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二氧化钛(钛白粉)
二氧化钛(钛白粉)4 a& V5 F o+ @- i o; }0 f" o
: H3 d! v* Y v# GJECFA关于二氧化钛(钛白粉)的结论
/ b2 y9 N }/ x* f. k4 O* ?
0 d8 s4 n7 `$ T* |( q3 j摘要: 2006年JECFA关于二氧化钛的结论$ n" R j. [) d( F/ m
ADI值:不作限制。
. |8 C. }* ?" A0 ?# E* Y( X; |! z功能:着色剂
9 e+ E# D/ i: y, m: N
3 y$ d' `, `" e& N" hTITANIUM DIOXIDE
& N8 M. l# G* J. ?9 P. GPrepared at the 67th JECFA (2006) and published in FAO JECFA
; V8 h; x7 I7 J, @. C6 A4 mMonographs 3 (2006), superseding specifications prepared at the 63rd
: I9 ]" x9 M% g7 ?) ^) J' x+ }: QJECFA (2004) and published in FNP 52 Add 12 (2004) and in the# u) \% R3 H& u' ]: ~
Combined Compendium of Food Additive Specifications, FAO JECFA
, ?8 J7 S3 a" o! H9 Y/ wMonographs 1 (2005). An ADI “not limited” was established at the 13th& }$ U) [+ g$ V8 P
JECFA (1969).4 ~) d; `9 O1 |0 {2 q3 c+ }
SYNONYMS
" ^3 N7 I2 ^5 B$ v: eTitania, CI Pigment white 6, CI (1975) No. 77891, INS No. 171# F) u/ J4 X: D" w5 i
DEFINITION s! e ~8 J1 n
Titanium dioxide is produced by either the sulfate or the chloride: x( c; J7 h5 o
process. Processing conditions determine the form (anatase or rutile& W0 K4 G! V! R% D [7 O- `
structure) of the final product.6 P! b' T& z5 F% b1 ^7 }3 y* d
In the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3)7 c3 }! b' N/ }, R3 J
or ilmenite and titanium slag. After a series of purification steps, the
' U4 x# @4 B7 Uisolated titanium dioxide is finally washed with water, calcined, and
$ z2 C; B7 H( ?, x8 g0 T* U ~$ {0 x5 T0 Nmicronized.
& F' B- g9 t9 D A5 nIn the chloride process, chlorine gas is reacted with a titaniumcontaining
; M# e4 ^& z6 A- N+ `. ^mineral under reducing conditions to form anhydrous
3 i& E# M+ P/ _) W4 F0 ?titanium tetrachloride, which is subsequently purified and converted to
+ e6 X: [9 J. E# ttitanium dioxide either by direct thermal oxidation or by reaction with. a! H1 A( r8 Y. l2 [) \
steam in the vapour phase. Alternatively, concentrated hydrochloric
4 [5 e% J( E! j9 B0 I9 bacid can be reacted with the titanium-containing mineral to form a
- g$ s% f8 K6 x; E) @ z: qsolution of titanium tetrachloride, which is then further purified and
7 I1 M0 |* C7 }. k8 s$ yconverted to titanium dioxide by hydrolysis. The titanium dioxide is+ n3 M6 {3 y' K$ u3 ? ~+ W. [4 `
filtered, washed, and calcined.
9 k& e/ z* C8 p8 ]6 H: |Commercial titanium dioxide may be coated with small amounts of$ h2 U; b6 [+ f8 H
alumina and/or silica to improve the technological properties of the2 ^# n1 ~- }7 C
product.7 P. g% H' n+ v3 @/ M
C.A.S. number 13463-67-7: @% e- u7 Q9 i% |9 A/ I
Chemical formula TiO2
( U4 q0 F/ {7 I8 X+ |! bFormula weight* M/ Q* I; g: ^- o* a
79.88
/ o. b( z* v3 ^/ i% P4 T8 GAssay
; ~& q- u! R5 H. PNot less than 99.0% on the dried basis (on an aluminium oxide and) X8 t0 Y: }. T$ f+ E" c8 z9 t
silicon dioxide-free basis)
) Z3 L* s0 w' x5 g/ H; FDESCRIPTION
, E2 F; O5 h' t! r9 T( B0 LWhite to slightly coloured powder) ]& T2 Z# c1 d4 _
FUNCTIONAL USES
' @: p8 B4 A9 \Colour: v2 g( ~: k, K9 x6 q+ _
CHARACTERISTICS
$ F3 A: c7 F' U/ {* O9 b1 LIDENTIFICATION1 b9 e' b$ {7 N8 X. n: F5 y; @
Solubility (Vol. 4)5 D6 \) v1 `5 G* |
Insoluble in water, hydrochloric acid, dilute sulfuric acid, and organic
7 ?8 x; k% z! `+ F( { R4 |solvents. Dissolves slowly in hydrofluoric acid and hot concentrated
" M- `' w: v: F" Z' Rsulfuric acid.7 q# d( n; P3 l! w. h, d% \4 Z
Colour reaction
* d* i! C7 ?) n4 p: Q* ]' l) sAdd 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of) }) J* n$ z: i. d* E
sulfuric acid appear, then cool. Cautiously dilute to about 100 ml with$ {) B* \0 T4 u* m7 G
water and filter. To 5 ml of this clear filtrate, add a few drops of7 A+ H7 g4 D; E2 g
hydrogen peroxide; an orange-red colour appears immediately.
: z, O( w# K- DPURITY
+ T u; B; ?8 |Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h)
6 O+ g# j+ z9 X( @+ X+ K' jLoss on ignition (Vol. 4)( }8 q' J# r6 y
Not more than 1.0% (800o) on the dried basis
+ l: m( t# w) @8 [- [Aluminium oxide and/or7 L$ V K1 M, C5 v4 ], R
silicon dioxide
$ L8 h$ ]( E7 K, {+ xNot more than 2%, either singly or combined
, @# V8 k1 A) F+ `See descriptions under TESTS
3 o0 I" c+ m) U& ?( D' n' ]4 LAcid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing
5 [7 c7 k k" v0 @2 i) m; M; ?alumina or silica.
2 Q3 {( R8 v# O N" j. c/ d" ?Suspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and
; a, b: ^1 x, l% ~6 splace on a steam bath for 30 min with occasional stirring. Filter9 K4 D1 {$ D1 c
through a Gooch crucible fitted with a glass fibre filter paper. Wash- @/ i$ ?9 A5 h0 y( i* ], S
with three 10-ml portions of 0.5 N hydrochloric acid, evaporate the& E* ~! ?3 m6 k7 I. n, K9 g w
combined filtrate and washings to dryness, and ignite at a dull red2 m8 K& u- j: i9 d- x0 O
heat to constant weight.
6 C$ _) U: D4 n: i/ v; tWater-soluble matter2 Z& }. @- ~, \' v
(Vol. 4)# i; n1 Z0 W$ L6 | f1 S
Not more than 0.5%# Q6 u5 p( c# s+ V5 l* ~6 L# ^- h
Proceed as directed under acid-soluble substances (above), using
6 J/ q8 n. U: W8 fwater in place of 0.5 N hydrochloric acid.
v$ l# g, l' w; J( bImpurities soluble in 0.5 N
0 ]' Y1 g% @, G1 Dhydrochloric acid
+ |$ C. H7 U6 J/ J' KAntimony Not more than 2 mg/kg
# j& m+ h* r* G( |# y0 hSee description under TESTS
" t/ T Q0 @' f# ^Arsenic Not more than 1 mg/kg
% i H7 ^" ~* kSee description under TESTS
" @3 l9 t$ n' A( ~7 u3 NCadmium Not more than 1 mg/kg
% p4 D* z( U+ Q. C+ t/ p/ a, nSee description under TESTS
; b' _$ g X) Y& V$ MLead3 g8 z& w& j; m& s1 y
Not more than 10 mg/kg+ E! k% O' m0 U) u5 ]+ K+ N7 P$ W
See description under TESTS/ I9 M# ^ j' b4 N
Mercury (Vol. 4) Not more than 1 mg/kg
) r; \( L/ n4 M, e. W. Z& r7 ]* HDetermine using the cold vapour atomic absorption technique. Select a
6 y, b2 d9 m, a4 n' W6 ysample size appropriate to the specified level4 h" s' e3 x4 Q3 J8 v
TESTS
0 }. R! Y* `" l: f; Q9 O; PPURITY TESTS% u# E- m5 C. c+ s) J- f
Impurities soluble in 0.5 N" q) M5 r L! A% V6 N
hydrochloric acid
( |2 k% C4 g# h9 U4 ^Antimony, arsenic,
' o2 e$ j' H3 u3 P% Xcadmium and lead( k$ Z6 y% K5 \$ ?: Q1 p* }+ S
(Vol.4)
. C) F3 k* |1 [6 dTransfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N
p0 m) `" X# P4 N9 M- Qhydrochloric acid, cover with a watch glass, and heat to boiling on a, R. j& h2 r* l/ Q8 x
hot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml
/ {! ^% b+ H0 k1 rcentrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved. P6 D7 v! j3 Y) U0 y ^; P0 K
material settles. Decant the supernatant extract through a Whatman) J8 I' Q: @8 }
No. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml9 t6 u$ s% k9 n
volumetric flask and retaining as much as possible of the undissolved) }+ |2 i0 s1 u( M+ Y6 U; C' F
material in the centrifuge bottle. Add 10 ml of hot water to the original! ?! d/ ]& }- ?/ e9 `% W
beaker, washing off the watch glass with the water, and pour the
, v. {" a" r9 R+ N/ z* I' r3 C8 dcontents into the centrifuge bottle. Form a slurry, using a glass stirring w0 ^+ F1 ^, b( } J
rod, and centrifuge. Decant through the same filter paper, and collect
. z3 P$ o$ z) ~- X) V9 k0 z$ Othe washings in the volumetric flask containing the initial extract.
% U' U' v; o, }, R0 q$ [Repeat the entire washing process two more times. Finally, wash the
. g) d: s) P. zfilter paper with 10 to 15 ml of hot water. Cool the contents of the flask
2 B3 O+ W- C6 e5 F2 m2 q2 V& sto room temperature, dilute to volume with water, and mix.5 U; B2 U% p6 U& B" O5 j& g: F
Determine antimony, cadmium, and lead using an AAS/ICP-AES9 X& i8 w# `5 C. Y* }2 D9 c8 b
technique appropriate to the specified level. Determine arsenic using the
9 [4 D, |6 _8 P; OICP-AES/AAS-hydride technique. Alternatively, determine arsenic using( |3 u7 R/ Q! x: D& e# w
Method II of the Arsenic Limit Test, taking 3 g of the sample rather than
8 N- }! m% c6 q) g1 g. The selection of sample size and method of sample preparation
/ a/ R+ D3 X9 a5 f P: i$ Imay be based on the principles of the methods described in Volume 4./ D7 f& K+ y" |9 Z+ J
Aluminium oxide Reagents and sample solutions% Y+ q' ?( W- J3 g
0.01 N Zinc Sulfate
8 C3 g) A9 z- i9 a; LDissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to
' q+ k9 G0 ~5 y& m2 f6 Kmake 1000 ml. Standardize the solution as follows: Dissolve 500 mg1 O! z ~1 Q6 w) I! N
of high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of# h$ @; }+ g6 R# I- O) E
concentrated hydrochloric acid, heating gently to effect solution, then
- m4 {+ N2 F: \; N$ p) M/ Gtransfer the solution into a 1000-ml volumetric flask, dilute to volume
& w* z% g0 B5 }1 P% O- z9 z1 x! Jwith water, and mix. Transfer a 10 ml aliquot of this solution into a 500
4 l' z) a7 K6 L7 J( w7 o& Dml Erlenmeyer flask containing 90 ml of water and 3 ml of
/ q& i5 C3 E3 Q% I0 aconcentrated hydrochloric acid, add 1 drop of methyl orange TS and, [1 `6 x T0 T3 B3 s) ]
25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add,- |1 B. G2 W, H8 h/ V0 P% O$ o3 Q7 F
dropwise, ammonia solution (1 in 5) until the colour is just completely9 s! l4 {( |$ K7 o# v5 y. D
changed from red to orange-yellow. Then, add:
; T; B+ d5 |8 ~0 ?; \7 Y ^6 m% H) r(a): 10 ml of ammonium acetate buffer solution (77 g of7 q2 k( m' w& ?% L/ T, x0 T
ammonium acetate plus 10 ml of glacial acetic acid, dilute to3 ]. n# l( g- u5 V) T2 d5 d6 U
1000 ml with water) and
% Q4 D. `7 o$ X(b): 10 ml of diammonium hydrogen phosphate solution (150 g/ N% t0 @2 V& h+ Y0 q9 ~( W
of diammonium hydrogen phosphate in 700 ml of water,
0 M5 J" Y- u( ?, ~" r( zadjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,8 d" M, H0 A& E& T4 b6 j
then dilute to 1000 ml with water).
' u$ H, }5 [1 ]! c) j' JBoil the solution for 5 min, cool it quickly to room temperature in a
; g5 L: t$ x- @/ X8 f' D2 Dstream of running water, add 3 drops of xylenol orange TS, and mix.
# o+ g9 |7 i$ y& qUsing the zinc sulfate solution as titrant, titrate the solution to the first
- t. s% }9 M' Z' t, cyellow-brown or pink end-point colour that persists for 5-10 sec. (Note:" D6 R; g& V, M& d. z0 A3 P
This titration should be performed quickly near the end-point by
6 [. f2 u8 w, U; x* radding rapidly 0.2 ml increments of the titrant until the first colour- n% n, O/ A: G. L! o, E7 b2 \4 w
change occurs; although the colour will fade in 5-10 sec, it is the true# }& T1 @0 q- y6 D
end-point. Failure to observe the first colour change will result in an
Y {" Y, O S! m- oincorrect titration. The fading end-point does not occur at the second
6 i2 A8 G/ D$ P7 }3 }end-point.)9 `. v$ \) f, X$ v- B" ?/ Z! _5 n
Add 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a
3 ?/ S0 q0 ]/ Z- b* l7 \stream of running water. Titrate this solution, using the zinc sulfate$ u2 e, b* x/ g& w3 V
solution as titrant, to the same fugitive yellow-brown or pink end-point" B& `5 f$ n( W
as described above.
* n& o. V2 A8 S zCalculate the titre T of zinc sulfate solution by the formula:/ m) e) k' S3 M7 b, {) r- w8 L
T = 18.896 W / V# k2 _$ ?- e* q/ l
where; e; ]2 m( x; I3 R4 K0 }
T is the mass (mg) of Al2O3 per ml of zinc sulfate solution4 B1 W2 f) J) S$ E
W is the mass (g) of aluminium wire
* d0 K5 c* c! ~: C; bV is the ml of the zinc sulfate solution consumed in the
, `/ h- X5 `! D9 U( i+ [7 F* g( U0 ?second titration, R: _# i$ V: O: @# Q
18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and) v) F+ b! C; {
R is the ratio of the formula weight of aluminium oxide to
$ m' X! N6 k1 N3 g7 lthat of elemental aluminium.
( ~0 ]9 }+ E0 U mSample Solution A" f( e" l5 s* q
Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica
A$ x5 O2 d3 n6 zglass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).3 F9 Z$ F# J8 f o
(Note: Do not use more sodium bisulfate than specified, as an excess
5 n9 T \6 e8 ]concentration of salt will interfere with the EDTA titration later on in the
5 K! Q, b1 ^5 o# L; A: qprocedure.) Begin heating the flask at low heat on a hot plate, and: s/ c- h; h, w! ^- C
then gradually raise the temperature until full heat is reached." l# M1 R' w! J0 W; n
(Caution: perform this procedure in a well ventilated area. ) When
% l4 J( [- Z8 C7 A4 V) ~" v# R$ H+ nspattering has stopped and light fumes of SO3 appear, heat in the full* i/ {1 f7 g8 V& o- g1 S) o
flame of a Meeker burner, with the flask tilted so that the fusion of the3 j4 i- X( L4 o( @% R. _
sample and sodium bisulfate is concentrated at one end of the flask. _9 g6 U) d7 o: i. {) c2 [8 D
Swirl constantly until the melt is clear (except for silica content), but
& S" x0 ?$ n2 vguard against prolonged heating to avoid precipitation of titanium
: x3 ~4 ^0 f8 |4 _& ^3 o7 E4 zdioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until4 Y" R9 O: {& P9 N2 P. z( H
the mass has dissolved and a clear solution results. Cool, and dilute to
3 l9 B" ]' ~. i; k2 {- \; h120 ml with water. Introduce a magnetic stir bar into the flask.5 e8 `( j8 T) J0 p
Sample Solution B8 r' p/ W( U( J. A
Prepare 200 ml of an approximately 6.25 M solution of sodium$ z1 @. G$ E2 N- \ E* d R3 e) R
hydroxide. Add 65 ml of this solution to Sample Solution A, while/ S) W, p: z; L8 M- l( n
stirring with the magnetic stirrer; pour the remaining 135 ml of the! j' ?7 c; h: {
alkali solution into a 500-ml volumetric flask.
8 T: X* e( p3 U, OSlowly, with constant stirring, add the sample mixture to the alkali
3 l. h, `9 K" R isolution in the 500-ml volumetric flask; dilute to volume with water,
& B y# H) K! ^and mix. (Note: If the procedure is delayed at this point for more than' c0 \; W6 ^: l4 x) `" X
2 hours, store the contents of the volumetric flask in a polyethylene
; u4 X7 B- O6 M; K/ t: P1 ]bottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),
; E$ m6 g2 p: A6 I& a$ athen filter the supernatant liquid through a very fine filter paper. Label$ m. P- ] h4 ?
the filtrate Sample Solution B.) @) Z9 j8 G- f; p/ f8 [1 A: w3 P; ~
Sample Solution C3 N R" F+ f. }/ J
Transfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer. e1 F7 K" J$ Q3 i$ g
flask, add 1 drop of methyl orange TS, acidify with hydrochloric acid
; s, v0 i, [! g5 Q# w; ysolution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.02& t. o5 ~- J' ]6 x+ W0 v
M disodium EDTA, and mix. [Note: If the approximate Al2O3 content is; `( }7 h1 d3 U
known, calculate the optimum volume of EDTA solution to be added
1 d6 @8 N6 D$ \) K9 pby the formula: (4 x % Al2O3) + 5.] ^* Y( N4 j3 Y
Add, dropwise, ammonia solution (1 in 5) until the colour is just
& R: n1 z% t; j( A; o3 J7 ~completely changed from red to orange-yellow. Then add10 ml each
4 c& X. J: n6 K7 _) @of Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to
- {( |- ?5 f( [8 Vroom temperature in a stream of running water, add 3 drops of xylenol+ `0 i8 v, Z4 i* N
orange TS, and mix. If the solution is purple, yellow-brown, or pink,' f+ ~8 g+ V7 v6 n% I4 u' }) x
bring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired
5 G; A* w0 f* x. h2 s/ l( ]% DpH, a pink colour indicates that not enough of the EDTA solution has. G7 K: ^' J+ e6 [
been added, in which case, discard the solution and repeat this
. v# ` A) r! \, D- A8 Eprocedure with another 100 ml of Sample Solution B, using 50 ml,
( [5 K! M- r, r5 rrather than 25 ml, of 0.02 M disodium EDTA.
8 t0 y2 Y$ y2 F; K* k4 z* N3 N# uProcedure
3 F: Z# y) @, c. f2 X; k; VUsing the standardized zinc sulfate solution as titrant, titrate Sample
' `8 a8 h' G) ]# e/ B: `& nSolution C to the first yellow-brown or pink end-point that persists for
9 v% X% g8 m! J' ^& b5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first M$ |4 {* k, a- S
titration should require more than 8 ml of titrant, but for more accurate
& p7 i+ w- S' I" _! a) k) Qwork a titration of 10-15 ml is desirable.
0 w* Y9 y. B+ I1 N* T. H; }1 N! a dAdd 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5
( U, F+ I( E1 a6 E- Z" P0 Hmin, and cool in a stream of running water. Titrate this solution, using* i2 x9 k8 s. Y4 S" V
the standardized zinc sulfate solution as titrant, to the same fugitive4 ]+ V8 J' Q+ A j
yellow-brown or pink end-point as described above.& R% a r, A& g/ t1 ?* j/ }5 s+ i
Calculation:
; `) K9 s4 z$ Z9 I! ICalculate the percentage of aluminium oxide (Al2O3) in the sample
$ O5 t* k0 p4 L' v: A2 }' x* ^) \taken by the formula:! u: ~) R- a, u1 N
% Al2O3 = 100 × (0.005VT)/S
% `2 P! F; y# b7 e, zwhere' P. p( {4 W/ [5 @
V is the number of ml of 0.01 N zinc sulfate consumed in
$ K# m: I5 y3 e) Fthe second titration,% H$ k# v/ p" l9 ]7 x t3 _
T is the titre of the zinc sulfate solution,
" r- D' d9 D! e* Q( K; FS is the mass (g) of the sample taken, and1 C6 ]) X* H7 K" T7 ]9 b
0.005 = 500 ml / (1000mg/g × 100 ml).
2 v. u& N3 N+ _3 M2 zSilicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica/ U9 A2 F+ }: r r; c$ z
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).8 A# w+ b; r. J& ^
Heat gently over a Meeker burner, while swirling the flask, until
7 w8 D" o. `; O! h) \, kdecomposition and fusion are complete and the melt is clear, except/ p8 u% x* Y+ c/ e6 S6 b
for the silica content, and then cool. (Caution: Do not overheat the" {+ _" j, t5 h4 z. ?' ?0 ~3 m
contents of the flask at the beginning, and heat cautiously during0 q( G, J* k+ Q% L# L! V& E
fusion to avoid spattering.)) i, g7 ^. N% P" H6 ^
To the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat& T* ^( W" g6 G
carefully and slowly until the melt is dissolved. Cool, and carefully add
7 T2 q. }7 v7 K150 ml of water by pouring very small portions down the sides of the
. ^: b& u F( r! Iflask, with frequent swirling to avoid over-heating and spattering. Allow% J3 b; P$ c% b# |" K. x# m
the contents of the flask to cool, and filter through fine ashless filter, T& V$ \" O4 w6 ^+ O. l
paper, using a 60 degree gravity funnel. Rinse out all the silica from% ]2 s$ W, W/ A$ s
the flask onto the filter paper with sulfuric acid solution (1 in 10)." z, y- g$ _+ z2 j6 t
Transfer the filter paper and its contents into a platinum crucible, dry in
N% @3 @+ X4 C7 }7 ean oven at 1200, and heat the partly covered crucible over a Bunsen
$ X$ Q3 o1 U. u6 a* X# aburner. To prevent flaming of the filter paper, first heat the cover from) J/ S7 g+ h1 ~0 H4 g8 n
above, and then the crucible from below.
7 M7 U5 B, z/ G3 U+ I; a4 k8 YWhen the filter paper is consumed, transfer the crucible to a muffle
5 ]7 X) D) Z2 M; @' ]- V: Dfurnace and ignite at 1000o for 30 min. Cool in a desiccator, and" x; e/ e* |! `% L3 Q0 n
weigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated0 i/ W9 f4 j+ E. h
hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first. x9 k: V* Q( h9 Z( H. k8 E- G
on a low-heat hot plate (to remove the HF) and then over a Bunsen
7 |3 `% R1 w% j( ^burner (to remove the H2SO4). Take precautions to avoid spattering,/ }+ ]" |3 |; l# D
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a
7 q/ j+ f# B# ~. W. }6 ?) Q* Ydesiccator, and weigh again. Record the difference between the two
$ M$ {3 ^1 s/ j9 R1 Nweights as the content of SiO2 in the sample.- E; g/ Y6 W) }* _
METHOD OF ASSAY, ]* _, @1 s+ f$ S
Accurately weigh about 150 mg of the sample, previously dried at 105o( {, X' h. A2 j7 s
for 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water/ h$ ~$ X1 y9 d/ [& O$ k+ o
and shake until a homogeneous, milky suspension is obtained. Add 30
2 z3 p3 v+ `+ O9 a3 w; J' [ml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially# l6 C: ~2 Y! [: F
heat gently, then heat strongly until a clear solution is obtained. Cool,
) j6 l; Y* e4 n0 U: Pthen cautiously dilute with 120 ml of water and 40 ml of hydrochloric# D+ f. u0 u* Z* J" E
acid, and stir. Add 3 g of aluminium metal, and immediately insert a
/ k1 j6 x3 @. O" ?4 ~) wrubber stopper fitted with a U-shaped glass tube while immersing the
+ N* W' w2 W. N2 M0 _, d; qother end of the U-tube into a saturated solution of sodium! q1 n8 v; P+ L2 L0 Y4 J" v
bicarbonate contained in a 500-ml wide-mouth bottle, and generate5 }7 X/ @* z2 ^7 {6 a3 ?# t
hydrogen. Allow to stand for a few minutes after the aluminium metal6 c) s) R3 M/ v! m
has dissolved completely to produce a transparent purple solution.
- k: p' r& T3 Z2 P6 W5 P' xCool to below 50o in running water, and remove the rubber stopper. J8 b7 u, I9 X1 X
carrying the U-tube. Add 3 ml of a saturated potassium thiocyanate
' f( J+ y( F( Jsolution as an indicator, and immediately titrate with 0.2 N ferric3 p" H% u/ A$ h
ammonium sulfate until a faint brown colour that persists for 30
' P* t. X3 O- }seconds is obtained. Perform a blank determination and make any
F% X7 m6 `( O: B q3 i7 j3 Nnecessary correction. Each ml of 0.2 N ferric ammonium sulfate is
% d; L7 l g2 `0 Z* nequivalent to 7.990 mg of TiO2.( B/ G- n. Q/ M8 C" B3 Q0 }
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