10.3 nh lng nc bng thuc th Karl Fischer

Nguyn tc

Phng php nh lng nc ny da trn phn ng ton lng ca nc vi lu hunh dioxyd v iod trong dung mi khan cha mt cht base hu c thch hp.

Dung mi hu c thng dng l methanol khan nc, cng c khi c thay bng dung mi hu c khc thch hp ho tan ch phm. Cht base hu c l pyridin, nhng hin nay ngi ta dng nhng cht kim hu c khc thay th nh imidazol, 2- methylaminopyridin.

Dng c

Hin nay c nhiu loi dng c, nhng nguyn tc u phi cu to sao cho thao tc thun tin v trnh m. Dng c gm c mt cc chun dung tch khong 60 ml, c np gn in cc kp platin, mt ng dn kh nitrogen, c l cm vi buret v l cm ng thng hi cha cht ht m. Ch phm cho vo bnh chun qua l trn np hoc ming bn cnh c nt mi. Trong qu trnh chun , khuy bng my khuy t hoc bng lung kh nitrogen kh i qua dung dch. im kt thc phn ng c xc nh bng in k gn trong mch c bin tr 2000 m, ni vi mt ngun pin 1,5 von. Lc bt u kim in k ch im khng, v dng in chy qua 2 in cc platin khng ng k. Khi nh thuc th Karl Fischer vo dung dch, do hin tng kh cc nn kim in k lch i nhng lp tc tr v v tr ban u, ch khi n im dng th mt git thuc th tha s lm kim lch i v duy tr t nht 30 giy.

Thuc th

Thuc th Karl Fischer gc gm 4 thnh phn chnh l lu hunh dioxyd, iod, pyridin hoc mt base hu c khc v methanol pha thnh mt dung dch, hoc hai dung dch. Trng hp pha thnh hai dung dch th dung dch A cha lu hunh dioxyd v pyridin pha trong methanol khan, dung dch B cha iod pha trong methanol khan. Trc khi dng 1 gi, trn u 1 th tch dung dch A vi 1 th tch dung dch B, sau xc nh ng lng nc ca thuc th. Lng thuc th thu c ch dng trong ngy. Hin nay trn th trng va c cc loi thuc th nh trn, va c loi thay pyridin v methanol bng cht kim hu c khc v dung mi hu c khc. Do vy, cn kim tra k thnh phn thuc th v cch s dng cho ng mc ch. Cc thuc th Karl Fischer v dung mi dng trong phng php u phi khan nc, bo qun trong l mu, trnh nh sng, chng m v phi xc nh li ng lng nc trc khi dng.

Xc nh ng lng ca thuc th

ng lng ca thuc th Karl Fischer d b thay i theo thi gian nn trc khi dng phi xc nh li v phi t ti thiu 3,5 mg nc cho 1 ml thuc th. C th xc nh theo 2 cch:

a. p dng xc nh hm lng nc nh hn 1%.

Dng mt ho cht c hm lng nc kt tinh xc nh, sau khi sy nhit quy nh n khi lng khng i loi ht m, cho tc dng vi thuc th ri tnh ra ng lng. Thng dng natri tartrat dihydrat ( C4H4Na2O6.2H2O).

Tin hnh:

Cho mt lng methanol hoc dung mi thch hp dng cho thuc th Karl Fischer vo cc chun ngp in cc platin ri chun bng th Karl Fischer n im dng. Cho nhanh khong t 150 mg n 350 mg natri tartrat dihydrat cn chnh xc vo cc v chun bng thuc th Karl Fischer n im dng, tnh h s ng lng nc F (tnh bng mg nc/ml thuc th) ca thuc th theo cng thc:

 

F = 2. (18,02/230,08) . (W/V)

 

Trong :

18,02 v 230,08 l khi lng phn t ca nc v ca natri tartrat dihydrat.

W l khi lng natri tartrat dihydrat (tnh bng mg).

V l th tch ca thuc th dng (tnh bng ml).

b. p dng xc nh hm lng nc ln hn hoc bng 1%

 

Dng nc tinh khit chng ct t tiu chun lm cht chun ho vo methanol hoc dung mi thch hp loi dng cho thuc th Karl Fischer ri dng thuc th Karl Fischer chun .

Tin hnh:

Cho 25 ml methanol vo cc chun , chun bng thuc th Karl Fischer n im dng. Thm nhanh khong 50 mg nc tinh khit cn chnh xc vo cc chun trn v chun bng thuc th Karl Fischer n im dng. Tnh h s ng lng nc F (tnh bng mg nc /ml thuc th) ca thuc th theo cng thc:

 

F=W/V

 

Trong :

W l khi lng nc (tnh bng mg).

V l th tch thuc th dng (tnh bng ml).

Tin hnh nh lng

Chun b mu th

Nu khng c ch dn g khc trong chuyn lun ring, cn hoc ly chnh xc mt lng ch phm c lng cha khong 10 mg n 50 mg nc em nh lng. Thao tc phi nhanh v thc hin trong phng c m thp trnh m ngoi nh hng n cht phn tch.

Phng php nh lng trc tip

Nu khng c ch dn g khc trong chuyn lun ring, cho khong 20 ml methanol khan hoc dung mi c ch dn trong chuyn lun ring vo cc chun , chun bng thuc th Karl Fischer n im dng. Cho nhanh mt lng ch phm ch dn trong chuyn lun ring vo cc chun , ng nt ngay, khuy u phn ng tc dng trong khong 1 pht ri tip tc chun bng thuc th Karl Fischer n im dng. Tnh hm lng nc X (tnh bng mg) ca ch phm theo cng thc:

X = N.F

Trong :

N: Th tch thuc th dng cho ln chun sau khi cho ch phm (tnh bng ml)

F: H s ng lng nc ca thuc th Karl Fischer (tnh bng mg/ml.

Phng php nh lng gin tip

Pha dung dch nc chun: Pha long 2ml nc tinh khit vi methanol hoc dung mi thch hp thnh 1000ml. Ly chnh xc 25,0 ml dung dch ny cho vo cc nh lng v chun bng thuc th Karl Fischer va mi xc nh chun. Tnh hm lng nc W(tnh bng mg/ml) ca dung dch nc chun theo cng thc:

 

W = V.F/25

 

Trong :

V: Th tch thuc th Karl Fischer dng (ml).

F: H s ng lng nc ca thuc th Karl Fischer (tnh bng mg/ml).

Tin hnh: Nu khng c ch dn g khc trong chuyn lun ring, cho mt lng methanol, hoc dung mi c ch dn trong chuyn lun ring vo cc nh lng va ngp in cc, chun bng thuc th Karl Fischer n im dng. Cho nhanh mt lng ch phm ch dn trong chuyn lun ring vo cc, ng nt ngay, thm tip mt lng chnh xc thuc th Karl Fischer vo cc sao cho tha khong 1 ml, hoc theo mt th tch ch dn trong chuyn lun ring. ng nt yn 1 pht, trnh nh sng, thnh thong khuy. Chun phn thuc th Karl Fischer tha bng dung dch nc chun va mi pha trn. Tnh hm lng nc A (tnh bng mg) c trong ch phm theo cng thc:

 

A = F.V1- W.V2

 

Trong :

F: ng lng nc ca thuc th Karl Fischer (tnh bng mg/ml).

V1: Th tch thuc th Karl Fischer thm vo (ml)

V2: Th tch dung dch nc chun dng (ml).

W: Hm lng nc ca dung dch nc chun trn (tnh bng mg/ml).

Ch :

Cn phi kim tra xem cht th c tng k vi thuc th Karl Fischer khng trc khi p dng phng php ny. Nhng cht c kh nng phn ng vi mt hay nhiu thnh phn ca thuc th nh acid ascorbic, cc mercaptan, cc sulfid, cc mui hydrocarbonat v carbonat kim, cc oxyd v hydrat ca oxyd kim loi.. khng p dng c phng php ny. i vi cc aldehyd v ceton, hin nay c loi thuc th dnh ring nh lng nc trong cc cht ny.

Nhng dung mi hu c sau y c th dng thay th methanol trong thuc th Karl Fischer khi cht th khng tan trong methanol: Cloroform, methyl celosolve, diethylen glycol monoethyl ether. Trc khi s dng phi lm khan bng zeolit t tiu chun cho nh lng nc.

Nhng cht base hu c sau y c th thay pyridin trong thuc th Karl Fischer: Imidazol,
2- methylaminopyridin. Phi kim tra hm lng nc trc khi dng.

 

10.4 phng php chun bng nitrit

 

Chun bng nitrit l phng php nh lng th tch, trong dung dch chun l dung dch natri nitrit.

Phng php ny c dng ch yu nh lng cc ch phm c cha nhm amin thm bc nht hoc nhng ch phm khc m qua bin i ho hc chuyn c thnh hp cht c nhm amin thm bc nht.

Cch tin hnh

Cn chnh xc 1 lng ch phm tng ng vi khong 0,002 phn t gam hot cht (hoc theo ch dn trong chuyn lun ring) ho tan trong 20 ml acid hydrocloric (TT), v 50 ml nc. Cho thm 2 g kali bromid (TT), lm lnh trong nc ri chun bng dung dch natri nitrit 0,1 M (C) ( Phi u nh dung dch chun ca buret ngp di mt dung dch cn chun ) . Trong qu trnh chun phi lc bnh lin tc, nh nhng sao cho khng to ra dng xoy khng kh trong dung dch (tt nht l dng my khuy t). Nh dung dch chun vi tc lc u chng 2 ml trong mt pht, n trc im tng ng khong 1 ml th nh tng 0,1 ml mt v yn t nht mt pht sau mi ln thm dung dch.

im kt thc c xc nh bng phng php o in (theo hng dn trong cc chuyn lun "Chun o ampe" hoc "Chun o in th" hoc bng ch th mu thch hp.

 

10.5 Phng php chun complexon

Nhm (Al)

Ly mt lng dung dch nh ch dn trong chuyn lun ring cho vo mt bnh nn 500 ml. Thm 25 ml dung dch trilon B 0,1 M (C) v 10 ml hn hp ng th tch ca dung dch amoni acetat 15,5% v acid acetic 2 M (TT). un si dung dch 2 pht, ngui, thm 50 ml ethanol v 3 ml dung dch dithizon 0,025% (kl/tt) mi pha trong ethanol (TT) ri chun dung dch trilon B 0,1 M tha bng dung dch km sulfat 0,1 M (C) n khi mu ca dung dch chuyn t xanh sang tm .

1 ml dung dch trilon B 0,1 M tng ng vi 2,698 mg Al.

Bismuth (Bi)

Nu khng c ch dn g khc, pha long 1 lng dung dch ch phm trong acid nitric 2 M (TT), nh ch dn trong chuyn lun vi nc thnh 250 ml. Va lc va thm tng git amoniac 13,5 M (TT) cho n khi ta bt u xut hin. Thm 0,5 ml acid nitric (TT) v un nng n 70 oC, duy tr nhit ny cho n khi dung dch trong sut. Thm khong 50 mg hn hp da cam xylenol (TT) ri chun bng dung dch trilonB 0,1 M (C) n khi mu ca dung dch chuyn t tm hng sang vng.

1 ml dung dch trilon B 0,1 M tng ng vi 20,90mg Bi.

 

Calci (Ca)

Ly mt lng dung dch ch phm nh ch dn trong chuyn lun cho vo mt bnh nn 500 ml ri pha long vi nc ct thnh 300 ml. Thm 6 ml dung dch natri hydroxyd 10 M (TT) v khong 15 mg hn hp calcon (TT) ri chun bng dung dch trilon B 0,1 M (C) n khi mu ca dung dch chuyn t tm sang xanh hon ton.

1 ml dung dch trilon B 0,1 M tng ng vi 4,008mg Ca.

 

Ch (Pb)

Ly mt lng dung dch ch phm nh ch dn trong chuyn lun cho vo mt bnh 500 ml ri pha long vi nc ct thnh 200 ml. Thm khong 50 mg hn hp da cam xylenol (TT) v mt lng hexamin (TT) va thu c dung dch c mu hng tm ri chun bng dung dch trilon B 0,1 M (C) n khi dung dch chuyn sang mu vng.

1 ml dung dch trilon B 0,1 M tng ng vi 20,72 mg Pb.

 

Magnesi (Mg)

Cho vo mt bnh nn 500 ml mt lng dung dch ch phm nh ch dn trong chuyn lun v pha long vi nc thnh 300 ml hoc ho tan mt lng ch phm nh ch dn trong chuyn lun trong 5 - 10 ml nc hoc trong mt lng nh acid hydrocloric 2 M (TT) ri pha long vi nc thnh 50 ml. Thm 10 ml dung dch m amoniac pH 10,0 (TT) v khong 50 mg hn hp en eriocrom T (TT) vo dung dch cui cng. un nng dung dch n 40 oC v chun nhit bng dung dch trilon B 0,1 M (C) n khi mu ca dung dch chuyn t tm sang xanh lam hon ton.

1 ml dung dch trilon B 0,1 M tng ng vi 2,431mg Mg.

 

Km (Zn)

Cho vo mt bnh nn 500 ml mt lng dung dch nh ch dn trong chuyn lun ri pha long vi nc thnh 200 ml. Thm khong 50 mg hn hp da cam xylenol (CT) v mt lng hexamin (TT) va lm dung dch chuyn sang mu hng tm. Thm 2 g hexamin (TT) na ri chun bng dung dch trilon B 0,1 M (C) n khi mu ca dung dch chuyn sang vng.

1 ml dung dch trilon B 0,1 M tng ng vi 6,54 mg Zn.

10.6 Phng php chun trong mi trng khan

Chun trong mi trng khan l phng php chun acid v base yu hoc nhng mui ca chng trong mi trng khng phi l nc.

Phng php 1

(p dng cho base v mui ca chng)

Ho tan mt lng ch phm nh ch dn trong chuyn lun ring trong mt th tch thch hp acid acetic khan (TT) c trung tnh ho trc vi ch th quy nh trong chuyn lun ring, nu cn thit c th lm m hay lm lnh, hoc chun b mt dung dch nh ch dn trong chuyn lun ring.

Khi ch phm l mui ca acid hydrocloric hoc acid hydrobromic th thm 15 ml dung dch thu ngn (II) acetat 5% (TT) trc khi trung tnh dung mi, tr khi c nhng ch dn khc trong chuyn lun ring.

Chun bng dung dch acid percloric 0,1 N trong acid acetic khan (C) n khi c s chuyn mu ca ch th, iu ny tng ng vi gi tr tuyt i cao nht ca dE/dV trong chun o th ca ch phm th, y E l th in ng v V l th tch dung dch chun (Ph lc 10.2).

Vic trung tnh ho dung dch thu ngn (II) acetat v chun ho dung dch chun cng phi dng cng mt ch th c quy nh trong chuyn lun ring cho chun ch phm.

Khi nhit t2 ca dung dch chun thi im nh lng khc vi nhit t1 ca dung dch chun lc c chun ho th tnh kt qu nh lng cn c vo th tch dung dch chun hiu chnh.

V(h) = V(c) x [1 + 0,0011 (t1 - t2)]

Trong :

V(h): Th tch dung dch chun hiu chnh

V(c): Th tch dung dch chun dng.

Tin hnh chun mu trng khi cn thit.

Phng php 2

(p dng cho acid yu)

Dung dch chun , dung mi v ch th c ch dn trong chuyn lun ring

Bo v dung dch th v dung dch chun khi s thm nhp ca carbon dioxyd v m ca khng kh trong sut qu trnh chun .

Ho tan ch phm trong mt th tch thch hp dung mi trung tnh ho trc vi ch th quy nh, nu cn c th lm m hay lnh, hoc chun b mt dung dch ch phm nh ch dn trong chuyn lun ring.

Chun cho n khi c s chuyn mu ca ch th, iu ny tng ng vi gi tr tuyt i cao nht ca dE/dV trong chun o th ca ch phm, y E l th in ng v V l th tch dung dch chun (Ph lc 10.2).

Dung dch chun c chun ho bng cch s dng dung mi v ch th ging nh s dng cho chun ch phm.

Tin hnh chun mu trng khi cn thit.

 

10.7 nh lng cc khng sinh h Penicilin bng phng php o iod

Phng php sau y c dng nh lng phn ln thuc khng sinh h penicillin trong Dc in v nhng dng bo ch ca chng m php chun bng o iod l c bit thch hp. Tin hnh th nghim nhit 25 2 oC.

Dung dch chun

Cn chnh xc mt lng thch hp cht chun quy nh trong tng chuyn lun, ho tan vo dung mi c ghi trong Bng 10.7 v pha long vi cng dung mi c mt dung dch c nng khong nng quy nh trong Bng 10.7.

Dung dch th

Nu khng c ch nh g khc trong chuyn lun ring, cn chnh xc mt lng mu th thch hp ho tan trong dung mi ghi trong Bng 10.7 v pha long vi dung mi c mt dung dch c nng vo khong nng ghi trong Bng 10.7.

Phng php tin hnh

Lm mt hot tnh v chun : Ht ring r 2,0 ml dung dch chun v 2,0 ml dung dch th vo 2 bnh nn nt mi dung tch 125 ml tng ng. Thm vo mi bnh 2,0 ml dung dch natri hydroxyd 1,0 N(TT), lc u v yn 15 pht. Tip tc cho vo mi bnh 2,4 ml dung dch acid hydrocloric 1,0 N(TT), thm 10,0 ml dung dch iod 0,01 N (C), y ngay nt bnh v yn 15 pht. Chun bng dung dch natri thiosulfat 0,01 N (C) n gn im kt thc, thm 1 git dung dch h tinh bt (TT) v tip tc chun n khi mt mu xanh.

Mu trng

Ht 2,0 ml dung dch chun cho vo mt bnh nn nt mi, thm 10,0 ml dung dch iod 0,01 N (C). Nu dung dch chun l amoxicilin hay ampicilin th thm ngay lp tc 0,12 ml dung dch acid hydrocloric 1,0 N (TT). Chun ngay bng dung dch natri thiosulfat 0,01 N (C) n gn im kt thc, thm 1 git dung dch h tinh bt (TT) v tip tc chun n khi mt mu xanh. Cng lm nh vy i vi bnh c cha 2,0 ml dung dch th.

Tnh ton

Tnh ng lng F [s microgam (hay n v) khng sinh chun tng ng vi mi ml dung dch natri thiosulfat 0,01 N (C)] bng cng thc :

 

Trong :

C l nng cht chun tnh bng mg trong 1 ml ca dung dch chun.

P l hot lc tnh bng microgam (hay n v ) trong 1 mg ca cht chun.

B l th tch tnh bng ml ca dung dch natri thiosulfat 0,01 N (C) tiu th trong mu trng ca dung dch chun.

I l th tch tnh bng ml ca dung dch natri thiosulfat 0,01 N (C) tiu th trong php th lm mt hot tnh v chun ca dung dch chun.

Tnh hm lng ca mu th bng cng thc c cho trong tng chuyn lun.

 

Bng 10.7. Nhng dung mi v nng cui cng ca dung dch khng sinh

Cht khng sinh

Dung mi

Nng cui cng

Amoxicilin

Nc

1,0 mg /ml

Ampicilin

Nc

1,25 mg /ml

Ampicilin natri

m phosphat pH 6,0

1,25 mg /ml

Cloxacilin natri

Nc

1,25 mg /ml

Dicloxacilin natri

m phosphat pH 6,0

1,25 mg /ml

Methicilin natri

m phosphat pH 6,0

1,25 mg /ml

Oxacilin natri

m phosphat pH 6,0

1,25 mg /ml

Penicilin G kali

m phosphat pH 6,0

2000 n v /ml

Penicilin G natri

m phosphat pH 6,0

2000 n v /ml

Penicilin V kali

m phosphat pH 6,0

2000 n v /ml

10.8 nh lng cc steroid bng tetrazolium

 

Phng php ny c p dng nh lng cc steroid cha cc nhm chc c tnh kh.

Cc sn phm ca phn ng mu ny c khuynh hng hp ph ln b mt ca thu tinh. trnh s sai lch kt qu, nn x l cc bnh phn ng thu tinh bng cch chng cha chnh cc sn phm ca phn ng mu ny trc khi dng. Nn gi cc bnh thu tinh c x l dng ring cho php nh lng ny v ch ra bnh bng nc gia cc ln nh lng. Phng php ny c tin hnh trong iu kin trnh nh sng.

Dung dch th

Tr khi c ch dn c th trong chuyn lun ring, ho tan mt lng ch phm trong ethanol khng c aldehyd (TT) thu c mt dung dch th c nng t 30 n 35 mg/ml.

Dung dch chun

Chun b mt dung dch cht chun tng ng trong ethanol khng c aldehyd (TT) c nng tng ng vi nng ca dung dch th.

Tin hnh

Ly chnh xc 10 ml mi dung dch th v dung dch chun cho vo hai bnh nh mc 25 ml ring bit v cho 10 ml ethanol khng c aldehyd (TT) vo mt bnh nh mc 25 ml th ba. Ln lt thm vo cc bnh 2 ml dung dch triphenyltetrazolium clorid (TT), 2 ml dung dch tetramethylamoni hydroxyd long (TT). y bnh, trn u bng cch lc xoay trn nh nhng v ngm cc bnh phn ng ny trong cch thu 30oC trong 1 gi, tr khi c quy nh c th trong chuyn lun ring. Lm lnh nhanh, thm ethanol khng c aldehyd (TT) n nh mc 25 ml. Lc u v o ngay hp th nh sng ca cc dung dch thu c trong hai bnh u (theo th t khi cho thuc th) cc i 485 mm (Ph lc 4.1), trong cc o c np y, ly dung dch thu c trong bnh th ba lm mu trng.