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二氧化钛(钛白粉)
二氧化钛(钛白粉)% Y: u( A9 Q6 a+ D' b. Y4 K
, v. o& H3 j( Y- mJECFA关于二氧化钛(钛白粉)的结论
) m; |2 `. v1 I* D$ x, j% }8 n0 r; y) U0 g) O3 r& u7 u
摘要: 2006年JECFA关于二氧化钛的结论. Z% t! g! p- b# I5 L* ~
ADI值:不作限制。' ^! z( O; f2 ~2 w
功能:着色剂
# A C P/ `6 c- x% s# S! R: y9 {; f1 Q: S: u9 X) M
TITANIUM DIOXIDE
5 P! t9 O6 D* n3 G U: s. uPrepared at the 67th JECFA (2006) and published in FAO JECFA6 E; `2 N* s* T* p. R0 L- z
Monographs 3 (2006), superseding specifications prepared at the 63rd
% d9 A7 g |. G+ |9 n- mJECFA (2004) and published in FNP 52 Add 12 (2004) and in the2 t+ W9 M3 B% A! r+ s8 y
Combined Compendium of Food Additive Specifications, FAO JECFA5 t- n/ ~- h1 u' L# y- m6 @; x
Monographs 1 (2005). An ADI “not limited” was established at the 13th
% n8 N% ^6 M* S2 U N+ ?JECFA (1969).
5 H" z' c) j( s$ A6 I7 U. ESYNONYMS
% V! ?3 V# }1 q# VTitania, CI Pigment white 6, CI (1975) No. 77891, INS No. 1713 L) r; b4 B- s3 z8 B, K
DEFINITION
4 l* x2 u; M" E" h. L4 ~' @' XTitanium dioxide is produced by either the sulfate or the chloride+ b, t! K2 K8 r- @# a
process. Processing conditions determine the form (anatase or rutile# d8 }% V: j% Y$ Y5 C$ G% ^2 u1 u
structure) of the final product.
- W& {4 s, b2 Q/ H$ a7 BIn the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3)( P4 I3 R/ `$ z# j
or ilmenite and titanium slag. After a series of purification steps, the, \4 I$ [8 G; \6 o) Q
isolated titanium dioxide is finally washed with water, calcined, and' n, \3 _4 n8 m2 N
micronized.
9 x [1 }4 e( }% ?In the chloride process, chlorine gas is reacted with a titaniumcontaining+ B' E R2 m) c2 I2 t
mineral under reducing conditions to form anhydrous
W. x3 a S. F+ Gtitanium tetrachloride, which is subsequently purified and converted to
. }) w- Q8 |! E6 G# j5 ltitanium dioxide either by direct thermal oxidation or by reaction with* ^' }" B: h2 [, ~& s) M" L8 @) b! T
steam in the vapour phase. Alternatively, concentrated hydrochloric4 O+ }- n7 \7 o* a9 {. t1 r) n* h. c
acid can be reacted with the titanium-containing mineral to form a' g3 l4 g- N2 N* S4 ~+ o
solution of titanium tetrachloride, which is then further purified and3 G' j. H$ T0 w; [
converted to titanium dioxide by hydrolysis. The titanium dioxide is9 J! _$ Q' V' u/ S
filtered, washed, and calcined.
% ?8 y% Y1 e% @Commercial titanium dioxide may be coated with small amounts of
. x1 c3 p# O- j) `7 N8 Calumina and/or silica to improve the technological properties of the
1 {" h& V# q/ p) Bproduct.
n5 l( G" R1 C5 J- a2 u* I, J" YC.A.S. number 13463-67-7
8 ` o3 O _1 |' a2 D8 DChemical formula TiO2
0 W* `+ I; z; OFormula weight
) R. s, \7 K' s79.88
. ?) ^% S' `2 a% zAssay
% c7 [) x: D! p7 P, f( oNot less than 99.0% on the dried basis (on an aluminium oxide and
- ]" M9 w( _1 K+ K4 [silicon dioxide-free basis) h( D" z" Y6 ~6 o* S1 B9 m
DESCRIPTION4 U. K" `/ r6 A
White to slightly coloured powder4 g- O4 G& a5 o! I
FUNCTIONAL USES
$ }3 a( V4 E& f- F8 cColour
5 Z) {" m7 O9 B+ o! o; JCHARACTERISTICS
, F% |+ N8 Y8 ]0 gIDENTIFICATION
- I# a) X, U! V* oSolubility (Vol. 4)
0 A: f/ E3 [; Z( I0 ^, Y. G1 E) B' qInsoluble in water, hydrochloric acid, dilute sulfuric acid, and organic7 L+ b0 g; ~" c! K& @* C
solvents. Dissolves slowly in hydrofluoric acid and hot concentrated
4 `/ U- O6 q Dsulfuric acid.
3 t( u) h I$ P0 p; {) I0 sColour reaction0 ?( q1 |# N% y# V
Add 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of; v% e% c: S ^; c4 h
sulfuric acid appear, then cool. Cautiously dilute to about 100 ml with: |* `+ A! ^+ z, [6 |- `
water and filter. To 5 ml of this clear filtrate, add a few drops of9 r" D- g/ X1 T8 Z& N$ J
hydrogen peroxide; an orange-red colour appears immediately.
. s# c0 e L6 B) u1 o6 h& HPURITY
$ p1 d- \& }" O; Q4 I5 \Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h)
: r* K3 t* ]: x* l; G: O0 kLoss on ignition (Vol. 4)
2 p: K/ y' J4 \& mNot more than 1.0% (800o) on the dried basis
8 f) G) R' @1 L0 K$ VAluminium oxide and/or
) {) S' E+ y8 G% w8 Usilicon dioxide5 K' P6 o* L% o$ [2 l
Not more than 2%, either singly or combined" G# o) g" ?7 P7 d4 z9 y7 |
See descriptions under TESTS/ S1 T2 Y+ ^. v* H. b% {5 c
Acid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing
& n( S$ h: |8 ~* m1 Oalumina or silica.. ]0 H$ K, p8 [* y+ m, Z" J$ I. ?
Suspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and3 H1 B8 o+ L5 c+ ^
place on a steam bath for 30 min with occasional stirring. Filter! o z$ ~ h% I6 ~ f
through a Gooch crucible fitted with a glass fibre filter paper. Wash* ]' X! o* ^; K
with three 10-ml portions of 0.5 N hydrochloric acid, evaporate the
: L# W; ]2 r: {; Gcombined filtrate and washings to dryness, and ignite at a dull red; [# v, U. d; J! B' P7 f
heat to constant weight.
, x0 S' m$ x: K' tWater-soluble matter
( J" B; R4 t+ ^4 ~& Q3 A. ~(Vol. 4)
' F6 }: Z# a8 U- H. mNot more than 0.5%- F% Q( R! q, Z% d8 R
Proceed as directed under acid-soluble substances (above), using
% W- P+ `5 r- B% W1 U" Hwater in place of 0.5 N hydrochloric acid.6 T0 S5 n g( \" d% \
Impurities soluble in 0.5 N% h9 L4 n5 m/ C9 F" L
hydrochloric acid4 c, |/ Z9 B6 n* C9 D
Antimony Not more than 2 mg/kg
! F) M7 l/ X: }9 ZSee description under TESTS' t# w: b0 x; F! }/ \
Arsenic Not more than 1 mg/kg$ v# s8 S) J( _( c
See description under TESTS* B' [' e* u/ d! ?4 T
Cadmium Not more than 1 mg/kg0 S7 x( E4 J7 P6 A* r( g
See description under TESTS! d- Q: F2 R! q. k4 O {
Lead
5 f2 G$ V. T& b& X& ~Not more than 10 mg/kg
( B" r3 J& c) c- W& GSee description under TESTS# h0 S. X, p1 S x1 U B$ _) o
Mercury (Vol. 4) Not more than 1 mg/kg
, T* g+ E8 W+ ~( t& m( BDetermine using the cold vapour atomic absorption technique. Select a
5 u+ X1 l8 g: T# f0 z9 zsample size appropriate to the specified level1 J' p/ S7 k' [. ^# w# L2 [
TESTS Q# Y; L9 U5 I! W; d( G
PURITY TESTS
+ y2 U" w7 G$ v& J" n, c" l* d: ZImpurities soluble in 0.5 N
% K) ] {4 U2 c7 Q0 o) z2 t8 s$ dhydrochloric acid" s- l# g6 h, `" W2 b9 H* t4 s
Antimony, arsenic,
+ O! Q. \( _% R; L# Scadmium and lead
/ u0 G5 P8 ]6 l) B(Vol.4)& {5 Y! g/ C z4 S) V
Transfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N
2 h( k. c0 E- [5 ehydrochloric acid, cover with a watch glass, and heat to boiling on a' v" f4 p$ K& z" J0 U+ K
hot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml
( f# F) q3 P* F; ~5 e9 E: ~* rcentrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved
" C4 F- s7 U2 A( C6 a, Jmaterial settles. Decant the supernatant extract through a Whatman
# t# H/ w9 ~# [ X; j/ N' O* U# ENo. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml
& o; s5 ~4 v/ W6 `7 Rvolumetric flask and retaining as much as possible of the undissolved
4 {* \( y; Z3 j- b8 w; Pmaterial in the centrifuge bottle. Add 10 ml of hot water to the original4 v; f% y' w" }2 A
beaker, washing off the watch glass with the water, and pour the I7 ?5 D7 l7 U- R5 \
contents into the centrifuge bottle. Form a slurry, using a glass stirring+ g3 M# l: \: _ ~2 o8 T
rod, and centrifuge. Decant through the same filter paper, and collect m' g! s+ O, J$ Q/ S3 ~+ q( S" c
the washings in the volumetric flask containing the initial extract.0 v. W! e. b; r' A2 v& H
Repeat the entire washing process two more times. Finally, wash the
4 B, \$ H' p3 ]" _* u3 Hfilter paper with 10 to 15 ml of hot water. Cool the contents of the flask
3 b) y# ?# I$ ~, q' M/ |( @9 a- Uto room temperature, dilute to volume with water, and mix.- X# N9 _8 X2 `2 i# M
Determine antimony, cadmium, and lead using an AAS/ICP-AES
/ \. f; b) b) a" J2 l( g; a% e$ g ztechnique appropriate to the specified level. Determine arsenic using the5 t5 r$ h _2 A1 h, B+ D
ICP-AES/AAS-hydride technique. Alternatively, determine arsenic using6 h) q v5 U# c1 V8 j% r/ ^
Method II of the Arsenic Limit Test, taking 3 g of the sample rather than
2 e& ~) Y) y* U" C- j, T: y; C1 g. The selection of sample size and method of sample preparation8 O+ u7 x8 g4 I* j+ t
may be based on the principles of the methods described in Volume 4.
! E6 V8 K# F( z4 G( s! xAluminium oxide Reagents and sample solutions6 W( P2 _0 Y% z! \1 H2 v
0.01 N Zinc Sulfate- p# s5 B8 J$ I) t( K- `# ]
Dissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to
: U0 T% Y0 x: q/ w9 Kmake 1000 ml. Standardize the solution as follows: Dissolve 500 mg
/ r; O/ X8 i! e* \0 ]8 ?of high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of' k* w7 h/ F1 h5 q5 ^
concentrated hydrochloric acid, heating gently to effect solution, then
6 ]: O: f4 i0 @* Ytransfer the solution into a 1000-ml volumetric flask, dilute to volume. g B! Z" v6 E2 t
with water, and mix. Transfer a 10 ml aliquot of this solution into a 500
8 ?- T- s5 k U7 p7 W: Iml Erlenmeyer flask containing 90 ml of water and 3 ml of
9 R* v+ M1 [2 [; R1 `) Yconcentrated hydrochloric acid, add 1 drop of methyl orange TS and
: w5 P; n9 C/ D( O6 \25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add,
6 x: P2 O4 @& I% B5 ?2 sdropwise, ammonia solution (1 in 5) until the colour is just completely& H' n: k! D1 e$ N7 K" e. {
changed from red to orange-yellow. Then, add:; p/ y7 T* w3 Y" I1 u
(a): 10 ml of ammonium acetate buffer solution (77 g of6 O- `. D8 Q# I$ H& o
ammonium acetate plus 10 ml of glacial acetic acid, dilute to9 K# M9 @2 Z" Q+ r! \5 ?, u% {
1000 ml with water) and
) b# g2 _2 @- K1 E; ~(b): 10 ml of diammonium hydrogen phosphate solution (150 g8 t2 V. O/ G8 q# e7 \: a) J) i# f
of diammonium hydrogen phosphate in 700 ml of water,, W5 P. G4 A- N- F0 j+ J
adjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,
% V1 `3 x r- C' xthen dilute to 1000 ml with water).% Q* Y& z3 W4 d
Boil the solution for 5 min, cool it quickly to room temperature in a; Y) F9 U) _0 n- u5 I$ |3 N" F
stream of running water, add 3 drops of xylenol orange TS, and mix.
9 u. x* [+ I K- ~Using the zinc sulfate solution as titrant, titrate the solution to the first
/ S( Q5 F1 C0 g9 B1 V: c* Gyellow-brown or pink end-point colour that persists for 5-10 sec. (Note:3 D2 a) }1 \' f* `" k; k+ o a
This titration should be performed quickly near the end-point by* U' R9 N9 v5 A$ G
adding rapidly 0.2 ml increments of the titrant until the first colour
; z' Y* f$ V; T' ochange occurs; although the colour will fade in 5-10 sec, it is the true+ l& @8 K+ W) C' T: R6 x3 u y+ v, {
end-point. Failure to observe the first colour change will result in an
% n, X% { M, D7 kincorrect titration. The fading end-point does not occur at the second; C1 g1 b- ^; c5 T8 x: g
end-point.)) x" u; e; x$ Y" O5 ?
Add 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a
# P+ t1 F5 ]# {$ N+ L9 T% ystream of running water. Titrate this solution, using the zinc sulfate* `6 }7 H! }( _* Z
solution as titrant, to the same fugitive yellow-brown or pink end-point4 I: |7 i/ C4 x
as described above.
$ K1 u8 j! o1 cCalculate the titre T of zinc sulfate solution by the formula:6 |8 {% L) x. s- ~5 a6 B2 {
T = 18.896 W / V
9 ]- U# i8 K/ Wwhere. u) @6 W3 @9 c! V! w8 {
T is the mass (mg) of Al2O3 per ml of zinc sulfate solution
- a5 ]5 _2 I( Z2 L* {W is the mass (g) of aluminium wire) Y+ `7 R) o. C2 i: {
V is the ml of the zinc sulfate solution consumed in the3 [" O. G; W, t Y2 L: H* |+ n; W6 v
second titration
; w8 ~/ g% `% b1 u$ `18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and' W7 m" d: k* v0 a( y
R is the ratio of the formula weight of aluminium oxide to% i& T9 U" N, V; l; q0 {
that of elemental aluminium.
9 u3 I% L. {1 j6 lSample Solution A- W1 f) u# N( ~0 ?! x8 h p- s
Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica0 S4 b6 w$ Q% _0 X7 C; Q
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).
$ C: y0 }2 w @4 d) l" N(Note: Do not use more sodium bisulfate than specified, as an excess" z+ W8 B& e4 V' f4 o/ M
concentration of salt will interfere with the EDTA titration later on in the
2 R, l0 w+ x. ?& Q- Oprocedure.) Begin heating the flask at low heat on a hot plate, and
4 f0 M& Z- L( v7 }. u2 b" |then gradually raise the temperature until full heat is reached.& [ L; X& o/ f
(Caution: perform this procedure in a well ventilated area. ) When
7 m& X( p, V1 ^" G# mspattering has stopped and light fumes of SO3 appear, heat in the full
5 {4 H/ W/ i7 [9 H- wflame of a Meeker burner, with the flask tilted so that the fusion of the
7 C. A7 A$ _3 Y: V, m c2 R; Fsample and sodium bisulfate is concentrated at one end of the flask.
8 `& a7 k+ s1 R) ySwirl constantly until the melt is clear (except for silica content), but4 Y: b/ s) M4 {$ V2 N8 R* V
guard against prolonged heating to avoid precipitation of titanium. l+ B# E+ ?7 y
dioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until- O, t# Y+ l; n+ A" I
the mass has dissolved and a clear solution results. Cool, and dilute to0 s" [( Z9 P {3 W Q& K1 V
120 ml with water. Introduce a magnetic stir bar into the flask.' t3 R: |1 k& O6 x; E) P* {
Sample Solution B
+ R2 s. Z: g I: \Prepare 200 ml of an approximately 6.25 M solution of sodium3 `; b7 r( ?% a& }5 s; [! V) ]
hydroxide. Add 65 ml of this solution to Sample Solution A, while
3 b8 n. G: \& t! g; T Istirring with the magnetic stirrer; pour the remaining 135 ml of the
! L7 M9 h* x. I3 _alkali solution into a 500-ml volumetric flask.
# \& l8 ?3 ^& }* g" h2 W: VSlowly, with constant stirring, add the sample mixture to the alkali2 L2 i6 J# U/ V% n
solution in the 500-ml volumetric flask; dilute to volume with water,
- _. }1 { f# ~and mix. (Note: If the procedure is delayed at this point for more than
8 J. J. c9 t2 t& m5 r0 g# y% p3 P2 hours, store the contents of the volumetric flask in a polyethylene+ w' q% I% t% X7 m( K8 b" \, g
bottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),9 `: J& @" ~) C! F9 ]: ~* E3 p" F
then filter the supernatant liquid through a very fine filter paper. Label) Y9 {6 p( v, F% E$ O
the filtrate Sample Solution B.* s5 K# \. v/ r# \
Sample Solution C- q7 v6 y2 L6 k) v- j, @
Transfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer7 S7 U1 K e( t" b
flask, add 1 drop of methyl orange TS, acidify with hydrochloric acid
- O) W, K4 ?, Psolution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.02
: r# y a9 d/ D, b& Z: Z* mM disodium EDTA, and mix. [Note: If the approximate Al2O3 content is
4 \2 l0 e6 Q. A! zknown, calculate the optimum volume of EDTA solution to be added' X5 A( u8 P4 I/ _
by the formula: (4 x % Al2O3) + 5.]
) K1 a( ]0 T, N# c& J; K2 LAdd, dropwise, ammonia solution (1 in 5) until the colour is just l. @( d5 B- P& B6 X. D" O
completely changed from red to orange-yellow. Then add10 ml each3 b4 ~. Z- j$ P
of Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to
- p L! w5 o! C9 qroom temperature in a stream of running water, add 3 drops of xylenol+ W% e: ^: P. k" y8 C" G
orange TS, and mix. If the solution is purple, yellow-brown, or pink,- F3 ` i8 S ?
bring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired3 F6 E) k( |' d4 s6 z2 ^8 |4 E
pH, a pink colour indicates that not enough of the EDTA solution has% J; a8 t. X2 d
been added, in which case, discard the solution and repeat this0 _9 J1 ~. C1 J) o" ^+ i& a% b( p
procedure with another 100 ml of Sample Solution B, using 50 ml,3 V# \* b1 T( W4 w
rather than 25 ml, of 0.02 M disodium EDTA.5 {6 g! W( A6 k' [; K3 I
Procedure( S: A( \0 P5 F7 z: T
Using the standardized zinc sulfate solution as titrant, titrate Sample
9 d: `7 W+ d1 H4 n6 oSolution C to the first yellow-brown or pink end-point that persists for
9 b: f- U( ^$ F3 Q0 S5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first7 C2 F9 M' Z' K- l' J8 Z9 t
titration should require more than 8 ml of titrant, but for more accurate
- t Q. k+ t$ swork a titration of 10-15 ml is desirable.
; `1 ?0 t# K# f' EAdd 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5; j2 A0 l4 J6 ^0 B" f7 [# f9 d; t
min, and cool in a stream of running water. Titrate this solution, using
: G! o o- C- M, G& Q+ Mthe standardized zinc sulfate solution as titrant, to the same fugitive
9 v+ m) V* Q4 I- Xyellow-brown or pink end-point as described above.2 I3 P( [4 V0 j& b; h9 M$ b8 f
Calculation:
$ A8 j( _& T9 A+ e+ ACalculate the percentage of aluminium oxide (Al2O3) in the sample
1 K1 u7 `6 F; M* b# X* m5 Ataken by the formula:/ }+ y7 j- D) o
% Al2O3 = 100 × (0.005VT)/S
5 k/ U. [. P9 twhere$ O# e4 U1 v( Y! T
V is the number of ml of 0.01 N zinc sulfate consumed in
7 F' k) A. M3 a! a% d/ m9 q, H0 }the second titration," g$ P( }; t8 T4 e8 e% f) U
T is the titre of the zinc sulfate solution,
( v/ U/ ]2 G/ j' J. U# X5 mS is the mass (g) of the sample taken, and
7 T8 P5 U" O( x( j) t7 B) E0.005 = 500 ml / (1000mg/g × 100 ml).* }# J6 \ M; M/ x# P' M
Silicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica
/ x" B6 t$ E# ]; y- Wglass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).9 U7 ]0 d2 u* d
Heat gently over a Meeker burner, while swirling the flask, until9 g) y7 O6 \- o# A
decomposition and fusion are complete and the melt is clear, except
3 ^5 b/ r7 Z5 x) wfor the silica content, and then cool. (Caution: Do not overheat the
/ K0 E$ @$ M# A$ @contents of the flask at the beginning, and heat cautiously during
1 B! c; ]" Z. j3 t% Z) kfusion to avoid spattering.)3 a6 k( z ~3 W7 O! N! @
To the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat2 q4 e8 K( M6 s3 }: @/ n7 T8 Y
carefully and slowly until the melt is dissolved. Cool, and carefully add* A. L; d1 c! x1 {0 r2 k
150 ml of water by pouring very small portions down the sides of the
& M" P) B0 H( B2 g: o, X8 K Q9 ]flask, with frequent swirling to avoid over-heating and spattering. Allow
# o: v+ E) ^' w6 Qthe contents of the flask to cool, and filter through fine ashless filter% m! W5 d) z" Y. l
paper, using a 60 degree gravity funnel. Rinse out all the silica from
/ D1 u+ G3 e0 ?, }' _, e7 E Gthe flask onto the filter paper with sulfuric acid solution (1 in 10).. W5 [1 ]) [7 o. Y$ s* V
Transfer the filter paper and its contents into a platinum crucible, dry in, c* E& c+ b# C c$ `1 J8 K9 g
an oven at 1200, and heat the partly covered crucible over a Bunsen' E* a% Y7 b& Y7 B. I* L- M
burner. To prevent flaming of the filter paper, first heat the cover from, D, k+ O/ b- H! Z" {6 z0 Z
above, and then the crucible from below. D1 G' i. ]' _/ `/ K! ]
When the filter paper is consumed, transfer the crucible to a muffle
3 A( o' H) n& qfurnace and ignite at 1000o for 30 min. Cool in a desiccator, and
2 v, I& f! {( {0 Eweigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated
. b3 u' V# [8 `8 _2 p' q5 E) V2 [; v' {hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first! Z$ w0 p2 W- n- L
on a low-heat hot plate (to remove the HF) and then over a Bunsen3 {1 B6 P5 e/ v4 K: J
burner (to remove the H2SO4). Take precautions to avoid spattering, M; H: a8 {! ]
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a
3 o) D4 Q- \' d& P* U. F! sdesiccator, and weigh again. Record the difference between the two- @9 a$ D: [% G6 l" x. S0 @
weights as the content of SiO2 in the sample.
- o7 I! u! e( E0 D. m: {% hMETHOD OF ASSAY
3 J) d' N3 U8 C/ a4 E9 \Accurately weigh about 150 mg of the sample, previously dried at 105o
# K) ]$ s0 b- P+ L3 Ifor 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water
]. e+ C) ]9 }- |& h+ fand shake until a homogeneous, milky suspension is obtained. Add 30
- E! V# t6 y& l9 B ^ml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially4 u) d! M( D' K4 d1 `0 a k
heat gently, then heat strongly until a clear solution is obtained. Cool,
2 S0 ~, v( Q/ B6 r, Ythen cautiously dilute with 120 ml of water and 40 ml of hydrochloric
% O7 M: u4 Y4 r ]0 T' g1 }. @acid, and stir. Add 3 g of aluminium metal, and immediately insert a
# F: l3 ~* j9 Crubber stopper fitted with a U-shaped glass tube while immersing the
. F9 @& d! A5 Q% E: [! i& L+ `# Kother end of the U-tube into a saturated solution of sodium
& }9 Q1 W) w' j$ l( qbicarbonate contained in a 500-ml wide-mouth bottle, and generate
7 p8 F: z5 ^, ]1 R" Xhydrogen. Allow to stand for a few minutes after the aluminium metal( d1 j$ M8 U7 {8 w2 O$ A4 U
has dissolved completely to produce a transparent purple solution.
1 u( ~) y3 L o" sCool to below 50o in running water, and remove the rubber stopper
" J1 A1 [- }+ a4 y0 icarrying the U-tube. Add 3 ml of a saturated potassium thiocyanate
% K# l7 m3 N8 T# P4 \solution as an indicator, and immediately titrate with 0.2 N ferric
/ L0 W+ {- U3 aammonium sulfate until a faint brown colour that persists for 30
. H4 l& Z6 ~& B/ c! \seconds is obtained. Perform a blank determination and make any
# ?4 e1 N% T4 N3 K8 I' A8 _0 _+ Rnecessary correction. Each ml of 0.2 N ferric ammonium sulfate is
" y9 s* F4 t6 U2 w# c6 |+ gequivalent to 7.990 mg of TiO2.' n g: O3 E3 W6 ~6 e
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