Acid-Base Titrations Chapter 8 PDF

Summary

This document outlines acid-base titrations. It covers topics such as strong acid vs strong base titrations, calculations, and methods for determining the pH before and after the equivalence point of a titration. The document emphasizes volumetric analysis, equilibrium principles, and the use of indicators in chemical procedures, including important chemical concepts such as acidimetry and alkalimetry.

Full Transcript

Chapter 8 Acid-Base Titrations: 1. Strong acid vs strong base 2. Strong acid vs weak base 3. Strong base vs weak acid 4. Strong acid vs weak polyprotic base 5. Strong base vs weak polyprotic acid 6. Amino acid titration 7. Strong base vs mixture of weak acids Acid-base titration...

Chapter 8 Acid-Base Titrations: 1. Strong acid vs strong base 2. Strong acid vs weak base 3. Strong base vs weak acid 4. Strong acid vs weak polyprotic base 5. Strong base vs weak polyprotic acid 6. Amino acid titration 7. Strong base vs mixture of weak acids Acid-base titration ILO’s ‫مخرجات التعلم‬ 1- calculate and construct titration curves for strong acids with strong bases 2- recognize the concept of acid-base color Indicators 3- calculate and construct titration curves of weakly acidic drugs vs strong base. 4- calculate and construct titration curves of weakly basic drugs vs strong acid 5- calculate the equations governing weak acidic and basic drugs titration 6- calculate and construct polyfunctional acidic/basics including drugs titration curves Titration Titrant standard solution (in buret) Titration: A standard solution (titrant) with a known [conc] is used to determine the [conc] of an unknown solution (analyte or API). The reaction that occurs is a neutralization reaction. unknown analyte (API) solution Acidimetry involves the quantitative determination of basic drugs. Alkalimetry involves the quantitative determination of acidic drugs. Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids and Bases Strong acids & bases completely dissociated in HOH (e.g.) HCl, HClO4, NaOH, KOH) Consider only one equilibrium, Kw = [H3O+][OH-] Know the stoichiometric Ration of the acid-base reaction (S.R.) (e.g. HCl + NaOH  HOH + Na+ Cl- S.R. =1:1 Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids & Bases What is the pH before a titration begins? 100.0 mL of 0.1000M HCl (anayte) [H3O+] = 1.000 x 10-1M [OH-] = Kw/[H3O+] = 1.000 x 10-13 M pH = - Log [H3O+] = 1.0000 (4s.f.) pOH = 13.0000 (4 s.f.) How do we find the pH of a titration before the equivalence point is reached? When add 1.00 mL of 0.1000M NaOH to 100.0mL of 0.1000M HCl [H3O+] = ((CacidVacid – CbaseVbase)/(Vacid + Vbase) [H3O+] = (100.0 mLx 0.1000M–1.0mL x 0.1000M)/(100+1mL) [H3O+] = 0.09802M pH = 1.0087 (4s.f.) [OH-] = Kw/[H3O+] = 1.020 x 10-13M pOH = 12.9913 (4s.f.) Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids & Bases How do we find pH at equivalence point? HCl + NaOH  HOH + Na+ Cl- Autodissociation of water governs the pH at equivalence point. HOH + HOH  H3O+ + OH- Kw = [H3O+][OH-] = 10-14 CH+ =(Kw)1/2 = 1.000 x 10-7M pH = 7.0000 (4s.f.) How do we find pH after equivalence point? Excess base add [OH-] = ((CbaseVbase – CacidVacid)/(Vbase + Vacid) [H3O+] = Kw /[OH-] pH = -Log [H3O+] pOH = -Log [OH-] then pH = 14 - pOH The text page 283 shows Table summarizes the equations governing the different portions of the titration curve. A strong acid – strong base titration curve has a large end point break. Phenolphthalein is used as an indicator because the colorless to pink transition is easy to see. This titration curve was constructed using a spreadsheet (next slide). ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.1. Titration curve for 100 mL of 0.1 M HCl versus 0.1 M NaOH. Compare the species and equations for the strong acid with the calculations in the previous spreadsheet. A strong base-strong acid titration is treated similarly, but we start with excess base, and end with excess acid. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) As the concentrations of acid and titrant decrease, the end point break decreases. So the selection of indicator becomes more critical. 0.001 M 0.01 M 0.1 M ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.2. Dependence of the magnitude of end-point break on concentration. The concentrations of acid and titrant are the same. Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids & Bases add standard HCL solution to NaOH solution: HCl + HOH  H3O+ + Cl- [H3O+] = C(HCl) = Ca NaOH  Na+ + OH- [OH-] = C(NaOH) = Cb HCl + NaOH  Na+ + Cl- + HOH H3O+ + OH-  HOH net ionic reaction When add 1.00 mL of 0.1000M HCl to 100 mL of 0.1000M NaOH (pH = 13.0000) Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids & Bases What is the pH before titration begins? 100.0 mL of 0.1000M NaOH [OH-] = 1.000 x 10-1M [H3O+] = Kw/[OH-] = 1.000 x 10-13 pH = - Log [H3O+] = 13.0000 (4s.f.) How do we find the pH of a titration before the equivalence point is reached? When add 1.00 mL of 0.1000M HCl to 100.0mL of 0.1000M NaOH [OH-] = ((CbVb – CaVa)/(Vb + Va) [OH-] = (100.0mL x 0.1000M – 1.0mL x 0.1000M)/(101mL) [OH-] = 0.09802M [H3O+] = Kw/[OH-] = 1.020 x 10-13M pH = 12.9913 (4s.f.) Volumetric Analysis Acid-Base Titrations Titration Curves for Strong Acids & Bases How do we find pH at equivalence point? HCl + NaOH  HOH + Na+ Cl- Autodissociation of water governs the pH at equivalence point. HOH + HOH  H3O+ + OH- Kw = [H3O+][OH-] = 10-14 c =(Kw)1/2 = 1.000 x 10-7 pH = 7.0000 (4s.f.) How do we find pH after equivalence point? Excess acid added [H3O+] = ((CaVa – CbVb)/(Va + Vb) pH =-Log [H3O+] pOH = 14 – pH This is the mirror image of the HCl titration curve. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.3. Titration curve for 100 mL of 0.1 M NaOH versus 0.1 M HCl. Volumetric Analysis Acid-Base Titrations Acid-Base Color Indicators Organic weak acids or bases Strongly colored acid and/or conjugate base form(*) Distinct color change from acid to base form Acid conj. Base HIn + OH-  In- + HOH Color A Color B (*) Only very small amount HIn required to give color ( 10-3 M Volumetric Analysis Acid-Base Titrations Acid-Base Color Indicators Color A Color B HIn + HOH  In- + H3O+ Ka = [H3O+][In-]/[HIn] [H3O+] = Ka x [HIn]/[In-] NOTE: [HIn]/[In-] determines solution color [H3O+] determines [HIn]/[In-] High [HIn]/[In-] (>10:1) gives Color A Low [HIn]/[In-] ( 10:1 pH > pKa + Log(10/1) Color B, pH = pKa + 1 Indicator color change occurs at pH = pKa ± (1) pH transition range = pKa ± 1. We select an indicator with a pKa near the equivalence point pH. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.4. pH transition ranges and colors of some common indicators. Titration of weak acid and strong base @ midpoint pH=pKa Example: Titrating 50.0mL 0.1M HOAc with 0.1M NaOH: HOAc + NaOH  HOH + OAc- + Na+ Initial Concentration of HOAc = CHA = 0.1M Equilibrium Concentration HOAc = Ca Equilibrium Concentration OAc- = Cs (a) Initial pH: [H3O+]  (Ka x CHA)1/2 = (1.75 x 10-5 x 0.1)1/2 [H3O+] = 1.323 x 10-3 M pH = 2.8785 (b) Buffer Region: (e.g. 10.0 mL NaOH added): [OAc-] = Cs = CbVb/(Va + Vb); Ca = (CHAVa – CbVb)/(Va + Vb) Cs = (0.1M x 10.0mL)/(60.0mL) = 0.0167M Ca = 0.0667M [H3O+] = Ka[HOAc]/[OAc-] = Ka Ca/Cs [H3O+] = (1.75 x 10-5)(0.0667/0.0167) = 6.99 x 10-5M pH = 4.1556 Acid-Base Buffers Buffer Region of Titration Curves Titrating 50.0mL 0.1M HOAc with 0.1M NaOH: HOAc + NaOH  HOH + OAc- + Na+ Buffer Region (e.g. 25.0mL NaOH added): [OAc-] = Cs = CbVb/(Va+Vb); Ca = (CHAVa- CbVb)/(Va+Vb) Cs= (0.1M)(25.0mL)/(75.0mL) = 0.0333M; Ca = 0.0333M [H3O+] = Ka[HOAc]/[OAc-] = KaCa/Cs [H3O+] = (1.75 x 10-5)(0.0333/0.0333) = 1.75 x 10-5 M pH = - Log[H3O+] = 4.757 (3s.f.) Note: at 50% titration, ½ neutralization point, pH = pKa Volumetric Analysis Acid-Base Titrations Titrating 50.0mL 0.1M HOAc with 0.1M NaOH: HOAc + NaOH  HOH + OAc- + Na+ (c) Equivalence Point pH: [HOAc]  0 = Ca [OAc-]  CbVb/(Va+Vb) = Cs = 0.1M/2  0.05M OAc- + HOH  HOAc + OH- Kb = Kw/Ka = 5.71x 10-10 [HOAc][OH-] = Kb[OAc-] = KbCs; [HOAc]  [OH-] [OH-]  (KbCs)1/2  (5.71 x 10-10(0.05))1/2 = 5.34 x 10-6M [H3O+] = Kw/[OH-] = 1.87 x 10-9M ; pH = 8.728 (3 s.f.) Volumetric Analysis Acid-Base Titrations Titrating 50.0mL 0.1M HOAc with 0.1M NaOH: HOAc + NaOH  HOH + OAc- + Na+ (d) pH After Equivalence Point: Bases present are OAc- & OH- (from excess NaOH) Excess NaOH is stronger base and determines pH Excess [OH-] = (CbVb – CHAVa)/(Va + Vb) At 0.1 mL past equivalence point. Vb = 50.1 mL [OH-] = ((0.1M)(50.1mL) – (0.1M)(50.0mL))/(100.1mL) [OH-] = 9.99 x 10-5M; [H3O+] = Kw/[OH-] = 1.001 x 10-10 pH = 9.9996 Titration of weak base and strong acid Titration of weak acid and strong base Salt is basic so, equivalence point comes at a pH > 7. The start of the graph shows a relatively rapid rise in pH but this slows down Titration of weak base & strong acid Salt formed is acidic, hence, equivalence point comes at a pH < 7. At the very beginning of the curve, the pH starts by falling quite quickly as the acid is added, but the curve very soon gets less steep. A weak acid gives a smaller end point break.. A stong base titrant is always used. We start with HOAc. Then we have a buffer mixture of OAc- and HOAc. At the equivalence point, we have OAc-, a weak base. Beyond the equivalence point, we have excess NaOH (suppresses OAc- hydrolysis), and the curve follows that for a strong acid titration. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.5. Titration curve for 100 mL 0.1 M HOAc versus 0.1 M NaOH. For a weak acid, we start with ionization of HA. In the buffer region, use the Henderson-Hasselbalch equation. At the equivalence point, A- hydrolyzes as a weak base. Beyond the equivalence point, excess OH- dominates. A weak base is treated similarly, but beginning with base and ending with acid. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) The buffer regions are about the same for all concentrations. The equivalence point pH increases with increasing concentration. 0.1 M 0.01 M 0.001 M ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.6. Dependence of titration curve of 100 mL acetic acid on concentration. NaOH concentration the same as HOAc concentration. The weaker the acid, the smaller the break and the more alkaline the equivalence point. Visual indicators can be used for Ka of 10-6. A pH meter provides better precision for weaker acids. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.7. Titration curves for 100 mL 0.1 M weak acids of different K values versus 0.1 M NaOH. This is the reverse of the HOAc titration curve. We start with NH3 weak base. Then we have a buffer mixture of NH4+ and NH3. At the equivalence point, we have NH4+, a weak acid. Beyond the e.p., we have excess HCl, which suppresses the hydrolysis of NH4+, and the curve follows that for a strong base titration. Fig. 8.8. Titration curve for 100 mL 0.1 M NH3 versus 0.1 M HCl. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) The weaker the base, the smaller the break and more acid the equivalence point. Visual indicators can be used for Kb of 10-6. A pH meter provides better precision for weaker bases. ©Gary Christian, Fig. 8.9. Titration curves for 100 mL 0.1 M weak Analytical Chemistry, 6th Ed. (Wiley) bases of different Kb values versus 0.1 M HCl. Polyfunctional Acids/Bases Equilibria Polyfunctional Acids: H3PO4 , H2CO3 , H2SO4 , H2SO3 , H2C2O4 , H2S H2S + HOH  H3O+ + HS- Ka1 = 5.7 x 10-8 HS- + HOH  H3O+ + S2- Ka2 = 1.2 x 10-15 Polyfunctional Bases: ethylenediamine (NH2C2H4NH2), CO32-, PO43-, HPO42-, S2- PO43- + HOH  HPO42- + OH- Kb1 = Kw/Ka3 = 2.4 x 10-2 HPO42- +HOH H2PO4- + OH- Kb2 = Kw/Ka2 = 1.6 x 10-7 H2PO4- +HOH H3PO4 + OH- Kb3 = Kw/Ka1 = 1.4 x10-12 Polyfunctional Acids/Bases Titration Curves Polyfunctional acids titrated with strong base: Separate equivalence points observed if ratios of successive dissociation constants > ~104 Equivalence point can be observed for dissociation step where Ka > ~10-8 e.g. H3PO4 Ka1 = 7 x 10-3, Ka2 = 6 x 10-8, Ka3 = 4 x 10-13 – First two end points can be observed – Third end point is not observed Polyfunctional Acids/Bases Titration Curves Polyfunctional acids titrated with strong base: [H3O+] in solution of amphiprotic anion (HA-) OH- + H2A  HA- + HOH At first equivalence point, [HA-] = Cs  added OH- [H3O+] = ((Ka2Cs + Kw)/(1+ Cs/Ka1))1/2 If Cs/Ka1 >> 1; Ka2Cs >> Kw; then [H3O+]  (Ka1Ka2)1/2 Or pH = (pKa1 + pKa2)/2 We start with a weak acid, H2A, followed by a buffer region of HA- and H2A. The first equivalence point is HA- ([H+] ≈ constant). Then we have a A2-/HA- buffer region, and A2- (a fairly strong base) at the second equivalence point, followed by excess titrant. Fig. 8.12. Titration of diprotic acid, H2A, with sodium hydroxide. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) We start with ionization of H2A, a weak to moderately strong acid. In the two buffer regions, use the Henderson-Hasselbalch equation. At the first equivalence point, HA- has [H+] ≈ √Ka1Ka2. At the second equivalence point, A2- hydrolyzes as a fairly strong base. Then excess OH- dominates. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) The strong acid titrates first. At its equivalence point, we have a mixture of NaCl and HOAc, and the pH is acidic. This is followed by a buffer region of OAc- and HOAc, and then the HOAc equivalence point, where we have OAc-, a weak base. The weak acid Ka must be no larger than 10-5 to give a sharp second end point. For two weak acids, the Ka’s should differ by 104 or more. Fig. 8.13. Titration curve for 50 mL of mixture of ©Gary Christian, Analytical Chemistry, 0.1 M HCl and 0.2 M HOAc with 0.2 M NaOH. 6th Ed. (Wiley) Titration of weak triprotic acid vs strong base We start with CO32-, a quite strong base. Then we have a HCO3-/CO32- buffer. At the first equivalence point, we have HCO3- ([H+] = √Ka1Ka2). Then we have a HCO3-/H2CO3 buffer, and H2CO3 at the second equivalence point. The first e.p. is used to approximate the second, which is more accurately used. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.10. Titration curve for 50 mL 0.1 M Na2CO3 versus 0.1 M HCl. Dashed line represents a boiled solution with CO2 removed. Titrate till the methyl red indicator (gradually) changes from yellow through orange to red (occurs just before the equivalence point). Then boil to remove CO2 and continue titration for a sharp end point to a pink color. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 8.11. Titration of 50 mL 0.1 M Na2CO3 with 0.1 M HCl using methyl red indicator. Amino acids are polyprotic The resulting structure, with positive and negative sites, is called a zwitterion Titrations of polyprotic amino acids Titration of glutamic acid The isoelectric point is the pH at which there is zero net charge pI = (pKa1+pKa2)/2 Titration of lysine Appendix: Aqueous Titrations from USP ‫المعايرة كما في دساتير الصيدلة‬ Direct titration methods Sodium bicarbonate chemical compound with the formula NaHCO₃ Sodium salicylate C7H5NaO3. Its chemical structure: Indirect titration methods (residual or back titrations) Calamine: ZnO Sodium lactate: Ephedrine: Alkalimetry (direct titration method) Some of the acidic drugs structure Benzoic acid Chlorpropamide Indomethacin Ibuprofen Citric acid Frusemide Non-aqueous titration of basic drugs Some of the basic drugs structure Nicotinamide Metronidazole benzoate Noscapine Hydrochloride Hydrate

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