Phn th hai: Thc nghim ha hc

Phn th hai: Thc nghim ha hc

PGS TS T Th Tho

Khoa Ha hc Trng i 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 c bn 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 c bn 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 t c.

1.1. An ton trong PTN:

Tun th ng 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 ch dn 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)

Phi bit cc k hiu cnh bo v c, 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 mnhCht 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 th nh 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)

+ 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

+ C s l 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 ch gii thiu mt s k thut c bn 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 c bn trong phn tch ha hc

1.3.1. Dng c dng trong phn tch trng lng

1. 3.1.1. Cn

Trong phn tch, nh gi chnh xc, ng ca mt php o nht thit phi thng qua cc dng c o 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 c bn ca php cn, cn g v cn 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 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 s b 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 c hc v in t.

Cn c hc 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.

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 tr li 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, c hng 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 c hc 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: Cn in t v nguyn l hot ng

Nguyn l hot ng ca cn in t nh sau:

Vt cn ko a cn xung vi lc l F = m(G, 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 s dng cn phn tch

1.Trc khi cn phi kim tra thng bng ca cn thng qua bt nc ca b 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 s dng cn c hc.

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 vi cn in t v thang o ch im 0 i vi cn c hc.

Nht thit khng xp vt cn ln cn qu gii hn ti trng ca n, s qu ti c th gy nn nhng bin dng hoc gy n cn; i vi cn hin s, hin tng trn lm chy cun y in t do khng th b tr c vt cn.

4. t vt cn gia a cn trnh dao ng (i vi quang cn) (do khi xut hin lc ly tm s nh hng n kt qu ca php cn).

5. ng kn t cn trc khi m hm cn/ bt cn, ch cc con s hin th n 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 th dn n n cn v dao cn b nh hng.

- Ch c th cn khi vt cn c nhit bng nhit ca khng gian xung quanh cn. Khi cn vt cn nng, dng khng kh gy ra do s chnh 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 vy 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 s giai on phn tch c lin quan vi nhau ch nn 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 ln a cn. Cn s dng cc cn, thuyn cn trong mi trng hp v cn hai ln: ln th nht cn cc cn, sau cn cc cn c cha cht cn cn v ly hiu s khi 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 b n mn, v sinh sch s cn v v tr xung quan cn sau khi cn xong. - Khi cn cht lng khng n mn, khng bay hi c th cn trc tip bng cch s dng l cn c nt y va kht.

- Khi cn cc cht lng bay hi v c tnh n mn cao th n phi c ng trong ng thy tinh kn. ng thy 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 tr li 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 v cht lng i ra. Mt th tch hiu chnh cho ng thy tinh c th l cn thit nu ng thy tinh ny c chia vch o th tch.

1.3.1.2. T sy

T sy l thit b c 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.

Tuyt i cm sy cc cht d chy, n trong t sy.

1.3.1.3. L nung

Trong 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 th t ca chng.

Loi l nung c th t 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 c hn ch v nhit , mt s hp cht khng th phn hu nhit 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 tr t gia cc thanh cacbua silic. Hnh 3 l hnh nh l nung trong PTN .

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 trng lng 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 ch c 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:

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 anh

Chn 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 nng chy lm hng chn thch anh nh l Al, Cu, Ag v.v.

Hnh 4 : Chn nung

a- Chn s thp ; b- chn s cao; c- chn platin

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 (1000oC), tr i vi cc ho 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 .

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 k thut lc kt ta

Giy lc l loi giy c bit c kch thc mao qun nht nh cht lng c th i 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 xenluloz c x l vi axit clohyric HCl v axit flo hidric HF loi b cc kim loi v silic. Sau dng amoniac trung ha cc axit ny. Phn d mui amoni cn li trong nhiu giy lc c th c 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 v hp ng giy lc hoc bng bng dn trn hp.

Giy lc bng xanh: rt mn, chy chm, dng lc cc kt ta kch thc 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 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.

T in

(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

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 m chic 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 gi giy lc ca chng ta c gp li thnh bn lp, m n 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 t giy lc c kch thc ph hp, mp t giy lc cch mp phu khong 0,5cm. Sau khi t giy lc vo phu, dng bnh nc ct thm t u t giy sau y nc ct vo phu cho chy. Nu giy lc t ng, cung phu s c 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 c o th tch v cch s dng

1. 3.2.1. Cc loi dng c ng, ong, o th tch 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 chun , s dng buret t ng 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 c o th tch trong PTN.

1: Buret 2 : Buret t ng 3 : Pipet

4 : ng ong 5 : Bnh nh mc

Hnh 7: Cc dng c o th tch trong phn tch th tch

Cc dng c o 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 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 sai ny tnh ton khng m bo o (KB) (uncertainty) v bo co vo kt qu phn tch.

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

Cn ch cc dng c thy tinh thng u d b kim mnh n mn, b HF ph hy, d b gin n khi un nng v d v khi 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 c o th tch khng ng nh nh sn xut ghi. Nu khng tin hnh hiu chnh li th tch dng c o (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 s dng mt s dng c c bn trong chun

* Bnh nh mc: c dng pha ch cc dung dch c nng xc nh bng 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 v ngn 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 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 s dng pipeta- buret b- thao tc khi chun

Hnh 10: Cch s dng 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. Kp buret vo gi buret v tr thng ng. Trc mi ln chun cn trng buret bng chnh dung dch s ng 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.

(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 chun

Hnh 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)

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 s c 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 s dng 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 s ng vai tr ch yu trong vic dn ion qua mng theo c ch trao i ion.

in cc thy tinh gm ng thy tinh pha di l bu thy tinh c thnh mng, trong cha dung dch c hot H+ c nh 0,1 M v in cc so snh trong Ag/AgCl (hnh 14).

Hnh 14: S cu to in cc thy tinhHnh 15: ng chun xc nh pH da trn gi tr th o c ca in cc

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

Ag(AgCl bo ho, HCl 0,1M (Mng 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 l i 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).

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 t in ho l:

Ag(AgCl bo ho, HCl 0,1M(Mng thu tinh(H3O+o a2M ((KCl bo ha, Hg2Cl2 (Hg 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 Eb c gi l th i 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 Em c xc nh bi biu thc sau:

Em = 0,059 log

(2)

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

E ct. = E0 + 0,059log = 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 th o c theo pH (hnh 15) . Da trn ng chun ny s xc nh c pH ca dung dch cha bit khi o c gi tr E. Gi tr pH s c 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 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 c in 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 mng khng 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 s ehirat 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 ch dung dch chun t lng 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 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 ch c 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 k cho 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 oxalicCh : 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 ch dung dch chun t fixanal

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

(Cc im 1, 2 trn hnh v l cc im cn phi c chc thng bng a thu tinh chuyn vo bnh nh mc)

Trn mi ng Fixanal ngi sn xut ghi r dung tch cn pha thu c nng nht nh. Th d Fixanal ng Na2B4O7.10H2O trn c ghi N/10 nh vy, ngi ta cn 19,0685gam Na2B4O7.10H2O pha thnh 1lit dung dch c nng 0,1N (khi nng dung dch Na2B4O7 l 0,05M).

1.4.3. Pha ch dung dch ca cc cht khng phi cht gc

Vic pha ch cc dung dch chun hoc thuc th t cc cht khng phi cht gc,nh cc cht ht m mnh, cc axit c, cc dung dch c nng ln c tin hnh vi lng cn trn cn k thut hoc th tch dung dch ly t ng ong, pha long bng dung mi thch hp trong ng ong, bnh nn hoc bnh nh mc sau phi chun li xc nh nng cc cht va pha nu dng n lm cht chun.

1.5. Tnh ton v biu din kt qu nh lng

1.5.1. S c ngha v cch ly gi tr gn ng

Mt gi tr s hc dng biu din kt qu o trc tip nh gi tr c c trn cn, c th tch buret, ghi gi tr pipet hoc kt qu tnh gin tip nh khi lng cht, nng , s khng c ngha nu khng bit chnh xc ca n, ni cch khc khng phi c ghi mt cch ty tin. Do vy, khi biu din kt qu cn phi ghi r tin cy ca s liu v cc s liu cn c lm trn ch chnh xc ca n ni cch khc s liu ch c cha cc s c ngha.

1.5.1.1. Khi nim s c ngha

S c ngha trong mt dy s l tt c cc s chc chn ng v s khng chc chn ng (s gn ng) u tin.

Th d: - Khi c th tch dung dch ng trong buret 50 ml (cp chia 0,1 ml), chng ta c th thy vch cht lng v tr ln hn 30,2 ml v nh hn 30,3 ml. Nu c th c on v tr vch cht lng cp chia khong 0,02 ml th c th ghi th tch l 30,24 0,02 ml. Trong th d ny, 3 con s u tin l s chc chn ng, s cui cng l s khng chc chn ng. Nh vy c th vit 30,24 ml hoc 0,03024 lit hay c hiu l kt qu c 4 ch s c ngha.

- Khi lng mt cht trn cn phn tch hin th n bn ch s sau du phy ( chnh xc n 0,0001gam) l 15,0980 gam. Trong gi tr ny s 8 l s chc chn ng, s 0 l s khng chc chn ng u tin. Nh vy, nu ghi 15,098 gam th c ngha s 8 s l s khng chc chc ng (ngi c s hiu cn trn cn phn tch c chnh xc l 0,001 gam). Nh vy, kt qu khi lng o c 15,0980 s gm 6 s c ngha, trong c 4 s c ngha sau du phy.

S c ngha c qui c nh sau:

+ Gm cc ch s t nhin 1,2,. 9

+ S 0 c th l s c ngha hoc khng phi l s c ngha tu thuc vo v tr ca n trong dy s.

Nu s 0 nm gia cc s c ngha l s c ngha.

Nu s 0 nm cui dy s th ch l s c ngha nu ng sau du phy.

Nu s 0 nm trc du thp phn th khng phi l s c ngha.

Th d : 25,24 c 4 s c ngha 0,15 c 2 s c ngha

15,05 c 4 s c ngha 360 c 2 s c ngha

0,0241 c 3 s c ngha 150,00 c 5 s c ngha

Mt th d khc, nu ghi th tch bnh l V= 2,0 lit th khi chuyn sang n v ml khng th ghi l 2000 ml (v trong con s ny ch ghi 1 s c ngha) m phi ghi l 2,0.103ml.

* Lm trn s: Khi bo co kt qu, ngi phn tch cn ghi ng s c ngha, tc l phi l loi b cc s khng c ngha trong kt qu. V vy, cn phi lm trn s.

Nguyn tc lm trn s l nu b cc s 6,7,8,9, th tng ga tr trc n ln 1 n v. Nu loi b cc s 1,2,3,4, th khng thay i con s ng trc n. Nu loi b s 5 th lm trn s trc v s chn gn nht.

Th d s 2,25 c lm trn thnh 2,2; 2,35 thnh 2,4; gi tr nng tm c t cng thc tnh l 0,035785 M th khi vit 4 ch s c ngha s l: 0,03578M

1.5.1.2. Cch ly gi tr gn ng

* i lng o trc tip: y l cc gi tr nhn c do c hoc o, m c t cc dng c o. S liu th nghim c ghi theo nguyn tc s cui cng l s gn ng v s trc s cui cng l s chnh xc.

Th d khi lng cn cc cn trn cn phn tch (c cp c l 0,0001 gam) l 1,3501gam th s 1 cui cng l s gn ng; cc bnh nh mc c ghi l: 100,0 ml; 250,0ml; mt pipet chia vch trn thnh nh sn xut ghi 10ml, ti 200C, dung sai ( 0,1 ml tc l gi tr th tch ghi ng s c ngha l 10,0 ml hay 5,0 ml. Mt pipet bu 25 ml, dung sai ghi ( 0,03 ml th th tch o c s ghi ng s c ngha l 25,00 ml. Ni cch khc, ch cn nhn vo cch ghi th tch c th bit cn phi s dng dng c no c chnh xc mong mun.

Mt im lu khc, khi c th tch trn buret hoc pipet vch cn xem v tr ca mt cong pha di cht lng m bo ng s c ngha. C th thy cch ghi s liu trong th d di y:

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 th tch cht lng trong buret

* i lng o gin tip: y l kt qu tnh c t cc i lng o trc tip nh cc 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 s c 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) =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( 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)88,547

1.5.2. Biu din kt qu phn tch

Cc kt qu nh lng (nng hoc khi lng, phn trm) trong phn tch ha hc thu c khi bo co bao gi cng 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. 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.

B. MT S BI TH NGHIM XC NH NNG CC CHT BNG PHNG PHP PHN TCH TH TCH

Bi 1: Xc nh nng HCl v H3PO4 trong cng hn hp

1.1. Phng php chun s dng cht ch th mu

1.1.1. Nguyn tc phng php xc nh:1.1.1.1. Nguyn tc xc nh hai axit trong cng hn hpHCl l axit mnh phn ly hon ton cn H3PO4 l axit yu phn ly 3 nc vi cc hng s phn ly axit Ka1 = 7,11.10-3, Ka2 = 6,32.10-8, Ka3 = 4,5.10-13. V vy c th s dng dung dch chun NaOH chun tng nc khi c mt ca cc cht ch th khc nhau xc nh nng ca chng.

Khi thm t t NaOH vo dung dch hn hp cn chun , nu la chn c ch th ph hp ton b HCl v nc 1 ca H3PO4 tham gia phn ng: NaOH + HCl ( NaCl + H2O

(1-1)

NaOH + H3PO4 ( NaH2PO4 + H2O

(1-2)

th th tch dung dch chun NaOH tiu tn (V1) chnh l phn NaOH tham gia c phn ng (5-1) v (5-2).

Sau nu thm tip NaOH phn ng s tip tc nh sau:

NaOH + NaH2PO4 (Na2HPO4 + H2O

(1-3)Bng vic chn ch th thch hp c th xc nh c th tch NaOH tiu tn (V2) v chnh l VNaOH phn ng vi H3PO4 trong hn hp. Khi (V1-V2) chnh l th tch NaOH tham gia phn ng vi HCl.

Nu qu trnh chun kt thc sau phn ng ( 5-2) th ti im tng ng th nht ny, pH ca dung dch c tnh theo pH ca cht lng tnh NaH2PO4 (gi s c nng 0,1M).

H2PO4- H+ + HPO42- Ka2

H2PO4- + H+ H3PO4

pH = (pKa1 + pKa2) = (2,15 + 7,20) = 4,68

Cht ch th metyl da cam (khong pH i mu 3,1 n 4,4) c th c 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-baz v n l hp cht n nh, bn trong khng kh, c khi lng mol ln (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.Nghe

1. c ng m

2.

3. T in - Xem t in chi tit

Khi chun KHph vi NaOH, ti im tng ng, pH ca dung dch c tnh theo pH ca mt a baz yu Phthalat. Vi Kb1 v Kb2 tng ng ca baz Phthalate 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

( = 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 ch dung 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

Cn KHPh trn cn phn tch mt lng c gi tr gn m (g) tnh trn cc cn vi chnh xc 0,0001g, cho vo cc c m 100ml sch. Sau thm khong 40-50ml nc ct, khuy cho tan bng a thy tinh. Chuyn ton b lng dung dch ha tan vo bnh nh mc (rt qua a thy tinh). Trng cc v a thy tinh 3-4 ln bng nc ct vo bnh nh mc. nh mc bng nc ct n vch mc. Lc tht u dung dch trc khi s dng.T lng cn thc t trn cn phn tch, chng ta tnh li nng dung dch KHPh pha (M).

1.1.2.2. Chun dung dch NaOH

Np dung dch NaOH va pha ch (t lng cn trn cn k thut, nng khong 0,05M) vo buret, ui ht bt kh, chnh n vch 0

Ly chnh xc 10,00 ml KHPh cho vo bnh nn, thm 3-5 git cht ch th phenolphtalein, dung dch khng mu

Chun bng dung dch NaOH cho ti khi dung dch chuyn mu t khng mu sang mu hng, ghi th tch NaOH tiu tn V (ml) Lm 3 ln, ly gi tr trung bnh tnh nng 1.1.2.3.Xc nh nng tng axit trong hn hp bng dung dch chun NaOH

Ly chnh xc 10,00 ml dung dch hn hp HCl v H3PO4 cn phn tch cho vo bnh nn c 250ml; thm 2-3 git dung dch cht ch th metyl da cam, lc u v chun bng dung dch chun NaOH c nng C(M) ( khong 0,05M) ti khi dung dch chuyn sang mu vng. Ghi s ml NaOH chun (V1). Thm tip vo dung dch ang chun ba git dung dch cht ch th phenolphtalein v tip tc chun ti khi dung dch chuyn sang mu hng. Ghi s ml NaOH chun ln 2 (V2). Lm 3 ln ri ly kt qu trung bnh nng ca hai axit.

Nng mol/l ca hai axit trong dung dch nh phn c tnh nh sau:

1.2. Phng php chun in th s dng thit b o pH

1.2.1. Nguyn tc:

C s php xc nh ging nh php chun s dng cht ch th mu ch khc l pht hin im im tng tng thay v dng cht ch th pht hin ra s bin i t ngt v pH th pH ca dung dch trong trng hp ny c theo di bng thit b o 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

- M np bo qun in cc pH, nhc in cc ra, ra in cc o pH bng nc ct vo mt cc khc v lp vo h o (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 th xc nh HCl v H3PO4Thm 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 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.

- V ng 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 s im khc trn ng nh phn. So snh gi tr pK2 thu c t thc nghim v ti liu tham kho.

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