Document nmr3qM8k9ZZmKnJqBVgBRR5wa

D IFFER EN TIA L THERMAL ANALYSIS FOR SERPEN TIN E (CHRYSOTILE) IN TALC L The D ifferential T h e rm a l A nalyzer (DTA) p e rm its the detection of ch ry so tile and other serp en tin e m in erals in talc for co sm etic use. T h e t a l c m u s t not c o n t a i n g r e a t e r t h a n 10% (w /'w) c h l o r i t e f o r t h i s m ethod to be applicable becau se of the p o ssib le in te rfe re n c e of c h lo rite th e rm a l tr a n s itio n s . The m in im u m level of detection is 0 . 5 - 1% b y w e i g h t of c h r y s o t i l e a s b e s t o s . 2. The instrum ental p a ra m e te rs m ust be determ ined for the particular d ifferential th e rm a l a n a ly z e r which is em ployed, with r e g a r d to g eo m etry of the sam p le h o ld er, sensitivity of the differential th e r m o c o u p l e , e t c . , s u c h t h a t 0 . 5 - 1% b y w e i g h t c h r y s o t i l e m a y b e e a s i l y detected. 3. The su ccessfu l developm ent of the DTA m ethod is based on the p rep aratio n of hom ogeneous c h ry so tile-ta lc stan d ard sam p les. S t a n d a r d s c o n t a i n i n g 0 . 5 , 1, 3, a n d 5% b y w e i g h t c h r y s o t i l e in t a l c are prepared as follows: a . F i v e g m . of t h e s t a n d a r d t a l c is s l u r r i e d in a b o u t 100 m l . of * (R) ethanol in a W aring Blend or . b. T h e a p p r o p r i a t e w e i g h t of c h r y s o t i l e a s b e s t o s of a t l e a s t 9 5% purity is then added to the talc s lu rry , and the m ix tu re is blended for 5 m inutes at low speed. c. The solution is w ashed into a b eak er with ethanol, and the solvent is evap o rated on a steam bath. d. The resulting caked sam ple is broken and m ixed in a plastic vial^by shaking for 5 m inutes in a m ix e r such as the Spex M ix e r/ M i l l ' 3'. e. The hom ogeneity of the ch ry so tile-ta lc stan d ard s should be v e r i fied by optical or electro n m icro sco p y . 4. The ex p erim en tal conditions for the Stone DTA unit (Models LA-XYH and RC-202C}, for exam ple, are as follows: High tem p eratu re powder sam ple holder of nickel or stainless steel construction (Model SH-8BE2) with exposed-loop differential th e rm o couple (Platinel II). - 2- T e m p e r a t u r e ra n g e : a m b ie n t to 1000 C. R eference therm ocouple: P latin el II or C h ro m e l/A lu m el, exposed-loop Sam ple: 130-140 m g . talc, using a loose, co n siste n t packing technique. R e fe re n c e m a te ria l: A lum ina, ground to pass a 32 5 - m e s h sieve (m ax im u m p article size of 44 m icro n s) Inert dynamic atm osphere: helium or nitrogen at 0.05 SCFH H e a t i n g r a t e : 10 C . / m i n u t e S ensitivity: 40 ^ivolts full s c a le F u rn ace: L T F , w ater cooled to 1200C. The pu rp o se of the in e rt dynam ic a tm o s p h e re is to p rev en t oxidative reactions and to sweep out gaseous m in e ra l decom position products. The Stone powder sam ple holder yields m axim um sensitivity by p e r m itting d ire c t contact of th e rm o co u p le and sam p le. T ru e dynam ic gas flow through the sam ple is also attained. It m ay be possible to use a m o r e ra p id heating r a te with s o m e DTA in s tr u m e n ts , if it can be asc e rta in e d that constant th e rm a l equilibrium exists between s a m p le h o ld e r and f u rn a c e . R e p ro d u c ib ility of the m ethod is b a s e d on standardizing the ex p erim en tal conditions. E x p erim en tal conditions for the duPont DTA unit (Models 900 and 990) v/ould be as follows: T e m p e r a t u r e r a n g e : a m b i e n t to 1000 C. F u rn ace: 1200C. S a m p le : about 40 m g . talc Reference M aterial: alumina A tm osphere: static air Heating rate: 30C ./m inute Sensitivity: A T 0.004 mv. /in. T 0.8 mv. /in. 5. Initially , a t h e r m o g r a m of the b lank s t a n d a r d s a m p le is run in o r d e r to d e te r m in e th e t h e r m a l p ro file of the ta lc . 6. A t h e r m o g r a m is ru n on c h r y s o tile a s b e s to s by diluting the m a t e r i a l a p p r o x i m a t e l y 1:4 in f in e a l u m i n a . T h e t h e r m a l t r a n s i t i o n s i n v o l v e a dehydroxylation endotherm at about 650C and a recrystallization ex o th erm at ab o u t 820 C. T h e se two peaks estab lish the p re s e n c e of serpentine m ineral. 7 . T h e 5, 3, 1, a n d 0 , 5 % c h r y s o t i l e in t a l c s t a n d a r d s a r e t h e n a n a l y z e d . B o t h t h e r m a l t r a n s i t i o n s of t h e c h r y s o t i l e in t h e 1% s t a n d a r d s h o u l d be easily d etecta b le. If not, the ex p erim en tal conditions m ust be altered to im p ro v e detectability. - 3- 8. N o r m a l l y , ta lcs for c o s m e t i c p u r p o s e s w ill not r e q u i r e a d d itio n a l grinding for DTA . The a n aly sis of talc ro c k or the p r e s e n c e of lumps or gritty m ateria l will, however, necessitate grinding the sam p le to pass a 3 2 5 -m esh sieve. 9. The p r e s e n c e of la rg e am o u n ts of c a r b o n a te s or b ic a rb o n a te s (10% w / w o r g r e a t e r ) in ta lc w ill o b s c u r e th e s e r p e n t i n e e x o t h e r m at a p p ro x im a te ly 820 C. A m ild acid leaching is used to re m o v e these carbonates while leaving any chrysotile present intact: a . V /e i g h o u t 2 g. of t h e t a l c in t o a 100 m l . b e a k e r . A d d 25 m l . of 10% HC1 ( v / v ) s l o w l y ( t o p r e v e n t e x c e s s i v e e v o l u t i o n of gas if carb o n ates a r e p re s e n t) and h eat, with o cc a sio n a l s tirrin g , on a s te a m bath for 5 m in u tes . b. F ilte r on a vacuum filter, in e ith e r a po rcelain cone with glass fiber filter m at or a porous glass bottom cup, and w ash several times with hot w ater. Dry the talc. 10. W hen t h e r e is in su fficien t talc a v a ila b le to fill the e n tir e s a m p le cavity or cup, a le s s e r am ount of the talc m a y be placed in the bottom of the cup and covered with fine alum ina, or packed as a "sandw ich" su rrounding the th e rm o co u p le , between lay ers of fine alum ina. In this case, the stan d ard sam ples should be packed and run by the s a m e technique to d e te rm in e the co rresp o n d in g d e c r e a s e in peak in ten sities. 11. If serpentine mineral is indicated in a talc sample by the presence of the two characteristic thermal transi tions, the thermogram should be rerun on another sample of the powder to verify these peaks. Because of the sensitivity of the powder sample holder with exposedloop thermocouple, spurious heat effects caused, for example, by slight shifting of the sample during the run, can occasionally produce minor baseline fluctuations. Therefore,,- very weak thermal peaks should be checked for reproducibility.