Presentation on theme: "5. Electrochemistry Depends On chemistry Ion Selective Electrodes"— Presentation transcript:
1 5. Electrochemistry Depends On chemistry Ion Selective Electrodes Anodic Stripping VoltammetryInstrumentSampleSamplePrepInstrumentOut putSignal (Data)Abraham Lincoln was born Feb. 12, 1808.
2 β phase phase---Mobility, u, is greater, in this example, for negative than positive species+++---++++--+Liquid JunctionConcentration gradientβ phase phaseNet charge develops due toDifference in mobilities---+++--++++---+Nov at Gettysburg: “Four score and seven years ago our fathers brought forth
3 Henderson Equation for a liquid junction diffusion potential concentrationmobilityZ, charge on ionJ, Junction potentialAlso develop potential across a membranePotential is due to both a diffusion gradient andto interfacial equilbrium.on this continent a new nation, conceived in Liberty, and dedicated to the proposition
4 Consider a membrane in which only a single ion can move within the membrane Bzb+-+Membraneβ phase-- phase+Aza++--+-+Aza+Interfacial regions where work is required to bring ions toThe surfaceThat all men are created equal. Now we are engaged in a great civil war, testing whether
5 This term represents the work it takes to move an ion of charge a in a potential field at the interfaceThis term representsThe diffusion gradient betweenThe two phases phaseβ phaseVreferenceFix this activityRepresents the work at the interfaceAnd the composition of the inner compartmentthat nation, or any nation, so conceived and so dedicated, can long endure. We are met.
6 Membrane potential equation Often have to worry about the contribution to the potential from competitive ionson a great battle-field of that war. We have come to dedicate a portion of that field, as a
7 ISE EquationDifference in theInterfacial workrequiredfinal resting place for those who here gave their lives that that nation might live. It is
8 Selectivity coefficient, converts the impact Of the activity of the competitorto the corresponding activityOf the analyteDifference in work at the interfaceBetween the two competing ionsaltogether fitting and proper that we should do this.
9 If you have a competing ion, you can convert it’s activity to that of the analyte by The selectivity coefficient and sum it upNikolskii- Eisenman EquationKey PointsActivitiesTemperatureDifference in interfacial work controlsthe selectivity coefficient4. Also means response is SLOWBut, in a larger sense, we can not dedicate—we can not consecrate—we can not
10 Activities concentration Activity coefficient Charge on the ion Ionic strengthhallow—this ground. The brave men, living and dead, who struggled here, have
11 If activity was directly proportional to conc If activity was directly proportional to conc. the ISE response would be linear withConcentration.If there is no ionic strength buffer, increasing the concentration changes the ionicStrength, which changes the activity coefficient, which drives the activity away fromThe concentration, and, therefore, the ISE response is non-linear with concentrationconsecrated it, far above our poor power to add or detract. The world will little note, nor
12 Temperature Effects Suppose you calibrate at 50oF but make A measurement of -140mV at 80oFYou would conclude the concentrationIs log(conc.) = -5.2=6.3x10-6The real log conc. From the80oF calibration curve wouldbeLog(conc) = -4.8 = 1.6x10-5That is an errorOf a factor of 2!50oF80oFlong remember what we say here, but it can never forget what they did here. It is for us
13 Selectivity – impact is concentration dependent Pb2+Manufacturer’s dataLet’s calculate the selectivity coefficient for H+ vs Pb2+ to check the manufacturer’sdataH+the living, rather, to be dedicated here to the unfinished work which they who fought here
14 Electrode is 1/0.0665Or 15 more responsiveTo lead than to protonshave thus far so nobly advanced. It is rather for us to be here dedicated to the great task
15 Equation does not give model lines that Match the manufacturer’s reported dataremaining before us—that from these honored dead we take increased devotion to that
16 Calculate a value for z from the slopes cause for which they gave the last full measure of devotion—that we here highly resolve
17 Using the manufacturer’s data and the equation pH 1pH 2pH 3pH 4Looks like a good match!that these dead shall not have died in vain—that this nation, under God, shall have a
18 2009 experimental Resultsnew birth of freedom—and that government of the people, by the people, for the people,
19 2009 ISE resultsBlank: -235-218-216-223+/-10.44LODsignal = (10.44)=-192LODconc=3(10.44)/25=1.25ppmTotal concentration used:1.13 ppm!!Experimentalconditionsshall not perish from the earth.”
20 Modeled behavior using PbTotal = 6x10-5 ConstantPb = -115 KPbH = 3545 ZPb = 2ZH=1Allowed Pb2+ to changeWith pHData, 2009Both terms contribute! So you have Pb2+ and H+ decreasingWith pH making a complicated plot~0.06~10-6~10-4Mar 4, 1865, 2nd Inaugural address, “Fellow Countrymen:……On the occasion corresponding to this four years ago all thoughts were anxiously directed to an impending civil war. All dreaded it, all sought to avert it. While the
21 Types of Ion Selective Electrodes CrystallinePolymeric MembraneGas sensingField Effect transitorsinaugural address was being delivered from this place, devoted altogether to saving the Union without war, insurgent agents were in the city seeking to destroy it without war—seeking to dissolve the Union and divide
22 Silver moves in membrane down a concentration gradient Charge develops across the membrane leading to a voltage differenceThe interfacial concentration of silver in the bulk solution is controlled by solubilityPb2+S2-Ag+S2-S2-H+S2-SO42-Pb2+S2-S2-Fixed internal Ag+ activityS2-Pb2+Pb2+Ag+Pb2+S2-Ag+Cl-effects by negotiation. Both parties deprecated war, but one of them would make war rather than let the nation survive, and the other would accept war rather than let it perish. And the war came.
23 Similar crystalline commercial electrodes: Analyte ion Conc. Range, pM InterferencesBr-0 to 4.3CN-, I-, S2-Cd2+1 to 7Fe2+, Pb2+, Hg2+, Ag+, Cu2+Cl-0 to 3.3CN-, I-, Br-, S2-, OH-, NH3Cu2+1 to 8Hg2+, Ag+, Cd2+CN-2 to 6S2-, I-F-Sat’d to 6OH-I-0 to 7.3Pb2+1 to 6Hg2+, Ag+, Cu2+Ag+/S2-0 to 7Hg2+SCN-0 to 5.3I-, Br-,CN-, S2-One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was
24 Other common ion selective electrodes? pH electrode Exchange reaction at the surface:somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict
25 Image of Na “hopping” through glass Physical Chemistry Chemical Physics DOI: /b201721kAlkali ion migration mechanisms in silicate glasses probed by molecular dynamics simulationsA. N. Cormack, J. Du and T. R. Zeitler School of Ceramic Engineering and Materials Science, New York State College of Ceramics, Alfred University, 2 Pine St., Alfred, NY 14802, USAReceived 15th February 2002, Accepted 9th May 2002First published on the web 13th June 2002Image of Na “hopping” through glassTo different coordinate sites with oxygenthe territorial enlargement of it. Neither party expected for the war the magnitude or the duration which it has already attained. Neither anticipated that the cause of the conflict might cease with or even before the conflict
26 Types of Ion Selective Electrodes Link to PVC structureTypes of Ion Selective ElectrodesCrystallinePolymeric MembraneGas sensingField Effect transitorsMay have greater selectivityMay have faster response (rise) time phaseβ phaseVreferencePb2+Pb2+PVCPb2+Create a “plastic” membranewith an ionophoreWhose job is to ferry the ion across the membraneitself should cease. Each looked for an easier triumph, and a result less fundamental and astounding. Both read the same Bible and pray to the same God, and each invokes His aid against the other. It may seem strange that
27 From the Critical Reviews any men should dare to ask a just God's assistance in wringing their bread from the sweat of other men's faces, but let us judge not, that we be not judged. The prayers of both could not be answered. That of neither has been
28 Clips from 3 recent articles follow Authors are promoting Sensitivity (slope)Selectivity (selectivity coefficients)Response TimeLinear Rangeanswered fully. The Almighty has His own purposes. 'Woe unto the world because of offenses; for it must needs be that offenses come, but woe to that man by whom the offense cometh.' If we shall suppose that American
29 Response to Pb2+ Response to Pb2+ slavery is one of those offenses which, in the providence of God, must needs come, but which, having continued through His appointed time, He now wills to remove, and that He gives to both North and South this terrible war
30 Next works on right ionophore to plastic composition Linear RangesensitivitySelectivityAs the woe due to those by whom the offense came, shall we discern therein any departure from those divine attributes which the believers in a living God always ascribe to Him? Fondly do we hope, fervently do we pray,
31 Selectivity Response Time that this mighty scourge of war may speedily pass away. Yet, if God wills that it continue until all the wealth piled by the bondsman's two hundred and fifty years of unrequited toil shall be sunk, and until every drop of blood
32 SelectivityLinear RangeResponse TimesensitivityDetection LimitThey haven’t reported thesblank so we cannotConfirm their LODconc.
33 Selectivity – depends upon mobility and interfacial equilibria Key conceptsSelectivity – depends upon mobility and interfacial equilibriatherefore often slow responsemay be complicated by solution chemistryPotential depends upon activity therefore dependent upon ionic strength, IPotential depends temperature!Signal depends on physics of membrane transportWhich ALSO depends upon chemistry of leadInstrumentSampleSamplePrepInstrumentOut putSignal (Data)drawn with the lash shall be paid by another drawn with the sword, as was said three thousand years ago, so still it must be said 'the judgments of the Lord are true and righteous altogether'.
34 5. Electrochemistry Ion Selective Electrodes Anodic Stripping VoltammetryAbraham Lincoln was born Feb. 12, 1808.With malice toward none; with charity for all; with firmness in the right, as God gives us to see the right, let us strive on to finish the work we are in; to bind up the nation's wounds; to care for him who shall have borne the battle, and for his widow, and his orphan – to do all which may achieve and cherish a just and lasting peace, among ourselves, and with all nations.”
35 Electrons “roll” away from negative If the applied potential is more positive than then Zn metalWill oxidize (electrons will flow from Zn metal into an acceptor in soln.)ReactionEo at 25oC, V vs NHESolubility Metal in Hg Wt(%)5.6-0.4445-0.2570.0022.214.171.12480.3400.0080.7991NHE = Normal Hydrogen ElectrodeIf the applied potential is more negative than then silver ionWill be reduced (electrons will flow from electrode to aqueous ion)Mercury is stable as a liquid at potentials less than V vs NHELead, tin, nickel, cadmium, iron, and zinc metals are soluble in liquid mercury
36 Hold voltage negative of 0.268 V vs NHE Reduce Mercury to form a film (small volume container) of thickness, ℓVeHg deposition timeReference electrode (comparison point)Counter Electrode (sacrificial reactions)Working electrode where current is measuredeeHgHg+ℓ
37 Hold voltage between negative of 0.268 V and positive of V vs NHERemove Hg2Cl2 solution and replace with analyte solution; holding potential such that no current flowsVeReference electrode (comparison point)Counter Electrode (sacrificial reactions)Working electrode where current is measured“quiet time” allowsSolution to revert toBulk solution like conditionsHgPb2+ Sn2+
38 Hold voltage between negative For example, -0.8 V vs NHE Step potential negative to reduce metals into the liquid HgVeAnalyte deposition time, tdReference electrode (comparison point)Counter Electrode (sacrificial reactions)Working electrode where current is measuredeeHg
39 Hold voltage between negative For example, -0.8 V vs NHE Step potential negative to reduce metals into the liquid HgVeAnalyte deposition time, tdReference electrode (comparison point)Counter Electrode (sacrificial reactions)Initially we remove all the Pb2+ and Sn2+ near the Hg film ….If we don’t stir then we set up a concentration gradient and deposition will be determined bydiffusion to the surfaceWorking electrode where current is measuredeDeposition currentAnd time of deposition control theAmount depositedeHg
40 Hold voltage between negative For example, -0.8 V vs NHE Step potential negative to reduce metals into the liquid HgVeAnalyte deposition time, tdReference electrode (comparison point)Counter Electrode (sacrificial reactions)Working electrode where current is measuredeHanging mercury drop electrodespherically shapedNotice that we have transferredMoles from a large volume toA small volume = pre-concentration!eHgConcentration in the drop dependsOn deposition time
41 Scan voltage between -0.8 and 0 V vs NHE 4. Scan potential negative to remove metals from the liquid Hg (oxidize back into aqueous phase)VThe rate at which you scan, v, affects the shape and peak current of the oxidationeReference electrode (comparison point)Counter Electrode (sacrificial reactions)Working electrode where current is measuredeMercury Film ElectrodeArea of electrodeHg film thicknesse# e transferredScan rate
42 Deposition time, tdChange to analyte containing solution;Change potential with time to oxidizeSet potential to reduce the metalsScan rate = ν= dV/dtPotential range to deposit Mercury thin liquid filmAND to NOT reduce the analytesStarting Potential; soln contains only salts and Hg2Cl2Do not keepScanning to thesePotentials, why?Set potential to NOT oxidize the deposited Hg
48 Note the selectivity in potential and the discrimination between various metals8 mmol/L ofPb2+, Cu2+, Cd2+ and Zn2+Au particle modifiedAu electrodeblankOrdinary Au electrodeNote scaleChange!!Measures Pb2+ and Cu2+Selective against Cd2+And Zn2+;Increased sensitivity