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Electroanalytical – Introduction Ch 21, 7 th e, WMDS) Four Basic Electroanalytical Methods of Analysis 1) Potentiometric 2) Voltammetric, Polarographic,

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Presentation on theme: "Electroanalytical – Introduction Ch 21, 7 th e, WMDS) Four Basic Electroanalytical Methods of Analysis 1) Potentiometric 2) Voltammetric, Polarographic,"— Presentation transcript:

1 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) Four Basic Electroanalytical Methods of Analysis 1) Potentiometric 2) Voltammetric, Polarographic, Amperometric 3) Electrolysis: electrogravimetric and coulometric 4) Conductiometric We will briefly describe each one, then focus individually on each.

2 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) 1) Potentiometric - The electrical potential of a galvanic (spontaneous cell) is related to the concentration of the analyte by the Nernst Equation. Examples of potentiometric include pH measurements and the use of ion-selective electrodes. Potentiometric methods may also be used to monitor the course of redox, acid/base/complexometric, and precipitation reactions.

3 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) 2) Voltammetric, Polarographic, Amperometric – in these methods, the electrical current at an electrode in an electrolysis (nonspontaneous) cell is related to the concentration of the analyte. They vary according to the parameter varied and the quantity measured. Generally, calibration curves are constructed to show the relationship between the measured quantity and the concentration of the analyte.

4 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) 3) Electrolysis: electrogravimetry and coulometry - electrolysis is done on the nonspontaneous electrochemical cell by an external power supply. a - In electrogravimetric methods the sought analyte is electrochemically deposited onto a previously weight electrode (generally the cathode) and the amount of analyte determined by the increase in the mass on the electrode.

5 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) 3) Electrolysis: electrogravimetry and coulometry b - In coulometric methods, the amount of an electrochemical species oxidized or reduced is related to the amount of current, time and number of electrons in that half cell reaction. The number of coulombs (C) of electricity Q = it where i is current in amperes and t time in sec. One Faraday ( F ) (96,485 C) of charge is equivalent to 1 mole of electrons, so in combination with the half cell one can quantify the amount of electrochemical work done.

6 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) 4) Conductiometric – The electrical conduction of a solution is related to the number of ions present in solution. Method is especially useful to monitor the purity of deionized water. It is also useful to monitor titrations if the electrical conductance of the titrant and analyte are significant different, or if the product at the equivalence is different than the reactants.

7 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Potentiometry – In order to make potentiometric measurements one must have both a reference and an indicator electrode. Reference Electrodes The point of beginning for the construction and definition of reference electrodes is with the standard hydrogen electrode (SHE), the primary reference standard electrode. It is based on the half-cell reaction H + + e = ½H 2. Whenever the [H+ ] = 1.00 m and p H2 = 1.00 atm (or in SI, 1 bar), this electrode has a defined potential of volts.

8 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Potentiometry – Reference Electrodes SHE Unfortunately the SHE is not a very convenient reference electrode. 1) It is quite fragile 2) hazardous because of the hydrogen gas 3) difficult to maintain the standard acid solution at 1.00 m. Although it is not used in the laboratory for measurements, it is the primary reference electrode against which all other electrodes are ultimately standardized.

9 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) Potentiometry – Reference Electrodes Secondary Reference Electrodes. There are 2 commonly used secondary reference electrodes. The first is the saturated calomel electrode (SCE) whose half-cell reaction is Hg 2 Cl 2 (s) + 2e = 2Hg(l) + 2Cl - (aq). Measured against the SHE with saturated KCl as the electrolyte, its potential is o C. The potential is also dependent of the concentration of chloride, so the saturated KCl is the most commonly used. Literature gives the potential at other temperatures, as well as with other concentrations of KCl.

10 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Potentiometry – Reference Electrodes Secondary Reference Electrodes. The second widely used laboratory reference electrode is the silver-silver chloride electrode whose half-cell reaction is AgCl(s) + e = Ag(s) + Cl - (aq). The potential of the Ag-AgCl system is o C and a saturated solution of KCl is used as its electrolyte. Literature gives the potential at other temperatures.

11 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Indicator Electrodes Indicator electrodes are classified according to the process that produces the electrode potential. Metal indicator electrodes develop a potential dependent on the position of the equilibrium of the redox half-reaction at the surface of the electrode. Membrane indicator electrodes develop a potential determined by the difference in the concentration of particular ions across a special thin layer known as a membrane.

12 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Metal indicator electrodes are classified as either first-order or second-order electrode. A first order electrode involves the metal in contact with its own ions, such as Ag, Ag + or Zn, Zn. Only a few metals give reproducible potentials as first-order electrodes due to crystalline irregularities on their surfaces and the ease of forming oxides. A second-order electrode is one that responds to the presence of precipitating or complexing ions. For example, silver wire could serve as the indicator electrode for chloride. The use of second-order indicator electrodes are limited.

13 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) The most commonly used indicator electrodes are known as inert. These electrodes are not involved in the half-cell reactions of the electrochemical species. Typical inert electrodes are platinum, gold, and carbon. Inert electrodes are responsive to any reversible redox system; these are widely used in potentiometric work.

14 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) There are several different types of membrane indicator electrodes according to the type of membrane and the specific use of that electrode. Membranes may be made of glass, polymers, or crystals. The first and most studied membrane indicator electrode is the glass electrode, most commonly used as the pH electrode. The membrane is a very thin layer of specially composition glass, generally containing CaO, BaO, Li 2 O and Na 2 O as well as SiO 2. The internal solution is of known constant hydrogen ion activity, such as pH 7.

15 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) When this system is placed in an external solution other hydrogen ion activity, a Nernstian potential develops across the glass membrane. Although the response follows the Nernst equation E = /1 log ([H + ] external /[H + ] internal ) there are three important facts that must be considered.

16 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Factors in the Use of Glass electrodes 1 - The electrical resistance of the glass membrane is very high (~ 100 M  ), so special high input resistance voltmeters (electrometers with R input ~ 106 M  ), are required to obtain accurate potential readings.

17 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Factors in the Use of Glass electrodes 2 - In use, a small error, known as the asymmetry potential develops across the membrane so that frequent re- standardization of the electrode is required. Standardization consists of placing the electrode in a buffer of known pH and then electrically adjusting the meter to give the reading of that buffer.

18 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Factors in the Use of Glass electrodes The composition and structure of the glass membrane affects the selectivity of the glass membrane electrode. Membrane electrodes are not exclusive in their response to one single ion, but response to other ions of similar size and charge, so the potential may be a combination of the presence of several ions. The response for a desired ion A to the response to B is given by the selectivity coefficient k B/A.

19 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Selectivity Coefficient k B/A. Small values of k B/A, such as are desirable. This means that only at equal concentration of A and B, only 1/1000 of the potential comes from B. A typical glass electrode for pH measurements may have appreciable Na ion error, especially at high pH (when [H + ] is low). Low sodium error electrodes are available when needed.

20 Electroanalytical – Introduction Ch 21, 7 th e, WMDS)

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22 Ion-selective electrodes (ISE) In addition to the glass electrode for pH measurements, there is a wide range of membrane electrodes for the measurement of specific ions. Although sometimes called ion-specific electrodes one must realize that they are really selective for definite ions, but have the same limitations of selectivity as the Glass electrodes discussed earlier. They should really be termed as ion-selective electrodes (ISE). All of the types of membranes described earlier are used in their construction.

23 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) Polymer membrane ISE, similar to the Calcium ISE

24 Electroanalytical – Introduction Ch 21, 7 th e, WMDS) The Flouride ISE, a solid crystal membrane ISE;

25 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) A calibration curve for the F  ISE; other electrodes would have different y-intercept values. The theoretical slope is 59.16/n where n is the charge of the ion.

26 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) IonConc Range, M Membran e pH Range Interferences F–F– 10 –6 – 1LaF 3 5 – 8OH – Cl – 10 –4 – 1AgCl2 – 11CN –, S –2, I –, S 2 O 3 –2, Br – Br – 10 –5 – 1AgBr2 – 12CN –, S –2, I – I–I– 10 –6 – 1AgI3 – 12S –2 CN – 10 –6 – 10 –2 AgI11 – 13S –2, I – S –2 10 –5 – 1Ag 2 S13 – 14 Typical Solid-state (Crystal-membrane) ISE (from Harris, 2e, p 316)

27 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) At the present time the following Ion-selective electrodes have been developed: NH 4 +, Ca +2, Mg +2, K +, Cl –, S –2, F –, ClO 4 –, NO 3 –, Br –, I –, CN –, Ag +, Cu +2, Pb +2, Cd +2 ; In addition to the above, by using permeable membranes over glass electrodes that measure the amounts of various gases or of specific metabolites have been developed, such as the CO 2, urea, or glucose electrodes.

28 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) A CO 2 secondary membrane electrode

29 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) The measurement of the potential of ion-selective electrodes is accomplished by making an electrochemical cell consisting of the ion-selective electrode and a suitable reference electrode. E measured = E indicator – E reference. Since the potential of the reference electrode is known, E indicator = E cell + E reference. There are three different ways that potentiometric measurements may be done in the laboratory: 1 - Direct 2 - Electrode Calibration 3 - Standard Addition

30 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) 1 - Direct – the measured potential is related to the concentration of the analyte by the Nernst equation, /n pA where pA is the –log [A]. Although this method is straight forward and simple, it does not account for matrix effects, i.e., the effect of other ions, etc in the sample. This method is useful for continuous monitoring and is thus used in the analysis of industrial and natural processes such as effluent streams.

31 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) 2 - Electrode Calibration – electrode response is calibrated against solutions of known [analyte]. Often a plot is constructed of E measured vs pA and used as both a check of range of linearity of the response of the electrode, and to use as a calibration curve.

32 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Standard Addition – in this method the electrode is used to first measure the unknown sample and then a small known amount of the analyte is added and the measurement taken. The incremental change in potential was caused by the known addition. This then allows the analyst to calculate the concentration of the analyte in the original sample. This technique is best to assure that the ionic strength (I or  ) of the solution (and therefore the activity) is not causing a significant error.

33 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Standard Addition –. The following relationship is useful in the calculation of the concentration of the analyte using the standard addition method;  E = E 2 – E 1 = S log ((C 1 +  C/C 1 ))Eqn where S is the slope (emf/pC).

34 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Standard Addition –. Alternately, I find the following relationship from Hargis to be friendlier in working with real data: [A] = ___________[S] x V S ________ _ [(V A + V s ) 10 –n(  E/ ] – V A where A is the analyte, S the standard solution, V volume, n number of electrons, and  E = E 2 – E 1. Hargis, “Analytical Chemistry “, p 344, ©1988, Prentice-Hall

35 Electroanalytical – Introduction Ch 22, 7 th e, WMDS) Sample Standard - Addition Problem The wastewater from an industrial processing plant was routinely analyzed for lead as required by EPA. A Pb ISE and a SCE reference were placed in a 50.0 mL sample. The potential was measured to be –0.118 volts; after the addition of 5.00 mL of a M solution of Pb +2 standard solution was added, the potential was –0.109 volts. What is the approximate concentration of Pb +2 in the wastewater? Also express the [Pb +2 ] in ppm Solution: [Pb +2 ] = (6.00 x 10 –3 )(5.00) /[( ) 10 –(2)( (-0.109))/ ] – 50) = (3.00 x 10 –2 ) /(55.0 x – 50.0) = (0.0300/(110.7 – 50.0) = M = ( mmol/mL) x 207.2mg/mmol = mg/mL ≈ mg/g =  g/g = 102 ppm


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