Version 2012 Updated on 042312 Copyright © All rights reserved Dong-Sun Lee, Prof., Ph.D. Chemistry, Seoul Women’s University Chapter 12 Principles of Neutralization Titration Standard solutions for Acid-Base Titrations Analyte Titrant vs Standard solution N V = N’V’ The standard reagents used in acid-base titrations are always strong acids or strong bases, most commonly HCl, HClO4, H2SO4, NaOH, KOH. Weak acids and bases are never used as standard reagents because they react incompletely with analyte. N= unknown V= measure N’= known V’= known Primary standards for standardizing Acids Potassium acid phthalate Sulfamic acid ( H2NSO3H) HCl Potassium hydrogen iodate Bases TRIS(hydroxymethylaminomethane) Sodium carbonate Borax (= sodium tetraborate ) HgO General goal and procedure of acid-base titration 1) Standardization 2) Determination 3) Titration curve Plots of pH versus volume of titrant Acid or base standard solution unknown normality(N’) unknown volume(V’) 14 12 10 pH 8 6 4 Acid or base standard solution known normality(N’) unknown volume(V’) 2 0 0 5 10 15 20 25 Volume of NaOH added (ml) 4) Interpret titration curve , Primary standard solution known normality(N) known volume(V) Analyte Sample solution unknown Ns known Vs understand what is happening during titration Finding the end point 1) Indicator 2) Potentiometry : Titration curve ( pH or mV vs Va ) 1st derivative titration curve 2nd derivative titration curve Gran plot 3) Conductometry 4) Spectrometry Indicator An acid-base indicator is itself an acid or base whose different protonated species have different colors. Ex. Thymol blue H2In HIn– Red Yellow H2In Transition range pKa11 In2 – Blue H+ + HIn– pKa1 = 1.7 pH = pKa1 + log [HIn– ]/[H2In] if [HIn– ]/[H2In] 1:10 HIn– pH = 0.70 Red color [HIn– ]/[H2In] = 1:1 pH = pKa1 = 1.7 Orange color [HIn– ]/[H2In] 10:1 pH = 2.70 Yellow color H+ + In2 – pKa2 = 8.9 The approximate pH transition range of most acid-type indicator is roughly pKa 1 Choosing an indicator 14.00 Phenolphthalein 12.00 acid form : 10.00 colorless PP 8.0~9.0 8.00 transition range(pH): pH 6.00 8.0~9.0 4.00 Phenolphthalein 2.00 MR MO 4.8~6.0 3.1~4.4 base form : pink 0.00 0 5 10 15 20 25 30 35 Volume of 0.1000M HCl (ml) The calculated pH titration curve for the titration of 10.00ml of 0.1000M Na2CO3 with 0.1000M HCl. Calculated titration curve for the reaction of 100 mL of 0.0100M base (pKb = 5.00) with 0.0500 M HCl. Indicator color as a function of pH (pKa=5.0) Experiments. Standardization of 0.1000N NaOH 5 . 0 0 1 4 . 0 0 6 . 0 0 1 2 . 0 0 4 . 0 0 4 . 0 0 pH 8 . 0 0 6 . 0 0 pH/ Va)/ Va pH/ Va 1 0 . 0 0 3 . 0 0 2 . 0 0 2 . 0 0 0 . 0 0 2 . 0 0 4 . 0 0 1 . 0 0 4 . 0 0 2 . 0 0 0 . 0 0 0 5 1 0 1 5 2 0 2 5 3 0 V o l u m e o f0 . 0 9 2 7 0 N N a O H ( m l ) F i g .E x p e r i m e n t a lt i t r a t i o n c u r v e . 0 . 0 6 8 6 0 N K H P 2 5 . 0 0 m lv s 0 . 0 9 2 7 0 N N a O H 0 . 0 0 0 5 1 0 1 5 2 0 2 5 3 0 6 . 0 0 0 5 1 0 1 5 2 0 2 5 3 0 V o l u m e o f0 . 0 9 2 7 0 N N a O H ( m l ) F i g .T h e 1 s td e r i v a t i v e e x p e r i m e n t a lt i t r a t i o n c u r v e . 0 . 0 6 8 6 0 N K H P 2 5 . 0 0 m lv s 0 . 0 9 2 7 0 N N a O H V o l u m e o f0 . 0 9 2 7 0 N N a O H ( m l ) F i g .T h e 2 n d d e r i v a t i v e e x p e r i m e n t a lt i t r a t i o n c u r v e . 0 . 0 6 8 6 0 N K H P 2 5 . 0 0 m lv s 0 . 0 9 2 7 0 N N a O H Primary standard KHP Titration of End point=18.50ml 204.22g/1000ml=1.0000N 25.00ml of 0.06860N×25.00ml= x N×18.50ml 0.35g/25ml = xN KHP with x=0.09270N x = 0.06860N NaOH Calculation of concentration (Finding of NaOH concentration) HOOCC6H4COOK (mw 204.44) NaOH (mw 40.01) 1 Eq wt. 204.22 g 1 N × 1000 mL 20.422 mg 0.1 N × 1 mL a g x N’ × Ve mL x N’ = (a g × 1000 mL) / (204.22 g × Ve mL) Experiments. Determination of acetic acid in venigar 14.00 4.00 5.00 3.00 12.00 4.00 2.00 8.00 (pH/Va)/Va pH/Va pH 10.00 3.00 2.00 6.00 1.00 0.00 -1.00 -2.00 1.00 4.00 -3.00 2.00 0 10 20 30 40 50 60 Volume of 0.0927N NaOH(ml) Fig. The 1st derivative experimental titration curve. 0.1094N Venigar 25.00ml vs 0.0927N NaOH -4.00 0.00 0 5 10 15 20 25 0 30 20 30 40 50 60 Volume of 0.0927N NaOH(ml) Volume of 0.09270N NaOH (ml) Fig. The 1st derivative experimental titration curve. 0.06860N PHP 25.00ml vs 0.0927N NaOH 10 Fig. The 2nd derivative experimental titration curve. 0.1094N Venigar 25.00ml vs 0.0927N NaOH Standardization of NaOH Titation of End point = 29.50ml 0.09270N 25.00ml of Venigar x ´ 25ml = 0.09270N ´ 29.50ml with NaOH Venigar : x = 0.1094N Gran plot HA = H+ + A– Ka = [H+] fH+ [A–]fA– / [HA]fHA HA vs NaOH [A–] = (moles of OH– delivered) / (total volume) = VbFb / (Vb + Va) [HA] = (original moles of HA – moles of OH –) / (total volume) = (VaFa – VbFb ) / (Vb + Va) [H+] fH+ Vb = (fHA / fA– ) Ka{(VaFa – VbFb )/ Fb} 10–pH Vb = (fHA / fA– ) Ka(Ve – Vb) 10–pH Vb Ka = [H+] f H+ VbFb fA– / (VaFa – VbFb ) fHA Slope =–(fHA / fA– ) Ka Ve = –(fHA / fA– ) Ka Vb + (fHA / fA– ) KaVe Vb(l) Gran plot for the first equivalence point. This plot gives an estimate of Ve that differs from that in Figure 12.7 by 0.2μL (88.4 versus 88.2μL). The last 10-20% of volume prior to Ve is normally used form a Gran plot. Conductometric end-point detection Titration of strong acid with strong base A Neutralization : 14 HCl + NaOH NaCl + H2O B 12 Ve mL×0.1000M = 50.00 mL × 0.02000 M 10 Ve = 10.00 mL pH C 8 6 0.1000M HCl D 4 2 0 0 2 4 6 8 10 12 14 16 18 20 V a (mL) 0.02000M NaOH 50.00 mL Calculated titration curve for the reaction of 50.00 mL of 0.02000 M NaOH with 0.1000 M HCl. A. Before the beginning of titration : Va = 0.00 mL. NaOH Na+ + OH–, [OH–] = 0.02000 M 0.02000 M [H+] = 1.00×10–14/ 2.000×10–2 pH = 12.30 B. Before the equivalence point : 0 < Va < Ve remaining NaOH Initial NaOH amount =F × Vi added HCl amount = used NaOH amount = F× Va Remaining NaOH amount = Va Ex. Ve [OH–] = {(Ve – Va )/ Ve}FNaOH {Vi /(Vi + Va)} Va = 3.00 mL [OH–] = {(10.00 – 3.00)/10.00}(0.02000){50.00 /(50.00 +3.00)} = 0.0132 M [H+] = 1.00×10–14/ 0.0132 = 7.58×10–13 pH = 12.12 C. At the equivalence point : Va = Ve + – H2O = H + OH , [H+] [OH–] = 1.00×10–14 [H+] = 1.00×10–7 D. After the equivalence point : pH = 7.00 Va > Ve excess HCl NaOH added HCl amount = FHCl× Va initial NaOH amount = used HCl = F × Vi Vi Excess HCl amount = FHCl(Va–Vi) Ve Va = FHCl (Va–Ve) [H+] = FHCl {(Va– Ve) /(Vi + Va)} Ex. Va = 10.50 mL [H+] = (0.1000) {10.50 –10.00) /(50.00 + 110.50)} = 8.26×10–4 pH = 3.08 Titration curves for NaOH with HCl. A. 50.0 mL of 0.0500 M NaOH with 0.1000 M HCl. B. 50.00 mL of 0.00500 M NaOH with 0.0100 M HCl. Titration curves for HCl with NaOH. A. 50.0 mL of 0.0500 M HCl with 0.1000 M NaOH. B. 50.00 mL of 0.000500 M HCl with 0.00100 M NaOH. pH The effects of titrant and analyte concentrations on neutralization titration curves. 14.00 13.00 12.00 11.00 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 B A 0 5 10 15 20 25 30 35 Va of NaOH (mL) Changes in pH during the totration of strong acid with strong base. A: 50 mL of 0.0500 M HCl vs 0.1000 M NaOH B: 50 mL of 0.000500 M HCl vs 0.001000 M NaOH For the titration with diluted concentrations, the change in pH in equivalence point region is markedly less than concentrated solution Weak acid titrated with strong base 1) Titration reaction : CH3COOH + NaOH CH3COO– Na+ + H2O 14.00 strong + weak complete reaction 12.00 K = 1/Kb = 1.76 ×109 A 10.00 pH B 8.00 6.00 2) Calculation of equivalence point : 4.00 2.00 [CH3COOH] Vi mL = [NaOH] Ve mL 0.1000 M×5.00 mL = 0.1000 M × Ve mL Ve= 5.00 mL 3) Titration curve : 0.00 0 1 2 3 4 5 6 7 8 Va of NaOH(mL) Changes in pH during the titration of a weak acid with a strong base. A: 5.00 mL of 0.1000M HOAC vs 0.1000 M NaOH B: 5.00 ml of 0.001000M HOAC vs 0.001000M NaOH Titration curves for the titration of acetic acid with NaOH. A. 0.1000 M acetic acid with 0.1000M NaOH. B. 0.001000 M acetic acid with 0.00100 M NaOH. A. Before adding any titrant : Va = 0.00 HAC = H+ + AC– [H+] = KaF = 1.75 ×10–5 ×0.1000 14.00 = 1.323 × 10–3 12.00 pH= 2.88 10.00 pH 8.00 0.1000M NaOH 6.00 4.00 A 2.00 0.00 0 0.1000M HAC 25.00ml 1 2 3 4 5 6 7 Va of 0.1000 M NaOH (mL) Titration curve. 0.1000 M HAC vs 0.1000 M NaOH. B. Before equivalence point : 0 < Va < Ve HAC + NaOH H2O + Na+AC– [H+] = Ka[HAC] / [AC–] 14.00 initial HAC amount = F×Ve – added NaOH amount 10.00 = used amount HAC= F×Va 8.00 Remaining HAC amount =F(Vi–Va) 6.00 [HAC] = {(F×Ve) – (F×Va)}/(Vi +Va) 4.00 [AC–] = (F×Va) / (Vi +Va) 2.00 pH = pKa + log [AC–] /[HAC] 0.00 at Va = Ve/2 B 12.00 pH= pKa = 4.76 Vi Ve/2 0 1 2 3 4 5 6 7 Va of 0.1000 M NaOH (mL) Titration curve. 0.1000 M HAC vs 0.1000 M NaOH. Va Ve C. Equivalence point Va = Ve 14.00 HAC + NaOH H2O + Na+AC– 12.00 10.00 C 8.00 initial 1 1 final 0 0 6.00 4.00 AC– = HAC + OH– 2.00 [OH–] = KbF’ = KwF’ /Ka 0.00 0 1 2 3 4 5 6 7 Va of 0.1000 M NaOH (mL) Titration curve. 0.1000 M HAC vs 0.1000 M NaOH. F’=( F×Vi) / (Vi +Va) 1 D. After equivalence point Va> Ve 14.00 added NaOH amount = F×Va D 12.00 initial HAC amount 10.00 = used NaOH amount = F×Ve 8.00 Excess NaOH amount = F×(Va –Ve) 6.00 4.00 2.00 [OH–] = FNaOH ×(Va– Vi) / (Vi +Va) 0.00 0 1 2 3 4 5 6 7 Va of 0.1000 M NaOH (mL) Titration curve. Vi 0.1000 M HAC vs 0.1000 M NaOH. Va Ve The effect of acid strength (dissociation constant) on titration curves. Each curve represents the titration of 50.00 mL of 0.1000 M acid with 0.1000 M base. Weak base titrated with strong acid 1) Titration reaction : NH3 + HCl NH4+Cl– 14.00 2) Equivalence point : A 12.00 NH3 HCl 0.1100 M×20.00 mL = 0.1000 M×Ve mL Ve = 22.00 mL 10.00 8.00 6.00 3) Titration curve : 4.00 A. Initial 2.00 Va = 0 NH3 + H2O = NH4 + + OH– 0.00 0 10 20 30 40 0.1000M HCl volume (mL) Kb = Kw / Ka = [NH4+][OH– ]/[NH3] 1.79×10–5 = [OH– ]2 / 0.1100–[OH– ] [OH– ]= KbF =1.4×10–3 pH = 11.15 The calculated titration curve for the titration of 20.00 mL of 0.1100 M ammonia with 0.1000 M HCl. 50 B. Before the equivalence point 0<Va< Ve Major constituents: NH3 + NH4+Cl– [OH]= Kb [NH3]/ [NH4 1 4 . 0 0 +] 1 2 . 0 0 initial NH3 amount = F×Ve 1 0 . 0 0 – added HCl amount pH 8 . 0 0 = used amount NH3 = F×Va 6 . 0 0 Remaining NH3 amount =F(Vi–Va) 4 . 0 0 [NH3] = {(F×Ve) – (F×Va)}/(Vi +Va) 2 . 0 0 [NH4+] = (F×Va) / (Vi +Va) 0 . 0 0 0 pH = pKb + log [NH4+] / [NH3] at Va = Ve/2 pOH= pKb = 4.74 pH = 9.26 Vi Va B Ve/2 Ve 1 0 2 0 3 0 4 0 5 0 0 . 1 0 0 0 M H C lv o l u m e ( m l ) F i g .1 2 4 .T h e c a l c u l a t e d t i t r a t i o n c u r v e f o r t h e t i t r a t i o n o f 2 0 . 0 0 m lo f 0 . 1 1 0 0 M a m m o n i a w i t h 0 . 1 0 0 0 M H C l . C. Equivalence point Va = Ve 1 4 . 0 0 NH3 + HCl NH4+Cl– 1 1 final 0 0 1 0 . 0 0 1 C 8 . 0 0 pH initial 1 2 . 0 0 6 . 0 0 NH4+ = NH3 + H+ 4 . 0 0 [H+] = KaF’ 2 . 0 0 F’=( F×Vi) / (Vi +Va) 0 . 0 0 0 1 0 2 0 3 0 4 0 5 0 0 . 1 0 0 0 M H C lv o l u m e ( m l ) F i g .1 2 4 .T h e c a l c u l a t e d t i t r a t i o n c u r v e f o r t h e t i t r a t i o n o f 2 0 . 0 0 m lo f 0 . 1 1 0 0 M a m m o n i a w i t h 0 . 1 0 0 0 M H C l . D. After equivalence point Va> Ve added HCl amount = F×Va 1 4 . 0 0 initial NH3 amount 1 2 . 0 0 1 0 . 0 0 = used HCl amount = F×Ve 8 . 0 0 pH Excess HCl amount = F×(Va –Ve) D 6 . 0 0 4 . 0 0 [H+] = FHCl ×(Va– Ve) / (Vi +Va) 2 . 0 0 0 . 0 0 0 Vi 1 0 2 0 3 0 4 0 5 0 0 . 1 0 0 0 M H C lv o l u m e ( m l ) Ve Va F i g .1 2 4 .T h e c a l c u l a t e d t i t r a t i o n c u r v e f o r t h e t i t r a t i o n o f 2 0 . 0 0 m lo f 0 . 1 1 0 0 M a m m o n i a w i t h 0 . 1 0 0 0 M H C l . The effect of base strength (dissociation constant) on titration curves. Each curve represents the titration of 50.00 ml of 0.1000 M base with 0.1000 M HCl. Titration of Weak Base with Strong Acid Comparison of Weak Acid/ Base with Strong Base/Acid Weak Acid with Strong Base Weak Base with Strong Base Titration reaction HA + OH- → H2O + A- B + H2O → BH+ + OH- Initial [H+] = KaF [OH-] = KbF =1.4×10–3 Before the equivalence point (0<V a<V e) pH = pKa + log [A–] /[HA] pH = pKb + log[NH4+]/[NH3] Equivalence point [OH–] = KbF’ = KwF’ /Ka [H+] = KaF’ F’=( F×Vi) / (Vi +Va) F’=( F×Vi) / (Vi +Va) After equivalence point (Va>Ve) [OH-] = FNaOH (Va– Ve) { (V + V ) } i a [H+] = FHCl { (Va– Ve) (Vi + Va) } Determining the pK values for amino acids Amino acids contain both an acidic and a basic group. alanine All naturally occurring amino acids are left-handed (L) form. The amine group behaves as a base, while the carboxyl group acts as an acid. Aspartic acid Curves for the titration of 20.00ml of 0.1000M alanine with 0.1000 M NaOH and 0.1000M HCl. Note that the zwitterion is present before any acid or base has been added. Adding acid protonates the carboxylate group with a pKa of 2.35. Adding base reacts with the protonated amine group with a pKa of 9.89. Plots of relative amounts of acetic acid and acetate ion during a titration. Acid base titration in non-aqueous media HOOCCHNH2 + HClO4 R CH3COOH HOOCCHNH3+ClO4– R (solvent) 0.010M CH3COOK in acetic acid Amino acid in acetic acid + 0.020M HClO4 in acetic acid HClO4 + CH3COOK CH3COOH + K+ClO4– CH3COOH(solvent) Titrants used in non-aqueous titrimetry Acidic titrants Perchloric acid p- Toluenesulfonic acid 2,4-Dinitrobenzenesulfonic acid Basic titrants Tetrabutylammonium hydroxide Sodium acetate Potassium methoxide Sodium aminoethoxide Selected solvents for non-aqueous titration Solvent Amphiprotic Aprotic or basic Autoprotolysis constant Dielectric (pKHS) constant Glacial acetic acid 14.45 6.1 Ethylenediamine 15.3 12.9 Methanol 16.7 32.6 Dimethylformamide Benzene 36.7 2.3 Methyl isobutylketone 13.1 Pyridine 12.3 Dioxane 2.2 n-Hexane 1.9 Acid and base strengths that are not distinguished in aqueous solution may be distinguishable in non-aqueous solvents. Ex. Perchloric acid is a stronger acid than hydrochloric acid in acetic acid solvent, neither acid is completely dissociated. HClO4 + CH3COOH = ClO4– strong acid strong base + CH3COOH2+ weak base HCl + CH3COOH = Cl– K = 1.3×10–5 weak acid + CH3COOH2+ K = 5.8×10–8 Differentiate acidity or basicity of different acids or bases differentiating solvent for acids …… acetic acid, isobutyl ketone differentiating sovent for bases …… ammonia, pyridine Acid Name Ho H2SO4 (100%) Sulfuric acid –11.93 H2SO4 SO3 Fuming sulfuric acid –14.14 H SO3F Fluorosulfuric acid –15.07 H SO3F+10%SbF5 Super acid H SO3F+7%SbF53SO3 Hammett acidity function, Ho, for aqueous solutions of acids. –18.94 –19.35 Titration of a mixture of acids with tetrabutylammonium hydroxide in methyl isobutyl ketone solvent shows that the order of acid strength is HClO4 > HCl > 2-hydroxybenzoic acid > acetic acid > hydroxybenzene. Measurements were made with a glass electrode and a platimum reference electrode. The ordinate is proportional to pH, with increasing pH as the potential becomes more positive. Summary Acid-Base titration Titration curve Indicator Detection of end-point Potentiometry Gran plot Conductometry Non-aqueous titration Hammett acidity function, Ho