Volumetric analysis To understand volumetric analysis, we must understand the types of reaction that happen in it. Types of reactions used in volumetric analysis : I- Ionic combination reactions:- - The reaction goes to completion due to formation of slightly ionizable or slightly insoluble products. a- Neutralization reaction : In which acid reacts with base to form slightly ionized water. H+ + OH- H2O b- Formation of precipitate : Ag+ + Cl- AgCl c- Formation of slightly ionizable complex : Ag+ + 2 CN- [Ag(CN)2]- Ca+2 + H2Y-2 [EDTA] 2H+ + CaY-2 [Ca-EDTA complex] • II- Electron transfer reactions : • In which electron transfer from one reactant to another. It is called • (oxidation -reduction reactions) Ce+4 + Fe+2 Ce+3 + Fe+3 i.e. Fe+2 Fe+3 + e oxidation (loss of es.) Ce+4 + e Ce+3 reduction (gain of es)
Acid-Base Acid- base theories 1- Arrhenius theory :- Acid : Is the substance which ionize to give H+ eg. HCl Base : Is the substance which ionize to give OH- eg NaOH 2- Bronsted - Lowry theory :- Acid : Is the substance which donate proton. Base : Is the substance which accept proton. Every acid has a conjugate base and the base has conjugate acid. The stronger the acid the weaker its conjugate base and vice versa. Eg. HCl + H2O Cl- + H3O+ Acid base conj.base conj.acid Eg. NH3 + H2O NH4+ + OH- base acid conj.acid conj.base N.B. Water behave as acid or base because it is neutral.
3- Lewis theory :- Acid : Is substance which accept lone pair of electrons eg. BF3, AlCl3. Base : Is substance which donate lone pair of electrons eg NH3, amines. Acid-base titration in aq. medium Solns. are classified into :- Electrolytes: Which desociate (ionize) and conduct electricity. orNon electrolytes : Which doesn't ionize and doesn't conduct electricity. Dissociation of water H2O H+ + OH- Dissociation const. Kw = [ H+] [OH-] / [H2O] - Since H2O is weakly dissociated , therfore H2O is considered unity. therfore Kw = [H+] [OH-] = 10 -14 at 25oc Kw : it is called ionic product of water. At 25oc [H+] =[OH-] = 10-7 If [H+] = [OH-] , therfore soln. is neutral If [H+] > 10-7 eg 10-6, 10-5 , therfore soln. is acidic If [H+] < 10-7 eg 10-8, 10-9 , therfore soln. is alkaline.
Hydrogen exponent : pH pH = -log [H+] i.e. If [H+] = 10-7 pH = - log 10-7 = 7 In acidic side i.e. If [H+] = 10-6 pH = - log 10-6 = 6 In basic side i.e. If [H+] = 10-8 pH = - log 10-8 = 8 i.e. as pH value inc. [H+] conc. decrease. Therefore acid soln has pH less than 7 , alkaline soln. has pH more than 7 and neutral soln. has pH = p OH = 7 pH of acid and bases :- 1- pH of strong acids :- Since strong acids are strongly ionized. Therfore pH = pCa where Ca ( conc. of acid) i.e. 0.1 N HCl pH = - log 0.1= - log 10-1 = 1 2- pH of strong bases :- Since strong bases are completely ionized. Therfore p OH = p Cb where Cb (conc.of base) pH = p Kw – p OH i.e. pH = p Kw – p Cb. i.e. 0.1 N NaOH pH = 14 _ 1 = 13 3- pH of weak acids : pH = 1/2 pCa + 1/2 pKa 4- pH of weak bases :- pH =pKw - 1/2 pCb - 1/2 pKb
5- pH of salts :- a- Salt of strong acid and strong base eg. NaCl Always neutral i.e. pH = 7 b- Salt of strong acid and weak base eg.NH4Cl Always pH is in the acidic side , calculated from eq. pH = 1/2 pKw - 1/2 pKb + 1/2pCs where Kb (dissociation constant of weak base) Cs (conc. of salt) c- Salt of weak acid and strong base eg. Na Ac Always pH is in the alkaline side, calculated from eq.: pH = 1/2 pKw + 1/2 pKa - 1/2pCs where Ka (dissociation constant of weak acid) Cs (conc. of salt) d- Salt of weak acid and weak base eg. NH4Ac pH is calculated from eq. : pH = 1/2 pKw + 1/2 pKa - 1/2pKb Buffer solutions -Def : They are solns which resist changes in pH upon addition of small amount of acid or base. -They consist of weak acid and its salt or weak base and its salt Type 1- weak acid and its salt eg. HAc and Na Ac pH of this buffer is calculated from the eq.: pH = pKa + log salt / acid
Type 2- weak base and its salt eg. NH4OH and NH4Cl pH of this buffer is calculated from the eq.: pH = pKw - pKb - log salt/base log salt/acid or log salt/base is called buffer ratio if [salt] = [acid] therefore pH =pKa Examples 1- Calculate the pH of a buffer soln. containing 0.1 M acetic acid and 0.1 M sodium acetate pKa =4.76 soln. pH = pKa + log salt / acid pH = 4.76 + log 0.1 / 0.1 = 4.76 Neutralization indicators Color indicators: - Substance which change their color with change in pH are used as neutralization indicators. eg. phenol phthalein"Ph.Ph" (one color indicator), methyl orange"M.O" (2 color indicator) eg. Ph.Ph. =8-10 M.O. =3.3-4.4 M.R. = 4-6 N.B. the indicator is chosen according to pH of the product.
Neutralization titration curves For neutralization reaction titration . The titration curve is plot of pH versus the mls of titrant. Types of neutralization curves: 1- Strong acid -strong base titration :- eg. HCl and NaOH we have sample of 100 ml HCl and titrate against NaOH. • Before the titration pH is due to • the sample i.e HCl (strong acid) • therfore pH = pCa • b- At the equivalent point : HCl +NaOH NaCl + H2O pH is due to NaCl i.e. pH= 7 (salt of strong acid and strong base) c- After the equivalent point : pH is due to excess titrant i.e. NaOH (strong base) pH = pKw-pCb N.B. The pH rises slowly till 99.9 % of acid is titrated by adding 0.1 ml NaOH pH rises from 4 to 7 then another 0.1 ml after end point pH rises from 7 to 10 i.e. at e.p. pH rises from 4 to 10. So we can use M.O.(3.3 - 4.4)- M.R.(4 - 6)- Ph.Ph.(8 - 10) indicators