Lecture 8 Application of Acid-Base Titration PDF

Double Indicator Titrations It is a direct titration in which two indicators are used. Double indicator titrations are used for: 1- Mixture of two monobasic (monoprotic) acids e.g. HCl / CH3COOH. 2 - Dibasic (diprotic) acids e.g. H2SO4 and H2CO3. 3- Polybasic (polyprotic) acids e.g. H3PO4....

Double Indicator Titrations It is a direct titration in which two indicators are used. Double indicator titrations are used for: 1- Mixture of two monobasic (monoprotic) acids e.g. HCl / CH3COOH. 2 - Dibasic (diprotic) acids e.g. H2SO4 and H2CO3. 3- Polybasic (polyprotic) acids e.g. H3PO4.  Requirement of Double Indicator Titrations The difference between pKa of two acids equal to 4 or more, or the ratio of ionization constant (Ka) of two acids must be at least equal to 104. Ka1 / Ka2 ≥ 104 or pKa1 - pKa2 = 4 or more  1-Determination of mixture of two monobasic acids a-Determination of mixture of HCl and CH3COOH By common ion effect, HCl suppress the ionization of HAC, so titration with standard NaOH using M.O → HCl. HCl H+ + CL- CH3COOH H + + CH3COO - Add ph.ph and complete titration with standard NaOH. ph.ph reading will equivalent to HAC. If new sample is titrated in presence of ph.ph, the reading will equivalent to total acidity. b- Determination of mixture of HCl and Boric acid Titration sample of mixture with standard NaOH in presence of M.O as indicator. M.O reading → HCl ------- (1) New sample is titrated with standard NaOH in presence of ph.ph after addition of glycerol. ph.ph reading will equivalent to total acidity ---- (2) Boric acid only = (2) – (1) 2- Determination of Dibasic acids: Dibasic acid contains two replaceable H+. Its ionization occurs in two successive steps and each step has its characteristic Ka Usually Ka2 less than Ka1. If the ratio of Ka1 / Ka2 is ≥ 104, the determination of each step separately can be done. If the ratio of Ka1 / Ka2 < 104 the inflection in titration curve is small and no sharp end point is observed and the acid titrated in a single step. According to pH of the step, suitable indicator is used. Examples: H2SO4: The two ionization constants in H2SO4 are near to each other Ka1 / Ka2 < 104 so, available H+ titrated in one step. H2CO3: Also has two separate ionization constants Ka1 / Ka2 >104 but H+ from second step is too small to be titrated directly and acid titrated with a base to first stage only using ph.ph as indicator where pH of first step is 8.5. 3- Determination of Tribasic acids e. x. H3PO4: H3PO4 H+ + H2PO4- pK1 = 2.12 H2PO4- H+ + HPO4-- pK2 = 7.12 HPO4-- H+ + PO4--- PK3 = 12.3 Ka1 / Ka2 > 104. pH at first equivalence point = ½pKa1 + ½pKa2 = 4.6 M.O pH at second ionization step = ½pKa2 + ½pKa3= 9.7 ph.ph From titration curve, pH break is sufficient to determine 1st step by M.O. and also to determine the 2nd by ph.ph. while the third proton in H3PO4 cannot be determined directly since ka3 is very small, it can be titrated indirectly by adding CaCl2 to precipitate PO43- as Ca3(PO4)2 and release an equal amount of HCl which can be directly titrated. 2HPO42- + 3Ca2+ → Ca3(PO4)2 + 2H+  Determination of Easily Hydrolysable Salts a-Determination of Na2CO3: Na2CO3 + HCl = NaHCO3 + NaCl pH =8.3 NaHCO3 + HCl = H2CO3 + NaCl pH=3.8 Na2CO3 + 2HCl = CO2 + H2O + 2NaCl Titration of Na2CO3 takes place on two steps. First reading (ph.ph) give ½ neutralization of Na2CO3 so, that gives ½ CO32- , While second reading (M.O) gives total CO32-. b- Determination of NaHCO3: Titration against HCl using M.O. indicator. C- Determination of Borax: Borax when dissolved in water, hydrolyses to sodium hydroxide and boric acid, the amount of sodium hydroxide produced is equivalent to borax and equivalent to half boric acid. Na2B4O7 + 7H2O → 4H3BO3 + 2NaOH Liberated NaOH # standard HCl using M.O. as indictor H3BO3 not affect it. After complete titration of NaOH, boric acid can be titrated in presence of glycerol against standard NaOH using ph. ph. as indicator. Readings: M.O ≡ NaOH ≡ Borax Ph.ph ≡ Boric acid ≡ 2 borax d- Determination of Mixtures: 1-Mixture of CO32- / HCO3- 1- For total alkalinity (CO32- / HCO3-), solution is titrated against HCl using M.O as indicator. --------(1) 2- For half CO32- , another solution is titrated against HCl using ph.ph as indicator. --------(2) 3- (2) x 2 = CO32- 4- (1) – (3) = HCO3-  2-H3BO3 / Na2B4O7 Mixture 1- Borax: Liberated NaOH against standard HCl using M.O. -------- (1) 2- Total boric acid: After complete titration of NaOH, total boric acid can be titrated after addition of glycerol against standard NaOH using ph. ph. as indicator. -------- (2) 3- (1) x 2 = boric acid from borax. ------ (3) 4- (2) – (3) = original boric acid Biphasic titration Used for water soluble salt the acid of which is water insoluble e.g. sodium benzoate and sodium salicylate. During titration with HCl, the librated acid (benzoic or salicylic acids) gives acidic pH and causes color change before end point. To overcome this problem, we use organic solvent e.g. ether in which solubility of salicylic acid is 250 times more than water, to isolate salicylic acid from the sphere of reaction and accordingly not affect pH of the solution. Indicator used bromophenol blue at end point gives green color.