Document Lg33ZeLvKZojJLR0dgB424jb3

DIFFERENTIAL THERMAL ANALYSIS FOR SERPENTINE (CHRYSOTILE) IN T A L C 1. T h e D i f f e r e n t i a l T h e r m a l A n a l y z e r ( D T A ) p e r m i t s the d e t e c t i o n of chryso tile and other serp en tin e m in e rals in talc for c o sm e tic u se. 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 than 10% ( w / w ) c h l o r i t e f o r t h i s method to be applicable b e ca u se of the p o s sib le in terfere n ce of chlorite therm al tran sitions. The minimum level of detection is 0 . 5 - 1% by w ei 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 differential th erm al a n a ly z e r which is em ployed, with r e g a rd to geom etry of the sam p le holder, sensitivity of the differential th e rm o c o u p l e , e t c . , s u c h t h a t 0 . 5 - 1% by w e i g h t c h r y s o t i l e m a y be e a s i l y detected. 3. The s u c c e s s f u l d evelopm ent of the DTA method is b a s e d on the preparation of homogeneous chry so tile-talc standard sa m p le s. S t a n d a r d s c o n t a i n i n g 0 . 5 , 1, 3, an 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 e iv th e an g o lm i..n of a the War st in a g ndard Blend ot arlc . is slurried in a b o u t 100 m l . of 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 slu rr y , and the m ixture is blended for 5 m inutes at low speed. c. The solution is w ashed into a beaker with ethanol, and the solvent is evaporated on a ste a m bath. d. The resu ltin g caked sa m p le is broken and m ixed in a plastic vial^by shaking for 5 minutes in a m ix e r such as the Spex M ix e r / Mill . e. The homogeneity of the ch ry so tile-talc stan dards should be v e r i fied by optical or electron m ic ro sc o p y . 4. The e x pe rim e n tal conditions for the Stone DTA unit (M odels LA -X Y H and RC-202C}, for exam ple, are as follows: High tem perature powder sam ple holder of nickel or stainless steel construction (M odel SH -8B E 2) with exposed-loop differential th erm o couple (Platinel II). - 2- T e m p e r a tu r e ran ge: am bien t to 1000 C . R efe ren ce therm ocouple: P latin el II or C h ro m e l/A lu m e l, exposed-loop Sam ple: 130-140 m g . talc, using a lo ose, consistent packing technique. R e fe r e n c e m a t e r i a l : A lu m in a , ground to p a s s a 3 2 5 - m e s h sieve (m axim um particle size of 44 m icrons) Inert dynamic atm osphere: helium or nitrogen at 0.05 SC F H Heating rate: 10C./m inute Sensitivity: 40 jrvolts full s c a le F u rn ac e: L T F , w ater cooled to 1200C. The p u rpo se of the inert dynamic a tm o sp h e re is to prevent oxidative reaction s and to sw eep out g a se o u s m in e r a l decom position p ro du cts. The Stone powder sam ple holder yields m axim um sensitivity by p e r mitting direct contact of thermocouple and sam ple. T ru e dynamic g a s flow through the sa m p le is a l s o attained. It m ay be p o s s ib le to use a m o r e rap id heating rate with s o m e DTA in str u m e n ts , if it can be a sc erta in ed that constant th erm al equilibrium exists between sa m p le holder and furn ace. R ep rod u cibility of the method is b a sed on stan dard iz in g the e x p e rim e n ta l conditions. E x p e rim e n ta l conditions for the duPont DTA unit (Models 900 and 990} would be as follows: T e m p e r a tu r e range: am bient to 1000C. Furnace: 1200C. S a m p le : about 40 m g . talc Reference Material: alumina Atm 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 blank stan d ard sa m p le is run in o rd e r to determ ine the th e rm a l profile of the talc. 6, A th e r m o g r a m is run on ch ry so tile a s b e s t o 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 fine 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 in volve a dehydroxylation endotherm at about 650C and a recrystallization exoth erm at about 820 C . T h e se two p eak s e sta b lish the p r e 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 then 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 the c h r y s o t i l e in the 1% s t a n d a r d s h o u l d be easily d etectab le. If not, the experim en tal conditions m ust be a lte re d to im p r o v e detectability. - 3- 8. N o r m a lly , talc s for c o s m e t i c p u r p o s e s w ill not r e q u ir e ad d itio n al grinding for DTA . The a n a ly sis of talc ro c k or the p r e s e n c e of lumps or gritty m a te ria l will, however, n ecessitate grinding the sa m p le to p a ss a 3 2 5 -m e sh sie v e . 9. The p r e s e n c e of la rg e am ounts of c a r b o n a te s or b ic arb o n ate s (10% w /w or g r e a t e r ) in talc will o b s c u r e the se rp e n tin e e x o th e rm at ap p ro x im ate ly 820 C . A m ild acid leaching is used to r e m o v e these carbonates while leaving any chrysotile present intact: a . W e i g h out 2 g. of the t a l c into 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 (to 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 g a s if carb o n ates a r e p resen t) and heat, with o c c a sio n a l s t i r r i n g , on a s t e a m bath for 5 m in u te s . b. F i l t e r on a vacuum filter, in either a p orce lain cone with g lass fiber filter m at or a porous g la ss bottom cup, and w ash s e v e r a l tim es with hot w ate r. Dry the ta.lc. 10. When there is insufficient talc av a ila b le to fill the entire sa m p le cavity or cup, a l e s s e r am ount of the talc m a y be p laced in the bottom of the cup and covered with fine alum ina, or packed as a ''sandwich" surrounding the therm ocouple, between layers of fine alumina.. In this c a s e , the stan d ard s a m p le s should be packed and run by the s a m e technique to d e te rm in e the correspo 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. Thereforevery weak thermal peaks should be checked for reproducibility.