4.1. Phng php quang ph hp th t ngoi v kh kin

Phng php quang ph hp th t ngoi v kh kin cn c gi l phng php quang ph hp th in t, l mt trong cc phng php phn tch da trn s hp th bc x in t.

Xc nh hp th

hp th A ca mt dung dch l logarit thp phn ca nghch o truyn quang T khi cho nh sng n sc i qua. N c biu th bng phng trnh:

 

1 IO

A = Log10 = Log10

T I

 

Trong :

I: Cng nh sng n sc sau khi truyn qua dung dch.

Io: Cng nh sng n sc ti.

T: truyn quang.

Ngoi tr s c mt ca cc yu t l - ho hc khc, hp th A t l vi di quang trnh d ca nh sng truyn qua dung dch (b dy lp dung dch) v nng c ca dung dch cht kho st. S ph thuc ny c biu th bng phng trnh:

A = e c d

Trong e l hp th mol, d c biu th bng cm v c biu th bng mol/lt.

hp th ca dung dch cht tan nng 1% (kl/tt) hay 10 g/l trong mt cng o c chiu dy 1 cm v o mt bc sng xc nh l hp th ring ca cht tan v c k hiu l A (1 %, 1 cm). hp th ring ca cht tan c tnh bng cng thc:

 

10 e

A(1%, 1 cm) =

M

 

M l khi lng phn t ca cht th.

hp th ring ca mt cht trong mt dung mi xc nh v o mt bc sng xc nh l mt c tnh ca cht . Tr nhng ch dn khc trong chuyn lun ring, ngi ta o hp th di sng quy nh v s dng quang trnh di 1 cm 19 0C n 21 0C.

Ngoi tr cc ch dn trong chuyn lun ring, cc php o c tin hnh so snh vi dung mi hoc vi hn hp dung mi dng chun b mu th. hp th ca dung mi hoc hn hp dung mi o i chiu vi khng kh khng c vt qu 0,4 v tt nht l di 0,2. Khi v ph hp th, t hp th hoc hm s ca hp th trn trc tung v t di sng hoc hm s ca di sng trn trc honh.

Khi mt chuyn lun ring a ra mt tr s ring l cho v tr ca mt cc i, iu ny c hiu l tr s kho st c c th lch khng qu 2 nm.

Thit b

My quang ph thch hp dng cho vic o ph vng t ngoi v kh kin bao gm mt h thng quang hc c kh nng cung cp nh sng n sc trong di t 200 n 800 nm v mt thit b ph hp o hp th. Hai cng o dng cho dung dch th v dung dch i chiu cn phi c c tnh quang hc nh nhau. Khi o trn my t ghi hai chm tia, cng ng dung dch i chiu c t bn c chm tia i chiu i qua.

Kim tra di sng

Kim tra thang di sng bng cch s dng cc i hp th ca dung dch holmium perclorat. Vch ph ca n ngun hydro hoc n deuterium v cc vch ph ca n hi thy ngn c ch ra di y:

Gii hn sai lch cho php l 1 nm vng t ngoi v 3 nm vng kh kin.

241,15 nm (Ho) 404,66 nm (Hg)

253,70 nm (Hg) 435,83 nm (Hg)

287,15 nm (Ho) 486,00 nm (Db)

302,25 nm (Hg) 486,10 nm (Hb)

313,16 nm (Hg 536,30 nm (Ho)

334,15 nm (Hg) 546,07 nm (Hg)

361,50 nm (Ho) 576,96 nm (Hg)

365,48 nm (Hg) 579,07 nm (Hg)

Kim tra hp th

Kim tra hp th ca dung dch kali dicromat cc di sng ch ra bng 4.1.1. Trong bng mi di sng c mt gi tr chnh xc ca hp th ring A (1%, 1 cm) v cc gii hn cho php.

kim tra hp th, dng mt dung dch kali dicromat c chun b theo cch sau: Ho tan 57,0 n 63,0 mg kali dicromat c sy 130 o C n khi lng khng i trong mt lng va dung dch acid sulfuric 0,005 M to thnh 1000 ml dung dch.

Bng 4.1.1

di sng (nm)

A (1%, 1 cm)

Gii hn cho php

235

124,5

122,9 - 126,2

257

144,0

142,4 - 145,7

313

48,6

47,0 - 50,3

350

106,6

104,9 - 109,2

hp th ca dung dch kali dicromat chun cha ng 60,06 mg K2Cr207 trong 1000 ml acid sulfuric 0,005 M, o trn my vi cng o c quang trnh di 1 cm, c ch ra bng 4.1.2.

Bng 4.1.2

di sng (nm)

hp th

235

0,748

257

0,865

313

0,292

350

0,640

Dung sai hp th l 0,01

Gii hn nh sng lc

nh sng lc c th c pht hin di sng cho bng knh lc thch hp, hoc dung dch ha cht thch hp, th d hp th ca dung dch kali clorid 1,2% (TT) trong mt cng o c quang trnh di 1 cm phi ln hn 2,0 di sng 200 nm khi so snh vi nc ct.

phn gii (cho phn tch nh tnh)

Khi c quy nh trong chuyn lun, tin hnh kim tra phn gii ca my nh sau: Ghi ph ca dung dch toluen 0,02% (tt/tt) trong hexan (TT). Gi tr ti thiu ca t s gia hp th cc i 269 nm v hp th cc tiu 266 nm c quy nh trong chuyn lun ring.

rng khe ph (cho phn tch nh lng)

trnh sai s gy ra bi rng khe ph, khi s dng mt my c rng khe ph thay i di sng chn th rng khe ph phi nh so vi na rng ca bng hp th. Nhng rng ny phi ln thu c gi tr cao ca cng nh sng ti I0 v vic thu hp rng khe ph sau phi khng lm cho hp th c c tng ln.

Cng o ( Cc o)

Dung sai v di quang trnh ca cng o l 0,005 cm. Khi c np y cng mt dung mi, cc cng o c nh s dng cha dung dch mu th v cha dung dch mu trng phi c truyn quang nh nhau. Nu tiu chun ny khng c p ng th cn c s hiu chnh thch hp.

Cc cng o cn c lm sch v thao tc thn trng.

Phng php ph o hm bc hai

Phng php ph o hm l s chuyn dng ca ph hp th sang ph o hm bc mt, bc hai hoc bc cao hn. Ph o hm bc mt l th ca gradient ng cong hp th (tc ca s thay i hp th theo di sng, dA/dl) theo di sng. Ph o hm bc hai l th ca cong ca ph hp th (d2A/ dl2) theo di sng. Nu hp th tun theo nh lut Lambert Beer, o hm bc hai ti bt k di sng l no cng lin quan ti nng bi phng trnh sau:

 

d2A

 

=

d2A(1%, 1 cm)

 

c d

 

dl2

dl2

Trong :

A: hp th bc sng l

A(1%, 1cm): hp th ring bc sng l

c: Nng ca cht hp th biu th di dng % (kl/tt)

d: B dy ca lp cht hp th tnh bng cm.

Thit b

L mt my quang ph p ng cc yu cu v kim tra di sng, kim tra hp th ni trn v c ghp ni thm mt modul vi phn tr - t cho tn hiu tng t, hoc mt vi phn k k thut s, hoc mt thit b khc thch hp, to ra c ph o hm bc hai. My c s dng theo s hng dn ca hng sn xut.

Mt vi phng php to ph o hm bc hai a n s tri sng so vi v tr ca di sng ph bc khng, iu ny cn c tnh n khi cn thit. Tr khi c nhng ch dn khc trong chuyn lun ring, rng khe ph ca my phi c iu chnh nh ch dn trong mc rng khe ph trn.

Cc cng o phi p ng cc yu cu quy nh trong mc Cng o.

Nhit ca tt c cc dung dch dng trong php th khng c khc nhau qu 0,5oC.

phn gii

Khi c quy nh trong chuyn lun ring, ta ghi ph o hm bc hai ca dung dch toluen 0,020% trong methanol (TT) trong di di sng t 255 nm n 275 nm, dng methanol (TT) trong cng i chiu. Ph phi thy r mt cc tiu nh gia hai cc tiu ln trong khong 261 nm v 268 nm.

Tr khi c ch dn khc trong chuyn lun ring, t l A/B gia khong cch A tnh t cc i ti 263 nm n cc tiu ti 265 nm (cc tiu nh ni trn) vi khong cch B, tnh t cc i ti 263 nm ni trn n cc tiu ti 261 khng c nh hn 0,2 ( xem hnh 4.1.1).

 

Hnh4.1.1