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  • 1

    4500-O C. Phng php Winkler (APHA et al., 1995)

    1. Nguyn l

    Trong mi trng baz mnh, oxy ha tan (DO) trong nc s oxy ha ion Mn2+ thnh

    Mn4+ to kt ta nu.

    Mn2+ + 2OH- + O2 = MnO2 + 2H2O

    Trong mi trng acid v c s hin din ca ion I-, Mn4+ b kh thnh Mn2+ v gii

    phng I2 tng ng vi lng O2 c trong mu nc lc ban u.

    MnO2 + 2I- + 4H+ = Mn2+ + I2 + 2H2O

    I2 c gii phng ra s ha tan trong nc v c xc nh bng phng php chun

    vi dung dch Na2S2O3. H tinh bt c s dng lm cht ch th xc nh im

    dng chun (I2 to phc mu xanh vi h tinh bt).

    I2 + Tinh bt-I2 (xanh) + Na2S2O3 Na2S4O6 + NaI + H2O + Tinh bt (khng mu)

    2. Cc cht gy nhiu

    Cc cht oxy ha s oxy ha I- thnh I2 lm tng kt qu phn tch (nhiu dng). Cc

    cht kh th kh I2 thnh I- lm gim kt qu phn tch (nhiu m). Hu ht cht hu c

    b oxy ha trc khi M4+ b kt ta. Theo APHA et al. (1995), phng php wincler c mt s sa i loi b cc cht gy nhiu: (i) Phng php dng NaN3 (4500-O C.

    Azide modification) loi b cc cht oxy ha nh NO2-; (ii) Phng php x l mu

    nc vi KMnO4 v K2C2O4 (4500-O D. Permanganate modification); (iii) Phng php

    x l mu nc vi KAl(SO4)2.12H2O v NH4OH (4500-O E. Alum flocculation

    modification) loi b vt cht l lng trong mu nc Trong cc phng php

    Winkler sa i th phng php dng NaN3 l thch hp phn tch nc ao.

    3. Thu mu v bo qun

    Thu mu nc vo l nt mi nu 125 mL, c nh bng 1 mL MnSO4 v 1mL dung dch KI-NaOH, y np l li, lc u, trong l xut hin kt ta. Ch , khi thu mu v sau

    khi c nh khng bt kh xut hin trong chai khi thu mu nc.

    4. Thuc th

    a) Dung dch Mn2+: Ha tan 50 g MnSO4.5H2O hay 41 g MnCl2.4H2O vi nc ct thnh 100 mL.

    b) Dung dch KI-NaOH-NaN3: Ha tan 50 g NaOH v 15 g KI (hay 14 g NaI) v i nc ct thnh 100 mL. Ha tan 10 g NaN3 trong 40 mL nc ct, sau trn vi dung dch KI-NaOH.

    c) H2SO4 (d =1,84) hay H3PO4 c (d = 1,88). d) Dung dch Na2S2O3 tiu chun 0,1N: Pha mt ng Na2S2O3 chun 0,1N trong

    1000mL nc ct

    e) Dung dch Na2S2O3 0,01N: Pha long 50 mL dung dch Na2S2O3 0,1N vi nc ct thnh 500 mL.

  • 2

    f) Ch th h tinh bt 1%: Ha tan 1 g tinh bt trong 100 mL nc m (t 80-90 oC) khuy u cho n khi dung dch mu trong sut, cho vo 0,5 mL formaline nguyn cht s dng c lu.

    5. Tin hnh

    a) Thm 2 mL H2SO4 , lc u mu ha tan kt ta

    b) ong 50 mL cho vo bnh tam gic 100 mL.

    c) Chun bng dung dch Na2S2O3 0,01N cho n khi dung dch c mu vng

    nht, cho 3 git ch th h tinh bt, lc u dung dch c mu xanh, tip tc chun

    cho n khi dung dch chuyn t mu xanh sang khng mu th dng li. Ghi

    th tch (V1) dung dch Na2S2O3 chun . Lp li qu trnh phn tch mt ln

    na,ghi th tch (V2) dung dch Na2S2O3 chun ln 2. T V1 v V2, tnh th tch

    V trung bnh ca dung dch Na2S2O3 s dng.

    6. Tnh kt qu

    Tnh hm lng CO2 t do theo cng thc sau:

    Trong :

    V l th tch trung bnh dung dch Na2S2O3 chun .

    N l nng ng lng ca dung dch Na2S2O3.

    Vm l th tch mu (50 mL)

    mV

    xxNxVLmgDO

    000.18)/( =

  • 3

    4500-CO2 C . Phng php chun (APHA et al., 1995)

    1. Nguyn l

    CO2 t do phn ng vi NaOH hoc Na2CO3 to thnh NaHCO3. Phn ng xy ra hon

    ton c xc nh bng ch th in th hoc s to phc mu hng vi ch th

    phenolphthalein pH tng ng 8,3.

    NaOH + CO2 NaHCO3

    Na2CO3 + CO2 + H2O 2NaHCO3

    Nh vy, c 2 cch xc nh CO2 t do l chun bng NaOH hoc Na2CO3 vi ch th

    l phenolphthalein im dng l pH=8,3, khi dung dch chuyn t khng mu sang

    mu hng.

    2. Cc cht gy nhiu

    Cc cation v anion gy nh hng n cn bng CO2-CO32-. Cc ion kim loi b kt ta

    trong dung dch kim nh nhm, chronium, ng, st l tng kt qu phn tch, Fe2+

    khng c vt qu 1 mg/L. Cc ion kim yu nh ammonia hay amine, cc mui ca acid yu hay baz mnh nh borate, nitrite phosphate, silicate v sulfide gy nhiu dng

    (tng kt qu phn tch). Cc cht ny khng nn vt qu 5% ca hm lng CO2.

    Phng php chun khng p dng cho nc thi c cha acid khong. Tng cht rn

    ha tan (TDS) cao s gy nhiu m (gim kt qu phn tch), c bit l nc bin.

    3. Thu mu v bo qun

    Dng chai nt mi thy tinh thu mu nc, trnh b bt kh trong chai. Tt nht l phn

    tch ngay sau khi thu mu, c th gi mu trong 2-3 gi trong iu kin nhit thp (4 oC) hoc cho vi git chloroform ngn cn qu trnh h hp ca vi sinh vt lm tng hm lng CO2.

    4. Thuc th

    a) Nc ct khng cha CO2: un si nc ct hoc nc kh ion trong 15 pht,

    lm ngui bng nhit phng, pH phi ln hn 6 v dn in phi nh hn

    2mhos/cm. Dng nc ny pha thuc th v pha long mu.

    b) Dung dch m NaOH 0,1N: C 2 cch chun b dung dch m.

    (i) Pha 1 ng NaOH chun 0,1N (do nh sn xut cung cp) vi nc ct thnh

    1.000mL.

    (ii) Ha tan 4 g NaOH vi nc ct khng c CO2 t do thnh 1.000 mL. Phi

    chun ha dung dch m bng dung dch H2C2O4 0,1 N. Cch tin hnh nh sau: Cho 20mL dung dch H2C2O4 0,1N v 3 git ch th phenolphthalein vo bnh tam

    gic 100mL, lc u, chun bng dung dch NaOH mi pha trn cho n khi

    dung dch chuyn t khng mu sang mu hng nht. Ghi th tch V1 ca dung

    dch NaOH s dng. Lm li nh trn ln na ly gi tr trung bnh. Sau

    hiu chnh li nng dung dch NaOH cho chnh xc theo cng thc:

    N1V1=N2V2.

    c) Dung dch NaOH 0,01N: Pha 100mL NaOH 0,1N vi nc ct thnh 1.000mL

  • 4

    d) Dung dch chun Na2CO3 0,02N: Sy Na2CO3 140oC v ngui trong bnh ht

    m. Ha tan 1.06 g vi nc ct 1000 mL. Dung dch ny nn chun b mi mi

    khi phn tch.

    e) Dung dch H2C2O4 0,1N: Ha tan 0,63gram H2C2O4.2H2O vi nc ct thnh

    100mL.

    f) Dung dch m pH= 8,3:

    Dung dch Na2B4O7 0,05M: Ha tan 1,91gram Na2B4O7.10H2O vi nc ct thnh

    100mL.

    Dung dch H3BO3 0,2M: Ha tan 1,24 gram H3BO3 vi nc ct thnh 100mL.

    Ly 20mL dung dch Na2B4O7 0,05M cho vo 30mL dung dch H3BO3 0,2M. Ta

    s c dung dch m c pH=8,3.

    g) Dung dch ch th Phenolphthalein 1%: Ha tan 1g ch th Phenolphthalein

    (C20H14O4) trong 100ml cn 60o.

    5. Tin hnh a) Xc nh mu ti im dng chun theo cc bc sau:

    ong 50 mL dung dch m pH= 8,3

    Thm 3 git ch th phenolphthlein, lc u, dung dch c mu hng nht.

    b) Xc nh hm lng CO2 t do trong mu nc

    ong 50 mL mu nc.

    Thm vo mu nc 3 git ch th phenolphthlein, lc u. Nu dung dch c mu

    hng th trong nc khng cha CO2 t do. Nu dung dch khng mu, trong nc

    c cha CO2 t do, tip tc thc hin bc tip theo.

    Dng dung dch chun NaOH 0,01N chun t t cho n khi dung dch trong

    bnh c mu hng nht ging nh mu ca dung dch m (Ch : c th dng dung dch Na2CO3 0,02 N thay cho dung dch NaOH 0,01N). Ghi th tch V1 (mL)

    dung dch NaOH 0,01N s dng. Lp li cc bc trn, ghi th tch V2 (mL).

    T gi tr V1 v V2 tnh gi tr trung bnh V (mL).

    6. Tnh kt qu

    Tnh hm lng CO2 t do theo cng thc sau:

    Nu dng dung dch Na2CO3 chun th hm lng CO2 t do c tnh theo cng

    thc sau:

    Trong :

    V: l th tch trung bnh dung dch NaOH hoc Na2CO3.

    N: l nng ng lng ca dung dch NaOH hoc Na2CO3.

    mV

    xxNxVLmgCO

    000.144)/(2 =

    mV

    xxNxVLmgCO

    000.122)/(2 =

  • 5

    2340 B. kim - Chun acid (APHA et al., 1995)

    1. Nguyn l

    Lng acid chun trung ha baz trong nc dng xc nh kim. Cc cht gy

    kim bao gm HCO3-, CO3

    2-, OH-, SiO32-, PO4

    3-, NH3 v mt s cht hu c khc, nhng HCO3

    -, CO32-, OH- chim phn ln trong tng kim. Nc c pH>4,5 c th cha

    HCO3-, nc s c mu vng vi ch th methyl cam (methyl orange). Nc c mu hng

    vi ch th phenolphthalein khi trong nc c cha CO32- hoc OH- (pH>8,3). Do ,

    kim tng cng v kim ca cc thnh phn c xc nh qua 2 bc: Bc 1, chun

    acid vi im dng (im tng ng) ca ch th phenolphthalein (pH=8,3); Bc

    2, chun acid vi im dng ca ch th methyl cam (pH=4,5). Phn ng xy ra qua

    cc bc chun nh sau:

    CO32- + H+ = HCO3

    -

    HCO3- + H+ H2O + CO2

    2. Thu mu v bo qun:

    Thu mu trong chai nha hoc thy tinh v gi nhit thp (4oC), hot ng ca vi sinh vt c th lm thay i hm lng kh trong mu nc. Ly mu y chai, y kn,

    trch bt kh bn trong bi v nh th c th lm mt hoc tng kh CO2 hoc kh khc

    khi tip xc vi khng kh. Phn tch mu trong vng 1 ngy.

    3. Thuc th

    a) Nc ct khng cha CO2: un si nc ct trong 15 pht, lm ngui bng nhit

    phng, pH phi ln hn 6 v dn in phi nh hn 2mhos/cm. Dng nc ny pha thuc th v pha long mu.

    b) Dung dch m H2SO4 hoc HCl 0,1N: c 2 cch pha dung dch m

    (i) Pha long 1 ng axt chun (do nh sn xut cung cp) vi nc ct thnh

    1000mL.

    (ii) Ha tan 2,8 mL H2SO4 hoc 8,3 mL HCl m c vi nc ct thnh 1000mL. Chun ha nng ca dung dch m bng dung dch chun NaOH hoc Na2CO3

    0,1N. Cho 20 mL (V2) dung dch NaOH hoc v 3 git ch th phenolphthalein vo

    bnh tam gic 100 mL, lc u, chun bng dung dch acid mi pha trn cho

    n khi dung dch chuyn t mu hng sang khng mu. Ghi th tch ca dung

    dch acid s dng. Lp li qu trnh trn mt ln na, tnh th tch trung bnh V1

    ca dung dch acid s dng. Sau hiu chnh li nng dung dch acid cho

    chnh xc theo cng thc: N1V1 = N2V2.

    c) Dung dch chun H2SO4 hoc HCl 0,01N: Pha 100 mL ca dung dch m vi nc ct thnh 1000 mL.

    d) Dung dch ch th Phenolphthalein 1%: Ha tan 1g ch th Phenolphthalein

    (C20H14O4) trong 100ml EtOH 60%.

    e) Dung dch methyl orange 0,1%: ha tan 0,1g methyl orange vi nc ct thnh

    100ml

  • 6

    4. Tin hnh phn tch

    a) Xc nh kim phenolphthalein (phenolphthalein alkalinity): ong 50 mL mu nc

    Thm vo mu nc 2-3 git ch th phenolphthalein, lc u. , thc hin bc chun tip theo.

    Nu dung dch c mu hng th kim phenolphthalein ln hn 0, dng dung

    dch chun H2SO4 hoc HCl 0,01N chun n khi dung dch chuyn t mu

    hng sang khng mu. Ghi th tch V1 (mL) dung dch H2SO4 hoc HCl 0,01N

    s dng. Lp li cc bc trn, ghi th tch V2 (mL). T gi tr V1 v V2 tn