Fe = Fe + e MnO + 8H + 5e = Mn

Transcription

1 Redox Titrtions Net trnsfer of electrons during the rection. xidtion numbers of two/more species chnge. Stisfies requirements for rections in quntittion;. lrge K b. fst rection Blncing redox rections: 1. identify the oxidtion & reduction species nd/processes. 2. blnce ech of them seprtely for mss, then chrge. (, -, 2 my be needed to blnce for species contining, ). multiply hlf rections to mke the number of electrons the sme in both hlf rections 2 e Mn 8 5e Mn 4 Mn 8 5 Mn oxidizing gent, oxidnt reducing gent, reductnt Note the chnges in oxidtion number. Mn 8 5 Mn oxidizing gent, oxidnt Mn C Mn C Mn 8 5e Mn 4 Cr Cr 7 2 N of C 1 N of C -1 N of C 2 C C 2 C C C C 6 C C 6 6e (Mn 8 5e Mn 4 ) 6 ( C C 6 6e) 5 6Mn 18 5C 6Mn C 2 2 Mn C Mn C Mn 8 5e Mn 4 Cu N Cu(N ) 2 N 2 C C 2 C C C C 6 Cu Cu 2 2e N -1 4 e N 2 2 C C 6 6e Cu 8N Cu(N ) 2 2N 4 2 (Mn 8 5e Mn 4 ) 6 ( C C 6 6e) 5 6Mn 18 5C 6Mn C 2 2 Note some nitrte ions re specttor ions.

2 Potentiometric monitoring lectrochemistry (Potentiometry - very bbrevited version): lectrochemistry dels with rections, where electrons exchnge between species. xidtion-reduction rections: reduced oxidized in the process [] [R] Indictor electrode If this rection cn be mde to occur t two sites, lectrochemicl cell. cell (V) lectrochemicl ll Voltic ll V lectrodes Convention: Left-[] Right-[R] cell >0 ZnCu 2 Zn 2 Cu Zn Zn 2 2e Cu 2 2e Cu spontneous, genertes energy. ZnCu 2 Zn 2 Cu Zn Zn 2 2e Cu 2 2e Cu spontneous, genertes energy. oxidtion (hlf) rection t one site nd reduction (hlf) rection t the other site electrochemicl cell cell 2 e node 4 e cthode node nd cthode hlf rections If chemicl species hs propensity to oxidize nd nother to reduce, combintion of them will result in redox rection; rection products - weker oxidnt/reductnt. Inherent propensity of species to reduce is mesured by their stndrd reduction potentils. Note: A species tht cn be reduced esily ( propensity to be reduced) implies the reverse process, i.e. the oxidtion is difficult. Inherent bility to undergo reduction cn be glened from the stndrd reduction potentil vlues. Stndrd (reduction) potentil vlues of species re expressed referenced to stndrd proton reduction potentil, 25 o C potentil for 2 (q, 1) e 2 (g, p1tm) 0 /2 0.00V

3 lectrode potentil tbles re Stndrd Reduction Potentil tbles nd the hlf rection written s reduction. Good e - cceptor Poor e - donor Poor e - cceptor Good e - donor Construction of redox electrodes: Stndrd electrodes Inspect the hlf rection, identify the species involved nd their physicl sttes. Mix them up in solution, the ctivities of ech species must be t their stndrd sttes (unity). If there re no species tht is metllic use Pt wire (for electricl contct). Silver-silver chloride electrode: Ag Cl(1.0) AgCl(s) AgCl(s) e Ag(s) Cl (q) Nottion: Cl - (1)/AgCl(s)/Ag e.g. Mn 8 5e Mn 4 Pt Nottion: Mn 2 (1),Mn 4 (1), (1)/Pt KMn 4 (1) 2 S 4 (0.5) MnS 4 (1)

4 o cell A mesure of the inherent bility for rection to occur lectrodes non stndrd: In non-stndrd electrodes the concentrtions/ctivity of one or more of the species involved is not unity. Consider the redox rection 2 4 Would this redox rection occur spontneously? ow quntittive is this rection (K?)? Clculte 0 cell for the rection s written; if positive - rection occurs; spontneous nd if lrge the K is lrge. 2 e node e 4 0 cell cthode from tbles (reduction potentils) / / 2 hlf rections Prediction of the propensity to rect, K, relted to free energy chnge,... G -RTln K -cell cell reduction.pot. oxidtion.pot. electrode cthode cell cthode node node tbulted s reduction potentils - (from tbles) cell red.pocess oxd.process o cell Consider the redox rection e node 4 e cthode cell red.pocess oxd.process cell cthode - node - (from tbles) cell 4 / / V hlf rections (Lrge nd) positive o cell processes re good cndidtes for redox titrtion rections; lrger o cell gives shrper end points (lrge K vlues). lectrode potentil (in generl); Nernst eqution for electrode clcultes the propensity for reduction hlf rection to occur when species re t given set of concentrtions. It is mesure of the reducibility of the species in solution. Q rection quotient. 0 RT h ln(q) 0 RT rxn cell cel l ln(q) lfrxn el el el cell oxidized ne reduced ln el 0 reduced el oxidized Note: Q for electrode is for the electrode rection written s reduction. Q lrge for system ~ el smll (nd or negtive) implies poor electron cceptor/ implies good electron donor - reducing system (nd vice vers) Q

5 (IV) For the rection: (II), (III),(III) Soln.pot. ln 0 cthode 4 4 / / 4 Soln.pot. ln node 2 2 / / Note Q for electrode is for the electrode rection written s reduction. In potentil clculted from ny one of the two equtions bove re the sme nd is clled the solution redox potentil. nly one eqution is useful t given instnt for clcultion, however. During the titrtion the Q vlues chnge in systemtic mnner. ln / / 4 ln 2 2 / / The solution potentil clculted from the perspective of ny of the two redox equilibri tht exist in the system; is numericlly the sme. Mesuring the vrition of the solution potentil llows the monitoring of the progress of the rection/titrtion. To clculte the redox potentil in the solution, consider the pproprite equilibrium system, it is one of the two equilibri nd the one tht cn be esily hndled, rithmeticlly (mthemticlly). Potentils re clculted s reduction potentils (convention). Need to write the equilibri s reduction equilibri, e.g. e 2 Mn 8 5e Mn 4 The solution potentil clculted from the perspective of of ny redox equilibri tht exist in the system is numericlly the sme. Mn 8 5 Mn e Mn 8 5e Mn 4 Use Nernst eqution. Use Nernst eqution. ither, or e 2 Mn 8 5e Mn 4 ln 2 2 / / F RT ln Mn Mn 4 / Mn Mn 8 4 / Mn 5F 1 Mn4 cell mesures the propensity of (overll) rection, where the species re t ny given concentrtion. xmple 1. RT 0 ln(q) rxn cell cell cell Mn ( ) 8 ( ) 5 ( ) Mn ( ) 5 ( ) ln RT 2 Mn cell cell 8 5 5F 2 Mn RT 45 cell cell ln 8 5 5F 12

6 xmple 2 0 RT hlfrxn el el ln(q) el 0 RT rxn cell cell ln(q) cell For the two hlf rections, el ; ln 0 cthode / / 4 ln node 2 2 / / Solution potentil is determined by Q; chnging Q chnges the solution potentil. The rection progress is followed by following the redox potentil of the solution, which depends on Q, with n oxidtion-reduction potentil electrode. Pick the process (nodic/cthodic) to follow, judiciously. For Before eq. point Indictor rection After eq. point Indictor rection (IV) ln 2 2 / / (IV) ln / / 4 (II), (III),(III) (III), (IV),(III) n1 Before eq. point (IV) ~ very smll, 0; 2 chnges. After eq. point (II) very smll, ~0; 4 chnges. At the eq. point (II), (IV) very smll, ~0; But, 2 4 Q 4 2 concentrtions of species t end point Considering the redox potentil t eq. pt. from both perspectives RT ln 0 ep 4 4 / / 4 RT ln ep 2 2 / / At the eq. point (II), (IV) very smll, ~0; concentrtions of species t end point Considering the redox potentil t eq. pt. from both perspectives RT ln 0 ep 4 4 / / 4 RT ln ep 2 2 / / 2 1

7 ' ' ' ' 4 1 RT ln 0 ep 4 / 4 / 4 RT ln ep 2 2 / / Adding the bove two equtions: RT RT 2 ln ln ep 2 4 / 4 / 4 RT 2 ln ep 2 4 / 4 / ep 2 4 / 4 / RT ln1 2 Solution potentil t equivlence point. ep In generl, where n n ; / / 1 2 ep 2 n n n n The potentil of single electrode is not mesurble. nly differences in potentils cn be mesured. Thus coupling of the sensor electrode with nother electrode of known, unchnging potentil is employed, - stndrd reference electrode cell Note: The (reduction) potentil of the solution is mesured by coupling with reference electrode, ref (which remins constnt). cell el - ref. (convention) mesured cell sensor - SC SC cell vries in the sme fshion s el. sensor cell - SC sensor RP electrode

8 In generl, where rection is not 1:1 nd/or the rections re p dependent, clculte strting from Nernst qution. Study the hnd out. sensor In generl, where n n ; n1 1 n22 ep n n 1 2 Comprison of titrtion curves with sme titrnt 4 (q) (III)/ 0 (III) 0.68V U U(IV) / 0 U(VI) cell clcultion: / / 2 4/ / U 6/ U 4 4/ 0.4 > / 4/ cell chnge t end point higher if titrnt nlyte is lrge. This is lso mesure of the 'completeness' (i.e K) of the rection.

9 Redox Indictors: Substnces tht re reduced nd oxidized reversibly, two forms hve different colors. It must undergo reduction/ oxidtion s well. In the proximity of the end point, eq. In ox ne In red color I color II 2.0RT ind - log [ Inred ] [ In ] oxd [ Inred ] [ In ] oxd [ Inred ] [ In ] Must chnge by 100, t minimum to detect end point visully. e.g. 0.1 to 10 or vice vers. In the trnsition rnge solution potentil mesured s An electrode potentil must chnge; i.e. oxd el Ind ± n ± - n cell Ind ref The rnge dictted s el Ind ± n The indictor stndrd reduction potentil must fll in the rnge bove Simulted Titrtion Curve (el) mv/ ind Volume of Titrnt (Vt, ml)

10 ther indicting strtegies: Strch-Iodine complex for rections involving I 2. Self indictors where one of the rectnts is strongly colored, e.g. KMn 4. Smple Preprtion Some nlyte smples must be prepred for redox titrtion. Anlyte must be present in the correct oxidtion stte. e.g. For nlyzing iron with Sn 2, reducing titrnt ll the iron must be present s in order to be reduced to 2. If some iron is lredy present s 2, then it will not consume titrnt nd so escpes mesurement. Anlyte must be pre-oxidized or pre-reduced to convert the nlyte to single oxidtion stte. Adjusting oxidtion sttes of nlytes: 1. Chnge oxidtion sttes (oxidtion nd reduction) by chemicl rection with excess of pre-djustment chemicl gent. CrCl 2 2 S Zn/g (Jones reductor) Wlden Reductor 2. Remove un-rected chemicl gent. (red text) xmples: Mn(II) to Mn 4- (Ag 2 ) V(V) to V(IV) ( 2 S ) (III) to (II) (SnCl 2 ) Co(II) to Co(III) ( 2 2 )

11 Jones Reductor Zinc mlgm (strong) Wlden Reductor Ag in 1M Cl (mild/selective) Grnulr Cd (mild/selective)