Titration, Buffers, Indicators PDF

Acid-base titration Continuation Titration of a weak acid by a strong base Titration of a weak acid - Starts at higher pH. - The equivalence point is in the basic pH rang...

Acid-base titration Continuation Titration of a weak acid by a strong base Titration of a weak acid - Starts at higher pH. - The equivalence point is in the basic pH range: salt of a weak acid and a strong base. pKa - The pH range in which the rapid change occurs is Titration of a strong acid shorter. Titration of a polyprotic weak acid by a strong base I. Diprotic acid: Titration of a polyprotic weak acid by a strong base II. Triprotic acid:  Titration of bases by a strong acid A strong base by a strong acid A weak base by a strong acid Equivalence point: Equivalence point: pH 7 pH below 7 Salt of a strong acid and a weak base Salt of a strong acid and a strong base  Indicators Acid-base indicators are weak acids or bases having differently colored acidic and basic forms. HInd ⇌ H+ + Ind different colors Phenolphthalein Methyl orange in acid in base in acid in base  Indicator range HInd ⇌ H+ + Ind [H+][Ind] KInd = Indicator dissociation constant [HInd] Indicator range: within ±1 of the pKlnd value Answer the questions Buffers  Buffers A buffer is a solution that has the ability to resist changes when a limited amount of acid or base is added to it.  Biological fluids (e.g. blood) are usually buffer solutions, the control of pH is vital to proper functioning of these fluids.  Buffers contain either a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer solution contains both an acid species and a base species in equilibrium.  Buffers resist changes in pH 14 12 víz water 10 8 pH buffer puffer 6 4 2 0 0 0,02 0,04 0,06 0,08 0,1 Amount of (bázis) erős sav added strong acid or mennyiség base (mmol) / mmol Buffers work well only for limited amounts of added strong acid or base.  Examples for buffers Acetate buffer For example, CH3COOH ⇌ CH3 COO + H+ CH3COOH Weak acid Conjugate and base CH3COONa+ Ammonia/ammonium chloride buffer NH4OH ⇌ NH4+ + OH NH3 Conjugate and Weak base acid NH4Cl A buffer solution contains either a weak acid and its salt or a weak base and its salt.  pH of buffers – Henderson- Hasselbalch equation I. Acidic buffers Basic buffers HA ⇌ H+ + A BOH ⇌ B+ + OH [H+][A] [B+][OH] Ka = Kb = [HA] [BOH] [HA] [BOH] [H ] = Ka   + [OH] = Kb  [A ] [B+] [HA] [BOH] log[H+] = logKa  log  log[OH] = logKb  log [A ] [B+] [A] (Note that A and [B+] pH = pKa + log B+ come mostly pOH = pKb + log [HA] [BOH] from the salts.)  pH of buffers – Henderson- Hasselbalch equation II. Acidic buffers Basic buffers [A] [B+] pH = pKa + log pOH = pKb + log [HA] [BOH] Important notes: 1.) A and B+ come mostly from the salts. 2.) It is possible to use numbers of moles rather than molarity. Since everything occurs in the same volume of solution, the ratio of moles is the same as the ratio of molarities. E.g.: nA [A] Vbuffer nA = = [HA] nHA nHA Vbuffer  Henderson-Hasselbalch equation Acetate buffer [CH3COO] nacetate ion pH = pKa, acetic acid + log [CH3COOH] nacetic acid [CH3COONa] pH = pKa, acetic acid + log [CH3COOH] Ammonia/ammonium chloride buffer [NH4+] nammonium ion pOH = pKb, ammonia + log [NH3] nammonia [NH4Cl] pOH = pKb, ammonia + log [NH3]  Buffer capacity Buffer capacity is the number of moles of a strong monoprotic acid or base added to 1L of a buffer solution causing 1 unit change in the pH. Can only be used for the linear part of the titration curve. More precise definition: Buffer capacity is the reciprocal of the slope of the fitted line on Linear part of the curve the part of the titration curve where the system acts as a buffer: dn dpH  The factors that affect buffer capacity Buffer capacity depends on two factors:  the amount of acid and conjugate base (or base and conjugate acid) Having more of these, the buffer capacity is greater.  the ratio of amounts of acid and conjugate base (or base and conjugate acid) This ratio should be close to 1 (between 1:10 and 10:1), otherwise the buffer capacity is too low. pH optimum of buffers  How do buffers work? – The common-ion effect Common-ion effect is a shift in an ionic equilibrium caused by the addition of an ion that takes part in the equilibrium. CH3COOH ⇌ CH3COO + H+ Adding HCl to it, that provides H+ ions (H+ ion is a common ion), the equilibrium is shifted to the left. (LeChatelier’s principle)  How do acidic buffers work? HA ⇌ H+ + A Adding an acid (H+ ions) to it Equilibrium is shifted (common-ion effect): HA H+ + A Very small decrease in pH. Adding a base (OH ions) to it OH ions form H2O with H+ ions, so [H+] decreases. Equilibrium is shifted (Le Chatelier principle): HA H+ + A The produced H+ ions form H2O with almost all of the added OH ions. Very small increase in pH.  How do basic buffers work? BOH ⇌ B+ + OH Adding a base (OH ions) to it Equilibrium is shifted (common-ion effect): BOH B+ + OH Very small increase in pH. Adding an acid (H+ ions) to it H+ ions form H2O with OH ions, so [OH] decreases. Equilibrium is shifted (Le Chatelier principle): BOH B+ + OH The produced OH ions form H2O with almost all of the added H+ ions. Very small decrease in pH.  Physiological buffers Hydrogencarbonate/carbon dioxide buffer: HCO3/CO2 It keeps a constant pH in the blood. (In a healthy person: around 7.4) Deviations from the normal pH cause serious problems: pH > 7.4 alkalosis pH < 7.4 acidosis Phosphate buffer: H2PO4/HPO42 One of the most important intracellular buffers of living systems.  Hydrogencarbonate/carbon dioxide buffer Also called: bicarbonate/carbon dioxide buffer. Three equilibria: 1.) Dissociation of carbonic acid H2CO3 ⇌ HCO3 + H+ 2.) Equilibrium between the dissolved CO2 and carbonic acid CO2(aq) + H2O ⇌ H2CO3(aq) 3.) Solution process of CO2 gas CO2(g) ⇌ CO2(aq)  Regulation of the pH of blood Shift in equilibrium when [H+] decreases. Shift in equilibrium when [H+] increases. Increase in [H+] (acidosis) will increase the partial pressure of CO2 until there is enough HCO3. The body will increase Blood breathing, expelling CO2. Decrease in [H+] (alkalosis) will decrease the partial pressure of CO2. Compensatory mechanism: Lung alveolar hypoventilation with a rise in arterial CO2 tension.  Phosphate buffer The second ionization step of H3PO4 is involved: H2PO4 ⇌ HPO42 + H+ Ka,2 = 6.2  108 Weak Conjugate acid base [HPO42] pH = pKa,2 + log [H2PO4] For example, NaH2PO4 and Na2HPO4 can be used. In living systems: H2PO4 and HPO42 anions are mostly the anions of nucleotides, ATP and sugar phosphates. October 28, Monday: Consultation October 29, Tuesday: MTO