Redox Titration Notes - Ma'am Nabeela PDF

# Presentations: - Gaussian distribution - Tests for significance - Least square method - Outliers - Analysis of variance ANOVA. # Redox Titration - Titration involving $K_2Cr_2O_7$ and $KMnO_4$ are termed as oxidation - reduction titrations because oxygen evolved by treating $K_2Cr_2O_7$ or $KMnO_...

# Presentations: - Gaussian distribution - Tests for significance - Least square method - Outliers - Analysis of variance ANOVA. # Redox Titration - Titration involving $K_2Cr_2O_7$ and $KMnO_4$ are termed as oxidation - reduction titrations because oxygen evolved by treating $K_2Cr_2O_7$ or $KMnO_4$ with dil. $H_2SO_4$ is utilized for oxidation of substance. As oxidation and production go on side by side, so the titrations are termed as oxidation - reduction or Redox titrations. # Oxidation Redox reaction - Involve changes in the oxidation number or state of oxidation of the reacting substances. Electrons are transferred from the substance undergoing oxidation to that undergoing reduction. - There are two components to a redox reaction. Each is known as a half reaction and involves a transfer of electrons. - The half rex. that involves a gain of $e^-$ is called a reduction reaction. The half rex. that involves a loss of $e^-$ is called oxidation reaction. These two rex. always occur simultaneously. - If the substance gains $e^-$, substance is reduced, and it caused oxidation to the other reacting species. - $MnO_4^- + 8H + 5e → Mn^2+ + 4H_2O$ - (permanganate ion) (manganese ion) - If the substance loses $e^-$, substance is oxidized and caused reduction to the other reacting species. - $Fe^2+ → Fe^3+ + 1e$ - (Ferrous ion) (Ferric ion) - $Fe^2+ + Ce^4+ → Fe^3+ + Ce^3+$ - (Ferrous ion) (Cerric ion) (Ferric ion) (Cerrus ion) - $2CaO → 2Ca^2+ + 2O^{2-}$ - $2Cl^- → Cl_2 + 2e$ (reduction rex) - $Ca$ acts as reducing agent bcoz it donate $e^-$ to $O$ and causing $O$ to be reduced. - The $O$ act as oxidizing agent since accepts $e^-$ from the $Ca$, causing $Ca$ to be oxidized. # Oxidation Number - Oxidation number refers to the number of charges an atom would have in a molecule (or on ionic compound) if $e^-$ were transferred completely. - Oxidation can be defined as an increase in oxidation number while Reduction can be defined as decrease in oxidation number. # Important Rules for Assigning Oxidation Number 1. In free element (uncombined state), each atom has oxidation number zero. 2. For ion composed of only one atom oxidation number is equal to charge on the ion. - Alkali metal ion = oxi. no = +1 - Alkaline earth metals = oxi no = +2 - Al = oxi no = +3 3. Oxi no of $O$ in all = -2 but in peroxide = -1. 4. $F$ always = -1, no other halogens may have +1, except when bonded to metal , $H$ = +1. 5. Oxi. no of $Na$ = +1. 6. In neutral molecule the sum of oxi. no of molecules must be zero. In polyatomic ion the sum of oxi. no (in ion) = to the net charge of the ion. - Example: $MgO$ - Oxi No of $Mg$ = +2 - Oxi no of $O$ = -2 - Oxidation half rex: $2Mg → 2Mg^2+ + 4e$ - Reduction half rex: $O_2 + 4e → 2O^2-$ - Explanation: $O → O^2-$ and $Mg → Mg^2+ 2e$ # Balancing Oxi-Red. Equations - Can be balanced by the typical balancing steps of normal equations. - Ion-electron method can be used. - In this method two half rea, Oxi-rex and red. half rex are balanced separately, and then added together to given an overall balanced equation. # Redox Indicators - Redox Titration: The titrations which incorporate the processes of oxidation and reduction are known as redox titration. - Several methods are used for detection of end-point in redox titrations: - Internal redox indicators. - External indicators. - The reagent may serve as its own indicator (Self-indicator). - Potentiometric method. # What are the Internal Indicators? - Redox indicators are organic compounds which have different colors in the oxidized and reduced forms. - Redox indic- possess the property of being coloured M oxidized form and colourless in the reduced form or stale as " leuco compounds". - They change colour when a certain emf is reached in solution while an oxidizing agent is oxidizing a species. - The general indicator rea. may be written as. $In_{ox} + ne → In_{red}$ - The colour of the soln depends on the ratio $(In_{ox}) / (In_{red})$. - Where - $In$ = Indicator. - $e$ = electrons. - $n$ = usually 1-2. - Examples of redux indicators: - Diphenyl amine - Diphenyl benzidine - Methylene blue - Diphenyl sulphonic acid - $KMnO_4$ (in $K_2Cr_2O_7$) - Ferroin (Ferrous phenanthroline) - Methy red - Methyl orange # Redox Indicators: - Two Types: - General Red. Indicators - Specific Red. Indicators # General Red. Indicators: - General redox indicators are substances that change colour on being oxidized or reduced. # Specific Indicator: - These indicators, the change of color in analyte and titrant, and depend on the independent of the chemical nature of the analyte and titrant and depends on the changes in electrode potential of the system that occurs as the titration progresses. - The best known specific indicator in complex form is a dark blue complex starch which forms showing end pt in titration in which iodine either consumed or produced. # Example: - Potassium thiocyanate end pt. Involve disappearance of red. color of iron. # Internal Indicators - Oxidized form of these subs. has different colors from the reduced form. - Types - Reversible Internal Indicators. - Irreversible Internal Indicators. # Reversible Internal Indicators: 1. Ferroin (Ferrous phenanthroline) 2. Methyl red 3. Methyl Orange # Ferroin: - Ferroin is intense red-coloured coordination complex formed by combination of the base orthophenanthroline with 3 ferrous ions. - The molecular ratio is 3 base : 1 ferrous. - This complex is reversibly oxidized to the corresponding phenanthroline ferrous ion complex called "Ferrin" which is pale blue in color. - The complex is used as an indicator in the titration of ferrous ions by cerric sulphate. $(Phen)_3Fe^2+ → (Phen)_3Fe^3+ + e$ # Self redox Indicators: - The coloured standard when used in the titration, can serve as its own indicator. - Example: $KMnO_4$ , Cerric sulphate jodime solution. # Q: Why is $K_2Cr_2O_7$ preferred to $KMnO_4$ in redox titration? - $K_2Cr_2O_7$ is a primary standard substance and its standard soln can be easily prepared directly by weighing, while $KMnO_4$ is secondary subs. so its standard soln cannot be prepared easily hence $K_2Cr_2O_7$ is preferred to $KMnO_4$ solution. - Acidified $KMnO_4$ is a stronger oxidizing agent than acidified $K_2Cr_2O_7$. # Q: Can $K_2Cr_2O_7$ be wed as self-indicator? - No, because in the reaction, $Cr_2(SO_4)_3$ is produced which is green. So at the end point, the yellow colour of $K_2Cr_2O_7$ cannot be distinctive. # Q: Why $KMnO_4$ in not a primary standard? - $KMnO_4$ is not a primary standard. 1. It cannot be obtained in a pure state. 2. It undergoes decomposition in the presence of impurities to $MnO_2$ on standing. - Hence, it is standardized with the soln. of a suitable secondary standard such as oxalic acid, sodium oxalate etc. # Stability of permanganate solution: - A soln of $KMnO_4$ once properly prepared is quite stable as long as protected from dust, excess of reducing gases and direct sunlight or other strong light sources. It should never come in contact with organic matter such as rubber, paper etc. # Equivalent weight: - Eq. wt. of an oxidant or reductant is the molecular weight divide by the number of electrons 1 mol. substance gains or losses in the reaction. - Six Sigma concept.