Semester –I Chapter 4: Redox Titrations SHREE H. N. SHUKLA INSTITUTE OF PHARMACEUTICAL EDUCATION AND RESEARCH B.PHRAM (SEMESTER –I) SUBJECT NAME: PHARMACEUTICAL ANALYSIS -I SUBJECT CODE: BP102TP CHAPTER 4: REDOX TITRATIONS H.N. Shukla Institute of Pharmaceutical Education and Research Page 1 Semester –I Chapter 4: Redox Titrations Content Redox titrations: (a) Concepts of oxidation and reduction (b) Types of redox titrations (Principles and applications) Cerimetry, Iodimetry, Iodometry, Bromatometry, Dichrometry, Titration with potassium iodate INTRODUCTION Concept of oxidation and reduction As discussed before, in titrimetric analysis we can find out the quantity of pure component based on measurement of volume of standard solution that reacts completely with the analyte. This measurement of standard solution can be possible in different reactions, and if the reaction involved in this measurement is oxidation-reduction reaction, that method is called ns "oxidation reduction titration" or "Redox titration. In Redox titration oxidation & Reduction reaction occurs simultaneously. Oxidation Combination of the substance with oxygen is termed as oxidation. C (s) + O2 (g) CO2 (g) Removal of Hydrogen H2S + O S + H2O Loss of electron(s) is known as oxidation. By loosing electron positive valency of element increases and negative valency of element decreases. Fe2+ Fe3+ + e- Increase in oxidation number Reduction Removal of Oxygen from substance CuO + 2H Cu + H2O Additon of Hydrogen C2H2 + 2H C2H4 Gain of electron, by taking on electron positive valency is decreased and negative valency is increased. Fe3+ + e- Fe2+ Decrease in Oxidation number. H.N. Shukla Institute of Pharmaceutical Education and Research Page 2 Semester –I Chapter 4: Redox Titrations Oxidation-Reduction Reaction Oxidation-reduction reactions are the chemical processes in which a change in the valency of reacting elements or ions takes place. The valency of an element represents the number of electrons which it atoms take on or give up on reacting with other elements to form the compound. Depending on the compound in which element is available, the valency of some elements varies e.g. Iron can be bivalent or trivalent (in FeCl2, FeCl3, respectively), the manganese can have valencies from 2 to 7 (MnO, MnO2, Mn2O3, Mn2O7). Oxidation-reduction reaction is thus a process involving the transfer of electrons from one element or ion to another resulting in the change of the valency of reacting atoms or ions. Oxidizing agents oxidizes reducing agent by accepting their electron and itself get reduced, whereas Reducing agent reduces oxidizing agent by giving up their electron and itself get oxidised . (oxidized) (Reduced) Fe2+ + Ce4+ Fe3+ + Ce3+ (Reducing agent) (Oxidizing agent) Oxidation State/Oxidation Number Oxidation number is positive /zero/negative integer. Comparative +ve oxidation State (O.S) reflects LEO and –ve OS reflects GER. K +1 NaCl 0 Cl -1 Rules for assigning oxidation state The sum of the OS of all the atoms in a molecule/ion must be equal in sign and value to the charge on that molecule or ion. + 2- H2SO4 2H + SO4 {S +6, O 4 (-2) = -8} (zero) 2 X (+1) (-2) Certain elements assume the same oxidation state in different compounds. Halogens (F, Cl Br, I) = -1 Alkali Metals (Li, Na, K) = +1 Alkali earth metals (ca, Mg, Ba, Be, Sr, Ra) = +2 Oxgen is having -2 OS in general. But if in the form of Hydrogen peroxide oxygen has -1 OS. Many elements (specially nonmetals) can assume a variety of oxidation state. For e.g. NH3 = X x (+1)x 3 = 0 HNO3 = +1 x X x (-2)x 3 = 0 X = -3 X = -5 H.N. Shukla Institute of Pharmaceutical Education and Research Page 3 Semester –I Chapter 4: Redox Titrations Half Reactions: As seen in acid-base reactions, an acid is defined as proton donor and a base as a proton acceptor. The acid-base properties of a conjugate pair are not possible in absence of a second conjugate pair, Acid-base reaction is thus a transfer of a proton from one conjugate pair to another. The redox reactions have similar situations. In these reactions also two half reactions must be involved, each half reaction includes a redox conjugate pair and the net result of redox reaction will be transfer of one or more electrons from one pair to the other. General redox half reaction can be written as – oxidation Reducing agent Oxidizing agent Reduction In other words, it is not possible to observe a redox half reaction. Two half redox reactions are required, one to liberate electrons and one to accept it. 2+ 3+ - Fe Fe + e - - 2I I2 + 2e + H2 2H + 2e REDOX POTENTIAL It can be calculated by measuring the potential difference of a cell in which oxidation reduction half cell is coupled with standard reference cell, i.e. standard hydrogen electrode. Oxidising agents gain electrons and get reduced while reducing agents lose electrons and get oxidised. This transfer of electrons leads to the changes in the valency of the atoms or ions. The positive valency of oxidised atom or ion is increased while that of reduced atom or ion is decreased. Oxidising and reducing agents may differ in strength i.e. chemical activity. Strong oxidising agents have a pronounced tendency to accept/gain electrons and hence, they are having ability to take up the electrons from many reducing agents even relatively weak one. Weak oxidising agents have a much less pronounced tendency to gain electrons i.e. they can oxidise only strong reducing agents. The direction of a redox reaction can be predicted provided some quantitative characteristic of the relative force involved is known. This characteristic is known as the 'Redox Potential. H.N. Shukla Institute of Pharmaceutical Education and Research Page 4 Semester –I Chapter 4: Redox Titrations It is possible to measure the potential difference between two systems by connecting them into a galvanic cell. Any galvanic element consists of two half elements. Each of which is oxidation-reduction couple i.e. a system consisting of the oxidised and the reduced form of the chemical element or ion. The more powerful the oxidant of the pair, the weaker its reductant should be and vice versa; if Cl2, is said to be a powerful oxidising agent, this means its atoms possess the pronounced ability to accept electrons, changing to Cl-. In other words, Cl- should keep a strong hold on these electrons i. e. should be a weak reducing agent. One never comes across an absolutely pure oxidising or reducing agent. Their solutions always contain the products of their reduction or oxidation respectively. Reaction: At Zn anode, oxidation takes place (the metal loses electrons). This is represented in the following oxidation half-reaction. 2+ - Zn(s) Zn + 2e At the Cu cathode, reaction takes place (electrons are accepted). This is represented in the following reduction half-reaction. 2+ - Cu + 2e Cu(s) Combined reaction: Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s) Equivalent weight of oxidizing and reducing agent In a redox reaction, one of the reacting entities is oxidizing agent and the other entity is reducing agent. There are two methods to calculate equivalent weight in redox reaction. H.N. Shukla Institute of Pharmaceutical Education and Research Page 5 Semester –I Chapter 4: Redox Titrations 1. The number of electrons involved in the reaction. (Ion-Electron Balance Method) 2. The change in the oxidation number of significant element in the oxidation or reductant. (Oxidation Number Method) 1. Ion-Electron Balance Method The oxidizer is recipient of electrons, whereas reducer is releaser of electrons. The number of electrons transferred from one entity to another to balance the redox recation. So equivalent weight is calculated. 푴풐풍. 푾풆풊품풉풕 Equivalent weight of OA = 푵풖풎풃풆풓 풐풇 풆풍풆풄풕풓풐풏풔 품풂풊풏풆풅 풃풚 풐풏풆 풎풐풍풆풄풖풍풆 For example- Ex.1 Potassium permanganate in acidic condition a strong oxidizer It means it gains five electrons during redox rection. - + - 2+ MnO4 + 8H + 5e Mn + 4H2O 158 Equivalent weight of KMnO4 = = 31.6 gm 5 Ex.2 Potassium permanganate in neutral condition gives following rection. - + - MnO4 + 4H + 3e MnO2 + 2H2O 158 Equivalent weight of KMnO4 = = 52.66 gm 3 Ex.3 Potassium dichromate in acidic condition a strong oxidizer It means it gains six electrons during redox rection. Potassium dichromate in acidic solution results in: + - + + K2Cr2O7 + 14H + 6e 2K + 2Cr3 + 7H2O 294.26 Equivalent weight of K2Cr2O7 = = 49 gm 6 푴풐풍. 푾풆풊품풉풕 Equivalent weight of RA = 푵풖풎풃풆풓 풐풇 풆풍풆풄풕풓풐풏풔 풍풐풔풕 풃풚 풐풏풆 풎풐풍풆풄풖풍풆 For example: Ex.1 Redox reaction of ferrous sulphate; ferrous (Fe2+) ions lose its electron during redox rection. Fe2+ Fe3+ + e- Equivalent weight of Ferrous Sulphate = 278/1 = 278 gm FeSO4 H.N. Shukla Institute of Pharmaceutical Education and Research Page 6 Semester –I Chapter 4: Redox Titrations 2. Oxidation Number Method In Redox Titration, Equivalent weight is also calculated by taking the change in valency or oxidation no. of oxidizing or reducing agent during redox titration. Valency of an element represents the no. of electron, which its atom takes on or gives on to reacting element to form compound. 푴풐풍. 푾풆풊품풉풕 Equivalent weight of OA/RA = 푪풉풂풏품풆 풊풏 풐풙풊풅풂풕풊풐풏 푵풖풎풃풆풓 풑풆풓 풎풐풍풆 Ex.1 Potassium permanganate in acidic condition a strong oxidixzer. It is reduced and its oxidation number is reduced from +7 to +2. Therefore change in oxidation number is 5. - + 2+ MnO4 + 8H + 5e- Mn + 4H2O (Change in O.N. = 5) O.N= +7 O.N= +2 158 Equivalent weight of KMnO4 = = 31.6 gm 5 Ex.2 Redox reaction of ferrous sulphate; ferrous (Fe2+) ions converted into ferric ion in which oxidation number increases by 1. 2+ 3+ - Fe Fe + e (Change in O.N. = 1) O.N= +2 O.N= +3 Equivalent weight of Ferrous Sulphate = 278/1 = 278 gm FeSO4 Thus it indicates that, the valence factor for either an oxidizing or reducing agent is equal to the numbers of electron transferred from one entity to another.