Chuong 14 Hóa học phân tích

8/14/2019 Chuong 14 Ha hc phn tch

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Lm Ngc ThChng 14 . Chun Oxi ha kh

Csha hc phn tch. NXB i hc quc gia H Ni 2005.

T kho: Chun Oxi ha kh, Phng trnh Nerst, Phn ng to phc, Phn ng

kt ta, Nng cht phn ng, Chun hn hp, Cht ch th.

Ti liu trong Thvin in tH Khoa hc Tnhin c th c sdng cho mc

ch hc tp v nghin cu c nhn. Nghim cm mi hnh thc sao chp, in n phc

v cc mc ch khc nu khngc schp thun ca nh xut bn v tc gi.

Mc lc

Chng 14 Chun Oxi ha kh........................................................................... 3

14.1 Nhng khi nim cbn ................................................................................ 3

14.1.1 nh ngha ................................................................................................... 3

14.1.2 Phng trnh Nerst ......................................................................................5

14.2 Nhng yu tnh hng n th oxi ha kh ............................................... 5

14.2.1 nh hng ca axit ................................................................................ 5

14.2.2 nh hng ca phn ng to phc.............................................................. 6

14.2.3 nh hng ca phn ng kt ta................................................................. 7


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2

14.5 Hng s cn bng ca phn ng oxi ha kh................................................. 8

14.5 ng chun oxi ha kh ....................................................................... 10

14.5 Cc yu tnh hng ln ng chun oxi ha kh .............................. 12

14.2.1 Nng cht phn ng..............................................................................13

14.2.2 hon ton ca phn ng ....................................................................... 13

14.2.3 Tc phn ng v thin cc...............................................................14

14.7 Chun hn hp ........................................................................................15

14.7 Cht ch th oxi ha kh ............................................................................... 17

14.7.1 Nhng ch th oxi ha kh thng thng .................................................. 17

14.7.2 Ch thc bit........................................................................................... 21


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Chng 14

Chun Oxi ha kh

14.1 Nhng khi nim cbn

14.1.1nh ngha

Cht oxi ha l cht c kh nng nhn electron.

Cht khl cht c kh nng cho electron.

Cht oxi ha (Ox) sau khi nhn electron trthnh cht kh (Kh) gi l cht kh lin hpvi n. Mi cp oxi ha kh lin hp c th biu din bng phng trnh:

Ox + ne Kh

Mt vi v d v cp oxi ha kh lin hp:

Dng oxi ha Dng kh Cp oxi ha kh lin hp

Zn2+ + 2e Zn 0 Zn2+/Zn

Fe3+ + e Fe 2+ Fe3+/Fe2+

2H+ + 2e H 2 2H+/H2

Cl2 + e 2Cl Cl2/2Cl

4MnO

+ 5e + 8H

+

Mn

2+

+ 4H2O 4MnO

/Mn

2+

36Fe(CN)

+ e 46Fe(CN) 36Fe(CN)

/ 46Fe(CN)

AgCl + e Ag 0 + Cl Ag+/Ag0

Electron khng tn ti trng thi t do trong dung dch, do mt cht ch th hin tnhoxi ha khi c cht kh cho electron ca n hay ngc li.

Phn ng trao i electron gia cc cht oxi ha v cht kh gi l phn ng oxi ha kh.


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aOx1 + bKh2 cKh1 + dOx2

V d, khi nhng mt thanh kim loi hot ng hn ng, v d km vo dung dch ngsunfat th s xy ra phn ng oxi ha kh.

Trong phn ng oxi ha kh c tnh cht ha hc di y, ion Cu2+ nhn 2e ca nguynt km, b kh thnh nguyn tng, cn nguyn t km b oxi ha thnh ion Zn2+.

Chng ta c th lm mt th nghim in ha nu r rng trong phn ng trn c schuyn electron t cht kh l km kim loi sang cht oxi ha l ion Cu2+ cch xa nhau,hay ni mt cch khc l c mt dng in i t cht oxi ha l ion ng sang cht kh lkm.

Chiu quay ca kim in k ch r rng c dng in i t cc ng sang cc km. iu c ngha l th bn cc ng ln hn th bn cc km v electron chuyn t cc km sang

cc ng.

Sau mt thi gian, kim in k trv v tr khng, trong h khng cn dng in. emcc dung dch ra phn tch th thy nng dung dch km tng ln v nng dung dchng gim i, cc km sng ra, cc ng c ph mt lp ng kim loi. Chiu ca dngin trong th nghim trn chng t rng Cu2+ ca cp oxi ha kh lin hp Cu2+/Cu0 c thcao hn nn oxi ha km kim loi ca cp Zn2+/Zn0 c th thp hn. Th oxi ha kh cacp oxi ha kh lin hp quyt nh chiu ca phn ng:

Zn0 + Cu2+ Zn 2+ + Cu0

Hnh 14.1


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H oxi ha khng km

14.1.2Phng trnh Nerst

Th oxi ha kh ca mt cp oxi ha kh lin hp c tnh bng phng trnh Nerst.

i vi hn gin:

Ox + ne Kh

o Ox

Kh

aRTE E ln

nF a= +

trong : E l th oxi ha (Von), R l hng s kh (8,331 Jun), T l nhit tuyt i, F ls Faraday (96500 c), n l s electron trao i, Eo l th oxi ha kh tiu chun, th khi biuthc sau logarit bng 1, ln l logarit cs e (logarit t nhin) aOx, aKh l hot ca dng oxiha v dng kh. Eo ch ph thuc vo bn cht ca h oxi ha kh lin hp.

Trong thc t, ngi ta khng th xc nh c gi tr tuyt i th ca mt cp oxi hakh lin hp m ch xc nh c gi tr tng i ca n (xem th nghim trn).

so snh th ca cc cp oxi ha kh lin hp vi nhau cn phi xc nh gi tr tngi th ca mt cp oxi ha kh lin hp c quy c bng khng. Ngi ta quy c th oxiha kh tiu chun ca cp 2H+/H2 bng khng.

14.2 Nhng yu tnh hng n th oxi ha kh

Th oxi ha kh tiu chun quyt nh chiu ca phn ng oxi ha kh, nhng trong thct c nhng yu tnh hng rt mnh n th oxi ha kh ca cc cp oxi ha kh lin hpv do nh hng n chiu ca phn ng. Sau y ta xt tng trng hp c th.

14.2.1nh hng ca axit

Tnh th oxi ha kh iu kin ca cp 34AsO / 33AsO

trong mi trng natri hiro

cacbonat - ngha l pH = 8; bit Eo ca cp ny pH = 0 l +0,57 V.

Vi cp ny c th xy ra phn ng:

34AsO

+ 2H+ + 2e 33AsO + H2O

Th oxi ha kh ca cp ny:


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34

33

AsOo 2

HAsO

a0,059 0,059E log a log

2 2 aE

+

= + +

Th chun iu kin Eo

khi pH = 8 ngha l th khi:

[ 34AsO ] = [ 33AsO

] v [H+] =108

( )2

o 80,059 0,059E' E log 10 0,57 8 2 0,10(0,098)2 2

= + = + = +

Nh vy, pH cng tng th th chun iu kin cng gim ngha l kh nng oxi ha ca34AsO

gim khi pH tng, cn kh nng kh ca 33AsO li tng cng vi pH. Chnh v vy

m th oxi ha chun ca cp 34AsO / 33AsO

= 0,57 V khi pH = 0 nn 34AsO c th oxi ha

c I

v th chun ca cp I2/2I

= 0,54 V.34AsO

+ 2I+ 2H+ 33AsO + I2 + H2O

Nhng trong dung dch NaHCO3 (pH 8) th I2 li oxi ha c 33AsO v nh trn

tnh l th chun ca cp 34AsO / 33AsO

ch cn l +0,10 V. Phn ng xy ra theo chiungc li:

33AsO

+ I2 + H2O 34AsO + 2I+ 2H+

14.2.2nh hng ca phn ng to phc

Khi cht oxi ha hoc cht kh ca mt cp oxi ha kh lin hp tham gia vo phn ngto phc th th oxi ha kh cng bin i. Chng ta xt mt v d c th.

V d 1: Tnh th oxi ha kh tiu chun iu kin ca cp Fe3+/Fe2+ trong dung dch c

d florua to phc. Phc c hng s bn = 1036FeF 16; .3 2

o

Fe / FeE 0,+ + = 77 V

Vi h ny xy ra hai phn ng:

Fe2+ e Fe 3+ (1)

Fe3+ + 6F 36FeF (2)

Cng li: Fe2+ + 6F e 36FeF

Lc ny:36o

62

FeFE E 0,059log

Fe F

+

= +


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E =Eo'= th oxi ha kh tiu chun iu kin khi36

62

FeF1

Fe F

+

=

(3)

Theo phng trnh to phc (2):36 16

63

FeF10

Fe F

+

=

(4)

Chia (3) cho (4) ta c:3

162

Fe 1

10Fe

+

+

=

3

o' o

162

Fe 1E E 0,059log 0,77 0,059log 0,17 V

10Fe

+

+

= + = + =

Nh vy, trong mi trng c d F kh nng oxi ha ca Fe3+ gim i v kh nng khca Fe2+ li tng ln.

14.2.3nh hng ca phn ng kt ta

Phn ng kt ta cng lm thay i th oxi ha kh ca cc cp oxi ha kh lin hp vdo lm thay i chiu ca phn ng.

V d: Tnh th oxi ha kh tiu chun iu kin ca cp Cu2+/Cu+ khi c d I to thnhkt ta CuI, cho bit TCuI= 10

12, , E .2o

Cu /CuE 0,17V+ + = +

2

o

I /2I0,54V =

Th oxi ha kh tiu chun ca cp I2/2I cao hn cp Cu2+/Cu+ nn I2 s oxi ha Cu

+trthnh Iv Cu2+ ngha l phn ng cn xy ra theo chiu:

2CuI + I2 2Cu2+ + 2I

Nhng trong thc t phn ng li khng xy ra nh vy v th oxi ha kh tiu chuniu kin ca cp Cu2+/Cu+ khi c d I thay i rt nhiu. Ta tnh th. Khi khng c

ion I th th oxi ha kh ca cp Cu2+/Cu+ng vi phng trnh:

Cu2+ + e Cu +

c xc nh bng cng thc:

2

oCu

E E 0,059logCu

+

+

= +

;


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trong Eo = 0,17 V

Khi c ioua lm kt ta CuI theo phn ng:

Cu2+ + I+ e CuI

th

E = Eo + 0,059log[Cu2+][I]

E =Eo': th oxi ha kh tiu chun iu kin khi [Cu2+][I] =1

Chia v vi v ca biu thc ny vi biu thc tnh tch s tan ca CuI:

[Cu+][I] = TCuI = 1012

ta c:

2

12

Cu 1

10Cu

+

+

=

12

12

1E' E 0,059log 0,17 0,059log10

10= + = +

0,17 0,050 12 0,86 V= + = +

Nh vy l khi c Id, v phn ng thc t xy ra

theo chiu sau y:

22

o' o

Cu /Cu I /2IE 0,88 V E 0,+ + = > = 54 V

2Cu2+ + 4I 2CuI + I 2

14.5 Hng s cn bng ca phn ng oxi ha kh

Gi s c phn ng:

aOx1 + bKh2 aKh 1 + bOx2

trong :

aOx1 + ne aKh 1

bOx2 + ne bKh 2


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[ ]

[ ]

[ ][ ]

a1o

1 1 a1

b

2o2 2 b2

Ox0,059E E log

n Kh

Ox0,059E E log

n Kh

= +

= +

Khi phn ng t cn bng, E1= E2 nn:

[ ]

[ ]

[ ]

[ ]

a b1 2o o

1 2a b1 2

Ox Ox0,059 0,059E log E log

n nKh Kh+ = +

[ ]

[ ]

[ ]

[ ]

a b1 2o o

1 2 a b

1 2

Kh Ox0,059E E log

n Ox Kh =

[ ]

[ ]

[ ]

[ ]

( )

a b1 2

a b1 2

o o1 2

Kh OxK

Ox Kh

n E ElogK

0,059

=

=

V d: Tnh hng s cn bng ca phn ng:

Ce4+ + Fe2+ = Ce3+ + Fe3+

Cho bit: 4 3 3 2o o

Ce / Ce Fe / FeE 1,55 V;E 0,+ + + += = 77 V

p dng cng thc trn:

13,41,55 0,77logK 13,4 K 100,059

= = =

thy mc hon ton ca phn ng ny ta hy tnh cc t s nng [Fe3+]/[Fe2+] v[Ce4+]/[Ce3+] khi nng ban u bng nhau.

Khi cn bng th [Ce3+] = [Fe3+] v [Ce4+] = [Fe2+], do :

2 23 3 3 3

13,4

2 22 4 2 4

Fe Ce Fe Ce10 K ;

Fe Ce Fe Ce

+ + + +

+ + + +

= = = =


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3 3

13,4 6,7

2 4

Ce10 10

Ce

+ +

+ +

= = =

Fe

Fe

+ + = 24

o

MnO / Mn,51 V + =

Nh vy l khi phn ng t cn bng lng Fe2+ v lng Ce4+ ch cn di mt phntriu lng ban u.

14.5ng chun oxi ha kh

V d: Tnh th ca dung dch khi chun 50 ml FeSO4 0,100 M bng dung dch

KMnO4 0,020 M [H+] lun bng 1 M. Cho bit E 1 .3 2

o

Fe /FeE 0,77 V;

Trc im tng ng tnh th theo cp Fe3+/Fe2+ theo bn phn ng:

Fe2+ e Fe 3+

3Fe0,059

E 0,77 log1 2Fe

+= +

+ (14.1)

Sau khi qua im tng tng ng, lc ny cp 4MnO /Mn2+ d nn tnh th theo cp

ny da vo bn phn ng:

MnO4

+ 8H+ + 5e Mn 2+

8MnO H0,059 4E 1,51 log

5 2Mn

+= +

+(m [H+] = 1) (14.2)

im tng ng th ca 2 cp bng nhau, nn th l th chung cho 2 cp v phnng t cn bng:

3Fe0,059

E 0,77 log1 2

Fe

+= +

+

8MnO H45E 5.1,51 0,059log

2Mn

+= +

+

83

4

2 2

Fe MnO H6E 0,77 5.1,51 0,059log

Fe Mn

+ +

+ +

= + +

(14.3)


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(im T)

50,1 0,1

0,059 3E 1,51 log 2.10

5

= +

1,49

51,0 1,0 0,059 2

E 1,51 log 2.105

= + 1,50

Hnh 14.2

ng chun 50,00 ml dung dch Fe2+ 0,100 M bng dung dch KMnO4 0,020 M

14.5Cc yu tnh hng ln ng chun oxi ha kh

Trong cc chng trc y chng ta ni v hiu ng nng cht phn ng v hon ton ca phn ng ln ng chun . Trong chng ny chng ta s bn lun v hiung ca cc thng s ln ng chun oxi ha kh.


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14.2.1Nng cht phn ng

Trn y chng ta thy rt r rng, E ca h chun oxi ho kh thng thng

khng ph thuc vo s pha long. V vy, ng chun ca nhng phn ng oxi ha khthng khng ph thuc vo nng cht cn phn tch v cht phn ng. iu hon tontri ngc vi nhng iu thy v cc ng chun loi khc m chng ta gp.

14.2.2 hon ton ca phn ng

Phn ng ca mt php chun oxi ha kh ch xy ra cng hon ton khi s bin ith ca hvng im tng ng cng ln. Hiu ng v thin cc nh nu trn yc minh ho rt y trn hnh 14.3. Hnh 14.3 dn ra nhng ng chun mt chtkh ginh c thin cc l 0,20 V bng mt s cht oxi ha ginh c thin cc

chun t 0,40 n 1,20 V v hng s cn bng tng ng nm trong khong 2.10

3

n 8.10

16

.R rng l s bin i th ln nht ca h gn lin vi phn ng xy ra hon ton nht. Vphng din , ng chun oxi ha kh tng tng chun ca nhng phn ngloi khc.

Hnh 14.3

Hiu ng thin cc ca cht chun ln hon ton ca phn ng


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14.7 Chun hn hp

Cc dung dch c hai cht oxi ha hoc hai cht kh sinh ra cc ng chun c hai

im un nu th chun ca hai cht cn phn tch khc nhau ln. Nu s khc nhau lnhn 0,2 V th hai im un thng c phn bit r rng xc nh ring tng hpphn. Tnh hung hon ton ging php chun hai axit c hng s phn li khc nhau ln hoc php chun hai ion to kt ta c tan khc nhau ln.

Thm vo , tnh cht ca mt s t h oxi ha kh tng t nh tnh cht ca a axit.V d nh, xt hai na phn ng:

VO2+ + 2H+ + e V3+ + H2O ; Eo = + 0,369 V

V(OH)4+ + 2H+ + e VO 2+ + 3H2O ; E

o = + 1,00 V

ng chun V3+ bng cht oxi ha mnh, v d nh pemanganat, c hai im un,im un tng ng vi s oxi ha V3+n VO2+ v im th hai tng ng vi s oxi haVO2+n . S oxi ha tng bc ca molipen (III), bc u n trng thi oxi ha+5 v bc tip theo n trng thi oxi ha +6, l mt v d thng dng khc. y, mt lnna, im un xut hin r rng trn ng chun bi v s khc v th chun ca na

phn ng tng ng l 0,4 V.

4V(OH)+

Phn tch chi tit ngun gc ca ng chun cho tng cht ring bit trong hn hpca nhng cht phn ng loi ny khng c g kh khn nu s khc bit v th chun ln.Chng ta ly php chun dung dch cha ion st (II) v titan (III) bng kali pemanganat lm v

d. Th chun ca hai cp ng vai tr cht kh trn l:

TiO2+ + 2H+ + e Ti3+ + H2O ; Eo = +0,099 V

Fe3+ + e Fe 2+ ; Eo = +0,77 V

Nhng phn thm pemanganat u tin c dng cho ion titan (III) v d dng b oxiha hn. Cho n khi nng ca cht ny cn ln trong dung dch, th ca h khng thcao lm bin i nng ion st (II). Nh vy l, c th thu c nhng im to nn

phn th nht ca ng chun bng cch thay th nhng nng hp thc ca ion titan(III) v titan (IV) vo phng trnh:

3

22

TiE 0,099 0,059log

TiO H

+

+ +

= +

on u ca ng chun titan (III) bng kali pemanganat l ng nht. Sau imtng ng th nht, nng c hai ion st (II) v st (III) u ln nn c th c cc imtrn ng chun mt cch thun li nht theo h thc:


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2

o

3

FeE 0,059log

FeE

+

+

=

Khp c vng v vng sau im tng ng th hai, ng chun thc cht lng nht vi ng chun ion st(II) ring bit. Nhng php tnh nh vy khng tnhc thim tng ng th nht. C mt cch thun tin tnh gi tr l cng cc

phng trnh Nerst biu din th ca st(II) v titan(III). V thin cc ca hai h bng nhauim cn bng nn ta c th vit:

3 2

22 3

Ti Fe2E 0,099 0,77 0,059log

TiO Fe H

+ +

+ + +

= + +

Cc ion st (III) v titan (III) tn ti lng nh v bng nhau do kt qu ca cn bng:2H+ + TiO4

2+ + Fe2+ Fe 3+ + Ti3+ + H2O

nn [Fe3+] = [Ti3+]

Thay kt qu ny vo phng trnh th trn y ta c:

[ ]

2

2

2

Fe0,87 0,059E log

2 2TiO H

+

+

= +

Hnh 14.4


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ng chun 50,0 ml dung dch hn hp Ti3+ 0,050 M v Fe2+ 0,200 M bng dung dch KMnO40,050 M. Nng H+ ca dung dch lun bng 1,0 M

Cui cng, nu [ ] v [Fe22TiO+ 2+] c ginh l c nng phn tch ng nht ta c

th tnh th tng ng.

ng chun hn hp st (II) v titan (III) bng dung dch pemanganat c dn ratrn hnh 14.4.

14.7 Cht ch th oxi ha kh

C hai loi ch th ha hc c s dng pht hin im cui ca nhng php chun oxi ha kh: nhng cht ch th oxi ha kh thng thng v nhng ch th oxi ha khc bit.

14.7.1Nhng ch th oxi ha khthng thng

Nhng ch th oxi ha kh thng thng l nhng cht chuyn mu khi b oxi ha hockh. Khc vi cc cht ch thc bit c mu bin i ph thuc nhiu vo bn cht ha hcca cht cn phn tch v cht chun, s chuyn mu ca cc cht ch th tht s oxi ha kh

ph thuc vo s bin i thin cc ca h xut hin do s tin trin ca php chun .

C th vit na phn ng tng ng vi s chuyn mu ca mt cht ch th oxi ha khthng thng nh sau:

Inox + ne Inkh

Nu phn ng ch th thun nghch, c th vit:

[ ][ ]

Kho

Ox

In0,059E E log

n In= (14.4)

Mt ngi ch c kh nng nhn bit s bin i mu khi

[ ][ ]

Kh

Ox

In 1

In 10 chuyn thnh[ ][ ]

Kh

Ox

In

10In

C th tm thy s bin i th cn thit to nn s chuyn mu hon ton ca cht chth thng thng in hnh bng cch thay th hai gi tr trn y vo phng trnh (14.4).

oIn

0,059E E

n=


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Phng trnh trn ch r rng, mt cht ch th thng thng ch c th biu lkh nng pht hin s chuyn th ca mnh khi cht chun gy nn s chuyn dch th cah t + 0,059/n n 0,059/n hoc khong (0,118/n) V. i vi nhiu ch th n = 2

v s

chuyn d

ch 0,059 V l

.

oInE

oInE

Bng 14.2 di y lit k th dch chuyn ca mt s cht ch th oxi ha kh. Cu trcv phn ng ca mt s ch th c lit k trong bng nu trn sc bn lun mcdi y.

Bng 14.2 Cc cht ch th oxi ha khla chn

Mu

Cht ch thDng ox

haDng kh

Th chuyndch

iu kin

Phc Fe(II)-5-nitro-1,10-phenantrolin

Xanh nht tm +1,25 H2SO4 1 M

Axit iphenylaminicacboxilic

Xanh tm Khng mu +1,12 H2SO4 7 10 M

Phc Fe(II)-1,10-phenantrolin

Xanh nht +1,11 H2SO4 1 M

Phc Fe(III)-5-metyl,1,10-

phenantrolinXanh nht +1,02 H2SO4 1 M

Erioglancin A Xanh Vng lc +0,98 H2SO4 0,5 M

iphenylaminsunfomic axit

tm Khng mu +0,85 Axit long

iphenylamin Tm vng Khng mu +0,76 Axit long

p-Etoxicrisidin Vng +0,76 Axit long

Metylen xanh Xanh Khng mu +0,53 Axit 1 M

Indigo tetrasunfonat Xanh Khng mu +0,36 Axit 1 M

Phenosafranin Khng mu +0,28 Axit 1 M

Phc ca st (II) vi o-phenantrolin


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Mt lp cc hp cht hu c c bit nh 1,10-phenantrolin (hoc o-phenantrolin)to phc bn vi st (II) v mt vi ion khc. Hp cht ny c hai nguyn t nitc phn

bv tr c th to lin kt cng ha tr vi ion st (II). Ba phn to-phenantrolin lin ktvi mt ion st (II) to thnh phc c cu trc:

Phc ny i khi c gi l "feroin" c cng thc ha dng (phen)3Fe2+.

Phc st feroin tham gia phn ng oxi ha kh thun nghch:

(phen)3Fe3+ + e (phen) 3Fe

2+ ; Eo = +1,06 V

Xanh nht

Trong thc t mu ca dng oxi ha rt nht, kh pht hin nn s chuyn mu lin hpvi dng kh l t dng gn nh khng mu n mu . V s khc nhau v cng munh vy nn im cui thng l lc ch 10% ch th dng phc vi st (II). Th dch

chuyn khong +1,11 V trong axit sunfuric 1 M.

Trong tt c cc ch th oxi ha kh, feroin l cht gn nht vi ch th oxi ha kh ltng. Ch th ny phn ng nhanh v thun nghch, chuyn mu r rng, dung dch ch th

bn v c iu ch sn. Khc vi nhiu ch th oxi ha kh khc, dng oxi ha ca feroinc bit tri vi cc cht oxi ha mnh. nhit khong 60oC feroin b phn hy.

Nhng tnh cht ch th ca mt s dn xut ca phenantrolin cng c nghin cu vmt s trong chng chng t rng, cng hu dng nh hp cht gc. Trong s, ngch l cc dn xut 5-nitro v 5-metyl:

N

N

O2NN

N

H3C

Th dch chuyn tng ng ca chng l +1,25 V v +1,02 V.


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20

iphenylamin v cc dn xut ca n

iphenylamin, C12H11N, l mt trong nhng cht ch th oxi ha kh c pht hinu tin v c Knop s dng nm 1924 chun st (II) bng kali icromat. Khi c mt

cht oxi ha mnh, chc chn l iphenylamin s tri qua nhng phn ng:

N

H

2 N

H

N

H

+ 2H+

+ 2e

iphenylamin

(khng mu)

iphenylbenziin

(khng mu)

N

H

N

H

+ 2H+ + 2eiphenylbenziin

(khng mu)

N

H

N

H

iphenylbenziin tm

(mu tm)

Phn ng th nht khng thun nghch, nhng phn ng th hai l thun nghch v tonn phn ng ch th tht s.

Th kh ca phn ng th hai vo khong 0,76 V. Do ion hiro xut hin trong phngtrnh, s dao ng v axit c nh hng nhn gi tr th, c l do sn phm mu l cht

proton ha.

iphenylamin khng phi l cht tan nhiu trong nc, nn phi iu ch dung dch chth trong axit sunfuric. Cn ch l, khng c dng ch th cho cc dung dch c cha ionwonfram v s to thnh kt ta mu tm. Ion Hg(II) cng gy cn trv km hm phn ngch th.

Dn xut sunfonic ha iphenylamin c cu trc:

N

H

HO3S

khng gp phi nhng trngi nu trn. Dung dch ch th trong nc c iu ch tmui kali hoc natri ca axit ny c nhng tnh cht cbn ging nhiphenylamin. Tuyvy, s chuyn mu hi khc mt cht: chuyn t khng mu qua lc sang tm thm. Th dchchuyn khong +0,8 V v ph thuc vo nng axit. Ngy nay, dn xut sunfonic c sdng rng ri trong chun oxi ha kh.

iphenylbenziin, sn phm trung gian ca phn ng oxi ha iphenylamin, c tnh chtging nhiphenylamin trong cc phn ng oxi ha kh nhng tiu th t cht oxi ha hn.ng tic l, do tan thp trong nc v trong axit sunfuric nn iphenylbenziin khngc s dng rng ri. C th hy vng rng, dn xut sunfonic ha ca iphenylbenzidin s lcht ch th thch hp.


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21

Dung dch iot-h tinh bt

H tinh bt to thnh mt phc mu xanh tri vi triioua, nn c s dng rng rilm cht ch thc bit trong cc phn ng oxi ha kh sinh ra io do mt cht oxi ha hoc

sinh ra ioua do mt cht kh. V vy, mt dung dch h tinh bt cha mt lng nh iontriioua hoc ioua cng c th lm chc nng ca mt cht ch th oxi ha kh thc s. Khid cht oxi ha, t s nng iot trn ioua cao lm cho dung dch c mu xanh. Khi d chtkh, nng ion ioua chim u th nn mu xanh bin mt. Nh vy l, h ch th bin it khng mu n xanh trong php chun nhiu cht kh bng cc cht oxi ha. S chuynmu ny hon ton c lp vi thnh phn ha hc ca cc cht phn ng, ch ph thuc voth ca h ti im tng ng.

La chn cht chthoxi ha kh

T hnh 14.3, r rng l tt c ch thbng 14.2, tr ch thu tin v cui cng, u c

th s dng vi thuc th A. Ngc li, vi thuc th D ch c th s dng indiotetrasunfonat. Th chuyn i (dch chuyn) vi thuc th E qu nh nn pht hin cch c mt ch th.

14.7.2Ch thc bit

Mt ch thc bit c bit n nhiu nht l h tinh bt. Ch th ny to phc xanhthm vi triioua. Phc ny cho tn hiu vim cui ca php chun nhphn ng lmxut hin hoc bin mt iot.

Mt ch thc bit khc l kali tioxianat. C th s dng tioxianat, v d nh, chunst (III) bng titan (III) sunfat. im cui c pht hin theo s bin mt phc mu st(III)-tioxianat do nng st (III) b gim ti im tng ng.

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Chuong 14 Hóa học phân tích