ANALYTICAL CHEMISTRY Acid Base Titration by Wan Norfazilah Wan Ismail Faculty of Industrial Sciences & Technology [email protected] Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Chapter Description • Expected Outcomes – Understand and state the principles of titrations. – Define and identify Arrhenius and Brönsted-Lowry acids and bases – Define and identify the conjugate of a given acid or base – Describe and apply the titration curves, calculations and indicators to solve the problem regarding acid base titration. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Contents • Acid-Base Theories • Autopyrolysis of Solvents • Acidity of Solution • Acid-Base Titration • End Point Detection • Indicators for titration Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 APPLICATIONS OF NEUTRALIZATION TITRATIONS General flow: ◦ Preparation of standard solution (acid/base) Standardization of solution with primary standards Titration Results Applications: ◦ Elemental analysis ◦ Determination of inorganic substances ◦ Determination of organic functional groups ◦ Determination of salts Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACID – BASE THEORIES Arrhenius Theory (Nobel Prize 1894) + • Acid: any species that can produce hydroxonium ions (H3O ) – abbreviated H+ or proton. • Base: produce hydroxyl ions (OH-) in aqueous solution. • Does not include acids or bases that can not produce H+ and OH- ions. Brönsted – Lowry Theory • Acid = proton (H+) donor • Base = proton acceptor • Amphoteric substance = function as an acid or a base Lewis Theory • Acid = accept a pair of electrons • Base = donate a pair of electrons Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 Strong electrolyte : completely dissociated. Weak electrolyte : partially dissociated. Strong acid Weak acid Strong base Weak base HCl CH3COOH NaOH NH3 HBr H2CO3 KOH N2H4 HI HOCN LiOH CH3NH2 HNO3 HCN RbOH H2SO4 HF CsOH HClO4 H2S Sr(OH)2 HOOH Ba(OH)2 HOCl Ca(OH)2 HON=O Mg(OH)2 HOOCCOOH H3PO4 Source: Christian G.D., Dasgupta, P., Schug, K. (2014) CH3CH2COOH Analytical Chemistry. Wiley-VCH Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 AUTOPYROLYSIS OF SOLVENTS • Autopyrolysis : self-ionization – acts as both an acid and a base. + + = 1.0 × 10 + − −14 2 −2 퐻2푂 퐻2푂 ⇌ 퐻3푂 푂퐻 퐾푤 푚푚푚 퐿 K = + − 퐻 푂퐻 2 = 퐻 푂 + − 퐾푤 퐻 푂퐻 • Protic solvent : a solvent that involves the transfer of H+ from one molecule to another. Can undergo self-ionization. i.e. H2O, CH3OH, C6H5OH 2 + + − 3 3 2 Acid3 Base Titration 퐶퐻 퐶� ⇌ 퐶퐻 퐶 푂퐻 퐶퐻by퐶 Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 AUTOPYROLYSIS OF SOLVENTS • Polyprotic acids and bases: compounds that can donate or receive more than one proton. i.e. phosphoric acid, phosphate + + = 7.11 × 10 − + −3 퐻3푃푂4 퐻2푂 ⇌ 퐻2푃푂4 퐻3푂 퐾푎1 + + = 6.32 × 10 − −2 + −8 퐻2푃푂4 퐻2푂 ⇌ 퐻푃푂4 퐻3푂 퐾푎2 + + = 7.1 × 10 −2 −3 + −13 퐻푃푂4 퐻2푂 ⇌ 푃푂4 퐻3푂 퐾푎3 • Aprotic solvent : a solvent that does not have an acidic proton. i.e. CH3CN, (C2H5)2O Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 ACIDITY OF SOLUTIONS For an aqueous solution of 0.10 M HCl: + + − = 0.01 = 1.0 × 10 퐻퐶푚 → 퐻 퐶푚 + + − −14 ∴0.10퐻 =푀1.0 × 10 퐻[ 푂퐻] = 1.0 × 10 − −14 − −13 Acidity is related푂 퐻to pH scale: ∴ 푂퐻 = log = log + 푝퐻 =− 퐻 − 푝 푂퐻 푎 푎 Because = 1.0 × 10푝퐾 , pH− is related푙퐾 to pOH by + = 14 + − −14 퐻 푂퐻 푝퐻 푝 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF STRONG ACIDS & BASES 100% ionization Example: calculate the pH of a 2.0×10-3M HCl [H+] = 2.0×10-3 -3 pH = -log(2.0×10 ) = 2.70 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 EXAMPLE What is the pH of a solution containing 0.10 M NaOH? The final concentrations, = = 0.10 + = −[ ] 푁푁 푂퐻 푀 + − = =푤1.0 × 10 0.10 = 1.0 × 10 퐾 퐻 푂퐻 + − −14 −13 퐻 퐾푤⁄ 푂퐻 = 1.0 × 10⁄ = 13 푀 −13 푝퐻 −푙 @ = log [ ] + = 14 − = log 0.10 = 1 푝 − 푂퐻 푝퐻 푝 푝 =−14 1 = 13 푝퐻 − Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID The dissociation of weak acid + (weak acid) + (conjugate− base) 퐻퐻 ⇌ 퐻 퐻 HA (conjugate acid) and A- (conjugate base) are conjugate acid-base pair. [ ] = [+ ]− 퐻 퐻 퐾푎 퐻퐻 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 EXAMPLE A solution of acid HA (0.030 M) was found to have [H+] = 6.5 × 10-4 M. Calculate the Ka value for the acid. + + [ −] 퐻퐻=⇌ 퐻 퐻 [+ ]− 퐻 퐻 푎 퐾 = = 6퐻퐻.5 × 10 4 + − = 0.030퐻 퐻6.5 × 10 4 =− 2푀.9 × 10 + −2 퐻 (6.5 ×푀10−4)(6.5 × 10− 푀4) 푀 = = 1.5 × 10 2.9−× 10 − −5 푎 −2 퐾 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK BASE For every weak acid, there is always an associated weak base. + ( ) + − 푎 The anion퐻퐻 A⇌- acts퐻 as 퐻a weak base. This anion퐾 ℎ푁 푎can푁 푣푣푣푣 undergo� hydrolysis푣푣푣 : + + (conjugate− base) (weak acid) (weak acid) (conjugate −base) 퐻 퐻2푂 ⇌ 푂퐻 [ ] = = [ ] − 퐻퐻 푂퐻 퐾ℎ 퐾푏 − 퐻 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES Partially ionized + - HA + A Initial C0 0 0 ⇌ H Equilibrium C0-C1 C1 C1 Ka = acidity constant Assuming 1. Contribution of [H+] from water is negligible 2. Ka<<1, C1<<C0 [HA] = C0-C1 0 2 Ka = C1 /C0 + ≈ C [H aC0) ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 + - B + OH Initial C0 0 0 ⇌ BH Equilibrium C0-C1 C1 C1 Assuming 1. Contribution of [OH-] from water is negligible 2. Kb<<1, C1<<C0 [B] = C0-C1 0 2 Kb = C1 /C0 - ≈ C [OH bC0) ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES Partially ionized + - HA + A Initial C0 0 0 ⇌ H Equilibrium C0-C1 C1 C1 Ka = acidity constant Assuming 1. Contribution of [H+] from water is negligible 2. Ka<<1, C1<<C0 [HA] = C0-C1 0 2 Ka = C1 /C0 + ≈ C [H aC0) ] = √(K Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 IONIZATION OF WEAK ACIDS & BASES - [OH Kw / Ka) * C0) + [H ] = √(Kw / Kb) * C0) ] = √( Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 NEUTRAL IONS • Do not reacts with water to form H+ or OH-. • Are not affected by pH. • Not many, neutral ions are only from strong acid or strong base. Species Neutral Anion − − 3 퐶푚 − 푁푂 − 퐵푣 퐶푚푂4 Cation − 2− 퐼 푆푂4 + 2+ 퐿퐿 + 퐶 푁 2 + 푁푁+ 퐵푁 • When an acid and a base react, they퐾 neutralize each other to form a salt. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID VS STRONG BASE *Completely dissociated Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID VS WEAK BASE *Cation hydrolysis occurs affect the pH value at equivalence point (pH < 7) Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID VS STRONG BASE *Anion hydrolysis occurs affect the pH value at equivalence point (pH > 7) Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 WEAK ACID VS WEAK BASE Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STANDARD SOLUTIONS Strong acids or strong bases ◦ Complete reaction with analyte ◦ Sharp end points ◦ Never use weak acids & bases as standard reagents (incomplete reaction) Standard solutions of acids ◦ Dilution of concentrated sulfuric, hydrochloric or perchloric acid. Standard solutions of bases ◦ prepared from solid sodium or potassium and occasionally barium hydroxides. The concentrations of these bases must be established by standardization. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 TITRATION OF STRONG ACIDS AND STRONG BASES A strong acid – strong base titration curve has a large end point break. Figure shows the titration curve for 100 mL of 0.1 M HCl versus 0.1 M NaOH. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 As the concentrations of acid and titrant decrease, the end point break decreases. So the selection of indicator becomes more critical. Dependence of the magnitude of end-point bread on concentration. The concentrations of acid and titrant are the same. 0.001 M 0.01 M 0.1 M Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 This is the mirror image of the HCl titration curve. Titration curve for 100 mL of 0.1 M NaOH versus 0.1 M HCl. Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 STRONG ACID - STRONG BASE TITRATIONS Approaching the equivalence point, the concentration of [H+] gets very small small Small additions of base large relative change in the concentration of [H+] Thus, near the equivalence point, greater change in pH observed This behavior make it easy by just using an indicator dye to show when we are approaching the equivalence point The indicator may change color at close to pH 7.0 Acid Base Titration by Wan Norfazilah Wan Ismail http://ocw.ump.edu.my/course/view.php?id=467 TITRATION CURVES OF WEAK ACIDS WITH A STRONG BASE .