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Ch15-Note - Chapter 15 Elect-odes and Potenfiomtry page I...

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Unformatted text preview: Chapter 15 Elect-odes and Potenfiomtry page I Ch. 15 Electrodes and Potentiometry - In potentiometer. electrodes are used to measure voltages that provide chemical information about a specific analvte. - An indicator electrode or working electrode responds to changes in the activity of the analyte while a reference electrode (self-contained half cell} produces a constant reference potential. - Ion-selective electrodes {e.g. pH glass electrode} respond mainlyr to one ion that selectively interacts with a specialized membrane of the electrode. - The potential difference across the membrane (E) depends on the activity {.14} of the analyte target ion {of charge a} in the external analvte solution. At 25°C: E = const. + (W) log(fl) Elam-dos and Potcnflomclry Reference Electrodes Aglfigcllfll‘ plus Anod : Ag + Cl’ —r AgGl + e‘ Cathode: Fe“ + e‘ —r Fe2+ AQ’iS} | AQGHS} I GHaq. 531d} ll Feflfaq}, Fehfaq] I Ft{s]l Chapter 15 Chapter 15 Elect-odes and Patenfin‘mltl'y Silver-silver chloride reference electrode 1lr‘ll‘ire lead Air inlet to allow . . . Double-junction 33$?gtgfiggram reference electrode pornug pmg Inner electrode Outerccmpartment Ag wire bent Into a loop Pmm Plugs Aqueous solution saturated with illgtS} IAQCHS) | Cit—{EGO || KCI and A90! AgCI paste Agles) + e' 7—“- Ag(s} + Cl'taq} Solid KCI plus _ some AgCl fig? KCI} — +0.19? V Porous plug for contact _ with external solution (salt bridge) Electrodes and Fourier-merry Saturated calomel electrode (S.C.E.) Wire lead Pt wire Hole to allow drainage through porous plug Heir) HQ, HQECIZ + KCI Glass wool Opening Saturated KCI solution KC|{s} Glass wall Porous plug {salt bridge} W5} | Half} IHflsHQzC'stCIU'JaSte] | KCI{ao1I || H2 HQ:CI2{S} + e‘ -——" Hgti} + Cl‘iaq} Mercury {I} chloride {calomel} E {satd KCI} = +0241 tr tool E") Chapter 15 Electrodes and Potenliorrtlelrr}I Relating reference electrodes A worklng electrode. A, has a potentlal w.r.t. a saturated calomel electrode {50E} of 41.436 V. What 1would thle voltage he w.r.t. SHE or eilver-ellver chloride electrodes? A ;— 41.436 [SC-E} {—-fl.392 Assigned (AQIAEICIJ 1Iralue of Ail IHQCI E.C.E 5.H.E. 1-11.19? +0241 1 l l {I I 4.2 5.H.E. scale in Potential versus 5.H.E. [trolls] A different electrode, B, has potential {Augmggfilnlts of +0318 ‘9' w.r.t. a slltre r-eiltrer chloride electrode. What are Ite potentials +0 315 bear.r.t. calomel and SHE? ' tSHEl Chapter 15 Elton-odes and Potenflomcrry Junction Potential Depends on Ion Mohiiitiee Free-1m Huerta-1D morzflfizgm“ ' 32:149. g§glqul=Eflwmmflc+mi+m+++gi dinnl'fiowl -++* *+ i 3 .5735 o‘” E. EIIIIMTT ”I g 55.8.: H E. EEEFEs-e eauweaw= Hoe ear-1:45:19? 3“ gig—jammy. hbhgmoqh Me—meomch 3-”; £113- :- g g-xxx xxxxxxxx XXXXXXX‘KXXEQ H 9.555 EEEEEEEE EEEEEEEEEEE augeeteeeteeeteeeteeteeea :3“ e a.“ . 5 . «a: E' 4e“. ‘.-" o: E :5 2: E EL 9., E .3 sun! JD caquow [-51 amt-:1 Chapter 15 Elect-odes and Flutenll'urrnetryI Junction potential ”13-5 |_.-:'l...'.5-"-.r-'F 5r.- —F Region rich Region rich in Na“ in El— Development of thejunction potential caused by unequal mobilities of Na+and Cl' {CI' 50% faster than Na"'} Liquid junction potentials at 15”C Mohilitics ofions in water at 25 0C Potential Ion Mobility [max-Ts * VJJ” Junllinn {IN} fl+ 35.3 x 1-3—3 {1.1 M Natl | M MKCI —c.4 Li: 4'0 X 19:: c.1 M Natl | 3.5 M Itcl -c.2 N“ 5'3 x 1" 1 n1 Natt|1s M “CI —1.9 K“ 115 X HTS _ {3.1 M HCI | M M KEI +2? DH” 20.5 mot o1chI ssmxcl 31 or 19 x m-S ' I ‘ + ' + men-n: the right side of thejuncfion heroines positive if H the left. How ionuselective “" $333“ eladru-da electrodes work am new hon-selective nun-Item E: const. + (W) loglfl) where n is the charge of the analyte [DH and A Is Its aclnr'ltjlr In the outer is} 5:13: 3:333» ?? I} are. g fl [unknown] solution. and E Is given ci- in Volts at 25°C. li'met aLl'faDe I:II membrane l' c+ 1ll'alirlomgwcin, K” complex Emcee; newline charge Emcee: posture charge Chapter 15 Electrodes and Potenfliirrnet'rj.I page it pH measurement with a glass electrode {+1 4 Lead: to pH meter ii- inlet uni-id level of flmm dot-node Liquid lent o'l itnlln'lerenoe m Ag | AgCI ire ': " aqueousfilllng solution *9 " -. : ll‘tulltlttl use iigci end no Sehtlon level April ”mended between twee-ides of ”I mlh Salt folded A3 wire r hrid E Pointer-plug to Illflfl' slow 9 drainage of electrolyte out ofelectro-de _ . _ Andi-1+ scan {a} |Eorn bination glass electrode with 0.1 II l-ICI . . . Glue membrane an...“ ,1... a silver-silver chloride reference not! electrode. {b} Electrode with plastic sleeve to protect th e delicate glass membrane. Chapter 15 Electrodes and Potenttometry Ion exchange equilibria on surfaces of glass is. ). {IL rf 1tlnm fl.1mrnt1l}’ninit 1flnrn ' ' Internal l l I I External i I A A a y ‘( solution solution A .f . ‘f' {flu-I- '- fl-‘ll W is variable} i! 'y be ‘9’ Hydrated gel Hydrated gel *- . V I. Y layer layer i If T '* - ti. " i . H” if“ '3 m V Ht '-o 0:,_ Schematic structure of glass. 1- O 0/“ Gray = cations H H" H+ Dark = silicon ":0 0" I oxygen Exchange sites All sites Exchange sites Cation in the glass electrode Mfupied Er occupied by Mar Clef-used I1? is Na” H and Na H and Na Migration of Na+ through the glass. no matter hfl‘w slowly. establishes electrical contact across the glass membrane. Chapter 15 Electrodes and Potenfimnetfil page ]] Solid-state ion-selective electrode To potentiometer O.‘ O y 7 :‘0.0 .01... O. .000. O 0.0.0 99 o .0000 Eu" Migration of F‘ establishes electrical contact across the crystal Silver-silver chloride electrode Filling solution Linear range is limited by solubilityr of LaFa (Km = a x 10'19) Inorganic crystal Fluoride electrode uses LaFa doped with Eu 2" Chapter 15 Electrodes and P'otenfiormeta-g-I page I! Using other inorganic crystals and varying dopants changes selectivity Table l5—5 Properties of solid-state ion-selective electrodes {omen mitten Membrane [III lnterierllg Ion mane [Ill 111quch Hi It“ spetles F" 1 11—5-1 LaF1 s-s oH-ton M] CI‘ 1 (rt—1 AgCI 2—1 1 CM’. 51-, 1-. 5203’: Br Br 1o'5—1 Agar 2—11 Cir. 51-, I- l' 1 o'fi-t Agl 3-1 2 51' SC N' 1 IT 5-1 AgSrCN 2-1 2 51', I', EH', Br, 5203* CN' 1o—5-1 11—1 A91 11-1 3 5231' 5 13-1-1 57-- 1o-5-1 iig2 Chapter 15 Eileen-odes and I'otenlierrrrel'fl.I Liquid-based ion-selective electrodes To potentiometer F31: CL 0 a- H u =3 F3: 4 Hydrophobic onion [Ir] 11' Tot-akile,S—hisltrifi uammuthyophanytJhm-ar. "fl“! “""m lea... om M reel2 U attire-ted with MCI for (onion- Hyd rophoblr: Can—binding ligand {Li selectlre elemde N. N-DicyclohexyI—H'.N*—dloctadequ-3mxapentanediamide Silver-silver chloride electrode 0 Hydrophobic poiymer membrane containing we2 hydrophobic Ion exchanger and iomselectlue ionophore Hyd rophoblr. liquid solvent 2-Hitrophem'l octyl ether Chapter 15 Electrodes and Petentiemetry 002 gas-sensing electrode Compound Electrodes }To potentiometer Contain conventional electrode surrounded .11 M Hel by a membrane that isolates (or generates) the analyte to which the electrode responds Internal silver-silver chloride electrode During ' - ' - t}.1 M KCI electrolyte Sllwirgéflgmfi With weal: bicarbonate electrolyte butler Glass electrode Spacer membrane (302 permeable membrane Chapter 15 Electrodes and Potentiometn' Ion-selective electrodes in medicine Table l5-4 Critical care profile function Imlyte Conduction Etta“ Eontraction Ci“: ”91+ Energyr level Glucose. Pu: lactate, hematocr'rl: Ventilation P“:I ,Pm Perfusion Laitatef soles hemetocrit Acid—base pH. Pm lHm; Dsmolellty Hatgldcose Electrolyte Illa", Kt Ca“. Mg“ balance Renal function Blood urea nitrogen, creatinine Using Ion-Selective Electrodes Composition of the standard solution 4 must oloseljl,r match the composition of the unknown solution: Use the standard addition method J; = em 8): + 0.439 2 Standard addition graph Concentration ofunlmowrn or from "if e and the x—interoept D? s-1 was {mL} Intercept = —U.59U mL Chapter 15 Electrodes and Potenflomchjl Solid-State Chemical Sensors pH sensitive field effect trans1stor Thermistor (FET) Fteference FET pH eiectrode sensor Chapter 15 Eteclrodes and Pmnttonntry Semiconductors and Diodes \ / 3:111:ng Will!!! I pair to} Forward bias [Current flows} I'll WI \ >flj‘? / ‘\ ‘X /‘\. /"‘\ /"\ "" /\ f‘x p-Si n—Si \ / Conduchmotectron \ / m \ /‘>/I/ \ /”\‘:*/ ‘\ /"\_/’\.fl/ \ /"\s;*/"\fl/ [xxx/\ /\f\/\t-i /”\ /"\ /"\ /“\ _\ /\ _ _ n-It'fpllllimn 155' Depletion " 5‘ '5" Negative electron ”9:1" A“hule”[£le¢hunval:anq} [“10 I11] \Ei \ _,/ {E Positiveliole \ >l<l '5'»: / \ >fl<ijj§f / Behavior of arm jucntion /"\_/‘\_x"\ ‘\ /“\ /“\ f s. \ /§>§|/§\ Doping stroogty effects \ Conductivity of semiconductors Chapter 15 Electrodes and Potenflnmctrj-I page 19 Band Gaps Material eV* Diamond 5.4? SiC 3.00 Si 1 .1 2 Ge 0.66 Sn 0 [metallic] BN 7.5 [approx] BF 2.0 GaN 3.36 GaP 2.26 GaAs 1.42 InAs 0.36 *1 at! = 93.5 kJimole = 23.08: koah’mole Chapter 15 Electrodes and Potenttnnutry Operation of a Field Efiect Transistor (FET) H Depletion region The potential of the gate regulates the current flow between the the source and drain. The more positive the gate, the more current can flow. Conducting channel induced but positive III-m erg-h Chapter 15 Electrodes and Potcnflomehj' page 1] Operation of a Chemical-Sensing Field Effect Transistor (FET) IiZI'IIIlarnicitlligqlr sensitive layer Circuit adjusts potential difference between the reference electrode and the source, in response to changing analyte concentration, to maintain constant drain-source current Protective coating Metal contact luau hi 1 at“. siam‘ em warm ml SID: insulator sue Ag‘i can 5‘ son E. 3 me This voltage sets g “I“ the drain-sou rce current at a “cl r ain ti _ _ desired level _ -'I m Br- 4-5-4-3-2-1 o ...
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