: r! e2 h" [/ K; w9 e) YTITANIUM DIOXIDE # Z' Y" i" t- @9 @1 i2 E" mPrepared at the 67th JECFA (2006) and published in FAO JECFA5 N0 k6 ?- Z1 A( `+ W
Monographs 3 (2006), superseding specifications prepared at the 63rd 9 R, }! _# {4 \+ PJECFA (2004) and published in FNP 52 Add 12 (2004) and in the 5 k5 d6 I0 Q3 eCombined Compendium of Food Additive Specifications, FAO JECFA2 U) Y6 E$ ~: l! P' M; O* {9 \* @2 V
Monographs 1 (2005). An ADI “not limited” was established at the 13th r \4 Y/ W& ^8 w2 m2 m: `JECFA (1969). ^0 |2 O! S4 l9 h
SYNONYMS- A6 z. C$ R* O, D7 P+ ]
Titania, CI Pigment white 6, CI (1975) No. 77891, INS No. 171 & f( Q& @2 @9 c8 M& d; XDEFINITION : D7 `& |+ d1 [$ \1 _8 b& w8 bTitanium dioxide is produced by either the sulfate or the chloride 2 ?* p- ^+ k9 I ^; Qprocess. Processing conditions determine the form (anatase or rutile/ M1 C: C6 Q& \! I0 t
structure) of the final product. 8 X- G8 b$ I6 H- ] L% x C5 K3 UIn the sulfate process, sulfuric acid is used to digest ilmenite (FeTiO3) M& [& ~- _9 t8 For ilmenite and titanium slag. After a series of purification steps, the * y( R3 R( v5 h+ Q1 z7 s' `isolated titanium dioxide is finally washed with water, calcined, and9 o; Y' o2 C8 [% P% L( m3 q
micronized. 2 y+ I; p! N. o6 m4 P! xIn the chloride process, chlorine gas is reacted with a titaniumcontaining$ O6 \9 D4 l* f: I
mineral under reducing conditions to form anhydrous1 M# G# `8 S: k5 b
titanium tetrachloride, which is subsequently purified and converted to" _" h/ M0 S O" x
titanium dioxide either by direct thermal oxidation or by reaction with # Y+ X/ O! b! K3 w' i. c4 ^0 Q2 qsteam in the vapour phase. Alternatively, concentrated hydrochloric B9 I: r+ R- B1 `2 ?# b; `acid can be reacted with the titanium-containing mineral to form a / O2 s8 @( o; b9 {. Zsolution of titanium tetrachloride, which is then further purified and7 V' g5 A+ P+ k; N
converted to titanium dioxide by hydrolysis. The titanium dioxide is5 a+ l+ S+ e. @6 x' Z+ E
filtered, washed, and calcined.2 I' k- B$ k# L# z7 u$ Y
Commercial titanium dioxide may be coated with small amounts of , ^2 p# a' W6 Q8 g# ^ Kalumina and/or silica to improve the technological properties of the # c7 z4 y) Z3 |. b, zproduct. 4 \" P7 ?7 A$ q! F+ o6 UC.A.S. number 13463-67-7 4 l% |3 n* k) @7 ?( x- GChemical formula TiO2 8 ]- a" y2 j5 i1 w& BFormula weight * G2 X- H9 A3 _2 a# q79.88 * G( p" V# ~2 N k1 d5 n' G& q7 OAssay, B: v" j) Y, s! n, q0 D4 S$ Z' Q
Not less than 99.0% on the dried basis (on an aluminium oxide and! Z& m1 n( ^/ v
silicon dioxide-free basis) 2 Y* j8 o1 @$ D: V% bDESCRIPTION J2 K, c; k3 I" D2 |) t1 qWhite to slightly coloured powder. ]5 u3 {5 {( V0 @. |2 h
FUNCTIONAL USES/ ]& d# N$ n1 x$ _3 @1 d: c
Colour 2 C* e% W% U5 P& ZCHARACTERISTICS , I+ D3 ? T3 T$ [8 H+ a, F7 [IDENTIFICATION ' r- Y% F; E( _# S8 y7 QSolubility (Vol. 4)9 C! F# K8 u; d! U1 g O* A
Insoluble in water, hydrochloric acid, dilute sulfuric acid, and organic 5 {1 H8 ^9 X q) H7 Asolvents. Dissolves slowly in hydrofluoric acid and hot concentrated ! ^0 t l G/ n- ^sulfuric acid.5 @1 r/ g9 \- Z7 {
Colour reaction # A# M1 U# ]& t* q/ A* S' ]3 JAdd 5 ml sulfuric acid to 0.5 g of the sample, heat gently until fumes of$ O( v# \/ B2 P4 L% {7 \
sulfuric acid appear, then cool. Cautiously dilute to about 100 ml with 6 O8 ?0 t8 D( [) q8 D* fwater and filter. To 5 ml of this clear filtrate, add a few drops of" e5 W% M" f. }5 Q$ m6 _" `
hydrogen peroxide; an orange-red colour appears immediately. 4 {4 p5 M8 T, z, V( ]% }4 l5 D" c0 lPURITY C) Q0 u$ e' w
Loss on drying (Vol. 4) Not more than 0.5% (105°, 3 h) 7 I/ ~2 k( E" r* dLoss on ignition (Vol. 4)! z _# D: ]( B& `! X
Not more than 1.0% (800o) on the dried basis 9 z8 f6 r. I4 e$ V# XAluminium oxide and/or 2 X7 C3 i: Z. W8 x) ]1 {silicon dioxide # ^- q" l9 T/ _( g7 V- R- kNot more than 2%, either singly or combined % c0 f+ ?; V4 k8 ^: XSee descriptions under TESTS ! X' Z D* c! A: U; E" q3 ~& D' VAcid-soluble substances Not more than 0.5%; Not more than 1.5% for products containing ' `7 K: \2 g1 S! F! F4 Aalumina or silica./ ~# s4 m4 x1 w; v& N/ k M
Suspend 5 g of the sample in 100 ml 0.5 N hydrochloric acid and4 d1 W4 S* T L" P2 N4 ?, r
place on a steam bath for 30 min with occasional stirring. Filter$ A/ K+ c' }: X6 H! _ K! S" j+ [
through a Gooch crucible fitted with a glass fibre filter paper. Wash $ k. d: |$ f1 h* e( W3 Vwith three 10-ml portions of 0.5 N hydrochloric acid, evaporate the6 ?/ u8 X5 K# M4 r/ z3 b1 D, [+ ?
combined filtrate and washings to dryness, and ignite at a dull red # k- J1 ^# N$ J2 |1 K. H1 [. g1 ^heat to constant weight. ) M: k# b0 p; F0 s! x4 q% Q. K% c8 }Water-soluble matter# R8 Q6 T; U' P3 Q' W
(Vol. 4) 3 s7 F l2 e, |, c9 T+ N* xNot more than 0.5%$ y, o8 o: h- h
Proceed as directed under acid-soluble substances (above), using 6 \& B+ h5 w) v; q) xwater in place of 0.5 N hydrochloric acid. ) b. u3 A6 b) c* k: i; F6 ]Impurities soluble in 0.5 N " f, K9 Z) s( B; t: jhydrochloric acid% _ m2 H3 u7 y. E; p) f
Antimony Not more than 2 mg/kg/ j! T0 l8 X& J; w% v
See description under TESTS 4 M! R4 \* Y w5 CArsenic Not more than 1 mg/kg/ v9 t# D8 G; l" c2 \
See description under TESTS$ S) r9 [6 I9 {% U. n* w( x! T
Cadmium Not more than 1 mg/kg8 K: _& l% c4 M t# R, f% a
See description under TESTS ! Q" x8 d M% n ]Lead7 k7 x# v$ ?2 o# I( X' S
Not more than 10 mg/kg : E) x6 y |: ~& g' F' KSee description under TESTS5 {' @/ s) @) S5 [
Mercury (Vol. 4) Not more than 1 mg/kg 3 K! u4 z, P4 F! gDetermine using the cold vapour atomic absorption technique. Select a . L$ r V9 W/ u! K) j9 v Xsample size appropriate to the specified level4 p) t Y+ E' G$ c' t2 ]) t5 x( {
TESTS # T) J% X# b, u6 C6 }( j- c( t* j, E6 f8 `PURITY TESTS+ }; r$ U& K' \# r3 H3 L. E
Impurities soluble in 0.5 N l* @9 W6 p; x6 qhydrochloric acid$ m) i0 x$ }4 O' q7 X4 Q O7 L
Antimony, arsenic,- {# g; ^, M+ ^$ I" s
cadmium and lead% H4 B/ U" q; S! _# {. j$ H
(Vol.4)2 E; S' A' h. q* L' l
Transfer 10.0 g of sample into a 250-ml beaker, add 50 ml of 0.5 N * a8 c' S, I I4 I) k7 r& vhydrochloric acid, cover with a watch glass, and heat to boiling on a 0 a$ V" X0 ]+ k! uhot plate. Boil gently for 15 min, pour the slurry into a 100- to 150-ml- a3 J3 k, E5 O( p- k6 q' U
centrifuge bottle, and centrifuge for 10 to 15 min, or until undissolved+ q7 B1 |6 w9 K
material settles. Decant the supernatant extract through a Whatman + Y# }; f. l5 ~- I) NNo. 4 filter paper, or equivalent, collecting the filtrate in a 100-ml - h3 u. p( Q1 J- V5 z0 zvolumetric flask and retaining as much as possible of the undissolved% }: e. A( T' X# ?0 L" s- s
material in the centrifuge bottle. Add 10 ml of hot water to the original/ N$ i, |1 ^0 ~6 G/ N* P
beaker, washing off the watch glass with the water, and pour the - d' T1 h) Y2 tcontents into the centrifuge bottle. Form a slurry, using a glass stirring7 y3 ~) ?& u7 l# y
rod, and centrifuge. Decant through the same filter paper, and collect) o0 y$ Q6 v' j- o& K5 u& i) ?/ I
the washings in the volumetric flask containing the initial extract.' M3 v% K7 l% |' n r% w2 q
Repeat the entire washing process two more times. Finally, wash the - n9 B' H3 z% v( i3 U$ N, k) K" Afilter paper with 10 to 15 ml of hot water. Cool the contents of the flask % m3 k: @( T9 R# }/ `8 Ito room temperature, dilute to volume with water, and mix. ; V, m) t. {8 ?: X% SDetermine antimony, cadmium, and lead using an AAS/ICP-AES( @! ~9 \) O, ^' E
technique appropriate to the specified level. Determine arsenic using the7 A% V& b6 s0 h* e
ICP-AES/AAS-hydride technique. Alternatively, determine arsenic using 0 h, s8 M- `& H+ B8 ~5 ] TMethod II of the Arsenic Limit Test, taking 3 g of the sample rather than 3 k" O3 ~( v- ]7 [6 d1 g. The selection of sample size and method of sample preparation 3 E& i7 Z, ^4 W% N- D* }may be based on the principles of the methods described in Volume 4. l7 U0 }3 {# [; ?5 _7 R. K$ I
Aluminium oxide Reagents and sample solutions 7 e7 \% t1 u' ]4 j5 a0.01 N Zinc Sulfate+ b# [6 v7 M: [! g
Dissolve 2.9 g of zinc sulfate (ZnSO4 ? 7H2O) in sufficient water to9 N, E+ T" x0 G1 h3 T. u# q: [) h5 ]! E
make 1000 ml. Standardize the solution as follows: Dissolve 500 mg3 X% q! I! `7 H
of high-purity (99.9%) aluminium wire, accurately weighed, in 20 ml of# O1 X8 l4 P3 j3 n. x& z
concentrated hydrochloric acid, heating gently to effect solution, then! A, F4 H5 ?1 v$ |7 W! i; m J
transfer the solution into a 1000-ml volumetric flask, dilute to volume( B% W% B$ F7 p+ x' P
with water, and mix. Transfer a 10 ml aliquot of this solution into a 500 ; F0 j0 ` U1 c; K$ a3 v' \ml Erlenmeyer flask containing 90 ml of water and 3 ml of" B2 y7 t ^: \* R
concentrated hydrochloric acid, add 1 drop of methyl orange TS and( e1 _! B% n- R9 p( b$ U5 D3 R
25 ml of 0.02 M disodium ethylenediaminetetraacetate (EDTA) Add, r. V3 m/ X: q" B5 q: udropwise, ammonia solution (1 in 5) until the colour is just completely 1 h* A+ g' L. v* e! r: hchanged from red to orange-yellow. Then, add: - G! S( u7 R, p3 q4 Y8 ]( x(a): 10 ml of ammonium acetate buffer solution (77 g of* T3 G3 G& y$ J
ammonium acetate plus 10 ml of glacial acetic acid, dilute to/ `) B9 F9 D! h& F# x
1000 ml with water) and2 N/ s" U, j* B1 a9 {9 @
(b): 10 ml of diammonium hydrogen phosphate solution (150 g8 x! [+ Z. z. W
of diammonium hydrogen phosphate in 700 ml of water,5 |! v" T3 M* @7 v$ @
adjusted to pH 5.5 with a 1 in 2 solution of hydrochloric acid,# J3 Z" ^7 l) i4 C
then dilute to 1000 ml with water).9 U7 q# t4 `: j! K! y3 f
Boil the solution for 5 min, cool it quickly to room temperature in a: a( m0 F( f" a5 P' e! N
stream of running water, add 3 drops of xylenol orange TS, and mix.2 @! B! e: K* l5 g
Using the zinc sulfate solution as titrant, titrate the solution to the first 3 i# o( {: l" d6 x2 }+ yyellow-brown or pink end-point colour that persists for 5-10 sec. (Note: 4 Z, l& M* u% I1 C* [& I- I/ iThis titration should be performed quickly near the end-point by 3 H: P8 Q7 C3 {5 D& ]adding rapidly 0.2 ml increments of the titrant until the first colour ! @% |4 v8 Q$ e% u0 n7 ichange occurs; although the colour will fade in 5-10 sec, it is the true* @' k6 ?$ _7 ^
end-point. Failure to observe the first colour change will result in an ' P6 ~ u% M9 ]incorrect titration. The fading end-point does not occur at the second : V# ~& {+ X) F3 c" Pend-point.)4 x; x }' M# {: o; g) Y
Add 2 g of sodium fluoride, boil the mixture for 2-5 min, and cool in a ! V. a) I% O7 C. E; Istream of running water. Titrate this solution, using the zinc sulfate5 G! O9 x) H! _+ J4 p; Z, U7 X
solution as titrant, to the same fugitive yellow-brown or pink end-point; D- k0 ~& y+ z# X
as described above.! X$ a3 w; |( ]# H" o# S
Calculate the titre T of zinc sulfate solution by the formula: ' O) Y' j: Z) [( f+ Y4 c, xT = 18.896 W / V: A$ ]) | y+ @' U8 m
where + o8 N1 @: N2 L: B$ T5 IT is the mass (mg) of Al2O3 per ml of zinc sulfate solution 2 m) w5 P6 R+ F/ C. {' iW is the mass (g) of aluminium wire & t! x& w6 D X% YV is the ml of the zinc sulfate solution consumed in the . m4 G4 L5 W! [# [3 ~% p& fsecond titration9 q! t1 N: }0 h1 \' Y
18.896 = (R × 1000 mg/g × 10 ml/2)/1000 ml and( V- q) s! J" s4 ]7 ?3 V$ s
R is the ratio of the formula weight of aluminium oxide to . W+ [! W6 C+ i' O# d9 Kthat of elemental aluminium.% H8 r& m9 h) A8 E! {( V5 S
Sample Solution A4 p7 Q* c* E* v3 {/ R
Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica( O: l+ N. E- g1 b$ g3 U
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O). ; |' v! S8 |/ E5 k& N( U1 h(Note: Do not use more sodium bisulfate than specified, as an excess' h" w& q" T$ \! g' B5 ?# |
concentration of salt will interfere with the EDTA titration later on in the8 N9 o) M+ Y9 C/ @& O, ?
procedure.) Begin heating the flask at low heat on a hot plate, and1 T2 M6 Z, C, I
then gradually raise the temperature until full heat is reached. ! v7 e! Y" h( F(Caution: perform this procedure in a well ventilated area. ) When1 \: T# y2 a# g6 K: L
spattering has stopped and light fumes of SO3 appear, heat in the full/ [! s4 |1 k' z
flame of a Meeker burner, with the flask tilted so that the fusion of the4 w3 C2 L# n: ^8 |+ u; z: X
sample and sodium bisulfate is concentrated at one end of the flask. R6 W- C" L! v9 n6 FSwirl constantly until the melt is clear (except for silica content), but ! j+ U' r0 d+ w% xguard against prolonged heating to avoid precipitation of titanium+ d2 R4 Z& z) P# E
dioxide. Cool, add 25 ml sulfuric acid solution (1 in 2), and heat until/ b9 s7 {1 W! X
the mass has dissolved and a clear solution results. Cool, and dilute to$ `+ M5 K: C5 m5 p* S9 ^- p! @
120 ml with water. Introduce a magnetic stir bar into the flask." S2 Z, F$ O. P. ?0 O& O, e
Sample Solution B y' c+ J5 z5 N* _/ U- E2 C2 ~
Prepare 200 ml of an approximately 6.25 M solution of sodium 5 B; i4 a; U/ E6 s3 `4 w7 I* @2 O4 |& |hydroxide. Add 65 ml of this solution to Sample Solution A, while1 b1 M! P% @# H3 v
stirring with the magnetic stirrer; pour the remaining 135 ml of the0 A7 {9 t1 a+ y4 G: K% l. n
alkali solution into a 500-ml volumetric flask. 8 A) |& K1 ]2 ?. ^$ rSlowly, with constant stirring, add the sample mixture to the alkali " F8 d6 k0 j3 x5 F& T3 n; osolution in the 500-ml volumetric flask; dilute to volume with water, 5 R c$ t, z* Z2 _2 `; band mix. (Note: If the procedure is delayed at this point for more than3 o6 ] w9 C+ W2 X
2 hours, store the contents of the volumetric flask in a polyethylene 4 d; L$ ^9 k* N A- x8 T# Tbottle.) Allow most of the precipitate to settle (or centrifuge for 5 min),8 G5 t$ i- Z. X: B
then filter the supernatant liquid through a very fine filter paper. Label 1 Z. Z3 F. A5 N& U0 D2 Dthe filtrate Sample Solution B.- r4 r2 H8 S$ p3 t2 P
Sample Solution C1 t+ O5 }: `' t& m. \) R, \6 _
Transfer 100 ml of the Sample Solution B into a 500-ml Erlenmeyer% b( A& w9 f( M
flask, add 1 drop of methyl orange TS, acidify with hydrochloric acid! e* O/ A. u8 \, o) t, w
solution (1 in 2), and then add about 3 ml in excess. Add 25 ml of 0.025 K' F* h, J$ w8 a( z5 w% J, |
M disodium EDTA, and mix. [Note: If the approximate Al2O3 content is 0 ?) W! P0 W% N6 I, _+ z5 C$ Z9 pknown, calculate the optimum volume of EDTA solution to be added ; @4 q9 e! w$ ?. h& ?* N, Kby the formula: (4 x % Al2O3) + 5.]- ?: b+ f% ?" G0 ]
Add, dropwise, ammonia solution (1 in 5) until the colour is just) D. K- P+ X& v& H# t
completely changed from red to orange-yellow. Then add10 ml each * u: C: ]+ `. {! H; Y% k; Qof Solutions 1 and 2 (see above) and boil for 5 min. Cool quickly to1 x0 M" b1 |- o7 _4 b
room temperature in a stream of running water, add 3 drops of xylenol ( f8 C: I. @: t8 h( v( korange TS, and mix. If the solution is purple, yellow-brown, or pink,8 K; \8 d, k$ I, A( M# Q
bring the pH to 5.3 - 5.7 by the addition of acetic acid. At the desired ! @! V. a. U) M5 X; _+ M& ZpH, a pink colour indicates that not enough of the EDTA solution has ( B: }. p8 ?% x/ e" p5 d$ v4 Hbeen added, in which case, discard the solution and repeat this - L8 ]9 f" U" R. h0 Dprocedure with another 100 ml of Sample Solution B, using 50 ml, ; g8 k- r/ H% e% o$ p) \8 `2 T: Qrather than 25 ml, of 0.02 M disodium EDTA./ H- D7 u6 J5 z
Procedure ' Z( S' G1 ~* O- ?3 F% x9 EUsing the standardized zinc sulfate solution as titrant, titrate Sample2 A) a. Y% X+ t* {
Solution C to the first yellow-brown or pink end-point that persists for" `- R! J1 @2 [3 T5 d& `
5-10 sec. (Important: See Note under “0.01 Zinc sulfate”.) This first 5 o: r- j0 _; B5 I9 g, s# M( x' a; P5 Htitration should require more than 8 ml of titrant, but for more accurate $ C8 ]9 Z8 z( E' Swork a titration of 10-15 ml is desirable.$ E# g; l r+ \; @( p* J
Add 2 g of sodium fluoride to the titration flask, boil the mixture for 2-5' m, r% m9 l/ z; r. D6 \0 R
min, and cool in a stream of running water. Titrate this solution, using0 h, m, N7 W/ G' v; Q
the standardized zinc sulfate solution as titrant, to the same fugitive+ v2 X& Q4 v% e( O' D# ?& U
yellow-brown or pink end-point as described above. * T- M( j. m0 R+ K9 y8 }% PCalculation:& T. [, p( E) h! Q9 B
Calculate the percentage of aluminium oxide (Al2O3) in the sample# \8 @/ f: s- ?: ^: i9 \
taken by the formula:4 K1 ?2 z$ C; f8 w/ ]6 @
% Al2O3 = 100 × (0.005VT)/S ! B! k& N1 I( w8 o( Nwhere - ?7 F7 e( V; |7 U# g2 RV is the number of ml of 0.01 N zinc sulfate consumed in: @/ M( q7 I! t3 h
the second titration, 5 i5 G2 F1 G8 b% G1 ZT is the titre of the zinc sulfate solution,+ E# P2 s+ M8 p
S is the mass (g) of the sample taken, and# p# Z4 C# b+ R+ U1 D
0.005 = 500 ml / (1000mg/g × 100 ml).- P# z$ }# X# D( ?9 A
Silicon dioxide Accurately weigh 1 g of the sample and transfer to a 250-ml high-silica) e |( b- x5 B
glass Erlenmeyer flask. Add 10 g of sodium bisulfate (NaHSO4 ? H2O). ; [' b3 i; Z9 P7 w# ], H& aHeat gently over a Meeker burner, while swirling the flask, until% w) `7 J9 H; Q- n
decomposition and fusion are complete and the melt is clear, except* b# c+ B5 r6 c, F! ~6 I
for the silica content, and then cool. (Caution: Do not overheat the4 ^# ^1 z* \0 a$ s* h! C8 W. q4 m7 `. x& r
contents of the flask at the beginning, and heat cautiously during1 O/ n* u9 v: x+ @, x
fusion to avoid spattering.) M" u+ }% J6 t, R9 c7 m: q) Z N
To the cooled melt add 25 ml of sulfuric acid solution (1 in 2) and heat ; y- ~3 u: y P/ o3 |( c- ~ h p% dcarefully and slowly until the melt is dissolved. Cool, and carefully add - V0 p" j8 @- ^% }# i: ~" W150 ml of water by pouring very small portions down the sides of the 2 O! X5 x+ W! G, M0 O& Z* cflask, with frequent swirling to avoid over-heating and spattering. Allow: b- X) X" u& B( s7 I( _6 D
the contents of the flask to cool, and filter through fine ashless filter ! a+ ]- S Q$ @7 bpaper, using a 60 degree gravity funnel. Rinse out all the silica from3 R( ]) y8 D2 ?, x0 k1 c
the flask onto the filter paper with sulfuric acid solution (1 in 10). ' J; {# w0 Q: q; ^% m7 w' s2 ATransfer the filter paper and its contents into a platinum crucible, dry in # _7 G+ E7 h- H+ s$ D# aan oven at 1200, and heat the partly covered crucible over a Bunsen " j" m F( u: ^burner. To prevent flaming of the filter paper, first heat the cover from 0 [" ]3 z9 T( u7 I1 s: labove, and then the crucible from below. # c1 Q3 a2 Y, R- F/ D& [0 D S. oWhen the filter paper is consumed, transfer the crucible to a muffle 9 V$ P. e( @: b0 Ifurnace and ignite at 1000o for 30 min. Cool in a desiccator, and 1 h7 ~: f& k r: Kweigh. Add 2 drops of sulfuric acid (1 in 2) and 5 ml of concentrated A' v! f r/ c( Y/ z; k+ U
hydrofluoric acid (sp.gr. 1.15), and carefully evaporate to dryness, first 0 v( {! M, @8 |" q1 u jon a low-heat hot plate (to remove the HF) and then over a Bunsen 0 Q {5 y( n- ?/ g" Kburner (to remove the H2SO4). Take precautions to avoid spattering,' \6 A' q. |) n6 `
especially after removal of the HF. Ignite at 1000o for 10 min, cool in a# N, J- W7 s) Q6 G& M! s
desiccator, and weigh again. Record the difference between the two- z5 S/ R" Y# C' V3 F
weights as the content of SiO2 in the sample.+ _2 W0 `: S8 {
METHOD OF ASSAY . K( Z, K; Q; Q, v, b- cAccurately weigh about 150 mg of the sample, previously dried at 105o' q9 N! Z5 I/ K' Z' m' l7 |
for 3 hours, and transfer into a 500-ml conical flask. Add 5 ml of water 3 U8 A# i, ]8 v6 [4 I v$ Uand shake until a homogeneous, milky suspension is obtained. Add 30& @" U+ A8 f* |2 S% M9 M2 o2 M
ml of sulfuric acid and 12 g of ammonium sulfate, and mix. Initially% k1 b4 `3 m: E! a
heat gently, then heat strongly until a clear solution is obtained. Cool, , N( p4 `6 B2 A# b- \then cautiously dilute with 120 ml of water and 40 ml of hydrochloric 8 c6 Q. F0 H* @acid, and stir. Add 3 g of aluminium metal, and immediately insert a* y4 V! p/ c" a8 U0 ]
rubber stopper fitted with a U-shaped glass tube while immersing the 7 @' t3 J2 [- M; s, R2 o! ?other end of the U-tube into a saturated solution of sodium - P5 O! z* h" s7 Pbicarbonate contained in a 500-ml wide-mouth bottle, and generate9 X! i9 l: S$ U: Q8 \# `
hydrogen. Allow to stand for a few minutes after the aluminium metal 9 K; k. S6 {4 J7 M$ shas dissolved completely to produce a transparent purple solution.& @; p/ L2 y+ d1 L( S6 [
Cool to below 50o in running water, and remove the rubber stopper # [8 K- z- _5 z, acarrying the U-tube. Add 3 ml of a saturated potassium thiocyanate ' h% B3 m) t6 j0 Esolution as an indicator, and immediately titrate with 0.2 N ferric 7 g. d0 b; a4 D! d/ \- Fammonium sulfate until a faint brown colour that persists for 303 x$ b7 P5 Q/ V8 m5 N
seconds is obtained. Perform a blank determination and make any# C7 X/ _5 `7 n& _; W
necessary correction. Each ml of 0.2 N ferric ammonium sulfate is # u1 i, d; ]- O, X$ Q8 _equivalent to 7.990 mg of TiO2., S$ @& b$ @/ D$ P/ `2 a/ _