F AC 21 08 Potentiometry

8/13/2019 F AC 21 08 Potentiometry

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Dong-Sun Lee / cat - lab / SWU 2012 -Fall version

Chapter 21

Potentiometry

Copyright


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Electroanalytical techniques

1) Ionics---- Conductance

2) lectro!ics

a) Static (I = 0) --- Potentiometry

b) Dynamic (I 0)

Controlled current --- Coulometric titration

Controlled potential

Stirred solution Controlled potential

Hydrodynamic oltammetry

!mperometry --- !mperometic titration

"uiescent solution Potential scan --- Cyclic oltammetry# Polarography

Small amplitude pulse techni$ue

Di%%erential pulse oltammetry

S$uare &ae oltammetry

Potential step --- Chronoamperometry# Chronocoulometry

Pulse oltammetry# Chronoabsorptometry


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Classification of electrochemical methods

1" P#$%$I#&$'(

'easure electrical potential deeloped by an electrode in an electrolyte solution at ero current

%lo& *se +,+S. ,"*!.I/+ relating potential to concentration o% some ion in solution

2" #L$*&&$'(

Determine concentration o% ion in dilute solutions %rom current %lo& as a %unction o% oltage &hen

P/!I1!.I/+ o% ion occurs around the electrode

P/!I1!.I/+ = depletion o% concentration caused by electrolysis

I% using a dropping mercury electrode# method is termed P/!/2!PH3

+" ,#UL#&$'(

,lectrolysis o% a solution and use o% 4araday5s la&6relating $uantity o% electrical charge to amount

o% chemical change

76essentially states that it ta8es 9:; < 0>Coulombs o% electrical charge to cause electrolysis o%

mole o% a unialent electrolyte species?

" ,#%DU,$I&$'(

'easure conductance o% a solution# usingINERT ELECTRODES, ALTERNATING CURRENT, AND

AN ELECTRICAL NULL CIRCUIT- thereby ensure no net current %lo& and no electrolysis .he

concentration o% ions in the solution is estimated %rom the conductance

+/.,@

&et.o!s 1 an! #

%# L,$'#L(SISo% solution Sample recoerable# unaltered by analysis

&et.o!s 2 an!+ must cause L,$'#L(SIS #F $ S*&PL" http@AA&&&scienceutseduauAsubBectsA9:ASectionAsectionhtml


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Potentiometry

!n electroanalytical techni$ue based on

the measurement o% the electromotie

%orce o% an electrochemical cell

comprised o% a measuring and a re%erence

electrode

.he simplest e


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eneral Princiles

e%erence electrode E salt bridge E analyte solution E indicator electrode

,re% ,B ,ind

,cell = ,indF ,re% G ,B

e%erence cell @

a half cell having a known electrode otential

Indicator electrode@

ha! aotential that varie!in a known wa"

with variation! in the concentration of an anal"te

! cell %or potentiometric determinations


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) Saturate! calomel electro!e(S","")

Hg(l) E HgCl(satd)# Cl (satd) E E

electrode reaction in calomel hal-cell

HgCl(s) G e = Hg(l) G ClF

Eo= G 0:JK

E=EoF (00;9:A) log7ClF?= 0>> K

.emperature dependent

! calomel electrode saturated &ith Cl is called a saturated

calomel electrode# abbreiated S",""

!dantage @ using saturated Cl is that 7Cl-? does not changei% some li$uid eaporates

'eerence electro!e

# $aintain! afi%ed otential#

a half cell having a known electrode otential


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HgCl= HgG

G ClF

sp = J L0FJ

Saturated Cl = >: ' Cl

.he crystal structure o% calomel(HgCl)#

&hich has limited solubility in &ater

(sp = J L0FJ)


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4ig - Diagram o% a typical commercial

saturated calomel electrode

4ig - ! saturated calomel electrode

made %rom materials readily aailable in

any laboratory


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) Silver-silver c.lori!e electro!e

!g(s) E !gCl (satd)# Cl (%') E E

!gCl(s) G e = !g(s) G ClF

Eo= G0>>K

E=EoF (00;9:A) log 7ClF?

E(saturated Cl) = G 099K (;oC)


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) stan!ar! .y!rogen electro!e 3S)

.he most %undamental re%erence electrode in electrochemistry MNy

de%initionM its e$uilibrium potential is considered ero at any temperature#

because this electrode &as chosen as an arbitrary ero point %or electrodepotentials ! ero point is needed since the potential o% a single electrode

cannot be measured# only the di%%erence o% t&o electrode potentials is

measurable !ll electrode potentials are e


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oltage ,onversions 4et5een Dierent 'eerence Scales

I% an electrode has a potential o% 0>:K &ith respect to a calomel electrode# &hat is the

potential &ith respect to a siler-siler chloride electrode Qhat &ould be the potential &ith

respect to the standard hydrogen electrode


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Li6ui!-7unction otential

! potential di%%erence bet&een t&o solutions o% di%%erent compositions separated

by a membrane type separator .he simplest e


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8unction otential@

a small potential that e


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Fig" 21-9" Sc.ematicreresentation o a li6ui!

7unction s.o5ing t.e source

o t.e 7unction otential: 7"

$.e lengt. o t.e arro5s

correson!s to t.e relative

mobilities o t.e ions"

Fig 21-Diagram o% a silerAsiler chloride electrodesho&ing the parts o% the electrode that produce the

re%erence electrode potential ,re%and the Buction

potential ,B


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Li6ui! 7unction otential

Cells &ithout li$uid Bunction

PtAH&g'# HClA!gClA!g

are to hae this type o% cell

Cells &ith li$uid Bunction

2lass %rit

Salt bridge

Deelop a potential by di%%erential migration rates o% the cation and anion

Tunction potential

HCl(0)AHCl(00) ,B= >0 mK (HG%aster than ClF)

Cl(0)ACl(00) ,B= F0 mK (G slo&er than ClF)

*sually e


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In!icator electro!es

(etallic indicator electrode re!ond! to anal"te activit")

lectro!e o t.e irst tye

Direct e$uilibrium &ith analyte

!g %or !gG# !u %or !uG# etc

Potential described by +ernst e$uation

!s 7'? # ,

+ote potential linearly related to logo% the concentration U

emember - indicator N3 D,4I+I.I/+ cathode

measurement theoretically under ero-current (steady state)

lectro!e o t.e secon! tye

Indirect e$uilibrium &ith analyte

'A'VAVF

SilerASiler chloride %or chloride

also +ernstian response

as 7VF? # ,

Inert &etallic electro!e or 'e!o; systems

Proides a sur%ace %or the electrochemistry to occur

Pt# !u# Pd# C

VnG(a*) G ne = V(!)

,ind = ,oF (0;9:An) log (A7VnG?)

!gCl(s) G e = !g(s) G ClF(a*)

,ind = ,oF 0;9: log 7ClF?

! plot o% ,$uation - %or an

electrode o% the %irst 8ind

! plot o% ,$uation ->

%or an electrode o% thesecond 8ind %or ClF


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In!icator electro!es

Indicator electrodes %or potentiometric measurements are o% t&o basic types#

namely#$etallicand $e$+rane

1) &etallic in!icator electro!es ?


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Secon!-or!er electro!es or anions

! metal electrode can sometimes be indirectly responsie to the concentration o%

an anion that %orms areciitateor co$le% ion&ith cations o% the metal

,F Eo= 0K

E= 0 F (00;9:A) log (73>F? A7Hg3F?)


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!g(s) E !gCl7satd?# Cl7%'? E E 4eG#4eG) E Pt

4eGGe = 4eG Eo= G00K

Ecell=EindicatorFEre%erence

= X00F (00;9:A) log 74eG?A74eG?Y F X0F (00;9:A) log 7ClF?Y

Inert electro!es

Chemically inert conductors such asgold, latin$# or car+onthat do

not participate# directly# in the redo< process are called inert electrodes .he

potential deeloped at an inert electrode depends on the nature and

concent-ration o% the arious redo< reagents in the solution


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2) &embrane in!icator electro!es

.he potential deeloped at this type o% electrode results %rom an une$ual chargebuildup at opposing sur%ace o% a special membrane .he charge at each sur%ace is

goerned by the position o% an e$uilibrium inoling analyte ions# &hich# in

turn# depends on the concentration o% those ions in the solution

.he electrodes are categoried according to the type o% membrane they employ @

glass#

polymer#crystalline#

gas sensor .he %irst practical glasselectrode (Haber and

lemensie&c#.) /h"!)

Che$# 909# :;# J;


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&embrane in!icator electro!es

lass membrane electro!es

.he internal element consists o% siler-siler chloride electrode immersed in a

pH bu%%er saturated &ith siler chloride .he thin# ion-selectie glass

membrane is %used to the bottom o% a sturdy# nonresponsie glass tube so that

the entire membrane can be submerged during measurements Qhen placed in a

solution containing hydrogen ions# this electrode can be represented by the hal%-

cell @

!g(s) E !gCl7satd?# ClF(inside)# HG(inside) E glass membrane E HG(outside)

E=EoF (00;9:A) log7ClF? G (00;9:A) log(7HG(outside)?A7HG

(inside)?)

E= Q G (00;9:A) log7HG(outside)?


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&eter

pH meter is a olt meter that measures the electrical potential di%%erence

bet&een a pH electrode and a re%erence electrode and displays the result in

terms o% pH alue o% the sample solution in &hich they are immersed

Intro!uction

.he pH meter measures the pH o% a solution using an ion-selectie electrode

(IS,) that responds to the HGconcentration o% the solution .he pH electrode

produces a oltage that is proportional to the concentration o% the HG

concentration# and ma8ing measurements &ith a pH meter is there%ore a %orm

o% potentiometry .he pH electrode is attached to control electronics &hich

conert the oltage to a pH reading and displays it on a meter

Instrumentation

! pH meter consists o% a HG-selectie membrane# an internal re%erence

electrode# an e


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.ypical electrode system %or measuring pH (a) 2lass electrode (indicator) and saturated calomel electrode

(re%erence) immersed in a solution o% un8no&n pH (b) Combination probe consisting o% both an indicator glass

electrode and a silerAsiler chloride re%erence ! second silerAsiler chloride electrode seres as the internal

re%erence %or the glass electrode .he t&o electrodes are arranged concentrically &ith the internal re%erence in the

center and the e


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pH Meters


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meter #A gla!! co$+ination electrode

E 0 12 + (3)34567)log (Ain8 Aot)

1@A!"$$etr" otential

+ @electro$otive efficienc" ( close to 00)

A @Activit" of h"drogen ion


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Composition o% glass membranes

0Z Si/

0Z Ca/# Na/# i

/# +a

/#

andAor !l/

Ion e


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Potential o t.e glass electro!e

.he potential di%%erence across the glass pH electrode

depends on the actiity o% HGon each side o% the

glass membrane

,m= KF K = (.A4)lnaF (.A4)lna

= ,asymG 00;9: log(aA a)

i%A= constant#

,m= G 00;9: loga

= F 00;9: pH

Standardiationat pH=00 # , = 0 K

pH >00# ,= ;9: mKApH unit

Potential pro%ile across a glass membrane %rom the analyte solution to

the internal re%erence solution .he re%erence electrode potentials are

not sho&n


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Isopotential

point,(mK)

pH

00oC > mKApH unit

0oC ;> mKApH unit

>

;00

0


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,alibrating a glass electro!e

!l&ays 8eep the electrodes in distilled &ater# saturated Cl solution(') or bu%%er &hen not in

use

Po&er /+ S&itch to [S.!+DN3\ @ allo& to &arm %or 0 min

inse the electrode thoroughly &ith distilled &ater and then &ith pH 00 bu%%er solution

Nlot &ith clean tissue

> Determine the temperature o% the bu%%er solution &ith a thermometer

!dBust [.,'P,!.*,\ 8nob on the unit to the temperature

; Place the electrode inpH 00(isopotential point) bu%%er solution

otate the selector s&itch to [pH\ Qait %or a stable display

Ny using [C!IN!.I/+\ 8nob# set the meter to the pH alue o% the bu%%er at its measured

temperature S&itch to [S.!+DN3\

: inse the electrode thoroughly &ith distilled &ater and then &ith pH >00 bu%%er solution

Nlot &ith clean tissue

Place the electrode inpH >00 bu%%er solution otate the selector s&itch to [pH\

Qait %or a stable display *sing [S/P,\ 8nob# set the meter to the pH alue o% the bu%%er at its

measured temperatureS&itch to [S.!+DN3\


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,alibration o t.e &eters 5it. an! 2 4uers

Select the pH 'ode and set the temperature control 8nob to ;]C !dBust the cal 8nob

to read 00Z

inse the electrode &ith deionied &ater and blot dry using a piece o% tissue (Shur&ipesor im&ipes are aailable in the labs)

Place the electrode in the solution o%pH bu%%er# allo& the display to stabilie and# then#

set the display to read by adBusting cal emoe the electrode %rom the bu%%er

> inse the electrode &ith deionied &ater and blot dry using a piece o% tissue (Shur&ipes

or im&ipes are aailable in the labs)

; Place the electrode in the solution o%pH bu%%er# allo& the display to stabilie and# then#

set the display to read by adBusting cal emoe the electrode %rom the bu%%er

: inse the electrode &ith deionied &ater and blot dry using a piece o% tissue (Shur&ipes

or im&ipes# as be%ore)

%#$- Nu%%er solution are made aailable to you in indiidually labeled o bottles .he

bu%%ers are to be used in these containers# onlyU Do not pour them into other containers at

any time !%ter use# cap the bottles so that the bu%%ers can be re-used


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&easuring

'a8e sure that the meter is set to the pH 'ode and adBust the temperature to ;]C

Place the electrode in the sample to be tested

.he pH o% the solution appears in the display

+/.,@ !llo& the display to stabilie be%ore ta8ing your readingU

> inse the pH electrode and place it bac8 in the storage solution


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rrors t.at aect measurements 5it. glass electro!e

.he al8aline(sodium) error @ lo& readings at pH alues greater than 9

.he acid error @ some&hat high &hen the pH is less than about 0;

Dehydration may cause erratic electrode per%ormance

> Kariation in Bunction potential @ ^ 00 pH unit; ,rror in the pH o% the standard bu%%er @ 00 pH unit

,leaning glass electro!e@ Qashing &ith :' HCl

0 &A&Z a$ueous ammonium bi%luoride (+H>H4)


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!cid and al8aline errors %or selected glass electrodes at ;

(4rom 2 Nates#Deter$ination of 9,nd ed# p :; +e& 3or8@ Qiley# 9)


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Ion-Selective lectro!es 3IS)

Intro!uction

!n Ion-Selectie ,lectrode (IS,) produces a potential that is proportional to the

concentration o% an analyte 'a8ing measurements &ith an IS, is there%ore a %ormo% potentiometry .he most common IS, is the pH electrode# &hich contains a thin

glass membrane that responds to the HGconcentration in a solution

$.eory

.he potential di%%erence across an ion-sensitie membrane is@

, = F (0.An4)log(a)

&here is a constant to account %or all other potentials# is the gas constant# . is

temperature# n is the number o% electrons trans%erred# 4 is 4araday5s constant# and a

is the actiity o% the analyte ion ! plot o% measured potential ersus log(a) &illthere%ore gie a straight line

IS,s are susceptible to seeral inter%erences Samples and standards are there%ore

diluted @ &ith total ionic strength adBuster and bu%%er (.IS!N) .he .IS!N

consists o% ' +aCl to adBust the ionic strength# acetic acidAacetate bu%%er to

control pH# and a metal comple


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IS 3ion selective electro!e)

!ny electrode that pre%erentially responds to one ion species

2lass membrane electrode @ HG

i$uid membrane electrodes

Solid state and precipitate electrodes

> 2as sensing electrodes

; ,nyme electrodes

Selectiity coe%%icient

kV#3

= (response to 3) A (response to V)

2eneral behaior o% IS,

, = constant (00;9:AnV) log 7AV G(kV#3A3nV n3)?

I t t ti


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Instrumentation

IS,s consist o% the ion-selectie membrane# an internal re%erence electrode# an

e


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Li6ui! IS

Ca IS,

Calcium didecylphosphate dissoled in dioctylphenylphosphonate

7(CH(CH)JCH/)P/?Ca 7(CH(CH)JCH/)P/?FG CaG

Diagram o% a li$uid-membrane electrode %or CaG


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Comparison o% a li$uid-membrane calcium ion electrode &ith a glass pH electrode


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Photograph o% a potassium li$uid-ione


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! homemade li$uid-membrane electrode


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Soli! state crystalline membrane electro!e

'igration o% 4Fthrough a4doped &ith ,u4


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;amle< Fluori!e 3F-) electro!e

Internal re% electrode

!gA!gCl

4illing soln

!$ueous +aCl G +a4

'embrane

a4crystal disc

!pplications

,lectroplating industry# &ater treatment (%luoridation)# toothpaste

*lications o ion selective electro!es


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Ion-selectie electrodes are used in a &ide ariety o% applications %or determining the concentrations

o% arious ions in a$ueous solutions .he %ollo&ing is a list o% some o% the main areas in &hich IS,s

hae been used

Pollution 'onitoring@ C+# 4# S# Cl# +/etc# in e%%luents# and natural &aters

!griculture@ +/# Cl# +H

># # Ca# I# C+ in soils# plant material# %ertilisers and %eedstu%%s

4ood Processing@ +/# +/

in meat preseraties

Salt content o% meat# %ish# dairy products# %ruit Buices# bre&ing solutions

4 in drin8ing &ater and other drin8s

Ca in dairy products and beer

in %ruit Buices and &ine ma8ing

Corrosie e%%ect o% +/in canned %oods

Detergent 'anu%acture@ Ca# Na# 4 %or studying e%%ects on &ater $uality

Paper 'anu%acture@ S and Cl in pulping and recoery-cycle li$uors

,


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Semi-con!uctor

Imper%ections or impurities in &hat are normally insulators may gie rise to a

temperature-dependent conductiity( metallic conductiity decreases &ith rise in

temperature) arising because the highest occupied energy leel is ery close to an

unoccupied leel

Classi%ication speci%ic resistance

semiconductor 0F>^ 0_m

conductor ^0FJ

insulator 0^00


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,lemental or intrinsic semiconductor @ ten nine (9999999999Z purity)

II III IK K KIN C

!l Si P S

1n 2a 2e !s

Cd In Sn Sb Se

.e

Compounds or e


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p-tye an! n-tye semicon!uctor

Si

Si

Si

Si

Si

Si

SiSi

Si

Complete coalent

bond

SiSi

SiSi

Si

Si

Si

SiSiSi

!l

Displaced by

trialent(acceptor)

impurity atom

Si

Si

Si

SiSi

Si P

hole

Conduction

electron

Displaced by

pentaalent(donor)

impurity atom

carrier

Pure silicon crystal

structure-type n-type

Dio!e < electron tube eacuated glass or metal enelope containing t&o electrodes a cathode


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Dio!e < electron tube# eacuated glass or metal enelope containing t&o electrodes# a cathodeand an anode It is used as a recti%ier and as a detector in electronic circuits such as radio and

teleision receiers Qhen a positie oltage is applied to the anode (or plate)# electrons emitted %rom

the heated cathode %lo& to the plate and return to the cathode through an e


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Dr Tohn Nardeen# Dr Qalter Nrattain# and Dr Qilliam Shoc8ley discoered

the transistor e%%ect and deeloped the %irst deice in December# 9># &hile

the three &ere members o% the technical sta%% at Nell aboratories in 'urrayHill# +T .hey &ere a&arded the +obel Prie in physics in 9;:

http@AA&&&lucentcomAmindsAtransistorA Copyright 2002 Lucent Technologies. All rights reserved. *
http://c/


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$ransistor

!n actie component o% an electronic circuit &hich may be used as an ampli%ier#

detector# or s&itch

! transistor consists o% a small bloc8 o% semiconducting material to &hich at leastthree electrical contacts are made

.ransistors are o% t&o general types# bipolar and %ield e%%ect

:n:type n::ntype

emitter emitter base collector collectorbase

C C

, ,

Carrier@ hole Carrier@ conduction electron

N N


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.ransistor

,miter Collector Nase Nase Carrier

( ) !" #$ %&'( )* +, -. /01 +, -.2

*nipolar .ransistor Nipolar .ransistor2 34'5 # 67 # 89 # :" ; ?@A B

'/S('etal /


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ate

) p2 .ransistor Ge )* F 2 G5 # 9 g K

} F$ p 2

) +, > '/S .ransistor KA G = .2 Nipolar .ransistor

Nase }lG .

mitter

+P+# P+P O F .ransistor 2 K +,

,ollector

.ransistor 3

4ase &aterial3 )Z 2 F$ P $

-3 (carbon nanotube) p 2 (silicon microchip) A Y

,f MN2 1[ 00 J : { o1 3] (+ano etter) }

V IN' V- . T V] (. T Qatson esearch Center) -

> p< (Ph !ouris) (K Deryc8e)# ( 'artel)# &U (T

!ppeneller) 2 $ V {E -3 (Single &all carbon nanotube R SQC+.s)

@} RS X\'


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Fiel! eect transistor

2ate

Drain

Source

channel

'/S4,.

nchannel

! metal o


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/peration o% a iel! eect transister

(a) +early random distribution o% holes and electrons in the base in

the absence o% gate potential

(b) Positie gate potential attracts electrons that %orm a conductie

channel beneath the gate Current can %lo& through this channel

bet&een source and drain


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/peration o% chemical-sensing %ield e%%ect transistor .he transistor is coated &ith an

insulating Si/ layer and a second layer o% Si+> (silicon nitride)# &hich is imperious

to ions and improes electrical stability .he circuit at the lo&er le%t adBusts the potential

di%%erence bet&een the re%erence electrode and the source in response to changes in the

analyte solution# such that a constant drain-source current is maintained


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C/gas sensing electrode


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esponse o% a li$uid-membrane electrode to ariations

in the concentration and actiity o% calcium ion


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.itration o% >mmol o% chloride ion &ith 0000' siler nitrate

(a) .itration cure (b) 4irst-deriatie cure (c) Second-deriatie cure

!pparatus %or a potentiometric titration


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Kolume o% 0= + +a/H (m)

0 5 10 15 20 25

pH

2

4

6

8

10

12

14

Experimental titration curve of 0.1 N HOAc with 0.1 N NaOH ( f = 0.9720.

C!.-abASQ*# Dong-Sun ee

Kolume o% 0= + +a/H (m)

0 5 10 15 20 25

mK

-400

-300

-200

-100

0

100

200

300


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Q

E!

Thanks

Dong-Sun ee A C!. A SQ*

Documents

F AC 21 08 Potentiometry