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UNIT II, , , , , , Chapter a 2, , ACID BASE TITRATIONS, a, , ¢ LEARNING OBJECTIVES ¢, , © To describe, by using chemical equations and equilibrium constants, the chemical change(s), that occur during a strong acid/strong base titration., , © ‘To identify the equivalence point in an acid-base titra, , e To describe why the pH at the, generally equal to 7., , © To describe why the pH at the equivalence point for a weak acid/strong base (or strong, acid/weak base) titration is generally not equal to 7., , —_—-——_—_—_—$—S, , | OUTCOMES, , On satisfying the requirements of this course, students will have the knowledge and skills, to:, , , , , , tion from the pH titration curve., quivalence point for a strong acid/strong base titration is, , * Sufficient indepth knowledge of acid base concepts is acquired, so that the theory of acid, . _ base titrations can be developed, understood and applied. They should be able to, «differentiate between acid and base; identify their strength., ve Quantitative analysis of the concentration of an unknown acid or base solution. It makes, , use of the neutralization reaction that occurs between acids and bases,, , 2.1 INTRODUCTION) 205, , An acid-base titration (or neutralization titration) is based upon the chemical reaction, between an acid and a base; also known as neutralization reaction., , H* + OH «> H,0, (From acid) (From Base) (Neutralization of Ions), , Acidimetry: A standard acid solution is used for quantitative determination of the free, bases or those formed from salts of weak acid by hydrolysis., , Alkalimetry: A standard base or alkali solution is used for quantitative determination of, the acid or those formed from salts of weak base by hydrolysis., , This acid base titration method is used for various volumetric determinations, which, involves neutralization in one or another ways., , , , , , , , , , , , , , , , , , (2.1)

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=, , Pharmaceutical Analysis 2.2 Acid Base Tite, , Example: Determination salts like: NazCO3, Na2B,O; etc., , These salts have strongly alkaline reaction, that's why they are titrated with acid, Advantages of Acid Base titrations:, , * The reaction between acid-base is quite fast and practically instantaneous,, , * The reaction in acid base titrations will be single without the side reaction(s)., , * The reaction will always reach to the completion., , * The reaction will always be stoichiometric., , (Stoichiometric: The amount of the reactant and those of the products are as per cher, equation presenting same), , Various theories have been proposed to explain or classify acidity and basic properti:, substances. Most familiar theory is the Arrhenius theory, but it is applicable only to wat), solvent. Rest other theories are more generalized and there applicability was better. M, five theories are proposed for acid base titrations i.e., , 1, Arrhenius theory, , 2. Bronsted Lowry theory, , 3. Lewis theory, , 4. Usanovich theory, , 5. Lux flood theory, , ‘2.2 THEORIES OF ACID BASE TITRATIONS ., 1. Arrhenius Theory (H* and OH):, , Soluti, , , , , , Arrhenius, as a graduate student, introduced a radical theory in 1894 (for whict, received the Nobel prize). According to him an acid is any substance that ionizes (partial, , completely) in water to give hydrogen ions (which associates with the solvent to, hydronium ions, H;0*)., , HA + H2O <—>H30° + AT, HCl + HzO <— H* +Cl (Aqueous medium), H2SOq + H20 <>» 2H” + SO, * (Aqueous medium), HNOs + HzO <—» H* +NO;" (Aqueous medium), CH3COOHI + H20 <—> H* +CH3COO7 (Aqueous medium), , A Base ionizes in water to give hydroxyl ions. Weak (partially ionized) bases gene, ionize as follows:, , B + H,0 «> BH’* + OH”, NaOH + H,O <—> Na* +OH™ (Aqueous medium), NH,OH + HO «—» NH4* +OH" (Aqueous medium), KOH + H20 ¢— K* +OH™ (Aqueous medium), While strong bases such as metal hydroxide dissociate as, M (OH), <— M™ + nOH™, Ba(OH), + HO <> Ba’* + 20H” (Aqueous medium)

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pharmaceutical Analysis 2.3 Acid Base Titrations, , This theory is restricted to water as solvent; because hydrogen and hydroxide ion are, compulsory to define this theory., Nature of the water: Nature of the water is neutral as per the Arrhenius concept and, , acts as a solvent., Neutralization Reaction: Acid and base will react with each other and neutralization, reaction takes place; finally they will form a salt and water molecule after completion of the, , reaction., Example: HCl + NaOH e+ NaCl + H,O (Aqueous medium), , (Acid) (Base) (Salt) (Water), Advantage : This concept explain the behaviour of and base practically., , Limitations:, ¢ Inthis concept acid and base is defined in the aqueous medium only., , e This concept fails to explain the stability of the H” ion(s)., e It does not explain the conjugate acid-base theory., * It does not able to define those acids or base which does not contains H" or OH” ions, , like SO, CO? etc., 2. Bronsted-Lowry Theory (Taking and giving protons):, In 1923, Bronsted and Lowry separately described a theory known as the Bronsted-Lowry, , theory. This theory states that:, Acids are those compounds or species, which have the tendency to donate the proton(s), , (H’) in any type of solvent by any method., Example: H2SOz, HCI, HF, HClO4, CH;\COOH, H2CO3, HSO3, HCO3, NH; etc., Bases are those compounds or species, which have the tendency to accept the proton(s), (H’) in any type of solvent by any method, Example: NaOH, KOH, Ba(OH)2, Ca(OH)2,NHs, OH”, Cl, Br. TF, HSO3, HCO; etc., Conjugate Acid-Base Concept: Always a pair conjugate acid and base will be present., , HCl + HCO; =—— CI+ H,CO,, (Acid) (Base) (Base) (Acid), , HCl: = H+ GIF, Examples of some conjugate acid base pairs are given below:, HCl — Clr, H2SO4 -_- HSO,, HSOq” <9 SO,, H,CO; <> HCO;”, HCO" > CO;*"

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sometimes water acts as an acid sometimes, , , , , Nature of the water: Nature of the water is amphiprotic or amphoteric in nature,, sometimes water acts as an acid sometimes water acts as a base., Advantages: : :, * This concept can explain the acid base in any type of solvent,, ¢ This concept is able to explain the stability of the proton., Limitation:, * This concept is not able to define acid or base, where, SO; etc., 3. Lewis Theory (Taking and giving electrons):, In 1923, G. N. Lewis also introduced the electronic theory of acids and bases, In t, concept he has defined acid and base according to the electron pair donor acceptor conce,, Bases are those species which have self tendency to donate the lone pair of electrons ., , proton(s) are absent, ie. 5, , Lewis bases are nucleophiles., , Example: NHs, RNH2, CsHsNH>, HO, SO;,, CI, Br°, I, F-, CN- ete., Acids are those species which, Lewis acids are electrophiles., Example: AICI3, BCls, SnClz, CO2, SO», H*, Ag’, Al** etc., So in the Lewis theory, a base is a substance that can donate an electron pair and aci, @ substance that can accept an electron pair. Base, frequently contains oxygen or nitrogen, the electron donor. Thus, non-hydrogen containing substances are introduced as acids. jor, AICl; + : O-R; > CI3Al: OR2, , Here, Aluminium Chloride is an acid and ether a base. This theory is useful to describe, indicator colour change in non-protonic systems ex, , have self tendency to accept the lone pair of electrons a, , pina, , 2, , tt, , hibiting acid-base reaction. Al, , Nature of the water: Water is a Lewis base. Br, , Advantages: Th, e, , Defines acid and base without any type of solvent., * Explained acid and base, which can not be defined by others., 4. Usanovich Theory (Cation and anion donor acceptor):, In the year 1934, Usanovich Modified the Lewis concept of acid and base by removing ee, limitation of either acceptance or donation of the electron pair in a more acceptable, , According to this theory “Acid is a chemical species that reacts with a base and gives cati, Or accepts anions or electrons"., , Examples: HCI, H,SOs, H’, Ag”, Al**, Fe?*, Ce’ ate., , “Base 's a chemical species that reacts with an acid and gives anions or electrons)., combines with cations’, i, , Examples: NaOH, NH, Cl,.Br7,.T; F-,, , Sele, , =, , Fe**, Ce* etc, nom, , ea Ne, ;, As per this concept oxidizing agents are acids and reducing agents are bases. te, Fe*" ease) <> Fe "(acicy + 7, 3+ =, Fem lacigy +O” y Fe?* ase) A, , Nature of the water: Nature of the wat, , €r Is amphiprotic or amphoteric in nature, m, Water acts as a base.

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2602, Pharmaceutical Analysis 25 Acid Base Titrations, , 5, Lux Flood Concept (Oxide-ion acceptor-donor), Lux (1929) and supported by Flood (1947) first introduced the concept of acid-base, reactions with respect to the oxide ion., According to this theory, bases are those species which can donate the oxide-ions., CaO <> Ca** + 07, MgO <— Mg”* + 07, PbO <> Pb* + OF, Al,03 <> 2AI** + 307, And,, Acids are those species which can accept the oxide-ions., , SO; + 0 —>S0;", SiO, + OF <4 SiO,, CO, + 0* —5cOr, Some Examples of acid and bases related to Lux Flood concept., MgO ase) + SiO2 acig) > MgSiO3, CaO (ease) + SO3 (acigy <> CaSOy, As per Lux Flood concept ZnO is amphoteric in nature., Limitations: Bases must contain oxide ion and acids must be able to accept the oxide, ion., Summary of Acid-Base Theories:, , , , , , , , , , , , , , , , , , , , , , , , , , Name of Theory Acid Base, Arrhenius Theory _| Hydrogen ion donor Hydroxide ion donor, Bronsted-Lowry Proton donor Proton acceptor, Theory :, Lewis Theory Electron pair acceptor Electron pair donor, Usanovich Theory | Cation donor and anion acceptor | Cation acceptor and anion, donor ;, Lux-Flood Theory | Oxide-ion acceptor Oxide-ion donor, 12.3: LAW OF MASS ACTION, , , , , , , , This law was first proposed by Goldberg and Wage in 1867 and can be defined as:, , “The rate of a chemical reaction is proportional to the active masses of the reacting, substances”., , Active masses may be represented by the concentration of the reacting substances in, dilute solutions where conditions approach the ideal state. The constant of proportionality is, known as ‘velocity constant’., , Now,, , Let us consider a homogeneous, reversible reaction,, , A+Be>C+D, , According to the law of mass action,, , Vy = Ky [AJ[B], Vb = Ke[C[D]