Potentiometery BP 1st year.pptx

Potentiometry B.Pharm first year , Pharmaceutical analysis

1. PHARMACEUTICAL ANALYSIS UNIT-5 TOPIC: POTENTIOMETRY B.PHARM 1 ST YEAR Presented By: MS. SHWETA SINGH (Assistant Professor) RBMI, Bareilly

2. CONTENT Principle Reference electrode Standard Hydrogen electrode Saturated calomel electrode Silver-Silver Chloride electrode Indicator electrode Glass electrode

3. Definition Potentiometry is  the field of electro-analytical chemistry in which potential is measured without current flow. It is a method of analysis in which we determine the concentration of solute in solution and the potential difference between two electrodes. Potentiometry is an instrument used to determine the unknown concentration between the reference solution and the sample.

4. 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 titrant. Metal atoms may dissolve in the solution as positive ions leaving electrons on the electrode. Metal ions may take up electrons from the electrode and get deposited as neutral atoms In this way, A POTENTIAL DIFFERENCE is set b/w electrode and solution.

5. POTENTIOMETER   It is an instrument to determine the potential differences between a reference electrode and an indicator electrode. These two electrodes form an electrochemical cell that is dipped in solution to be analyzed. Measured potential can be used to determine the quantity of analyte in terms of concentration.

6. REFERENCE ELECTRODE A  reference electrode is a half-cell having a known potential. Has constant potential. Example: STANDARD HYDROGEN ELECTRODE (S.H.E), SILVER-SILVER CHLORIDE ELECTRODE INDICATOR ELECTRODE INDICATOR/TEST/SAMPLE ELECTRODE An indicator electrode is another half-cell with unknown potential. Immersed in analyte solution. Its potential is sensitive to the concentration of analyte. Potential is directly proportional to the ion concentration. EXAMPLE: METAL ELECTRODE, ION SELECTIVE ELECTRODE (I.S.E).

7.   The electromotive force  of the complete cell is given by the following equation: E cell = E (reference) + E (indicator) + E ( salt bridge ) E (reference) = is the electromotive force of the reference electrode E (indicator) = is the electromotive force of the indicator electrode, E ( salt bridge ) = is the electromotive force at the salt bridge

8. Salt Bridge It   is a laboratory device used to connect two half-cells of galvanic or voltaic cells. Salt should be inert. It should not react with anode/cathode/solution. Salt should not be precipitate. FUNCTION- To complete the circuit of the cell. To maintain electrical neutrality. Salt used: NaCl, KCl , KNO 3 , NH 4 OH, NH 4 Cl

9. Theory The main theory  involved in potentiometry is, that when the known potential electrode is immersed in the sample solution then the potential is given by the Nernst equation: E= E + ( 2.303 RT ) log 10 Q Where E is the electrode potential of the solution; E is the standard electrode potential; R= Gas constant (8.314 J/mol K) T= Temperature (Kelvin) n= No. of electrons transferred F = faradays constant (96500 coulombs) Q= Reaction quotient nF

10. Electrodes These are mainly used to measure the voltages. Mainly two electrodes are used in the potentiometry. They are as follows: •Reference/Standard electrode •Indicator/test/sample electrode Reference electrodes :  These are mainly used for the determination of the analyte by maintaining the fixed potential (0V). Eg : • Standard hydrogen electrode • Silver-silver chloride electrode • Saturated calomel electrode The reference electrodes are classified into two main classes they are as follows: 1. Primary standard electrodes Eg : Standard hydrogen electrode (SHE) 2. Secondary standard electrodes Eg : Silver-silver chloride electrode and Saturated calomel electrode (SCE).

11. Reference electrode Ⅰ. Standard hydrogen  electrode (SHE) Also known as Normal hydrogen electrode (NHE) It is a gas-ion electrode [H 2(g) /H + ( aq ) ] It is a reference electrode set by IUPAC . Ideal characters of the Reference electrode: Obeys the Nernst equation. Exhibit constant potential (0V) Exhibit little change with temperature. This electrode is conjugated with a sample/indicator electrode and measures the concentration of analyte in the solution.

12. Construction: It consists of a platinum electrode is covered in finely powdered platinum black (platinized platinum electrode). Hydrogen gas is passed through the tube at 1atm pressure . A platinum foil is attached at the end of the wire. The electrode is immersed in 1M HCl ion solution at 25°C . Working: The electrode potential of SHE is ZERO at all temperatures. Half cell reaction: ½ H 2(g) H + + e - It can act as an anode half-cell and a cathode. Its standard reduction potential (SRP) E O H+/H 2 = 0, as well as standard oxidation potential (SOP) E O H2/H+=0. Diagram of Standard Hydrogen Electrode (SHE)

13. Ⅱ. SATURATED CALOMEL ELECTRODE (SCE) Construction: Also known as Mercury-Mercurous electrode . It consist of an outer tube containing saturated solution of KCl . At the bottom of the tube pure mercury (Hg) is placed, which is covered with a slurry or paste of Hg-Hg 2 Cl 2 which is known as Calomel. The remaining portion of the cell is filled with a solution of normal saturated KCl (1M). Working: This electrode is used as a reference electrode. SCE is used as both cathode and anode. Hg 2 Cl 2 Hg 2 + + 2Cl - At Cathode: Reduction (GOE): Hg 2 2+ + 2e - 2Hg At Anode: Oxidation (LOE): 2Hg Hg 2 2+ + 2e - Electron (e - ) exchange between ionic Hg and molecular Hg takes place. Concentration of KCl Potential 0.1 M KCl 0.337V 1M 0.280V Saturated KCl 0.242V

14. Saturated Calomel Electrode

15. Ⅲ. Silver, Silver Chloride (Ag, AgCl) Electrode The silver/silver chloride reference electrode is composed of a silver wire, sometimes coated with a layer of solid silver chloride, immersed in a solution that is saturated with potassium chloride and silver chloride. Construction: Ag, AgCl electrode consists of Silver wire coated with silver chloride (AgCl) and is fully placed in a 3M KCl solution saturated with AgCl. Working: This electrode is used within the range of -10 to 110 C. The half-reaction will be at the Ag-AgCl electrode. Reduction (GOE): Ag + + e - Ag (s) Oxidation (LOE): Ag (s) Ag + + e -

16. Glass tube Silver- Silver Chloride (Ag, AgCl) Electrode

17. Difference between Indicator and Reference electrode

18. Indicator Electrode Definition: An  electrode that is useful for measuring the potential or pH of a solution is known as an indicator electrode. It is connected to the reference electrode and the potential of indicator electrodes is measured compared to the potential of the reference electrode. There are two types of Indicator electrodes: Metal indicator electrode First-order electrode Second order electrode 2. Ion-selective electrode Glass electrode

19. 1. Ion-Selective Electrodes (ISE) It is also known as Membrane electrodes. Ion-selective electrodes are not based on redox processes but on selective binding of one type of ion to a membrane, which generates a potential. The combination glass pH electrode contains a complete galvanic cell with two electrodes in a single device. Glass membrane is a most common example of ISE . Glass membrane electrode It is a type of ion-selective electrode made of a doped glass membrane (Doping is done to increase the electrical conductivity (and thus resistance is decrease) that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH of any solution. Construction: It consists  of a hard glass tube, one internal electrode (usually Ag wire coated with AgCl), and a reference electrode (usually calomel electrode). A sensing part of the electrode which is a bulb filled with 0.1 M solution of HCl , where transfer of ion occurs through thin and semipermeable membrane. An Ag wire is placed between both the electrodes. 0.1M AgCl solution as internal solution and reference internal solution usually 0.1 M KCl (calomel electrode).

20. Working of the glass membrane potential : For the electrode to become operative, it must be soaked in water. During this process, the outer surface of the membrane is coated with a hydrogel which has Na + . The metal cation ( Na + ) in hydrogel diffuses out of the glass and into the solution, while H + from the solution can diffuse into the hydrogel. H + does not cross through the glass membrane of the pH electrode it is the Na + that crosses and leads to a change in energy. When an ion diffuses from a region of activity to another region of activity there is an energy change and this is what a pH meter measures. It is the hydrogel that makes the pH electrode an ion-selective electrode. If external conc. of H + is high and internal H+ conc is low than, pH<7. If external conc. of H + is low and internal H+ conc is high than, pH>7 . If external conc. of H + is equal to internal H+ conc than, pH=7 . PROPERTIES: 1. Potential not affected by the presence of oxidizing or reducing agents. 2. Operates over a wide pH range. 3. It responds fast and functions well in the physiological system. 4 . Very selective. 5 . Long lifespan.

21. Analyte

22. Metal electrodes Metal electrodes develop an electric potential in response to a redox reaction at the metal surface. Platinum and gold electrodes are very common because they are not very reactive. Their purpose is to transfer electrons to and from the solution. Such electrodes work best when: They have a very large surface area. They are very clean (clean the electrodes with hot 8.0 M HNO 3 and rinse with distilled water)

23. METAL INDICATOR ELECTRODE Metal electrode develops electric potential as a result of redox reaction at its surface. 1. First-order electrode: A first-order electrode involves the metal involved the metal in contact with its ions, such as Ag, silver nitrate (Ag+) or Zn, Zinc chloride (Zn²+), and Copper rod in a copper sulfate solution. Ag + + e - Ag(s) Eº = 0.799 Construction: Electrode is made up of the sample element. Electrode is dipped in a solution containing the sample in ionic form. KCl -filled tube acts as a salt bridge.

24. These are a kind with two interfaces. Such as a metal rod coated with a thin layer of its sparingly soluble salt. The reaction involved on its surface. This category will include silver coated with a thin deposit of silver chloride or mercury with mercurous chloride. Electrode of this type can be used for direct determination of the activity of metal ions or the anion used in coating. E.g.- Chloride ion in Silver chloride. AgCl (s) + e - Ag (s) + Cl - Eº = 0.222V 2. Second-order electrode

25. Limitation Metallic indicator electrodes are not very selective and respond not only to their own cations but also to other more easily reduced cations. Many metal electrodes can be used only in neutral or basic solutions because they dissolve in the presence of acids.

26. METHODS FOR DETERMINING THE ENDPOINT BY POTENTIOMETRIC TITRATIONS It’s a volumetric method in which potential between two electrodes (reference & indicator) is measured as a function of added reagent volume. There are various methods to locate the endpoint or equivalence point. It is generally found graphically (curve). The methods for determining the endpoint by potentiometric titration are as follows: Normal titration curve First derivative curve Second derivative curve

27. 1. Normal titration curve Values of EMF or pH are recorded after the addition of each volume of titrant. The graph is plotted between the volume of titrant added v/s values of pH or EMF . At the endpoint there is a sharp increase in potential or there is a maximum slope of the curve. Generally S-shaped curve is obtained. E g : Weak acid vs strong base (CH 3 COOH Vs NaOH) Normal titration curve of potentiometric titration

28. 2. First derivative curve It indicates the change in pH per ml of titrant addition. Here change in pH for the volume of titrant added is plotted as ordinate (Y-axis) vs the volume of titrant (v) added as abscissa (X-axis) From the shape of the graph, it becomes clear that the maximum change in pH occurs at the equivalence point The end point can be readily recorded by drawing perpendicular from the peak of the graph on the volume axis (X-axis). First derivative titration curve of potentiometric titration

29. 3. Second derivative curve This method uses the second derivative square of the difference in potential or pH of the volume added vs the volume of titrant added is plotted on the graph The endpoint is shown as a zero point where the slope or curve of ∆pH 2 /∆ 2 V is in the middle of the curve . Second derivative titration curve of potentiometric titration

30. Advantages of endpoint detection by potentiometry over normal visual indicator method pH meter is versatile as compared to an indicator method because each indicator has a certain pH range. So same indicator cannot be used for all types of acid-base titrations but the same pH meter can be used for all types of acid-base titration. pH meter does not require any indicator.