The precipitation· of rubidium and ceasium with silicotungstic
a cid has been stu died w ith the aim of investigating such processes
as a possible separation method. The influence of hydrochloric acid,
ammonium chloride, and the aging of the precipitates was investigated.
The results show that there are diffe rences in the
precipitation of rubidium and ceasium with s ilicotungstic acid.
When accompanying ceasium, rubidium was found even in precipitates
under such experimental conditions for which it would not
precipitate itself. The results also indicate that precipitates formed
in the low concentration region of h ydrochloric acid are salts either
of rubidium or ceasium silicotungstate. At higher acid concentrations
there is a higher precipitation yield. This is due to the contributions
from two processes: precipitation and co -precipitation** . At higher
hydrochloric a cid concentrations, the precipitation of rubidium
or ceasium silicotungstate is followed by co-precipitation of rubidium
and ceasium. It is shown that rubidium can be removed
from rubidium-ceasium silicotungstates by an exchange reaction

J. Tuta

P . Strohal

Đ. Letić

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CCA-427 CR 0 AT IC A CHEM I CA ACT A 38 (1966) 205 543.243 :546.35 :546.36 Ori ginal Scientific Paper The Precipitation of Rubidium and Ceasium with Silicotungstic Acid P. Strohal, D . Letic*, and J. Tuta Institute »Ruder Boskovic«, Zagreb, Croatia, Yugoslavia Received, April 11, 1966 The precipitation· of rubidium and ceasium with silicotungstic acid has been studied with the aim of investigating such processes as a possible separation method. The influence of hydrochloric acid, ammonium chloride, and the aging of the precipitates was in-vestigated. The results show that there are differences in the precipitation of rubidium and ceasium with silicotungstic acid. When accompanying ceasium, rubidium was found even in precipi-tates under such experimental conditions for which it would not precipitate itself. The results also indicate that precipitates formed in the low concentration region of hydrochloric acid are salts either of rubidium or ceasium silicotungstate. At higher acid concentrations there is a higher precipitation yield. This is due to the contributions from two processes: precipitation and co-precipitation**. At higher hydrochloric acid concentrations, the precipitation of rubidium or ceasium silicotungstate is followed by co-precipitation of ru-bidium and ceasium. It is shown that rubidium can be removed from rubidium-ceasium silicotungstates by an exchange reaction. The quantitative determination of rubidium and ceasium is rather difficult because of the chemical similarities of the alkali metals. Procedures for the quantitative determination of rubidium and ceasium presented in the literature, therefore, are time-consuming and, in most of them, the interference of some other elements play an important role. It has been suggested that heteropoly acids are specific reagents for the precipitation of rubidium and ceasium. A number of authors have demonstrated the application of various heteropoly acids to the separation of rubidium and ceasium either by precipitation or by using various salts of h eteropoly acids as ion exchangers. O'Leary and Papish1 suggested the silicotungstic m ethod for the separation of ceasium from rubidium. Glendenin and Nelson2 applied this method to ceasium separation from the bulk of fission products. Kourim3, then Kourim, Lavrukhina, and Rodin4•5 described the coprecipitation of rubidium and ceasium with h eteropoly acids from strong mineral acid solutions, while Healy6- 8 investigated the preparation and p roperties of some heteropoly acid salts. Since no systematic and detailed studies describing precipitation processes of rubidium and ceasium silicotungstate have been presented as yet, the present work was undertaken with the aim of investigating such processes as a possible * Taken in part from the Chem. Eng. Thesis submitted to the Faculty of Technology, University of Zagreb. ** Term "co-precipitation" is used in this paper for any process in which certain ions are carried down by the precipitate. 206 P. STROHAL, D. LETIC, AND J. TUTA separation method. The concentrations of rubidium and ceasium used in this work are of interest for conventional analytical determinations of these ele-ments. Since ceasium concentrations of about 10-1 to 10-3 M are probable in reprocessing9, the investigated precipitation systems are also of interest in nuclear technology. Conditions for the precipitation of rubidium and ceasium silicotungstate following the proposed analytical procedure1 were investigated in this work using a wide range of concentrations and acidity. EXPERIMENTAL All solutions were prepared using analytical grade chemicals (Merck) and bidistilled water. Concentrations were tested by standard analytical methodsto. In each case the series of precipitation systems were prepared by mixing either rubidium or ceasium chloride solutions with silicotungstic acid solutions. Experiments were performed in the concentration range from 5 X 10-3 M to 5 X 10-2 M of rubidium and ceasium. Most of the experiments were made in an excess of silicotungstic acid, and the concentration of hydrochloric acid was varied from 10-2 to 10 M. Precipitates were separated from solutions using a laboratory centrifuge and the remaining activity was determined in the liquid phase. The precipitation yield in each system was determined radiometrically using .S6Rb and 1a1cs as tracers. Radioactivity measurements were performed using a 11;2 X 1 Y. inch well type NaI(Tl)scintillation crystal and an EKCO-Electronics scaler. In the case when both rubidium and ceasium were present, the activity due to each nuclide was determined using a scintillation spectrometer. The scintillation spectro-meter used in this work was a 256-channel pulse height analyser attached to a .3 X 3 inch thallium activated sodium iodide crystal. In the resultant gamma spectro-gram consisting of to succesive spectra, the heights of the photopeaks due to the 1.08 MeV gamma ray from S6Rb and the 0.662 MeV from 1a1cs were measured. RESULTS Figures 1. and 2. illustrate a typical reaction of rubidium chloride with silicotungstic acid as a function of hydrochloric acid concent ration. The preci-pitation yields for 2.5 X 10-2 M RbCl with various concentrations of silicotungs-(Rb Cl ) • 2 5 ·10- 2 M v [H,SiW,,o.oJ • 25 10"1M HCl concn. IMJ Fig. 1. The influence of hydrochloric acid concentration on the precipitation of rub idium from 2.5 X 10-2 M solution. PRECIPITAT-ION OF RUBIDIUM AND CEASIUM tic acid between 3.125 X 10-3 M and 2.5 X 10-2 M are shown in Fig. 1. Rubidium was found to be present in the precipitates in concentrated hydrochloric acid .solutions for all rubidium concentrations investigated. Moreover, rubidium did not precipitate in diluted HCl solutions when its concentration was 5 X 10-3 M~ as seen from Fig. 2. The yields of ceasium found in the precipitates are shown in Fig. 3 and 4. The concentrations of ceasium and silicotungstic acid used in these series of # 100 Cl [ RbCl] w • 5 10·• M f I-~ 80 t:; [H.S1W120'41) • 5 · 10-JM Q. 0 ( H.S1W11D ... ) · 25 ·1o·l M u w 60 a:: Cl. .a 40 # Q'. 20 . i t3---j r-~. , 0 0-0-0.oifJ 10-' 1 10 HCl concn . (M] Fig. 2. The influence of hydrochloric acid concentration on the precipitation of rubidium from 5 X 10-1 M solution. [CsCl] • 5 10 ·• M 0 ( H,SiWnD..o ) • 5 . 10·1 M x rH,SiW,.O..o ) • 2.5 10·1 M o [H.s;w,,o,,.) • 1.25 ·10-'M ll [H,s;w,.o..,) •6.25 10-'M • [H,s;w,,o .. ) • 3.125 10·' M 100 ~ 0 ·t q w I-<t 60 I-a: u w 40 a:: ll. .. • u 10 HCI concn . (M] ·J'ig. 3. The influence of hydrochloric acid concentration · on ther precipitation of ceasium · from li .X ltl-:! M .solutl<;m .. P. STROHAL, fl. LETIC, AND J. TUTA experiments were the same as described above in the rubidium precipitation experiments. Experimental data indicate that at higher concentrations of ceasium, e. g. 5 X 10-2 M, the process of precipitation is almost quantitative if an excess of silicotungstic acid is used. [cs ct] • s .10-> M o [ H,s;w110.,.] • s 10-J M x (H,SiW120..o J • 2.5 10-> M i. 100 0 IJJ 90 ..... ~ ~ u IJJ a: ~ 70 .. u 60 so 0 HCL concn. [M] Fig. 4. The influence o! hydrochloric acid concentration on the precipitation of ceasiura from 5 X io-s M solution. 0 w 100 I-~ 9= so u w a:: a.. 60 "' u -g l.D ro .a a:: 20 0 o [cs a ] • 10-1 M 0 [RbCl) • 10-1 M (H..S iWnO..O ] ·1 ·10"M 0-- J 0-------0-HCl concn . (M] 10 Fig. 5. Precipitation yields o! rubidium and ceasium when precipitated from io-• M solution a s a. function o! hydrochloric acid concentration . PRECIPITATION OF RUBIDIUM AND CEASIUM 209 Results concerning the precipitation of the rubidium-ceasium mixture are presented in Fig. 5 and 6. From both diagrams it can be seen that the precipi-tation yield of ceasium in all cases is over 900/o, and in concentrated acid solutions even close to 1000/o. On the other hand, the precipitation yield curves for rubidium are not of the same shape in comparison with the case when no ceasium is present. Moreover, when accompanying ceasium, rubidium was found even in precipitates formed under such experimental conditions for which it would not precipitate itself. In order to study the influence of the electrolyte on the precipitation of rubidium and ceasium with silicotungstic acid, the precipitation yields were measured in the presence of various concentrations of ammonium chloride. l;l 80 er a.. t'.l 60 'O c ., 40 J:l er 20 [H,s,w.,o...) • s . :ir•M O(RbC!) • 5 · lJ-l M 6 (CsC!) • 5 10"'M r---6~~~~-.o.~~~~-6---6-"'~ 10 HCL concn . fM] Fig . 6. Precipitation yields of rubidium and ceasium when precipitated from 5 X 10- 3 M solution as a function of hydrochloric acid concentration. [H. Siw.,o ... ] - 5 10"' M o[CsC! J • 5 10·1 M 0 a-100 0 - o w f- 80 <l: 6 [Rb Cl) • 5 10" 3 M ~~~-0----------0--~ ' f-0.. u 60 w er a.. \ "' 1.0 - 6 u 'O c ., 20 J:l Cl:: 10 Fig. 7. The influence of NH,CJ con centra tion on the precipitation yield of rubidium and ceasium. 21Q P. STROHAL, D. LETIC, AND J. TUTA Typical results are presented in Fig. 7. The results indicated that at high am-monium chloride concentrations, rubidium and ceasium precipitation yields decrease, and over a certain concentration of ammonium chloride there is no more rubidium in the precipitate. DISCUSSION As can be seen from the results presented above, there are some differences in the precipitation of rubidium and ceasium with silicotnngstic acid. In the low concentration region, e.g. 5 X 10-3 M, rubidium does not precipitate, whereas ceasium precipitates with a r easonable yield. At higher concentrations they were both found in the precipitates, but the ceasium yield was higher. That is due to the fact that the solubility of alkali element salts decreases with mass number, and salts formed by silicotungstic acid are among the most soluble heteropoly acid salts12• Since the experimental results indicate that at HCl concentrations up to about 1 N the precipitation yield is practically constant, but above 1 N it increases rapidly, it may be supposed that various processes take place. Precipitates formed in the low concentration region of hydrochloric acid are salts either of rubidium or ceasium silicotungstates. On the other hand, at higher acid concentrations a better precipitation yield is due to the sum of two processes present: precipitation either of rubidium or ceasium silicotungstate and a complex process in which the silicotungstic acid precipitation is followed by a coprecipitation of rubidium or ceasium. The explanation might be found in the well-known fact that heteropoly acids can be precipitated from con-centrated mineral acid solutions. Thus, according to the results of Lavrukhina et al.5, in such systems both a precipitation and a coprecipitation process occur. Contributions of each process can be seen from the diagrams shown in Fig. 4. The formation of either rubidium or ceasium silicotungstates in the low concentration region of HCl is in agreement with Healy's results relating ceasium and rubidium silicotungstate formation in nitric acid media7• Precipi-tates in this region may have various hydrogen to alkali metal ratios. Since Matijevic and Kerker11 demonstrated the tetravalent nature of silicotungstic acid, the substitution of the four hydrogen ions by alkali metal is possible. For this purpose we measured radiometrically the Rb+ or Cs+ to SiW120 404- ratio in precipitates formed from systems where concentration of each ion was 5 X 10-2 M, and 86RbCl, 137CsCl, and H4Si185W1 20 40 solutions were used. The results are presented in Fig. 8. It can be seen that with increasing acidity the content of the alkali metals in the precipitate decreases. The results also indicated that there is no region where either rubidium or ceasium can be precipitated separately. Because of the similarity in the ionic radii of rubidium and ceasium and their similar chemical behaviours, isomor-physm should also be taken into account. Rubidium can be removed from ceasium-rubidium silicotungstate by an exchange reaction. Fig. 9 illustrates the rubidium removal from such precipitates using 1 M HCL The proposed procedure1 recommends aging the precipitate for about ten hours. Investigations of the influence of aging on the Rb/Cs ratio in the · precipitates were therefore carried out. Fig. 10 illustrates the changes in the rubidium to ceasium ratios as a function of time when the rubidium-ceasium PRECIPITATION OF RUBIDIUM AND CEASIUM 211 silicotungstate'' precipitate was allowed to stay in the mother liquor. It can be assumed that a recrystallization process takes place, so that rubidium ions from the· mother liquor exchange with those of hydrogen. The aging of the precipitate is therefore not preferable in the separation process. Experiments concerning the precipitation of rubidiu~ and ceasium silico-tungstates in the presence of a large amount of ammonium chloride indicate that the n~pidium and ceasium yields in the solid phase decrease with increasing 0 ;a .. "O 2 c .., ~- 1 fl: ...... 0 0 ( RbCI) •5 · 10 -I M t> [ Cscl] • s .10·2 M (H,,S 1 W~Oo.0) •5 · 10"M o ____ _ 0----°'--J--t:Y-----t>- ~ -l>-t;....,._e,,.-6 1Q-· 10-' HCL concn, [M] ll Fig, B. The influence ot hydrochloric acid concentration on the Rb . or Cs. to SiWa0 .. 4- ratio. uJ I-~ Q: u w a:: Q. ~ (/) 1-z <t 1-z 0 u .0 a:: • 15 ------· ~-""'· ~ ... 10 s -TIME (MIN} Fig, 9, Rubidium removal from rubidium-ceasium silicotungstates using 1 M HCl. 212 P. STROHAL, D. LETIC, AND J. TUTA electrolyte concentration. This phenomenon is due to the effect of the NHt con-centration and the solubilities of the certain silicotungstates in concentrated electrolyte solution. It is possible to have such conditions that the precipitates does not contain rubidium. Moreover, under such conditions the ceasium yield in the precipitate is much lower. ~100 0 w .... 80 ~ Q. u 60 W , a: Q. "' 40 0 [ H,.S1W110.,.] .1 10-1 M o [csC l ] -1 10-1 M lo[RbCl] •110-1 M 10 10' 10 1 10• TIME (MIN) Fie 10. Rubidium and ceasium content in precipitates vs. contact time in mother liquor. In this respect, the precipitation of rubidium and ceasium silicotungstates is a complex process. Under the investigated conditions it is not possible to obtain rubidium free of ceasium silicotungstate precipitate. It is therefore evident that following this mechanism both rubidium and ceasium will be precipitated from 6 M HCl solution, as seen from Figs. 5 and 6. This also indicates that the procedure proposed in the literature1 is not suitable for the separation of ceasium from rubidium. Ack~owledgement. The authors wish to thank Dr M . J . Herak for helpful discus-sions concerning this work. REFERENCES 1. O' Leary and B. Papi s h, Ind. Chem., Anal. Ed. 6 (1934) 110. 2. L. E . G 1 end en in and C. M. N e 1 son, Nat. Nuclear Energy Ser., Div. IV. 9 Book 3 (1951) 1642. 3. V . Kouri m, J . Inorg. Nucl. Chem. 12 (1960) 370. 4. V. Kouri m, A. K. Lav r u k hi n a, and S . S. Rodin, J. Inorg. Nucl. Chem . 21 (1961) 375. 5. A. K. Lavrukhina, V. Kourim, and S . S. Rodin, J. Inorg. Nucl . Chem. 33 (1962) 309. 6. T. V. He a 1 y, Radiochimica Acta 2 (1964) 146. 7. T. V. He a 1 y, Radiochimica Acta 3 (1964) 100. 8. T . V. He a 1 y, Radiochimica Acta 3 (1964) 106. 9. P . Des n e i g es, Chim. Anal. 42 (1960) 120 ; 'i bid. 42 (1960) 195. PRECIPITATION OF RUBIDIUM AND CEASIUM 213 10. W. W. Scott, Standard Methods of Chemicai Analysis, D. Van Nostrand Co. Inc., New York, 1954. 11. E. Mat i j e vi c and M. Kerker, J. Am. Chem. Soc. 81 (1959) 5560. IZVOD Talozenje rubidiuma i ceziuma silikovolframatnom kiselinom P. Strohal, D. Letic i J . Tuta Ispitivano je talozenje rubidiuma i ceziuma s aspekata njihove separacije. Rezul-tati ukazuju na razlike u talozenju rubidiuma i ceziuma. Kada je u smjesi s ceziumom, rubidium talozi i kod eksperimentalnih uslova, gdje sam ne bi talozio Rezultati takoder ukazuju da su talozi nastali u nizim koncentracijama solne kiseline soli r ubidium ili cezium silikovolframata. Kod visih koncentracija solne kiseline, .medutim, prinos talozenja mnogo je veci sto pripisujemo sumi dvaju procesa: talo-zenju i sutalofonju. Prikazani su eksperimentalni uslovi kod kojih se moze dobiti talog, koji saddi samo cezijum. Ispitivan je uticaj solne kiseline, amonium klorida i starenja taloga na separaciju ceziuma . .INSTITUT ,.RUDER BOSKOVIC .. ZAGR!:B Primlje no 11. travnja 1966. 

The Precipitation of Rubidium and Ceasium with Silicotungstic Acid

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