1001
V O L U M E 28, NO. 6, J U N E 1 9 5 6 was 4%. T h e 2-tlieno~-ltrifliioroacetonemethod is rapid and can be performed in about one fourth of the time required for the h r i w n fluorozirconate method. Also, the 2-thenoyltrifluoroacetone technique is very useful when one desires to perform beta-roiinting, because the problem of self-absorption by t h r carrier is eliminated. T h e presence of free oxalate, fluoride, sulfatp, cind phosphate l o w r s the extraction efficiency in the 2thenoylti,ifluoroacetone inethod and, if present, these ions should he rc,movc.d before t h e extraction is performed. Separation of Inactive Zirconium. Zirconium in this form may he wparxted from aluminum, iron, rare earths, thorium,
Table S I I . Comparison of 2-Thenoyltrifluoroacetone and Hariuni FluoroAirconate IIethods for Determination o f Zirconium-95
1
and uranium. For iiistanc'e, by adjusting the aqueous phase t o 6 N hydrochloric arid concentration, zirconium may be removed very effectively from solutions of these elements in a 10-minute extraction \\-ith an equal volume of 0.5.V 2-thenoyltrifluoroawtone-xylene. LITERATURE CITED
Breiinan, 31. E., Flagg, J . F., C . S.Atoriiic Energy Coinniissioii Serret Report, KAPL-332 (April 1950). Connick. 13. E.. AIcT-ey, IT. H., J . A m . C'hern. SOC.71, 3182 (1949).
Connick. R. E.. Reas, IT.H., Jbzd., 73, 1171 (1951). Hudgens, J. E., Karren, R.. l l o o r e , F.L., U. S.Atomic Energy Cominiwion Declassified Report, Mon-N-234 (September 1947). Huffman. E. H.. Beaufait. L. J.,
VI. Chem. SOC.71, 31T9 (1949). Huffman. E. IT., Iddinps. G . 31.. Oshorne, K. S . , Shalimofl, G . T., Jbid., 77, 881 (1955). Hunie. D. S . , "Sational S u c l e a r Energy Series," Division I V , vol. 9, Book 3. 1499-1503, AIcGraw-Hill. New York.
1951.
Kraus, K. d.,T-an Winkle, 4.. U. 8 . .Itomic. Energy Commission Secret Report. ORNL-239 (February 1949). Larsen. E. .\I., Terry. G.. J . li?J?. Cheni. Soc. 75, 1560 (1953). lloore, F'. L., in U. 9 . Atomic, Euergy Commission Secret Heport, ORNL-286, by Swartout. J . A , . others (June 1940). J h i d . , ORNL-336, (3Iay 1949).
Thermogravimetric Pyrolysis of Ammonium and Alkali Metal Tetraphenylborates WESLEY W. WENDLANDT Department o f Chemistry and Chemical Engineering, Texas Technological College, Lubbock, Tex.
The therriiograbinietric p j rol) ses of ammonium, potassiuni, rubidium, and cesium tetraphenjlborates were determined on the thermobalance. It was found that the alhali mela1 compounds were more stable than previousl? suspected. The p j rolj sis of ammonium Letraphen>lborate proceeds in a different manner than plroljsis of alkali metal compounds. It mas n o t possihle to determine both ammonium and potassium b) the automatic therrriograrimetric niethod. i linear relationship w a s fouiicl to exist between the ionic radius of the olhali metal ion and the temperalure at which deconiposi tion began.
T
HE determination of potassium and the heavier alkali metals has always been one of the more difficult problems i n :malytical chemistry. l l a n y reagents have been proposed hiit have had disadvantages such as lack of selectivity, solubilit!. of precipitate, and sometimes doubtful composition of the precipitate. Recently, Wittig and coworkers (10-12) found that the tetraplienylhorate ion, [B(C,H,),] -, is characterized by the unusual pi,operties t h a t its sodium and lithium salts are soluble in n a t c r lvhile the corresponding ammonium, potassium, rubidium, and wsium salts are insoluble. Only mercurous and thallous ions interfere, but these can be easily removed. l l a n y articles have described both gravimetric and volumetric niethods, with emphasis on ammonium and potassium, using this newly developed reagent ( 1 , 5 ) . I n the gravimetric determinations, t h e precipitates were usually dried a t temperatures of 105' to 120" C. I n a volumetric method for potassium the precipitate was ignited a t "red heat" t o potassium metaborate
rind then titrated with 0.1S hydrochloric acid ( 4 ) . S o detiriitr temperature limits were given for the ignition. In view of the lack of knowledge as to the thermal stabilitj. uf the ammonium and alkali metal tetr:iphenylborates, these vompounds xvere prepared arid suhjrrtecl t o thermogravimetric pyroly sis on the thermohalance. EXPERIMEKTA L
Reagents. Sodium tetraphenvlborate \vas otitained from t h r J . T. Baker Chemical Co., Phihpsburg, K. J. X 39; solution \vas prepared as previously described (6, 7 ) . Rubidium and cesium chlorides, c.P., were obtained from A . 11. Jfackay, Inc., S e w Tork 38, S. Y. Thermobalance. The thermobalance used has been described (9). , T h e weight of the samples ranged from 100 to 150 mg. The heating rates on all t h e samples were 4.5" per minute with a maximum temperature limit of about 900" C. T h e weight of the sample was recorded t o r t O . 1 mg., while the temperature \vas recorded t o i1O C. The accuracy and reproducibility of the thermobalance agreed t o within ;:1 of the Chevenard recording thermobalance as checked against t,he thermolysis of Sa2HPOr 12H20 ( 2 ) . Duplicate samples Tvere run on each sample with a resulting agreement to each other within 17;. Preparation of Precipitates. T h e ammonium, potassium, rubidiuni, and cesium tetraphenylborate precipitates were prepared according to the method of Raff arid Brotz ( 8 ) . The ions were pevipitatpd from dilute acetic. acid solution with a 509; escess of a 3c,i sodium tetraphenylborate solution. After standing for 5 minutes, the precipitates were filtered off into sintered powelain c.ruc.ibles, washed twice with very dilute acetic acid solution, and dried a t room temperature for a t least 24 hoiire before pyrolysis on the thermobalanw. DISCUSSION
T h e pyrolysis curves (Figure 1) shoiv the decomposition of the tetrnphenylborate group in the temperature range 210" t o 265' C.
1002
ANALYTICAL CHEMISTRY
T h e potassium, rubidium, and cesium curves are all of the same general appearance. The decomposition temperature is defined as the minimum temperature in which there is 8 loss in weight of sample. This temperature is, of course, a function of the heating rate of the furnace and the sample size. Hence, the heating rates were kept constant and the sizes of the samples were all within 10 mg. of each other.
I
280
6
2 60
240 W II
I
I
I
t
I
220 200
1.30
1.40
1.50
1.60
1.70
1.80
IONIC RADIUS A.
Figure 2. Relationship between decomposition temperature and ionic radius
I
Table I.
t
Stability of Tetraphenylborate Precipitates Minimum JIB02
MB(CsHs)4 4"
IS
Decompa Temp., C. 1306
K
-WW
265 240 210
Rb CS
3 a
b
1 0
20MG
103
Figure 1. 1. 2. 3. 4. 5.
Level,
c.
h-one (B201 a t 5 2 5 ) 715
730 825
Heating rate 4.5' per minute. Sublimation temperature.
\1 2
200
300 400 500 TEMP 'C.
600
700
800
Pyrolysis curves of tetraphenylborate precipitates
Ammonium tetrapbenylborate Mixture of a m m o n i u m and potassium tetrapbenylborates Potassium tetrapbenylborate Rubidium tetrapbenylborate Cesium tetraphenylborate
After the initial decomposition of the precipitate the organic material of the tetraphenylborate ion is slovly oxidized until the level corresponding to the metal metaborate (MBOn) is reached in the temperature range of 715" to 825' C. T h e pyrolysis of the metal tetraphenylborate proceeds according t o the general equation
lI[B(CsHs)d] $. 3002
--+
AIBO2
+ 24C02 + 10HzO
where lI is K, R b , or Cs. T h e behavior of the ammonium precipitate x-as different from that of the other three compounds. This precipitate began to sublime a t about 130". After about two thirds of the material was removed, the remaining amount decomposed slowly, until a t 625' a small quantity of boric oxide (B203) remained in the thermobalance pan. T h e significance of the pyrolysis curves to the determination of these ions can easily be seen. Previous determinations were much too restricted as to drying temperatures] except in the case of the ammonium ion. Table I gives the temperature limits in which the precipitates m r e stable.
T h e drying temperatures previously described in the literature for the ammonium precipitate are correct. Care should be taken that the temperature does not exceed 130" C., where sublimation of the compound begins. Because theie was such a great difference in the stability temperatures for the ammonium and potassium precipitates, it was thought that perhaps this could be the basis for an automatic determination of the two ions ( 3 ) . Curve 2 of Figure 1 s h o m the pyrolysis of a mixture of 40.35m0 ammonium and 59.65% potassium tetraphenylborates. As the curve did not show tiyo separate levels, the method cannot be used for the automatic determination of these tn-o ions. .4n inteiesting fact is also revealed by the pyrolysis curves. There seems to be a linear relationship betn-een the size of the alkali metal ion and the temperature a t which decomposition begins. h plot of this relationship is shown in Figure 2. T h e ammonium ion, Tyhich has a radius about equal to the potassium ion (1.48 A), does not fall on the curve. This is perhaps due to the different type of behavior Fhich the ammonium precipitate undergoes on pyrolysis. T h e other three compounds all decompose in the same general manner. LITERATURE CITED
Barnard, A. J., Chemist Analyst 44, 104 (1955). (2) Duval, C., "Inorganic Thermogravimetric Analysis," chap. 1, (1)
Elsevier, Houston, Tex., 1953. (3) Ibid., chap. 4 . (4) Flaschka, H., Holasek, A., Amin, A. AI., 2. anal. Chem. 138, 161 (1953). (5) Gloss, G . H., Chemist Analyst 42, 50 (1953). (6) Gloss, G. H., Olson, B., Ibid., 43, 70 (1954). (7) Kohler, hf., 2. anal. Chem. 138,9 (1953). (8) Raff, P., Brota, W., Ibid., 133,241 (1951). (9) Wendlandt, W. W., ASAL. CHEX. 27, 1277 (1955). (10) Wittig, G., Angew. Chem. 62A, 231 (1950). (11) Wittig, G., Keicher, G., Ruckert, A , , Raff, P., Ann. 563, 110 (1949). (12) Wittig, G., Raff, P., Ibid.. 573, 195 (1951). RECEIVED for review Sovember 26, 1955.
Accepted March 13, 1956.
