UNIT 3 POTENTIOMETRY-II (ph METRY)

Transcription

1 Eletronlytil ethods -I UNIT 3 POTENTIOETRY-II (ph ETRY) Struture 3.1 Introdution Objetives 3.2 Conept of ph 3.3 Glss embrne Eletrodes 3.4 ph eter 3.5 esurement of ph 3.6 ph Titrtion 3.7 odified Glss nd Solid Stte embrne Eletrodes 3.8 Summry 3.9 Terminl Questions 3. Answers 3.1 INTRODUCTION In the previous unit, you hve seen tht potentiometri methods minly inlude two mjor types of nlysis. First being diret mesurement of n eletrode potentil from whih the tivity (or onentrtion) of n tive ion my be derived. The other type involves, mesuring the hnges in the eletromotive fore (emf) brought bout by the ddition of titrnt to the smple. The priniple of potentiometry n be pplied to mesure the emf, in terms of ph unit on ph sle by suitble modifying the ommon voltmeter to high input impedne mv meter. The id-bse titrtions, involving mesurement of hnge in ph to detet the end point, n be termed s ph metry insted of potentiometry. Objetives of developing ph metry lies in reognising the importne of ph, the very importnt test performed in studying hemil, biohemil, mirobiologil proesses involved in vrious unit opertions nd proesses in smll to lrge sle industril produtions. In every phse of wter nd wstewter tretment proesses, i.e. in id-bse neutrliztion, wter softening, preipittion, ogultion in the field of both prodution nd environmentl protetion tehnologies, ll these proesses re rried out t speifi rnge. Objetives After studying this unit, you should be ble to: drw nd level glss eletrode, write the Nernst eqution for glss eletrode, desribe how the ph meter n be used to mesure the ph, nd desribe how the ph dt n be used to determine the equivlene point in id bse titrtions, nd list the pplitions of ion-seletive eletrodes 3.2 CONCEPT OF ph At given temperture the intensity of the idi or bsi hrter of solution is indited by ph or hydrogen ion tivity. By definition it is the negtive logrithm of the hydrogen ion tivity. H 56

2 ph log H In dilute solution the hydrogen ion tivity is pproximtely equl to the onentrtion of hydrogen ion. Pure wter is very slightly ionized nd t equilibrium the ioni produt is [(OH w t 25 C or [OH A logrithmi form is Potentiometry-II (ph etry) log log [OH log w or ph poh p w From the bove equilibrium it is ler tht the ph sle for n queous solution lies between 0 nd 14. The ph of most rw wter soures lies within the rnge of , slightly bsi due to the presene of birbontes nd rbontes of the lkli nd lkline erth metls. The ph is generlly mesured with glss eletrode nd ph meter. The glss membrne eletrode or glss eletrode is one of the most ommon exmples of n ionseletive inditor eletrode. The overll ell, when the glss eletrode is used with n externl referene eletrode suh s stndrd lomel eletrode (SCE), n be represented by: Glss membrne H SCE To ompute the ell potentil, it ould be ssumed tht the SCE is more positive thn the glss eletrode. Thus, E ell E SCE E j (E o G log ) where E j is liquid-juntion potentil nd E o G is the stndrd eletrode potentil of the glss eletrode. In terms of ph, or E ell E * log (3.1) E ell E * ph (3.2) Thus, ell potentil is diretly proportionl to ph i.e. : E ell ph where E * inludes the stndrd eletrode potentil of glss eletrode, potentil of referene eletrode nd liquid-juntion potentils between the referene eletrode nd solution. Beside these three potentils there is nother potentil lled the symmetry potentil whih is lso ontributing to this. Asymmetry potentil is smll potentil whih exists ross the membrne, even when the inner referene solution nd the test solution re identil. The soures of symmetri re not muh ler. It my be due to the degree of hydrtion of the inner nd outer surfe of the glss membrne or due to its struturl design or due to mehnil nd hemil ttk whilst in use. The degree of hydrtion of outer surfe will lso hnge if the eletrode is llowed to dry out for some time. Beuse of this it is dvised tht the eletrode should be stored either in wter or in dmp otton wool. It is not possible to determine the vlue of symmetry potentil, therefore, it is neessry to librte glss membrne eletrode with suitble buffer solutions. As the vlue of the symmetry potentil n hnge with eletrode use, it is neessry to rry out librtion t lest dily. 57

3 Eletronlytil ethods -I SAQ 1 Why is it neessry to librte the glss eletrode before use? GLASS EBRANE ELECTRODES The ph glss eletrode lthough somewht mehnilly frgile, resists vriety of smple medi nd with the exeption of hydroxide is lrgely free from interferenes. oreover, ph-sensitive glss eletrodes form the bsis of mny suessful sensors for environmentlly sensitive gses. Thus, glss membrnes represent n importnt lss of solid-membrne ion seletive eletrodes (ISEs). The mjority of ph eletrodes vilble ommerilly re ombintion eletrodes tht hve both glss H ion sensitive eletrode nd dditionl referene eletrode onveniently pled in one housing (see figure below). For some speifi pplitions seprte ph eletrodes nd referene eletrodes re still used - they llow higher preision needed sometimes for reserh purposes. In most ses ombintion eletrodes re preise enough nd muh more onvenient to use. As illustrted in Fig. 3.1(), these eletrodes hve thin glss membrne fused to the end of glss or plsti body. The min body of the eletrode ontins n internl referene eletrode typilly Ag/AgCl nd is filled with solution tht is usully the 3 queous HCl of onentrtion round 1.0 mol dm. The seletivity oeffiient of glss-membrne is suh tht exellent dissemintion ginst ommon tioni speies is hieved. () (b) Fig. 3.1: Glss eletrode: () Typil glss eletrode onsisting of both n inditor glss eletrode nd silver/silver hloride referene; (b) illustrtion showing n ion exhnge Combintion eletrodes The ph eletrode responds to hydrogen ions s result of the thin ion-exhnge sites on the surfe of hydrted glss membrne. The eletrode onsists of thin lyer of glss, typilly bout 50µ m thik. Chrge is trnsported ross the membrne by sodium or lithium ions within the glss. The membrnes re primrily mde from Lithi (Lithium oxide) or sodium oxide, nd SiO 2, nd some mount of Al 2 O 3 nd B 2 O 3 or multi-omponent glsses whose sensitivity pttern depends on the omposition of the glss. The surfe lyer of the glss onsist of silite group N ssoited with sodium ion (- Si O ) s shown in Fig. 3.1 (b). When this eletrode is dipped in wter, the sodium ions exhnged with the protons in wter. 58

4 N - Si O H SiO H N Surfe of solution Glss surfe Glss eletrode fter hydrtion before hydrtion Potentiometry-II (ph etry) If glss eletrode is pled in test solution its glss membrne will hve n inner nd outer hydrted lyers nd potentil differene is developed due to the differene in hydrogen ion tivities between test solution nd outer hydrted surfe of glss eletrode s well s inner solution nd inner hydrted surfe. This potentil is lled boundry potentil nd it vries with the tivity or ph of the solution. Overll boundry potentil is the potentil differene between both the boundry potentils. We n write hemil eqution for both the boundry potentils H Gl (s) Outer surfe of glss H Gl (s) Inner surfe of glss H (q) Gl (3.3) Outer solution H (q) Gl (3.4) Inner solution For the Eq. (3.3), the boundry potentil is E 1 ( 1 h) where 1 nd h re the tivity of the hydrogen ions in the test solution nd the outer hydrted lyer, respetively. Similr for Eq. (3.4), the boundry potentil is E 2 ( 2 h) where the tivity of inner solution is 2. Thus, overll boundry potentil, E b E 1 E 2 ( 1 h ) ( 2 h ) If we ssume tht the tivities of the hydrogen ions in the inner nd outer hydrted lyer re the sme, the bove eqution n be written s: E b ( 1 2 ) But the tivity of the hydrogen ions in the inner solution is onstnt nd this eqution redues to: E b 1 E b n be expressed for n in Nernst form: E b E 1 E log 1 2 E b log ph (3.5) where 1 is onstnt, it inludes the onstnt ftor relted to hydrogen ion tivity of inside solution, 2, tht is, log 2. Thus, the boundry potentil is mesurement of the hydrogen ion tivity or the ph of the externl solution. For mesuring ph with glss eletrode the rrngement shown in Fig. 3.2 re ommonly employed. This rrngement hs glss eletrode with Ag/AgCl internl referene eletrode nd externl referene eletrode suh s 59

5 Eletronlytil ethods -I sturted lomel eletrode (SCE). For suh rrngement omplete ell my be represented s: Referene eletrode H Glss membrne H Referene eletrode (internl) (internl) (extenl) (SCE) Glss eletrode The ell potentil is expressed s Fig. 3.2: A typil eletrode system for mesuring ph E ell (E ind ) E SCE E j (3.6) where E SCE is the potentil due to externl referene eletrode, E j is liquid-juntion potentil, nd E ind is eletrode potentil of the glss eletrode, whih is tully ombintion of three omponents: (i) the boundry potentil, E b, (ii) the potentil of the internl Ag/AgCl referene eletrode, nd (iii) the symmetry potentil, E sy. From Eq. (3.5), substitute the vlue of E b. Thus, E ell (E b E Ag/AgCl E sy ) E SCE E j (3.7) Substitute the vlue of E b in this eqution E ell ( log 1 E Ag/AgCl E sy ) E SCE E j or E ell E * log 1 E * ph (3.8) where E * is onstnt, it inludes ll the onstnts nd ner onstnt soure of potentils, i.e. potentil of both referene eletrodes, liquid-juntion potentil between the externl referene eletrode nd the solution, symmetry potentil nd internl boundry potentil. It is very diffiult to determine the vlue of E * both experimentlly s well s theoretilly. Therefore, librtion method is used to eliminte this ftor. In this method, first test solution is omprised of stndrd buffer solution with the ph preisely known. Thus, for the stndrd, we n write following expression: (E ell ) s E * (ph) s (3.9) This step is followed by mesurement of ell potentil for unknown solution. The expression will be (E ell ) u E * (ph) u (3.) 60

6 To eliminte E * subtrt Eq. (3.9) from Eq. (3.), we find ( Eell) u ( Eell) s (ph) u (ph) s (3.11) Eq. (3.11) hs been dopted throughout the world s the opertionl definition of ph. Potentiometry-II (ph etry) ph determintion using glss eletrode is most urte nd widely used method despite few disdvntges viz. the glss membrne being very frgile, it requires gret re while using. The ordinry potentiometer nnot be used for mesuring the potentil of the glss eletrode. Thus, the eletroni potentiometers re required to be used, needs frequent stndrdiztion nd, nnot be employed in pure ethyl lohol, eti id nd geltin. The following fetures of glss eletrode hve mde it more verstile to be used be s inditor eletrode for ph mesurement. It my be used in the presene of strong oxidizing nd reduing solutions in visous medi nd in presene of proteins whih interfere with opertion of other eletrodes. It n be used for solutions hving ph vlues 2 to with some speil glss, mesurements n be extended to ph vlues greter thn. It is simple to operte nd immune to poisoning. The equilibrium is rehed quikly While mesuring ph you should be little re full s there re few ftors whih limit the ury of ph mesurements. We re listing few of them below: 1. The lkline error: It is notied tht the ordinry glss eletrode beomes sensitive to lkli ions nd gives low reding in high ph rnge bove 9 or ph units. The reson for the error is tht whilst the glss membrne is seletive to hydrogen ion, it lso responds to other ions. This beomes more signifint when the tivity of the other ions is higher to tivity of the hydrogen ion. Fig. 3.3 shows the error produed by different tions t different onentrtions. You my hve notied tht lkline error is reltively more in se of sodium ion. This is beuse of higher seletive oeffiient of sodium ion. This due to sodium ion n be redued by the use of Li 2 O glss in ple of N 2 O glss. Fig. 3.3: Devition in ph mesurement using glss eletrode under lkline onditions 2. The id error: At low ph rnge less thn 0.5, the vlues determined by the glss eletrode tend to be somewht higher. This error is due to the tivity of wter whih we hve ignored while writing Nernst eqution for the inditor eletrode. We hve ssumed tht tivity of wter my be tken to be unity s it is in lrge exess in the solution nd it behves s pure substne. However, in highly idi solution, the tivity of the wter beomes less thn unity beuse good mount is used in hydrting the protons. Similr effets re lso observed 61

7 Eletronlytil ethods -I on ddition of lrge mount of ny dissolved slt nd on ddition of misible non-queous solvent suh s ethnol. The net result in eh se is the mesured ph will be too high. 3. Vrition in juntion-potentil: In most of the ses the omposition of the stndrd buffer solution nd test solution re different. In suh sitution, the liquid juntion potentil will be different. 4. Error in the ph of the stndrd buffer: There my be possibility tht buffer solution is not prepred with full re or its omposition my hnge during storge. All these ftors will use n error in subsequent ph mesurements. 5. Temperture: A hnge in temperture my ffet on ph mesurements, beuse hnge in temperture ffets the tivities of the ions s well s the liquid-juntion potentils. Therefore, it is dvised to librte the eletrode t the temperture of the test solution. 6. Clibrtion proedures: Buffer solution nnot be prepred more urtely thn ± 0.01 ph units. Therefore, we nnot librte the eletrode better thn this. 7. Equipment relted: These errors my due to the power flututions, prllx errors in reding nlogue sles, et. With this theoretil bkground now we will see how the ph is mesured using ph meter, but before tht try following SAQs. SAQ 2 How will you define boundry potentil? SAQ 3 List some ftors whih my use errors in ph mesurements ph ETER The instrumenttion required to perform potentiometri mesurements inludes referene eletrode, n inditor eletrode nd high input-impedne mv (ph/pion) meter s depited in Fig 3.2. Ordinry lbortory volt meters nnot be used for the mesurement of the emf of glss eletrode ells beuse of the high eletril resistne of glss eletrode (typilly -200 megohms), speil high-impedne voltmeter iruits re required, whih drw -12 mperes or less from the iruit. The ph meter is voltmeter but with severl ritil ddition funtions. Not only does it mesure the potentil ross the ph-sensing nd referene eletrode system, but it lso onverts the potentil 62

8 differene mesurement t given temperture into ph terms, nd it provides mehnisms to orret for the non-idel behviour of the eletrode system. The opertionl mplifier not only serves s high-impedne voltmeter, but lso provides stbility nd utomti opertion through the use of the feed-bk loop. The opertionl ontrols on ph meter re best understood by referene to the opertionl mnul provided by mnufture of individul instrument. odern eletroni tehniques permit the prodution of simplified ph meter tht mesures ph with n ury of 0.1 ph unit. The miroproessor equipped ph meters inlude temperture probe to disply temperture ompenstion, memory for the ph vlues of stndrd buffers, witing period to llow drft before tking ph redings nd built in dignostis to lert for eletroni mlfuntions or defetive eletrodes. Along with ph meter we lso require referene eletrode nd glss membrne eletrode whih ts s inditor eletrode in ph mesurement. Detils of referene eletrode were disussed in erlier unit of this blok nd detils of glss membrne eletrode were disussed in previous setion. Referene Eletrode: The two most populr types of referene eletrode re the sturted lomel nd silver/silver hloride systems. Both types of referene eletrodes exhibit mny of idel hrteristis, those inlude mintining fixed potentil over time nd temperture, hving long-term stbility nd returning to the initil potentil fter being subjet to smll urrents. Potentiometry-II (ph etry) i) The sturted lomel eletrode (SCE) is omposed of metlli Hg, solid merurous hloride (Hg 2 Cl 2 ) nd sturted solution Cl equilibrium. Consequently, the potentil of the SCE (0.241/2 V versus the stndrd hydrogen eletrode) remins onstnt even if some of the liquid evportes over time. The SCE is more populr nd with onstnt temperture bth, the error used by flututing temperture n be eliminted. The SCE n be used s referene eletrode in smple tht does not exeed 80 C. ii) The Ag/AgCl eletrode inludes silver wire, oted on one end with the insoluble AgCl slt. When the eletrode is immersed in sturted Cl solution, its potentil t 25 C depends only on the Cl onentrtion nd is 0.192V versus the stndrd hydrogen eletrode. The Ag/AgCl eletrode should not be used in the solutions tht ontins speies tht n preipitte or omplex with silver. Referene eletrode should be prepred nd mintined so tht the level of the internl liquid is kept bove smple solution to void infusion of smple into referene eletrode. This is ommonly used preutionry mesure to void ny ontmintion of the smple by ions. Cl, Inditing Eletrode (glss eletrode): As desribed in Setion EASUREENT OF ph Ag or 2 Hg ph mesurements re prt of routine tests done to hek nd omtroll mny our dy to dy tivities like potble wter qulity, soil usbility for different plnts, wter qulity in quristis, they re done to ontrol industril proesses, in wine-mking nd beer-mking, to hek milk qulity, to hek osmetis - not to mention ll lbs throughout the world where ph mesurements re performed mny times dy to ontrol retions nd nlysis onditions. ph n be mesured in severl wys, of whih two re widely used. One - simple nd often enough preise - is use of olorimetri inditor methods, i.e. use of ph strips 63

9 Eletronlytil ethods -I (ph ppers). Seond, more ostly nd more demnding in terms of proedure tht hve to be used, but giving muh more preise results - is potentiometri method with usge of glss eletrodes nd ph meters. Colorimetri (spetrosopi) methods hve never gined muh populrity, lthough they re osionlly used in ples where glss eletrodes. The mil disdvtge of this proedure is tht it n't be immersed in the test solution. Thus, the ph determintion is usully done by potentiometri methods, whih is the most urte methods nd free of interferenes. Potentiometri ethod of ph esurement The ph is determined by mesurement of the emf of ell omprising n inditor eletrode (n eletrode responsive to hydrogen ions suh s glss eletrode) immersed in the test solution nd referene eletrode (usully sturted lomel eletrode, SCE) ontt between the test solution nd the referene eletrode is hieved by mens of liquid-juntion, whih form the prt of referene eletrode. The emf of this ell is mesured with ph. The desription of ph-meter hs lredy been mde in Setion 3.4, however proedure for stepwise librtion of ph meter nd mesurement of ph of test smple is given below: Apprtus ) ph meter: onsisting of potentiometer, glss eletrode, referene eletrode nd temperture ompensting devie. A blned iruit is ompleted through potentiometer when the eletrodes re immersed in the test solution. b) Referene eletrode: onsisting of hlf-ell tht provides stndrd eletrode potentil. Generl olomel (SCE) or silver/ silver-hloride eletrodes re used s referene eletrode. ) Glss membrne eletrode: Severl types of glss eletrodes re vilble. Even ombintion (glss nd referene) eletrodes re vilble for ph mesurement of different test solutions nd t vrying test onditions. d) Bekers: Preferbly use polyethylene or TFE bekers. e) Stirrer: Use mgneti TFE oted stirring br. Regents Clibrte the eletrode system ginst stndrd buffer solutions of known ph: Buffer tblets hving ph 4.0, 7.0 nd 9.2 re ommerilly vilble. Beuse buffer solutions my deteriorte s result of mold growth or ontmintion, prepre fresh s needed. Alterntively buffer solutions n lso be prepred s following: ph 4 buffer solution : Dissolve.2 gm nhydrous potssium biphthlte (HC 8 H 4 ) using boiled nd ooled distilled wter. Dilute to 1 dm 3. ph 7 buffer solution: Dissolve gm nhydrous potssium dihydrogen phosphte (H 2 PO 4 ), nd 1.42 gm nhydrous disodium hydrogen phosphte, N 2 HPO 4 both of whih hve been dried t 1ºC to 130ºC. Use distilled wter whih hve been boiled nd ooled. Dilute to 1 dm 3. ph 9.2 buffer solution : Dissolve: 3.81 gm borx (N 2 B 4 O 7 H 2 O) in distilled wter. Dilute to 1 dm 3. Proedure Numerous ph meters of vrious designs re mrketed by severl instrument mnuftures. Generl purpose ph meters re either line operted instruments tht re 64

10 redble to 0.05 ph unit or bttery operted instruments suitble for field job. These dys digitl ph meters redble to 0.01 ph unit re more populr s ompred to sle-needle instruments. Potentiometry-II (ph etry) esurement of ph of solution with the instrument (nlog meter) shown in Fig. 3.4 n be mde following the proedure given below in step-wise mnner. 1. eep the seletor swith on zero position nd djust the zero position by srewdriver if the pointer does not indite zero. 2. Before using ph meter, remove eletrodes from storge solutions (reommended by mnufturer) nd rinse with distilled wter. Dry eletrodes by gently blotting with soft tissue pper. 3. ount the eletrodes in the lip on the stnd. 4. Connet the power ble to 220V AC supply. Swith on the instrument nd wit for few minutes till the instrument wrms up. 5. Adjust the temperture/solution temperture vlue. 6. Tke the stndrd buffer solution of desired rnge (e.g. buffer of ph 4 for idi solutions) in beker. The eletrode ssembly is immersed in the ph referene buffer nd the solution is gitted gently by swirling the solution in the region of the glss eletrode surfe so s to bring it into ph equilibrium. It should be sertined tht the glss eletrode membrne is ompletely immersed in the solution. The eletrodes should not touh eh other or the side or the bottom of the beker. II III VII IV I VI V Fig. 3.4: A diret reding ph meter (front view) legend: I. On/off swith II. Set zero III. Seletor IV. Eletrode support V. Temperture ompenstion VI. Set buffer VII. eter 7. Put the seletor swith to suitble ph rnge (0-7 for idi or 7-14 for bsi solutions) nd djust set buffer knob in mnner tht the pointer reds the ph of the stndrd buffer solution (pled in the beker). 8. Put the seletor swith bk to zero position. Remove the eletrodes from the buffer solution, wsh the eletrodes with distilled wter nd wipe them gently with tissue pper. 9. Immerse the eletrodes in seond buffer below ph, pproximtely 3 ph units different from the first one; the reding should be within 0.1 units for the ph of the seond buffer. (If the meter response shows differene greter thn 0.1 ph unit from expeted vlue looks for trouble with the eletrodes or ph meter.. Trnsfer the stndrd buffers bk to the storge bottle nd wsh the beker well with distilled wter. 11. Tke the smple solution in the beker. Introdue the eletrodes in the solution nd swirl it gently. 65

11 Eletronlytil ethods -I 12. Set the seletor swith in the suitble rnge position nd red the ph on the sle. 13. Put the seletor swith bk to zero position. Remove the eletrodes from the solution, wsh them with distilled wter nd keep the eletrodes in distilled wter, when not in use. Preutions i) Never touh the membrne of the glss eletrode with nything else exept soft tissue pper sine it is frgile nd is esily ruined if srthed or bumped. ii) iii) iv) The eletrode(s) must not be removed from the solution unless the seletor swith is t zero. Never dip the glss eletrode in solution with dehydrting tion. For smple nlysis estblish equilibrium between eletrodes nd smple by stirring smple to ensure homogeneity nd mesure ph. v) If used for mesuring ph of lbuminous substnes, the glss eletrode must be lened with suitble solvents nd then the eletrode is pled in distilled wter for few hours before it is used to mesure the ph of the other solution. vi) For bsi solutions with ph more thn 11, glss eletrodes of speil omposition re required to void interferene due to sodium ion. vii) The glss eletrode my be overed with sleeve to sve it from jerks. viii) The stndrd buffer of ph vlue s lose s possible to the smple ph vlue must be tken for the librtion of the system. Commerilly vilble stndrd buffers of ph vlues 4, 7 nd 9.2 re ommonly used. 3.6 ph TITRATION Similr to potentiometri titrtions, in ontrst to diret ph mesurements, ph titrtions generlly offer inresed ury nd preision. Aury is inresed beuse, mesured ph re used to detet rpid hnges in tivity tht our t equivlene point of the titrtion. Furthermore, it is the hnge in ph versus titre volume rther thn bsolute vlue of ph tht is of interest. Thus, the errors due to liquid-juntion potentils nd tivity oeffiients re minimized. ph titrtions my be pplied to vriety of systems inluding those involving wek ids nd wek bses. In suh titrtion, it is diffiult to get end point using inditor method. A typil idbse titrtion using ph metry is briefed s follows. It is known tht the neutrliztion of ids nd bses is lwys ompnied by the hnges in the onentrtion of H nd OH ions. It is evident tht hydrogen eletrode my be employed in these titrtions. The referene eletrode used in these titrtions is 1 -lomel eletrode. The pprtus used for id-bse titrtions is s shown in Fig The ritil problem in titrtion is the reognition of point t whih the quntities of reting speies re presented in equivlent mounts, i.e. the equivlene point. The titrtion urve n be followed point by point plotting s the ordinte suessive vlues of the ph versus the orresponding volume of titrnt dded s the bsiss. Addition of the titrnt should be the smllest urtely mesurble inrements tht provide n dequte density of points, prtiulrly in the viinity of equivlene point. Over most of the titrtion rnge the ph vries grdully, but ner the end point the ph hnges very bruptly. The resulting titrtion urve resembles Fig. 3.6 (). 66

12 Potentiometry-II (ph etry) Fig. 3.5: Typil Instrumentl set up for ph titrtion By inspetion, the end point n be loted from the infletion point of the titrtion urve. This is the end point tht orresponds to mximum rte of hnge of ph per unit volume of titrnt dded (0.05 m 3 or 0.1 m 3 ). The distintion of the end point inreses s the retion involved beomes more nerly quntittive. One the ph hs been estblished for given titrtion, it n be used to indite subsequent end points for the sme hemil retion. The equivlene point n be more preisely loted from the 1st nd 2nd derivtive urves s illustrted in Fig. 3.6 (b) nd 3.6 (). Solutions more dilute thn -3 generlly do not give stisftory end points. This is limittion of ph metry nd potentiometri titrtions. () (b) () Fig. 3.6: ph titrtion urves; () Norml urve; (b) First derivtive urve; nd () seond derivtive urve ` Titrtion of Wek Aid with Strong Bse So fr titrtion urve shown in Fig. 3.6 () desribe the progress of the titrtion of strong id nd strong bse. We now onsider the titrtion of 25 m 3 of 0.05 CH 3 COOH solution with 0.05 NOH solution to explin how the ph of the titrtion is hnged t different stges of the titrtion. For this purpose we will be using mny expressions used in theory of neutrliztion titrtions whih you my hve studied in first ourse on Bsis of Anlytil Chemistry or in your undergrdute physil hemistry ourses. 67

13 Eletronlytil ethods -I Hydrolysis of slt of wek id nd strong bse my be represented s A H 2 O HA OH Hydrolysis onstnt is given by A [OH h [A A [OH [A w...(i) But t equilibrium, A [OH, therefore, [OH 2 [OH h [A slt 2 w.(ii) 2 slt Combining Eqs. (i) nd (ii) gives 2 w 2 slt i.e. w / w slt Strting point of the titrtion urve: The strting point of the titrtion urve of 0.05 CH 3 COOH solution is onsiderbly lower thn tht of 0.05 HCl solution. This is beuse eti id is dissoited lmost by 0 times less thn hydrohlori id. (the degree of eletrolyti dissoition of 0.05 CH 3 COOH solution α 1 per ent; for 0.05 HCl solution, α 90 per ent). Hene in 0.05 CH 3 COOH solution is lso 0 times less thn in 0.05 HCI solution. And ph will be 3, nd not unity. A more preise ph vlue of the strting point of the titrtion urve is found by the following wy. Write the expression for id dissoition onstnt for eti id: [A A At equilibrium, [A. As the degree of ioniztion of eti id is smll, therefore t equilibrium, the onentrtion of unionized id is pproximtely equl to totl onentrtion of id( id ), i.e. id [ HA [A A. Therefore, 6 2 A id id where id is the onentrtion of idi id. For eti id ph log [ H log (9.35 ) Equivlene Point: We n lso determine the ph t the equivlene point. At the moment when titrtion is ompleted, equivlent quntities of the CH 3 COOH nd NOH solution will be present. Hene, the titrting flsk will hve slt solution formed by wek id nd strong bse. For the solution of suh slt (see mrginl remrk): w slt The slt onentrtion in omprison with 0.05 will hlved; slt [ H ph log log Intermedite Points: After determining the strting nd end points of the titrtion, we will now onsider ph lultions for the intermedite points. These points of the urve orrespond to the simultneous presene in the solution of n un-titrted wek id nd slt whih is formed s result of its prtil neutrliztion. Hene, for lultion, we my use the formuls for finding the vlues of nd ph in 68

14 solutions of wek id in the presene of its slt of strong bse (see mrginl remrk): slt id Let us lulte the first intermedite point whih orresponds to 5 m 3 of 0.05 NOH solution poured in. We first determine the quntity (in m 3 of 0.05 solution) of the residul id. It will be m 3, sine 5 m 3 of 0.05 NOH solution hve titrted CH 3 COOH. Consequently, id in the titrtion flsk is not 0.05, but id Let us now determine the onentrtion of the slt formed t this moment of titrtion. As we hve seen erlier, 5 m 3 of 0.05 CH 3 COON solution were formed, but this quntity of the slt is lso in the totl volume of 30 m 3. Therefore, slt For eti id is Potentiometry-II (ph etry) A wek id in the presene of its slt of strong bse be be represented s H A HA H 2 O OH HA A H 2 O Sine for wek id, [A A [ slt [ id Substituting ll these dt in the formul for finding the ph of the solution, we obtin the following for the first intermedite point: ph log - log Let us lulte the next point of the titrtion urve for the moment t whih 12.5 m 3 of NOH solution will be poured in: id 0.05 ( ) slt ; ph log log Tht is ph p We now lulte the ph of the third intermedite point for V 15 m (25 15) id (25 15) slt ph log log After equivlene point (on ddition of NOH): In this sitution both the exess NOH nd ette re soure of hydroxide ion. We my onsider tht the ontribution from the ette ion will be smll, beuse the exess of strong bse repress the retion of ette with wter. We hve then 69

15 Eletronlytil ethods -I [OH NOH 0.05( ) ph [ log ( ) 8.99 On further titrtion, the ph of the solution will be determined only by exess 0.05 NOH solution. Thus, the urve of titrtion of wek id with strong lkli hs the following signs: 1. The strting point of titrtion is in medium whih is less idi thn when strong id is being titrted, 2. the equivlene point is in wekly lkline medium, 3. the middle prt of the titrtion urve is more slnting thn tht of the titrtion urve of strong id; 4. the titrtion jump is not gret, rnging from ph 8 to ph, nd ordingly, the vertil prt of the urve is onsiderbly smller thn tht when strong id is being titrted. Buffering tion of the CH 3 COO ion: One will see without ny diffiulty tht this urve is muh smoother thn the urve of titrtion of strong id with strong lkli, nd it does not hve shrp infletion. When HCl solution ws titrted with lkli, 22 m 3 of the titrnt hd to be dded in order to hnge ph by unit, while in this se, pproximtely 5 m 3 were enough (Fig. 3.7). This differene is due to the ft tht when titrting strong id with strong lkli, the onentrtion of H ions derese only s result of their ombintion with OH ions. () (b) Fig. 3.7: ph titrtion urves: () For 0.50 HCl with 0.50 NOH; (b) 0.50 Aeti id with 0.50 NOH. When titrting wek id with strong lkli, H ions ombine not only with OH ions, but lso with nions (in this se, with CH 3 COO ions), s result of whih dereses muh more rpidly t the beginning of titrtion, nd the titrtion urve bends downwrds muh more shrply. Suh n effet of the CH 3 COO ion, whih smoothens out the titrtion urve, is known s the buffering tion. The vlue of in the solution of wek id in the presene of its slt is determined by the following formul. 70

16 slt id It follows tht if the id onentrtion in the solution is equl to the slt onentrtion then slt id [ H nd ph p id This mens tht if id nd slt hve lose vlues in suh mixture, the ph of the solution will remin onstnt even when onsiderble quntities of id nd lkli re dded. Preisely suh reltionship between id nd slt is observed in the se of titrtion of wek id with strong bse t the seond intermedite point. id Potentiometry-II (ph etry) SAQ 4 Clulte the ph during the titrtion of m 3 of 0.05 HCl with 0. NOH t different stges of titrtion (i) initil point, (ii) fter ddition of m 3 of NOH, (iii) fter ddition 25 m 3 of NOH nd (iv) fter ddition of m 3 of NOH ODIFIED GLASS AND SOLID STATE EBRANE ELECTRODES Glss eletrode n be mde seletive for ions other thn hydrogen ion by some modifitions. This modifition is possible by hnging the omposition of the glss nd the internl solution of glss eletrode. By dding luminum oxide to sodium oxide nd silion oxide glss nd hnging internl filling solution from hydrohlori id to sodium hloride, suh eletrode beomes seletive to N ions. There is nother types of glss eletrode with omposition of Li 2 O, Al 2 O 3 nd SiO 2 is lso used s sodium eletrode. Sodium eletrode hs mny pplitions in mesurement of sodium in wter nlysis nd in biologil fluid nlysis. For mesuring potssium nd mmonium ions, modified glss ontining 27 % of N 2 O, 4 % Al 2 O 3 nd 69 % SiO 2. Potssium/mmonium eletrode is now repled by other ion seletive eletrodes using more seletive membrne. Solid Stte embrne Eletrodes In solid stte membrne eletrode we use doped single rystl membrne in ple of glss membrne. These hnges enble us to design eletrodes whih hve response to number of different nions nd tions suh s F, Cl, nd Ag. A typil design is shown in Fig In this eletrode system similr to glss eletrode internl solution nd eletrode form the internl referene. For exmple, fluoride ion seletive eletrode onsists of LF 3 membrne nd n internl Ag, AgCl referene eletrode immersed in n internl solution of F, Cl. LF 3 membrne is highly seletive nd responds to fluoride ion only. If this eletrode used with referene sturted lomel eletrode, the omplete ell my be written s: Hg,Hg 2 Cl 2 (s) Cl F (unknown) LF 3 (s) NF(0.1 ) NCl (0.1) AgCl (s), Ag 71

17 Eletronlytil ethods -I The potentil of ell is given by Fig. 3.8: A typil solid stte eletrode E ell E SCE ( log E E E E ell E * log F F AgCl sy j ) where E * inludes E AgCl, E SCE, E sy nd E j onstnt potentil representing internl referene eletrode, externl referene eletrode, symmetry potentil, nd liquid juntion, respetively. Clibrtion with the known fluoride tivity elimintes the need of knowing these onstnts. Fluoride eletrode hs mny pplitions suh s fluoride determintion in bone, ir nd stk gs smples, hromium plting bths, minerls, wter, nd toothpstes. Similr to fluoride ion seletive eletrode mny other solid stte membrne eletrodes n be developed. In Tble 3.1, we re listing few suh ion seletive eletrodes. Tble 3.1: Some exmple of solid stte membrne ion seletive eletrodes Seletive ion embrne Lower limit of mesurement/mol dm -3 F LF 3 7 Cl AgCl 5 Br AgBr 6 I AgI 8 S 2 Ag 2 S 7 SCN AgSCN 6 Ag Ag 2 S 8 Hg 2 HgS/Ag 2 S 8 Cu 2 CuS/Ag 2 S 9 Cd 2 CdS/Ag 2 S 7 Pb 2 PbS/Ag 2 S 7 Bi 3 Bi 2 S 3 /Ag 2 S 11 72

18 Bsed on the priniple of ion seletive eletrodes mny gs sensing eletrodes hve been developed in pst few yers. They re vilble primrily for the mesurement of mmoni, rbon dioxide, nd nitrogen oxide. This type of eletrode hs gs permeble membrne nd n internl buffer solution. The ph of the buffer solution hnges s the gs rets with it. The hnge is deteted by ombintion ph sensor within the housing. This type of eletrode does not require n externl referene eletrode. In Tble 3.2, we hve summrized some ommeril gs sensing eletrodes. Potentiometry-II (ph etry) Test ion Tble 3.2: Severl typil gs sensing eletrodes Internl eletrolyte Internl ion seletive eletrode (ISE) embrne NH 3 NH 4 Cl ph ptfe (polytetrfluoroethylene) CO 2 NHCO 3 ph ptfe NO x NNO 2 ph ptfe SO 2 2 S 2 O 5 ph silione rubber H 2 S Citrte buffer S 2- silione rubber Applitions Ion Seletive Eletrodes Ion seletive eletrodes long with ph-sensitive glss eletrode re widely used in linil, biologil, wter, ir, oenogrphi, nd phrmeutil reserh nd routine nlytil determintions. So fr there re relible ommerilly vilble eletrodes for deteting H, F, Cl, Br, I, Cd 2, Cu 2, CN, BF 4, Pb2, NO 3, ClO 4, Ag, S 2, N,, nd SCN, for NH 3, H 2 S, SO 2, CO 2, nitrogen oxides gses, nd for severl different enzymes. Glss membrne eletrode is the most ommonly used ion seletive eletrode. ph mesurements hve mny pplitions some of whih re summrized below: ph metri or eletrometri methods help in deteting the end point of id-bse titrtion more urtely nd preisely s ompred to inditor methods. ph metry is n importnt nlytil tool in studying the id-bse equilibri whih is ontrolled by the rbon dioxide-birbonte-rbonte equilibrium system in most nturl wters. The estimtion of lklinity nd idity bsed on ph metry serves useful informtion of buffering pity of wter. In wter nd wstewter tretment, it gives n estimte of vilble of lklinity to ret with the ogulnt viz lum, Ferrous sulphte et. or otherwise to be supplemented with lime. Industril proess ontrol espeilly in bth or flow-through onfigurtions; through online ph monitoring nd hemil dosing system. In neutrliztion of wstewters, to evlute the doses of id or lkli to be dded. Development of biosensors bsed ph sensitive immobilized enzymes nd other demi studies. Further the flexibility in vilble onfigurtions llows the ions mentioned bove to be monitored in single smple solution (bth mode) or ontinuously in flow through pprtus (flow-injetion nlysis). 73

19 Eletronlytil ethods -I 3.8 SUARY One of the most ommon nd erliest pplitions of potentiometry is ph determintion, nd titrtion for the determintion of eletro tive speies. Through development of eletrodes tht seletively determine trget ions, potentiometry in generl is repling mny older, more expensive nd time onsuming tehniques for nlytilly monitoring nd mesuring ion tivity. Reent developments in the ion seletive eletrodes (ISE) brnh of potentiometry hve mde the monitoring of inorgni, orgni, gseous nd biologilly importnt ions possible. The flexibility in vilble onfigurtions llows these ions to be monitored in single smple solution (bth mode) or ontinuously in flow through pprtus (flow-injetion nlysis). 3.9 TERINAL QUESTIONS 1. Disuss the soure of ph dependene in glss membrne eletrode. 2. Wht is the soure of the symmetry potentil in glss membrne eletrode? 3. Wht re the dvntges of ph metri titrtion over diret ph metry? 4. Drive n expression for the opertionl definition of ph. 5. Wht is the soure of the potentil of n ion seletive eletrode used to determine the onentrtion of fluoride ion? 6. Clulte the ph during the titrtion of m 3 NOH with 0. HCl fter the ddition of the following volume of id : (i) m 3, (ii) m 3 nd (iii) m Clulte the ph during titrtion of m 3 NCN with 0. HCl fter the ddition of the following volume of id : (i) 0.00 m 3, (ii) m 3, (iii) m 3 nd (iv) 26 m 3. (Hint: w b nd id dissoition onstnt for HCN 6.2 ) 3. ANSWERS Self Assessment Questions 1. It is not possible to determine the vlues of symmetry potentil s well s liquid- juntion potentil in glss/lomel eletrode, therefore, it is neessry to librte glss membrne eletrode with suitble buffer solutions before use. 2. When glss eletrode is pled in test solution its glss membrne will hve n inner nd outer hydrted lyer nd potentil differene is developed due to the differene in hydrogen ion tivities between test solution nd outer hydrted surfe of glss eletrode s well s inner solution s inner hydrted surfe. This potentil is lled boundry potentil nd it vries with the tivity or ph of the solution. Overll boundry potentil is the potentil differene between both the boundry potentils. 3. i) Alkline error ii) iii) iv) Aid error Vrition in juntion potentil Error in the ph of the stndrd buffer v) Temperture 74

20 vi) Clibrtion proedures vii) Equipment relted Potentiometry-II (ph etry) 4. Initil Point Before ny bse is dded, the solution is 0.05 in H, nd ph log log After Addition of.00 of id: The hydrogen ion onentrtion is deresed s result of the retion with the bse nd dilution. So the nlytil onentrtion of HCl is id No.mmol HCl remining fter ddition of Totl volume solution NOH Originl no.mmol HCl No. mmol Totl volume solution NOH dded 3 (50.00 m 0.05) (.00 m 0. ) m.00 m nd ph log. 2 2 log ( ) 1.60 After ddition of of id: The Equivlene Point At the equivlene point, neither HCl nor NOH is in exess nd so the onentrtions of hydrogen nd hydroxide ions must be equl. Substne equlity into the ion-produt onstnt for wter yields w ph log ( ) 7.00 After Addition of 25. of id: The solution now ontins n exess of NOH, nd we n write bse No.mmol NOH dded Originl no mmol HCl Totl volume solution nd the equilibrium onentrtion of hydroxide ion is [OH bse 1.33 poh log (1.33 nd ph Terminl Questions 4 4 ) When glss eletrode is pled in test solution its glss membrne will hve n inner nd outer hydrted lyers nd potentil differene is developed due to the differene in hydrogen ion tivities between test solution nd outer hydrted surfe of glss eletrode s well s inner solution nd inner hydrted surfe. 75

21 Eletronlytil ethods -I This potentil is lled boundry potentil nd it vries with the tivity or ph of the solution. Overll boundry potentil is the potentil differene between both the boundry potentils. Conentrtion of inner solution is kept onstnt, thus the boundry potentil is mesurement of the hydrogen ion tivity or the ph of the externl solution. 2. The symmetry potentil in membrne rises from differene in the struture of the inner nd outer surfes. These differene my be due to the mnufture reson or due to its use. 3. ph titrtions generlly offer inresed ury nd preision. Aury is inresed beuse mesured ph re used to detet rpid hnges in tivity tht our t equivlene point of the titrtion. Furthermore, it is the hnge in ph versus titre volume rther thn bsolute vlue of ph tht is of interest. Thus, the errors due to liquid-juntion potentils nd tivity oeffiients re minimized. 4. Cell potentil for the stndrd buffer n be expressed s (E ell ) s E * (ph) s (i) Cell potentil for unknown solution will be expressed s (E ell ) u E * (ph) u (ii) To eliminte E * subtrt Eq. (i) from Eq. (ii), we find ( Eell) u ( Eell) s (ph) u (ph) s Eq. (iii) is the opertionl definition of ph. (iii) 5. When ion seletive eletrode is dipped in the solution ontining fluoride ions, the equilibrium will be estblished between both the fluoride ions in test solution nd LF 3 rystl nd fluoride ions in inner solution nd LF 3. The tivity of the F t the inner surfe is likely to be different to tht t the outer surfe. This results in development of the diffusion potentil between two surfes similr to liquid juntion potentil. As the tivity of internl solution is onstnt vlue, the diffusion potentil is, thus, dependent on the tivity of F in the test solution. 6. i) At m 3 dded, is very smll nd nnot be omputed from stoihiometri onsidertions but n be obtined from [OH [ OH bse Originl no.mmol NoH Totl volume of w / ( ph log(1.49 ) ) No.mmol HCl dded solution /( ) ii) This is the equivlene point where [OH - 76

22 w Potentiometry-II (ph etry) iii) ph log ( ) 7.00 At m 3 dded, [ H HCl ( ) ph log ( ) i) The ph of solution of NCN n be lulted s CN H2O HCN OH b Sinenequivlentmountof [OH [OH [ OH CN w [CN 6.2 CN NCN [ CN [OH NCN 14 ndcn reformed 0.05 Substitution into the bove dissoition-onstnt expression gives, fter rerrngement, [OH 5 4 b NCN ii).00 m 3 of Regent ph ( log ).95 Addition of id produes buffer with omposition given by NCN HCN These vlues re then substituted into the expression for the id dissoition onstnt of HCN to give diretly: slt id 6.2 (1.000/ 60.00) / ph log (4.13 ) 9.38 iii) m 3 of Regent This volume orresponds to the equivlene point, where the prinipl solution speies is the wek id HCN. Thus, 77

23 Eletronlytil ethods -I HCN Applying following Eqution gives HCN ph log ( ) 5.34 iv) m 3 of Regent The exess of strong id now present represses the dissoition of the HCN to the point where its ontribution to the ph is negligible. Thus, HCl ph log( )

24 Stndrd Eletrode Potentils* Retion Appendix I no 4 8 H 2e n 2 4H 2 O 1.51 E 0 t 25 C,V Cl 2 (g) 2e 2Cl Cr 2 O 2 14 H 6e 2Cr O 2 (g)4h 4e 2H 2 O Br 2 (q) 2e 2Br Br 2 (l) 2e 2Br Ag e Ag (s) Fe 3 2 e Fe I - 3 2e 3I Cu 2 2e Cu (s) UO 2 2 4H 2e U 4 2H 2 O Hg 2 Cl 2 (s) 2e 2Hg (l) 2Cl AgCl(s) e Ag(s) Cl Ag(S 2 O 3 ) 3-2 e Ag (s) 2S 2 O Potentiometry-II (ph etry) 2H 2e H 2 (g) AgI(s) e Ag(s) I PbSO 4 2e Pb(s) SO Cd 2 2e Cd (s) Zn 2 2e Zn(s) Ce 4 e Ce 3 g 2 2e g (s) 1.70 V