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二氧化钛(钛白粉)
二氧化钛(钛白粉)
) L/ t3 Y, R, ^ r. O- H
" T0 S: t. ~9 @8 |6 eJECFA关于二氧化钛(钛白粉)的结论& d% I* g/ X* h" C$ ^
5 h" s$ g- }* o
摘要: 2006年JECFA关于二氧化钛的结论
" D W; x/ r9 R7 M& K$ bADI值:不作限制。% B$ u" _$ k( L
功能:着色剂& v" X! `7 d. |. m! f$ u" c
1 V- t6 D/ U7 X9 [
TITANIUM DIOXIDE
7 ^+ `: i5 `3 E* r# D8 E: yPrepared at the 67th JECFA (2006) and published in FAO JECFA
% h' K p6 m6 _) c% OMonographs 3 (2006), superseding specifications prepared at the 63rd' J* q/ J# C0 Q" m7 e) f7 G
JECFA (2004) and published in FNP 52 Add 12 (2004) and in the
; @: M8 M# f* k$ u* h+ n+ y, WCombined Compendium of Food Additive Specifications, FAO JECFA* |9 G( l) d9 B$ ]: H
Monographs 1 (2005). An ADI “not limited” was established at the 13th
3 \3 ^+ U2 M9 CJECFA (1969)./ v& y/ X% {3 z& o0 v S& h; ?
SYNONYMS
! L3 \9 S# v- P5 B4 j4 JTitania, CI Pigment white 6, CI (1975) No. 77891, INS No. 171# {. ~5 _2 M- v1 ^9 q+ H
DEFINITION
9 Z6 k+ y3 B, w3 J% N2 h8 S2 c" y8 aTitanium dioxide is produced by either the sulfate or the chloride
, ^: N5 Y" Z. Z b8 k0 K/ Jprocess. Processing conditions determine the form (anatase or rutile
# @) B1 ]# K0 b& V! T+ n7 O! Jstructure) of the final product.% `- N f( e7 u0 D) A9 l/ j
In the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3)" [& z' O6 t2 Z6 H
or ilmenite and titanium slag. After a series of purification steps, the
6 @/ m9 [! L1 e+ B4 D \: Cisolated titanium dioxide is finally washed with water, calcined, and' {4 ]. |1 {1 _" w# a
micronized.! Y- s/ f9 ~5 }( a% a
In the chloride process, chlorine gas is reacted with a titaniumcontaining- n: l6 j- u8 V" B, A
mineral under reducing conditions to form anhydrous
: X9 q3 U' Q2 m) |1 ]0 a l! c! v( ltitanium tetrachloride, which is subsequently purified and converted to/ w& x# s7 \ o, S1 x3 y$ [! k8 D- A
titanium dioxide either by direct thermal oxidation or by reaction with, N7 b- a+ h8 ]% p
steam in the vapour phase. Alternatively, concentrated hydrochloric
* T! S+ ]" c* R. l& c& Oacid can be reacted with the titanium-containing mineral to form a
, \5 T3 E9 ^1 U1 h6 o6 y3 `solution of titanium tetrachloride, which is then further purified and
8 [3 @$ V- s3 M" j/ N5 Sconverted to titanium dioxide by hydrolysis. The titanium dioxide is, i' `$ u6 y) ]6 }7 d I% {( h/ [! E
filtered, washed, and calcined.5 `: V- o9 m2 J0 Y7 L* E& ^
Commercial titanium dioxide may be coated with small amounts of
0 M$ Z* p9 w0 V4 q; I6 L7 k3 ?alumina and/or silica to improve the technological properties of the# L3 s$ Y1 t0 C6 l' b
product.9 x7 W& ^, x+ [
C.A.S. number 13463-67-7
$ M0 E$ _( R3 h: h$ V B! A: V2 qChemical formula TiO2
7 B6 E1 \$ w( t" H3 `) RFormula weight$ l4 C7 ]' Q3 y. a' P
79.88
2 x/ I! p7 l5 X7 M$ ?# V$ SAssay
- _" ~2 o" I0 C( N/ [Not less than 99.0% on the dried basis (on an aluminium oxide and, f3 A* v' e7 h& ^: p
silicon dioxide-free basis)3 j) T7 {, H+ P
DESCRIPTION8 u8 Q% S6 a6 j Y' w* [, l% o% m1 g
White to slightly coloured powder
2 T, P- p, J; S8 d, V5 nFUNCTIONAL USES
/ z" B3 _/ t1 H' C( [ u" FColour) |+ g# }# x& N4 P
CHARACTERISTICS0 [3 v2 |- v a2 V, k
IDENTIFICATION
8 t; x: O5 U; y( B0 c; [4 rSolubility (Vol. 4)7 R0 ~. V3 b$ T) @
Insoluble in water, hydrochloric acid, dilute sulfuric acid, and organic
& A' T/ \, t) Y, J# l: {2 Hsolvents. Dissolves slowly in hydrofluoric acid and hot concentrated
. Y. o7 Z/ p4 T7 s% P+ h1 Usulfuric acid.
* q/ O {9 K2 }$ k7 D7 o" J; [$ e0 mColour reaction
" x+ l7 Q1 U2 P, ^" v1 lAdd 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of
* a* _7 B& t. q8 g, c' d" tsulfuric acid appear, then cool. Cautiously dilute to about 100 ml with. M, q; [, i6 x3 B1 G$ K
water and filter. To 5 ml of this clear filtrate, add a few drops of4 o3 v/ F5 l/ X! m8 Y$ b
hydrogen peroxide; an orange-red colour appears immediately., U! c: S% h/ s7 Q( U: z
PURITY3 G# Y X8 X- p1 j" \4 b
Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h) x& r7 m6 t. ^% G) t |4 F# h% I
Loss on ignition (Vol. 4)
7 \. s( Z2 J% \& p4 a: tNot more than 1.0% (800o) on the dried basis. m* i) I0 B1 K7 c# y5 c
Aluminium oxide and/or
4 o2 m m. z: N9 Usilicon dioxide
$ q) Z+ }4 a, |; hNot more than 2%, either singly or combined' x5 P/ O+ ~# m. |0 S7 |, [- F6 a
See descriptions under TESTS
. S U9 r: c$ w- NAcid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing% k8 M5 X! V0 P1 u% V+ M0 T9 j" i
alumina or silica.
% I( t) h" [7 t9 CSuspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and
& L: W+ w' ~2 Mplace on a steam bath for 30 min with occasional stirring. Filter. s1 [3 A7 U% F$ B# G, F( A& c1 z
through a Gooch crucible fitted with a glass fibre filter paper. Wash
2 S! E# P3 E+ C0 Z5 W2 \with three 10-ml portions of 0.5 N hydrochloric acid, evaporate the
& n2 @: t. ~+ \: \9 K# @combined filtrate and washings to dryness, and ignite at a dull red6 }) D/ Z! E! M) Y. u
heat to constant weight., q0 Q) S9 g# ?, }( F2 D
Water-soluble matter
" e- E) \ j/ ^(Vol. 4)% S" n* O4 l/ L5 K4 a( J8 t. s+ g4 n# O
Not more than 0.5%
+ g' k2 k g3 z: ~' _ _Proceed as directed under acid-soluble substances (above), using
; o1 f$ R. c) ?9 k: h. e5 Nwater in place of 0.5 N hydrochloric acid.# s9 a4 O0 F1 d! T
Impurities soluble in 0.5 N
, m! y6 T3 ]- `2 i6 Whydrochloric acid! V' N6 x4 M& c+ s# S6 \ s
Antimony Not more than 2 mg/kg" e: o& b+ e% e' @
See description under TESTS( H% P J: i8 e5 D/ Z% ]4 g" r
Arsenic Not more than 1 mg/kg0 q& ^3 F% w5 U6 u/ u8 W, y; D
See description under TESTS3 ^; ~$ @4 @7 E0 B5 C
Cadmium Not more than 1 mg/kg: U8 l- B9 c6 y5 |
See description under TESTS
8 ^/ q C, s, m8 a( K6 P. tLead7 L) u: ]+ m+ o6 d7 [8 g, E3 l
Not more than 10 mg/kg$ M0 i1 W, F3 X0 N4 ^" B9 E
See description under TESTS
& c9 }, N1 J3 V* h! d# aMercury (Vol. 4) Not more than 1 mg/kg
5 ?! M6 ~) V/ g: o6 W2 l/ ^9 |Determine using the cold vapour atomic absorption technique. Select a
. h& w% x, Z' ^! A( Ssample size appropriate to the specified level: a) n* G' ?( l& E3 X( A
TESTS* a3 v. \8 M4 w7 }9 B$ p: G' Q8 W
PURITY TESTS* F( \1 o e! v3 D5 @0 u4 f( @
Impurities soluble in 0.5 N
2 B. T3 @4 z2 T9 I/ I8 Mhydrochloric acid
& {9 B9 e" C) r9 I% W* M2 ]Antimony, arsenic,
2 J9 ?* U, r2 ~( K8 scadmium and lead
0 w/ z1 c) @5 j(Vol.4)& z. b' j# l/ F+ M; [- V* Q
Transfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N
. O2 X9 ^, T, ghydrochloric acid, cover with a watch glass, and heat to boiling on a
% w6 K" G+ j |# ]- I2 ~# d/ ?9 yhot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml
, n, |$ ~8 z+ h" a& Xcentrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved
f1 _3 Y2 O! a" tmaterial settles. Decant the supernatant extract through a Whatman% N! B3 u" r4 K K* T0 \5 e$ k
No. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml$ V. X4 a1 q! H" i8 {; H, t+ g
volumetric flask and retaining as much as possible of the undissolved
! o& A" V P3 f8 ^material in the centrifuge bottle. Add 10 ml of hot water to the original" Y! {" D) O. M3 U7 S
beaker, washing off the watch glass with the water, and pour the
5 ~0 w* ~* ?4 t. R! _contents into the centrifuge bottle. Form a slurry, using a glass stirring
3 e7 e9 c- I5 Frod, and centrifuge. Decant through the same filter paper, and collect$ s" L1 d2 {3 z# X/ j5 p
the washings in the volumetric flask containing the initial extract.
3 m4 M% |% d- u$ X0 o6 @( P, P: W3 vRepeat the entire washing process two more times. Finally, wash the4 Z. j/ O+ M9 M2 X+ {
filter paper with 10 to 15 ml of hot water. Cool the contents of the flask& ~, ?7 `( l2 {2 w) h# i9 x
to room temperature, dilute to volume with water, and mix.
4 f" H! F$ N" Z, I3 I. bDetermine antimony, cadmium, and lead using an AAS/ICP-AES
) X7 o1 x! u: q! Ctechnique appropriate to the specified level. Determine arsenic using the
2 k1 g& t* l1 o, ?ICP-AES/AAS-hydride technique. Alternatively, determine arsenic using
: u. _9 a& J' T7 m6 {Method II of the Arsenic Limit Test, taking 3 g of the sample rather than" Q- T: x5 z' Y4 a, F
1 g. The selection of sample size and method of sample preparation: c8 {6 ^8 @. D1 g6 H& c, B
may be based on the principles of the methods described in Volume 4.
( _1 R+ m; X9 \9 }3 |. C) sAluminium oxide Reagents and sample solutions
* @' H4 w! ]2 e( k; j0.01 N Zinc Sulfate" I( h5 z1 u4 R- X) u
Dissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to% t2 l- t) E A; K
make 1000 ml. Standardize the solution as follows: Dissolve 500 mg1 F3 p/ H3 Z- s% l5 J9 M1 l& R
of high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of
! ^4 e. r/ J# {6 s3 G# Fconcentrated hydrochloric acid, heating gently to effect solution, then p5 ~+ E. b3 V
transfer the solution into a 1000-ml volumetric flask, dilute to volume% S; t4 a' _2 z) o$ F- L. i
with water, and mix. Transfer a 10 ml aliquot of this solution into a 500
( i& }+ |) N$ T1 F- P" `ml Erlenmeyer flask containing 90 ml of water and 3 ml of2 O0 r7 o0 `( Y5 _) g+ j- {
concentrated hydrochloric acid, add 1 drop of methyl orange TS and
, w z* I" B9 ~+ h/ V" v( x25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add,( _7 z2 T/ ~9 H! K
dropwise, ammonia solution (1 in 5) until the colour is just completely: {. R3 X5 y( p" {4 n' m6 A
changed from red to orange-yellow. Then, add:! B7 t4 ?/ D5 j) m. [) L( M, g6 n; t
(a): 10 ml of ammonium acetate buffer solution (77 g of
0 A( z, n# D, C/ fammonium acetate plus 10 ml of glacial acetic acid, dilute to D4 A% z- i# H7 w# a! Q2 |& K. D- M
1000 ml with water) and1 V! ?1 d, Z; z# I; I$ \
(b): 10 ml of diammonium hydrogen phosphate solution (150 g
; S* B7 I; h7 H* r! Q* Gof diammonium hydrogen phosphate in 700 ml of water,$ W0 [5 A9 E& d0 [1 K! p
adjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,3 v3 @1 J* W% @0 m
then dilute to 1000 ml with water).4 I7 j+ M2 p. u, Y
Boil the solution for 5 min, cool it quickly to room temperature in a
. C+ x8 d" Z5 c' `6 r. T) ?% Pstream of running water, add 3 drops of xylenol orange TS, and mix.3 ^, _- _7 t2 \. H+ U6 X
Using the zinc sulfate solution as titrant, titrate the solution to the first5 r3 L2 W$ \9 L8 b1 e6 ~3 H5 U6 a B
yellow-brown or pink end-point colour that persists for 5-10 sec. (Note:
G: ^/ ?( e* d0 o! `This titration should be performed quickly near the end-point by. o$ R, J2 n' ?8 ]
adding rapidly 0.2 ml increments of the titrant until the first colour
& }7 i3 Z9 O& z% m4 m7 \) \( ychange occurs; although the colour will fade in 5-10 sec, it is the true
# t6 c6 z. ^, `# M: u: t tend-point. Failure to observe the first colour change will result in an4 V+ S& n' n0 O- D
incorrect titration. The fading end-point does not occur at the second
3 ^: N+ z* }5 g2 fend-point.)
* u t9 f3 t c% _( r9 k" m- FAdd 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a2 F" Y1 j1 e+ `$ k! o4 L
stream of running water. Titrate this solution, using the zinc sulfate
* Q5 \7 K- ^% X7 k7 _! u) j7 csolution as titrant, to the same fugitive yellow-brown or pink end-point/ J+ H" p H0 x5 B7 L3 V' Q
as described above.
" v8 r k! o7 ]5 e2 q' u. f7 h& I1 G/ KCalculate the titre T of zinc sulfate solution by the formula:
& m( ], w" v1 D9 u1 {* TT = 18.896 W / V
9 l9 {" X* \$ nwhere
& Z+ }! ^ Z! u2 c$ D4 @( l9 ]0 {T is the mass (mg) of Al2O3 per ml of zinc sulfate solution3 k$ u. U4 z9 k2 z1 b
W is the mass (g) of aluminium wire
* G1 P% a. Z' qV is the ml of the zinc sulfate solution consumed in the
# X/ s$ D4 ?. o$ A% ^- d' `second titration
% Z* y! O# z3 _18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and
4 V/ A+ l$ e0 z- _R is the ratio of the formula weight of aluminium oxide to1 Q9 ]2 d! q7 }; w: g
that of elemental aluminium.
* }1 \' v& `9 zSample Solution A4 R7 O+ y7 ^, C; A, F m0 {+ S3 h( L
Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica5 G& G" O& u. w
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).6 ]/ P# V t1 A0 O, x; o& g6 m) C
(Note: Do not use more sodium bisulfate than specified, as an excess) I- ~3 b( G8 E) [* O; a& T
concentration of salt will interfere with the EDTA titration later on in the o& @# K! R s8 r2 O% X
procedure.) Begin heating the flask at low heat on a hot plate, and) p% P) _+ ]# w% H
then gradually raise the temperature until full heat is reached.
3 ~1 F D7 q% m/ f) P# S |(Caution: perform this procedure in a well ventilated area. ) When- F8 t) ~9 w2 ~; l% F4 y
spattering has stopped and light fumes of SO3 appear, heat in the full
4 V$ A* j9 v) G/ bflame of a Meeker burner, with the flask tilted so that the fusion of the
; l- A' e0 n& r* i fsample and sodium bisulfate is concentrated at one end of the flask.9 y6 k0 Z3 }. m; b6 M3 G
Swirl constantly until the melt is clear (except for silica content), but0 N+ o5 l9 l0 N3 ?0 b1 c B
guard against prolonged heating to avoid precipitation of titanium6 O- R; r! Q* I J
dioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until
`; B. M- k: G0 Tthe mass has dissolved and a clear solution results. Cool, and dilute to# n _2 u, Q! L: Y* B) z) u
120 ml with water. Introduce a magnetic stir bar into the flask.$ M: }% ?% L. D( Z
Sample Solution B
1 {! {( ^# b6 b) b" W4 uPrepare 200 ml of an approximately 6.25 M solution of sodium3 d6 e6 j5 q; R: D7 \# d
hydroxide. Add 65 ml of this solution to Sample Solution A, while
3 w% k$ Y. D; A3 c& _/ S( istirring with the magnetic stirrer; pour the remaining 135 ml of the
$ y$ | ~% i4 Ralkali solution into a 500-ml volumetric flask.
7 r' Q2 `7 C1 K" VSlowly, with constant stirring, add the sample mixture to the alkali0 J9 I7 E# G( I2 x5 a. g
solution in the 500-ml volumetric flask; dilute to volume with water,
) S; C4 I/ U6 J5 ~, i7 gand mix. (Note: If the procedure is delayed at this point for more than
# `2 J# E8 I( j( o2 K2 hours, store the contents of the volumetric flask in a polyethylene
) ]8 A/ B; p7 ]8 Mbottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),5 a& M# ^1 |2 p" G2 q' V
then filter the supernatant liquid through a very fine filter paper. Label
3 x5 M8 }8 n$ E& a" Qthe filtrate Sample Solution B.
5 l6 e5 ?, B" j; y( l$ G, F3 LSample Solution C9 o+ H! D" ]6 w
Transfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer) ~9 d' t+ T% w/ _( k+ O. f# D' }
flask, add 1 drop of methyl orange TS, acidify with hydrochloric acid; q2 Y _+ g& X- u; ]+ z5 J
solution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.02
- E& x5 w( @! A6 oM disodium EDTA, and mix. [Note: If the approximate Al2O3 content is0 y1 U6 F7 C- _. N
known, calculate the optimum volume of EDTA solution to be added
" Z$ E' e) ^1 M4 fby the formula: (4 x % Al2O3) + 5.]
/ Z% p8 F% M5 p& ]: g7 \# aAdd, dropwise, ammonia solution (1 in 5) until the colour is just! Q: k$ o6 G$ z3 R2 s
completely changed from red to orange-yellow. Then add10 ml each
. _. E2 f8 a5 Y: z" Tof Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to+ S) F' c( _+ x% O! X* X- J' Q
room temperature in a stream of running water, add 3 drops of xylenol
1 }% E3 d9 V( f2 |orange TS, and mix. If the solution is purple, yellow-brown, or pink,
4 L" M. b7 D0 J4 I/ Z. ibring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired% m/ h$ q8 A' i
pH, a pink colour indicates that not enough of the EDTA solution has* U1 r' K: \$ U
been added, in which case, discard the solution and repeat this
; C$ ?2 u b d* \8 b* N7 Y& hprocedure with another 100 ml of Sample Solution B, using 50 ml,
" m) v/ a S+ p5 M+ Z$ Jrather than 25 ml, of 0.02 M disodium EDTA.
$ [& o K# s2 k6 ]4 NProcedure* i3 O- w2 ]! ]2 n8 \/ T N/ D1 E( x8 W+ |
Using the standardized zinc sulfate solution as titrant, titrate Sample8 h" O x' m& K7 X# {
Solution C to the first yellow-brown or pink end-point that persists for" b# p7 F7 n5 y* P, y
5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first
1 ^( ?: l! F) c1 Ctitration should require more than 8 ml of titrant, but for more accurate& y( R! C3 ]. K# Z1 n
work a titration of 10-15 ml is desirable.2 ^, c! w4 m! }' A5 x, S k5 ~& z
Add 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5' ^$ R& @/ s1 t6 ~( m* }' b
min, and cool in a stream of running water. Titrate this solution, using
! D W% T# l, P2 s0 w' rthe standardized zinc sulfate solution as titrant, to the same fugitive i" b2 E, y; {! O
yellow-brown or pink end-point as described above.
( e* Z- w- e# W2 e6 B7 f5 @3 ACalculation:' I! Z% |& _$ N, @# E
Calculate the percentage of aluminium oxide (Al2O3) in the sample
) l1 n$ a# i9 H& htaken by the formula:% ?1 S2 s) ^. T2 l$ k1 L
% Al2O3 = 100 × (0.005VT)/S
3 J1 C2 y: N/ d Z) x1 H, O4 i. H7 Lwhere: ~+ X$ I6 H: U# Q9 B
V is the number of ml of 0.01 N zinc sulfate consumed in
, R' n% S+ z+ m9 B3 i {. D" Othe second titration,
, [& N0 ~) c! K5 M$ w- zT is the titre of the zinc sulfate solution,3 ]# k u4 Q& I. S8 R- Y
S is the mass (g) of the sample taken, and4 f" O K# w4 E( n! d
0.005 = 500 ml / (1000mg/g × 100 ml).1 F2 Q8 V0 F5 z3 i: t! n
Silicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica
5 p7 ^! q. _ J% r: ?. m+ fglass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).2 n' u* K2 e& I7 J! \% V. q
Heat gently over a Meeker burner, while swirling the flask, until- p: t' l9 D" E6 X" @ T6 Q
decomposition and fusion are complete and the melt is clear, except
/ x3 w' `+ a C( w6 V+ B8 {" W2 Ofor the silica content, and then cool. (Caution: Do not overheat the
# O( m, F! [0 z7 J5 lcontents of the flask at the beginning, and heat cautiously during
. Z8 F, [9 s8 x5 R( b/ M, Nfusion to avoid spattering.)
5 l0 y2 J1 |9 Z! w* tTo the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat
. W. @0 o3 Z" H ncarefully and slowly until the melt is dissolved. Cool, and carefully add
. I! H2 W6 ~ [ [: e% L150 ml of water by pouring very small portions down the sides of the
+ F- m0 L" |$ Y5 gflask, with frequent swirling to avoid over-heating and spattering. Allow
) V' _4 a, M1 Y1 A* Q3 nthe contents of the flask to cool, and filter through fine ashless filter
, c* n$ `: ^9 w) ^9 X E2 Tpaper, using a 60 degree gravity funnel. Rinse out all the silica from
- }6 \2 X$ ^3 g ~3 f2 g5 _; `0 xthe flask onto the filter paper with sulfuric acid solution (1 in 10).9 ]) D; k: B: p. ~
Transfer the filter paper and its contents into a platinum crucible, dry in
" ? ]9 T. }+ P; `+ Uan oven at 1200, and heat the partly covered crucible over a Bunsen
* h3 k. ]6 {7 |0 m( Z+ a/ Iburner. To prevent flaming of the filter paper, first heat the cover from1 r: O, o9 p6 s+ r- @+ v
above, and then the crucible from below.
; P5 A& Z" s+ fWhen the filter paper is consumed, transfer the crucible to a muffle
* \+ t9 h3 n0 ?3 Q1 w- Xfurnace and ignite at 1000o for 30 min. Cool in a desiccator, and
" W( g) ^. V2 D2 w* \' s! {* `weigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated
1 k& j9 M" {# W7 e* B4 [hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first
8 Z' ^; l$ K2 E2 h2 Aon a low-heat hot plate (to remove the HF) and then over a Bunsen2 q5 A! ?% f% ?- j9 R
burner (to remove the H2SO4). Take precautions to avoid spattering,4 F( o9 U, d# b5 l6 f1 V, h' r
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a5 ?+ w! `0 q) o7 O9 ]- l' u$ \8 ?% _
desiccator, and weigh again. Record the difference between the two
2 C# y+ Z7 y2 Rweights as the content of SiO2 in the sample.
' h: K& @' ?. N% L# h9 KMETHOD OF ASSAY
0 c6 U" I7 I+ E- s+ o+ tAccurately weigh about 150 mg of the sample, previously dried at 105o7 I( p3 K1 N7 j( {
for 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water% ? x. l1 p" c2 O# R/ M
and shake until a homogeneous, milky suspension is obtained. Add 30
$ b& t5 e6 Z2 \4 J& r5 w9 [ml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially
/ `( {& P' z& p! A- J: jheat gently, then heat strongly until a clear solution is obtained. Cool,
% ^5 n* U2 L, w8 o" B/ U/ f# ithen cautiously dilute with 120 ml of water and 40 ml of hydrochloric
. h& _) m- q. v( N' w# B$ o0 b! Hacid, and stir. Add 3 g of aluminium metal, and immediately insert a
$ C* I1 B0 a7 J* @2 B Zrubber stopper fitted with a U-shaped glass tube while immersing the* M+ i$ o6 r( F. s$ H0 L! g
other end of the U-tube into a saturated solution of sodium2 V+ R$ h* Z Q+ v2 A4 H1 w( G
bicarbonate contained in a 500-ml wide-mouth bottle, and generate7 E& b6 i, \1 S" n: }* Z7 [
hydrogen. Allow to stand for a few minutes after the aluminium metal) o) J2 F9 k9 ^
has dissolved completely to produce a transparent purple solution.- s. x& f* Z! K; v
Cool to below 50o in running water, and remove the rubber stopper. Y) q7 S) {" c, C. }* P8 G
carrying the U-tube. Add 3 ml of a saturated potassium thiocyanate
( e: W Q/ E' q, e$ w5 Q1 t7 Esolution as an indicator, and immediately titrate with 0.2 N ferric
, U# h9 y* o' j3 _' rammonium sulfate until a faint brown colour that persists for 30/ i2 U3 _, p2 V5 N
seconds is obtained. Perform a blank determination and make any
% g8 y+ Y# _/ U1 Inecessary correction. Each ml of 0.2 N ferric ammonium sulfate is6 B- C" t; R" V
equivalent to 7.990 mg of TiO2." T& R+ a$ R- l. s* ~
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