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5. Electrochemistry Depends On chemistry Ion Selective Electrodes

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1 5. Electrochemistry Depends On chemistry Ion Selective Electrodes
Anodic Stripping Voltammetry Instrument Sample Sample Prep Instrument Out put Signal (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 Junction Concentration gradient β phase  phase Net charge develops due to Difference in mobilities - - - + + + - - + + + + - - - + Nov at Gettysburg: “Four score and seven years ago our fathers brought forth

3 Henderson Equation for a liquid junction diffusion potential
concentration mobility Z, charge on ion J, Junction potential Also develop potential across a membrane Potential is due to both a diffusion gradient and to 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+ - + Membrane β phase - -  phase + Aza+ + - - + - + Aza+ Interfacial regions where work is required to bring ions to The surface That 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 interface This term represents The diffusion gradient between The two phases  phase β phase V reference Fix this activity Represents the work at the interface And the composition of the inner compartment that 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 ions on a great battle-field of that war. We have come to dedicate a portion of that field, as a

7 ISE Equation Difference in the Interfacial work required final 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 competitor to the corresponding activity Of the analyte Difference in work at the interface Between the two competing ions altogether 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 up Nikolskii- Eisenman Equation Key Points Activities Temperature Difference in interfacial work controls the selectivity coefficient 4. Also means response is SLOW But, 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 strength hallow—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 with Concentration. If there is no ionic strength buffer, increasing the concentration changes the ionic Strength, which changes the activity coefficient, which drives the activity away from The concentration, and, therefore, the ISE response is non-linear with concentration consecrated 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 80oF You would conclude the concentration Is log(conc.) = -5.2=6.3x10-6 The real log conc. From the 80oF calibration curve would be Log(conc) = -4.8 = 1.6x10-5 That is an error Of a factor of 2! 50oF 80oF long 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 data Let’s calculate the selectivity coefficient for H+ vs Pb2+ to check the manufacturer’s data H+ the living, rather, to be dedicated here to the unfinished work which they who fought here

14 Electrode is 1/0.0665 Or 15 more responsive To lead than to protons have 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 data remaining 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 1 pH 2 pH 3 pH 4 Looks like a good match! that these dead shall not have died in vain—that this nation, under God, shall have a

18 2009 experimental Results new birth of freedom—and that government of the people, by the people, for the people,

19 2009 ISE results Blank: -235 -218 -216 -223+/-10.44 LODsignal = (10.44)=-192 LODconc=3(10.44)/25=1.25ppm Total concentration used: 1.13 ppm!! Experimental conditions shall not perish from the earth.”

20 Modeled behavior using PbTotal = 6x10-5 ConstantPb = -115 KPbH = 3545
ZPb = 2 ZH=1 Allowed Pb2+ to change With pH Data, 2009 Both terms contribute! So you have Pb2+ and H+ decreasing With pH making a complicated plot ~0.06 ~10-6 ~10-4 Mar 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
Crystalline Polymeric Membrane Gas sensing Field Effect transitors inaugural 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 difference The interfacial concentration of silver in the bulk solution is controlled by solubility Pb2+ S2- Ag+ S2- S2- H+ S2- SO42- Pb2+ S2- S2- Fixed internal Ag+ activity S2- 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
Interferences Br- 0 to 4.3 CN-, I-, S2- Cd2+ 1 to 7 Fe2+, Pb2+, Hg2+, Ag+, Cu2+ Cl- 0 to 3.3 CN-, I-, Br-, S2-, OH-, NH3 Cu2+ 1 to 8 Hg2+, Ag+, Cd2+ CN- 2 to 6 S2-, I- F- Sat’d to 6 OH- I- 0 to 7.3 Pb2+ 1 to 6 Hg2+, Ag+, Cu2+ Ag+/S2- 0 to 7 Hg2+ SCN- 0 to 5.3 I-, 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: /b201721k                   Alkali ion migration mechanisms in silicate glasses probed by molecular dynamics simulations A. 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, USA Received 15th February 2002, Accepted 9th May 2002 First published on the web 13th June 2002 Image of Na “hopping” through glass To different coordinate sites with oxygen the 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 structure Types of Ion Selective Electrodes Crystalline Polymeric Membrane Gas sensing Field Effect transitors May have greater selectivity May have faster response (rise) time  phase β phase V reference Pb2+ Pb2+ PVC Pb2+ Create a “plastic” membrane with an ionophore Whose job is to ferry the ion across the membrane itself 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 Time Linear Range answered 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 Range sensitivity Selectivity As 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 Selectivity Linear Range Response Time sensitivity Detection Limit They haven’t reported the sblank so we cannot Confirm their LODconc.

33 Selectivity – depends upon mobility and interfacial equilibria
Key concepts Selectivity – depends upon mobility and interfacial equilibria therefore often slow response may be complicated by solution chemistry Potential depends upon activity therefore dependent upon ionic strength, I Potential depends temperature! Signal depends on physics of membrane transport Which ALSO depends upon chemistry of lead Instrument Sample Sample Prep Instrument Out put Signal (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 Voltammetry Abraham 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 metal Will oxidize (electrons will flow from Zn metal into an acceptor in soln.) Reaction Eo at 25oC, V vs NHE Solubility Metal in Hg Wt(%) 5.6 -0.444 5 -0.257 0.0021 1.3 1.2 0.268 0.340 0.008 0.7991 NHE = Normal Hydrogen Electrode If the applied potential is more negative than then silver ion Will be reduced (electrons will flow from electrode to aqueous ion) Mercury is stable as a liquid at potentials less than V vs NHE Lead, 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, ℓ V e Hg deposition time Reference electrode (comparison point) Counter Electrode (sacrificial reactions) Working electrode where current is measured e e Hg Hg+

37 Hold voltage between negative of
0.268 V and positive of V vs NHE Remove Hg2Cl2 solution and replace with analyte solution; holding potential such that no current flows V e Reference electrode (comparison point) Counter Electrode (sacrificial reactions) Working electrode where current is measured “quiet time” allows Solution to revert to Bulk solution like conditions Hg Pb2+ Sn2+

38 Hold voltage between negative For example, -0.8 V vs NHE
Step potential negative to reduce metals into the liquid Hg V e Analyte deposition time, td Reference electrode (comparison point) Counter Electrode (sacrificial reactions) Working electrode where current is measured e e Hg

39 Hold voltage between negative For example, -0.8 V vs NHE
Step potential negative to reduce metals into the liquid Hg V e Analyte deposition time, td Reference 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 by diffusion to the surface Working electrode where current is measured e Deposition current And time of deposition control the Amount deposited e Hg

40 Hold voltage between negative For example, -0.8 V vs NHE
Step potential negative to reduce metals into the liquid Hg V e Analyte deposition time, td Reference electrode (comparison point) Counter Electrode (sacrificial reactions) Working electrode where current is measured e Hanging mercury drop electrode spherically shaped Notice that we have transferred Moles from a large volume to A small volume = pre-concentration! e Hg Concentration in the drop depends On 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) V The rate at which you scan, v, affects the shape and peak current of the oxidation e Reference electrode (comparison point) Counter Electrode (sacrificial reactions) Working electrode where current is measured e Mercury Film Electrode Area of electrode Hg film thickness e # e transferred Scan rate

42 Deposition time, td Change to analyte containing solution; Change potential with time to oxidize Set potential to reduce the metals Scan rate = ν= dV/dt Potential range to deposit Mercury thin liquid film AND to NOT reduce the analytes Starting Potential; soln contains only salts and Hg2Cl2 Do not keep Scanning to these Potentials, why? Set potential to NOT oxidize the deposited Hg

43 Raw Data 2001 Student Data FFT data 120 microL Pb Std. addition 800 microL Pb Std addition

44 2001 Student Data Standard series (FFT filtered) Allows you to get a very Nice calibration curve!

45 2001 Student Data Cd2+ Pb2+ Difference in peak potential allows Both metals to be measured simultaneously! Good selectivity!

46 To make this method more acceptable to commercial testing need to
Get rid of the mercury. JChem Ed 2007

47 8 mmol Cu2+/L 8 mmol Pb2+/L blank 8 mmol/L Zn2+ 8 mmol Cd2+/L

48 Note the selectivity in potential and the discrimination between various
metals 8 mmol/L of Pb2+, Cu2+, Cd2+ and Zn2+ Au particle modified Au electrode blank Ordinary Au electrode Note scale Change!! Measures Pb2+ and Cu2+ Selective against Cd2+ And Zn2+; Increased sensitivity


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