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Ch. 23 Potentiometric Methods
Introduction: 1.) Potentiometric Methods: based on measurements of the potential of electrochemical cells in the absence of appreciable currents (i. 0) 2.) Basic Components: a) reference electrode: gives reference for potential measurement b) indicator electrode: where species of interest is measured c) salt bridge d) potential measuring device
Figure 23.1 A cell for potentiometric determinations
Ecell = (E (^) ind – E (^) ref ) + Ej For most electroanalytical methods, the junction potential is small enough to be neglected.
A) Reference Electrodes:
Need one electrode of system to act as a reference against which potential measurements can be made relative comparison.
Desired Characteristics: a) known or fixed potential, Eref b) constant response (even when there is a net current in the cell) c) insensitive to composition of solution under study d) obeys Nernst Equation e) reversible f) rugged and easy to assemble g) Always treated as the left-hand electrode
Common Reference Electrodes used in Potentiometry i) Calomel Electrodes (Hg in contact with Hg 2 Cl 2 & KCl) ii) Silver/Silver Chloride Electrode
Temperature coefficient is significantly larger
- "M" and "saturated" refer to the concentration of KCI and not Hg 2 Cl 2 •
Table 23-1 lists the composition and the potentials for three common calomel electrodes. Note that each solution is saturated with mercury(I) chloride (calomel) and that the cells differ only with respect to the potassium chloride concentrations.
2- Silver/Silver Chloride Electrode
- most widely used reference electrode system
- Ag electrode immersed in KCl solution saturated with AgCl
½ cell repr. : Ag | AgCl (satd) KCl (xM)||
½ cell reaction: AgCl (s) + e-^ Ag(s) + Cl-
Advantage – one advantage over SCE is that Ag/AgCl electrode can be used at temperatures > 60oC Disadvantage – Ag reacts with more ions,
- plugging of the junction between electrode (Ag) and analyte soln. - Precautions in the Use of Reference Electrodes
- need to keep level of solution in reference electrode above the level in analyte solution( to prevent rxn of Ag/Hg with analyte)
- need to prevent flow of analyte solution into reference electrode can result in plugging of electrode at junction erratic behavior second salt bridge ( non-interfering electrolyte: KNO 3 , NaSO 4 )
Vycor plug
A) Reference Electrodes:
B) Indicator Electrodes:
- Detects or Responds to Presence of Analyte Ideal indicator electrode responds rapidly and reproducibly to changes in the concentration of an analyte ion (or groups of analyte ions).
Three Common Types: a) Metallic Indicator Electrodes Electrodes of the First Kind Electrodes of the Second Kind Electrodes of the Third Kind Metalic Redox Indicators b) Membrane Indicator Electrodes Crystalline Membrane Electrodes Non-crystalline Membrane Electrodes c) Ion selective Electrode (field effect transistor) ISFET
B) Indicator Electrodes:
Metallic Indicator Electrode (Four Main Types)
a) Metallic Electrodes of the First Kind:
A pure metal electrode that is in direct equilibrium with its cation in the solution: i. Involves single reaction ii. Detection of cathode derived from the metal used in the electrode iii. Example: use of copper electrode to detect Cu^2 +^ in solution
½ reaction: Cu^2 +^ + 2 e-^ Cu (s)
Eind gives direct measure of Cu^2 +: Eind = EoCu – ( 0. 0592 / 2 ) log aCu(s)/aCu 2 + since aCu(s) = 1 : Eind = EoCu – ( 0. 0592 / 2 ) log 1 /aCu 2 + or using pCu = - log aCu 2 +: Eind = EoCu – ( 0. 0592 / 2 ) pCu
Metal electrode respond to the activities of anions that form sparingly
soluble precipitates or stable complexes.
i. Example: Detection of Cl-^ with Ag electrode
½ reaction: AgCl(s) + e-^ Ag(s) + Cl-^ EO^ = 0. 222 V
Eind gives direct measure of Cl-:
Eind = Eo^ – ( 0. 0592 / 1 ) log aAg(s) aCl-/aAgCl(s)
since aAg(s) and aAgCl(s)= 1 & Eo^ = 0. 222 V:
Eind = 0. 222 – ( 0. 0592 / 1 ) log aCl-
B) Indicator Electrodes:
Metallic Indicator Electrode (Four Main Types) b) Metallic Electrodes of the Second Kind:
ii. Another Example: Detection of EDTA ion (Y^4 - ) with Hg Electrode ½ reaction: HgY^2 -^ + 2 e-^ Hg(l) + Y^4 -^ Eo^ = 0. 21 V Eind responds to aY 4 - : Eind = Eo^ – ( 0. 0592 / 2 ) log aHg(l) aY 4 - /aHgY 2 - aHg(l)= 1 and Eo^ = 0. 21 V: Eind = 0. 21 – ( 0. 0592 / 2 ) log aY 4 - /aHgY 2 -
B) Indicator Electrodes:
Metallic Indicator Electrode (Four Main Types)
b) Metallic Electrodes of the Second Kind:
To use this electrode system, it is necessary to introduce a small concentration of HgY 2 -^ into the analyte solution at the outset. The complex is so stable (for HgY 2 -^ , Kf = 6.3 X 10^21 ) that its activity remains essentially constant over a wide range of Y 4 - activities. Therefore, the potential equation can be written in the form
This electrode is useful for locating end points for EDTA titrations.
If, in addition, a small volume of a solution containing the EDTA complex of calcium is introduced, a new equilibrium is established, equilibrium reaction: CaY2-^ Ca2+^ + Y4-^ Where: Kf =
Eind = K – ( 0. 0592 / 2 ) log.
aca2+.^ aY4- aCaY2-
ay4- = aca2+
Kf.^ aCaY2- aca2+
Kf. aCaY2-
Eind = K – ( 0. 0592 / 2 ) log.
Eind = K’ – ( 0. 0592 / 2 ) pCa.
aca2+
Kf.^ aCaY2-^ – (0.0592/ 2 ) log. 1
If a constant amount of CaY^2 -^ is used in the analyte solution and in the solutions for standardization, we may write
Where (^) K’ = K – ( 0. 0592 / 2 )log Kf. (^) aCaY2-
Thus, the mercury electrode has become an electrode of the third kind for calcium ion.
Metallic Indicator Electrode (Four Main Types) d) Metallic Redox Indicators i. Electrodes made from inert metals (Pt, Au, Pd) often serve as indicator electrodes for oxidation-reduction systems. ii. Electrode acts as e-^ source/sink for electrons transferred from a redox system in the solution Example: Detection of Ce^3 +^ with Pt electrode
½ reaction: Ce^4 +^ + e-^ Ce^3 +
Eind responds to Ce^4 +: Eind = Eo^ – ( 0. 0592 / 1 ) log aCe 3 +/aCe 4 + Thus, a platinum electrode can serve as the indicator electrode in a titration in which Ce(IV) serves as the standard reagent.
Problems:
- electron-transfer processes at inert electrodes are frequently not reversible
- do not respond predictably to ½ reactions in tables
B) Indicator Electrodes:
Desired properties of ISE’s
- membrane will not dissolve in solution during measurement
- Many membranes are formed from large molecules or molecular aggregates: silica glasses, polymeric resin, low solubility inorganic compounds (AgX) can be used as membranes
* need some electrical conductivity
- Generally, this conduction takes the form of migration of singly charged ions within the membrane. * selective reactivity with the analyte Three types of binding are encountered: ion-exchange, crystallization and
complexation. The former two are the more common, and we will largely
focus on these types of bindings.
Membrane Indicator Electrodes
a) pH Electrode i. most common example of an ISE based on use of glass membrane that preferentially binds H+ ii. Typical pH electrode system is shown Two reference electrodes here one SCE- outside of membrane one Ag/AgCl - inside membrane pH sensing element is glass tip of Ag/AgCl electrode
( b)Combination probe consisting of both an indicator glass electrode and a silver-silver chloride reference.
(a ) Glass electrode (indicator) and SCE (reference) immersed in a solution of unknown pH.
Selective binding of cation (H+) to glass membrane
The Composition and Structure of Glass Membranes
Cross-sectional view of a silicate glass membrane structure. Each silicon atom is shown as being bonded to three oxygen atoms in the plane of the paper. In addition, each is bonded to another oxygen above or below the plane. Thus, the glass consists of an infinite three-dimensional network of SiO, - groups in which each silicon is bonded to four oxygens and each oxygen is shared by two silicons. Within the interstices of this structure are sufficient cations to balance the negative charge of the silicate groups. Singly charged cations, such as sodium and lithium, are mobile in the lattice and are responsible for electrical conduction within the membrane.
- Glass composition affects the sensitivity of membranes to protons and other cations
- Corning 015 glass, which has been widely used for membranes, consists of approximately 22% Na 2 O, 6% CaO, and 72% Si02• This membrane is specific in its response toward hydrogen ions up to a pH of about 9. At higher pH values, however, the glass becomes somewhat responsive to sodium, as well as to other singly charged cations -. Other glass ormulations are now in use in which sodium and calcium ions are replaced to various degrees by barium and lithium ions. These membranes have superior selectivity at high pH.
The hydration of a pH-sensitive glass membrane involves an ion-exchange reaction between singly charged cations in the interstices of the glass lattice and protons from the solution:
The Hygroscopicity of Glass Membranes The surface of a glass membrane must be hydrated before it will function as a pH electrode. Nonhygroscopic glasses show no pH function. Even hygroscopic glasses lose their pH sensitivity after dehydration by storage over a desiccant. The effect is reversible, however, and the response of a glass electrode can be restored by soaking it in water.
Electrical Conduction across Glass Membranes
The equilibrium constant for this process is so large that the surface of a hydrated glass membrane ordinarily consists entirely of silicic acid (H +GI-) groups.
To serve as an indicator for cations, a glass membrane must conduct electricity. Conduction within the hydrated gel layer involves the movement of hydrogen ions. Sodium ions are the charge carriers in the dry interior of the membrane. Conduction across the solution-gel interfaces occurs by the reactions H+^ + Gl-^ H+Gl-
