1_PENDAHULUAN ELEKTROANALISIS-2010

Oleh: DR. ATIKAH, MSi LABORATORIUM KIMIA ANALITIK JURUSAN KIMIA FMIPA UNIVERSITAS BRAWIJAYA MALANG 2010

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KULIAH KIMIA ELEKTROANALISIS-1

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Metoda elektroanalisis elektroanalisis lebih disukai karena :  Cepat  Murah  Spesifik terhadap bentuk kmia analit k onsentrasi analit  Merespon terhadap aktivitas, bukan konsentrasi  Dapat digunakan secara insitu  Memberikan informasi tentang:  Tingkat

oksidasi  Mengungkapkan reaksi secara stoichiometri  Kecepatan transfer muatan  Konstanta kesetimbangan reaksi 3/1/2011


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Gambaran Frekuansi penggunaan teknik analisa dalam laboratorium ± laboratorium analisis

Metoda  Potensiometri  Polarografi (voltametri) 30%  Elektroda ion selektif  Spektroskopi Uv-Vis  AAS atau FES (Flame-ICP)

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 Kimia

elektroanalisis merupakan kelompok metoda analisis kuantitatif berdasarkan peng pe nguk ukur uran an sifat sifat listr listrik ik larut larutan an anali analitt (seb (sebag agai ai bagian dari sel elektrokimia) elektrokimia)  Sistem Pengukuran Terdiri :  Elektrolit,

sistem kimia yang mampu menghantarkan arus

listrik  Alat ukur (rangkaian luar), untuk mengukur sinyal listrik  Elektroda, konduktor yang berfungsi menghubungkan sistem alat ukur dengan elektrolit

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KOMPONEN SEL ELEKTROKIMIA

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Electroanalytical methods

Interfacial methods

Static methods i=0

potentiometry E

dynamic methods i > 0

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Conductometry G=1/R

Conductometric titrations

Potentiometric titrations volume Controlled potential

Constant electrode potential coulometry Q =  i dt

Voltammetry I=f(E)

bulk methods

Amperometric titrations volume

Electrogravimetry (wt)


Constant current

Coulometric titrations Q=it

Electrogravimetry (wt) 6



POKOK BAHASAN : 

    

Konsep dasar elektrokimia Proses elektroda Hubungan Arus- Potensial & Termodinamika elektrokimia Kinetika reaksi elektroda Lapis rangkap listrik Metoda elektrometri:    

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Potensiometri Voltametri Koulometri Impedansi KULIAH KIMIA ELEKTROANALISIS-1

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Buku Referensi: 1.A,J.,Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and, Application,2 nd Ed., john Wiley & Sons, New York,2001  2. Joseph Wang, Analytical Electrochemistry,VHC Publisher,Inc,1994  3. Christian & O.Relly,1986, Instrumental Analysis  4. Kennedy,JH,Analytical Chemistry,1990 

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Elektrokimia  merupakan

disiplin ilmu yang mengembangkan sistem hubungan antara sifat listrik dengan efek kimia Reaksi si yang yang terj terjad adii me meliliba batk tkan an re reak akta tann Reak elektron  Perubahan kimia disebabkan oleh adanya arus listrik yang mengalir  Suatu sistem elektrokimia merupakan sistem heterogen 3/1/2011


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PENGUKURAN ELEKTROKIMIA    

Berdasarkan reaksi elektrokimia yang melibatkan senyawa kimia Bagaimana prinsip ini digunakan dalam analisis kimia? Karena p.u reaksinya kimia redoks (transfer elektron) Yang diukur :  potensial yang dihasilkan oleh reaksi yang terjadi  Jumlah atau peningkatan reaksi yang diinduksi oleh proses elektrokimia  Bagaimana

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hubungannya dengan konsentrasi analit?


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 Dasar

reaksi : redoks memun memun penentuan keadaan oksidasi suatu unsur dengan Metoda kimia elektroanalisis

Oks + neRed  Ada 2 macam metoda elektroanalisis: Potensiometri

(Arus = nol, potensial kesetimbangan Voltametri ( pengukuran arus sebagai fungsi potensial yang terpasang) 3/1/2011


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Terminologi Elektrokimia Anoda

elektroda dimana terjadi reaksi oksidasi Katoda

elektroda dimana terjadi reaksi reduksi Elektrolit

medium kimia yang mampu menghantarkan arus (=migrasi muatan) 3/1/2011


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

WORKING ELECTRODE (ELEKTRODA INDIKATOR) Elektroda dimana pada permukaannya teradi transfer muatan

ELEKTRODA PEMBANDING Elektroda yang menjaga agar harga potensial konstan,tidak tergantung pada arus yang mengalir  COUNTER ELECTRODE (ELEKTRODA PENDUKUNG) Elektroda yang mensupport arus listrik, namun tidak mempengaruhi reaksi atau potensial yang timbul 

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Elektroda pada Potensiometri &Voltametri  Pada

Potensiometri karena reaksinya spontan maka digunakan 2 elektroda : Elektroda

Reference (pembanding) Elektroda Indikator (WORKING ELECTRODE)  Pada

Voltametri karena reaksinya tidak spontan maka digunakan 3 elektroda : ELEKTRODA

PEMBANDING WORKING ELECTRODE) COUNTER ELECTRODE (ELEKTRODA PENDUKUNG) 3/1/2011


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Proses Faradaik  Reaksi oksidasi atau reduksi (=transfer elektron) Proses non Faradaik  Adsorpsi  Perubahan struktur pada antarmuka elektrolitelektroda elektroda , misal arus muatan  Transpor massa 3/1/2011


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Electrode Processes

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Electrode Polarization  When

³slow´ nonfaradaic processes dominate, electrode becomes polarized , and more potential is required for redox than would be required at equilibrium  When all electrode processes are ³fast´, electrodes are nonpolarizable, and make good reference electrodes (=constant potential at any current) 3/1/2011


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Mass Transport Migration²movement  Migration²movement

induced by electrical gradient; polarization minimized by? adding excess inert supporting electrolyte

 Convection²fluid

flow or gross physical movement of solution; polarization minimized by? stirring or temperature gradients

 Diffusion²movement

induced induc ed by chemical potential (e.g., concentration) gradient

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Arus Muatan  Adalah arus yang timbul sesaat (arus non faradaik) yang yang terjadi terjadi dalam daerah lapisan rangkap listrik (electrical double layer)  Arus

pada permukaan elektroda, H r r E E/ H adalah linier (faradaik); namun pada jarak lebih jauah dari permukaan elektroda adalah eksponensial r r E E/ H H  Tebal lapisan rangkap listrik sekitar 20300 Å 3/1/2011


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Polarization  An

ideally polarized electrode does not allow faradaic processes to occur  Mercury (Hg) in a NaCl solution is a nearly ideally polarized electrode 3/1/2011


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Depolarization A

current flow induced at a aradaic aradaic f

polarized electrode electrode by the addition of a substance (depolarizer ) which can be oxidized or reduced at the existing potential  In

a reversible process redox is so rapid that oxidized and reduced species are at equlibrium (=fast process)

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Overvoltage  The

³excess´ potential (=voltage) required to accomplish a redox reaction compared to the equilibrium condition  Occurs when there is some lag in establishing chemical equilibrium

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Causes of Overvoltage Slow charge transfer, esp. for evolution of gases  diffusion of reactant to or product from electrode  adsorption intermediates  chemical reactions of intermediates 

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Electrode Potentials Ox + n e± ¾ Red reduction expressed by  Propensity for the reduction potential, E°, relative to Standard Hydrogen Hydrogen Electrode (SHE): 2 H+ (aq) + 2 e± ¾ H2 (g ) E°= 0.00 V  Consider: Cu2+ (aq) + 2 e± ¾ Cu (s) E°=+0.34 V Zn2+ (aq) + 2 e± ¾ Zn (s) E°=±0.76V 3/1/2011


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Concentration and Activity  At

low concentration (< 0.02 M ) activity is approximately equal to concentration ( a } ) c , where f c  At higher concentrations, a = f c c is the activity coeff at concentration c icient icient is essentially the corrected  Activity ³effective´ concentration of a substance 3/1/2011


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Non-Standard Conditions Described by the N ernst ernst equation , Q

E ! E 

RT ared

ln

nF

Q

$ E 

aox

F= 96,487 C mol ±1

RT cred

ln

nF

cox

R = 8.314 J mol±1 K±1

 How

can this relationship be utilized in principle to construct an electrochemical sensor?

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Electrochemical Cells  Galvanic

(Voltaic) cell²spontaneous; cell²spontaneous; redox chemistry produces electricity Electrolytic cell²non-spontaneous; cell²non-spontaneous; uses  Electrolytic electricity to cause redox chemistry

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A Galvanic Cell Zn (s)|Zn2+ (aq)||Cu2+ (aq)|Cu (s) anode cathode

Zn (s)

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KCl (a )

Cu (s)

1 M Zn(NO 1 M Cu(NO3)2 (a ) 3)2 (a ) KULIAH KIMIA ELEKTROANALISIS-1

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Sel Elektrokimia anoda oksidasi

katoda reduksi

reaksi redoks spontan

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Menarik anion


Menarik kation

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19.2

A Galvanic Cell Zn (s)|Zn2+ (aq)||Cu2+ (aq)|Cu (s) anode: Zn ( s) p Zn2+(aq) + 2 e± ±E °=+0.76 °=+0.76 V cathode: Cu2+(aq) + 2 e±p Cu (s) E °=+0.34 °=+0.34 V cell: Zn (s) + Cu2+(aq) p Zn2+(aq) + Cu (s) E°cell = 1.10 V

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Liquid Junctions  Ideally,

= E E cell cathode ± E anode

 In

= E practice, E cell cathode ± E anode + E LJ , the liquid junction potential, arises  E LJ from the differential mobility of the cation and anion in the salt bridge be minimized?  How can E LJ

Use a salt bridge containing concentrated salt with equal mobility ions, e.g ., ., KCl 3/1/2011


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How could the Zn/Cu cell be used to measure the Zn2+ concentration in a sample? [ Zn 2 ] 

!

Q



n

ln

[Cu 2 ]

Q

!



ln[Cu 2 ]  



n

ln[ Zn 2 ] 

n

If aCu2+ = 1, then Q

E ! E 

RT

Q ln[ n 2 ] ! E  

nF

!

Q



2.303 n

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2.303 RT log[ n 2 ] 

nF

pZn


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How could the Zn/Cu cell be used to measure the K sp of CuS? CuS (s)¾ Cu2+(aq) + S2±(aq)

K sp

= [Cu2+][S2±]

Place saturated CuS or small amount of Cu 2+ and known S2± concentration in cathode compartment [ Zn 2 ] 

!

Q



n

ln

[Cu 2 ]

!

Q



ln[Cu 2 ]  



n

ln[ Zn 2 ] 

n

If aZn2+ = 1, then Q

E ! E 

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RT

2

Q

ln[ u ] ! E 

nF


RT

K sp

nF

[S2 ]

ln

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Reference Electrodes 

2All cell potential measurements require two electrodes! 1.AgCl(s) 1.AgCl(s) + e-Ag(s) Ag(s) + ClE = Eo+ (0. (0.059/n)log1 059/n)log1/[ /[C Cl] 2.Hg2Cl2(s) + 2e-2C 2Cl-+2Hg +2Hg(l) (l) E = Eo+ (0. (0.059/2)log1 059/2)log1/[ /[C Cl-]2

n = number of electrons transferred per mole, 2.3 2.30 03 RT/F RT/F = 0. 0.059V 3/1/2011


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1_PENDAHULUAN ELEKTROANALISIS-2010

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