Analytical Chemistry I PCC 111 Lecture Notes PDF

LECTURE I Analytical Chemistry I PCC 111 Dr Marwa ibrahim Lecture content  Analytical Chemistry is divided into: Qualitative= Quantitative= Quality, Quantity, Identification Concentration  Methods used in Quantitative Analysis :...

LECTURE I Analytical Chemistry I PCC 111 Dr Marwa ibrahim Lecture content  Analytical Chemistry is divided into: Qualitative= Quantitative= Quality, Quantity, Identification Concentration  Methods used in Quantitative Analysis : 2- Gravimetric 3- Instrumental 1- Volumetric Analysis= Analysis= Analysis= Converting the sample to Using certain Titration an insoluble form instruments Types of Reactions used in Quantitative Analysis: I- Ionic Combination Reactions. II- Electron Transfer Reactions.  Requirements of Titrimetric Analysis: 1- The reaction between the sample and the standard must be simple and can be expressed by a chemical equation. 2- The reaction should be single, i.e.: with no side reactions. 3- A suitable standard solution must be available as a titrant for the sample. 4- The end point of the reaction must be easily detected. 5- The reaction must be rapid or a catalyst is used.  I- Ionic Combination Reactions: 1- Neutralization Reactions: H+ + OH- → H2O (slightly ionized) 2- Precipitation Reactions: Ag+ + Cl- → AgCl↓ (slightly soluble) Ba2+ + SO42- → BaSO4↓ (slightly soluble) 3- Complexation Reactions: Ag+ + 2CN- → [Ag(CN)2-] (slightly ionized) II- Electron Transfer Reactions: Ce4+ + Fe2+ → Ce3+ + Fe3+ Involve transfer of electron Reduction – Oxidation reaction To understand the concept of the titrimetric analysis, some definitions are needed:  NaOH + HCl ➝ NaCl + H2O Equivalence Factor (F)  Funnel 1- Transfer 10 ml sample Bulb Pipette 2- Add drops of Titrant indicator 3- Fill the burette with the standard sol. Standard Solution Known conc 4- Titrate till the endpoint Burette 5- Record the volume of the titrant Endpoint Sample Conical flask Indicator Unknown conc Titration It is the process of measuring the volume of titrant needed to reach the end point. Sample It is the solution of unknown concentration. Standard It is the solution of known concentration.  Titrant It is the standard solution used in titration. Equivalence point It is the point at which the reaction between the sample and the standard is complete. End point It is the point at which the indicator changes its color. Indicator It is a chemical substance that responds to the end point by changing colors.  Types of Standard Solutions: 1- Impirical 2-Molar Solutions: 3-Normal Solutions: Solutions: 1M= 1N= Used for specific 1M.Wt(gms)/L 1Eq.Wt(gms)/L samples  Equivalent Weight of Acids: Eq.wt = Molecular weight / Number of replaceable H+ Examples: Calculating Eq.wt of HCl = M.wt / 1 the Eq.wt of H2SO4 = M.wt / 2 Equivalent Equivalent Weight of Bases: Weights: Eq.wt = Molecular weight / Number of replaceable OH- Examples: Eq.wt of NaOH = M.wt / 1 Eq.wt of Ba(OH)2 = M.wt / 2 Types of Chemical Standards: 1- Primary Standards 2- Secondary Standards  Easily obtained in a very high grade of purity and of known composition. Easily tested for impurities. They are substances of definitely known composition and high Stable; non volatile, purity. non hygroscopic, non efflorescent. 1- Primary Primary Standard should have the following Reacts Standards: characteristics: stoichiometrically; in a quantitative way according to a balanced chemical Examples: equation. Readily soluble in solvents. Potassium acid phthalate, benzoic acid, anhydrous sodium carbonate Has high equivalent and constant boiling weight to minimize point hydrochloric weighing errors. acid.  2- Secondary Standards: They are substances which do not fulfill all the previous requirements and whose content of the active Examples Oxalic acid and borax. substance can be found by comparison against primary standards. Preparation of Standard Solutions: 1- Direct method: 2- Indirect method: Involves 1step; Involves 2 steps; preparation. preparation and Used for primary standardization. standards. Used for secondary standards. Acid - Base Titration in Aqueous Medium  Electrolytic Dissociation Theory: When an electrolyte is dissolved in water, it dissociates into positively charged fragments "Cations" and negatively charged fragments "Anions": For each concentration there is a state of a reversible equilibrium between un- dissociated molecules and ions: Molecules ⇋ Cations + Anions Degree of Dissociation: Arrhenius introduced a quantity "ɑ" called the degree of dissociation ɑ = Number of dissociated solute molecules Total number of solute molecules before dissociation  According to the value of "ɑ", electrolytes can be classified into: 1- Strong Electrolytes: If the value of ɑ is unity or near to unity. Examples: HCl ɑ = 0.92 NaOH ɑ = 0.91 NaCl ɑ = 0.86 2- Weak Electrolytes: If the value of ɑ is far from unity. Examples: H3BO3 ɑ = 0.001 NH4OH ɑ = 0.013 HgCl ɑ = 0.01  Acid-Base Theories: 1- Arrhenius Theory 2- Bronsted-Lowry 3- Lewis Theory Theory  1- Arrhenius Theory: An acid: any substance ionizes to give H+ A base: any substance ionizes to give OH- The theory suffers several points of weakness: 1- It didn't discus the role of solvent (water) in the ionization process. 2- Some compounds contain no OH-, and are more basic than NaOH.  2- Bronsted-Lowry Theory: An acid: substance that donates H+ A base : substance that accepts H+ When the acid gives the proton, the remaining species has a certain proton affinity and hence is a base. This base is known as the conjugate base of the acid: Acid ⇋ Proton + Conjugate Base  2- Bronsted-Lowry Theory: Exemples: HCl ⇋ H+ + Cl- HAc ⇋ H+ + Ac- H2O ⇋ H+ + OH- The stronger the acid, the weaker its conjugate base. The solvent, in this theory, is involved in the reaction as an acid or a base( water has Amphoteric characters ): HCl + H2O ⇋ Cl- + H3O+ (acid) (base) NH3 + H2O ⇋ NH4+ + OH- (base) (acid) 3- Lewis Theory: A base : substance that donates electrons, it contains an atom with unshared electron pair e.g.: N, O, P, S. eg:NH3. An acid: substance that accepts electrons. eg:BCl3 The End