Cam Nag Thi Nghiem Hoa Hoc

8/6/2019 Cam Nag Thi Nghiem Hoa Hoc

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Cm nang thc nghim ha hc

PGS TS T Th ThoKhoa Ha hc Trngi hc KHTN -HQG H Ni

T VN - Mc ch: Phn thc nghim ha hc nhm bi dng cho GV trng THPT

chuyn cc kin thc v k nng cbn v thc nghim ha hc trong phng th

nghim (PTN) phc v cho vic hng dn k thut thc nghim ha hc cho

hc sinh THPT chuyn, chun b cho phn thi thc nghim trong cho k thi

HSG quc gia (t nm 2012) v k thi olympic ha hc quc t (IChO).

- Ni dung:

+ Mt s k thut cbn trong PTN Ha hc (ch yu phn ha hc phn tch)+ Mt s bi th nghim v phng php chun v trc quang xc nh cc cht.

A. MT S K THUT C BN TRONG PHNG TH NGHIM HA HC

Cc ni dung gii thiu trong phn ny c bin son da trn yu cu

t ra trong phn ph lc km theo ca k thi IChO m cc th sinh khi tham d

k thi IChO phi tc.

1.1. An ton trong PTN:

- Tun thng ni qui PTN.

- Phng tin bo m an ton trong PTN phi tun theo:

+ Mc o chong (o blue), eo knh bo v mt, i giy hoc sandal, tc,

qun o gn gng;

+ Dng qu bp cao su ly dung dch vo pipet, nghim cm ht ha

cht bng ming.

-

Cm s dng cc ha cht rt c, cc ha cht khc c dng theo chdn ca gio vin hng dn th nghim.

- Ha cht thi sau th nghim phi c thu gom vo cc bnh (c phn

loi c th)


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- Phi bit cc k hiu cnh bo vc, tnh an ton v yu cu phi

tun theo ca cc ha cht khi s dng.

Cht c (T) v rt

c (T+)Cht d chy (F) v rt d chy

(F+)Cht d bt la (Xi) v c

(Xn)

Cht gy n (E) Cht oxi ha mnh Cht n mn (C) Cht gy nguy him

vi mi trng (N)

1.2. Yu cu v k nng PTN:

Khi lm vic trong PTN, hc sinh cn bit/thnh tho cc k nng/ thao tc sau:+ Cc th nghim c lin quan n un nng, un hi lu

+ Thao tc vi my khuy t

+ Cch tin hnh cc phn ng trong ng chia

+ Cc php thnh tnh nhm chc hu c.

+ Phn tch nh tnh cc cht v c

+ Cc php o th tch (cch s dng ng ong, pipets, buret, bnh

nh mc v cn khi lng (cn in t).

+ Cch pha ch cc dung dch chun v cc dung dch thuc th

+ Cch tin hnh cc php phn tch th tch (chun ), cch dng

qu bp cao su

+ Php o pH (dng giy ch th v my o pH)


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+ Cc k thut tng hp cht: lc, sy kt ta, sc k lp mng, tng

hp cht trong cc thit b kch thc nh

+ Phn tch trng lng

+ Cch s dng thit b trc quang

+ Csl thuyt v thc hnh chit vi dung mi khng trn ln.

+ Sc k ct

+ Bit c cc loi sai s thng gp trong qu trnh thc nghim v

cch ghi kt qu th nghim ng s c ngha

Sau y chng ti chgii thiu mt sk thut cbn v cn ch

trong qu trnh chun xc nh nng /hm lng cc cht.

1.3. Mt s dng c, thit b v k thut thc nghim cbn trong phntch ha hc

1.3.1.Dng c dng trong phn tch trng lng1. 3.1.1. Cn

Trong phn tch, nh gi chnh xc, ng ca mt php o nht

thit phi thng qua cc dng co trng lng, l chic cn. N c dng

cn cht chun, cn mu phn tch, cn ho cht phn ng, cn sn phm v.v.

Tuy nhin, ngi phn tch cn bit nguyn tc cbn ca php cn, cn g vcn nh th no?

Php cn phn tch c thc hin cn cc vt cn l cht gc, cc cht c

thnh phn xc nh dng pha cc dung dch chun; cn cc sn phm ca

mt qu trnh bin i tnh kt qu. Vt cn c th l cht rn, hoc lng, tuy

nhin phi tun theo cc nguyn tc nht nh sao cho php cn va chnh xc

va c ngha.

Mt php cn ng phi bao gm c hai yu t trn; chnh xc nhng phi

c ngha. C nhng cht nht thit phi cn bng cn phn tch nhng cng c

nhng cht iu khng cn thit; hn na c nhiu cht nu cn trn cn

phn tch l hon ton v ngha. Th d mt ho cht rt ho nc nh NaOH,

hay FeCl3 v.v. s thay i trng lng ngay trong qu trnh cn; mt ho cht


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qu hn s dng b bin cht hoc c hi m bm trn b mt thm ch

chy nc th s dng cn phn tch trong trng hp l khng cn thit.

Mc ch s dng cng quyt nh loi cn cho ph hp: ch cn pha mt

dung dch m cho phn ng chun m bin dao ng v pH cho php

ln, nh vy cn chnh xc ho cht pha ch dung dch bng cn phn tch c

cn khng ? Mt th d khc l pha dung dch bng ho cht m khng th c

nng chnh xc ngay c, th d pha dung dch Na2S2O3, nng chnh xc

ch c c sau khi c thi gian yn 3-5 ngy v chun li; trong cc

trng hp nh vy cn cc ho cht bng cn phn tch l khng c ngha.

Chng ta c th tm thy cc cch pha dung dch trong cc sch tham kho.

Cc loi cn trong phng th nghim Ho phn tch

Cn k thut: Cn dng cho cc php cn km chnh xc, c th l cn sb

vt cn trc khi cn phn tch; cn cc ho cht c hi m khng cn sy

sau xc nh li nng dung dch bng cc cht chun; cn pha cc dung

dch khng cn chnh xc nng .v.v. Kh nng c kt qu ca cn loi ny l

0,01gam hoc 0,1g.

Cn phn tch: Cn phn tch thng cn cc vt cn c khi lng ti a ti

200 gam, c chnh xc ti 10-4 -10-5 gam, gm hai loi chnh l cn chcv in t.

Cn chc mt quang ngy nay vn cn nhiu phng th nghim s dng.

Nguyn l hot ng ca cn n gin l dng qu cn v n cn o trng

lng ca vt cn. Quang cn v n cn c trn cc li dao bng hp

kim hoc chu mi mn tt. Cn Mettler c cu to gm 2 li dao cn, mt

li dao dng treo quang v qu cn, li dao cn li dng nng n cn

v tr thng bng.


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Hnh 1: Nguyn l lm vic ca cn Mettler

Mt c im ca cn Mettler l qu cn v quang cn cng mt pha ca

n cn. Khi a vt cn vo a cn, ta phi ly bt qu cn ra vi khi lng

tng ng gi cho n cn trli v tr gn vi cn bng ban u (tt nhin

nu trng vi v tr ban u cng tt). Phn khng trng khp vi v tr ban u

thng c gi tr nh, chng chc miligam, c th hin bng thang quang

hc. Kt hp phn thang quang hc vi qu cn ly ra chnh l trng lng

ca vt cn. Hnh 1 cho bit nguyn l hot ng ca cn chc mt quang

Mettler.

Ngy nay, cn in t ra i s dng k thut s v hin th bng mn hnh

tinh th lng, gim c rt nhiu thao tc cho ngi phn tch.

Hnh 2: Cnin tv nguyn l hotng


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Nguyn l hot ng ca cn in t nh sau:

Vt cn ko a cn xung vi lc l F = mG, trong m l trng lng

ca vt cn; G l gia tc trng trng. Cn in t dng mt lc phn hi in

t ko a cn v v tr ban u ca n. Sau khi xc lp v tr cn bng, nu

t vt cn vo a cn, trng lng ca vt cn ko a cn xung, dn n tn

hiu sai lch m u d ca cn pht hin ra v gi n b chnh dng, t

dng i ph c sinh ra a n ng c tr. Dng in cn thit sinh ra

lc phn hi t l vi trng lng ca vt cn, c hin th qua mn hin s

(hnh 2).

* Quy tc sdng cn phn tch

1.Trc khi cn phi kim tra thng bng ca cn thng qua bt nc cab phn iu chnh thng bng (bt nc gia vng trn gii hn).

2. Khi cn, ngi ngi trn gh i din vi cn, mi thao tc phi nh

nhng trnh va p.

Tt c cc qu cn phi c sp xp ng v tr ban u, t 0 vi

trng hp sdng cn chc.

3. Ni ng ngun in cho cn, bt cng tc ngun, cc n bo hiu v

mn hin th sng, i cho n khi n nh, mn hin th ch 0,0000g i vicn in t v thang o chim 0 i vi cn chc.

Nht thit khng xp vt cn ln cn qu gii hn ti trng ca n, squ

ti c thgy nn nhng bin dng hoc gy n cn; i vi cn hin s, hin

tng trn lm chy cun y in tdo khng thb trc vt cn.

4. t vt cn gia a cn trnh dao ng (i vi quang cn) (do khi

xut hin lc ly tm snh hng n kt qu ca php cn).

5. ng kn t cn trc khi mhm cn/ bt cn, chcc con s hin thn

nh mi bt u c gi tr trng lng.

-Khi t vo hay ly ra vt cn nht thit phi hm cn. Khng tun theo quy

tc ny c thdn n n cn v dao cn bnh hng.


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- Chc thcn khi vt cn c nhit bng nhit ca khng gian xungquanh cn. Khi cn vt cn nng, dng khng kh gy ra do schnh lch nhit

nng vt cn v a cn, trng lng o c thp hn vt cn. Nu vt cn

lnh hn nhit phng, m ca khng kh kt hp dng khng kh ngc li

lm tng khi lng ca vt cn. V v y phi mang vt cn vo phng th

nghim trc khi cn t nht t 30-40 pht v trong bnh ht m

(desicator).

- Mi ln tin hnh phn tch hay cc qua mt sgiai on phn tch c lin

quan vi nhau chnn tin hnh trn mt cn v cn cng nhng qu cn

dng.

- Trong bt k trng hp no, khng c t trc tip ho cht cn cn lna cn. Cn sdng cc cn, thuyn cn trong mi trng hp v cn hai ln:

ln thnht cn cc cn, sau cn cc cn c cha cht cn cn v ly hiu

skhi lng gia hai ln cn.

6. Khi cn xong, phi kho/ tt cn (a cn v trng thi khng dao ng),

sp xp cc qu cn v v tr ban u, v sinh cn v khu vc cn sch s, ng

ca t cn.

Ch :Phi bo v cn phn tch trnh bn mn, v sinh sch scn v v tr

xung quan cn sau khi cn xong. - Khi cn cht lng khng n mn, khng bay

hi c thcn trc tip bng cch sdng l cn c nty va kht.

- Khi cn cc cht lng bay hi v c tnh n mn cao th n phi c ng

trong ng th y tinh kn. ng th y tinh c lm nng ln v u ng c

nhng vo mu, khi ngui, cht lng ngng t trn ng. ng thy tinh sau

c quay ngc trli v u ng c bt kn bng ngn la nh. ng thy

tinh v cht lng cha trong , cng vi phn thy tinh b tch ra nu c trong

qu trnh bt kn c lm ngui n nhit phng v cn. ng thy tinh sau

c chuyn n bnh cha yu cu v c lm vcht lng i ra. Mt

thtch hiu chnh cho ng thy tinh c thl cn thit nu ng thy tinh ny c

chia vch o thtch.


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1.3.1.2. T sy

T sy l thit bc s dng rt rng ri trong cc phng th nghim. T

sy thng c nhit nh hn 200oC, c tc dng sau:

- Dng lm kh cc dng c thu tinh,

- Loi nc hydrat ho hoc nc ht m ca mt s mui hoc cht ht m

- Lm kh mu trc khi nung, lm kh mt s loi mu phn tch.

Nguyn l cp nhit cho t sy l s dng si t niken-crom, c b phn

iu chnh nhit mong mun. Ngi phn tch cn phi cp in cho t sy

ng (220V hoc 110 V), ng thi t nhit sy cn thit.

Tuyti cm sy cc cht dchy, ntrong t sy.

1.3.1.3. L nungTrong phng th nghim ho phn tch, c nhiu loi l nung, tuy nhin

ngi ta chia ra lm ba loi chnh tu theo nhit ti a c tht ca chng.

- Loi l nung c tht 800oC - 1000oC. Loi ny thng dng si t

niken-crom qun xung quanh mt hp lm bng vt liu chu la (silicat), cn

gi l l muf. iu chnh nhit , ngi ta s dng cp nhit in (to ra

mt dng in rt nh), ni vi cc r le cn thit v mt ngun cung cp in

p. y l loi l nung ph thng tht v n d trang b, tuy nhin n cng chn ch v nhit , mt s hp cht khng th phn hunhit ny.

- Loi l nung t 11000C-1200oC. L loi l muf thng thng nhng

cc vt liu ch to l cng c bit hn chu nhit cao hn. Si t cho

loi l ny l hp kim c bit Th d hp kim Tantan. tng cng hiu qu

nung cc nh ch to c gng sp xp si t gn vi vt nung hn.

- Loi l nung c nhit 1350oC - 1400oC. y l loi c ng cp v

nhit hon ton khc hai loi trn. Lc ny khng th dng cc si t thng

thng, phi dng cc thanh t bng vt liu l hp cht silic, l cc thanh

cacbua silic. Vt nung thng c xp vo ng hnh trt gia cc thanh

cacbua silic. Hnh 3 l hnh nh l nung trong PTN .


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L nung l thit b quan trng hng u trong phn tch, n c s dng vo

cc mc ch sau:

+ Nung cc sn phm phn ng, a cht phn tch v dng cn, th d

chuyn Fe(OH)3 v dng Fe2O3, a Al(OH)3 v Al2O3 v.v.

+ Phn hu mu theo phng php kim chy, l cc mu khng tan trong

cc axit v c, th d cc mu silicat, i vi cc mu ny, ph mu theo li

kim chy l mt cch d lm v rt c hiu qu.

Hnh 3: L nung trong phn tch trng lng

L nung l thit b d s dng, tuy nhin phi tun theo cc quy tc sau:

-

Nung trc chn nung trong iu kin tng t khi nung mu n trnglng khng i.

- Lm kh mu bng cc thit b ph hp (t sy hoc bp in) v a vo

chn nung bit trng lng.

- S dng chn nung ng loi, ph hp v nhit v yu cu ca cht phn

tch. Tu theo nhit cn thit phn hu mu, chn chn nung c nhit

cho php ph hp. Th d chn s chc nung n 1000oC, chn thch

anh chu c n 1200oC v.v. Chn nung cng phi ph hp vi cht phn

tch, trnh nung cc cht c th b nhim bn do chn nung gy ra hoc

ngc li cht phn tch c th lm hng chn nung, c bit l cc chn

nung bng kim loi qu (Au, Pt).

Tuyt i trnh:


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- Nung cc ho cht khi phn hu to thnh cc cht c hi cho l nung

v ngi s dng, th d nung cc mui nitrat, cc mui Hg v.v.

- Lm ho cht c bit l cc cht kim ra l nung. Chng ta bit

rng, muf l lm bng vt liu silicat, d dng b ph hu bi kim.

Nu iu ny xy ra, l nung xem nh b hng hon ton do nhit

cao kim lm chy muf l, ng thi vi vic phng in lm t

si t.

1. 3.1.4. Chn nung

C th chia chn nung c s dng trong phng th nghim thnh hai loi

chnh l chn nung h silicat v chn nung kim loi (hnh 4).

- Chn nung h silicat bao gm chn s v chn thch anhChn s c thnh phn ngoi oxit silic t thch anh c thm oxit nhm t

cao lanh. Chn c th chu nhit ti 1000oC, bn vi cc axit v c , bn c

hc tt. Tuy nhin n chu kim km, khng th s dng chn s thc hin

phn ng kim chy.

Chn thch anh c thnh phn l SiO2 nng chy cho nn rt bn vi axit,

hn na n c th chu nhit ti 1200oC. Cng nh chn s, chn thch anh

khng chu c kim c bit l phng php kim chy. Mt s kim loi nngchy lm hng chn thch anh nh l Al, Cu, Ag v.v.

Hnh 4 : Chn nung

a- Chn sth p ; b- chn scao; c- chn platin


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- Chn nung kim loi: gm 5 loi chnh

+ Chn st chu c hc tt, khng chu axit, tuy nhin chu kim tt,

thng c s dng cho mc ch kim chy. St c nhit nng chy

1539oC cho nn n hon ton p ng c cho phn ng kim chy. Tuy nhin

nhit cao st b ph hu, hnh thnh cc lp oxit, mt phn tan trong kim,

v vy i vi cc chn mng thng s dng mt ln.

+ Chn niken c nhit nng chy thp hn nhit nng chy ca

st khng nhiu (1455oC), tuy nhin niken bn hn st nhit cao, c th

dng li c.

+ Chn vng c nhit nng chy tng i cao (1000o

C), tri vi ccho cht, s dng cho phn ng kim chy, tuy nhin cng cn trnh cc ho

cht c hi lm hng chn, l nc cng thu, c bit cc kim loi c kh

nng to hp kim.

+ Chn bc chu axit km hn chn vng (Ag tan trong HNO3, trong

khi Au ch tan trong cng thu). Chn bc cng c s dng thc hin

phn ng kim chy, tuy nhin nhit chu c n 900oC. Nhng iu cn

trnh i vi chn vng cng p dng i vi chn bc.+ Chn Platin l chn tt nht trong cc chn kim loi, n c th chu

nhit ti 1770oC, n chu cc axit v c rt tt, k c HF, cho nn ngi ta

thc hin phn ng xc nh thnh phn Silic trong cc mu silicat khi cho tc

dng vi HF trong bt Pt. Tuy nhin n b tan trong nc cng thu, v vy c

4 im cn trnh khi s dng chn platin :

- Khng cho chn Pt tip xc vi cng thu

- Khng nung cc kim loi Cu, Fe, Bi, Pb, v hp cht ca n trong

chn Pt v chng to hp kim vi Pt khi nung nhit cao.

- Khng thc hin phn ng kim chy c cht oxy ho trong chn Pt v

n cng b ho tan trong iu kin .


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- Khng t chn Pt trn ngn la c nhiu mui than, khi to cacbua

Pt lm hng chn.

1.3.1.5. Giy lc v kthut lc kt ta

Giy lc l loi giy c bit c kch thc mao qun nht nh cht lng

c thi qua v c mn thch hp. Trong phn tch trng lng, ngi ta lc

kt ta bng giy lc khng tn/ tro. Giy lc khng tn l loi giy khi t

chy, khi lng cn li t 0,00003-0,00008g tro tu theo tng loi. Thc t

trng lng ny khng nh hng ti kt qu phn tch ( chnh xc ca cn l

0,0001g)

Giy lc khng tro c sn xut t si xenlulozc x l vi axit

clohyric HCl v axit flo hidric HF loi b cc kim loi v silic. Sau dngamoniac trung ha cc axit ny. Phn d mui amoni cn li trong nhiu giy

lc c thc xc nh bng phng php Kjeldahl.

C nhiu loi giy lc khng tro (nh giy lc bng xanh, bng trng,

bng vng, bng ) c s dng cho mc ch lc cc loi kt ta c kch

thc khc nhau. Nh sn xut thng nh du cc loi giy lc theo mu sc

ca vhp ng giy lc hoc bng bng dn trn hp.

- Giy lc bng xanh: rt mn, chy chm, dng lc cc kt ta kchthc ht nh

- Giy lc bng trng, bng vng: mn va phi, tc chy trung bnh

- Giy lc bng : l to, chy nhanh, dng lc cc kt ta kch thc

ln, cc kt ta v nh hnh.

Nhc im ca giy lc l khng c kch thc l r rng nn ngy nay

nhiu phng th nghim s dng mng lc polime c kch thc l chnh xc

hn.

Ngoi ra, ty mc ch lc ly kt ta hay lc b kt ta ly nc lc m

cn gp giy lc theo cc kiu khc nhau. Chng ta c th gp giy lc theo hai

cch hoc gp giy lc dng rnh (hnh 5a) hoc dng cn (hnh 5b). Dng rnh

s cho php cht lng i qua giy lc nhanh chng do c din tch b mt ln


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hn khi gp giy lc dng hnh cn. Tuy nhin, giy lc gp dng hnh cn li

cho php tch cht rn d dng hn.

(a) Gp giy lc ly nc lc (b) gp giy lc ly kt ta

Hnh 5. Giy lc c gp dng rnh (a) v dng cn (b)

(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (k) (l) (m)

Hnh 6. Cch gp giy lc dng rnh


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Cch gp giy lc:

Hnh 6 ch ra cc bc gp giy lc dng rnh. u tin ta gp hnh trn

thnh mt na (a), ri mt phn t (b, c). Cn thn vi cc np gp, khng mit

qu mnh vo cc rnh bi s lm giy lc c th b rch sau . Tip tc cc

np gp tng s rnh ca giy lc (d-h). Chng ta thy giy lc c gp nh

hnh ci qut giy (i). Cui cng ta mchic qut giy ra v s c giy lc dng

rnh (m).

i vi cch gp giy lc dng cn, u tin ta cng gp thnh mt na, ri

thnh mt phn t nh vi giy lc dng rnh. By gigiy lc ca chng ta

c gp li thnh bn lp, mn thnh dng hnh cn (ging nh chic nn

vy) v iu chnh sau cho tht va kht khi t vo phu. lc tt, cn chn tgiy lc c kch thc ph hp, mp tgiy lc cch

mp phu khong 0,5cm. Sau khi t giy lc vo phu, dng bnh nc ct

thm t u tgiy sau y nc ct vo phu cho chy. Nu giy lc

t ng, cung phu sc lp y nc tin hnh lc ngay khi cung phu

vn cn y nc. Chnh ct nc ny lm gim p sut thu tnh phn cung

phu v v vy tng nhanh qu trnh lc.

1.3.2. Dng co thtch v cch sdng

1. 3.2.1. Cc loi dng cng, ong, o thtch dung dch:

C rt nhiu loi dng c thy tinh dng trong phng th nghim phn tch

phc v cc mc ch khc nhau. C loi ch dng ng v bo qun nh

chai, l, bnh; c loi dng un nng nh cc c m, bnh nn (hay bnh

tam gic) chu nhit hoc dng ly th tch khng cn chnh xc nh cc, ng

ong, bnh nn... Cn cc dng c thy tinh dng o th tch l cc dng c

dng ly th tch chnh xc dung dch nh gm bnh nh mc, pipet vch

hoc pipet bu, buret . Ty theo mc ch s dng m dng cc loi dng c

o th tch khc nhau. Th d, ly chnh xc th tch dung dch chun gc,

nht thit phi dng pipet bu, hoc micropipet, nu khng cn tht chnh xc c

th dng pipet chia vch. Dng c chun l buret, nu c nhiu php


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chun , s dng buret tng s thun li hn. Bnh nh mc l dng c

pha dung dch gc c nng chnh xc, n cn c s dng pha cc dung

dch mu phn tch v.v. ly cc dung dch m, pha ch cc dung dch c

nng khng cn chnh xc s dng ng ong thun li hn.

Hnh 5 minh ha mt s dng co th tch trong PTN.

1: Buret 2 : Buret tng 3 : Pipet

4 : ng ong 5 : Bnh nh mc

Hnh 7: Cc dng co thtch trong phn tch thtch

Cc dng co th tch c hiu chnh bi nh sn xut c ghi trn

nhn vi hai loi TD v TC ghi r th tch chnh xc iu kin chun 200C v

1 atm. Nu nh sn xut ghi trn pipet l TD (to delivery) chng ta c th dng

ly th tch dung dch m khng cha dung dch trong cc loi dng c thy

tinh khi c k hiu TD. Khi ly th tch trong pipet c k hiu TD, khng bao


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199

gi thi ly git dung dch cui cng hoc trng ra ly git dung dch

ny. Vi bnh nh mc c k hiu TC (to contain), khng c php dng

ong th tch (Th d bnh nh mc 50 ml) ri chuyn sang bnh khc. Nhng

dng c ny thng dng pha dung dch chun hoc gi dung dch thi gian

ngn. Nu l pipet c ghi TC th khi ly dung dch cn ly c git cui cng,

thng thng l cc Gilson pipetman ly th tch qua cc u tip (ch khng

dng ming thi).

Vi cc dng c thy tinh, chnh xc php o th tch thng c th

hin dung sai (tolerance) i km trn thnh bnh ti 200C (hnh 8), hoc c

ghi r loi dng c (A hay B, AS v ngi th nghim c th tm c dung sai

qua ti liu tham kho). Khi lm th nghim, cn ghi r li cc ga tr dung sainy tnh ton khng m bo o (KB) (uncertainty) v bo co vo kt

qu phn tch.

Hnh 8: Mt sloi pipet v bnh nh mc c ghi km dung sai


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Cn ch cc dng c thy tinh thng u d b kim mnh n mn, b

HF ph hy, d b gin nkhi un nng v d vkhi va chm mnh. Do ,

tuyt i khng un nng cc bnh nh mc, sy cc pipet, buret

Cng cn ch , iu kin th nghim khng phi iu kin chun nh

nh sn xut ghi trn nhn mc th th tch dung dch cha trong cc dng co

th tch khng ng nh nh sn xut ghi. Nu khng tin hnh hiu chnh li

th tch dng co (m cn trng lng nc cha trong dng d thy tinh l

cch ph bin) trc khi th nghim s c th gy ra sai s h thng php phn

tch.

1. 3.2.2. Cch sdng mt sdng c cbn trong chun

* Bnh nh mc:c dng pha ch cc dung dch c nng xc nhbng cch thm nc ct n vch mc. Khi nh mc, trnh tip xc bng tay

vo bu bnh v nhit s truyn t tay vo thnh bnh lm thay i dung tch

bnh. Khi lm y bnh nh mc cn t bnh v tr bng phng v c chiu

sng r sao cho mt cong pha di ca dung dch chm vo vch chia.

* Pipet: dng ly chnh xc th tch dung dch. Khi thao tc vi pipet trnh

nm c tay vo pipet v nhit t tay s lm thay i th tch ca cht lng trong

pipet. Khi ly dung dch bng pipet, tay cm u trn ca pipet bng ngn ci vngn gia ca tay thun ri nhng u di ca pipet vo dung dch (gn y

bnh). Tay kia cm qu bp cao su, bp li ri a vo u trn ca pipet ht

dung dch vo pipet n khi dung dch trong pipet cao hn vch mc 2-3 cm.

Dng ngn tay tr bt nhanh u trn ca pipet li cht lng khng chy khi

pipet. Dng tay khng thun nng bnh ng dung dch ln, iu chnh nh ngn

tay tr cht lng chy t t ra khi pipet cho n khi mt cong pha di ca

cht lng trng vi vch ca pipet th dng ngn tay tr bt cht u trn ca

pipet l v chuyn pipet c cha mt th tch chnh xc cht lng sang bnh

chun . Khi ly dung dch v khi cho cht lng chy khi pipet cn gi cho

pipet v tr thng ng. Khi cht lng chy xong cn chm nh pipet vo phn


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bnh khng c dung dch (hnh 9) nhng tuyt i khng thi git dung dch cn

li trong pipet (nu thnh pipet c ch thch l loi TD).

a- Ht dung dch ln

b- iu chnh mc cht lng trong

pipet

Hnh 9: Cch sdng pipet

a- buret b- thao tc khi chun

Hnh 10: Cch sdng buret

Buret: Khi lm vic vi buret cn kim tra cu kha buret c m bo kn v

trn, nu cn th bi kha vi mt lp mng vaselin tng kn v trn. Kpburet vo gi buret v tr thng ng. Trc mi ln chun cn trng buret

bng chnh dung dch sng trong buret v phi dung dch vo buret ti

vch 0 pha trn v ch lm y c phn cui v c kha buret. Khi c th

tch buret, mt phi v tr ngang mt cong pha di dung dch trong sut

hoc phn trn mt li vi dung dch khng mu. Khi tin hnh chun phi

cho dung dch chy khi buret t t tt c cht lng chy ra ht khi buret

v sau 30s k t khi kha dung dch mi c kt qa. Cui qu trnh chun

phi nh tng git dung dch v lm vi ln ly gi tr trung bnh. Php chun

c coi l kt thc khi hiu th tch gia cc ln chun song song khng

qu 0,1 ml.


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(a) np dung dch

vo buret

(b) Kim tra xem

c cn bt kh

kha van khng

(c) ra u buret

bng nc ct

(d) lm sch v

kh buret trc

khi chun

Hnh 11. Cc thao tc vi buret trc khi chun

* Bnh nn v cch ly dung dch chun : (ch dng nc ct trng,

khng c dng dung dch cn ly trng bnh nn).Chng ta s dng pipet ly dung dch chun hoc dung dch phn tch

vo bnh nn (hnh 12).

(a) trng pipet

bng chnh dung

dch cn ly

(b) lau pha ngoi

pipet bng giy

thm

(c) pipet thng

ng v nghing

bnh nn dung

dch chy vo

(d) Tia nc ct

xung quang bnh

nn m bo tt

c th tch chnh

xc dung dch

ly c phn ng

vi cht chunHnh 12. Cc thao tc ly dung dch vo bnh nn bng pipet

Cch tin hnh chun :

- Tay khng thun cm kha van (hnh 13a)

- Tay thun cm bnh nn (hnh 13b)


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203

- Chun vi tc nhanh trc im tng ng mt vi ml

- u buret chm vo bnh nn (hnh 13c)

- Tia nc ct xung quanh dung dch ca cht chun nu c bm trn

thnh ca bnh nn sc i xung (hnh 13d)

- Khi gn n im tng ng chun vi tc chm

- Du hiu kt thc chun l khi dung dch va chuyn t mu A sang

mu B

(a) (b) (c) (d)

Hnh 13. Cc thao tc trong qu trnh chun

1.3.3.My o pH v cch sdng my o pH

My o pH l thit b dng xc nh nng ion H+ thng qua vic o

in th gia hai in cc gm in cc ch th l in cc thu tinh v in cc

so snh ngoi.

in cc thu tinh c lm t thy tinh Corning mang nhn hiu 015

thng c dng ch to mng chn lc vi ion H+. Thnh phn ca n gm

22% Na2O; 6 % CaO; 72% SiO2. Khi nghin cu cu trc ca mng thu tinh

ny ngi ta thy v bn cht mng tng t nh mt mng khung silicat SiO42-

trong mi nguyn t Si lin kt vi 4 nguyn t oxi, mi nguyn t O trong

ng thi lin kt vi 2 nguyn t Si. gia nhng khe ca mng SiO42-

c nhng cation (th d Na+ v Ca2+) c in tch trung ho vi cc nhm

SiO42- c trong mng. Cc cation c mt in tch nh Na+ l nhng cation linh

hot sng vai tr ch yu trong vic dn ion qua mng theo cch trao i

ion.


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in cc thy tinh gm ng thy tinh pha di l bu thy tinh c thnh

mng, trong cha dung dch c hot H+ cnh 0,1 M v in cc so snh

trong Ag/AgCl (hnh 14).

Hnh 14: S cu to in cc

thy tinh

Hnh 15:ng chun xc nh pH da

trn gi tr tho c ca in cc

S cu trc thnh phn ca in cc thu tinh c thc biu din nh sau:

AgAgCl bo ho, HCl 0,1MMng thu tinh

Khi mng thu tinh tip xc vi dung dch cn o, do n ngn cch hai

dung dch c nng ion H+ khc nhau th hai bn b mt ca mng thu tinh s

b hidrat ho, trong cc lp hidrat ho mng khung silicat ng vai tr nh

mt ionit. Phn ng trao i ion trn ranh gii tip xc ch xy ra chn lc vi

ion H+ ca dung dch v ion Na+ ca mng thu tinh:

H+

dd + Na+

Th Na+

dd + H+

ThHng s cn bng ca cn bng trn ln n mc b mt lp thu tinh

ngm trong dung dch hon ton tr thnh b mt ca axit silixic (H+Th). Mt

ngoi li vi iu ch xy ra cc dung dch kim c nng ion kim loi

kim qu cao (tc l nng ion H+ v cng nh).


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Khi mun o pH ca dung dch no th nhng cc thu tinh vo dung

dch o v nhng thm vo cc so snh ngoi (cc calomen) o th ca

cc thu tinh vi cc so snh. S ca nguyn tin ho l:

AgAgCl bo ho, HCl 0,1MMng thu tinhH3O+o a2M KCl bo ha,

Hg2Cl2Hg

Gi s th cn bng ca in cc thu tinh so vi in cc so snh

calomen l E ct th gi tr th ny bao gm 3 thnh phn:

- Th tip xc trn ranh gii mng thu tinh v dung dch, c th gi l th mng

Em, th ca in cc so snh nhng vo dung dch ni, EAg/ AgCl v mt th c

gi tr nh Ebc gi l thi xng, n thay i chm theo thi gian v tn

ti l do 2 b mt ca thu tinh khng hon ton ging nhau:Ect. = E Ag /AgCl + Em + Eb (1)

Th mng Emc xc nh bi biu thc sau:

Em = 0,059 loga

a

1

2

(2)

Nh vy, thin cc thu tinh l hm s ca pH dung dch phn tch:

E ct. = E0 + 0,059loga

a

1

2

= E.T - 0,059 pH (3)

Trong E0 l hng s v ph thuc vo nhit . Thc t cc in cc khng

tun theo hon ton phng trnh Nerst nn cn a thm h s nhy s. Khi

phng trnh c dng y=a+bx thay cho phng trnh

Ect= E.T-0,059 s.pH

E v s c th tm c khi o th Ect ca hai hoc nhiu dung dch c gi tr pH

bit trc v dng ng chun biu th s ph thuc ca tho c theo pH

(hnh 15) . Da trn ng chun ny s xc nh c pH ca dung dch cha bitkhi o c gi tr E. Gi tr pH sc tnh ton v a ra mn hnh hin s.

Cn ch phng trnh (3) ch nghim ng trong cc dung dch c pH

nm trong khong 1 n 11 i vi cc in cc thu tinh tt nht. Thng

thng trong cc dung dch qu axit hoc qu kim th s tuyn tnh ca phng


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trnh (3) khng cn c tun theo na, ta ni in cc thu tinh mc sai s axit

v sai s kim. Cht lng ca in cc thu tinh ph thuc nhiu vo thnh

phn v k thut ch mng thu tinh to in cc.

Vi cc my o pH hin nay, in cc o pH c tch hp cin cc

thy tinh v in cc so snh ngoi nn khi o ch cn nhng in cc o pH

vo dung dch cn o. Trn mn hnh my pH s hin ln gi tr pH o c vi

chnh xc ti 0,01pH. Sau mi php o cn ra in cc bng nc ct

hoc nc eion.

Chun ha my o pH.

y l thao tc cn thit trc khi o pH (hoc hng tun) hiu chnh cc

sai s gy ra do s thay i yu t ha hc ca in cc lm cho th mngkhng lp li gi tr trc khi xy dng ng chun. Cc dung dch m

c pH = 4.01; 7.00 v 10.01 thng c dng. C th hiu chnh theo phng

php hai im hoc 1 im v ty vng gi tr pH cn o.

Bo qun in cc pH: Khi khng s dng in cc pH cn c bo

qun trong dung dch trnh sehirat ha mng thy tinh. in cc cn

c nhng trong dung dch KCl 3,8M (tuyt i khng nhng trong nc) v

vn cht nt bo qun.

1.4. Pha ch cc dung dch chun v dung dch thuc th.

1.4.1. Pha chdung dch chun tlng cn ca cht gc:

- Mt cht c xem l cht gc trong phn tch nu tha mn cc yu cu sau:

+ C tinh khit phn tch (P.A. )hoc tinh khit thuc th (P.R.)

+ C thnh phn ng ng vi cng thc ha hc k c nc kt tinh

+ Cht gc v dung dch pha t cht gc phi bn

+ Khi lng mol phn t cng ln cng tt.

- Mt s cht gc thng dng nh H2C2O4.2H2O, Na2B4O7.10H2O,

Na2CO3 K2Cr2O7, KIO3

pha dung dch chun t cht gc, ngi ta cn mt lng xc nh ph

hp cht gc trn cn phn tch c chnh xc 0,0001 hoc 0,00001g, ho tan


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207

nh lng lng cn trong bnh nh mc c dung tch thch hp ri pha long

bng nc ct hoc dung mi thch hp ti vch mc.

Th d pha dung dch chun H2C2O4 0,02500 M (M=126,066), trc tin

cn tnh khi lng H2C2O4.2H2O cn thit pha chc 250 ml dung dch

H2C2O4 nng 0,025M theo cng thc:

m= 0,250x0,025x126,066 = 0,7879gam.

Cn 0,7879 gam H2C2O4.2H2O c tinh khit phn tch trong cc cn trn

cn phn tch, chuyn cht rn qua phu vo bnh nh mc 250,0 ml (hnh 14).

Trng cc cn 3 ln bng nc ct vo bnh nh mc. Thm khong 150ml

nc ct na v lc kcho tan ht sau thm nc ct n vch mc, lc k

trn u, ta c dung dch chun axit oxalic 0,02500M.

Hnh 16. Pha dung dch axt oxalic

Ch : C th cn chnh xc trn

cn phn tch mt lng axit oxalic c

gi tr khong 0,8 gam, ha tan vo

bnh nh mc 250,0 ml v tnh nng

dung dch chun thu c theo

lng cn va cn, ghi chnh xc

n 4 ch s c ngha.

1.4.2. Pha chdung dch chun tfixanal

Nguyn tc: Dng nhng lng cht rn hay nhng th tch cht lng tinh khit

ho hc c cn, o chnh xc ng trong ng fixanal (hnh 17) pha cc

dung dch c chun xc nh.

Fixanal l ng bng thu tinh c cu to c bit: c hai v tr rt mng

gip ngi phn tch c th d dng dng a thu tinh chc thng, sau chuyn ton b cht trong fixanal vo bnh nh mc bng tia nc ct sau

ho tan v nh mc.

Hnh 17: Fixanal cha cht chun

1 2


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211

Vi buret hoc pipet vch, trng

hp ny vch cht lng nm gia 1,4

v 1,5 ml. Nu gi thit n c 1/5

khong chia (0,1 ml) th ghi gi tr l

1,42 ml.

Vch cht lng buret ny

c th ghi th tch l 42,25

ml (tc l ri vo na vch

chia)

Hnh 18: Cch c thtch cht lng trong buret

* i lng o gin tip: y l kt qu tnh c t cc i lng o trc

tip nhcc php cng tr, nhn chia, logaritnh kt qu cn khi lng mu

t hai ln cn cc v cn c cc ln mu, tnh nng t khi lng cn v th

tch dung dch

- Php tnh cng v tr : tnh kt qu cui cng sau lm trn s v ghi sc ngha sau du phy theo gi tr no c t s c ngha nht sau du phy.

- Php nhn v chia: kt qu ca php nhn v php chia c lm trn s

sao cho n cha s c ngha nh gi tr c t s c ngha nht.

- Php tnh logrit v ngc logrit:

+ logrit: ly cc ch s sau du phy bng tng cc s c ngha trong s

ban u ly logarit

+ ngc logarit: ly cc s c ngha bng s cc ch s sau du phy.Th d: a) 3,4+0,020+7,31=10,73=10,7 y v 3,4 l s ch c 1 s c

ngha sau du phy nn trong kt qu ch ghi 1 s c ngha sau du phy.

b)%5470578,88%100.

1689,1

05300.0.5481,0.63,35=

=88,55%

Trong biu thc ny c 3 gi tr c 4 ch s c ngha nn kt qu cui cng

phi c ghi l 88,55%

c) log(9,57.104)=4- log 9,57= 4,981 (gi tr 4 c 1 s c ngha;

gi tr 9,57 c 3 s c ngha nn kt qu cui cng ly 3 ch s c ngha sau

du phy )

log(4,000.10-5)=-5+ log4,000= -4,397940=-4,3479

Antilog (12,5)=3,162277.1012=3.1012= 30.1011= 300.1010


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212

( gi tr 12,5 c 1 ch s c ngha sau du phy nn s cui cng ca php

ngc logarit ch c 1 ch s c ngha

Ch : Trong qu trnh tnh ton, i khi gi tr thu c ca i lng o

gin tip li c dng tnh ton trong cc php tnh khc na. Do vy,

trnh sai s gy ra do vic lm trn s, chng ta c th gi li thm 1 hoc nhiu

hn mt s khng c ngha na trong kt qu tnh, khi s ny c vit thp

xung pha di (subscipt) ca s c ngha. S khng c ngha c ghi thm

ny cng nhc chng ta bit s con s c ngha cn dng trong kt qu tnh cui

cng.

Trong th d 3 trn c th ghi cc kt qu cc s nh sau: a) 10,73 ;

b)88,5471.5.2. Biu din kt qu phn tch

Cc kt qunh lng (nng hoc khi lng, phn trm) trong phn tch

ha hc thu c khi bo co bao gicng l kt qu trung bnh s hc ca nhiu

ln th nghim lp li gim sai s ngu nhin. Do , cn ghi ng s c ngha

cho gi tr trung bnh. Tuy nhin, gi tr thc t hm lng cht phn tch khng

phi gi tr thc m tn ti trong khong x x. i lng x c gi l

khng m bo o (uncertainty). C rt nhiu cch tnh i lng ny v s

cp trong ti liu nng cao chuyn kho khc.


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214

Cht ch th metyl da cam (khong pH i mu 3,1 n 4,4) c thc s dng

xc nh im tng ng ny.

im tng ng th hai, khi kt thc phn ng (5-3), pH ca dung dch

c tnh theo pH ca cht lng tnh Na2HPO4.

HPO42-

H+ + PO43- Ka3

HPO42- + H+ H2PO4

-

pH = (pKa2 + pKa3) = (7,20 + 12,35) = 9,80

Do c th dng cht ch th phenolphtalein (khong pH i mu 8,0 n 9,6)

xc nh im tng ng trong qu tnh chun hn hp.

1.1.1. 2. Xc nh nng dung dch NaOH s dng lm dung dch

chun.

Nng ca NaOH trong dung dch c xc nh thng qua dung dch

chun kalihyddro phtalat.

Axit phthalic (C4H6(COO)2 l axit hai nc c Ka1 = 1,12.10-3 v Ka2 =

3,9.10-6. Kali hydro phthalat, thng c gi n gin l KHPh, l mt hp

cht thuc loi mui axit, dng bt trng, tinh th khng mu, d tan trong nc.

N c tnh axit v thng c s dng nh l mt cht chun gc trong chun

acid-bazv n l hp cht n nh, bn trong khng kh, c khi lng molln (204,233g/mol), d cn chnh xc. Tuy nhin, n c tnh ht m nh, nn

thng c gi trong bnh ht m trc khi em s dng. Ngoi cc tnh cht

va nu trn, pH ca KHPh trong dung dch rt n nh nn n cng c

dng nh mt cht gc chun ha thang pH.

Trong nc KHPh phn ly hon ton thnh cation K+ v anion hydro

phthalat HPh c tnh axit nn KHPh c s dng chun mt baz, v d

nh NaOH.


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Khi chun KHph vi NaOH, ti im tng ng, pH ca dung dch c

tnh theo pH ca mt a bazyu Phthalat. Vi Kb1 v Kb2 tng ng ca bazPhthalate l 2,5641.10-9 v 8,9.10-12 th c th coi s phn ly ra OH

- ca a baz

phathalat do nc th nht l ch yu. Gi s nng ca baz phathalat l

0,10M, khi

Ph2- + H2O HPh- + OH-

Cb 0,1-x x x

b

xK

, x

=

2

10 1

x, x

2

0 1

= 2,5641.10-9

x = [OH-] = 10-4,80 M pOH = 4,80 pH = 9,20

pH ti im tng ng l 9,20 nn cht ch th thch hp xc nh im

tng ng ca phn ng chun c th l phenolphthalein.

Cng c th s dng cht gc axit oxalic (H2C2O4.2H2O) thay cho KHPh vi

cht ch th phenolphtalein chun .

1.1.2. Tin hnh th nghim

1.1.2.1. Pha chdung dch chun KHPh

pha ch 250 ml dung dch chun KHPh 0,05000M ( =

204,233g/mol) cn tnh c khi lng KHPh cn ly theo cng thc

gamxxm 5529,2233,204250,005000,0 ==


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s bin i t ngt v pH th pH ca dung dch trong trng hp ny c theo

di bng thit bo pH ca dung dch trong qu trnh chun . Bng cch theo

di chnh xc gi tr pH trong qu trnh nh phn cng c th xc nh c cc

hng s phn ly axit ca H3PO4.

1.2.2.Cch tin hnh-Dng pipet ly 1,00 ml dung dch hn hp hai axir vo cc thy tinh dung

tch 50 ml v dng pipet chia vch thm 19,0 ml nc ct.

- Cho con t vo cc

- Mnp bo qun in cc pH, nhc in cc ra, ra in cc o pH bng

nc ct vo mt cc khc v lp vo ho (hnh 19).

- Np y dung dch chun NaOH 0,05000M vo buret v lp vo gi buret( nh hnh 19). Cn chc chn in cc pH khng chm vo con t.

Hnh 19: S thit b chun in thxc nh HCl v H3PO4

Thm t t tng lng nh ( ban u 0,5 ml, gn n im tng ng thm

0,1 ml) dung dch chun NaOH t buret v ghi s bin i pH vo th tch

NaOH thm vo.

Cn ch khng thm tip NaOH khi gi tr pH > 11,5 v c th lm hng in

cc thy tinh.

- Sau khi chun xong, ly in cc ra khi dung dch, ra bng nc ct.

in cc trong nc ct t nht 15 pht trc khi tin hnh chun ln th


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2. Chun ha li my o pH bng dung dch m pH=7 trc khi tin hnh php

chun khc.

1.3. Bo co kt qu v tho lun- Tnh nng HCl v H3PO4 trong c hai th nghim v so snh kt qu.

- Vng chun biu th s ph thuc pH vo th tch dung dch NaOH(ml).

- Xc nh hai im tng ng trn ng cong chun v xc nh th tch

NaOH ti mi im tng ng.

- Tnh gi tr pKa2 ca H3PO4 v kim tra tnh ph hp ca phng trnh tnh

pH dung dch m ( phng trnh Henderson-Hasselbalch: pH= pKa-log

(Ca/Cb)) bng cch tnh mt sim khc trn ng nh phn. So snh gi tr

pK2 thu c t thc nghim v ti liu tham kho.

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Cam Nag Thi Nghiem Hoa Hoc