CHAPTER 14: ELECTRODES AND POTENTIOMETRY

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1 CHAPTER 14: ELECTRODES AND POTENTIOMETRY

4 Chapter 14 Electrodes and Potentiometry Potentiometry : The use of electrodes to measure voltages that provide chemical information. (The cell voltage tells us the activity of one unknown species if the activities of the other species are known). I. Reference Electrode : It maintains a fixed potential (a known, fixed composition) II. Indicator Electrode : It responds to analyte, an electroactive species How to use potentiometry : that can donate or accept electron at the electrode) 1) We connect two half-cells (indicator & reference electrodes) by a salt bridge. 2) We measure the cell voltage which is the difference between the potential of the indicator electrode and the constant potential of the reference electrode. 3) We make an equation of first degree to get the unknown concentration of analyte.

5 2 [Fe How 3 [Fe Reference Electrodes to measure in Fig.15-1? ] ]

6 14-1. Reference Electrodes Figure 14-2 Another view of Fig 15-1 Figure Another view of Fig The dashed box = reference electrode.

7 I. Common Reference Electrodes 1) Silver-Silver chloride Electrode AgCl(s) + e - Ag(s) + Cl - E o = From Nernst Equation E = E o log [Cl - ] (0.197V) (0.222V) E (saturated KCl) at 25 C = V (E = V when A Cl = 1.0)

8 I. Reference Electrodes The two half reactions : Right electrode: Fe 3+ + e - Fe 2+ E+ 0 = 0.771V Left electrode: AgCl(s) + e - Ag(s) + Cl - E- o = 0.222V E (Cell Voltage) = E + - E - [Fe log [Fe 2 o ( E 3 [Fe log [Fe ( 3 ] ) ( E ] 2 o log[Cl ]) ] ) ( log[Cl ] 2 - [Fe ] log[Cl ] log 3 [Fe ] i) [Cl - ] = const., because of saturated KCl solution. ii) As the value of [Fe2+]/[Fe3+] changes, the cell voltage (E) changes. ])

I. Common Reference Electrodes 2) Calomel

S.C.E) 1/2 Hg 2 Cl 2 (s) + e - Hg(l) + Cl

9 I. Common Reference Electrodes 2) Calomel Electrode (Saturated Calomel electrode = S.C.E) 1/2 Hg 2 Cl 2 (s) + e - Hg(l) + Cl - Mercury (I)Chloride (Calomel) E o = V (A Cl = 1) E E o log [Cl - ] (+0.241V at 25 o C) (0.268V) *Saturated KCl solution make the conc. of Cl - does not change if some of liquid evaporates.

10 Voltage conversions between different reference scales

11 14-2. Indicator Electrodes Indicator Electrodes 1) Metal Electrode i) It responds to a redox reaction at the metal surface. ii) It does not participate in many chemical reactions (Inert). iii) Simply transmit e - to or from a reactive species in solution. iv) It works best when its surface is large and clean (A brief dip in conc. HNO 3,followed by rinsing with distilled water is effective for cleaning). v) Au, Pt, Ag, Cu, Zn, Cd and Hg can be used as indicator electrode (The reaction of M M n+ + ne - should be fast)

12 14-2. Indicator Electrodes 2) Ion Selective Electrode i) It is not based on redox processes ii) Selective migration of one type of ion across a membrane generates an electric potential.

13 14-3. What Is a Junction Potential? Junction Potential : Any time two dissimilar electrolyte solution are placed in contact, an electric voltage ( Junction Potential) develops at their interface. -There is an electric potential difference at the junction of NaCl and H 2 O phases.

15 - In Table 14-2, a high concentration of KCl in one solution reduces the magnitude of the potential. - This is why saturated KCl is used in salt bridges.

16 14-3. What Is a Junction Potential? - There is an electric potential difference at the junction of NaCl and H 2 O phases. - Steady-state junction potential : A balance between the unequal mobilities that create a charge imbalance and the tendency of the resulting charge imbalance to retard the movement of Cl -. - Junction Potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we don t know the contribution of the junction to the measured voltage: E(measured) = E(cell) + E(junction)

17 14-3. What Is a Junction Potential? - Junction Potential exists at the interface between the salt bridge and each half-cell. - The junction potentials at each end of a salt bridge often partially cancel each other. - Although a salt bridge necessarily contributes some unknown net potential to a galvanic cell, the contribution is fairly small (a few mv).

18 14-4. How Ion-Selective Electrode Work? Ion Selective Electrode i) It is not based on redox processes ii) It responds selectively l to one ion. iii) Key feature of an ideal ion selective electrode: - A thin membrane ideally capable of binding only the intended ion. - Selective migration of one type of ion across a membrane generate an electric potential.

19 14-4. How Ion-Selective Electrode Work? For example, Liquid-based ion-selective electrode - ion-selective electrode : a hydrophobic organic polymer impregnated with a viscous organic solution containing an ionophore (L, ligand) which h selectively l binds the analyte cation (C + ). - Inside of the electrode : filling solution with C + (aq) and B - (aq) - Outside of the electrode : it is immersed in analyte solution with C + (aq) and A - (aq). -Two reference electrodes: to measure the electric potential difference (voltage) across the membrane depending on [C + ] in analyte.

20 Inside the membrane: Membrane : polymer (polyvinyl chloride) impregnated with a nonpolar liquid that dissolves L, LC +, and R - (hydrophobic anion) L : Ionophore. it has high affinity for C+. It binds only C+(ideal electrode).howeve it has some affinity for other cations (Real). L is soluble in the organic phase. R- : hydrophobic anion for charge neutrality. R- cannot leave membrane because it is soluble in membrane, but poorly soluble in water.

21 Bold colored dions; excess charge in each phase A - : it cannot enter the membrane because it s not soluble in the organic phase. C + : it diffuses from the membrane to the aqueous solution because of the favorable solvation of the ion by water. In the membrane, LC + is in equilibrium with L + C + (small amount).

22 - As soon as a few C + ions diffuse from the membrane into the aqueous solution, there is excess positive charge in the aqueous phase. -This imbalance creates an electric potential difference that opposes diffusion of more C + into the aqueous phase.

23 First, Let s see what happens when C + diffuses from membrane to outer solution -When C + diffuses from membrane (activity in membrane, A m ) to the outer aqueous solution (A 0 0), the free energy change is G G solvation RT G due to change in solvent ln A m A o G due to change in activityit (concentration) - G is always negative when a species diffuses from a region of high activity to one of lower activity. - When C + diffuses from membrane to the outer aqueous solution, there is a buildup of a positive charge in the water immediately adjacent to membrane. - The charge separation creates an electric potential ( E outer ) across the membrane. - The free energy difference for C + in the two phases is G = -nfe outer.

24 First, Let s see what happens when C + diffuses from membrane to outer solution - At equilibrium, the net change G from all processes with diffusion of C + across the membrane must be 0. G solvation RT ln A m A o ( nfe outer ) 0 G due to transfer between phase and activity difference G due to charge imbalance Electric potential G solvation RT difference across phase E outer nf boundary between nf membrane and analyte (outer solution): -E outer is proportional to the concentration of C+ in outer analyte solution, because Am is nearly constant. ln A m A o A o

25 Why does is constant during the process? A m The reason why A m is very nearly constant : i) C + + L LC + In this equilibrium in the membrane, C + is very small, but LC + is high concentration. ii) R - ( hydrophobic) is poorly soluble in water and thus cannot leave the membrane. Very little C + can diffuse out of the membrane because each C + that enters the aqueous phase leaves behind one R -. iii) As soon as a tiny fraction of C + diffuse from the membrane into solution, further diffusion is prevented by excess positive charge in the solution near the membrane.

26 Second, Let s see what happens when C + diffuses from membrane to inner filling solution - At equilibrium, the net change G from all processes with diffusion of C + across the membrane must be 0. G solvation RT ln A m A i ( nfe inner ) 0 G due to transfer between phase and activity difference G due to charge imbalance Electric potential difference across phase E inner boundary between membrane and inner filling solution G nf solvation RT nf ln A m A i - E inner is constant because A i and A m are constant.

27 14-4. How Ion-Selective Electrode Work? - The potential difference between the outer and inner solution : E E E G RT solvation A E ln E F F m outer inner nf nf A o G RT nf solvation lna - lna o m nf RT nf E inner inner Constant Constant Constant - Combing the constant terms, Electric potential difference for ionselective electrode: E E RT constant ln nf constant n A o log A o (volts at 25 C)

28 14-4. How Ion-Selective Electrode Work? Electric potential difference for ion- selective electrode: E E constant RT ln A o nf constant log n A o (volts at 25 C) Remarks: i) This equation applies to any ion-selective electrode (including a glass ph electrode). ii) If the analyte is anion, the sign n (charge of analyte ) is negative. iii) A difference of 4.00 ph units would lead to a potential difference of 4.00 x = 237 mv. iv) For every factor - of - 10 change in activity of Ca +2 would lead to a potential difference of 59.16/2 = mv.

29 Ion Selective Electrodes Nitrate Chloride

30 Ion Selective Electrodes

31 Ion Selective Electrodes

32 14-6. Ion-Selective Electrodes *Doctor needs blood chemistry information quickly to help a critically ill patient make a diagnosis and begin treatment. Ion-selective electrodes are the method of Ion-selective electrodes are the method of choice for Na +, K +, Cl -, ph, and P co2.

33 14-6. Ion-Selective Electrodes Selectivity Coefficient : the relative response of the electrodes to different species with same charge. * No electrode responds exclusively to one kind of ion. Selectivity coefficient: (14-9) A: analyte ion, X: interfering ion Pot: potentiometric

34 14-6. Ion-Selective Electrodes Response of ion-selective electrode: (14-10) A A k A, X : the activity it of primary ion, A : the activity it of interfering i species, A X : the selectivity coefficient, + : A is a cation, - : A is an anion Z A : the magnitude of charge A Reminder : How Ion-Selective Electrode Work - Changes in A A in the solution change the potential difference, E across the outer boundery of the ion selective electrode. - By using the calibration curve, E is related to - An ion-selective electrode responds to the activity of free analyte, not complexed analyte. A A

35 14-6. Ion-Selective Electrodes Classification of Ion-selective electrodes : 1) Glass membranes for H + and certain monovalent cations. 2) Solid-state electrodes 3) Liquid-based electrodes 4) Compound electrodes

36 2) Solid State Electrodes

37 2) Solid State Electrodes Site to site transportation t ti of F inside id the crystal. Filling solution ; 0.1 M NaF and 0.1 M NaCl

38 2) Solid State Electrodes Response of F - electrode: (14-12) - The electrode is more responsive to F - than to most other ions by > 1,000. -OH - is the only interfering species ( k F -, OH - = 0.1 ) - At low ph, F - is converted to HF (pka = 3.17), to which the electrode is insensitive. -At ph is 5.5, no interference by OH- and little conversion of F- to HF. - Citrate complexes Fe 3+ and Al 3+, which would otherwise bind F- p, and interfere with the analyte, F -.

39 2) Solid State Electrodes Response of F - electrode: E = constant - ( ) log A AF- F (outside) (14-12) 12) - A routine procedure is to dilute the unknown in a high ionic strength buffer containing acetic acid, sodium citrate, NaCl, and NaOH to adjust the ph to The buffer keeps all standards d and unknowns at a constant t ionic i strength, th so the fluoride activity coefficient is constant in all solutions (and can therefore be ignored). E = constant - ( ) log [F - ] F- = constant - ( ) log F- - ( ) log [F - ] This expression is constant because p F- is constant at constant ionic strength

40 2) Solid State Electrodes

41 2) Solid State Electrodes

42 3) Liquid Based Ion Selective Electrodes -A liquid-based ion-selective electrode is similar to the solid-state electrode (Fig ), 19) except that the liquid-based electrode has a hydrophobic membrane impregnated with a hydrophobic ion exchanger (ionophore) that is selective for analyte ion (Fig ), (Fig ) 19) (Fig )

43 3) Liquid Based Ion Selective Electrodes L; Ionophore (neutral) R - ; anion for charge neutralization

44 3) Liquid Based Ion Selective Electrodes - A liquid-based ion-selective electrode is similar to the solid-state electrode Response of Ca 2+ electrode: (14-13) 13) β is close to 1.00

45 Breakthrough in Ion-Selective Electrode Detection Limits Black curve: the electrode detects changes in the Pb +2 concentration above 10-6 M, but not below 10-6 M when PbCl 2 = 0.5 mm in the internal solution. Blue Curve: the elctrode responds to change in Pb +2 concentration down to ~10-11 M. when PbCl 2 =10-12 M in the internal solution by a metal ion buffer. go to 14-7 metal ion buffers

46 Breakthrough in Ion-Selective Electrode Detection Limits - The sensitivity of liquid-based ion selective electrodes has been limited by the leakage of fthe primary ion (Pb +2 ) from the internal filling solution through h the ion selective membrane. - Leakage provides a substantial concentration of primary ion at the external of the membrane (10-6 M) and thus the electrode always responds to 10-6 M although the real analyte concentration is far below 10-6 M.

4) Compound Electrodes - Compounds electrodes contain a conventional electrode surrounded by a membrane that isolates (or generates) the analyte to which h the electrode responds.

47 4) Compound Electrodes - Compounds electrodes contain a conventional electrode surrounded by a membrane that isolates (or generates) the analyte to which h the electrode responds. -CO 2 diffuses through the semi-permeable rubber membrane, it lowers the ph in the electrolyte. -The response of the glass ph electrode to the change in ph is a measure of the CO 2 concentration outside the electrode. 2

48 14-7. Using Ion Selective Electrodes Advantages of ion-selective electrodes: 1) Linear response to log over a wide range (4 to 6 orders of magnitude). A 2) Nondestructive ( they don t consume unknown ). 3) Noncontaminating (they introduce negligible contamination.) * They can be used inside living cells. 4) Short response time (seconds or minutes) 5) Unaffected by color or turbidity (unlike spectrophotometry and titrations).

49 14-7. Using Ion Selective Electrodes Disadvantages of ion-selective electrodes: 1) They respond to the activity of analytes. *we usually want concentrations, not activities 2) They respond to the activity of uncomplexed ions (advantage or disadvantage?) 3) Precision is rarely better than 1%. *I mv error 4% error in monovalent ion activity. it It doubles for divalent ions and tripled for trivalent ions. 4) They can be fouled by organic solutes like proteins which leads to sluggish, drifting response. 5) They are fragile and have limited shelf-life.

50 2) They respond to the activity of uncomplexed ions (advantage or disadvantage?)

51 14-7. Using Ion Selective Electrodes Disadvantages of ion-selective electrodes: 1) They respond to the activity of analytes. *we usually want concentrations, not activities. How to solve this problem: An inert salt is added to all the standards and samples to bring them to constant and high ionic strength. If the activity coefficients remain constant, the electrode potential gives concentrations directly because we know the concentrations of standards.

52 Standard Addition with Ion - Selective Electrodes Matrix: the medium in which the analyte exists. Standard Addition Method: - It is used when the matrix of unknown sample is different from that of standard. - The electrode immersed in the unknown and then a small volume of standard solution is added intermittently until the concentration of the analyte increases to 1.5 to 3 times its original concentration. - The graphical procedure based on the equation for the response of the ion-selective electrode. RT ln 10 E k β log[ X] (15-9) nf V at 25 0 C E : meter reading in volts [X] : concentration of analyte k, β : constants depending on particular ion-selective electrode

53 15-7. Standard Addition with Ion - Selective Electrodes RT ln 10 E k β log[ X] (14-14) 14) nf Let V 0 : the initial volume of unknown C x : the initial concentration of analyte V s : the volume of added standard C s : the concentration of standard Then the concentration of analyte after standard is added: (V 0 C x +V s C s )/(V 0 +V s ) Substituting this expression for [X] in equation 15-9 gives (V 0 + V S )10 E/S = 10 k/s V 0 c X + 10 k/s c S V S (14-15) Where S = y b m RT ln10 β nf x

54 x (V 0 + V S )10 E/S = 10 k/s V 0 c X + 10 k/s c S V S (14-15) 15) y b m intercept b m V x c k / S 0 X k / S c S V 0 c c S X (14-16) From equation (15-12), C x is obtained from x-intercept, C s and V 0.

55 Metal Ion Buffers - It is pointless to dilute CaCl Cl 2 to 10-6 M for standardizing di i an ion-selective electrode. At this low concentration, Ca +2 will be lost by adsorption on glass or reaction with impurities. * Glass vessels are not used for very dilute solutions, because ions are adsorbed on the glass. Plastic bottles are better than glass for dilute solutions. * Adding strong acid (0.1 1M) to any solution helps minimize adsorption of cations on the wall of container. It is because H + competes with other cations for ion-exchange sites. * An alternative is to prepare a metal ion buffer from the metal and a suitable ligand. Metal Ion Buffers is used to keep metal ion concentration so low!

56 Metal Ion Buffers MtlI Metal Ion Buffers Bff is used dto keep metal tlion concentration ti so low. - How to keep pca +2 concentration less than 10-6? Ca 2+ + NTA 3- CaNTA - ( at high ph) K f [Ca [CaNTA 2 ][NTA ] 3 ] in 0.1 M KNO 3 (15-14) If you add NTA at high ph into the high concentration of Ca +2 solution and make [CaNTA - ] = [NTA 3- ], then you obtain so low Ca 2+ concentration which does not vary substantially due to the metal buffer action. [Ca [CaNTA ] ] K 3 [NTA ] f M

Ch. 14. ELECTRODES AND POTENTIOMETRY

Ch. 14. ELECTRODES AND POTENTIOMETRY 14.1 Analytical chemists design electrodes (voltage sensitive to conc. change) galvanic cells ion-selective electrodes ion-sensing field effect transistors potentiometry