I. S LC V PHNG PHP CHUN KARL FISCHER:

I. S LC V PHNG PHP CHUN KARL FISCHER:

c im ca phng php Karl Fischer

Phng php trc tip xc nh nc, thch hp vi mu c m thp (nh hn 0.1%) v c th xc nh ti 0.01%. Phng php ny cn thch hp vi mu c hm lng ng cao hay protein cao.Trong phng php chun ny nc c xc nh thng qua phn ng vi thuc th l: dung mi methanol c cha Iod, SO2 v pyridine.Phng trnh phn ng:

Cc bc tin hnh trong chun Karl Fischer:

B1: Thm dung mi KF khan vo bnh chun cho ngp in cc v y np li ( Nn chn v s dng thuc th KF thng mi v tnh n nh cao v an ton, v y l cht c).

B2: Chy thit b chun m khng cn thm mu lm v hiu ha nc t dung mi KF v m trong bnh chun .

B3: Cn chnh xc 125mg nc v pha long thnh 10ml trong bnh h v sch bng dung mi KF.

B4: Ht v x 1 lng nc tiu chun vo kim tim loi m c trong kim tim. Lp li 1 ln na. (Nh y np sau khi ly).

B5: Tim 1 ml nc tiu chun vo bnh v chun xc nh h s chuyn i. Cn thc hin iu ny hng ngy hoc khi no s dng thuc th KF hoc dung mi mi.

B6: Ht v x mt lng thuc th KF vo kim tim loi m trong kim tim. Lp li 1 ln na.

B7. Tim 1 ml thuc KF khan vo bnh chun v chun hiu chnh blank. Nn chnh blank thm 1 ln na.

B8: S dng kt qu o blank c chnh xc dung mi KF v xc nh nng chun ca thuc th KF (mg H2O/ml thuc th KF).

II S LC V PHNG PHP CHUN IN TH:

c im phng php chun in th:

1. Da trn vic o s bin thin ca th trong qu trnh chun . M s bin thin ny l mt s thay i t ngt ti gn im tng ng.

2. Vi nhiu u im nh nhy cao (ppm), chun c dung dch c mu, v c trng hp khng c cht ch th.

Cc phng php chun in th: Khi chun in th cn phi nhng vo dung dch nghin cu hai cc. Mt cc th ca n cho bit nng ca cht b chun, cc gi l cc ch th . cc kia tr i vi ion b chun, ch dng xc nh th ca cc ch th, cc ny gi l cc so snh .

Da vo bn cht ca phn ng xy ra trong qu trnh chun m ngi ta chia thnh cc phng php chun in th sau :

1.1 Phng php kt ta:

C th minh ha phng php kt ta trong chun in th qua v d xc nh bc. C th dng cc bc lm cc ch th i vi ion bc. Cc so snh l cc calomen.

Eo = Ech th - Echun(Pt)Hg.HgCl2/ Cl- || Ag+ |Ag

Gi s ly 100ml dung dch AgNO3 0,1N chun . Th ca cc bc trong dung dch u c xc nh bng cng thc:

Ech th = 0,81 + 0,059lg0,1 = 0,742VKhi thm dung dch Cl 1N vo, nng ion bc gim i v ion bc chuyn vo kt ta.

Gi s thn 5ml dung dch Cl- 1N tng ng vi na hm lng ion bc tham gia phn ng, nng ion bc gim xung mt na cn 0,05N. (b qua s tng th tch ) v th ca cc bc s l :

Ech th = 0,81 + 0,059lg0,05 = 0,725VKhi thm n 9ml Cl- 1N tng ng vi qu trnh chun t 90%

Ta c : Ech th = 0,81 + 0,059lg0,01 = 0,684VKhi thm 9,9ml Cl- 1N vo, tng ng t 99% qu trnh chun

Ta c : Ech th = 0,81 + 0,059lg0,001 = 0,626VKhi thm 9,99ml Cl- 1N vo, tng ng t 99,9% qu trnh chun

Ta c : Ech th = 0,81 + 0,059lg0,0001 = 0,574VTi im tng ng lc ny nng bc c tnh nh sau:

[Ag+] = TAgCl / [Cl-]= 1,7.10-10/ [Cl-]

Theo tin trnh phn ng ta thy: [Ag+] = [Cl-]

Suy ra : [Ag+] = Th ti im tng ng : Ech th = 0,81 + 0,059lg = 0,516VSau im tng ng nng ion bc ph thuc vo lng d ion Cl- v cng c tnh bng tch s tan ca AgCl . Nu thm 0,01ml dung dch Cl vo dung dch tng ng vi nng Cl- t c l 10-4M.

Lc ny [Ag+] = 1,7.10-10/ 10-4 = 1,7.10-6

Tng ng vi E = 0,81 + 0,059lg1,7.10-6 = 0,465 v

Khi cho d 0,1ml dung dch Cl vo dung dch, tng ng vi nng Cl- t c l 10-3M.

Lc ny [Ag+]= 1,7.10-10/ 10-3 = 1,7.10-7

Tng ng vi E = 0,81 + 0,059lg1,7.10-7 = 0,408 v

Nhn xt :

+ Ti nhng thi im ban u ca qu trnh chun , cng ging nh nhng thi im sau qu trnh chun , gi tr th ca h thay i khng ng k mc d lng thuc th cho l kh ln.

+Ti nhng thi im st trc v st sau im tng ng, khi thiu 0,01% v d 0,01% th gi tr th ca cc ch th c s thay i r rt t 0,574v n 0,465v. Ngi ta gi l bc nhy in th ng vi 0,109v.

Nu ta o hiu th gia hai cc ch th v so snh th bc nhy gim th E = (0,574 - Eso sanh ) - (0,465 - Eso sanh ) = 0,109 v

y l im gip cho ngi kim nghim vin phn tch nhn bit v kt thc s chun ti y. T nng , th tch tiu tn ca thuc th v th tch ca dung dch em i xc nh, ngi ta tnh c nng ca cht cn nh phn.

th s chun dung dch Ag+ bng dung dch Cl- c m t nh sau :

th c tnh i xng qua im tng ng

Nhn xt:

+ Bc nhy th ca ng nh phn ph thuc vo tch s tan ca kt ta to thnh. Nu kt ta cng c tch s tan cng ln th bc nhy ng nh phn cng di. V d khi chun Ag+ bng I- th bc nhy ng nh phn di hn do TAgI = 10-16

+ Bc nhy ng nh phn ph thuc vo nng cc dung dch chun . Nng dung dch chun cng ln th bc nhy ng nh phn cng di

1.2 Phng php to thnh cc phc cht t phn ly: Phn ng to thnh trong phng php ny to ra phc cht c bn tng i ln.

Ta xt v d chun cation kim loi bng EDTA

Phn ng chun M + Y = MY (1)

Hng s bn: in cc ch th: in cc Hg

in cc so snh: in cc calomen

Pin to c (Pt)Hg.HgCl2/ Cl- // Hg2+/Hg(Pt)

Eo = Ech th - EchunTnh [Hg2+]

Nu ngay t u ta thm mt lng chnh xc HgY2- vo dung dch. Ta c cn bng: HgY2- = Hg2+ + Y4-= 1021,8 (2)

Khi thm dung dch chun Y vo ta c Y s tham gia vo c hai cn bng (1) v (2)

Ta c : [Hg2+] = = Thay vo biu thc tnh Echi thi ta c :

Nu nng HgY2_ ln th c th xem [HgY2-] khng i

Ta c c dng phng trnh Nerst

Ta thy E chi thi ph thuc vo [M] v [MY]

Dng th thu c khi chun :

Ta xt mt v d khc : Ag+ + 2CN- = Ag(CN)2-

Hng s khng bn ca phc: Ta dng cng thc ny tnh nng ca bc v th ca cc bc trong qu trnh chun . Gi s thm 0,5ml dung dch AgNO3 1N vo 100ml dung dch KCN 0,1N. y vic to phc, nng ion CN- gim ti khi [CN-] = 0,09N cn nng ion phc mi to thnh s bng [Ag(CN)-2] = 0,005N . T y ta c :

[Ag+]= 10-21 . Echi thi = 0,8 + 0,058lgKhi thm 2,5ml Ag+

Ta c : [Ag+]= 10-21 . Echi thi = 0,8 + 0,058lgKhi thm 4,5ml Ag+

Ta c : [Ag+]= 10-21 . Echi thi = 0,8 + 0,058lgKhi thm 4,95ml Ag+

Ta c : [Ag+]= 10-21 . Echi thi = 0,8 + 0,058lgTi im tng ng nng ion bc c tnh nh sau :

T phng trnh to phc trn ta thy ti im tng ng nng Ag+ v CN- c mi quan h l : 2[Ag+]= [CN-]

Thay vo Kkb ta c

Th tng ng :

Echi thi = 0,08 + 0,058lg2,32.10-8 = 0,415V

Nu tip tc thm Ag+ th lc ny c phn ng to kt ta :

Ag(CN)-2 + Ag+ = 2AgCN

Lc ny th ca dung dch khng i cho n khi kt Ta to thnh ht c ngha l kt ta AgCN l hon ton. Nu tip tc thm Ag+ th th tip tc tng do nng ion Ag+t do trong dung dch tng.

Echi thi = 0,8 + 0,058lgC th m t th chun nh sau :

1.3 Phng php oxy ha kh: Xt qu trnh chun 50ml Fe3+ 0,1N bng dung dch Sn2+ 0,1N ta c phng trnh chun nh sau :

2Fe3+ + Sn2+ Fe2+ + Sn4+

Th tiu chun ca cc in cc nh sau :

E0Fe3+/Fe2+ = 0,77v , E0Sn4+/Sn2+ = 0,15v

Nu nhng mt in cc Pt vo dung dch v tin hnh kho st s bin thin th trong dung dch. Ta thy th bin thin ca dung dch c th tnh :

+ Khi qu trnh chun c mt na c ngha l thm c 25ml Sn2+ lc ny nng Fe3+ cn li l 0,05N v nng Fe2+ cng to c l 0,05N

E = 0,77 + 0,058lg = 0,77 + 0,058lgE = 0,77V

+Khi chun c 90% tc l tng ng vi 45ml Sn2+. Lc ny nng Fe3+ cn li l 10% tng ng vi 0,01N. Nng Fe2+ to thnh l 0,09N

E = 0,77 + 0,058lg = 0,715v

+Khi chun c 99% tc l tng ng vi 49,5ml Sn2+. Lc ny nng Fe3+ cn li l 1% tng ng vi 0,001N. Nng Fe2+ to thnh l 0,099N

E = 0,77 + 0,058lg = 0,654v

Ti im tng ng th c tnh nh sau :

Sau im tng ng nu tip tc thm Sn2+th th dung dch lc ny ph thuc vo nng Sn4+ v nng Sn2+ cn nng Fe3+ v Fe2+ khng thay i. Nng Sn4+ lc ny l 0,05N

Khi thm d 0,1ml Sn2+ 0,1N th nng [Sn2+] = 10-4 ( )

E = 0,15 + lg= 0,15+ lg=0,24v

Tng t khi d 1ml Sn2+ 0,1N th nng [Sn2+] = 10-3E = ,15+ lg= 0,21v

Nu tnh bc nhy th bt u t thi im thiu 1% v d 1% so vi qu trnh chun ta c : bc nhy th = 0,654 0,21= 0,444v

1.4 Phng php trung ha: Phn ng xy ra trong qu trnh chun ny l phn ng gia axit v baz. y trong qu trnh chun [H+] s bin thin v thc cht ca qu trnh chun l H+ + OH- = H2O

Nn in cc dng o th qu trnh phi lin quan ti nng [H+] v d nh in cc Hydro, in cc thu tinh, in cc quihyron.

ng cong chun trong chun trung ha c tnh i xng qua im tng ng v bc nhy th ph thuc vo :

+ Hng s phn ly ca axit, hng s phn ly ca axit cng ln th bc nhy cng di.

+ Nng axit, baz tham gia vo qu trnh chun cng ln th bc nhy cng di.

1.5 Chun hn hp ion:y l phng php rt hu dng , c th ng thi xc nh cc ion khc nhau trong cng mt dung dch do kh nng to phc bn ca chng vi thuc th l khc nhau , do tch s tan ca chng vi kt ta l khc nhau hay do th oxy ha kh ca chng khc nhau. Nn khi phn ng vi thuc th phn ng xy ra c tnh cht tun t cho tng ion. V vy m ta c th nh lng tng ion mt trong hn hp nhiu ion.

V d: trong dung dch c ng thi 2 ion Cl- v I- th ta c th xc nh ng thi c hai ion m khng cn tch chng ra, bng cch cho tc dng vi thuc th Ag+ . V TAgI = 10-16 < TAgCl = 10-10 nn phn ng xy ra tun t nh sau:

Ag+ + I- = AgI

Sau : Ag+ + Cl- = AgCl

Khi AgCl bt u kt ta cng vi AgI th [Ag+] trong dung dch l :

[Ag+]= 10-16/[I-] = 10-10/[Cl-] suy ra : [I-]/[Cl-]= 106. Chng t khi bt u qu trnh chun Cl- th I- chun hon ton.

Cc yu cu i vi cc phn ng trong chun in th

1.Cc phn ng ion phi xy ra nhanh. Khi tc phn ng chm cn phi c bin php thc y phn ng nh : dng xc tc, tng nhit .

2.Cc phn ng cn xy ra mt cch hp thc. Trong trng hp cc cht c thnh phn phc tp v khng xc nh th khng dng phn ng hay phi thay i iu kin c thnh phn xc nh.3.Phn ng cn phi xy ra theo mt chiu xc nh. Trong qu trnh oxy ha kh thng c cc phn ng ph xy ra nn ta thng tin hnh chun trong nhng iu kin khng c cc phn ng ph .4.Phn ng cn phi xy ra nhanh, xy ra hon ton . cho phn ng xy ra hon ton th phi chn kt ta c T l nh nht, hng s bn ca phc l cao nht hay hiu th oxy ha kh chun ca cc cht l ln nht.5.i vi phn ng chun cn phi chn c in cc ch th c trng. y l iu kin phn no hn ch phm vi ng dng ca phng php v khng phi phn ng no cng tm c cc ch th .

2 Cc phng php o th ca dung dch:

2.1 Phng php chun ti im khng:

Phng php chun ti im 0 l phng php ph bin nht trong cc phng php chun khng b chnh. Bn cht ca phng php l khi thm t t dung dch chun vo v khuy u c th nhn thy rng cng gn im tng ng lch ca kim in k cng gim dn v cui cng khi d mt got dung dch chun kim in k lch theo hng ngc li.

in tr, 2. Cng tc, 3. Ac quy, 4. in k, 5. in cc calomen, 6. Bnh chun , 7. in cc Pt

u im ca phng php ny l cc thit b n gin v vic tin hnh nhanh. Thiu st ca phng php l th ca cc ch th v qu trnh bin thin th ph thuc vo cc cht in ly l c trong dung dch.

2.2 Phng php chun vi hai cc kim loi

Phng php chun vi hai cc kim loi da trn tnh tr ca cc lm bng cc kim loi tr nh Pt. Pd, W i vi s thay i thnh phn im tng ng. Nhng cc ny c th xem nh nhng in cc so snh .

Bnh chun , 2. in k, 3. c quy, 4. Cng tc, 5. in tr, 6. in cc Wonfram, 7. Cnh khuy, 8. in cc Platin

Khi chun theo s trn vic thm mt git thuc th cui cng im tng ng gy ra s sai lch bt thng ca kim in k. Phng php chun vi hai in cc kim loi tin dng cho tt c cc loi chun in th. Phng php ny cho php tin hnh phn tch nhanh, thit b chun n gin. Nhc im ca phng php l khng c du hiu chng t sp ti im tng ng.

XC NH NACL TRONG THC PHM BNG CHUN IN TH:

I. Nguyn tc v phm vi ng dng:

Sn phm c ha tan trong nc v c axit ha, clorua ha tan c chun bng ch th in th vi dung dch chun AgNO3. thun tin trong vic tnh kt qu, khi lng hoc th tch v nng mol c quy nh sao cho 1 ml AgNO3 = 0,1 % NaCl. Nu khng c sn cn cn nhanh khi lng quy nh th c th s dng khi lng mu th v dung dch AgNO3 nng mol thch hp. Phng php xc nh hm lng natri clorua (NaCl) ln hn hoc bng 0,03 %, bng phng php chun in th.

II. Chun b:

1.1. Chun b mu th:

1.1.1. Cc sn phm dng dung dch trong c snh thp: Nc p qu, nc xp trong, ru vang, v.v: c s dng trc tip lm mu th.

1.1.2. Cc sn phm dng vn: Nc p c chua, nc st c chua, rau nghin v.v: Lc k hp cha m trn u mi cht lng. Chuyn ton b lng cha sang bnh thy tinh hoc bt s ln v trn k, khuy lin tc hn 1 min. Sau chuyn ht sang bnh thy tinh c np y, lc hoc khuy k mi ln trc khi ly cc phn mu th phn tch.

1.1.3. Sn phm khng ng nht (c, tht v.v), c m thp (sn phm ng cc v.v) v kh phn tn, cc thc phm ng nht (phomat, b lc v.v): Cn 50,0 g mu th, cho vo bnh 1000 ml (4.5) ca my nghin tc cao v thm 450 g nc. y np, bt my nghin tc thp bng cch dng bin p bin i c phn tn s b v nghin k vi tc cao (thng 1 min n 2 min l ). Dng pipet 50 ml tho b u tip chuyn mt lng tng ng vi 5 g mu th (tng ng vi 50,0 gam hn hp sau khi nghin). Trn k huyn ph ca mu th ngay trc khi dng pipet ly phn mu th phn tch, sao cho phn cht rn c phn tn u.

1.1.4. Cc thc phm dng khc: Chun b mu th theo 1.1.1, 1.1.2, 1.1.3 hoc bng phng php thch hp khc.

bo qun cc mu th hoc cc huyn ph mu th dng cho phn tch sau ny, thm 0,5 ml dung dch HCHO khong 37 % cho 100 g mu th hoc huyn ph mu th, trn k v bo qun nhit phng. Chng ta chnh pha long bi dung dch HCHO bng cch nhn % NaCl vi 1,005.

2. Thuc th: 2.1. Axit nitric long, pha long 20 ml HNO3 bng nc n 1000 ml.

2.2. Dung dch chun bc nitrat: 0,0856 M. Ha tan 14,541 g AgNO3 trong nc v pha long bng nc n vch 1000 ml ng trong bnh nh mc (4.5). Chun ha li theo iu 5 v chnh n nng mol chnh xc quy nh sao cho vi lng mu th yu cu th 1 ml = 0,1 % NaCl. Bo qun dung dch trong bnh Pyrex, trnh nh nng trc tip. Dung dch ny n nh trong iu kin nh sng ca phng.

2.3. Dung dch chun natri clorua: (0,0856 M) Ha tan 5,000 g NaCl c sy trc 2 h 110 oC vo nc ( y nu hm lng NaCl < 100,0 % th chia 5,000 g cho % NaCl/100 thu c khi lng chnh xc) v pha long n 1000 ml trong bnh nh mc (4.5).

2.4. Nc: Nc ct hoc loi ion, khng cha nhm halogen, c kim tra theo cch sau:

Cho 1 ml dung dch AgNO3 0,1 M v 5 ml HNO3 (1 + 4) vo nc v thm nc n 100 ml. Dung dch ch c php hi c.

3. Thit b, dng c:3.1. Cn: c th cn c trn 200 g v c c n 0,01 g.

3.2. in cc: in cc t hp thanh Ag hoc cc in cc ch th Ag v in cc so snh thy tinh. Trc khi bt u v sau mi ngy s dng, lm sch u tip in cc thanh Ag, nu cn, dng bt lm sch hoc vt liu thch hp khc v trng ra k bng nc (i vi mt s mu phng th nghim c th phi dng nc nng ra). Lm sch cc in cc khc theo khuyn co ca nh sn xut. Lm sch li nu cn, trnh tri s c cui. i vi mt s mu th, trng ra nh k cc in cc bng nc v lau kh bng giy thm trnh tch t thnh mng trn in cc. Khng cn thit phi ph cc in cc thanh Ag bng AgCl.

3.3. My khuy t:My khuy t c vn hnh qua my bin p cho php n nh di tc ci t.

3.4. My o pH:Tt nht l loi c th c trc tip, c cc thang chia nh hn hoc bng 10 mV v c di o t nht l 700 mV, v d nh loi k thut s.

3.5. Bnh nh mc: dung tch 100 ml v 1000 ml.

3.6. Cc c m: dung tch 250 ml.

4. Chun ha: Dng pipet ly 25 ml dung dch chun ha NaCl cho vo cc c m 250 ml (4.6), pha long bng nc n khong 50 ml v b sung 50 ml dung dch HNO3 (1 + 49). t cc in cc vo, bt my khuy t (4.3) v lin tc khuy mnh vi tc n nh trong qu trnh chun m khng lm bn dung dch ra ngoi. Chun bng dung dch chun AgNO3 v iu chnh s tng tc thay i in p (bng cch thay i tc thm thuc th) sao cho thu c th chnh xc ca mV da theo ml dung dch AgNO3. B sung tt c 50 ml dung dch AgNO3 c c ng cong hon chnh.

Xc nh im un bng cch v 2 ng thng c dc 450 vi cc trc v tip tuyn vi ng chun ti 2 im c cong ln nht. im un l im giao gia ng chun vi ng v song song v gia 2 ng tip tuyn ni trn. T th tch dung dch AgNO3 s dng, tnh nng mol v hiu chnh n 0,0856 M. Thnh thong chun ha li. Dng im un l im kt thc chun mu th. Thnh thong kim tra li in th cui v khi thay in cc n l hoc in cc t hp, hoc my o pH th xc nh li bng cch chun b ng chun mi.

c chnh xc ln nht, khi thc hin mt dy cc php xc nh trn cng mt loi thc phm, th tin hnh xc nh v s dng im kt thc t ng chun ca loi thc phm hn l s dng im kt thc thu c bng dung dch chun NaCl.

5.2. Phng php xc nh:5.2.1. i vi cc sn phm cha t hn 5 % natri clorua: Cho 5,00 g (hoc 5,00 ml nu nng c tnh theo khi lng trn th tch) mu th chun b trong 1.1.1 hoc 1.1.2, hoc 50,0 g trong 1.1.3 cho vo cc c m 250 ml. Thm nc n khong 50 ml nu s dng mu th chun b trong 1.1.1 hoc 1.1.2 ( lm tan chy cht bo ca mu nh b th dng nc si). Thm 50 ml HNO3 (1 + 49). Chun nh trn, s dng buret 10 ml nu hm lng natri clorua nh hn hoc bng 1 %.

% NaCl = ml AgNO3 nng 0,0856 M/10

Khi nng AgNO3 khc vi 0,0856 M th tnh nh sau

% NaCl = ml AgNO3 x C/0,0856/10

Trong C l nng mol ca dung dch AgNO3 dng chun

5.2.2. i vi cc sn phm cha bng hoc ln hn 5 % natri clorua:Cho 5,00 g (hoc 5,00 ml nu nng c tnh theo khi lng trn th tch) mu th chun b trong 1.1.1 hoc 1.1.2, cho vo bnh nh mc 100 ml v pha long bng nc n vch. Trn u v chuyn dch lng cha t 50 mg n 250 mg NaCl cho vo cc c m 250 ml . Nu mu th c chun b theo 1.1.3 th chuyn phn dch lng cn cha t 50 mg n 250 mg NaCl sang cc c m 250 ml. Chun theo nh trn, bt u t on pha long bng nc n khong 50 ml.

% NaCl = F x ml AgNO3 nng 0,0856 M/10

Trong F l h s pha long = 100/ml phn dch lng chun nu mu th chun b trong 6.1.1 hoc 6.1.2, hoc 50/g mu th c chun b theo 6.1.3.

Khi nng AgNO3 khc vi 0,0856 M th tnh nh sau:

% NaCl = F x ml AgNO3 x C/0,0856 / 10

Trong C l nng mol ca dung dch AgNO3 dng chun .

5.2.3. Trng hp chung:Cn chnh xc phn mu th nu (cn di 5 g mu nu hm lng NaCl bng hoc ln hn 5 %, pha long n 100 ml). S dng dung dch AgNO3 khong 0,1 M, c chun ha chnh xc nh trn, khng iu chnh nng mol v chun nh trn% NaCl = ml AgNO3 x M AgNO3 x 0,05844 x 100/g mu th

Trong M l nng mol ca dung dch chun AgNO3 dng.

Nu mu th chun qu th thm chnh xc mt th tch dung dch chun NaCl v hon thnh chun . iu chnh th tch dung dch AgNO3 theo th tch dung dch chun NaCl thm vo.

im tng ng

DD Fe3+

ddSn4+

_1234567890.unknown