Successfully reported this slideshow.

Potentiometry

37

Share

Upcoming SlideShare
Potentiometry
Potentiometry
Loading in …3
×
1 of 37
1 of 37

Potentiometry

37

Share

Download to read offline

pH and Potentiometry, Potentiometric titrations, Electrodes used in Potentiometry, Standard Hydrougen electrode, calamel electrode, silver silver chloride electrode, glass electrode

pH and Potentiometry, Potentiometric titrations, Electrodes used in Potentiometry, Standard Hydrougen electrode, calamel electrode, silver silver chloride electrode, glass electrode

More Related Content

Related Books

Free with a 14 day trial from Scribd

See all

Related Audiobooks

Free with a 14 day trial from Scribd

See all

Potentiometry

  1. 1. Potentiometry Deokate U. A. Copyright © by U. A. Deokate, all rights reserved.
  2. 2. Points to be covered • Electrochemical cell, construction and • Working of reference (Standard hydrogen, silver chloride electrode and • calomel electrode) and indicator electrodes (metal electrodes and glass electrode) • methods to determine end point of potentiometric titration and • applications 11/1/2018 2Deokate U.A.
  3. 3. Electroanalytical methods • involve the measurement of either the electrical current flowing between a pair of electrodes immersed in the solution tested (voltammetric and amperometric methods) or • An electrical potential developed between a pair of electrodes immersed in the solution tested (potentiometric methods). • In either case, the measured parameter (current or potential) is proportional to the concentration of analyte 11/1/2018 Deokate U.A. 3
  4. 4. 11/1/2018 Deokate U.A. 4 Electro analytical chemistry • Electro analytical chemistry encompasses a group of quantitative analytical methods that are based upon the electrical properties of an analyte solution when it is made part of an electrochemical cell. • These methods make possible the determination of a particular oxidation state of an element.
  5. 5. 11/1/2018 Deokate U.A. 5 Advantages of electro analytical tech. • Electrochemical measurements are specific for a particular oxidation state of element, so determination of con from a individual species from mixture is possible. • Instruments are relatively inexpensive as compare to other • They provide information about activates rather than conc. of chemical specifies • High degree of sensitivity, selectivity, and accuracy. • Concentration less than 10-10 moles can be determine using these methods • Automation is possible for these methods.
  6. 6. 11/1/2018 Deokate U.A. 6 Electro Chemical Methods • pH metry & Potentiometry • Conductometry • Voltametry • Polarography • Amparometery • Coulombmetry
  7. 7. Potentiometry Principle: • The principle involved in the Potentiometry is when the pair of electrodes is placed in the sample solution it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions 11/1/2018 7Deokate U.A.
  8. 8. 11/1/2018 Deokate U.A. 8 Electro chemical Cell • A chemical cell is system in which chemical energy is transferred in to electrical energy. • Each cell is made up of 2 electrodes one liberating e- is called oxidizing electrode (anode) while the other absorb e- is called reducing electrode (cathode)
  9. 9. 11/1/2018 Deokate U.A. 9 Types of electrochemical cells: • There are two types of electrochemical cells: 1. Galvanic or Voltaic cell (ones that spontaneously produce electrical energy) and 2. Electrolytic (ones that consume electrical energy).
  10. 10. 11/1/2018 Deokate U.A. 10 Galvanic cell
  11. 11. 11/1/2018 Deokate U.A. 11 An electrolytic cell
  12. 12. 11/1/2018 Deokate U.A. 12 Electrodes used in Potentiometry •Reference Electrodes: the electrode whose potential is independent of nature and composition of surrounding •Indicator Electrodes: Electrode whose potential depends on Con/ activity of one component in solution.
  13. 13. • The reference electrode is the electrode which contains of its own potential value and it is stable when dipped into sample solution. • The salt bridge is used to prevent the interference of the analyte solution with that of reference solution. • Here analyte solution is the solution whose potential is to be measured. • The indicator electrode is the electrode which responds to change in the potential of analyte solution • The electromotive force of the complete cell is given by the following equation: Ecell = Ereference + Eindicator + Ejunction • where E reference is the electromotive force of the reference electrode ,E indicator is electromotive force of indicator electrode, E junction is the electromotive force at the junction of the liquid. 11/1/2018 Deokate U.A. 13
  14. 14. Theory: • When the known potential electrode immersed in the sample solution then the potential is given by Nernst equation: E= E0 +(0.592/n) log c • Where E is the potential of the solution; E0 is the standard electrode potential; n is the valency of the ions; c is the concentration of the sample solution; • 0.592 is the value obtained from the RT/F; where R is the gas constant, T is the temperature in Kelvin, F is the faradays constant. 11/1/2018 Deokate U.A. 14
  15. 15. Reference Electrodes • The reference electrodes are classified into two main classes they are as follows: • Primary standard electrodes ex: Standard hydrogen electrode • Secondary standard electrodes ex: silver-silver chloride electrode, saturated calomel electrode 11/1/2018 Deokate U.A. 15
  16. 16. 11/1/2018 Deokate U.A. 16 Characteristics of Ideal Reference Electrode: 1. Reversible and follow Nernst equation 2. Potential should be constant with time 3. Should return to original potential after being subjected to small currents 4. Little effect with temperature cycling 5. Should behave as ideal nonpolarized electrode 6. Made of some stable chemical species, is readily available and usually simple to use. 7. Should be non-toxic if you are going to use it in a biological system. 8. Should be rugged and portable if you are going to use it in the field
  17. 17. 11/1/2018 Deokate U.A. 17 Standard Hydrogen reference electrode • This is the reference half cell against which all others are compared. • It must be easy to construct, reversible and be highly reproducible • The standard hydrogen electrode (SHE) meets these specifications • It consists of a piece of platinum coated with finely divided platinum black to increase its specific area. • It is immersed in aq acid solution and hydrogen is bubbled
  18. 18. 11/1/2018 Deokate U.A. 18 The Standard Hydrogen Electrode (SHE) • The standard H2 electrode potential is defined as the potential that is developed between the H2 gas adsorbed on the pt metal and H+ of the solution when the H2 gas at a pressure of 760 mm of Hg is in equilibrium with H+ of unit concentration
  19. 19. Limitations SHE • It is rather difficult to regulate the pressure of the H2 gas to be at exactly 1atm throughout the experiment. • If the solution contains any oxidizing agent, the H2 electrode cannot be used. • Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration decreases. In such a system, it is difficult to maintain the concentration of HCl at 1M. • Platinum foil gets easily poisoned by the impurities present in the gas and HCl. In fact, the attainment of equilibrium is ensured by trial and error. 11/1/2018 Deokate U.A. 19
  20. 20. 11/1/2018 Deokate U.A. 20 Ag/AgCl • Constructed of a silver wire, coated with silver chloride, in a solution containing silver chloride – Sometimes AgCl is in solution saturated with KCl – Saturated KCl keeps the chloride activity constant and helps keep the electrode stable log!tenbasewithlnreplaceweIF C25@0.05916 nF RT ][Clln nF RT EE S.H.E)to(relativemV197KCl)(sat.E S.H.E)to(relativemV222E ClAgeAgCl -o o (aq) - (s) - (s)    
  21. 21. 11/1/2018 Deokate U.A. 21
  22. 22. 11/1/2018 Deokate U.A. 22
  23. 23. 11/1/2018 Deokate U.A. 23 Calomel (S.C.E) • Calomel is mercury (I) chloride • One of the most common reference electrodes 2-0 o (aq) - (l) - (s)22 ][Clln nF RT EE S.H.E)to(relativemV241KCl)(sat.E S.H.E)to(relativemV268E Cl2Hg2e2ClHg    
  24. 24. 11/1/2018 Deokate U.A. 24 Calomel electrode
  25. 25. 11/1/2018 Deokate U.A. 25 Indicator Electrodes • Used for indicating the potential caused by some chemical species as compared to the reference electrode. • Usually connected to the + (cathode) side of the potentiometer • Made of a variety of often allegedly, but never actually, inert materials. • Metal Electrodes – Electrode of first kind – Electrode of Second kind – Electrode of third kind – Electrode of redox type • Platinum • Ion Selective (membrane) – Glass membrane – Crystalline • Solid State – Liquid Membrane • Ecell=Eindicator-Ereference
  26. 26. 11/1/2018 Deokate U.A. 26 Metallic electrode • 1st kind – Used for determination of activity of cation derived from electrode metal – respond directly to changing activity of electrode ion – Direct equilibrium with solution – Not very selective – simple • Eg: Ag, Hg, Cu, Zn, Cd, Bi, Tl, Pb
  27. 27. 11/1/2018 Deokate U.A. 27 2nd kind • A metal which can often made responsive to activity of ion to which it form ppt or stable complex of ion – Ag for halides – Ag wire in AgCl saturated surface • Complexes with organic ligands – EDTA
  28. 28. 11/1/2018 Deokate U.A. 28 Indictor electrodes • 3rd kind – Electrode responds to different cation – Competition with ligand complex – Eg. Hg is used for determination of calcium • Inert metals (Redox electrode) – Pt, Au, Pd • Electron source or sink • Redox of metal ion evaluated – May not be reversible
  29. 29. 11/1/2018 Deokate U.A. 29 Typical Metal Electrode Arrangement
  30. 30. 11/1/2018 Deokate U.A. 30 Membrane Indicator electrodes – Non-crystalline membranes: • Glass - silicate glasses for H+, Na+ • Liquid - liquid ion exchanger for Ca2+ • Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3- – Crystalline membranes: • Single crystal - LaF3 for F Polycrystalline • or mixed crystal - AgS for S2- and Ag+ • Properties – Low solubility - solids, semi-solids and polymers – Some electrical conductivity - often by doping – Selectivity - part of membrane binds/reacts with analyte
  31. 31. 11/1/2018 Deokate U.A. 31 pH (Glass Membrane) Electrodes • One of the simpler ion-selective electrodes (ISE) • Hydrogen Ion imparts a charge across a hydrated glass membrane • Generally include an internal reference electrode (Ag/AgCl) and a separate Ag/AgCl electrode for sensing the charge imparted by the hydrogen ions • Not as simple to use as you think!
  32. 32. 11/1/2018 Deokate U.A. 32 Glass Membrane Electrode
  33. 33. 11/1/2018 Deokate U.A. 33
  34. 34. Advantages & Disadvantages • ADVANTAGES : – Response is very rapid – Chemically resistant to oxidizing & reducing agents, dissolved gases, salts etc. – When Lithia -silica glasses are used, it can be used over the entire pH range. • DISADVANTAGES – It is extremely fragile – Minute abrasions on the surface of the tip, damages the electrode – It cannot be used with simple potentiometers, because of the high resistance. 11/1/2018 Deokate U.A. 34
  35. 35. Types of Potentiometric titrations: • Acid-base titration • Redox titration • Complexometric titration • Precipitation titration 11/1/2018 Deokate U.A. 35
  36. 36. APPLICATIONS • Clinical chemistry: Ion selective electrodes are present sensors for clinical samples because of their selectivity for analyte in complex matrices. The most common analytes are electrolytes such as Na, k ,Ca ,H, and Cl and dissolved gases such as CO2 • Environmental chemistry: For analysis of CN- ,NH3, NO3, F3 in water and waste water. • Potentiometric titrations: For determining the equivalence point of an acid base titration. • possible for redox, precipitation, acid-base, complexation as well as for all titrations in aqueous n non aqueous solvents. • Agriculture: NO3 ,NH4 ,I ,Ca, K ,CN, Cl in soils, plant materials, feed stuffs, fertilizers. • Detergent manufacturing: Ca, Ba, F for studying effects in water quality. 11/1/2018 Deokate U.A. 36
  37. 37. Applications • Salt content of meat fish dairy products fruit juices brewing solutions • Ca in dairy products and beer • K in fruit juice and wine making • Corrosive effects of NO3 in canned foods • F in drinking water and other drinks • NO3 and NO2 in meat preservatives • Assay of bisacodyl suppositories and assay of sulpha drugs can be estimated potentiometrically. • Food processing: Automatic potentiometer 11/1/2018 Deokate U.A. 37

×