1 / 7

Volumetric analysis

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

inge
Télécharger la présentation

Volumetric analysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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)

  2. 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. 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.

  4. 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. 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

  6. 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.

  7. 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

More Related