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二氧化钛(钛白粉)
二氧化钛(钛白粉)5 e0 T, C6 c" M1 N+ M5 K& S
4 }6 c, X3 \2 z4 i
JECFA关于二氧化钛(钛白粉)的结论
! Y2 e0 _) {1 n
H+ J& S! A8 z0 i, \" l4 z摘要: 2006年JECFA关于二氧化钛的结论- ]) T0 j5 p. c" c0 \/ d' G
ADI值:不作限制。- R: Z" i. G. [) a8 G; {
功能:着色剂, L5 w% \; l) X2 W3 b5 o5 v
" l% Y d, u5 K+ @/ Z1 G) L
TITANIUM DIOXIDE
: z" E# W0 K1 T0 v$ d% aPrepared at the 67th JECFA (2006) and published in FAO JECFA( T O8 ` J: x) v
Monographs 3 (2006), superseding specifications prepared at the 63rd( R9 j/ J* W$ p( v# ?; @+ _7 v
JECFA (2004) and published in FNP 52 Add 12 (2004) and in the: P' [; V4 h% p
Combined Compendium of Food Additive Specifications, FAO JECFA3 Z3 s0 A P% y0 b& G5 H
Monographs 1 (2005). An ADI “not limited” was established at the 13th
! M+ D4 J; }8 f3 B& U# _JECFA (1969).
6 A, M" o4 K+ pSYNONYMS
T# g1 @+ d/ y6 ^- |" vTitania, CI Pigment white 6, CI (1975) No. 77891, INS No. 171 J3 _0 O& U D4 O# z
DEFINITION; w4 i' p3 D7 ?' F: G6 |
Titanium dioxide is produced by either the sulfate or the chloride
+ ?5 D. U" F+ s- mprocess. Processing conditions determine the form (anatase or rutile3 }: E) d( m* H8 p
structure) of the final product.# f4 f2 J" e6 \/ V
In the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3)
. k# q' J% X( H+ O6 P" uor ilmenite and titanium slag. After a series of purification steps, the
) S: i+ a$ v+ w3 Y# |5 n; _isolated titanium dioxide is finally washed with water, calcined, and; J3 a1 B" U$ x# s
micronized.
B: a7 m% q n( Y. hIn the chloride process, chlorine gas is reacted with a titaniumcontaining
5 ?4 m9 k3 a" z: e9 P3 ^mineral under reducing conditions to form anhydrous
& I0 x# n. r- \2 `5 S3 X$ }3 N1 Etitanium tetrachloride, which is subsequently purified and converted to
1 H6 F5 v$ P9 D* r2 C( d/ G+ Otitanium dioxide either by direct thermal oxidation or by reaction with
& m2 v# d; L+ ~# @0 {# vsteam in the vapour phase. Alternatively, concentrated hydrochloric
8 w- q5 R, H( x! R* oacid can be reacted with the titanium-containing mineral to form a9 e( a" ?# U( @& }- g8 c2 ?; M
solution of titanium tetrachloride, which is then further purified and
; k m/ k% e5 i& W4 F1 Y; iconverted to titanium dioxide by hydrolysis. The titanium dioxide is
" ]! [0 p. P7 r2 }filtered, washed, and calcined.( S3 ? \9 \- W7 W
Commercial titanium dioxide may be coated with small amounts of
2 Y$ `$ ^/ Y6 y2 |) @+ ~; A4 c- kalumina and/or silica to improve the technological properties of the
" H8 D u: s: G. `+ iproduct.3 y& h P) W7 v! d- ~/ m
C.A.S. number 13463-67-7
; l3 u! T$ M3 K$ vChemical formula TiO2
' \ B% Z7 Y3 B2 W( r# YFormula weight
" F% p+ D) o' l$ X+ k6 N) L3 a+ c79.881 b7 n z, l* e) p+ x. ~6 g& _, J
Assay+ K1 M2 V S- S: n) T: X* ]
Not less than 99.0% on the dried basis (on an aluminium oxide and! I; _/ C$ i; u+ Z( n# L
silicon dioxide-free basis)# L' v8 z3 K6 U6 u) R" A
DESCRIPTION8 q! O! J; b1 ^$ N8 z! n: }
White to slightly coloured powder
( q( P( j0 l/ L! n$ s2 I8 uFUNCTIONAL USES
3 w% z4 s8 b k6 E' M0 `Colour
! j0 [) x8 F7 B5 a) s0 ICHARACTERISTICS
4 B; ^) t4 n S& P& b8 G; m, k1 x/ Y; `" h+ XIDENTIFICATION
) d, A% L8 d" u6 b7 \Solubility (Vol. 4)! o7 v3 m1 W$ i. }. z& J0 ]
Insoluble in water, hydrochloric acid, dilute sulfuric acid, and organic
6 v8 k. B, m8 i+ Wsolvents. Dissolves slowly in hydrofluoric acid and hot concentrated
2 u+ W5 O( f# x; \# xsulfuric acid.0 D: H0 [# d8 s1 N I& S
Colour reaction; x3 K% s# k* S) @* \: I
Add 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of! Y% X& }9 W: h
sulfuric acid appear, then cool. Cautiously dilute to about 100 ml with
5 Z+ }; E) a1 r& u- j# X5 Twater and filter. To 5 ml of this clear filtrate, add a few drops of
! }& i+ P( C: b5 @hydrogen peroxide; an orange-red colour appears immediately.
& W J& T0 C) k/ f4 a+ I* A( w9 jPURITY( j# p r# X4 ~1 b! e
Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h)
5 x/ P8 B ]6 c1 J2 qLoss on ignition (Vol. 4)3 ^- u8 }4 I- x' z
Not more than 1.0% (800o) on the dried basis
+ {& \1 k: M- O+ bAluminium oxide and/or
3 b! T( m4 w6 G! q0 wsilicon dioxide
0 G% {" @; e, G* KNot more than 2%, either singly or combined
5 p1 w$ q3 R( ZSee descriptions under TESTS
$ D, S, C1 A+ H* g/ ~Acid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing
s* f ~. {. }; ~# ?; Z" t- C$ |alumina or silica.0 ]" p$ S$ ^) J9 Q4 a
Suspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and) w8 ?% P+ _, J
place on a steam bath for 30 min with occasional stirring. Filter) g" y5 j- E) q! `; r
through a Gooch crucible fitted with a glass fibre filter paper. Wash# ~: }; Y+ K# V0 i0 v
with three 10-ml portions of 0.5 N hydrochloric acid, evaporate the7 W: _ z% E0 ^, N; q
combined filtrate and washings to dryness, and ignite at a dull red! ~: m* k# S8 b7 Q
heat to constant weight.9 V1 }# a# k: P( V6 ^4 G
Water-soluble matter" }" M1 D% [! c% p$ U
(Vol. 4)
, h1 j C1 x* m! ^1 q5 `Not more than 0.5%* x% f) X2 v6 x/ r! e
Proceed as directed under acid-soluble substances (above), using; T4 [6 J2 Z8 k& y! R3 W% R4 N
water in place of 0.5 N hydrochloric acid.. t, K- P8 a$ e- b
Impurities soluble in 0.5 N9 U0 f* t" o3 R9 p$ f5 v- h
hydrochloric acid
0 k2 \) T" {& a, J' lAntimony Not more than 2 mg/kg& h$ }1 I9 M+ F8 k. c; ^ L J5 N
See description under TESTS. M6 F6 X$ T% h
Arsenic Not more than 1 mg/kg
" u0 q5 E% c( PSee description under TESTS" T- {- @3 {% K1 p0 u
Cadmium Not more than 1 mg/kg
6 m/ C" A1 }3 u1 M V1 R4 d% C5 `See description under TESTS
& P# x, a' L+ b2 oLead. z/ w# [- ]( V
Not more than 10 mg/kg0 P: ~' l% E% g$ `
See description under TESTS! ]" F. N% u* u0 V2 M# U4 i$ Q
Mercury (Vol. 4) Not more than 1 mg/kg; { v" C! `) b2 {8 F |
Determine using the cold vapour atomic absorption technique. Select a, l( X3 _9 M& C0 R
sample size appropriate to the specified level
$ ]/ P: d s1 k/ C/ JTESTS5 s6 y$ i+ v/ ?5 t
PURITY TESTS+ Z! N6 `, S) n* J% q" R7 j
Impurities soluble in 0.5 N
/ P1 F9 T9 C2 n; O- c! y2 |/ I |hydrochloric acid. B- A4 g2 O3 |$ |
Antimony, arsenic,
5 g! m) F+ C, v# O0 N- W. Ncadmium and lead E+ p6 Z# C7 e4 E. m5 z% N
(Vol.4)
5 W/ a1 M8 w4 z# N7 m; uTransfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N
1 d: i; X: h) zhydrochloric acid, cover with a watch glass, and heat to boiling on a
7 F/ ]; \* \1 C# Dhot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml& B( l7 U# i Q E! p( Z7 e
centrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved
9 m5 I( W" ^! n3 D9 ?material settles. Decant the supernatant extract through a Whatman
) U( L- a4 V* Q+ C. K; }4 s6 bNo. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml
0 e( }' `$ ?* bvolumetric flask and retaining as much as possible of the undissolved
% w) f6 ^% ^( Amaterial in the centrifuge bottle. Add 10 ml of hot water to the original
3 r j; i. d3 q0 l* N8 T- gbeaker, washing off the watch glass with the water, and pour the
+ A6 U# e; R ^6 m8 Pcontents into the centrifuge bottle. Form a slurry, using a glass stirring
: M0 V1 |/ U- I2 I- r/ c9 Arod, and centrifuge. Decant through the same filter paper, and collect7 K* q$ b. ?6 I* v- i Y
the washings in the volumetric flask containing the initial extract., e6 S& F* ?% B- I
Repeat the entire washing process two more times. Finally, wash the( R9 u( ~2 X, L; G9 X
filter paper with 10 to 15 ml of hot water. Cool the contents of the flask
" E1 J5 X/ i6 ~. U c# r3 q7 c; gto room temperature, dilute to volume with water, and mix.
9 K: B" _* f: r: s KDetermine antimony, cadmium, and lead using an AAS/ICP-AES
6 B3 H- B& w4 jtechnique appropriate to the specified level. Determine arsenic using the
/ o( g7 e9 W8 Q8 q9 K9 HICP-AES/AAS-hydride technique. Alternatively, determine arsenic using% z6 H4 Z& q+ y; F* M
Method II of the Arsenic Limit Test, taking 3 g of the sample rather than
- l3 c2 I) M$ R! t q9 n# I1 g. The selection of sample size and method of sample preparation5 T+ N0 P% Z: [; y8 c
may be based on the principles of the methods described in Volume 4.* p4 r* }5 O. a1 ?
Aluminium oxide Reagents and sample solutions3 W4 P- `, _; y& p
0.01 N Zinc Sulfate$ ^& _# r3 U2 _! }
Dissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to
8 ]8 @4 Q; b5 Y' ?$ s6 F" xmake 1000 ml. Standardize the solution as follows: Dissolve 500 mg
' C$ G. l+ H2 E, R; aof high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of
8 m8 r6 |& o9 r& V3 Dconcentrated hydrochloric acid, heating gently to effect solution, then' f2 `6 `& ~* L0 z
transfer the solution into a 1000-ml volumetric flask, dilute to volume
5 x M- f8 v& |4 z( q/ [with water, and mix. Transfer a 10 ml aliquot of this solution into a 500& R) H' q$ O; K9 c
ml Erlenmeyer flask containing 90 ml of water and 3 ml of
) H. I" B; g9 K! E1 v4 k4 ^5 I( Tconcentrated hydrochloric acid, add 1 drop of methyl orange TS and% R, ^# o. q7 |) A$ e+ l$ a! D
25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add,
( K- r( K* l# D& @# x4 h8 [: qdropwise, ammonia solution (1 in 5) until the colour is just completely
. H v6 D! |7 _changed from red to orange-yellow. Then, add:$ ?5 x. H8 I `# ]8 C- D: e, V c
(a): 10 ml of ammonium acetate buffer solution (77 g of$ V: P o: ~+ a. F
ammonium acetate plus 10 ml of glacial acetic acid, dilute to
! N6 B" J6 B {8 ?9 z! z/ F0 Q1000 ml with water) and3 D% }% u8 p: l% t' C
(b): 10 ml of diammonium hydrogen phosphate solution (150 g
& [ l* Z* s1 K7 h7 r7 w% K0 Eof diammonium hydrogen phosphate in 700 ml of water,6 }% I( }& V' E0 {' i, p+ b: P
adjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,+ M9 |. R: g2 Q& n) y1 L6 j7 r. F
then dilute to 1000 ml with water).
# Z( i1 Q% C; d7 Q) hBoil the solution for 5 min, cool it quickly to room temperature in a
" O* g% D5 g' {' V+ cstream of running water, add 3 drops of xylenol orange TS, and mix.
: l" Z2 c" C0 V' qUsing the zinc sulfate solution as titrant, titrate the solution to the first
# |; u$ R* u5 ^/ u. x7 Tyellow-brown or pink end-point colour that persists for 5-10 sec. (Note:
" t- l) {" n) l7 O! x0 ~This titration should be performed quickly near the end-point by
$ p# h+ E* t. _adding rapidly 0.2 ml increments of the titrant until the first colour& j# B/ k6 q6 T- `8 {
change occurs; although the colour will fade in 5-10 sec, it is the true! \2 O& q* c, v7 _+ P' s1 {4 E3 k' \
end-point. Failure to observe the first colour change will result in an* Y f; J5 A# @$ V5 F( q
incorrect titration. The fading end-point does not occur at the second
; F$ R2 R! x' F _, [# xend-point.)+ c8 |/ S& G: ?- e% s9 G% \
Add 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a
" o/ A7 J. ~/ z% {! z) |8 Fstream of running water. Titrate this solution, using the zinc sulfate
, D) P* ~' S3 ?, n. Q' \( x+ Y* m+ ksolution as titrant, to the same fugitive yellow-brown or pink end-point
: |$ R$ W+ B$ J$ C/ ^as described above.6 F! z: Y) x' A2 ^
Calculate the titre T of zinc sulfate solution by the formula: ?- p# n& @: `% P' L
T = 18.896 W / V$ T! R5 A/ \2 h. G$ r
where3 G v: n( B/ V% Q+ T3 \
T is the mass (mg) of Al2O3 per ml of zinc sulfate solution
, z/ U' y8 V0 `; e, _& ZW is the mass (g) of aluminium wire
$ b& F* L% L4 pV is the ml of the zinc sulfate solution consumed in the" ?/ i$ }8 W% M+ h) P$ a
second titration
" L1 L% m/ z6 [18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and3 _* L3 k3 p8 y$ o& G
R is the ratio of the formula weight of aluminium oxide to5 W2 {8 k% j, Q" s" }1 V7 r
that of elemental aluminium.' P' O [1 I, L5 Y$ a; F
Sample Solution A
0 T+ X* i$ e0 N: R* R/ G/ rAccurately weigh 1 g of the sample and transfer to a 250-ml high-silica7 d7 g( H2 e9 [8 s
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).8 S, g* E% U, k6 E6 a g g
(Note: Do not use more sodium bisulfate than specified, as an excess1 t$ Q& r: {) z* I. S% x
concentration of salt will interfere with the EDTA titration later on in the. {7 ?& e! q3 U: o+ _" ]- W
procedure.) Begin heating the flask at low heat on a hot plate, and: G) E+ T8 P3 w0 A5 G# \
then gradually raise the temperature until full heat is reached." j# h5 O" X: h! c
(Caution: perform this procedure in a well ventilated area. ) When5 O' C r1 z8 e, X1 K6 ^
spattering has stopped and light fumes of SO3 appear, heat in the full
+ m$ _0 c0 Z% d' F: Vflame of a Meeker burner, with the flask tilted so that the fusion of the7 M f; q$ K7 v
sample and sodium bisulfate is concentrated at one end of the flask.' f E/ c7 ~6 S: H6 ]) {
Swirl constantly until the melt is clear (except for silica content), but; F9 I8 {; Y% {& C1 T' j# y
guard against prolonged heating to avoid precipitation of titanium
3 Y Z) ]" _1 c. |3 X* qdioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until
+ m/ I; o) S% B" V$ A* p% _. nthe mass has dissolved and a clear solution results. Cool, and dilute to
. n& U/ q( W5 M) D c. A120 ml with water. Introduce a magnetic stir bar into the flask.
" {# n5 K, k6 |2 X+ ?( f8 Z% TSample Solution B
7 d5 o6 J% R" J: ZPrepare 200 ml of an approximately 6.25 M solution of sodium
6 w3 t" f! s# u4 Q' v- chydroxide. Add 65 ml of this solution to Sample Solution A, while
0 g$ K7 H& L: a/ K! B' i7 istirring with the magnetic stirrer; pour the remaining 135 ml of the, _6 ^- ?/ v) ?' N0 f$ g4 [
alkali solution into a 500-ml volumetric flask.- s# |: V0 q, y$ t8 {. _/ {. d
Slowly, with constant stirring, add the sample mixture to the alkali0 L# p( F) y5 G# N% p6 r
solution in the 500-ml volumetric flask; dilute to volume with water,- _, P& E l5 E4 d# N/ Q
and mix. (Note: If the procedure is delayed at this point for more than
' d" l; H+ N' }. O) W2 hours, store the contents of the volumetric flask in a polyethylene
$ b: c; H- p8 c& `7 q! B* nbottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),
' v8 b9 q8 E [- P. e* J% zthen filter the supernatant liquid through a very fine filter paper. Label2 ]0 a& w, G; }! M5 [
the filtrate Sample Solution B.
1 l" X( q! C% N' D4 ySample Solution C0 n4 ^. m8 T" r0 g% R
Transfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer/ h0 M$ A+ b+ }/ h1 @
flask, add 1 drop of methyl orange TS, acidify with hydrochloric acid
3 m$ g: w5 d1 W" R4 psolution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.02
9 G2 r' R3 r7 hM disodium EDTA, and mix. [Note: If the approximate Al2O3 content is5 P) x. P* I$ v; J( u- u8 ^! d8 s) H
known, calculate the optimum volume of EDTA solution to be added* `9 G- S. y) E% ~, d
by the formula: (4 x % Al2O3) + 5.]
5 n0 a% O: Z8 L5 G; {- lAdd, dropwise, ammonia solution (1 in 5) until the colour is just
) |. K( K; }& jcompletely changed from red to orange-yellow. Then add10 ml each! ~) A B Q& G+ P" j! s: ^
of Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to
$ g' `6 y2 C% Z: Zroom temperature in a stream of running water, add 3 drops of xylenol n# V5 j- |/ Q
orange TS, and mix. If the solution is purple, yellow-brown, or pink,! e7 E2 H& k6 m) U
bring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired
1 m! v a- c# DpH, a pink colour indicates that not enough of the EDTA solution has
1 p5 ]. Q7 O7 a# N) O; b5 o, Ibeen added, in which case, discard the solution and repeat this$ w% ^# e! [; r2 x/ Q$ u) W
procedure with another 100 ml of Sample Solution B, using 50 ml,
6 N: \; f7 o6 V4 f! vrather than 25 ml, of 0.02 M disodium EDTA.8 Y( D: t: m& u, j0 w i! A- G
Procedure) ^) n. c6 z* o, |9 A* U
Using the standardized zinc sulfate solution as titrant, titrate Sample
5 j+ A0 _& O% u, dSolution C to the first yellow-brown or pink end-point that persists for8 A; E& B6 ~/ \. j% t6 ~% R
5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first9 o& s; d2 I7 i' E0 C' O/ z( I( Q1 L$ @
titration should require more than 8 ml of titrant, but for more accurate
& Q9 `1 X1 O1 d' ]. awork a titration of 10-15 ml is desirable.0 x; f% W- q' g
Add 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5
0 M ] P$ ^5 C$ C7 l; lmin, and cool in a stream of running water. Titrate this solution, using# O' B; k% `# G7 {* l% Y
the standardized zinc sulfate solution as titrant, to the same fugitive
. `5 v* \9 R# ^/ s' o9 V/ Pyellow-brown or pink end-point as described above.0 ^/ W" g4 X$ w4 s/ r) N
Calculation:8 H p9 S( Y* i$ d
Calculate the percentage of aluminium oxide (Al2O3) in the sample, Y1 d* i7 q' ], l. S" S8 i/ l
taken by the formula:
: h! k' H7 I) `( x& \3 r% Al2O3 = 100 × (0.005VT)/S
) X/ V1 a" a' ?. f* ?6 J1 a) p. X7 awhere
2 G, t ~% R2 N5 D9 kV is the number of ml of 0.01 N zinc sulfate consumed in8 v, [& k' r# {& \0 i/ I& @
the second titration,
3 R, L; G/ j7 B# _ Q! pT is the titre of the zinc sulfate solution,% C# W6 B" V" z2 y; z
S is the mass (g) of the sample taken, and9 M) {% D# E$ }; o
0.005 = 500 ml / (1000mg/g × 100 ml).
1 \9 Y2 ^1 c+ W/ q: nSilicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica6 W5 s# ?% V- I9 D" d; i0 o
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).
. S; _( ?! k L0 V5 C4 DHeat gently over a Meeker burner, while swirling the flask, until
$ q9 f& [1 J+ m; N* `decomposition and fusion are complete and the melt is clear, except& ] A+ J) ]# g3 m+ N
for the silica content, and then cool. (Caution: Do not overheat the
' z! g W- S, Rcontents of the flask at the beginning, and heat cautiously during% J D. O9 R- c z
fusion to avoid spattering.)
! \( b4 m7 e0 Q4 S8 ]To the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat J$ q: [) A1 [# Y- x" L
carefully and slowly until the melt is dissolved. Cool, and carefully add9 k) ^' M3 O/ C! P4 n; f
150 ml of water by pouring very small portions down the sides of the
) ]- V- G8 |: O+ U+ [flask, with frequent swirling to avoid over-heating and spattering. Allow
$ r: n, U4 d) z4 _# L6 ythe contents of the flask to cool, and filter through fine ashless filter
6 z+ I7 }6 _& j% i ipaper, using a 60 degree gravity funnel. Rinse out all the silica from# u ]. ?+ x; a" X
the flask onto the filter paper with sulfuric acid solution (1 in 10).9 R+ g3 [/ p9 R; V7 u1 D0 ^9 B
Transfer the filter paper and its contents into a platinum crucible, dry in; c2 W8 O$ ]6 ]# Z; k/ Q
an oven at 1200, and heat the partly covered crucible over a Bunsen5 w5 _% _* v) C7 p
burner. To prevent flaming of the filter paper, first heat the cover from" o7 d, o7 ?/ C" j k3 k& y
above, and then the crucible from below.
9 M+ C; t7 k4 } t/ nWhen the filter paper is consumed, transfer the crucible to a muffle
4 x `' o1 ^! T& @* P/ h& U! Tfurnace and ignite at 1000o for 30 min. Cool in a desiccator, and y7 F& v7 b8 D- h
weigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated( c+ ?3 l3 M9 Q. X+ H
hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first
+ D) A, a' p! F, D% T& Uon a low-heat hot plate (to remove the HF) and then over a Bunsen+ H- G/ s) e. o% f
burner (to remove the H2SO4). Take precautions to avoid spattering,5 A6 T: p8 P% e+ c+ ]/ X
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a
0 [. P$ f. G! wdesiccator, and weigh again. Record the difference between the two
9 ]& [1 E$ n' |. y4 ?2 wweights as the content of SiO2 in the sample.5 j: L, S, \8 E4 e" L
METHOD OF ASSAY
# ]2 l0 e; |$ {, R4 ^$ s( @& xAccurately weigh about 150 mg of the sample, previously dried at 105o/ ~0 V0 {8 m$ X. g
for 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water
8 H3 t5 o7 E, H" M. d3 e0 g- jand shake until a homogeneous, milky suspension is obtained. Add 30" i6 ~6 z8 R+ E5 E6 _2 H; e1 R
ml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially
( C$ h; b3 G! G# b( A$ w1 S z* M, Sheat gently, then heat strongly until a clear solution is obtained. Cool,
8 e9 Q: t7 M+ v, C" F# rthen cautiously dilute with 120 ml of water and 40 ml of hydrochloric" d, N1 u2 \ K. ~9 B: g4 U; K
acid, and stir. Add 3 g of aluminium metal, and immediately insert a
4 ^+ B7 Q/ f( f5 d& m: orubber stopper fitted with a U-shaped glass tube while immersing the) H) t; f' O# _8 @0 F
other end of the U-tube into a saturated solution of sodium( E( u' T) q) U, ?; J) x6 P
bicarbonate contained in a 500-ml wide-mouth bottle, and generate
/ J' Y1 Z0 S8 X; J# A# dhydrogen. Allow to stand for a few minutes after the aluminium metal$ a u4 i9 w% K2 [, I
has dissolved completely to produce a transparent purple solution.
2 Q/ m+ D* Z) m+ S& vCool to below 50o in running water, and remove the rubber stopper
* Z$ N3 E) g9 q5 W: Acarrying the U-tube. Add 3 ml of a saturated potassium thiocyanate
- u, G F6 \0 Y! K4 isolution as an indicator, and immediately titrate with 0.2 N ferric
2 \" `* n& V z* \ammonium sulfate until a faint brown colour that persists for 30% w. H* `2 F& E) B$ V& M
seconds is obtained. Perform a blank determination and make any
( h$ e& `3 ~2 R) S, V; o* [1 rnecessary correction. Each ml of 0.2 N ferric ammonium sulfate is% _$ Y/ P7 l! ~5 | n
equivalent to 7.990 mg of TiO2.: n1 N U0 T: d# p7 w. b
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