Separation of Zirconium from Other Elements by liquid-liquid Extraction F. L. MOORE O a k Ridge National Laboratory, O a k Ridge, Tenn.
The mechanism of the reaction involves hydrogen replacement and coordinate bonding. Both the 2-thenoyltrifluoroacetone :tnd the zirconium chelate have low solubility in aqueous acid solutions but are soluble in benzene, toluene, or xylene. I n this work an aqueous acid solution containing zirconium was ertracted with a solution of 2-thenoyltrifluoroacetone in syl(9ne itt room temperature. This extraction of zirconium from aqiicous solutions of 2M nitric or hydrochloric acid (or higher :iciditicxs) effects an excellent separation of zirconium from othrr fission products and metal ions escept hafnium. Hafnium is not :t fission product and ordinarily will not have to be considerrti.
The extraction behavior of zirconium at both tracer and macro concentrations from aqueous solutions of nitric acid, hydrochloric acid, and oxalic acid into 0.5.M 2-thenoyltrifluoroacetone-xylene is described, as well as the separation of zirconium from other elements. Peroxy complexing of niobium and protactinium is particularly effective in achieving clean separations of these elements from zirconium. Several general applications of this liquid-liquid extraction technique to problems involving the purification of radiozirconium, the removal of radiozirconium when present as an interference, the rapid determination of radiozirconium in radioisotope and fission product solutions, and the separation of inactive zirconium from interferences prior to its determination are discussed.
PURIFICATION OF ZIRCONIUM-95 TRACER
The zirconium-95 tracer ( f l = 65 days) used in these cxperiinents was purified from a zirconium-95-niobium-95 mixture rontaining 0.570 oxalic acid. This product is available from thc Operations Division of Oak Ridge Sational Laboratory. The method of purification mas similar to that employed by Connick and McT'ey ( 2 ) . The zirconium-niobium mixture was diluted twentyfold or more with 6 J 1 hydrochloric acid: this solution \vas extracted for 10 minutes with an equal volume of 0.5d1 2thenoyltrifluoroacetone-xylene solution. The organic phase containing the zirconium was then washed three times for 5minute periods with equal volumes of 6 M hydrochloric acid. The organic phase was diluted tenfold or more and re-extracted with equal volumes of concentrated hydrochloric acid, 8.11 nitric acid, or 5Vc oxalic acid, depending on the nature of the subscquent experiments. The purity of the zirconium-95 tracer w i s established by aluminum absorption curves and decay measurements. The purified zirconium-95 tracer was used for 2 days before repurification. Appropriate corrections for the growth of a small amount of niobium-95 were applied when necessary.
0
S E of the major fission products for nhich analysec are made at this laboratory is zirconium-95. Because the barium fluozirconate method ('7) employed at present requires about 4 hours, a faster method was desired for the determination of zirconium-95. B liquid-liquid extraction techniqiir n as sought because it offers the advantages of speed, clean separation, and adaptability t o remote control. While the extraction of zirconium by 2-thenoyltrifluoroacetone (TTA) is a ell knon n, the optimum conditions for an analytical method have not h e n established previously. l l o s t of the open literature piih1ic.ation (5, 6, 9) describe the fractional separation of zirconium and hafnium. Several project reports of the Btomic Energy Commission ( I , I O , 11) have described the use of 2-thcnoyltrifluoroacetone in the determination of zirconium-05.
METHOD OF EXTRACTION OF ZIRCONIUM-95 TRACER
The extraction of zirconium with a solution of &-thenoylti,ifluoroacetone in xylene may he done manually, by mixing wii h a glass stirrer connected to a suitable motor, on a mechanical shaker, or bq- vacuum mixing. The latter technique has been found by several m-orkers ( 4 ) to be satisfactory; it also eliminates m-ashing of the stirrer. Vacuum mixing was used in all of the work described and gave reproducible mixing in the various experiments. The solution was agitated by applying a vacuum through a standard taper joint connected to the top of a suitable separatory funnel and allowing air to enter the system through the stopcock a t a rate just adequate to effect intimate mixing of the phases. The loss of xylene in a 10-minute extraction period was found to be negligible. Xylene is preferred t o benzene hecause it has a higher boiling point, is less flammable, and less toxic. Obviously, a low-hoiling solvent such as diethyl ether cannot be used in the vacuum mixing technique.
Table I. Extraction of Zirconiuni-95 Tracer from Nitric Acid Solution with 0.5M 2-ThenoyltrifluoroacetoneXylene HSO?,
sr
1.3 2.1 3 .2 c, , 3
Zr-93 E x t r a c t e d , Cr 97.5 98.8 99.6 99.R
99.3 98.3
Y.5 12.(in 0
Ilrnc.tn w i t h organic phase a t tl1i.c acidity.
Fundamental studies on the extraction of zirconium \Tith 2thenoyltrifluoroacetone have been made by Connick and AIcVey (2). The principle of the method described here is that the zirconium ion forms a very stable chelate complex a i t h 2-thenoyltrifluoroacetone even in strong aqueous concentrations of nitric. acid, hydrochloric acid, or perchloric acid. I n conrentrated arid solutions in the absence of complexing anions the ovei -:dl reaction may be stated as follow: Zr"4
+ 4HT,
F?
ZrT4,
From Nitric Acid Solution. Because fission products are often contained in a nitric acid medium, the extraction of zirconium-95 tracer from various nitric aeid solut'ions was investigated. An aqueous nitric acid solution containing 3 X 105 counts per minute, gamma radioactivity (gamma c.p.m.), of zirconium95 tracer was extracted for 10 minutes n-ith an equal volume (0 ml.) of 0.5M 2-thenoyltrifluoroaretone-xylene. Each phasc wap then measured for zirconium-95 by counting suitable aliquots in a gamma ficintillation counter having a sodium iodide crystal (thallium activated). Table I indicates that zirconium-95 tracer is readily extracted over a wide range of nitric acid concentration. An aqueous phase 2 M in nitric acid was selected for further work; hydrolysis of zirconium results in decreased extraction efficiency at lower nitric acid concentrations.
+ 4H-.d
where H T = 2-thenoyltrifluoroacetone in the xylene phase, and ZrTa = the zirconium chelate. Subscripts A and X refer to aqueous phase and xylene phase, respectively. 997
ANALYTICAL CHEMISTRY
998 Table 11. Extraction of Zirconium-95 Tracer with 0.5M 2-Thenoyltrifluoroacetone-Xylene as Function of Time Minutes Zr-95 Extracted, % 0.5 1 2 5
8 12 16
57.8 69.5 71.6 91.7 98.9 98.3 98.7
Table 111. Effect of 2-Thenoyltrifluoroacetone Concentration on Extraction of Zirconium-95 Tracer from 2M Nitric Acid Solution T T A in Xylene, M Zr-95 Extracted, 70 0.01 0.03 0.05 0.10 0.20 0.30 0.40 0.50
1.9 15.1 33.1 65.7 78.7 95.0 96.9 98.1
Table IV. Extraction of Zirconium-95 Tracer from Hydrochloric Acid Solution with 0.5M 2-Thenoyltrifluoroacetone-Xylene HCI, M Zr-95 Extracted, % 1.7 3.4 5.2
6.9 8.6 10.3
99.3 98,5 98,5 97.5 96.0 94.0
Table IT shows the extraction of zirconium-95 tracer as a function of time. The same procedure was used as before, except that an aqueous 2 M nitric acid solution was chosen and the extraction time was varied. Based on the results obtained, a lo-nlinute extraction period was selected as a standard mixing time for further work. It was not established that equilibrium had been attained under all experimental conditions. The effect of the concentration of 2-thenoyltrifluoroacetone in xylene on the extraction of zirconium-95 tracer is shown in Table 111. An aqueous 2M nitric acid solution containing 3 X 106 gamma c.p.m. of zirconium-95 tracer was extracted for 10 minutes with an equal volume of xylene containing varying concentrations of 2-thenoyltrifluoroacetone. A solution of 0.5M 2-thenoyltrifluoroacetone in xylene was selected as the standard extractant because i t alloas an excellent recovery of zirconium-95 tracer in a 10-minute mixing period. From Hydrochloric Acid Solution. The results of extraction of zirconium-95 tracer from various concentrations of hydrochloric acid (Table IV) indicate that zirconium can also be extracted by 2-thenoyltrifluoroacetone from these aqueous solutions. While the extraction of zirconium is still very efl'ective a t high hydrochloric acid concentrations, the results indicate that chloride complexing of zirconium in the aqueous phase lowers the extraction efficiency slightly. From Oxalic Acid Solution. It is well known that the oxalate ion strongly complexes zirconium. Zirconium-95 tracer is ordinarily stored in oxalic acid solution to prevent hydrolysis; therefore, several experiments were performed t o test the interference of oxalic acid and find methods of circumventing it. By raising the aqueous solution acidity from 2M nitric or hydrochloric acid t o the 8 to IOM concentration range of either of these acids, it was possible to recover about 50% of the zirconium-95 tracer from 0.5% oxalic acid solution. I n several other experiments attempts were made t o use the aluminum ion to complex the oxalate ion. The adjustment of the aqueous solution to 0.2% oxalic acid-l.2M aluminum nitrate resulted in an increase in efficiency of extraction of the zirconium-95 tracer; however, yields were less than 50%. It was found possible to extract the zirconium-95 tracer
readily from oxalic acid solutions, if the oxalic acid was first destroyed by the addition of excess potassium permanganate. I n a typical experiment a slight excess of 5 % potassium permanganate solution was used to destroy the oxalic acid present. Any manganese dioxide formed was dissolved by the addition of a small excess of 1M hydroxylamine hydrochloride solution. The solution was then extracted for 10 minutes &h an equal volume of 0.531 2-thenoyltrifluoroacetone-xylene. Table V shows that the destruction of oxalic acid with potassium prrmanganate results in an excellent recovery of zirconium. Stripping Zirconium-95 from 0.5M Z-ThenoyltrifluoroacetoneXylene. While the analytical radiochemist often may measure directly the radioactivity of the organic phase in the determination of zirconium-95, a t times it is desirable to re extract (strip) the zirconium-95 tracer into an aqueous solution. This is particularly desirable if one wishes to perform a colorimetric analysis for zirconium or is purifying zirconium-95 tracer. A series of aqueous solutions was investigated as possible stripping agents for zirconium. The 0.5M 2-thenoyltrifluoroacetone-xylene phase containing 1.2 X lo6 gamma c.p.m. of zirconium-95 tracer was stripped with an equal volume (4 ml.) of various aqueous solutions. The results shown in Table T'I indicate that dilute hydrofluoric-nitric acid mixtures are very effective stripping agents for zirconium. An alternate method for stripping the zirconium-95 tracer is to reduce the effective thenoyltrifluoroacetone concentration by a factor of 10 or more by dilution with xylene and then strip the zirconium-95 into a strong mineral acid solution. For instance, when the thenoyltrifluoroacetone concentration in xylene was reduced to 0.05M, a 10-minute extraction with an equal volume of concentrated hydrochloric acid or 8M nitric acid stripped approximately 90% of the zirconium-95 tracer into the aqueous acid solution. Increased extraction periods may be used for greater yields. SEPARATION O F ZIRCONIUM FROM OTHER ELEMENTS
The extraction of zirconium-95 from aqueous solutions of 2M nitric or hydrochloric acid (or higher acidities) effects an
Table V. Extraction of Zirconium-95 Tracer from Oxalic Acid Solutions Pretreated with Potassium Permanganate (All solutions pretreated except the first) Aqueous Phase Zr-95 Extraoted, % 3.5 2 M "01 0 5% (COOH), 98.7 2 M HNOs:0:5% (COOH)? 95.1 98.5 98.0 98.3 95.1 95.9 97.9 96.8 94.6 96.5
Table VI. Effect of Various Aqueous Solutions for Reextraction of Zirconium-95 from 0.5M P-Thenoyltrifluoroacetone-Xylene Stripping Agent 0.25M "01, 0.25.M H F
0 . 5 M HKOr, 0.5M IIF 2 . 7 5 7 (COOH): 1 1 . 0 8 (C0OH)t 4.5M HzSOc 3 % Dieodium Versenate 37.7% HzOn 25.0% HnOn 2 . 6 M HNOs, 25% KnOz
Minutes 1
3 1 3 5
5 5 10 4
10 10 10 10
Zr-95 Extracted, % 89.6 99.4 91.3 99.5 3.0 6.5
10.8 16.0 0.1 10.2 0.6 0.4 1.2
999
V O L U M E 2 8 , NO. 6, J U N E 1 9 5 6 Table 1’11. Extraction of Niobium-95 from Aqueous Nitric Acid Solution with 0.5M 2-ThenoyltrifluoroacetoneXylene Nb-95 Extracted, % H S O I , M HzOz, % KHsOH.HC1, M 1.8 2.2 2.2 2.2 2.7
5.5
0.9
1.25
Organic Organic phase washed phase 3 minutes with unwashed appropriate HNOa s o h . 1.2 1.1 0.9 0.8 2.2 1.6 1.5 1.4 2.5 2 2 lene
__-_ HYO,. If
1 8 2 2 6 2 2 4 10 4
a
iqueous Phase
Zr. m g per ml
Zr Extractrd
2 2 2 2 2 2
98,lQ 99 1 99.3
4 4
77.8 98 7
4 4 .Iqueous phase became sliglitly clortdy
c(
stripped the iron out of the 0.5A1 2-thenoyltrifluoroacetonexylene phase. However, it is recommended, if one wishes to separate zirconium from iron, that the original aqueous phase be adjusted to 6 M in hydrochloric acid. Zirconium extraction is essentially quantitative a t this acidity, while the ferric iron extraction (iron-59 tracer or 2.3 mg. of iron per ml.) was found to be approximately O.lyo in a typical 10-minute extraction period with an equal volume of 0.5X 2-thenoyltrifluoroacetonexylene. The decrease in the extraction efficiency of ferric iron is probably due to the formation of the chloroferric acid complex in the aqueous hydrochloric acid solution. Free iodine is ext.racted into 2-thenoyltrifluoroacetone-xylene. Radioiodine is present only in relatively short-cooled solutions and ordinarily does not have to be considered. However, if a short-cooled fission product solution containing radioiodine is used, it is recommended that the iodine be removed before estraction of the zirconium, because iodine has been found to exhibit erratic behavior in the 2-thenovltrifluoroacctone-xylene system. EXTRACTION OF ZIRCONIUM CARRIER WITH 0.5.+f 2-THEiYOY LTRIFLUOROACETONE-XY LENE
The thenoyltrifluoroacetone extraction techniqllc has mainly been used in this laboratory for thr drtermination or purificat,iori of zirconium-95 tracer in solutions of the fission product type. Some exprriments n-ere performrd in this study to determine if ma(~roamounts(milligrams) of zirconium could be used in instances where the chemist desires to use conventional radiochemical techniqurs :ind apply yield corrections. For thew experiments aqueous nitric acid solutions containing carrier amounts of zirconium (nith zirconium-95 tracer) were extracted for 10 minutes with an equal volume of 0.553 2-the~ioyltrifluoroacetone-xylene solution. Both phases were anal?-zed for zirconium by measuring the zirconium-95 radioactivity. Table XI shows that one may use as much as 2 mg. of zirconium per milliliter of 231 nitric acid solution and get an excellent recovery of zirconium. The table also indicates that it is possihle to carry out efficient extractions on higher c.oncentrations of zirconium (4.4 mg. per ml.) if the original aqueous acidity is increased. This may he explained by the fact that the zirconium ion is effectively monomeric in these solutions. At lower aqueous acidities ( 2 X nitric acid) the increase in zirconium concentration above approximatell- 2 mg. per ml. results in an appreciable formation of polymeric zirconium which is not available for chelation. This effect has been observed by other investigators iii perchloric acid solutions (3, 5 ) . It is recommended that an acidity in the range of 631 nitric acid (or 6'11 hydrochloric acid) he used for macro work. The extraction of zirconium is just as effective at this acidity and some of the hydrolytic problems which may occur a t lower acidities are avoided. GENERAL APPLICATIONS TO METAL SEPARATIONS
The use of 0.5M 2-thenoyltrifluoroacetone-xylene, the most highly selective known extractant for zirconium, offers many
practical applications t,o metal separations problems. Four applications for which the technique has been employed a t this laboratory are listed below; others ail1 doubtless occur to the reader. Purification of Zirconium-95 Tracer. This method may be used to purify zirconium-95 tracer, or for the rapid removal of radiozirconium interference in various separation methods. Rapid Determination of Zirconium-95 in Radioisotope Product Solutions. This method is emplo\-ed specificallj- for t'he determination of zirconium-95 in the zirconium-95-niobium-95 product from the Oak Ridge Sational Laboratory, Operations Division. The mixture is contained in 0.5% oxalic acid. The recovery of zirconium-95 has been found to be 09% =k 3. The gross gamma radioactivity is determined per milliliter of the original sample ( A ) on a gamma scintillation counter. .4 portion ( B ) of the sample of sufficient radioactivity such that an aliquot ( D )taken for measurement on the gamma scintillation counter will count between 1 X 104 to 1 X 106 gamma c.p.m. is pipetted into a 30-ml. beaker. Sufficient 0.331 potassium permanganate solution is added dropwise until a permanent purple color persists. I n order to ensure that all the oxalic acid is decomposed, the solution is swirled so that it washes the walls of the beaker. Hydroxylamine hydrochloride solution, 5J1, is added dropwise until the solution is decolorized, then several drops excess are added. The solution is transferred quantitatively to a 30-nil. separatory funnel (or other extraction vessel). The beaker is washed three times viith I-ml. portions of 211 nitric acid, and the washes are transferred to the fiinnel. The solution should be approximately 2.11 nitric acid. -4volume of 0.5.11 2-thenoyltrifluoroacetone-xylene equal t'o the volume of the aqueous phase is added to the funnel and a 10-minute extraction is performed. ilfter the phases have separated, the l o m r aqueous phase is dran-n off into another 30-ml. separatory funnel, and the organic phase is retained. -4n equal volume of fresh 0.5M 2-thenoyltrifluoroacetonexj-lene is added to the aqueous phase and a second extraction is performed for ten minutes. After the phases have separated, the lower phase is drawn off and discarded. The two organic phases are combined in a volumetric flask of suitable size. The combined solutions are made to volume ( C ) with xylene and mixed well. An aliquot ( D ) is removed and the gamma radioactivity ( E ) of the aliquot is measured on the gamma scintillation counter. The ealciilation is as follows: Let 1 = total gamma radioactivity of the original sample, gmima c.p.m. per ml. B = volume of sample taken for niial!.sis, nil. L' = volume of flask, nil. D = volume of separated solution measwed on the gamma scintillation counter, ml. E = gamma radioactivity of D , gamma c.p.m. ( , ; zirconium-95 gamma radioactivitj. in original sample =
CE
'4
x
100
Rapid Determination of Zirconium-95 in Fission Product Solutions. Several fission product solutions were analyzed for zirconium-95 by both the 2-thenoyltrifluoroacetone method and the standard barium fluorozirconate method ( 7 ) . For this determination a suitable aliquot of the fission product solution was adjusted to a concentration 2M in nitric acid and approximatel! 05-11 in hydroxylamine hydrochloride The solution was then extracted for 10 minutes with an equal volume of 2-thenoyltrifluoroacetone-xylene. After the phases had disengaged, the aqueous phase was drawn off and discarded. The organic phase was then Jvashed for 3 minutes with an equal volume of 1X nitric acid solution. After settling, the aqueous wash phase was drawn off and discarded. The organic phase was drawn into a suitable centrifuge cone and centrifuged for 3 minutes in a clinical centrifuge. The zirconium-95 radioactivity was then determined b) counting a suitable aliquot of the organic phase. Table S I 1 gives a comparison of the two methods. On a typical fission product solution the recovery of zirconium95 was found to be 99 & 2.4% by the thenoyltrifluoroacetone method, The precision using the barium fluorozirconate method
1001
V O L U M E 28, NO. 6, J U N E 1 9 5 6 was 4%. The 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 thr 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 the 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
and uranium. For iiistanc'e, by adjusting the aqueous phase to 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., 7 3 , 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.
1
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, O R N L - 2 8 6 , by Swartout. J . A , . others (June 1940). J h i d . , O R N L - 3 3 6 , (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 pj 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 that its sodium and lithium salts are soluble in natcr 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, the 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 thr 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 the samples were 4.5" per minute with a maximum temperature limit of about 900" C. The weight of the sample was recorded to r t O . 1 mg., while the temperature \vas recorded to i1O C. The accuracy and reproducibility of the thermobalance agreed to 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. The 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
The pyrolysis curves (Figure 1) shoiv the decomposition of the tetrnphenylborate group in the temperature range 210" to 265' C.
