Effect of Temperature on Volatilization of Alkali Salts During Dry Ashing of Tetrafluoroethylene Fluorocarbon Resin SIR: Teflon (DuPont's TFE fluorocarbon resin) has been used as the ablative surface on space vehicles. The plasma wake observed during the reentry of vehicles is attributed to trace alkali contamination of surface materials. Alkali metal analysis of Teflon by atomic absorption spectrometry (AAS) requires the organic material to be decomposed prior to elemental analysis. Because Teflon is virtually impervious to chemical attack, wet digestion techniques (3) cannot be employed. Dry ashing in air a t temperatures in excess of 500" C. is generally recommended (1-3, IO) to decompose organic material for trace analysis. However, the possibility of volatilizing inorganic material at elevated temperatures must be considered. The relative volatility of several alkali salts a t 1300" C. was first reported by Bunsen in 1866 (8). dlkali nitrates and nitrites have been distilled at 350" to 550' C. from their meltsunder reduced pressure in 1- to 5-gram quantities (4). Losses of antimony, arsenic (6), and cesium (7) salts during prolonged ashing of blood and food samples a t less than 500' C. have been reported. Teflon begins to decompose in air at 490' C. (5). At 525" C. the material undergoes destructive distillation and leaves no carbon residue. To establish saie ashing conditions for Teflon, the effect of temperature on the volatiliza-
tion of alkali salts was investigated.
AAS provided a convenient technique for obtaining retention data on heat treated samples. EXPERIMENTAL
Apparatus. A Temco electric muffle furnace, Model RCE, was ,modified by stacking l/r-inch alumina plates on the furnace floor with l/d-inch air space between the plates. This arrangement provided a raised platform for the samples at the center of the chamber, where the temperature was uniform. The temperature control as monitored by a chromelalumel thermocouple was 1 1 0 ' a t 600" C. Elemental analysis on the inorganic residue was performed with a Techtron atomic absorption spectrometer, Model AB-3. Procedure. VOLATILIZATION STUDIES OF ALKALISALTS. Salt solution containing 20 pg. of alkali metal was pipetted into 30-ml. platinum crucibles and the solution evaporated to dryness on a steam bath. The crucibles were heated in the furnace a t various conditions of temperature and time. The residue was dissolved with a few drops of hydrochloric acid, diluted to 10 ml. with distilled water, and analyzed by ABS. ASHING OF TEFLON.The samples were cleaned with hydrochloric acid (l:?), rinsed with water, and dried. Weighed samples in platinum crucibles
were placed in a furnace a t 500" C. The temperature was slowly increased to 525' C. until most of the sample was decomposed and then raised to 550' C. for 30 minutes. The furnace door was opened periodically to release the gaseous products. All ashings were performed in a well vented fume hood. The residue in the crucible was treated with hydrochloric acid, diluted, and analyzed by AAS. RESULTS A N D DISCUSSION
The apparent loss of sodium during high temperature ashing was indicated by the results of several sodium determinations of a Teflon sample as shown on Figure 1. Platinum crucibles containing 2 grams of Teflon were heated in the furnace a t various temperatures and the sodium in the residue was determined. Sodium loss was evident a t 750" C. and was almost complete a t 1200' c. Figure 2 shows the effect of temperature on 20 kg. of sodium as various salts heated for 1 hour. Small losses of sodium as nitrate, chloride, and fluoride were apparent a t 500' C. Loss of sodium sulfate was appreciable a t temperatures in excess of 700" C. At 1000' C. the loss was almost complete for all salts. Twenty micrograms of sodium chloride heated a t 600' C. for 6 hours was 70 per cent lost. Thus, the extent of loss from microgram quantities of sodium salt depends on the heating duration as well as the temperature.
TEMPERATURE I TEMPERATURE ('C)
Figure 1. Effect of dry ashing temperature on N o results of analysis of Teflon samples containing 13 to 14 p.p.m. Na
1596
ANALYTICAL CHEMISTRY
Figure 2. Effect of temperature on 20 fig. of N a as NaCI, NaN03, NazSOe and NaF NaZSOd, and NaF Heating time, 1 hour NaNOa; 0 Na2SOd; 0 NaF
0 NaClj A
TEMPERATURE ('c)
TEMPERATURE ('C) Figure 3. Effect of temperature on 20 pg. of alkali and alkaline earth metals as the respective chloride salts Heating time, 1 hour LiCI; 0 NaCI; H MgCIz
A KCl;
0 RbCI;
Figure 4. Effect of temperature on 20 pg. of alkali metals as the fluoride salts Heating time, 1 hour 0 LIF; 0 HaF; A KF
A CsCI; 0 CaC12;
Table I presents the results of weight loss studies on macro quantities of sodium salts. Appropriate amounts of dried reagent grade sodium chloride, sodium fluoride, and sodium sulfate were weighed into individual platinum crucibles so that each contained 1.0000 gram of sodium. The crucibles were heated in the furnace a t the temperatures and times listed and the per cent weight loss was determined. Appreciable quantities of sodium were lost in all cases as the temperature increased. However, the per cent weight loss was very small for the three salts even with prolonged heating a t high temperature. The large difference in the rates of volatilization of microgram and gram quantities of sodium salts indicates that the kinetics of volatilization is dependent on surface area exposed to the atmosphere. The surface to weight ratio of 20 pg. of sodium chloride, crystallized by evaporation from aqueous solution, was much larger than that of the reagent grade salt. Table I1 lists the results of replicate determinations, of a Teflon sample under various conditions of sample size, temperature, and heating time. Allowing for some nonuniformity of the sample, the agreement of results was good. The results were within the usual precision limits of the atomic absorption technique. It is evident that the best precision was obtained by ashing a t 550" C. for 1 hour. These data do not represent typical sodium concentration in Teflon. Most Teflon samples contained impurities at the 1-p.p.m. level. The volatilization of other alkali chlorides plus calcium and magnesium
Table 1.
Effect of Prolonged Heating on 1.0000 Gram of N a as Chloride, Fluoride, and Sulfate Salts
Hours
Wt. loss, yo
of
heating
Temp.,
O
C.
Table II.
NarSOc '
NaF 0.19 0.27 0.21 0.16 1.26
0.13 0.20 0.46 0.36
600 600 600 800 900
16 24 40 8 12
NaCl
Melted
0.00 0.03 0.10 0.00
Melted
Determination of N a by AAS in Teflon Sample Using Various Ashing Conditions and Sample Weights
' C.
Temp.,
Time, hr.
Sample wt., grams
Na, p.p.m.
550
1 1 1 1
2,1953 2.1598 4.2376 3.6697
13.7 14.4 14.1 14.0
575
1 2
2.8107 2.9601
600
AT. 1 4 . 1 f 0 . 2 12.2 13.9
2.4431 2.0686 2.0095 4.7594 4.8577 2.1227
Av. 13.1 f 0 . 4 Overall av. 13.4 f 0.6 Table 111.
Comparison of Order of Volatility of Alkali Chlorides and Fluorides with Vapor Pressure and Melting Points
Decreasina order of volat3ity at 550" C. CsCl RbCl KC1
NaCl LiCl
KF NaF
LiF
Decreasinn order of vapor pr&sure (9) at 550" C.. mm.
CsCl LiCl RbCl KCI
NaCl
KF
LiF
NaF
Increasing order
of melting points,
1.2 x 5.1 X 4.1 x 2.2 x 6.3 x 3.5 x 4.3 x 1.7 X
10-2 lo-* 10-3 10-8 10-4 10-4 10lo-'
LiCl CsCl RbCl KC1
NaCl LiF
KF
NaF
VOL 38, NO. 11, OCTOBER 1966
' C.
614 646 715 790 800 870 889 992
e
1597
chlorides was investigated and the results are shown on Figure 3. Twenty micrograms of each metal as the chloride were heated in platinum crucibles a t various temperatures for 1 hour. All the salts volatilized at temperatures above 750’ C. Loss of rubidium and cesium started abruptly a t 500’ C. and was almost complete at 700’ C. Serious lithium, sodium, and potassium losses were observed above 600’ C. Calcium and magnesium were not volatilized to the extent of the alkalis. Figure 4 shows the retention of alkali fluorides after heat treatment. Table I11 lists the decreasing order of volatility a t 550’ C. for the alkali chlorides and fluorides taken from Figures 3 and 4. The compounds are also listed in decreasing order of vapor
pressure and increasing order of melting point. The vapor pressures a t 550’ C. were extrapolated from data of Stull(9). With the exception of lithium chloride and fluoride, the alkali salts fall into three identical orders of decreasing formula weight. From the data in Table 111, it can be seen that the relative volatility of the alkali chlorides a t 550” C. can be predicted on the basis of melting point and vapor pressure. LITERATURE CITED
( 1 ) Assoc. Offic. Agr. Chemists, “Methods of Analysis,” 6th ed., p. 559, Washington, D. C., 1945. (2) ASTM Standards, Methods of Testing, D811-48, D1026-51, and D1318-64. (3) Gorsuch, T. T., Analyst 84, 135 (1959).
(7) Ritter, R., -Vuturwissenschujten 51, 144 (1964). (8) Roscoe, H. E., Schorlemmer, C., “Treatise on Chemistry,” Vol. 11, p. 121, Macmillan, London, 1913. (9) Stull, S. R., Ind. Eng. Chem. 39 517 (1947). (10) Thiers, R. E., “blethods of Biochemical Analysis,” Vol. 5, Chap. 6,
D. Glick, ed., lnterscience, Sew York, 1957.
T. Y. KOMETANI Bell Telephone Laboratories, Inc. Murray Hill, N. J. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., hlarch 1966.
Ra pid Rad ioche mica I Se pa rations of S tro nti um-90-Y tt rium-90 and Calcium-45-Scandium-46 on a Cation Exchange Resin SIR: Y90 is extensively used in medicine, both in research and as a means of providing intense local radiation to various parts of the internal body. Moreover, Y90 is often required in the purest form as a standard beta source for calibration in nuclear spectrometry. Numerous methods for its separation from Sr90 have been reported (5-6,11). Radioactive calcium of high specific activity is required as a tracer, like Y”, in many investigations, particularly biological ones. Neutron irradiation of natural calcium gives preparations of Ca45with low specific activity, because of low neutron capture cross section and low percentage (-2.1%) of Ca44in the natural mixture, Preparations of high specific activity could be obtained by irradiating an enriched sample of Ca4‘, but this method is costly. Ca45is more suitably produced by (n,p ) reaction on natural scandium. The production of Ca45 in a carrier-free form from scandium and its separation from other alkaline earths by various methods have been reported (6,Y). Recently, Macasek and Cech (8) have described a procedure for the separation of Y90 from Sr” using EDTA and Dowex 50-X2 resin. Strelow (IO), on the other hand, has published a paper in which is given equilibrium distribution coefficients of more than 40 elements between Dowex 5OW-XS resin and different normalities of nitric and sulfuric acids. Before this, dilute nitric acid was employed for the separation of Razz* from Ac22* and other decay products of RaZz4( I ) and also for the purification of Ba140 from Lala (8). 1598 *
ANALYTICAL CHEMISTRY
This work is the continuation of that described earlier ( I , 9). Most of these procedures were developed either for preparing Y90 tracer from Sr” or for purifying Ca45. This paper describes a simple and rapid procedure for separating SrgOfrom Yw and producing Ca45 from scandium target employing a cation exchange resin and dilute nitric acid. This acid could be removed easily and quickly to obtain clean and carrier-free sources of Srw and Ca45. Briefly, fast milking of Y” from Srw employing easily available lactate solution is described. EXPERIMENTAL
Reagents. Baker analyzed Dowex 50W-X8 resin (200-400 mesh) was used. lMost of this resin, when graded in a water column, settled within 20 minutes a t a height of 30 cm. This fraction was selected for the study. Merck guaranteed reagent nitric acid and lactic acid were employed. Dilute solutions from 0.5 to 2.5 moles per liter were prepared from the former after making it colorless. Ammonia was used to prepare solutions of the latter at various pH values. Phenol, the final concentration of which was O.ZOJ,, was added as a preservative for lactate solutions. Tracer solutjons, Srw-Yw, So4, Ca4s and SrW9, were supplied by the Atomic Energy Establishment Trombay (A.E.E.T.), Bombay. These were diluted as desired. Their assaying was done by counting the activity with either a NaI(T1) scintillation spectrometer or with an end-window GeigerMuller tube employing a utility scaler (Type DS 411) supplied by A.E.E.T., Bombay.
Distribution Studies. Employing mg. of accurately weighed air-dried Dowex 50W-X8 resin con taining 32% moisture and tracers such as Sr55!Sg Y”, etc., in a known volume of either dilute nitric acid or lactate solution, the distribution studies were carried out a t 30’ & 1’ C as described (9). The shaking of the resin with the tracer solution was continued for about 5 hours, and the liquid counted thereafter. They were filtered through borosilicate-glass filters to remove suspended resin particles before counting. A11 these experiments were conducted in duplicate but in some cases where the duplicates did not agree, they were repeated to ensure accurate results. From the volume of the solution, the amount of the resin taken, and the counting rate of the solution before and after shaking, the equilibrium distribution coefficient was calculated in each case. The results so obtained were plotted on log X log and semilog paper and are shown in Figures 1 and 2. Separation of Srw from Yw and Cad5 from Sc&. An aliquot of Sr” in equilibrium with Y90 was evaporated to dryness in a beaker and a few drops of dilute nitric acid of a known concentration were added to it. The solution containing most of the activity was then transferred to a resin column of diameter 0.58 cm. and length 3.3 cm. (bed volume -0.87 ml.). The resin column was previously equilibrated with the same acid that was used to dissolve the tracers. The adsorbed activities were then eluted with dilute nitric acid of a known concentration a t a flow rate of about 1 cm. per minute. Two-milliliter portions were collected in test tubes. They mere monitored immediately. The elution was con50-100
