

electrode?

Actual slope varies depending on electrode used


Hard water contains high concentrations of dissolved calcium and magnesium ions. Soft water contains few of these dissolved ions.

Hardness = [Ca2+] + [Mg2+]

A pipe with hard-water scale build up


Potentiometric Titration (Indirect Potentiometry)

• Since most (2/3) hard-water ions originate from calcium carbonate, levels of water hardness are often referred to in terms of hardness as CaCO3.

Note: also total hardness



eg. Ca2+ concentration of 30 mg/L, then its calcium hardness as CaCO3 can be calculated using the formula

(30 mg/L Ca2+) x (100 g CaCO3 / 40 g Ca2+)

= 75 mg/L (ppm) calcium hardness as CaCO3

(multiply Ca2+ by 2.5 to find equivalent CaCO3 hardness)

Exercise: measure hardness of water



• Volumetric method - potential, E (mV) between two electrodes is measured as a function of titrant, V (mL)

• Typical E vs V plot is S-shaped• Part of curve with maximum change marks equivalent

point of titration (dE/dV vs. V is asymptotic at this point)

• Used for:

redox, acid-base, precipitation and complexation titrations• Potentiometric titration can be automated


Conductometric Titration (Indirect Conductometry)

• Volumetric method - conductance, k (µS cm-1) between two electrodes is measured as a function of titrant, V (mL)

• Typical k vs V plot is V-shaped• Minimum marks equivalent point of titration

• Used for: Chloride determination

Electrochemical MethodsVoltammetric Applications in Environmental Analysis

• Environmental lab use of voltammetry:– Dissolved Oxygen (DO)– Chlorine– Metal species


Measurement of Dissolved Oxygen

• Two types:

(i) Galvanic – DO probe is Ag or Pb with KOH electrolyte (no external potential required)

(ii) Polarographic – DO probe has a silver anode surrounded by an gold cathode (requires external potential)

• Similar operational principles– (i) Dissolved oxygen diffuses across a permeable membrane and is reduced in a

reaction with a fill solution generating a current flow from cathode to anode

– (ii) Current flow is proportional to the amount of O2 crossing the membrane



• e.g. Galvanic DO cell

O2 + 2H2O + 4e- 4OH⇌ - (O2 reduced at silver cathode)

2Pb(s) → 2Pb2+(aq) + 4e- (oxidation of lead at anode)

• Flow of e- from anode to cathode is proportional to O2 concentration passing through the membrane

• The electrode requires a constant current of water across the surface since O2 is consumed



• e.g. Polarographic Clark cell



e.g. Polarographic Clark cell

O2 + 2H2O + 4e- 4OH⇌ - (O2 reduced at gold cathode)

4Ag(s) + 4Cl-(aq) 4AgCl(s) + 4e⇌ - (oxidation of silver at anode)

• Membrane is susceptible to degradation, must be replaced if it dries out

• Calibrated in air (O2), air saturated water (aerated water) or by Winkler method



• Calibrate the probe (in air)• Place the probe below the surface of the water• Set the meter to measure temperature and allow the

temperature reading to stabilize• Switch the meter to 'dissolved oxygen‘• For saline waters, measure electrical conductivity level

or use correction feature• Re-test water to obtain a field replicate result

NOTE: The probe needs to be gently stirred to aid water movement across the membrane


Measurement of Anions by Amperometric Titration

• Free chlorine: chlorine gas (Cl2)

• Residual chlorine: hypochlorous acid (HOCl), and/or hypochlorite ion (OCl-). The three forms of free chlorine exist together in equilibrium.

Cl2 + H2O HOCl + H⇌ + + Cl- ⇌ HOCl H⇌ + + OCl-

• Free and residual chlorine = Cl2 + HOCl + OCl-



• Free and residual chlorine (here represented by Cl2) can be titrated with sodium thiosulfate or phenylarsine oxide (PhAsO)

Cl2 + PhAsO + 2H2O → 2Cl- + PhAsO(OH)2 + 2H+

• Voltage of 0.10 V applied across two Pt electrodes generates a small current in solution

Reduced form (Cl-) is oxidized at anode: 2Cl- Cl⇌ 2+2e-

Oxidized form (Cl2) is reduced at cathode: Cl2+2e- 2Cl⇌ -

• Addition of reducing agent (PhAsO) irreversibly reduces the oxidized form (Cl2) terminating the reversible reaction (current drops to zero)



• Output: abrupt change in current is end-point• Concentration of free and residual chlorine calculated by determining amount of

titrant added to reach 0 amps


Measurement of Metals by Anodic Stripping Voltammetry (ASV)

• Anodic stripping voltammetry - Cations are stripped from anodes • Cathodic strippping voltammetry - Anions are stripped from cathodes

Common elements:



• ASV important for environmental work – Very low levels of detection (ppb) due to preconcentration step– can analyze multiple metals in a single run

• Assume elements of interest are cadmium (Cd) and copper (Cu)



• Step 1: pre-concentrate metals from solution onto electrode via electrodeposition at constant potential (reduction)

Electrode acts as cathode where metal ions are reduced to metallic form

Cd2+ + 2e- Cd(s)⇌ (E0 = -0.403 V)

Cu2+ + 2e- Cu(s)⇌ (E0 = +0.337 V)

• Step 2: Apply a changing potential to re-oxidize or ‘strip’ the metals from electrode. Current is measured during stripping.

Electrode reverses to act as anode where Cd / Cu are oxidized to ionic forms

Cd(s) Cd⇌ 2+ + 2e- (E0 = +0.403 V)

Cu(s) ⇌ Cu2+ + 2e- (E0 = -0.337 V)



• Potential must be low enough for both cations to reduce

• After deposition potential is increased linearly

• Cd starts to oxidize at -0.6 V causing increase in current

• As deposited Cd is consumed, current drops

• As potential increases the Cu peak appears



• Electrodes for ASV different to those described previously• Mercury provides good surface for REDOX reactions of metals• Hanging Mercury Drop Electrode (HMDE) and Mercury Film Electrode (MFE)

References

• Christian, G.D. (2003) Analytical Chemistry, 6th Edition, John Wiley & Sons, Hoboken, NJ.

• Hach Company (1998) Amperometric Titrator, Model 19300, Loveland, CO, pp. 1-84.

• Hanrahan, G., Patil, D.G., and Wang, J. (2004) Electrochemical sensors for environmental monitoring: Design, development and applications. Journal of Environmental Monitoring, Vol. 6, No. 8, pp. 657-664.

• Kellner, R., Mermet, J-M., Otto, M., Valcarcel, M., and Widmer, H.M. (2004) Analytical Chemistry: A Modern Approach to Analytical Science, 2nd Edition, Wiley-VCH Weinhein, Chapter 18, pp. 455-499.

• Kissinger, P.T., Heineman, W.R. (1996) Laboratory Techniques in Electroanalytical Chemistry, 2nd Edition, Dekker, New York.

• US EPA (1996) SW-846, Method 9210A, Potentiometric determination of nitrate in aqueous samples with an ion-selective electrode.YSI Incorporated (1999) YSI Model 5239, Dissolved Oxygen Instruction Manual, Yellow Springs, OH, pp. 1-23.

Questions

1. Explain why electrochemical methods measure the activity rather than concentration?

11. Give a list of common ions of environmental significance that can be analyzed by ion selective electrodes.

12. The potential (E1) in a 0.0015 mol/L F- standard was measured to be 0.150 V by a fluoride electrode in reference to a standard calomel electrode. For the same electrodes, a voltage of 0.250 (E2) was measured in a sample containing an unknown concentration of F-. Calculate the concentration of F- in the unknown sample.

14. Use a table to compare the similarities and differences between potentiometry, coulometry, and voltammetry. Consider the following comparisons: (a) The electrical measurement (e.g., current, potential, and charge), (b) The types of cells (galvanic and electrolytic), (c) The fundamental equation employed for quantitative measurement (Nernst, Faraday, and Ohm’s), and (d) The ability for qualitative determination.

16. Two major advantages of anodic stripping voltammetry (ASV) for metal analysis are (a) low detection limits (µg/L to ng/L), and (b) ability to analyze several elements in a sample run. Explain why?

air, water and land pollution chapter 11: electrochemical methods for environmental analysis...

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