|
沙发
楼主 |
发表于 2008-5-23 12:10:00
|
只看该作者
二氧化钛(钛白粉)
二氧化钛(钛白粉)
; U+ Z1 w. n2 V* k9 @$ e5 m
, _4 r7 |. b- {' x5 ~- ?4 ?8 z. R1 PJECFA关于二氧化钛(钛白粉)的结论
- q# k: a8 P" d. I$ b* e
; [2 b b+ C8 Z; o2 Z/ n摘要: 2006年JECFA关于二氧化钛的结论
( c+ g' D1 t1 g% G" k& O7 q' GADI值:不作限制。1 V2 G( x6 P( Y* a! t1 i
功能:着色剂
) c$ J1 b, d5 Y' d8 ~8 m, L: d
e. m! U. j4 }9 \( OTITANIUM DIOXIDE
/ }" Q$ l, p4 A _* [Prepared at the 67th JECFA (2006) and published in FAO JECFA
d% o5 ^, m9 t9 W' h, z- x7 N( NMonographs 3 (2006), superseding specifications prepared at the 63rd
; H. }* [* W. j2 B L8 c$ X- oJECFA (2004) and published in FNP 52 Add 12 (2004) and in the
" _1 P, e' q$ z+ [8 P. d- j0 ]/ jCombined Compendium of Food Additive Specifications, FAO JECFA
0 I9 g! a$ G' I6 X/ eMonographs 1 (2005). An ADI “not limited” was established at the 13th3 y4 Y) D C& j- n0 i8 I% {" e* |& o
JECFA (1969).+ l9 K: q0 V4 F7 }
SYNONYMS
* J8 ] [9 |9 x* I0 S! vTitania, CI Pigment white 6, CI (1975) No. 77891, INS No. 1714 Q; ~9 y- R6 v; \
DEFINITION8 p& g4 ^) F7 o: z' R
Titanium dioxide is produced by either the sulfate or the chloride$ x. a. h7 H+ D# r! X
process. Processing conditions determine the form (anatase or rutile
0 T( i; M2 ]5 D6 K! dstructure) of the final product.
: N: k0 p; Q: n" J- q* ]( o* LIn the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3)1 H6 g2 Y) |% j: t+ u+ [
or ilmenite and titanium slag. After a series of purification steps, the' q1 U F2 h; _1 s+ E, v
isolated titanium dioxide is finally washed with water, calcined, and
5 N7 y6 {0 A$ Y5 X9 |4 a0 ?micronized.
2 M9 D( Z5 u/ O8 mIn the chloride process, chlorine gas is reacted with a titaniumcontaining
6 A2 \* P/ }) imineral under reducing conditions to form anhydrous1 U+ Z a+ W( z
titanium tetrachloride, which is subsequently purified and converted to7 L+ H8 H! \/ t6 ?& [, X
titanium dioxide either by direct thermal oxidation or by reaction with
+ W7 G5 T0 F0 U$ F. [ l0 dsteam in the vapour phase. Alternatively, concentrated hydrochloric! B, E' C% m5 q% x1 c2 u
acid can be reacted with the titanium-containing mineral to form a
2 p# w! \# n2 \7 Q! e- A% z5 @solution of titanium tetrachloride, which is then further purified and: O) x1 n: ^' _- z5 T
converted to titanium dioxide by hydrolysis. The titanium dioxide is6 |% f, u, D, j# l
filtered, washed, and calcined.7 l* v7 z2 s9 @* A4 H
Commercial titanium dioxide may be coated with small amounts of
0 ]* u1 Z1 n' V# a; j6 D* J& R5 X& t( D+ Malumina and/or silica to improve the technological properties of the
8 w0 I- T4 Q U- ] @8 dproduct.6 _$ p L' T& r; `7 ?, C, g1 P P
C.A.S. number 13463-67-7+ A4 @- o' u' Y; x
Chemical formula TiO2' q9 w( H" t* f' Y6 ?8 |& g
Formula weight8 C# K& d% j) Z" o+ k$ Z N6 N
79.88
; x1 O8 @: z4 t! P2 f* B% zAssay
* |* P8 w4 M3 f. t5 d, X8 e0 ANot less than 99.0% on the dried basis (on an aluminium oxide and/ o$ u+ A* ]! ~& @3 B; T1 S
silicon dioxide-free basis)
9 o+ o3 a1 r; VDESCRIPTION
5 I! ^- J; l0 u+ ]9 eWhite to slightly coloured powder5 |2 D0 \" L& y- f& t$ U
FUNCTIONAL USES2 x8 v; C% K" ]* u
Colour& f* F% z# A# W( v: t, t
CHARACTERISTICS
5 ]7 N6 ^. n) k; WIDENTIFICATION
+ T" ?6 ^6 C8 k4 xSolubility (Vol. 4)) p6 x, V9 v& p- H
Insoluble in water, hydrochloric acid, dilute sulfuric acid, and organic
( @- T# `: ~/ tsolvents. Dissolves slowly in hydrofluoric acid and hot concentrated/ M- {& X% ^; Y4 o% \9 H. ?( _0 o
sulfuric acid.' d* \8 m* a/ \ J. a& b
Colour reaction
* |( V9 n) f7 q/ iAdd 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of
% |. b) u! n ysulfuric acid appear, then cool. Cautiously dilute to about 100 ml with
; X4 Q2 W1 {/ \. Zwater and filter. To 5 ml of this clear filtrate, add a few drops of
I; Q" g9 j# G# Mhydrogen peroxide; an orange-red colour appears immediately.5 E$ I- s: D: n$ f7 H
PURITY; _5 Q/ _. p& L( M# L
Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h)
7 Q5 h. F6 A6 g) s' ELoss on ignition (Vol. 4) l# {# h2 F& p4 e
Not more than 1.0% (800o) on the dried basis
8 i3 _8 Y& U7 D9 U! bAluminium oxide and/or
# @7 Z1 t) f2 J* t. _silicon dioxide
( d2 t7 J: _3 Y! F- lNot more than 2%, either singly or combined8 @3 M1 V4 I& M [3 a9 Z/ D9 p2 d1 _8 N
See descriptions under TESTS) Z. g- M4 ]- R. n( N) P6 W' W
Acid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing
. B' v6 H. z" X/ d* Q4 D9 r3 ralumina or silica.$ g* X; V3 R- u9 c6 i/ w+ F
Suspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and
2 c6 r6 m( {: k8 ~0 V' Pplace on a steam bath for 30 min with occasional stirring. Filter
0 H! ?; k& E$ R4 c- `/ t# R/ uthrough a Gooch crucible fitted with a glass fibre filter paper. Wash& r5 W4 V* F( C. @
with three 10-ml portions of 0.5 N hydrochloric acid, evaporate the7 Q4 r+ j) Y2 b4 w5 ~+ W
combined filtrate and washings to dryness, and ignite at a dull red6 \8 c ?+ D: v& E/ V1 b
heat to constant weight.7 A$ ]* r7 U1 v9 c) J$ @: I
Water-soluble matter
# d8 g l+ S7 v(Vol. 4)- m& z& f/ q5 @1 ^
Not more than 0.5%
# q4 w( V4 Q6 ]) H5 u. q+ pProceed as directed under acid-soluble substances (above), using: l8 V% J& M) s7 [
water in place of 0.5 N hydrochloric acid., i3 n1 u. U! }# a3 P
Impurities soluble in 0.5 N/ Y# w7 |3 N5 Y9 {3 }+ b
hydrochloric acid- H1 e4 V5 v2 d: n% Z6 r x
Antimony Not more than 2 mg/kg; U2 ^' b+ T: A
See description under TESTS( i" c; E; z+ w& W v# G
Arsenic Not more than 1 mg/kg
! ?; K0 ~( g4 {% h. c6 OSee description under TESTS; u0 }( h- s6 Q3 ^
Cadmium Not more than 1 mg/kg
/ i( C) f' `7 F- m7 @, w4 @See description under TESTS
$ ^$ x2 P0 U8 D3 u# {! S. i1 PLead2 f# q. u; U6 O
Not more than 10 mg/kg
+ i+ I9 K3 W$ L! p# J! HSee description under TESTS
. P- K, d7 @! u+ T5 g1 x- _0 h8 EMercury (Vol. 4) Not more than 1 mg/kg
2 i1 S7 O2 _2 fDetermine using the cold vapour atomic absorption technique. Select a
1 a7 |, z! S# psample size appropriate to the specified level
# y* R c5 g# W0 C/ ^0 x* ?/ K( |TESTS
" q# c, Z) p8 e& o0 o; o9 zPURITY TESTS
% A# A1 Y4 U: E) P$ h( uImpurities soluble in 0.5 N
' i1 B( K; E4 u% w" u6 qhydrochloric acid
0 A+ q/ S& R/ r- |+ P, i( EAntimony, arsenic,' Y! w/ F# g8 Y: H, Y& Q
cadmium and lead& X' N7 Q8 s! f+ @
(Vol.4)! V+ m- J0 @3 y
Transfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N9 y; x5 E1 k" ]* M1 p
hydrochloric acid, cover with a watch glass, and heat to boiling on a. H/ c, r! i' o- r9 K8 ]: ]3 S
hot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml
. d7 K+ I2 W0 N( @! H& Tcentrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved
3 V, E4 ^! p% e) @( P8 cmaterial settles. Decant the supernatant extract through a Whatman
1 u% I5 J( y0 C* ^& [$ w! ENo. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml. S* [% }- X/ a f
volumetric flask and retaining as much as possible of the undissolved
+ X. J- q- g) v& j5 \. l1 b+ l4 Rmaterial in the centrifuge bottle. Add 10 ml of hot water to the original
/ Y5 I0 A2 A2 i' [) ?beaker, washing off the watch glass with the water, and pour the
: \& K4 }; @& ]6 Ucontents into the centrifuge bottle. Form a slurry, using a glass stirring& j7 _: h9 L* M
rod, and centrifuge. Decant through the same filter paper, and collect
7 B+ h/ [& f5 [7 R! Kthe washings in the volumetric flask containing the initial extract., h( P7 X$ S) Q! R' A
Repeat the entire washing process two more times. Finally, wash the, J6 H/ M& u) |6 ^$ @- V
filter paper with 10 to 15 ml of hot water. Cool the contents of the flask
/ c$ T5 F% R6 a/ d) l: M( N) eto room temperature, dilute to volume with water, and mix.
?! r C/ M+ e, H s" G9 ], Y9 tDetermine antimony, cadmium, and lead using an AAS/ICP-AES
/ `; O8 [2 [$ a6 r0 y2 ^) otechnique appropriate to the specified level. Determine arsenic using the: Q( H* @0 [6 ^) Y7 J/ B
ICP-AES/AAS-hydride technique. Alternatively, determine arsenic using% y8 C8 T+ G( K) a4 e
Method II of the Arsenic Limit Test, taking 3 g of the sample rather than
- t( k( m8 b: S0 w! R1 g. The selection of sample size and method of sample preparation3 t3 z. l# y# u1 {
may be based on the principles of the methods described in Volume 4. C1 w) D" \8 O( a" x9 [' E6 X+ B7 h
Aluminium oxide Reagents and sample solutions7 r% Q! g7 \( a
0.01 N Zinc Sulfate
6 m- B0 h0 }( m- D% {2 }2 r' cDissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to
" I2 L1 e7 J( t. ^: c$ cmake 1000 ml. Standardize the solution as follows: Dissolve 500 mg
% `$ c$ a; H1 q# W! u/ q) ^1 o9 tof high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of
3 k/ k4 |/ f/ u0 xconcentrated hydrochloric acid, heating gently to effect solution, then
% g9 Q' ~0 a$ \7 j- g' q$ Ctransfer the solution into a 1000-ml volumetric flask, dilute to volume
( [# z0 Q: N* H% J; [with water, and mix. Transfer a 10 ml aliquot of this solution into a 5004 \9 t4 E. O8 l8 Y: N. d C
ml Erlenmeyer flask containing 90 ml of water and 3 ml of Z' b) t, n0 H1 v) c: V* Z
concentrated hydrochloric acid, add 1 drop of methyl orange TS and- f' U0 x- z$ L+ {) h- l
25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add,4 W+ j' @3 x- I- [: a+ Y" Z
dropwise, ammonia solution (1 in 5) until the colour is just completely1 M- M7 s2 i0 Y
changed from red to orange-yellow. Then, add:2 G- e/ `( k" `1 h# n3 R) R0 h6 y6 x
(a): 10 ml of ammonium acetate buffer solution (77 g of
2 l y4 O' D1 f+ n% ?+ `- W4 Dammonium acetate plus 10 ml of glacial acetic acid, dilute to
' l: K3 k7 m: U! t1000 ml with water) and) P" j. g9 I+ k) l, [" P; l8 s
(b): 10 ml of diammonium hydrogen phosphate solution (150 g
1 H* k. i' E! ]3 E e! ?of diammonium hydrogen phosphate in 700 ml of water,
) G1 A8 L. a' B8 d3 hadjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,
+ r) Z6 h: _$ V4 V1 j/ F: D. K- uthen dilute to 1000 ml with water).6 Y! d2 b( j* k& C& N9 l* P& K: y
Boil the solution for 5 min, cool it quickly to room temperature in a
9 Q( ^7 Y, [* o Q8 w3 l. fstream of running water, add 3 drops of xylenol orange TS, and mix.9 _7 Z8 m7 Q0 u
Using the zinc sulfate solution as titrant, titrate the solution to the first5 e$ D: {; S6 O# n# l" a( ]% f
yellow-brown or pink end-point colour that persists for 5-10 sec. (Note:2 [( `: Y: h; S! W. R! w: B- O/ w
This titration should be performed quickly near the end-point by% `( n6 Q/ X# f
adding rapidly 0.2 ml increments of the titrant until the first colour
, X$ [. x7 H- C1 Wchange occurs; although the colour will fade in 5-10 sec, it is the true
6 L: m8 z4 W& a4 O1 ~* E0 @% ~- Uend-point. Failure to observe the first colour change will result in an
2 U2 s! ^9 @6 z) Yincorrect titration. The fading end-point does not occur at the second! I0 e# ]* f1 K% ?
end-point.)) ]2 J. x4 L( S/ r Q s
Add 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a9 d& ]* B7 P7 M8 K7 F
stream of running water. Titrate this solution, using the zinc sulfate
+ O2 W4 Y1 y3 Y$ esolution as titrant, to the same fugitive yellow-brown or pink end-point
' S/ H! r3 s8 A! gas described above.
X- n! \9 s/ h- P# B6 |) uCalculate the titre T of zinc sulfate solution by the formula:
" ^; n/ ?* N" D# H' CT = 18.896 W / V
' @7 r( \% i% {( W# f: k$ Lwhere$ w3 F( A- |, ?" E/ ?
T is the mass (mg) of Al2O3 per ml of zinc sulfate solution* i4 J7 `* \5 i1 k
W is the mass (g) of aluminium wire
4 W5 l6 i1 j+ GV is the ml of the zinc sulfate solution consumed in the
' W) b2 t1 G8 V4 Z4 dsecond titration
+ W& @. W g1 q" R" W* R @6 y! [- f18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and- H# E4 U8 f6 E: I5 `! y) O
R is the ratio of the formula weight of aluminium oxide to0 Z( p$ C2 u3 J
that of elemental aluminium.
& v* y7 [6 U, s6 rSample Solution A+ |: {& c4 [ G9 A
Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica7 _8 G& z- n) u$ A0 P* Z% M$ d+ I4 r
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).
: n' @0 g7 S0 C6 G5 T(Note: Do not use more sodium bisulfate than specified, as an excess
' @* C" i, }& d3 @* ~: }3 Z( fconcentration of salt will interfere with the EDTA titration later on in the
4 ~8 N! R4 U$ M( sprocedure.) Begin heating the flask at low heat on a hot plate, and, T( a. O1 `4 a3 ?2 ]
then gradually raise the temperature until full heat is reached.6 E* x! G9 Q: u* k( F8 _9 l
(Caution: perform this procedure in a well ventilated area. ) When
8 Z0 `3 b/ z" n1 _. f3 N9 g, V" ^2 [spattering has stopped and light fumes of SO3 appear, heat in the full
' v, w4 l0 h9 K. @* _flame of a Meeker burner, with the flask tilted so that the fusion of the# E; [& s/ i+ q% S3 C
sample and sodium bisulfate is concentrated at one end of the flask.- g1 z7 {; \/ X( B# V
Swirl constantly until the melt is clear (except for silica content), but3 ^! ?9 [6 x b9 O) H2 B
guard against prolonged heating to avoid precipitation of titanium7 Q* T# w2 n4 |# k# a' g- f M6 L
dioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until, v1 c3 j! @$ m1 G9 _; B2 I A
the mass has dissolved and a clear solution results. Cool, and dilute to+ Y( E* Y% U. Y3 G5 J
120 ml with water. Introduce a magnetic stir bar into the flask.
( Z% p8 |) Y% D& d+ n6 j |6 TSample Solution B) p k$ a( P7 i8 e+ b, E6 W# r% T
Prepare 200 ml of an approximately 6.25 M solution of sodium& @* e# F; P7 l# }% d
hydroxide. Add 65 ml of this solution to Sample Solution A, while: G- d0 j1 R" o1 E( B
stirring with the magnetic stirrer; pour the remaining 135 ml of the
8 T* {0 Q: M0 D2 T5 k4 f2 balkali solution into a 500-ml volumetric flask.' N; Q/ i8 y$ f. T
Slowly, with constant stirring, add the sample mixture to the alkali
) K4 Z( S$ y0 D i& D% {solution in the 500-ml volumetric flask; dilute to volume with water,# a0 `$ k7 ^3 n4 e& d) J8 E" C( H
and mix. (Note: If the procedure is delayed at this point for more than0 m! D. A; @% o: Q/ }. {
2 hours, store the contents of the volumetric flask in a polyethylene
# k7 H* x t2 f4 x1 `% \; p- t/ Ybottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),
' m5 k: U3 t+ |" X# b9 dthen filter the supernatant liquid through a very fine filter paper. Label) u+ v, { i3 ~0 Y- A. e' J2 X
the filtrate Sample Solution B.# Q; Y+ m. ]6 v& f4 w4 s
Sample Solution C
5 Z W& P5 d3 S; E( v' s' ?0 G+ DTransfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer
$ J( X/ v K" v: @8 xflask, add 1 drop of methyl orange TS, acidify with hydrochloric acid8 x8 Y, V) r i* [
solution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.02
; v4 \0 G: v; {" g5 ^' E# |" i; lM disodium EDTA, and mix. [Note: If the approximate Al2O3 content is( c3 K6 \# ?, _2 P5 E( o6 U
known, calculate the optimum volume of EDTA solution to be added/ J0 @ N7 ?# M. x
by the formula: (4 x % Al2O3) + 5.]( H! F; \3 d" P/ H, k( K
Add, dropwise, ammonia solution (1 in 5) until the colour is just" y- y# h# b, N
completely changed from red to orange-yellow. Then add10 ml each
X& }! H" e5 b7 }0 V& q. mof Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to
: N, r1 B% {3 d4 z5 [6 }room temperature in a stream of running water, add 3 drops of xylenol5 o/ \! d7 U! W7 u+ q9 j9 M0 U
orange TS, and mix. If the solution is purple, yellow-brown, or pink, \5 D: o( M8 G/ L
bring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired0 O, t* i( L5 |) m* B
pH, a pink colour indicates that not enough of the EDTA solution has
( M5 s( N$ f; j+ X+ Tbeen added, in which case, discard the solution and repeat this t' q; ?. Y: d& [- i% \
procedure with another 100 ml of Sample Solution B, using 50 ml,. s6 S/ B7 K3 U$ r
rather than 25 ml, of 0.02 M disodium EDTA. G0 H) s( Q' H9 K; V+ P! E
Procedure( b3 ~1 l/ x6 T2 `8 j0 g6 S
Using the standardized zinc sulfate solution as titrant, titrate Sample3 y; U0 p- B( U5 i
Solution C to the first yellow-brown or pink end-point that persists for
2 k) @( X9 _1 c$ T& L$ r5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first
2 z& P! t8 T9 H( ctitration should require more than 8 ml of titrant, but for more accurate6 s+ U1 J* d( M X- P: u
work a titration of 10-15 ml is desirable.
8 |, z- }1 l3 T% {4 Q. O3 A( [/ yAdd 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5, K! n/ ]8 W) ^. j. n
min, and cool in a stream of running water. Titrate this solution, using: E6 ~% X v- g& i
the standardized zinc sulfate solution as titrant, to the same fugitive1 ^; F6 I( U: m* y
yellow-brown or pink end-point as described above.
: r& l- a, H% }1 U3 [Calculation:
% q8 T5 Z/ O% g" n2 u2 C9 M0 X1 ~7 ^Calculate the percentage of aluminium oxide (Al2O3) in the sample/ r! L5 k# l! [) s! |2 A/ s% y
taken by the formula:
( G9 `9 V8 M) I& j5 {% Al2O3 = 100 × (0.005VT)/S
" F+ d% X1 R/ ~; }$ q9 s9 z% M- ^! Rwhere, b" j' `( V6 f' K$ ?
V is the number of ml of 0.01 N zinc sulfate consumed in- F' K* D+ q& g6 g9 f; {" D/ Q
the second titration,$ \$ B# j. w1 h9 N3 l
T is the titre of the zinc sulfate solution,
$ N+ W$ C# g, Z7 U) C( j0 E8 u+ ]2 {S is the mass (g) of the sample taken, and
) B$ H; E4 n- W/ Y2 X0.005 = 500 ml / (1000mg/g × 100 ml).
- u1 G0 I. Q2 L8 P- O& [1 r/ eSilicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica ?" Y0 Q3 {& k3 E+ H# L
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O).# A; M4 A$ {; Q$ J& V$ }9 P# Z5 o
Heat gently over a Meeker burner, while swirling the flask, until5 v. e9 Q0 k% s! V$ r
decomposition and fusion are complete and the melt is clear, except
! @) T- G: q( V! D# cfor the silica content, and then cool. (Caution: Do not overheat the* R% O- G g2 @" J/ l' t2 }
contents of the flask at the beginning, and heat cautiously during: h$ |$ g( n$ K& A
fusion to avoid spattering.)
6 d$ s8 s! }; E+ {% m% q0 uTo the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat: f! j- Z( U/ ~ Z
carefully and slowly until the melt is dissolved. Cool, and carefully add7 U/ {0 s* i) q4 }1 a
150 ml of water by pouring very small portions down the sides of the) E% t [ f$ E$ j4 h1 Y: X
flask, with frequent swirling to avoid over-heating and spattering. Allow
. H7 i' Q/ _. M5 x o- y$ Lthe contents of the flask to cool, and filter through fine ashless filter
+ \/ R' R1 Z& X2 {& wpaper, using a 60 degree gravity funnel. Rinse out all the silica from0 a4 ]. N2 y; ]; Y
the flask onto the filter paper with sulfuric acid solution (1 in 10).
* p! H8 k& f4 BTransfer the filter paper and its contents into a platinum crucible, dry in- ]! Z: z/ V* N J
an oven at 1200, and heat the partly covered crucible over a Bunsen5 b2 K! |$ M( ]% f- [& S2 r% q7 O
burner. To prevent flaming of the filter paper, first heat the cover from
8 c) Y" V! k6 c8 O: S0 Vabove, and then the crucible from below.0 f& L8 k8 u$ f) o( n1 y; i2 W) s
When the filter paper is consumed, transfer the crucible to a muffle8 o+ g5 e7 G" q3 ~# b1 k
furnace and ignite at 1000o for 30 min. Cool in a desiccator, and, T" `4 o. M, j4 Y& h+ D; r
weigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated/ l$ m" A$ f7 U# n2 Z
hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first' }9 _8 o$ a4 m3 v, P) z7 ~
on a low-heat hot plate (to remove the HF) and then over a Bunsen! G/ J2 c" Z( a8 r) A
burner (to remove the H2SO4). Take precautions to avoid spattering,2 M# G' d5 y' O0 L3 H- v: o# S+ o
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a6 B/ t6 \) {! {2 {: y
desiccator, and weigh again. Record the difference between the two
" {9 }- z0 @7 v: k* \weights as the content of SiO2 in the sample.
; \" J% ` J% }) ]: z; J: PMETHOD OF ASSAY2 m7 {6 i4 {! t% i( u
Accurately weigh about 150 mg of the sample, previously dried at 105o
' _' P! u9 y! z! y. Wfor 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water
r' k& p+ L' Fand shake until a homogeneous, milky suspension is obtained. Add 30
7 ^( Z: K1 i& }6 pml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially. j0 U3 Y5 ?) t4 L) `6 p) V' r" O4 C
heat gently, then heat strongly until a clear solution is obtained. Cool,
; V& s2 J. U( n: F# e6 Qthen cautiously dilute with 120 ml of water and 40 ml of hydrochloric( |1 B3 ]$ J A1 A6 Z; n* R
acid, and stir. Add 3 g of aluminium metal, and immediately insert a
3 M( ^- f8 J: ~/ W9 V, X3 f2 f3 U# erubber stopper fitted with a U-shaped glass tube while immersing the2 V4 p; h( m% u) B
other end of the U-tube into a saturated solution of sodium
- Q( i; F6 L) ^) S) m( Dbicarbonate contained in a 500-ml wide-mouth bottle, and generate
& ~% x8 E+ i7 M. s" u/ Ahydrogen. Allow to stand for a few minutes after the aluminium metal$ f: [/ A" Y5 q0 m) v) e
has dissolved completely to produce a transparent purple solution.
. F* W, j0 D6 R4 O6 KCool to below 50o in running water, and remove the rubber stopper
$ N( v* I1 @$ T, j% W& c* \; Mcarrying the U-tube. Add 3 ml of a saturated potassium thiocyanate: W/ |# Z1 P# C$ Q' C$ A: V
solution as an indicator, and immediately titrate with 0.2 N ferric) d, n( y1 i8 D; q+ G
ammonium sulfate until a faint brown colour that persists for 30
0 q! c' n/ y8 g$ ]3 T kseconds is obtained. Perform a blank determination and make any* r, i$ A$ d$ b
necessary correction. Each ml of 0.2 N ferric ammonium sulfate is6 u8 v) v @: j) L( f- d2 o$ v
equivalent to 7.990 mg of TiO2.
1 a# |- F) K8 Q; a+ y |
|