Harris Quantitative Analysis, Quantitative Analysis

1. Potentiometry (Chapter 14)
Potentiometry
Ecell w/o current flow  info abt. a chemical system
• endpoint in a titration
• Measure [ion]
Rapid; Simple; Inexpensive
Need
Reference electrode
Indicator electrode
Potential measuring device

2. Potentiometry
Typical Cell
Reference electrode / salt bridge / analyte soln / Indicator electrode
Eref Ej Eind
Reference electrode: Anode by convention
Ecell = Ecath - Eanode
= Eind - Eref + Ej

3. Potentiometry Cell:

4. Potentiometry
A. Junction Potential (Ej)
– Potential that develops across the boundary between
2 electrolyte solns of different composition
– Fundamental problem in Potentiometry
– Limits accuracy
– Small unk. value
A junction potential of ~ 3 mV produces an error of ~ 0.05 pH units
(12% error in [H+])

5. 0.01 M HCL
-
+
1 M HCl
examples
Skoog etal

6. Caused differences in
mobility of the ions

7. Potentiometry
B. Reference Electrodes
Why is the SHE not useful as a reference in potentiometry
a. Limited practical use
b. Difficult to prepare electrode surfaces
c. Difficult to control the activity of H+
d. All of the above
Which one of the following is a arbitrary reference electrode
for measuring electrode potentials
a. Ag/AgCl
b. Saturated Calomel Electrode
c. Normal Hydrogen electrode
d. All of the above

8. 1. Ag/AgCl
Ag / AgCl, KCl (x M) //
AgCls + 1e-
 Ags + Cl-
E0
= 0.222 V
Potential determined by [Cl-
]
Sat’d, E = 0.197 V
Two common Reference Electrodes
Ag/AgCl
SCE

9. 2. calomel electrode
Hg / Hg2Cl2 (sat’d), KCl (x M) //
Hg2Cl2(s) + 2e-  2Hgl + 2Cl-
Potential determined by [Cl-
]
Standard conditions: E0
= 0.268 V
Sat’d calomel electrode (SCE) E = 0.241 V
Why are sat’d solutions used?

10. Voltage Conversions Between
Different Reference Scales
p. 331
SCE to SHE; SHE to SCE
SHE to AgCl; AgCl to SHE

11. C. Indicator Electrodes
Electrode that responds directly to the analyte
Ideal:
Responds rapidly, reproducibly
Selective
Two main types
1- metallic
2- membrane

12. Potentiometry
1. Metallic Electrodes
Example,
Pt in a solution of Fe2+
/Fe3+
Potential determined by the
Nernst Equation
Eind = E0
– 0.0592 log [Fe2+
]/[Fe3+
]
Ecell = Eind – E AgCl + Ej
Not selective

13. Potentiometry
2. Membrane Indicator Electrodes
– Ion selective electrodes (pIon electrodes)
– Respond “selectively” to one species in solution
Inside: soln containing the ion of interest, const. A
Outside: soln containing the ion of interest, var. A
Measure potential difference across the membrane
Thin membrane that separates the sample from the inside
of the electrode

15. pH measurement with a glass electrode
Glass combination electrode

16. pH electrode
Cell schematic
Ag l AgCl l Cl- (x M) ll H+ (outside) l H+ (inside), Cl- (x M) l AgCl l Ag
Cations (Na+
) bind oxygen in SiO4
structure

17. pH Electrode
Membrane must be hydrated

18. Functions by exchange of ions at the surface
H+
Gls = H+
aq + Gl-
s
Glass 1 Soln 1
H+
Gls = H+
aq + Gl-
s
Glass 2 Soln 2
Eb = E1 – E2 = 0.05916 log Ain / Aout
Eb = L – 0.0592pH
Position of these two equil. are determined by aH+ in soln
on the two sides of the membrane

19. Glass Electrode Potential
Three components
1- boundary potential
2- potential of internal Ag/AgCl reference electrode
3- small asymmetry potential
Eind = Eb + Eref2 + Easy
Eind = L’ + 0.0592 Log A1 + Eref2 + Easy
Eind = K + 0.0592 Log [H+]
Eind = K + β0.0592 Log [H+]
Electromotive efficiency

20. Activity becomes important when
a. Ions have divalent or trivalent charges
b. Ionic strength of the solution is high
c. All of the above
d. None of the above
Activity vs Concentration

21. Calibrating a glass electrode
Eind = K + 0.0592 Log [H+]
For every 10 fold change in
activity, the potential should
change by 59.2 mV
Slope = ?

22. Potentiometry
Errors (limitations) in pH measurements
1- must calibrate electrode
2- junction potential and drift
3- alkaline (sodium) error
4- acid error
5- allow membrane to equilibrate
6- membrane must be hydrated
7- Temperature

23. Other examples of ISE
Eind = K + (0.0592 / 2)Log [Ca2+
]
Liquid-based ISE

24. E = K – 0.0592 Log [F-]
Slope = ?
Solid State ISE
Linear range = ?

26. Potentiometry
Equation written from the point of view that the membrane
electrode responds to only one ion, maybe more
Full expression
E = const. + 2.303RT / zF log (Ai + Ki,jAj
zi/zj
)
K  ranges from 0 to values greater than 1
Selectivity coefficient
KA,X = response of X / response of A
The smaller, the less interference by X

27. Potentiometry
Monovalent cations (assume k= 0)
E = const + 0.0592 log Ax+
Monovalent anions (assume k = 0)
E = const – 0.0592 log Aa-

28. Constant made up of several constants
Determine experimentally
1- meas. Ecell for std sol of known conc.
2- meas Ecell for unknown conc.
make assumption that K is unchanged

29. Advantages (Figures of Merit) and other characteristics
a.Linear response over a wide range
b.Non destructive
c.Non contaminating
d.Short response time (min)
e.Unaffected by color and turbidity
f.Precision: OK (1% at best)
g.Sensitivity/Detection limits: (10-6
to 10-9***
M)
h.Standard addition method often used (Why ?)

30. Problems:
Questions to ask: Membrane or Metal ISE??
If membrane,
Ecell = Emem
–
Eref
Ememb = const +/− 0.0592 log a
If metal,
Ecell = Ecathode
–
Eref
Ecathode  Nernst Eq

31. Calculate the potential of the following cell when the
aqueous solution is 7.40 x 10-3
M Hg2+
SCE // aq soln / Hg
SCE is the saturated Calomel electrode, E0’
= 0.244 V

32. Example
A pH glass/calomel electrode was found to develop
a potential of –0.0412 V when used with a buffer of
pH 6.00. With an unk soln, the potential was
–0.2004 V. Calculate the pH of the solution.