Pharmaceutical Analysis Unit 4 Notes PDF

# Pharmaceutical Analysis ## Unit 4 Notes - Redox Titration ## Redox Titration - Redox titrations are those titrations that are based on the Redox Reactions. - Redox = Reduction + Oxidation. - Redox reactions are those reactions in which both oxidation and reduction reactions take place. - We ca...

# Pharmaceutical Analysis ## Unit 4 Notes - Redox Titration ## Redox Titration - Redox titrations are those titrations that are based on the Redox Reactions. - Redox = Reduction + Oxidation. - Redox reactions are those reactions in which both oxidation and reduction reactions take place. - We can also say Redox reactions are those in which transfers of electrons takes place between Analyte and Titrant. ### Oxidation Oxidation can be defined as three ways. - Addition of Oxygen - Loss of Hydrogen - Loss of electrons **Examples:** - In terms of oxygen: $SO_2 + O \rightarrow SO_3$ - In terms of Hydrogen: $H_2S + Cl_2 \rightarrow S + 2HCl$ - In terms of electrons: $Na \rightarrow Na^+ + e^-$ - Oxidation can be remembered by LEO (Loss of electrons) ### Reduction Reduction is just opposite to oxidation, it can be defined as: - Removal of Oxygen - Gain of Hydrogen - Gain of Electrons **Example:** - In terms of oxygen: $CuO + 2H \rightarrow Cu + H_2O$ - In terms of hydrogen: $C_2H_2 + 2H \rightarrow C_2H_4$ - In terms of electrons: $Cu^{2+} + e^- \rightarrow Cu^+$ - Reduction can be remembered by GEO (Gain of electrons) ## Redox Reactions - Redox reactions are those in which both oxidation and Reduction takes place. - In redox reactions one substance loses or gives up the electrons and other substance receives the electrons - The first substance is oxidized and other is reduced. | | | |---|---| | $Zn + Cu^{2+}$ | $Zn^{2+} + Cu$ | | Oxidation (loses e-) | Reduction (Gain e-) | ## Oxidizing Agents - Oxidizing Agents are those substances which do oxidation of others and itself gets reduced. - We can simply say substance which gain electrons are called as Oxidizing Agents. ## Reducing Agents - Reducing Agents are those substances which do reduction of others and itself gets oxidized. - We can simply say substance which loses or donates electrons are called as reducing agents. | | | |---|---| | $Zn + Cu^{2+}$ | $Zn^{2+} + Cu$ | | Reducing agent | Oxidizing Agent | ## Redox Indicators - Indicators that are used in Redox Titration are known as Redox Indicators. - They are also known as Oxidation-Reduction Indicator. - These are the substances that show visible colour change at the end point of the redox titration. - They show different colours in their oxidized and reduced form. ## Types of Redox Indicators Redox indicators can be generally classified into three categories: 1. Self Indicator 2. External Indicator 3. Internal Indicator ### Self Indicator - Self Indicators are those in which titrant itself act as an indicator. - In this we don't use any external indicator. - Self Indicators are involved in the titration as titrant and at the end point they themselves changes colour and indicating the completion of reaction. - Example: $KMnO_4$ (potassium permanganate) is the best example of self Indicator, at the end point of the titration it shows pink colour. ### External Indicator - External Indicators as the name says, they are not added to the titration, they are kept outside. - Titrated solution is taken and added dropwise time to time in the external indicator and the point at which it changes its colour simply determined by end point. - Example: Potassium Ferricyanide ### Internal Indicators - These are the actual Redox Indicators. - These are the indicators which have different colours in their Oxidized and reduced forms. - They are added in the analyte solution and shows visible colour change at the end point. - Internal indicators are generally colourless in reduced form. | **INDICATOR NAME** | **OXIDIZED** | **REDUCED** | |---|---|---| | Ferroin | Pale - Blue | Red | | Diphenylamine | Violet/Blue | Colourless | | Methylene Blue | Blue | Colourless | | Starch - Iodine | Blue | Colourless | | Nitroferroin | Pale- Blue | Red | ## Types of Redox Titration Based on the Titrant used redox titration can be classified into following categories: - Cerimetry - Iodimetry - Iodometry - Bromatometry - Dichrometry - Titration with Potassium Iodate ### Cerimetry | | | |---|---| | Titrant: | Cerric Ammonium Sulphate | | Analyte: | $Fe^{2+}$, $Cu^{2+}$ | | Indicator: | Ferroin | ### Bromatometry | | | |---|---| | Titrant: | Potassium Bromate | | Analyte: | $SO_3$, $As$ | | Indicator: | Methyl Red | ### Iodimetry | | | |---|---| | Titrant: | Iodine Solution | | Analyte: | Sodium thiosulphate | | Indicator: | Starch solution | ### Dichrometry | | | |---|---| | Titrant: | Potassium dichromate | | Analyte: | Ion salts ($FeSO_4$) | | Indicator: | Sodium diphenylamine sulphate | ### Iodometry | | | |---|---| | Titrant: | Sodium thiosulphate | | Analyte: | Iodine (Prepared) | | Indicator: | Starch Solution | ### Titration with Potassium Iodate | | | |---|---| | Titrant: | Potassium Iodate | | Analyte: | Vitamin-C, $CuSO_4$ | | Indicator: | Starch - Iodine solution | ## Iodimetry & Iodometry - Titration that involves iodine are reffered as iodine titration. - Iodine acts as as a Mild or Weak Oxidizing Agent. - Two types of iodine titrations are possible : 1. Iodimetry 2. Iodometry ### Iodimetry - Iodimetry is a type of Direct Titration. - It is a type of redox titration. - These are the titration in which free iodine is used. - The titration in which standard iodine solution is used for the determination of reducing agents like Sodium Thiosulphate ($Na_2S_2O_3$) is known as Iodimetry. - End Point of the titration is determined by change in colours from Blue to Colourless. **Methodology** - First iodine is mixed with KI because generally it is not easily soluble in water - $KI + I_2 \rightarrow KI_3$ - Now we use starch solution as an indicator, iodine will reacts with starch and forms Iodine - Starch complex which appears blue in colour. - $I_2 + Starch \rightarrow Blue \ Colour$ - Now this standard iodine solution is titrated with reducing agent ($Na_2S_2O_3$) - $I_2 + 2S_2O_3^{2-} \rightarrow S_4O_6^{2-} + 2I^-$ - Now at the endpoint all iodine reacts with thiosulphate ions and converted into iodide and blue colour disappears. - $I^- + Starch \ indicator \rightarrow No \ Colour$ ### Iodometry - Iodometry is a type of indirect titration. - It is a type of redox titration - These are the titration in which liberated iodine is used. - When a standard solution of sodium thiocynate ($Na_2S_2O_3$) is used for the determination of liberated iodine using starch solution as an indicator then it is known as Iodometry. - Iodometry is actually completed in two steps: **STEP-I** - The first step is done by the reaction between the oxidizing agent ($K_2Cr_2O_7$) and excess KI and as a result of the reaction iodine gets liberated. - $K_2Cr_2O_7 + 6KI + 7H_2SO_4 \rightarrow (Cr_2(SO_4)_3 + 4K_2SO_4 + 3I_2 + 7H_2O$ **STEP-Ⅱ** - In step - II: Sta first starch solution is added in the liberated iodine which form blue colour - $I_2 + Starch \rightarrow blue \ colour$ - Now we use standard sodium thiocynate solution which reacts with liberated iodine in the solution - $I_2 + 2S_2O_3^{2-} \rightarrow S_4O_6^{2-} + 2I^-$ - Now at the end point all the liberated Iodine reacts with thiosulphate and produce iodide ion, the Indicators don't show any reaction with iodide ions, hence blue colour of solution disappears (colourless) - $I^- + Indicator \rightarrow Colourless$ ## Major difference between Iodimetry and Iodometry | **IODIMETRY** | **IODOMETRY** | |---|---| | - Iodine is taken directly in the analysis | - Iodine is produced as a result of Redox Reaction | | - Direct Titration | - Indirect Titration | | - Only 1 Redox Reaction | - Two Redox Reaction | | - Iodine only gets reduced | - Iodine first get oxidized then get reduced | | - Less Common Method | - More common method |