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5.Electrochemistry Ion Selective Electrodes Anodic Stripping Voltammetry Sample Prep Instrument Out put Signal (Data) Depends On chemistry Abraham Lincoln.

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Presentation on theme: "5.Electrochemistry Ion Selective Electrodes Anodic Stripping Voltammetry Sample Prep Instrument Out put Signal (Data) Depends On chemistry Abraham Lincoln."— Presentation transcript:

1 5.Electrochemistry Ion Selective Electrodes Anodic Stripping Voltammetry Sample Prep Instrument Out put Signal (Data) Depends On chemistry Abraham Lincoln was born Feb. 12, 1808.

2 Liquid Junction + + - - + + - - + + - - + + - - Concentration gradient  phase β phase Mobility, u, is greater, in this example, for negative than positive species + + - - + + - - + + - - + + - -  phase β phase Net charge develops due to Difference in mobilities Nov 18 1863 at Gettysburg: “Four score and seven years ago our fathers brought forth

3 J, Junction potential Z, charge on ion Henderson Equation for a liquid junction diffusion potential concentration Also develop potential across a membrane Potential is due to both a diffusion gradient and to interfacial equilbrium. mobility 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 + + - - A za+ - + + - -  phase β phase B zb+ - - + + - Membrane B zb+ A za+ 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  phase β phase V reference Fix this activity 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 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 Often have to worry about the contribution to the potential from competitive ions Membrane potential equation 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 Difference in work at the interface Between the two competing ions Selectivity coefficient, converts the impact Of the activity of the competitor to the corresponding activity Of the analyte altogether fitting and proper that we should do this.

9 Nikolskii- Eisenman Equation 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 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 Activity coefficient concentration Ionic strength Charge on the ion Activities hallow—this ground. The brave men, living and dead, who struggled here, have

11 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 If activity was directly proportional to conc. the ISE response would be linear with Concentration. consecrated it, far above our poor power to add or detract. The world will little note, nor

12 Temperature Effects 80 o F 50 o F Suppose you calibrate at 50 o F but make A measurement of -140mV at 80 o F You would conclude the concentration Is log(conc.) = -5.2=6.3x10 -6 The real log conc. From the 80 o F calibration curve would be Log(conc) = -4.8 = 1.6x10 -5 That is an error Of a factor of 2! 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 Pb 2+ H+H+ Let’s calculate the selectivity coefficient for H + vs Pb 2+ to check the manufacturer’s data Manufacturer’s data 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 resultsBlank: -235 -218 -216 -223+/-10.44 LOD signal = -223+3(10.44)=-192 LOD conc =3(10.44)/25=1.25ppm Total concentration used: 1.13 ppm!! Experimental conditions shall not perish from the earth.”

20 Modeled behavior using Pb Total = 6x10 -5 Constant Pb = -115 K PbH = 3545 Z Pb = 2 Z H =1 Allowed Pb 2+ to change With pH Data, 2009 ~0.06 ~10 -6 ~10 -4 Both terms contribute! So you have Pb 2+ and H + decreasing With pH making a complicated plot Mar 4, 1865, 2 nd 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 1.Crystalline 2.Polymeric Membrane 3.Gas sensing 4.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 Pb 2+ S 2- Ag + Pb 2+ S 2- Pb 2+ S 2- Ag + S 2- Pb 2+ Ag + S 2- H+H+ Cl - SO 4 2- Fixed internal Ag + activity V 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 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 ionConc. Range, pMInterferences Br - 0 to 4.3CN -, I -, S 2- Cd 2+ 1 to 7Fe 2+, Pb 2+, Hg 2+, Ag +, Cu 2+ Cl - 0 to 3.3CN -, I -, Br -, S 2-, OH -, NH 3 Cu 2+ 1 to 8Hg 2+, Ag +, Cd 2+ CN - 2 to 6S 2-, I - F-F- Sat’d to 6OH - I-I- 0 to 7.3CN - Pb 2+ 1 to 6Hg 2+, Ag +, Cu 2+ Ag + / S2- 0 to 7Hg 2+ SCN - 0 to 5.3I -, Br -,CN -, S 2- 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 http://grandinetti.org/Research/Applications/GlassStudies/assets/GlassStructure.gif Exchange reaction at the surface: Other common ion selective electrodes? pH electrode 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 http://www.rsc.org/ej/CP/2002/b201721k/ Image of Na “hopping” through glass To different coordinate sites with oxygen Physical Chemistry Chemical Physics DOI: 10.1039/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 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 1.Crystalline 2.Polymeric Membrane 3.Gas sensing 4.Field Effect transitors May have greater selectivity May have faster response (rise) time  phase β phase V reference Create a “plastic” membrane with an ionophore Whose job is to ferry the ion across the membrane Pb 2+ PVC www.3dchem.com/molecules.asp?ID=327 Link to PVC structure 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 a.Sensitivity (slope) b.Selectivity (selectivity coefficients) c.Response Time d.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 Pb 2+ 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 sensitivity Linear Range 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 Response Time Selectivity Linear Range sensitivity Detection Limit They haven’t reported the s blank so we cannot Confirm their LOD conc.

33 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! Sample Prep Instrument Out put Signal (Data) Signal depends on physics of membrane transport Which ALSO depends upon chemistry of lead 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 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.” Abraham Lincoln was born Feb. 12, 1808.

35 ReactionE o at 25 o C, V vs NHESolubility Metal in Hg Wt(%) -0.76265.6 -0.444 -0.40255 -0.2570.0021 -0.13751.3 -0.12051.2 0 0.268 0.3400.008 0.7991 1.Mercury is stable as a liquid at potentials less than 0.268 V vs NHE 2.Lead, tin, nickel, cadmium, iron, and zinc metals are soluble in liquid mercury Electrons “roll” away from negative If the applied potential is more positive than -0.76 then Zn metal Will oxidize (electrons will flow from Zn metal into an acceptor in soln.) If the applied potential is more negative than 0.799 then silver ion Will be reduced (electrons will flow from electrode to aqueous ion) NHE = Normal Hydrogen Electrode

36 1.Reduce Mercury to form a film (small volume container) of thickness, ℓ Hg Hold voltage negative of 0.268 V vs NHE Working electrode where current is measured Reference electrode (comparison point) e Counter Electrode (sacrificial reactions) e e V Hg deposition time ℓ Hg +

37 2.Remove Hg 2 Cl 2 solution and replace with analyte solution; holding potential such that no current flows Hg Hold voltage between negative of 0.268 V and positive of -0.1205 V vs NHE Working electrode where current is measured Reference electrode (comparison point) Counter Electrode (sacrificial reactions) e V Pb 2+ Sn 2+ “quiet time” allows Solution to revert to Bulk solution like conditions

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

39 3.Step potential negative to reduce metals into the liquid Hg Hg Hold voltage between negative For example, -0.8 V vs NHE Working electrode where current is measured Reference electrode (comparison point) Counter Electrode (sacrificial reactions) e V Analyte deposition time, t d e e Deposition current And time of deposition control the Amount deposited Initially we remove all the Pb 2+ and Sn 2+ 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

40 3.Step potential negative to reduce metals into the liquid Hg Hg Hold voltage between negative For example, -0.8 V vs NHE Working electrode where current is measured Reference electrode (comparison point) Counter Electrode (sacrificial reactions) e V Analyte deposition time, t d e e Hanging mercury drop electrode spherically shaped Concentration in the drop depends On deposition time Notice that we have transferred Moles from a large volume to A small volume = pre-concentration!

41 4. Scan potential negative to remove metals from the liquid Hg (oxidize back into aqueous phase) Scan voltage between -0.8 and 0 V vs NHE Working electrode where current is measured Reference electrode (comparison point) Counter Electrode (sacrificial reactions) e V e e The rate at which you scan, v, affects the shape and peak current of the oxidation Scan rate # e transferred Mercury Film Electrode Area of electrode Hg film thickness

42 Starting Potential; soln contains only salts and Hg 2 Cl 2 Potential range to deposit Mercury thin liquid film Change to analyte containing solution; Set potential to NOT oxidize the deposited Hg AND to NOT reduce the analytes Set potential to reduce the metals Change potential with time to oxidize Deposition time, t d Scan rate = ν= dV/dt Do not keep Scanning to these Potentials, why?

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

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

45 Pb 2+ Cd 2+ Difference in peak potential allows Both metals to be measured simultaneously! Good selectivity! 2001 Student Data

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

47 8 mmol Cu 2+ /L 8 mmol Cd 2+ /L blank 8 mmol Pb 2+ /L 8 mmol/L Zn 2+

48 Note the selectivity in potential and the discrimination between various metals Au particle modified Au electrode Ordinary Au electrode Note scale Change!! blank 8 mmol/L of Pb 2+, Cu 2+, Cd 2+ and Zn 2+ Measures Pb 2+ and Cu 2+ Selective against Cd 2+ And Zn 2+ ; Increased sensitivity


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