Chương 8 - PHƯƠNG PHÁP HẤP THU PHÂN TỬ UV – VIS

8/13/2019 Chng 8 - PHNG PHP HP THU PHN T UV VIS

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CHNG 8

PHNG PHP HP THU PHN TUVVIS

8.1Tng quan

u th k 19, hu ht phn tch ho hc nh lng u s dng phng php trng lng

(gravimetry method) hoc phng php chun (titrimetry method). Vi nhng phngphp ny u t c ng (accuracy) cao, nhng kh c th xc nh c nhng thnhphn hp cht c nng thp trong nc. Trong sut thi gian ny nhiu nghin cu cbt u m rng kh nng phn tch nh lng c bit cc cc yu t vt trong mitrng. Mt trong s nhng pht minh l phng php so mu quang ph.

Phngphp so mu u tin l phng php Nessler phn tch hm lng ammonia trongnc vo nm 1856. Nessler khm ph ra rng khi thm HgI2v KI vo trong mi trng ccha NH4

+ s to thnh dung dch mu tmu vng n mu nu tutheo nng ca

NH4+

. Mu ca mu sc so snh vi mu ca mu chun xc inhnng tng ngtrong mu. Cho n ngy nay phng php ny c nghin cu bsung phn tchnc mt v nc thi trong Standard Methods. Cui thk19, mt sphng php mi btu c khm ph nh phng php hp thu, pht x, tn x, tia cc tm, in thng ngoi

pht x. Thk20 l giai on pht trin ca tia X, microwave, v sng radio, cc ht nnglng bao gm ht electron v ion.

8.2Lch s nghin cuquang ph hc

Quang ph hc l mt mn hc chnh yu trong thin vn hc, n c ng dng thnhcng nghin cu v kh quyn trong hnh tinh chng ta.Cch y 200 nm, Joseph vonFraunhofer (1787-1826) ln u tin sn xut loi my o quang ph m tnh nng khng cg snh kp lc by gi. ng y khm ph ra rt nhiu cc ng ti trong quang ph canh sng mt tri. ng y c th xc nh chnh xc di bc sng ca nhiu Fraunhoferlines (vch) v thut ng ny ngy nay vn c dng. Tuy nhin, trong thi gian ny ng ykhng hiu c nhng c s vt l v ngha v nhng vn m ng y khm ph ra.

Hnh 8.1. Thit b Spektralapparat thit k bi Kirchhoff v Bunsen (1833)


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Thnh tu quan trng k tip v Fraunhofer lines l qu trnh tm ra nguyn l vt l ca shp thu v pht x vo nm 1859 vi s cng tc ca nhiu nh vtl ni ting nh Gustav R.Kirchhoff v Robert W. Bunsen ti Heidelberg. Thit b m h s dng l Spektralapparat,h ghi nhn c qu trnh pht x rt c bit ca nhiu nguyn t khc nhau. Vi phng

php ny h tip tc khm ph ra 2 nguyn tmi l Csium v Rubidium, h chit c

mt lng rt nh (7g) t 44.000 lt nc khong gn ni Bad Nauheim, c. S khm phny l nn tng cho s khm ph tip theo v s hp thu v pht x ca hp thu phn t.

Nm 1879 Marie Alfred Cornu thy rng,nhng tia c bc sng ngn ca bc x mt tritrn b mt tri t b hp th bi kh quyn. Mt nm sau , Walther Noel Hartley m t rtt m v s hp th UV ca O3vi di bc sng 200 v 300 nm v n tr nn r rng hnkhi h pht hin ra rng O3 cha y trong bu kh quyn. Nm1880, Chappuis khm ph ras hp thu trong vng kh kin (400840nm). Nm 1925 Dobson pht trin mt my quang

ph mi rt n nh s dng lng knh bng thch anh.

8.3

i cng v quang phTrong quang ph hc, nh sng nhn thy (nh sng kh kin), tia hng ngoi, tia t ngoi, tiaRnghen, sng radio... u c gi chung mt thut ng l bc x.

Theo thuyt sng, cc dng bc x ny l dao ng sng ca cng in trng v cng t trng, nn bc x cn c gi l bc x in t.

Sau thuyt sng, thuyt ht cho thy bc x gm cc ht nng lng gi l photon chuynng vi tc nh sng (c = 3.108m/s). Cc dng bc x khc nhau th khc nhau v nng

lng h ca cc photon. y, nng lng ca bc x c lng t ha, ngha l nng

lng ca bc x khng phi lin tc m cc lng t nng lng t l vi tn s ca daong in t theo h thc Planck.

h

h = 6,625.1034J.s: hng s Planck.

Louis de Broglie a ra thuyt thng nht c khi nim sng v khi nim ht ca sng nhsng. nh sng va c tnh cht sng va c tnh cht ht. Tng qut hn l bc x c bncht sng ht. Ni dung nh sau:

Ht c khi lng mchuyn ng vi vn tc v c bc sng i i vi n l cho bi hthc:

p

h

mv

h

Trong : p = mv l ng lng ca ht

l bc sng (de Broglie)

h = 6,625.10-34J.s l hng s Planck.


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8.3.1 Cc i lng o bc x in t

Bc sng :L qung ng m bc x i c sau mi dao ng y .

n v: m, cm, m , nm,o

A . (1cm = 108o

A = 107m =104m)

Tn s : L s dao ng trong mt n v thi gian (giy)

Trong 1 giy bc x i c c cm v bc sng cm, vy:

c

Lu : Bc x truyn trong chn khng vi vn tc c = 2,9979.108m/s (thng ly trn 3.108m/s)

n v: CPS ( VNG DY), Hz, KHz, MHz. (1CPS=1Hz; 1MHz=103KHz=106Hz)

Nng lng bc x:Cc dao ng t (phn t chng hn) ch c th pht ra hoc hp thnng lng tng n v gin on, tng lng nh nguyn vn gi l lng t nng lng:

hchc

h

n v: Jun (J), Calo (Cal), electron von (eV).

8.3.2 Cc dng bc x

Bc x in t bao gm 1 dy cc sng in t c bc sng bin i trong khong rt rng:

t c mt sng raio n co

A (1010m) tia Rnghen hoc nh hn na. Ton b dysng c chia thnh cc vng ph khc nhau.

Bcsng(nm)

CamVngLcTm Chm Lam

(Kh kin)

Vi sng Sng rad ioTia hng ngoiTia XTia UV

Vng VIS

Hnh 8.2. Cc ph ca sng in t


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Mt ngi chcm nhn c mt vng phin trt nhgi l vng nhn thy (khkin)bao gm cc bc xc bc sng t396760 nm. Hai vng tip gip vi vng nhn thy lvng hng ngoi v vng tngoi UV.

8.3.3 S tng tc gia vt cht v bc x in t

iu kin bnh thng, in t ca phn t nm trng thi lin kt, nn phn t c mcnng lng thp, gi l trng thi c bnEo(Hnh 8.3).

Khi chiu mt bc x in t vo mt mi trng vt cht, s xy ra hin tng cc phn tvt cht hp th (absorption) hoc pht x (emission) nng lng, hay c gi l trngthi kch thch (Hnh 8.3). Nng lng m phn t pht ra hay hp th vo l:

E = E2E1= h

Trong , E1v E2l mc nng lng ca phn t trng thi u v trng thi cui

(hay cn gi l trng thi kch thch) l tn s ca bc x in t b hp th hay pht x ra.

Nu E > 0 th xy ra s hp th bc x in t.

Nu E < 0 th xy ra s pht x nng lng.

Hnh 8.3. S hp th(A) hoc pht x (B)nng lng ca mt photon

Theo thuyt lng t, cc phn t v cc bc x in t trao i nng lng vi nhau khngphi bt k v lin tc m c tnh cht gin on. Phn t ch hp th hoc pht x 0, 1, 2,3,n ln lng t h m thi. Khi phn t hp th hoc pht xs lm thay i cng

ca bc x nhng khng lm thay i nng lng ca n, bi v cng bc x in t xcnh bng mt cc ht phton c trong chm tia, cn nng lng bc x in t li ph

thuc tn s ca bc x.

V th khi chiu mt chm bc x in t vi mt tn s duy nht i qua mi trng vt chtth sau khi i qua nng lng ca bc x khng h thay i m ch c cng bc x thayi.Cc phn t khi hp th nng lng ca bc x s dn n thay i cc qu trnh trong

phn t (quay, dao ng, kch thch electron) hoc trong nguyn t (cng hng spinelectron, cng hng t ht nhn)

Mi mt qu trnh nh vy i hi mt nng lng c trng cho n, ngha l i hi bc xin t c tn s hay chiu disng nht nh kch thch. Do s hp th chn lc ny m

khi chiu chm bc x in t vi mt di tn s khc nhau i qua mi trng vt cht th sau

A B


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khi i qua chm bc x ny s b mt i mt s bc x c tn s xc nh, ngha l cc tia ny b phn t hp th.

Bng8.1.ng dng quang ph in hnh do s thay i nng lng

Kiu chuyn i nng lng Vng bc sng Thit b quang ph

Hp th (absorption)

Tia My quang ph Mossbauer

Tia X My hp th quang ph tia X

UV/VisMy so mu quangph UV/Vis

My hp thu nguyn t AAS

Hng ngoiMy so quang ph hng ngoi

My so mu hiu ng Raman

Microwave My quang ph vi sngSng radio My quang ph t ht nhn

Pht x (emission)

UV/Vis My pht x nguyn t

Tia X My pht x hunh quang tia X

UV/Vis

My quang ph hunh quang

My pht ln quang

My quang ph pht x hunh quang

nguyn t

Bng8.2. Quang ph in hnh khng dosthay i nng lng

Vng bc sng Kiu tng tc Thit b

Tia X Nhiu x My nhiu x tia X

UV/Vis Khc x My khc x

Tn x My o c

8.3.4 S hp th bc x v mu sc ca cc cht

nh sng nhn thy bao gm tt c di bc x c bc sng t 396 760 nm c mu trng(nh sng tng hp). Khi cho nh sng trng (nh sng mt tri) chiu qua mt lng knh, ns b phn tch thnh mt s tia mu (, da cam, vng, lc, lam, chm, tm). Mi tia mu ng vi mt khong bc sng hp hn (xem Bng 8.3). Cm gic cc mu sc l mt chuicc qu trnh sinh l v tm l phc tp khi bc x trong vng kh kin chiu vo vng mc

ca mt. Mt tia mu vi mt khong bc sng xc nh. Chng hn bc x vi bc sng


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400430 nm gy cho ta cm gic mu tm, tia sng vi bc sng 560 nm cho ta cm gicmu lc vng.

nh sng chiu vo mt cht no n i qua hon ton th i vi mt ta cht khngmu.

Th d, thy tinh thng hp th cc bc x vi bc sng nh hn 360 nm nn n trong sutvi cc bc x kh kin. Thy tinh thch anh hp th bc x vi bc sng nh hn 160 nm,n trong sut i vi bc x kh kin v c bc x t ngoi gn.

Mt cht hp th hon ton tt c cc tia nh sng th ta thy cht c mu en. Nu s hpth ch xy ra mt khong no ca vng kh kin th cc bc x khong cn li khi nmt ta s gy cho ta cm gic v mt mu no . Chng hn mt cht hp th tia mu

(= 610730 nm) th nh sng cn li gy cho ta cm gic mu lc (ta thy cht c mu

lc). Ngc li, nu cht hp th tia mu lc th i vi mt ta n s c mu . Ngi tagi mu v mu lc l hai mu ph nhau. Trn hai mu ph nhau li ta s c mu trng.

Ni cch khc, hai tia ph nhau khi trn vo nhau s to ra nh sng trng. Quan h gia muca tia b hp th v mu ca cht hp th (cc mu phnhau) c ghi bng sau:

Bng 8.3.Quan h gia mu ca tia b hp th v mu cht hp th

Tia bhp th Mu ca cht hp th

(mu ca tia cn li)(nm) Mu

400430

430490

490510510530

530560

560590

590610

610750

Tm

Xanh

Lc xanhLc

Lc vng

Vng

Da cam

Vng lc

Vng da cam

ta

Tm

Xanh

Xanh lc

Lc

Lu : Gia cc tia mu cnh nhau khng c mt ranh gii tht r rt.

Vic phn chia nh sng trng thnh 7, 8 hay 9 tia mu cn ty thuc vo lng knh v stinh t ca mt ngi quan st.

Mt cht c mu, th d nh mu chng hn l do n hp th chn lc trong vng khkin theo mt trong cc kiu sau:

- Cht hp thtia phca tia (tc l hp thtia mu lc)- Cht hp thcc tia trtia mu .- Cht hp thhai vng khc nhau ca nh sng trng sao cho cc tia cn li cho

mt ta cm gic mu .


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mt hp cht c mu, khng nht thit max ca n phi nm vng kh kin m ch cn

cng hp th vng kh kin ln. Ni mt cch khc tuy gi tr cc i ca vn hpth nm ngoi vng kh kin nhng do vn hp th tri rng sang vng kh kin nn hp cht

vn c mu. Tt nhin c c s hp th thy c vng kh kin th max ca cht

cng phi gn vi ranh gii ca vng kh kin.

Tng ng vi mt bc chuyn in t, ta thu c phhp thu c dng:

Hai i lng c trng ca phhp thu l vtr v cng

- V tr cc i hp thu, gi tr max ty thuc vo E m hp cht ny hp thu ccvng ph khc nhau. Bn chiu rng ca vn ph in t dao ng kh rng khong5060 nm.

- Cng th hin qua din tch hoc chiu cao ca nh biu (peak). Cng vn ph ph thuc vo xc xut chuyn mc nng lng ca in t. Xc sut ln chocng vn ph ln.

Mt hp cht mu c ph hp thu tt khi nh biu (peak) cao v bn chiu rng vn phhp.

Peak

rngBn vnph

max


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Hnh 8.4.nh(peak) v bn chiu rng vn ph

Khi bn chiu rng vn phhp, khi thay i nhth hp thu A thay i ln. iu nyrt c ngha trong phn tch nh lng. Gishp cht X c Amax500 nm. Khi chng tao bc sng 510 nm... th hp thu o c skhc rt xa i vi bc sng 500 nm.

T ta thy rng mi hp cht mu c mt gi tr max nht nh v n phn nh nhy

ca phng php.Mt khc, mt hp cht i hi nh biu cao ngha l khi ta o bc

sng max th ta c hp thquang cc i, khong lm vic rng.

8.3.5 nh lut Lambert Beer

Khi chiu mt chm tia sng n sc i qua mt mi trng vt cht th cng ca tia sngban u (Io) s b gim i ch cn l I

Ts TI

I

o

00100 c gi l truyn qua.

Ts AI

IIo

0

0100 c gi l hp th.

Nguyn tc ca phng php biu din theo s :

Mu

Cuvette

Io IA I

IR

Io = IA + IR + I

Hnh 8.5.S m t s hp th nh sng ca mt dung dch

Trong :

Io: Cng ban u ca ngun sngIA: Cng nh sng b hp thu bi dung dch

I: Cng nh sng sau khi qua dung dch.

IR: Cng nh sng phn x bi thnh cuvette v dung dch, gi tr ny c loib bng cch lp li 2 ln o.

Gia IA, I, dy truyn nh sng (l) v nng (C) lin h qua quy lut Lambert Beer lnh lut hp nht ca Bouguer:


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Lambert (1766) lKI

Io1lg

Beer (1852): CKI

Io1lg

truyn quang (T) hay hp th (A) ph thuc vo bn cht ca vt cht, dy truynnh sng lv nng C ca dung dch. C th vit:

nh lut LambertBeer : lCI

IA )lg(

0

Trong : l h s hp thu phn t, C nng dung dch (mol/L), l dy truyn nh sng

(cm), A l hp th quang. (Lu phng trnh trn ch ng i vi tia sng n sc).

Trong phn tch nh lng bng phng php trc quang ngi ta chn mt bc sng

nht nh, chiu dy cuvet lnht nh v lp phng trnh ph thuc ca hp th quang A

vo nng C.Kho st khong tun theo nh lut Lambert Beer:

Khi biu din nh lut Lambert Beer trn th ty theo cch thc hin php o, ta thnggp ng biu din s ph thuc hp thu A vo cng C ca dung dch c dng: y =ax + b

H s gc a cho bit nhy ca phng php, trong phng php trc quangngi ta ch odung dch trong khong tun theo nh lut Lambert Beer tc l khong nng m gi tr khng thay i. H s gc a cng ln v khong tun theo nh lut Beer cng rng

l iu kin thun li cho php xc nh.S lch khi nh lut Beer:

S lch khi nh lut Beer c biu din bng s sau:

Hnh 8.6.Gii hn ca nh lut Beer v s hp th quang

Khong tuyn tnh LOL (Limit of Linear Response) l khong nng tun theo nh lut

Beer )( ClA ngha l khi nng tng th hp th quang A tng. Ngoi gii hn

LOL l s lch khi nh lut Beer, ngha l khi nng tng th hp th quang A hu nh


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khng tng na. Nguyn nhn ca qu trnh ny l do nng dung dch qu ln. Ngoi ra,khong tuyn tnh LOL cn b nh hng ca mc n sc ca nh sng s dng, pH cadung dch, lc ion, s pha long...

ngha ca cc i lng:

-H s hp thu mol : ph thuc bn cht mi cht, bc sng , nhit , chit sut(theo nng ). Gi tr tnh l thuyt ca mt bc chuyn c php cho 1 electron l

= 105mol-1.cm-1.

lC

A (l. mol-1 cm-1)

cao cho ta bit c nhy ca phn ng, l thc o nhy ca phng php.

Trong phn tch trc quang, = 103105mol-1. cm-1l nhy dng cho phng

php trc quang, phthuc vo chit sut m chit sut li phthuc vo nng .

Khi chit sut tng ln th gim v khng thay i th phi thc hin C 10-2mol/L.

- hp th quang A: L i lng khng c n v, c tnh cht quan trng l tnhcng hp th quang.

Gis2 cht A v B c nng CAv CB, hp thu ti bc sng l:

A = AA+ AB= l (ACA+ BCB)

Nu mt cht tan X no c hp thquang l AX, dung mi c hp thquang l Adm,

ta c:A = Ax + Adm

o c chnh xc Axth Adm= 0, c ngha l phi chn maxca dung mi khc xa vi

maxcht tan. Nhng cht c chn lm dung mi thng c hp thu min ranh gii t

ngoi chn khng.

Bng8.4. Cc dung mi thng s dng trong vng UVVIS

Dung mi Bc sng (nm) Dung mi Bc sng (nm)

Nc ct 190 Benzen 280HCl 190 Cloroform 245

Etanol, metanol 210 Tetra Clorocarbon 265

n- Butanol 210 Dietyl Eter 218

n- Hexan 210 Aceton 330

Cyclohexan 210 1,4 Dioxan 215

Trong hn hp c nhiu cu tkhng lm thay i tng tc, khng phn ng ha hc, khng

dch chuyn cn bng, th c thxc nh hn hp cc cu ttheo hthc sau:


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nnii lClClClCA

.............2211

- truyn quang T:

oI

IT m )lg( 0

I

IA do TA lg

V T tnh theo % nn: TA lg2

Nu T = 100% th A = 0 (ngha l khng hp thnh sng (I = Io)

Nu T = 1% th A = 2

Nu T = 0 % th A (hp thu hon ton nh sng)

0.0

0.5

1.0

1.5

2.0

0 10 20 30 40 50 60 70 80 90 100

hpthA

truyn quang T (%) Hnh 8.7.Mi quan hgia truyn quang (T) v hp th(A)

V d8.1:

Mt mu c truyn quang l 50%, tnh hp thu A ca mu?

Gii quyt vn :

Ta c : A = -lg T = -lg(0,5) = 0,301

V d8.2:

Mt dung dch c nng 5 10-4M c phn tch v o bng cuvette 1 cm bc sng490 nm c hp thu l 0,338. Hy tnh hp thu phn tca cht bc sng ny ?

Gii quyt vn :

lC

A =

Mcm 410.51

338,0

= 676 cm-1M-1

8.3.6 Nguyn l cu to ca my quang ph

Ngun sng

Ngun sng cho my quang ph l chm bc x pht ra r n. My quang ph dng nhydro hay n Deuterium cho ph pht x lin tc trong vng UV t 160 380nm (nhng

thng s dng 200- 340 nm) v n tungsten halogen o vng 3801000 nm. lm vic


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cho c hai vng th phi c 2 loi n trn. Mt yu cu i vi ngun sng l phi nnh, tui th cao v pht bc x lin tc trong vng ph cn o.

n Deuterium: cu to sm mt si t ph xit v mt cc kim loi t trong mt bngthu tinh cha kh Deuteri hoc hydro c ca s bng thch anh bc x t ngoi i ra v nkhng truyn qua c thy tinh. Khi si t c t nng, electron sinh ra kch thch cc

phn t kh Deuteri (hoc hidro) bin thnh nguyn t v pht ra phton theo phn ng:

D2+ Ee*

2D D + D + h

Ee= *2D

E = ED + ED + h

y l nng lng electron kch thch, bc x pht ra l mt ph c bc sng t 160 nmn vng kh kin.

Bng 8.5.Ngun pht nng lng trong cc thit b quang ph

Ngun Vng bc sng S dng chon H2v D2 160380 nm Hp th phn t t ngoi

nTungsten 3202400 nm Hp thu phn t kh kin

n h quang Xe 2001000 nm Pht x hunh quang phn t

n Nernst 0,420 m Hp th phn t hng ngoi

n cc m hallow UV/Vis Hp th nguyn t

n hi nc Hg UV/Vis Pht x hunh quang phn t

Laser UV/Vis Hp th phn t, nguyn t, hunhquang, tn x

Bn sc

Bn sc c chc nng tch bc xa sc thnh bc xn sc, bao gm knh lc, lngknh hay cch t.

Cch tl mt bng nhm hay cc kim loi Cu, Ag, Au... c vch thnh nhng rnh hnhtam gic song song. Khi chiu nh sng qua cch t, phn cn li c tc dng to nn vn

nhiu xc bc sng khc nhau, khi quay cch tsto ra phnhiu xging nh trnghp nh sng qua lng knh. u im l cho phn gii tt, tn sc tuyn tnh, rng cadi n nh, chn bc sng n gin, gn nh, dchto nn hin nay sdng cch ttonh sng n sc c a chung. Cch tdng cho UV/Vis c 1200 vch/ mm (thng daong t3003600 vch/mm, svch cng nhiu th nng sut phn gii cng cao.


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Hnh 8.8S m phngcu to ca my quang ph

Lng knh ca my quang ph dng lng knh littrow (lng knh 30o) bng thch anh, c cim nh sng i qua lng knh hai ln do phn x mt sau. Khi phn tch, hn ch mc nhiu ca vn ph, tt nht nn chn bc nhy ca bc sng (bandwidth) mc thchhp tu vo hp cht(Hnh 8.9)

Bandwidth 0,25 nm Bandwidth 1,0 nm

Bandwidth 2,0 nm Bandwidth 4,0 nm

Hnh 8.9.Mc nhiu cc di Bandwidth khc nhau

Cuvet ng mu

Cuvet phi lm bng cht liu cho bc xvng cn o i qua. Cuvet thy tinh khng thchhp cho vng UV. Cuvet thch anh cho bc xi qua t1901000 nm. Cuvet nha chdngtrong vng Vis v chsdng c 1 vi ln.


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Detector

Detector l bphn o tn hiu nh sng trc v sau khi i qua dung dnh (ng trong cuvet).Cc tn hiu sau khi i ra Detector sc sc khuch i bng processor tn hiu (signalprocessor), lu gi v x l trn my tnh. C 2 loi detector l: b cm bin photon(photon transducer) v bcmbin nhit (thermal transducer) Bng 8.6.

Bng 8.6.c tnh ca mt sbchuyn i tn hiu (transducer)

Detector Lp Vng bc sng

n quang in (phototube) Photon 2001000 nm

in knhn (Photomultiplier) Photon 1101000 nm

Quang diot Silic (Si photodiode) Photon 2501100 nm

Quang dn (photoconductor) Photon 7506000 nm

Pin quang voltaic (photovoltaic cell) Photon 4005000 nmCp nhit in (thermocouple) Nhit 0,840 m

Nhit in tr(thermistor) Nhit 0,840 m

Hoin (pyroelectri) Nhit 0,31000 m

8.4S dng phng php trc quang trong nh lng ha hc

Yu cu v cc hp cht cn xc nh l phi bn, t phn ly, n nh, khng thay i thnhphn trong khong thi gian nht nh thc hin php o (1020 pht).

H s ln c gi tr t 1035.104 L. mol-1 cm-1, c th thc hin phn ng to mu vi ccthuc th v c v hu c.

Nng cc cht xc nh theo nh lut Lambert Beer. Khong xc nh nng theophng php l 10-210-6mole. Gii hn pht hin ca phng php 10-7 mole.

Cc hp cht l phc cn o phi c maxkhc xa vi maxca thuc th trong cng iu kin

tc l > 2 ln na bn chiu rng ca vn ph (khong 80 100 nm). Th d, khi phn

tch Fe

2+

bng phng php O-phenanthroline. Sau khi thm thuc th ta c phc muvng cam (max = 510 nm), trong khi thuc th 1,10- Orthophenanthroline c max= 250

nm.

Bng 8.7 Mt s ng dng phng php trc quang trong phn mt s yu t thngthngtrong ao nui thu sn

Ch tiu Phng php Phc mu max(nm)

Al Phn ng vi Eriochrome cyanide R pH 6,0 Hng 535

Cu Phn ng vi Neocuprine, ly trch bng CHCl3 Vng 457


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Fe2+ Phn ng vi O-phenanthroline, pH = 5 Cam 510

NH3 Phenate, to phc Indophenol Xanh 630

NO2- To mui diazonium, pH = 23 Hng ta 543

NO3- To mui salicylate, pH > 9 Vng 410

PO43- To phc moybden Xanh 690

SO42- To phc vi sodium rhodizonate 520

SiO2 To phc heteropoly Xanh 815

8.4.1 Phng php so snh

So snh cng mu ca dung dch cn xc nh vi cng mu ca dung dch chun bit nng .

iu kin: c hai dung dch trn phi c nng nm trong khong tun theo nh lut Beer.

Cx----------------------- Ax

Ctc---------------------- Atc

Ta cn xc nh Cx:tc

tcxx

A

CAC

Khi s dng 2 dung dch chun:

)( 112

121 AA

AA

CCCC xx

Vi A1, A2, C1, C2l hp thu v nng ca dung dch chun tng ng sao cho A1 0,999.

Hstng quan r bin i trong khong -1R 1 (R2= 01)

- Khi R 1 c s tng quan cht ch gia x v y theo t l thun.- Khi R -1 c s tng quan cht ch gia x v y theo t l nghch.- Khi R 0 hai i lng ny khng cn tng quan.

8.5 chnh xc trong phng php trc quang:

Trong phn tch trc quang cng nh bt k phng php no khc c th chia sai s thnh 2nhm:

- Sai s do tin hnh phn ng ha hc (ha cht, thao tc, dng c...)- Sai s ca tn hiu o hp thu ca dung dch (do h thng o).

chnh xc trong phng php ny ph thuc vo hng lot nguyn nhn khc nhau rtphc tp bao gm sai s ngu nhin v sai s h thng, trong sai s quan trng nht l sais ca tn hiu trong qu trnh o hp thu quang hc.

8.6Mt s v d p dng phng php nh lng trc quang

V d8.3:

hp th quang A ca dung dch anilin 2 10-4 M trong nc o bc sng = 280 nml 0,252. Chiu di nh sng i qua cuvet l 1cm. Tnh truyn quang ca anilin 1,03.10-3M

khi o cng di bc sng nhng dng cuvet 0,5cm.Gii quyt vn :

p dng cng thc lCI

IA )lg( 0 vi dung dch 1 ta c:

= 0.252/(2.10-4 1) = 1,26.103 l. mol-1 cm-1.

p dng cng thc lCI

IA )lg( 0 vi dung dch 2 ta c:

A = 1,26.103 0,5 1,03.10-3= 0.649

M: A = -lg T suy ra: lg T = -A = -0,649, do T = 0,224 = 22,4%

Vy truyn quang T = 22,4%

V d8.4:

hp th quang A o c t cc mu chun v mu nc thu t ao nui c cha ion PO43-

nh sau:

Nng mu chun (mg/L) 0,00 0,05 0,10 0,15 0,20 0,25


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hp thquang A 0,010 0,480 0,930 1,370 1,830 2,281

hp th quang A ca mu nc ao ca 3 ln lp li l: 1,256; 1,245; 1,264. Tnh nng PO4

3-trong mu nc ao.

Gii quyt vn :

T cc nng mu chun v hp th quang A. T kt qu thit lp phng trnh hi qui

ta c: 0184,00543,9 XY (R2= 0,9999).

T kt qu ca 3 ln phn tch lp li ta c yA = 1,255

T ta c 137,00543,9

0184,0255,1

0543,9

0184,0

y

x mg/L

Vy nng PO43-trong mu nc ao l 0,137 mg/L.

V d8.5: xc nh hng s phn ly ca Methyl da cam (k hiu HIn), ngi ta o hp th quangA ca 3 dung dch cng nng Methyl da cam cc pH khc nhau:

- Dung dch 1 trong HCl 0,1M; A1= 0,475.- Dung dch 2 trong NaOH 0,1 M; A2= 0,130.- Dung dch 3 c pH = 4,34; A3= 0,175

Cho bit o bc sng = 510 nm v chiu di nh sng i qua cuvet l 1cm. Tnh hng s

phn ly K ca Metyl da cam?Gii quyt vn :

hp th quang ca dung dch 3:

lHInlInA HInIn

3 (8.1)

Vi [In-] = x; [HIn] = y ta c: x + y = CHIn= C (8.2)

lC

AIn

2 v ton bcht chthdng In- (8.3)

lC

AHin

1 v ton bcht chthdng HIn (8.4)

Thay (8.3)v (8.4)vo (8.1)ta c:

C

yA

C

xAA 123 (8.5)

Qui c: )1(; C

y

C

x0,175 = 0,130+ 0,475 (1-) = 0,869

Hng s phn ly ca HIn:


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HIn H+ + In- ; Ka

1lglg

]][[pH

HIn

InpHpK

HIn

InHK

pK= 4,34 -689,01

869,0lg

= 7,340,82 = 3,52 K= 3,02.10-4.

Vy hng s phn ly ca methyl da cam lK= 3,02.10-4.

8.7Nguyn l phn tch cc yu tmi trng nc bng phng php quang ph

8.7.1 Xc nh tng m amoni TAN (Total Ammonia Nitrogen)

Phng php Nessler

Nguyn l

Trong mi trng baz mnh NH4+ sbin thnh NH3. NH3mi hnh thnh v NH3sn c

trong mu nc stc dng vi phc cht Indo-mercurate kalium (K2HgI4), hnh thnh phccht c mu vng nu, cng mu m hay nht ty thuc vo hm lng NH3c trongmu nc.Phng trnh phn ng:

2 K2HgI4 + NH3 + 3KOH Hg(HgIONH2) + 7KI + 2 H2O

(mu vng)

K2HgI4 + NH3 + KOH Hg(HgI3NH2) + 5KI + H2O

(mu nu)

Phng php Indophenol blue (Phenate)

Nguyn l

Trong mi trng kim mnh, NH4+ s chuyn thnh NH3. Ammonia phn ng vi

hypochlorite v phenol vi cht xc tc l sodium nitroprusside sto thnh phc indophenol

c mu xanh, phc ny hp thnh sng ti a (max) bc sng 640nm.

NH3

+ ClO-

NH2Cl + OH

-

NH2Cl + 2 OH + 2ClO

-

O N OH + 3HCl + 2OH-

O N OH O N O- + H+

Indophenol

NH3

+ ClO-

NH2Cl + OH

-

NH2Cl + 2 OHOH + 2ClO

-

O N OHO N OH + 3HCl + 2OH-

O N OHO N OH O N O- + H+

Indophenol

Cc cht gy nhiu v gii hn phn tch

Ca v Mg bkt ta trong mi trng pH cao (do NaOH cho vo mu nc trong qu trnhphn tch) lm dung dch bc lm nh hng n kt quo hp thquang. Cho vomu nc trisodium citrate trnh hin tng kt ta ca Ca v Mg. Nu mu nc cha


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H2S vi hm lng cao, cn phi loi bH2S bng cch gim pH xung 3 bng HCl v sckh n kh khng cn mi ca H2S.

Phng php Salicylate

Nguyn l

NH3 + OCl- NH2Cl + OH-

OH

COO-

NH2

OH

COO-

-O

COO-

N O

COO-

NH2Cl + + Cl-

Fe (CN)5NO2- + OH

COO-

NH2

Salicylate5-amonisalicylate

Nitroprusside5-aminosalicylate

Indosalicylate (mu xanh)

8.7.2 Xc nh NO2-

Phng php Ferrous SulfateNguyn l

Trong mi trng acid vi tc dng ca Fe(SO4)2, NO2- chuyn thnh NO (Nitrous oxide).

Fe3+lin kt vi NO to thnh phc mu nu.

2Fe2+ + 4H+ + 2NO2- 2Fe3+ + 2NO + 2H2O

NO + FeSO4 FeSO4.NO (mu nu)

Phng php to mui diazoniumNguyn l

NO2-trong mi trng acid mnh shnh thnh HNO2, HNO2mi hnh thnh skt hp vi

acid sulfanilique cho ra mui Diazonium sulfanilique. Sau mui diazonium sulfanilique s

kt hp vi thuc th-napthylammine cho ra -napthylammine diazonium sulfanilique. -napthylammine diazonium sulfanilique l mt hp cht c mu hng, cng m hay nhtty thuc vo hm lng NO2

-c trong mu nc lc ban u. Nng c xc nh bimy so mu quang phbc sng 543 nm.



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NCl3lm sai lch mu ca phc mu. Cc ion sau y gy kt ta nh Sb3+, Au3+, Bi3+,

Fe3+, Pb2+, Hg2+, Ag+, PtCl62-, VO3

2-. Ion Cu lm gim xc tc v phn hy mui diazonium.

NH2

HO3S

HO3S N

+

N

NO2- + + 2H+

+ 2H2O

OH OH

SO3HHO

3S

N

OH OH

SO3HHO

3S

N SO3H

HO3S N

+

N +

+ H+

. napthylammine

. napthylammine diazonium sulfanilique (hng)

acid sulfanilique

Diazonium sulfanilique

8.7.3 Xc nh NO3-

Phng php khbng ct cadmium

Nguyn lNO3

-bkhthnh NO2-khi i qua ct cadmium (Cd), phng trnh phn ng xy ra nh sau:

NO3- + H2O + Cd NO2

- + Cd2+ + OH-

NO2-hnh thnh c xc nh bng phng php so mu diazonium c m tphn trn


Hm lng vt cht l lng qu cao lm gim hiu sut ca ct kh, lc mu qua giy lc

0,45 m trc khi cho qua ct kh. Cc ion kim loi nh Fe2+, Cu2+v cc kim loi khc

cng lm gim hiu sut ca ct kh khi hm lng t vi mg/L, dng EDTA loi trnhhng ny. Du, mtrong mu nc sbao quanh bmt cc ht Cd l gim stip xc camu nc vi ht Cd, do mu nc cn loi du mbng dung mi hu c trc khi choqua ct kh. D lng chlorine soxy ha Cd cng lm gim hiu sut ca ct kh, c thkim tra d lng chlorine bng chthDPD v khchlorine bng Na2S2O3.

NCl3lm sai lch mu ca phc mu. Cc ion sau y gy kt ta nh Sb3+, Au3+, Bi3+,

Fe3+, Pb2+, Hg2+, Ag+, PtCl62-, VO3

2-. Ion Cu lm gim xc tc v phn hy mui diazonium.

Phng php salicylate

Nguyn l


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Trong mi trng kim NO3-shnh thnh phc mu vng anh vi acid sulfanilic.

+ NO3- + 2H+

Nitro-salicylate

Acid sulfanilic

8.7.4 Xc nh hm lng ln ho tan PO43-

Phng php Ascorbic acid

Phng php so mu quang phascorbic acid da trn nguyn tc ammonium molydate vpotassium antimonyl phn ng vi PO4

3-to thnh phosphomolybdate cng vi stham giaca ascorbic acid khthnh hp cht phc ca molydate c mu xanh c hp thu nh sng

bc sng 880nm.

Phcphosphomolybdate

12MoO3 + H2PO4- (H2PMo12O40)-

Phng php Clo Thic (SnCl2)

Nguyn l

Mui orthophosphate trong mi trng acid, ion PO43-s phn ng vi thuc th Molybdate

ammonium cho mt phc cht ammonium phosphomolybdate, mu vng chanh.

PO43-+ 12(NH4)2MoO4+ 24H

+= (NH4)2PO4.12MoO3+ 21NH4++ 12H2O

(Ammonium phosphomolybdate)

Vi s hin din ca cc cht kh nh SnCl2, dng ammonium phosphomolybdate b khthnh dng molybden blue c mu xanh. Cng mu m hay nht ph thuc vo hmlng ion PO4

3-c trong mu nc lc ban u.

(NH4)3PO4.12MoO3 + Sn2+ + 16H+ (NH4)3PO4.(4MoO2.2MoO3)2 + Sn

4+ + 8H2O

Phc mu ny hp th nh sng ti a (max) bc sng 690 nm.


SiO2 v AsO43- gy nhiu dng khi mu nc b un nng. Cc cht AsO4

3-, F-,

thorium (Th), bismuth (Bi), sulfide, thiosulfate, thiocyanate gy nhiu m. Fe2+ gynhiu khi hm lng ln hn 100 mg/L. Cl-gy nhiu khi hm lng ln hn 75 mg/Lv c sdng HNO3trong qu trnh phn tch. Hm lng lng thp nht c thpht

hin bng phng php ny l 3 g/L.

8.7.5 Xc nh hm lng H2S trong nc


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Phng php Methylence blue

Nguyn l

Nguyn l ca phng phpny da trn phn ng ca hydrogen sulfide (H2S) vi FeCl3vN,N-dimethyl-p-phenylenediamine to nn methylene blue (mu xanh). Ammoniumphosphate c thm vo sau khi hin mu kh mu ca FeCl

3. Methylene blue hp th

nh sng ti a (max) bc sng 664 nm.

NH2

(CH2)2N

+ HCl + 6FeCl3 + H2S

S

N

N(CH3)2(CH2)2N

Cl -+

(Methylene xanh)

(Dimethyl-p-phenylenediamine)

2

Cc cht gy nhiu

Sulfite (SO32-) lm chm phn ng hin mu nu hm lng ln hn 10 mg/L ngay c khi

hm lng H2S cao. khc phc sgy nhiu ca SO32-, tng lng Fe3+tham gia phn ng

ln 2-6 ln.

8.7.6 Xc nh hm lng Fe2+

Phng phpThiocyanate

Nguyn l

Phng php ny da trn nguyn tc: trong mi trng acid, Fe2+boxy ha thnh Fe3+bngmt tc nhn oxy ha thch hp. Fe3+mi c hnh thnh v Fe3+c sn trong mu nc skt hp vi ion SCN-hnh thnh mt phc cht c mu mu, cng mu phthuc hmlng ion Fe3+c trong mu nc.

10 Fe2+ + 10H+ + K2S2O8 = 10 Fe3+ + K2S2O3 + 3H2O

Fe3+ + 3SCN- = Fe(SCN)3 Phng php chun

Nguyn l

Phng php ny ch sdng trong trng hp hm lng st tng trong mi trng nccao. Fe2+ trong mu nc s b oxi ha thnh Fe3+, Fe3+ s c pht hin bi acidsulfosalicylic (to thnh phc mu ). Ta dng dung dch ny chun EDTA (khng mu)cho n khi dung dch chuyn sang mu vng. Phng trnh phn ng ha hc:

Fe2+ + 3HNO3 Fe(NO3)3 + 3/2H2


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OH

CO2H

SO3H

Fe3+

SO3H

O

OHO

SO3H

O

OH

O

Fe

SO3H

O

OH

O

+

Phng php O-phenanthroline

Nguyn l

St bkhthnh dng Fe2+bng cch un si vi acid v hydroxylamine v c xl vi1,10 phenanthroline pH 3,23,2. Ba phn tphenanthroline to hp cht cng cua vi mimt nguyn tFe2+ thnh dng phc cht c mu -cam. Phc mu hp thnh sng ti a

(max) bc sng 510nm.

NN

+

N

N

Fe

NN

N

N

Fe2+O-phenanthroline

3

Mu gch

Cc cht gy nhiu v gii hn

Cc cht gy nhiu trong phn tch gm: cht oxy ha, cyanide, nitrite, polyphosphate, Cr vZn (ln hn 10 ln ca Fe), Co v Cu ( ln hn 5 mg/L), Ni (ln hn 2 mg/L. Bi, Cd, Hg, Mo,v Ag gy kt ta phenanthroline. un mu vi acid chuyn polyphosphate thnhorthophosphate v loi bcyanide, nitrite. Xl hydroxylamine loi bcc cht oxy ha.

Trong trng hp bnhiu do kim loi cao nn tng thm lng phenanthroline khi phn tch.


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Hm lng Fe nhhn 10g/L c thxc nh bng my quang phvi di truyn quang5 cm hoc ln hn.

Phng php TPTZ

Nguyn l

2,4,6-tripyridyl-s-triazin (TPTZ) phn ng vi Fe2+ to thnh phc mu xanh ta (blue-purple).

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

Fe

2 + Fe2+

8.7.7 Xc nh hm lng SiO2

Phng php Molybdosilicate

Trong mi trng pH t 3 4, SiO2 v cc dn sut ca n s tn ti di dng H2SiO3(Silicic acid), H2SiO3 skt hp vi (NH4)2MoO4

(Molybdate ammonium) hnh thnh phccht Molybdosilicate c mu vng, cng m nht phthuc vo hm lng H2SiO3 ctrong mu.

H2SiO3 + 12(NH4)2MoO4 + 24HCl

H8Si(Mo2O7)6 + 24NH4Cl + 9H2O

Phng php Heteropoly blue

Nguyn l

Trong mi trng pH thp (~1) ammonium molybdate phn ng vi silic ha tan (reactivesilica) trong nc to thnh molybdosilicic acid (silicomolybdic acid) mu vng.

SiO2+ H2O H2SiO3(Silicic acid)

H2SiO3+ 3H2O H8SiO6(silicic acid hydrate)

H8SiO6 + 12(NH4)2MoO4 + 18 H2SO4 H8[Si(Mo2O7)6] + 12 (NH4)2SO4 + 12 H2O

Molybdosilicic acid b kh bi ascorbic acid hoc aminonaphtholsulfonic acid thnh

heteropoly blue (mu xanh). Heteropody blue hp thnh sng ti a (max) bc sng 815

nm.


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Phosphate cng phn ng vi ammonium molybdate to thnh phosphomolybdic acid (muvng). V vy, phosphate c trong mu nc sgy nhiu khi phn tch silic. Dng oxalic acidhoc citric acid ph hy phosphomolybdic acid trc khi kh Molybdosilicic acid thnh

heteropoly acid c thkhc phc snhiu do phosphate. Gii hn phn tch l 50 g/L vi

phng php so mu quang ph.

Cc cht gy nhiu

Dng cthy tinh v ha cht cha silic c thgy nhiu, nn dng ha cht v dng cchm lng silic thp. Tanin, phosphate, st, mu, nc c c thgy nhiu, oxalic acid cthloi trnhiu do phosphate v tanin. Nu cn thit th c thhiu chnh hp thquangi vi mu nc c v c mu.

8.7.8 Xc nh hm lng Phenol

Phng php 4-Aminoantipyrine

Trong mi trng pH = 10, vi s hin din ca Fe(CN)3phenol phn ng vi 4-

aminoantipyrine hnh thnh phc mu vng v c ly trch trong trong dch CHCl3

(chloroform). So mu bc sng = 500nm.

NN O

NH2

CH3

CH3

OH

NN OCH

3

CH3

N O

4-Aminoantipyrine Phenol

8.8Mt sthit bchnh trong phn tch quang ph

Cuvette thch anh 1 cm Cuvette nha 1 cm Cuvet thutinh 1cm


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Chương 8 - PHƯƠNG PHÁP HẤP THU PHÂN TỬ UV – VIS

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