De xac dinh photpho tong truêc hei ta phai pha maụ Cd nghia tien hanh qua trinh vd ca hoa m l u de photpho hiru ca, poliphotphat va cac
photphat vd ca chuyen thanh dang PO/ tan, sau đ xac dinh ndng dp
photphọ
IV.ỊL Cdc phuong phdp hod hoc IV.LỊL Phuong phdp trong luong
ạ Phumig phdp ket tua muoi kep: MgNH^PỘ
Nguyen tdc ciia phuemg phap dua tren phan ung tao ket tua ciia
MgNHjPÔ trong mdi trudng kiem yeu (NH_(OH).
L(;)c rira kei tiia bang dung dich NH_,OH 2,5% sau đ nung ndng d nhict đ 1000-1 1()0°C va dang can hinh thanh la Mg.P.Ọ (magie piropht)lphat).
2 M g N H , P 0 , — > Mg.P.O, + 2NH, + H.O
Can lupng Mg->P->07 thu dupe sau khi nung ta xac dinh ham lupng photpho theo he sd chuyen:
2P
=(),27S5 M g , P , 0 ,
Phuemg phap kei tiia ion PÔ^' duêi dang mudi kep MgNH^PÔ la phuemg phap tieu chuan. Sai se) ciia phuemg phap trong khoang ±0,13 -^ ±0,46%. Tuy nhien phucmg phap nay cd nhupc diem la thdi gian ket tiia lau, khe)ang 15-18 gid.
b. Phmmg phdp ket tua axit di da
Trong mdi tnrdng axit manh (HNÔ 2N-^3N) ion PÔ-^' cd the tao vdi amoni molipdat kei tiia dudi dang mudi amemi photpho molipdat mau vang.
3 N H / + PO,^- ±12MoO,-- + 24H^ - ( N H j . P M o i . O . o i + 12H.O Loc rira kei tiia rdi dem say khd d nhict d d l 6 0 ° C ^ISÓ^C thi dan^^ can lăNH,)3[PO,.12MoOJ.
Ham IU(mg photpho khi đ dupe tfnh theo he st) chuyen. P
(NHJJPO,.12MoOJ = 0,0165
Neu nung d nhiet do 500-550°C thi dang can la: HPO3.I2M0O3 ham lupng photpho dupe tfnh theo he sd chuyen:
P
= 0 , 0 1 7 1 HPO3.I2M0O3
IV.LL2. Phuong phdp chuan dp the tich xac dinh P [15]
a, Phuong phdp chuan do the lich MoUpdcu:
Trong mdi irudng axit manh, ion PÔ^ dupe kei tua vdi amoni molipdat d dang amcmi photpho molipdat. Loc, rira kei liia bang dung dich
NH^NÔ sau đ dem kei liia hoa lan trong dung dich NaOH da biei ndng đ
vdi the tfch du chfnh xac. Chuan do lupng NaOH du bang HCl lieu chuan. Sai sd phmmg phap nay la ±(),46^±L52%>.
h. Phuong phdp chuan do Complcxon
Day la phUcmg phap gian tiep xac dinh ham lupng photphọ Ion F^Ô^ dupe kei liia dudi dang mudi MgNH^PÔ bang mdt luong du chfnh xac dung dich Mg~^ da biei ndng dp. Loc iiJa ion Mg""^ bam livn kei tiia bang dung dich nUdc. Chuan do bang dung dich liilon B chuan vcii chi thi Eriocrom T. Tai diem tUtmg ducmg ciia phep chuan dp mau ciia dung dich dupe chuyen tir đ nho sang xanh bice, phan Ímg chuan do nhu sau:
M g - ' + H,Y-- = M g Y - + 2H"
IV.L2. Cdc phuong phdp hod ly
IV.L2.L Phuong phdp trdc quang [15,17]
ạ PhUitng phdp do man "Xcuih moJipden "
Trong mdi trudng Phucmg phap nay dUa tren phan ufng ciia amoni molipdat V(ii octo photphat de lao hpp chai di dạ Hpp chai nay dupe khir lao ihanh mau xanh dang Mo(V) goi la "Xanh Molipden'\
Cac chai khir cd the diing la axil ascobic (C^.H^Ô,) SnCL, ben/idin.
Neu lUcmg pholpho Irong khoang 0,0()l^l()mg/l ihi sir phu thudc vao mat eld quang ciia dung dich tuan iheo dinh lual Lambc-Beer.
ABS = £ . 1 . C
1 : chieu dajjcuvet
C: ndng dp photpho trong dung dich ABS: la dp hap thu cua dung dich Trong phuang phap nay cac iem asennat (As04"^), silicat ( S i O , ' ) cd anh hudng vi cac ion nay phan ung tuang l^ nhu PÔ-*" vdi amoni molipdat.
Ham lupng Fê"" Idn han 50mg/l, cac ion NỌ", S"' va axit vd ca (HCl,
H.SO4, HNO3) deu cd anh hudng den phucmg phap. Sai sd cua phuang phap la ±0,4%.
b. Phuong phdp do mau axit vanado molipdophotphoric
Trong mdi trudng axit manh thi octo photphat, vanadi (V) va amoni molipdat phan u^g vdi nhau tao axit vanado molipdophotphoric mau vang,
cd thanh phan ciia phirc nay la: PnÓs-^2^'^-^^^^^^-^^2^-
Cudng dp mau ciia axit nay ti le vêi ndng do POj^'. pH thfch hcTp de xac dinh ion P04^' theo phuemg phap nay la pH = 9-^10. Cac nguyen id cd anh hudng den viec xac dinh ion POj^ la asen (V), titan, crom (VI), đng (II), niken (II). Ndng dp photpho thap nhai trong dung dich mau cd the phat hien dupe theo phucmg phap nay la 0,2 mg/1.
IVẠ2.2. Phuong phdp sac ki trao doi ion
Phuang phap sac kf tracj đi ion dua tren vice tach ion PÔ^" khdi cac cation kim lêai nhu Fe(III), CăII), Mg(II), Cr(VI), Cu(II) va chuyen photpho ve dang H^POj. Khi đ cd the xac dinh true tiep ndng do PÔ^' bang phuemg phap chuan dp hoac do ABS.
IV.1.2.3. Phuong phdp cue pho
Vi ion VO/ khdng bi khir tren dien cue gipt thuy ngan nen ngudi ta
diing phuang phap chuan dp ampẹ Xac dinh gian tiep photpho theo sdng cua mdt iem nao đ bi khir tren dien cue gipt thuy ngan va cd kha nang tao kei tiia vdi P O / " .
Mot si) thucíc thir diing cho phucmg phap nay: muc")i uranyl, chi axetat, bismut nilrat.
rV.1.2.4. Phuong phdp pho hd'p thu nguyen tu
Ion PÔ-^' duêc kei tiia dueii dang magie amdniphotphat (MgNH^POJ. Them luemg du chfnh xac dung dich Mg""" da biei ndng dp, sau đ lien hanh
Ipc, tir do xac dinh Mg'"*" dụ Tir lupng Mg""*" du ta cd the xac dinh dupe
lupng Mg""" da phan img vdi PÔ^" va suy ra ham luemg photpho trong maụ Phuang phap nay cd dp nhay, de) chpn Ipc caọ Phuang phap nay cd dp nhay, de) chpn Ipc caọ
IV,2. Cac phurcmg phap xac dinh ion Fé*^ AJ'*% Cá^, F* [15]