TIU CHUN VIT NAM

TCVN 1978:1988 HP

XC NH HM LNG CH BNG PHNG PHP TRC QUANGCanned foods - Determination of lead content by photometric methodTiu chun ny thay th TCVN 1978 - 77 v ph hp vi ST SEV 5338 - 85.

1. Ni dung v phng php Loi tr km v mt s nguyn t cn tr bng kali xianua dng thuc th Dithizon to phc mu vi ch v chit bng tetraclorua cacbon ri o mt quang ca dung dch chit.

2. Quy nh chungTheo TCVN 1976 - 88.

3. Thit b dng cMy trc quang;

Phu chit chia ui ngn, 250ml;

Bnh nh mc 100, 500, 1000ml;

ng ht 5, 10, 20, 50ml;

ng ht 5 v 10ml khc vch 0,1ml;

ng ong 100ml;

ng nghim nt mi, chia , dung tch 15ml.

4. Thuc th v dung dchNatri metabisunfit (Na2S2O5), dung dch 1,25%;

Axit nitric, d = 1,14g/ml;

Axit sunfuric, d = 1,84g/ml v dung dch (1+4);

Axit clohidric, d = 1,19g/ml v dung dch (1+3);

Axit xitric (C6H8O7.H2O), dung dch 20%;

Hydroxylamin clohidrat (NH2OH.HCl) dung dch 20%;

Kali xianua (KCN), dung dch 10%;

Amoni hidroxit, d = 0,91g/ml v dung dch (1+1); (1+50); natri hidroxit, dung dch 0,4%;

Timol xanh, dung dch 1%: ho tan 0,1g timon xanh trong 1,5ml dung dch natri hidroxit 0,4% v thm nc n 10ml;

Tetraclorua cacbon (CCl4);

- Dithizon (C13H12N4S) dung dch c (0,2g/l hay 0,02%): ho tan 0,2 g dithizon trong 1000ml tetraclorua cacbon.

ng dung dch trong bnh mu sm nt mi, trong t lnh.

- Dithizon, dung dch long (0,02g/l hay 0,002%) pha long t dung dch c bng tetraclorua cacbon. Chun b dung dch ny dng tng ngy.

Nu thy dung dch dithizon pha long c mu vng th phi tinh ch li. Mun vy, ly 20ml dung dch dithizon c cho vo phu chia , thm 40ml dung dch amoni hidroxit (1+50) - Lc. Tch lp nc ra, lc qua giy lc. Trung ho nc lc bng dung dch HCl (1+3), sau cho vo phu chit, thm 40ml tetra cacbon vo phu chit v lc. Gp chung dung dch dithizon li v xc nh nng dithizon. Mun vy o mt quang ca dithizon (max = 620nm, dng cuvet 10mm, dung dch so snh l tetraclorua ccbon.

- Ly gi tr ca mt quang chia cho h s tt phn t 32,8.103, tm c nng dithizon, sau dng tetraclorua cacbon pha long sao cho c dung dch dithizon nng 0,002% (0,02g/l).

Ch khi no tinh ch mi cn xc nh li nng dithizon.

Ch nitrat, dung dch gc 100mg/ml, chun b nh sau: ho tan 0,16g ch nitrat ( sy kh 1050C) vo hn hp gm 10ml axit nitric v 500ml nc, chuyn ht dung dch vo bnh nh mc 1 lt ri thm nc n vch mc.

Ch nitrat, dung dch long 1mg/ml.

Ly 5ml dung dch gc cho vo bnh nh mc 500ml, thm nc n vch mc. Chun b dung dch ny dng cho tng ngy.

5. Chun b th5.1. Cn khong 30g hoc dng ng ht ly 30ml mu.V c ho mu theo phng php t trong TCVN 4622 - 88.

5.2. Xy dng th chunLy 5 phu chit chia , dng ng ht cho vo 1; 2; 4; 8; 10ml dung dch ch nitrat pha long, thm dung dch axit sunfuric (1+4) n 30ml. Sau khi lm ngui thm vo mi phu 2,0ml dung dch natri metabisunfit, 5ml dung dch axit xitric, 1ml dung dch hidroxilamin clohidrat v vi git dung dch timon xanh. Lc u dung dch, trung ho bng 15ml dung dch amoni hidroxit c sau thm tng git dung dch amoni hidroxit (1+1) cho n khi c mu xanh l (pH = 9 - 10), lm ngui, thnh thong h m nt phu. Sau khi dung dch ngui hn thm 2ml dung dch kali xianua, lc u. Sau cho vo phu 0,5 - 1ml dung dch dithizon long, lc cn thn.

Tho dung dch chit mu sang ng nghim chia c nt mi, y nt, trnh nh sng lt vo, tt nht l bc ng nghim bng giy nhm hay giy thic. Lp li qu trnh chit cho n khi thm lng dithizon mi vo dung dch vn cn mu xanh. Gp chung cc phn dung dch chit, cho lun phn dung dch dithizon c mu xanh vo, sau thm tetraclorua cacbon vo ng nghim n 10ml.

o mt quang ca dung dch chun bc sng (max = 525nm dung dch so snh l tetraclorua cacbon.

Tin hnh th nghim kim tra vi lng thuc th v iu kin nh trn nhng dung dch khng c ch.

Ly mt quang o c trn tr mt quang th nghim kim tra ri v th chun, trc honh t lng ch ((g) c trong dung dch chun, trc tung t cc gi tr mt quang tng ng.

6. Tin hnh thTu theo lng ch d kin c trong mu, ly ton b dung dch sau khi v c ho hoc mt phn dung dch cha t 2 - 10(g ch cho vo phu chit chia . Thm vo mi phu 30ml nc. Tin hnh tip nh iu 5.2. Tin hnh th nghim kim tra qua tt c cc giai on phn tch vi lng thuc th v cc iu kin nh trn nhng dung dch khng c ch.

Ly mt quang ca dung dch phn tch tr mt quang ca dung dch kim tra ri t gi tr mt quang tm c, theo th chun tm hm lng ch trong dung dch phn tch.

7. Tnh kt qu7.1. Hm lng ch (X) tnh bng mg/kg theo cng thc:Trong :

m - lng ch trong dung dch phn tch, tm c theo th chun, (g;

m1 - lng cn mu, g;

V1 - th tch dung dch sau khi v c ho, ml;

V2 - th tch phn dung dch ly phn tch, ml.

7.2. Nu mu l cht lng v th tch mu ly l V0 (ml) th hm lng ch (X) tnh bng mg/l mu theo cng thc trn nhng thay m1 bng V0.

7.3. Kt qu l trung bnh cng ca kt qu hai ln xc nh song song. Khi lng ch trong dung dch phn tch nm trong khong 2 - 10(g. Cc kt qu ny khng c chnh lch nhau qu 10% kt qu trung bnh.