determination of cesium in seawater by radiochemical methods

Analytica Chimicn Acta, 139 (1962) I77-IS6 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

DETERMINATION OF CESIUM IN SEAWATER HY RADIOCHEMICAL METHODS

V. I. SIlhhlAEV* and 1’. V. CIIUDINOVSKICII

Jlendeleeu Institute of Chemical Technology. hloscow (Li.S.S.R.)

(Received 26th October 198 1)

SUhlMARY

Radiochemical methods for the determination of ccsium in seawater are dcscribcd. Preliminary substoichiometric concentration with nickel hellcyarlofc!rrnte(II) and tetrn- phcnylborate and application of _selectivc radiochemical techniques arc used. The ccsium content in Atlantic Ocean .samplcs is 0:i.l : 0.03 ~6 I”. The method is applicable on hoard ship. The approach should be generally useful.

The determination of cesium in seawater is a difficult analytical problem, as the content of cesium in scawatcr is 4-9 X 10-‘” g ml-’ whereas the amounts of rubidium, potassium and sodium present are respectively about lo.‘, 2 X 10” and 10’ timesgreater, when molar ratios are considered. Cesium in seawater has been determined by flame photometry after preconcentration on molybdenyl phosphate and estraction with sodium tetraphcnylborate into nitrobenzene [l]. Activation analysis has been applied after concentration of cesium on Amberlite IR105 resin [2]. The low concentration of ccsium in seawater and the relatively large concentrations of other alkali elements make such preliminary concentrations essential but uncontrolled losses of ccsium are unavoidable.

The dctcrmination of ccsium in .seawatcr by radiochcmical methods has seemed impossible, partly because of insufficient sensitivity (the limit of detection for substoichiomctric variants of radiometric correction and iso- tope dilution methods with estraction processes is about 10” g ml ‘I), and partly because of inadequate selectivity; direct determinations of cesium by such methods are possible only in the presence of about equimolar quantities of potzsium. Cesium has been determined in the presence of a 60-fold amount of potassium [3] and in the presence of an S-fold amount of rubidium [4 1. Isolation with different quantities of reagent (51 allowed the determination of cesium in the presence of lOO-fold amounts of potassium and 20-fold amounts of rubidium; interpolation and comparative methods [6] made it possible to tolerate 150-fold amounts of potassium and 40-fold amounts of rubidium. Combination of these methods with substoichiometric prcconccntration [7-91 enabled cesium to be determined in the presence of a 400-fold amount of rubidium and a lo’-fold amount of potas-

1iS

sium. All these methods were based on extraction with tetraphenylborati or magnesium dipicryiaminatc into nitrobenzene. These data indicate that the application of estraction processes alone is not enough to solve the problems of determining cesium in seawater. Further, the exchange constants for cesium with other alkali metals in the tetraphenylborate (TPB) estraction (PcrlK = 25, pcSlxa - 2500, PCs,ILb = 5) do not provide the necessary selectivity_ Preliminary concentration of cesium by other methods was there- fort? examined.

First, the exchange constants (ratios of equilibrium constants) between cesium, rubidium and potassium for mised hcxacyanoferrate( II) precipitates with different metals were determined, and the dependence of these exchange constants (p) on the relative quantities of alkali metal, transition metal and hexacyanoferrat.e(II) ions was investigated. Precipitations were done at pH l--2 from hot solutions containing corresponding quantities of potassium (or rubidium), micro quantities of “‘Cs and corresponding quantities of transition metal ions, by slow addition of sodium hesacyanoferrate(II) solution with stirring.

The eschange constants were calculated by means of the equation [lo]

P = [(I - QcsWcsl {[r?lrc - mtdn + nks (1 -- adI /

[ nld~z - w3 (1 - a~~)1 ) (1) where mric, mcs and mK are the molar amounts of hesacyanoferrate(II), ccsium and potassium ions; acs = Argtratc/Ainic, is the unprecipitated fraction of cesium; and n is the stoichiometric ratio in the precipitate (HC/K).

In all experiments, the initial ratio used was rn&rrzHC = 3; when a large escess of alkali metal is present, this leads to the formation of a precipitate of the composition lMeJK,[ Fe(CN)6 j z [ 11). Therefore, in these experiments, R = 1. As micro amounts of cesium were used with mKtRbj % mrgc, Eqn. (1) can be simplified to

m tic

fiz l-cra,,X mK -n + m,, (l--&

&CS l.ElliC -mC: (1 --C3 n

(2)

It is easy to see that the exchange constant then coincides with the crystallization coefficient for the linear law of co-crystallization.

The results of these experiments are given in 'rablesl(/jCs,K) and 2 (PCSIRb)- In both cases, the best selectivity was attained with the mixed precipitate of nickel (JjcS,Ii = lo’, &s,Hb = 3 X 102). Even with a 2000-fold excess of potassium over hesacyanoferratc(II), precipitation of 13’Cs was nearly complete (97.8%). In Fig. 1 are presented the results of a study of the influence of pH on the degree of cesium sorption. The optimal range for the precipitations is pH 1-2.

Ii9

TABLE 1

Exchange constants (ocrK) for ccsium and potassium precipit:ilion lvith hrsncy:u~o- ferrate(II) salts of some transition elements - - _-.--. -----_____ ._____.- hlctal mu ?* HC nlK 1 -- ‘I ;I

(mmol) (mmol) *\ = - ‘nIIc ..-.

cu 6.16 3.096 x 10 ’ 1990 0.796 939s Ni 30.8 1.55 x 10” 1990 0.978 88320 iig 13.73 5.56 x 10-1 2469 0.726 6539 CO 27.72 1.38 x 10.’ 2016 0.949 37 4 6:! Cd 13.i3 1.11 x IO” 1’135 _ . 0.128 3 62 Zn 13.73 1.76 x 10-l 780 0.366 -151-l -.-.-.-._--_-.- .-_- -.-- -_-.- -..-

TABLE 2

Exchange cur&ants (P~,~,) for cesium and rubidium precipitation with hexncynno- ferra?c(II) salts of some transition elcmcnts

_- __-__ ---- --. - ._~

hlctal “1 Ub “‘IIC 1 --c. r’ (mmol) (mmol)

.\ : !%!? rn bit

-. --. - --- ---- cu 0.21 2.15 x 10” 9i .68 0:169 12-i Ni 2.75 2.75 x 10-T 100.0 0.768 363

h? .5.085 4.75 x IO” 1Oi. 1 0.391 GS.1 --. -. - --

The dependence of p on the composition of the precipitates was investigated for the mixed potassium-cobalt hesacyanoferrate(I1). Constant concentrations of potassium (mK = 25.74 mmol, V = 15 ml) and hcsacyano- ferrate(I1) (mric = 7.04 X lo-’ mmol) were used with a variablct concentration of cobalt ions. Results are given in Fig. 2, which also shows the? relationship

Fig. 1. The influence of pH on the sorption of “‘c‘s with cobalt hc~acl;anofcrratc(II). (1) Precipitation with mdium hesacyanofcrratc(I1); (2) f ,tccipitcltion \vith potw.sium hcxacyanofcrrate(I1).

Fig. 2. Dependence of the wchnngc constant (&-& ( CUNC 1) and of Lhc composition of the precipitate (curve 2) on the initial ratio of cobalt to ilexrrcyanofcrrate[II) ions.

between the composition of the precipitate and the initial (mc,/m& ratio [II]. The latter curve was used to calculate the n value. As can be seen from Fig. 2, the cschange constant increases with increasing (mcO/rnHc)in,c, ratio Up t0 (r?ZCc,/l?ll~C)ini~, ‘-, 6 and then remains constant. The precipitation of cesium must therefore be done with a big enough mMe/mHC ratio.

The influence of the mK/mlIc ratio on exchange constants was investigated for the precipitates formed with mercury or cobalt hcxacyanoferrate( II). The esperimental data from the mercury hexacyatloferrate(I1) tests are given in Table 3; clearly, p changes very little with varying X values.

Lleuelopment of the analytical method On the basis of the above results, as well as previous data for the separa-

tion of alkali metals by estraction with tetraphenylborate into nitrobenzene [61. a general #analytical scheme was worked out and model calculations were done by computer in order to choose the best variant of the scheme. The equations were as proposed earlier.

The degree of separation (DS) of cesium from its mixtures with potassium and rubidium was calculated from the equation [12]

D, L: 1 - a/cr = [4,01(1 + m,/mz)J/[(A - l)P, + Pz(m,/ml)l (3)

where m, and m, zue the molar amounts of the first and second macrocomponents of the mixture (potassium and rubidium); p, and pz are the eschange constants of the microcomponent (Cs) with the first (K) and the second (Rb) macrocomponents; o’ is the unisolated fraction of cesium; and X = (mK ” mRb)n/mHC-

For calculation of the deg-ree of separation of rubidium at the first stage of cesium preconcentration, when mK S mRb 3 mcs, the equation, proposed earlier [ 81 was applied

I - QRb = CHL,,K/[A + (flRb,K - 1)) (4)

For calculation of the degree of rubidium separation at the second stage of concentration when its quantity becomes comparable with the amount of pot:&um but still much greater than the amount of cesium, the equation of the calibration curve method [ 131 was used

‘I’hBLE 3

It~fluer~ce of the nrK/nltiC ratio on the ;I~,,~ value for the precipitation of potnssium mercury hc?tacyanofcrrate(II)J

tnK m tic mK I--o (mmol) (mmol) h-- P

m tic

13.i3 5.56 x 10” 2469 0.726 6539

2S.iJ 3.56 x 10-l 1629 0.600 6944

Z-1.02 4.45 x lo-’ 5401 0.550 6600

26.7'7 3.89 x lo-’ 6882 0.521 7484

27.11 3.34 x lo-’ 8117 0.499 8083 27.46 2.78 x lo-’ 9876 0.446 7950

181

.---

_/- -1 -.., .’ . .‘T--‘::

\ ,: I’. . . ,: \

.- . _... -._ . ._I .:.. : .,

Fig. 3. Alodcl ccrlculations of Iib/Cs and Ilb/K ratios after the sccwd ;~ntl Lhird slcps of cesium proconcentration for .\,A. .. 1.5 X 10‘ - COrIsL. ilnd .\, 7: 6 = cclnsl. (11) Second precipitation with nickrl Ilcsacyar;of~rrntr(Ii); (l-3) estracrtion wilh tctrai,tlc!n~ll)orntc.

ORb = ((1 + Id’ -1 4 J~~llbO~Z” - ~n,,,ln)(Cl,l*,,K - 111 ‘T21/2 ml(t,(lj,(t,!Cr - 1) (5)

where d = (flRblCc - - l)(mlIb t)l ric!n 1 m,, and w0 = /nh’ -i. tnilb_ The ratios between the alkali metals were calculated after each stage of

prcconcentration for two successive precipitations with nickel hesacyano- fwrate(I1) (A, and AZ) followed by estraction with tctraphcnylboratc into nitrobenzene (A ,)_ Figure 3 shows the results of these calculations for the conditions X ,A, = 2.5 X 10’ = const. and A, = 6 = const., where A is the ratio of the. total number of rno& of alkali metal to the number of moles of reagent. Table 4 gives the results of calculations for the condition X ,A :A ,, = 1.5 X tOG = const.

The model calculations show that the initial multicomponent system of alkali metals can be reduced by the three-step preconccntration to a two- component system containing cesium and rubidium; thct small quantities of potassium remaining at A, < 400 do not interfere with the determination of cesium. The content of ccsium in the final solution is only 3.-S-fold less than that of rubidium (see last two lines in Table 4). The cstraction step in

the schctme for ccsium preconcentration is necessary because of technical diificulties in the sctparation of the hcsacyanofcrratc( II) 1)rec:ipitat.c at such low concentrations, and because it is necessary to know the precise value of ??I 0 in the final solution in order to determine ccsium by the comparative or interpolation methods.

In the general scheme propcxcti, the solution of four alkali metals is first treated with nickel hesacyanofcrrat.e( II) (A, - 400). The precipitate containing rubidium and cesium and part of the potassium is dissolved in concentrated sulfuric acid and subjected to a second precipitation of nickel hexa- cya.noferrate( II) (A, = 450). which leaves most of the potassium and part of the rubidium in solution. Again the precipitate is dissolved in concentrated sulfuric acid, and the solution is estracted with tetraphcnylborate (A., = 10). The solution obtained by re+straction with 1 AI hydrochloric acid contains rubidium, cesium and a little potassium, and is used for the determination of ccsium.

\lotlel cdculations for preconcentration of cesium from seawater for h, A ) A, = 1.5 X IO*

and (m,), = 2.6i x 10.’ mmol

A , (m,), (mmol)

----

500 O.ROO -180 0.833

160 0.870 -I-IO 0.909

425 0.911 410 0.9i6 400 1 .oo

390 1.025 ---

A :

1500 1010

815 682 590 523 4 69 -1%

(4: (mmol)

---- 6.33 x lo-’

6.01 x 10 ’ 1.06 x 10‘. 1.33 x 10’ 1.59 x 10“ 1.66 x 10 2.13 x 10. 2.10 x lo-

(zifi): &)> ------- 3.05 2.08 4.50 1.82 6.99 1.76 7.55 1.77

9.09 1.79 10.7 1.81 12.25 1.82 13.9 1.84

-.. .-- 2.0 3.0 4 .o 5.0

6.0 7.0 8.0 9.0

(g), (2$), 1.12 1.06 1.35 1.35 1.75 1.46 2.15 2.56 2.62 3.67 3.13 5.65 3.65 10.03 1.22 28.50

X similar, but simpler, scheme can be applied for the determination of rubidium in seawater_ Model calculations show that for this purpose one precipitation with nickel hesacyanoferrate(I1) (X = 1000) and one extraction with tetraphenylborate (X = 10) arc sufficient.

The precipitated fraction of rubidium (PRb,k = 243) corresponds to 1 - Qeb (= 243/(1000 + 242) = 0.196). The amount of rubidium in the precipitate (for 1 1 of seawater) is m ~b = (mHb)idt. (1 - QH\,) = 2.4 X 10% X 0.196 - 4.6 X lo-’ mol. The amount of potassium in the precipitate is mK = mlfc = lo-’ mol. The estracted fraction of rubidium ((JRbjK = 5) is 1 - @Rb = S/l4 z 0.375. The amount of extracted rubidium is mRb = 4.6 X iO_’ X 0.375 = 1.72 X lo-’ mol. The amount of extracted potassium is nl, = (10 - 1.72) X lo-’ = 8.28 X lo-’ mol and the amount of extracted cesium (yield = 40%) is about 2.5 X lo- mol. At such relative quantities of alkali metals in the final solution, rubidium can easily be determined by any selective radiochemical method, and its concentration in the initial solution of seawater can be calculated by the radiometric correction method [ 141.

Preliminary work for the determination of cesium In preliminary esperimcnts the rubidium-cesium mixtures used were

prepared containing the absolute and relative concentrations which cor- responded to their expected concentrations in the final solution according to the model calculations. The reagents were suitably purified: nitrobenzene by partial freezing, and lithium hydroxide by a three-step fractional crystallization of lithium hydroxide at a molar ratio m,,Jm,,,. of 1:3.

The working pH region and the influence of lithium concentration (lithium hydroxide being used to achieve the necessary pH) on the extraction of trace amounts of cesium were first investigated. The results (Fig. 4) show a ma.-ximum on the curve, which can be explained as the competing action of hydrogen and lithium ions on the extraction of micro amounts of cesium. At Cc, = 10% M, this maximum disappears. The best working region was taken as pH lo--11.

183

The dependence of the unisolated fraction of cesium on its relative concentration (to rubidium) at low concentrations of both elements is shown in Fig. 5. With decrease in this total concentration (m,), the isolated fraction of cesium decreases and the slope of the curve acs = f(m,,/m,) decreases also. But, as can be seen, this slope (for the expected concentrations) still suffices for determinations of cesium by the comparative and interpolation methods [ 7 ] . In the comparative method

m, = m,,(a,J~,) + 1.25 mTps[(l - aAla, /I(1 - a,)(1 .- a,,)1 (6)

where CY, and Q,~ are the unisolated fractions of cesium in the test and standard solutions, and m, and m,, are the quantities of ccsium in the test and standard solutions.

For the interpolation method

m, =mz[(l-a,)-(1--a,)]/(l-cra-)-(l-~,)] +m,[(l-crz)

- (1 --a,)l/[(l -02) --(I -a,)1 (7)

where m, and m2 are the amounts of ccsium in the first and second standard solutions, respectively.

Application to seawater The volumes of the seawater were usually 4 1, but if necessary this could

be reduced to l-2 1. Precipitations of nickel hexacyanoferrate(I1) were done at pH 1-2 with rnNi/mIIc = 3; extraction was done at pH 10.5. The first precipitate of nickel hexacyanoferratc(I1) was decanted and finally filtered through a Cinpor membrane filter (0.45+tm pore size). The precipitates were then dissolved in the minimum volume of concentrated sulfuric acid (chemi-

Fig. 4. The influence of the lithium hydroxide concentration on the extraction of ccsium with tetraphcnylbomtc into nitrobenzene: (1) “‘GG without carrier; (2) 1.22 x 10” ;\I cesium ions.

Fig. 5. Dependence of unextracted fraction of ccsium on its absolute and relative (to rubidium) concentration: (1) C, = 1.157 x 10” M, nz~lBIs = 2.99 x lo’? mol, II, = 0.259. (2) C, = 3.85 x 1OM4 ILI, mTpB = 7.84 x lo-’ mol, 11, = 0.194.

tally pure) which was then evaporated nearly to dryness. Kadioactivities of final solutions (before determinations) were compared with initial radioactivities of the swples for the determination of cesium yield (usually about 30%). The total <amount of ccsium in the sample volume of seawater was calculated from the equation: mcr = m,?,.AtiJA,ti,_ where AintL and Ar,,, are the initial and final (before determination) radioactivities of cesium, and mdet. is the amount of cesium found in the “final” solution.

RESULTS AXD DISCUSSION

The results of analyses of rubidium-cesium mixtures at low total concentrations of both elements by the comparative and interpolation methods are listed in Table 5. The experime.ti.A data show that determinations of cesium ‘are quite possible at the expected cesium concentration in the final solution in the presence of about lo-fold amounts of rubidium.

The proposed scheme of cesium preconcentration and the comparative and interpolation methods for its final measurement were applied in the analyses of 34 samples of seawater taken in different parts of the Atlantic Ocean. The analyses were done directly on board the research ship. In all the tests, the results obtained were practically the same. A slight tendency to increasing cesium concentrations with depth was noted; samples were taken at depths of 10-300 m. However, these differences in cesium concentrations were actually inside the limits of experimental error. ‘I’able 6 shows the results obtained for a series of nine samples taken from a depth of 100 m.

The proposed method for cesium determination in seawater was verified by the method of standard additions. The results (Table 7) indicate the absence of systematic error in these determinations.

Determination of cesium in the presence of rubidium at low concentrations of both clcments by means of c?xtraclion with tetraphenylbor3te into nitrobcnzcne

(CTPR = 1.536 x 10 *AI. V,, -; V,,. = 5 ml, pI{ 10.5)

“ka (IO” mol)

mHh mol) -

mcs

1 -- tics mcr found (10“ mol)a

Comparative Interpolation method method

0 3.56 0.309 3.50.1 0.515 3.541 O.i21 3.569 1.03 3.522 1.339 3.564 1.548 3.506 1.85-I 3.563

- 0.41i -_ 10.4 0.372 0.4-I5 (44.2)

5.88 0.351 0.5352 (3.92) 3.99 0.332 0.7313 (3.20) 2.42 0.305 1.063 (3.20) 1.66 0.284 1.321 (1.34) 1.265 0.275 1.471 (4.97) 0.92 0.262 1.760 (5.07)

-

0.3511 (13.6) 0.5233 (2.0) 0.5272 (0.85) 1.069 (3.79) 1.393 (4.03) 1.549 (0.06) 1.837 (0.92)

aValues in parentheses are relative standard deviations (W) for n = 5.

185

TABLE 6

Determination of cesium in seawater (V = 4 1, (mHc), = 0.103 mmol, X, = 388, (m&: = 2.36 x IO-’ moi, h, = .I%. mTPB = 2.52 X 10.’ mol. ATPB = 9.4) --__ _ .____~_ ~__

Sample Cs yield Determination of cesium in final solution” -_

(lo) 1 - <* Comparative methodb Interpolation method= --

mcs Cs in water wn Cs in wa:er (lO.9 mol) (PC I”) (lo-’ mol) (PC I ‘)

1 31.68 0.368 6.55 0.69 6.61 0.69 2 32.13 0.364 6.91 0.72 7.20 0.75 3 32.53 0.363 6.99 0.7 1 7.35 0.75 4 33.07 0.360 8.0.1 0.8 1 i.80 0.78 5 32.46 0.362 i .87 0.8 1 7.50 O.ii 6 32.34 0.365 6.79 0.70 7.06 0.73 ‘7 35.06 0.359 8.13 0.77 i.95 0.75 8 33.21 0.366 6.74 0.67 6.91 0.69 9 32.89 0.362 7.08 0.72 i.50 Q.iG

_--. _._ - am0 :: 2.52 x 1O-a mol; nzTpB = 6.i2 x 10’ mol; pH 10.5; V<,=, = Vaca, = .5 ml. bstatistical treatment: Cs = O.i3 - 0.03 ~g I” (f’= 0.9). CStatistical treatment: Cs = 0.74 I 0.02 JIM? I” (P = 0.9).

The time required for the analysis is mainly determined by the mrlnipula-

tions with the first precipitate (filtration, decomposition with sulfuric acid and evaporation of the so!ution). The total time of analysis (with precipitation from hot solutions and the use of special v~ssols with narrow bottoms) is 8-10 h. Several parallel determinations arc possible, so that working time per analysis can be reduced significantly.

TABLE 7

Verification of the results by the method of standnrd additions (V= 4 l;msi/m,,C= 3..\, Y 388.A: z -136. (.\,)TIBB= 9.4)

_. -- _ --. _

CS Total Cs Cs Determination of ccsiunvl _. ._

added in sample yield

(r6) (rlz I-‘) (a)

0 0.74 31.62 0.361 0.795 0.84 13.5 1.44 1.10 28.37 0.352 0.875 1.03 6.3 2.88 1.46 24.65 0.333 1.140 1.5-t 4.9 4.33 1.82 23.42 0.325 1.160 1.65 9.3 5.77 2.19 18.46 0.314 1.280 2.29 4.6

1-a Comparative method

mCS Csin b (10’” sample (%) mol) (IJiz I-‘)

.- _- .__.- Interpolation method -._

mCS Cs in 5 (10 ’ sample (5%) mol) (/JE I”)

0.765 0.80 8.2 0.910 1.07 2.i2 1.13 1.53 .i.80 1.19 1.69 i.14 I.“8 2.39 -1.6

am, = 2.52 x 1O’8 mol, mTPB = 6.72 x lo9 mol. 6 = relative error.

186

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determination of cesium in seawater by radiochemical methods

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