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NEUTRALIZATION ANALYSIS TITRATION CURVES (A) analyte + (R) titrant K (P) product Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications Important points and regions: 2 points: before titration at the end point 2 regions: before the end point after the end point Titration curves: 1. Strong acid with strong base, 2. Weak acid with strong base, 3. Polyprotic acid with strong base (at 0%) (at 100 %) (0.00..1 – 99.99…%) (100.00..1 – ∞) I. [A] III. [A] = [R] II. [A] + [P] IV. [P] + [R] Strong base with strong acid Weak base with strong acid 1 TITRATION CURVES 1. Strong acid with strong base, Strong base with strong acid Outline NEUTRALIZATION ANALYSIS Introduction e.g. I. Titrants Titration curves II. End point detection Applications III. IV. HCl + NaOH Cl– acid1 + base1 + acid2 (very weak) base2 + Na+(H2O) At the start: [H+] = [H3O+]=[HCl]0 [OH–] = [NaOH]0 pH = – lg [HCl]0 pOH = – lg [NaOH]0 pH = 14 – pOH Before the end point: [H+] = [H3O+]=[HCl]unreacted [OH–] = [NaOH]unreacted pH = – lg [HCl]unreacted pOH = – lg [NaOH]unreacted At the end point: [H+] ≡ [OH–] KW = 10–14 pH ≡ 7 After the end point: [OH–] = [NaOH]excess [H+] = [H3O+]=[HCl]excess 2 pOH = – lg [NaOH]excess pH = – lg [HCl]excess TITRATION CURVES Titration curves: 1. Strong acid with strong base, 2. Weak acid with strong base, 3. Polyprotic acid with strong base Strong base with strong acid Weak base with strong acid EFFECTS ON THE TITRATION CURVE: 1. Effect of the temperature: Outline NEUTRALIZATION ANALYSIS 25°C [H+]·[OH–] = Kw = 10–14 Neutr. point: pH = 7 100°C [H+]·[OH–] = Kw = 10–12 Neutr. point: pH = 6 Introduction Titrants Titration curves 0 100 End point detection Applications 3 EFFECTS ON THE TITRATION CURVE 2. Dependence on the initial concentrations (e.g. [HCl]): Outline [HCl]0 ↓ 1N 0% 50% 90% 99% 99.9% 100% 100.1% 101% 110% 0 0,3 1 2 3 7 11 12 13 0,1 N 1 1,3 2 3 4 7 10 11 12 0,01 N 2 2,3 3 4 5 7 9 10 11 0,001 N 3 3,3 4 5 6 7 8 9 10 pH change around the end point ΔpH NEUTRALIZATION ANALYSIS 3 – 11 4 – 10 5– 9 6– 8 Introduction Titrants Titration curves End point detection Applications 0 100 4 EFFECTS ON THE TITRATION CURVE 3. Dependence on the acid strength (dissociation constants): Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection A. Weak acid with strong bases , 0 100 e.g. 10–1 N CH3COOH is titrated with NaOH (Ka = 2x10–5) % 0 50 90 99 99.9 100 100.1 101 110 pH 2.9 4.7 5.7 6.7 7.7 8.9 10 11 12 ΔpH pKInd ≈ 9 → PHENOLPHTALEIN B. Weak base with strong acid e.g. 10–1 N NH4OH is titrated with HCl (Kb = 2x10–5) % 0 50 90 99 99.9 100 100.1 101 110 pH 11.1 9.3 8.3 7.3 6.3 5.1 4 3 2 Applications 5 ΔpH pKInd ≈ 5 → METHYL RED TITRATION CURVES II. Weak acid with strong base Weak base with strong acid e.g. Titration of CH3COOH with NaOH , Titration of NH4OH with HCl: I. At the start: Weak acid Weak base pH H K a C acid H Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications K a CH 3 COOH OH K b C base OH K b NH 4 OH II. Before the end point: Buffer (acid / salt) Buffer (base / salt) pH H K a C acid C salt OH K CH COOH H KCH COO a 3 3 b C base C salt OH K NH OH b NH 4 4 III. At the end point: Hydrolysing salt (Brönsted base) pH Hydrolysing salt (Brönsted acid) OH OH Kw C salt Ka K b C salt H OH K b CH3 COO H Kw C salt Kb IV. After the end point: Excess of strong base pH [OH–] = Cexcess base [OH–] = [NaOH]excess K a C salt H K a NH 4 Excess of strong acid [H+] = Cexcess acid [H+] = [HClexcess 6 TITRATION CURVES III. Polyprotic acid with strong base e.g. Titration of H3PO4 with NaOH 1. 2. 3. H3PO4 + OH– H2PO4– + OH– HPO42– + OH– H2PO4– + H2O HPO42– + H2O PO43– + H2O Ka1 = 7x10–3 Ka2 = 6x10–8 Ka3 = 10–12 Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications 7 ACID / BASE INDICATORS 1. Azo-compounds Genearal structure: Outline NEUTRALIZATION ANALYSIS INDICATOR R1 R2 R3 ∆pH METHYLORANGE METHYL -RED p-ETHOXYCHRISOIDINE TROPAEOLIN 0 -N(CH3)2 H SO3Na -N(CH3)2 COOH -OC2H5 -SO3Na 3.1 – 4.4 Acidic color red Basic color orange H 4.4 – 6.2 red yellow NH2 NH2 3.5 – 5.5 red yellow OH OH 11.1 – 12.7! yellow orange Mechanism: Introduction Titrants Titration curves End p. detection - chemical - instrumental Applications Yellow Yellow Red (basic) (intermediate) (acidic) (aromatic) (protonated) (quinoid) 8 ACID / BASE INDICATORS 2. PHTHALEIN-derivatives General structure: INDICATOR R R1 R2 R3 PHTALEINS PHENOLPHTHALEIN COOH COOH H H H H CH3 THYMOLPHTHALEIN COOH CH(CH3)2 Outline NEUTRALIZATION ANALYSIS SULFONPHTHALEINS PHENOL RED SO3H SO3H H H H THYMOL BLUE SO3H CH(CH3)2 H CH3 Mechanism: ∆pH Acidic color colorless colorless Basic color colored red colorless blue basic / acidic colored colored 6.4 – 8.0 a)1.2 – 2.8 b)8.0 – 9.6 yellow red red yellow yellow blue basic 8.2 – 10.0 8.3 – 10.5 Thymol blue Introduction Titrants Titration curves End p. detection - chemical - instrumental Applications Colorless (acidic) Colorless (intermediate) Purple (basic) 9 INSTRUMENTAL DETECTION (Summary) Outline INSTRUMENTAL DETECTION Advantages Types Potentiometric end point detection The titration process is followed by electrochemical, photometric or other sensing devices. Method Sensing device POTENTIOMETRY (Potential vs %) Different types of electrodes Neutralization titr. Complexometric titr. Precipitation titr. Redox titr. AMPEROMETRY (Current vs %) Pt electrode (dead stop…) Redox titr. CONDUCTOMETRY Conductivity cell Neutralization titr. Precipitation titr. Spectrophotometer Complexometric titr. Neutralization titr. Complexometric titr. Precipitation titr. Redox titr. (Conductivity vs %) PHOTOMETRY (A = ε · c · l vs %) Conductometric end point detection Type of titration ENTALPHYMETRY (Q = f (c, ΔH) vs % Thermistor 10 POTENTIOMETRY Electrode potential developed between: Outline Indicator electrode Potential (Eind) varies Depends on the analyte concentration Reference electrode Known, constant potential (Eref) Independent of the analyte concentration Common reference electrodes: Solid metal / its „unsoluble” salt / saturated conc. of anion e.g. Ag / AgCl / KCl Hg / Hg2Cl2 / KCl Hg / Hg2SO4 / K2SO4 INSTRUMENTAL DETECTION Advantages Nernst equation: Types Glass electrode Potentiometric end point detection Conductometric end point detection Metal electrode Ion-selective electrode Nobel metal electrode E E0 0.059 lg c n Neutralization titration: E = E0 + 0.059 lg [H+] Complexometric titration: E = E0 + 0.059 lg [Mn+] n Precipitation titration: E = E0 + 0.059 lg [X−] Redox titration: E = E0 + 0.059lg [ox] [red] n 11 POTENTIOMETRY Neutralization analysis External reference electrode Indicator electrode: Outline Glass electrode GLASS ELECTRODE INSTRUMENTAL DETECTION H+ conc. to be determined Electrochemical cell for measurement of pH: Advantages Types Potentiometric end point detection Conductometric end point detection External reference || H+ conc. |pH-sensitive electrode || to be | glass(Hg/Hg2Cl2/KCl) ||determined | membrane | Internal | buffer sol. | Internal reference | electrode | (KCl) (pH = 7) | (Ag/AgCl/KCl) ███████████ External Dry glass Internal hydrated hydrated gel layer gel layer 12 POTENTIOMETRY Glass electrode Composition of glass: E.g. 22 % Na2O, 6 % CaO, 72 % SiO2. Outline INSTRUMENTAL DETECTION Advantages Types Na+ mobile membrane solution H+ Na++ H Na+ Ion-exchange reaction: between H++ Na H+ in the solution and Na+ in the glass: K H+ + Na+Gl− Na+ + H+Gl– K = LARGE! solution glass solution glass Combination glass electrode: Potentiometric end point detection Conductometric end point detection 13 POTENTIOMETRY Titration curve Potentiometric titration curve: Outline INSTRUMENTAL DETECTION Titration curve Measuring the potential of a suitable indicator electrode (pH) as a function of volume titrant. 1st derivative Advantages Determination of the end point: from the derivatives Types Potentiometric end point detection 2nd derivative Conductometric end point detection 14 CONDUCTOMETRIC TITRATION CURVES I. Titration of strong acid (a) with strong base e.g. HCl with NaOH (b) with weak base e.g. HCl with NH4OH Outline INSTRUMENTAL DETECTION Advantages Types % II. Titration of weak acid (c) with strong base e.g. CH3COOH with NaOH (d) with weak base e.g. CH3COOH with NH4OH Potentiometric end point detection Conductometric end point detection % 15 APPLICATIONS TITRATIONS Direct Outline NEUTRALIZATION ANALYSIS Introduction Back (indirect): Analyte Titrant in excess to calculate I. to measure Determination of strong acids / bases: Equivalence point: pH = 7 e.g. NaOH Titrants Titration curves End point detection Vphen. Vmeth.r. OH− H2O Applications 16 APPLICATIONS II. Determination of weak acids : Equivalence point: pH > 7 (phenolphtalein indicator) weak bases : Equivalence point: pH < 7 (methyl red indicator) Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications II. (a) Determination of weak acids : Ka ≥ 10–5. (10–7 - 10–4) Direct: e.g. carboxylic acids of low carbon atoms e.g. CH3COOH fatty acids (e.g. fat, wax, oil) Back : if the weak acid is volatile e.g. CO2 (as carbonate or hydrogencarbonate) bubble-free distillation CO2 known amount of Ba(OH)2 Distillation apparatus back titration of excess Ba(OH)2 (Maros- Schulek) with standard HCl Application of CO2 determination: Determination of organic materials Determination of CO2, HCO3– , CO32– content of natural waters Nonaqueous solvents: Ka < 10–7 17 > 10–12 APPLICATIONS II. (b) Determination of weak bases : Kb ≥ 10–5 (10–7 - 10–4) Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications Direct: e.g. NH4OH Back: NH4+ -salt strong base (NaOH) boiling NH3 distillation into known excess of acid Kjeldahl method: NH3 back titration of excess acid (HCl) known HCl with basic titrant (NaOH) Application of NH3 determination: N-containing organic compounds (e.g. amino acids, proteins,…) Decomposition (mineralization) with cc. H2SO4, 300 °C + catalyst: Se, or Cu2+ Ox. number: – 3 (NH4)2SO4 (e.g.. – NH2, –N(CH3)2, =NH, –N<) Ox. number: + 3, +1 HNO3 (+5) (e.g.,azo- (-N=N-), nitro-, nitrozo comp.) Reduction with Zn, Na2S2O4,.. NH4+ Nonaqueous solvents: Kb < 10–7 > 10–12 18 APPLICATIONS Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves III. Determination of salts: (a) Neutral salts: NOT MEASURABLE! (b) Salt hydrolyzing to acid: Brönsted acid (strong acid + weak base) MA + H2O MOH + A– + H+ if pK > 7! can be TITRATED with base E.g. Aniline · HCl; Benzidine ·H2SO4; Papaverine · HCl… (c) Salt hydrolyzing to base: Brönsted base (strong base + weak acid) MA + H2O HA + M+ + OH– if pK > 7 can be TITRATED with acid 2– – E.g. Na2B4O7 (B4O7 +7 H2O 4H3BO3 + 2OH ) methyl red (CO32– + H2O HCO3– + OH–) phenolpht. E.g. Na2CO3 (CO32– +2 H2O H2CO3 +2 OH–) methyl red – – NaHCO (HCO3 + H2O H2O + CO2 +OH ) methyl red 3 Na2CO3 End point detection NaHCO3 Applications Vphen Vmeth.r. CO32− HCO3− H2CO3 Vphen = 0 Vmeth.r. HCO3− H2CO3 19 APPLICATIONS (d) Specific determinations: NaOH – Na2CO3 in the presence of each other NaHCO3 – Na2CO3 in the presence of each other Outline NEUTRALIZATION ANALYSIS Introduction Titrants Titration curves End point detection Applications OH−, CO32− HCO3− H2CO3 Vphen Vmeth.r. Warder’s method : one sample : A. OH– + H+ CO32– + H+ B. HCO3– + H+ two samples : B. OH− + H+ CO32– +2H+ Winkler’s method : A. + BaCl2 CO32– +Ba2+ OH– + H+ Vphen Vmeth.r. H2O phenolpht. HCO3–. H2CO3 methyl red H2O H2CO3 methyl red BaCO3 H2O phenolpht. CO32− HCO3− HCO3− H2CO3 Warder’s method : two samples : A. CO32– + H+ HCO3– phenolpht. B. HCO. – + H+ H CO CO3 3 2− +2H+ 2 3 H2CO3 methyl red 20