The solvent extraction of alkali metal tetraphenylborates

Analylica Chimica Acla Elscvicr Publbhing Compnny, Amsterdam Printcdin The Nsthorlandr

THE SOLVENT EXTRACTION TETRAPHENYLBORATES

433

OF ALKALI

METAL

The use of tetraphenylborate (TPB) as an analytical reagent was, according to the review by FLASCHKA AND BARNARDI, extended very widely during the decade Ig3o-1960. In most of the previous work, the reagent has been used to form insoluble precipitates mainly with ammonium, potassium, rubidium and cesium ions, and the alkali metal ions have then been separated by filtration or centrifugation of these precipitates. Attempts have, however, also been made to separate alkali metal ions by solvent extraction of their tetraphenylborates. FIS” studied the extraction of rubidium and cesium into nitrobenzene from aqueous 0.1 M sodium tetraphenylborate solution. HANDLEY AND BURROS~ studied the extraction of cesium tetraphenylborate at tracer concentration into amyl acetate, and FINSTON~, also studying this system, showed that cesium in the organic phase could be stripped by 3 M hydrochloric acid. In a similar manner, TESTEMALE AND GIRAULT~ determined ra7Cs in radioactive wastes by extraction into 0.05 M NaTPB in isoamyl alcohol. The solvent extraction of cesium and francium tetraphenylborates has been studied by HARUYAMA et aLa and the extraction of ion-pairs into polar solvents has been described in the review by DIAMOND ANDTUCK'. of potassium, rubidium and cesium In the present work the extraction tetraphenylborates has been studied at low concentrations. In addition we have tried to clarify the equilibria involved in these systems and to assess the possibility of separating the alkali metal ions by solvent extraction. Most of the previous work seems to have neglected the details of the equilibria involved in the systems. ESPEIZIMENTAL

Reagents Cesium-134 and rubidium-86 were obtainecl from the Radiochemical Center, Amersham, England. Each of the tracer solutions obtained was diluted with water and used as a stock solution. Potassium carbonate (IOO mg) was irradiated with thermal neutrons in the R2 reactor at the Swedish Atomic Research Institute at Studsvik and was dissolved in dilute perchloric acid in order to provide a neutral potassium-42 stock solution. All the reagents were of C.P. grade. Sodium perchlorate was recrystallized twice from water. Hexone (methyl isobutyl ketone) was obtained from Fisher * Present address: Dcpartmcnt juku-ku, Tokyo, Japan.

of Chemistry,

Science University

Anal.

of Tokyo, Chim.

Ada,

Kaguraealca,

45

(1969)

Shin-

433-446

T.

434

Sl%KINE,

D.

DYRSSEN

Scientific Co., U.S.A., and I’BI- (trihutyl phosphate) was obtained from Eastman I~oclalc Chemicals, U.S.A. The solvents were washed with 0.1 M perchloric acid, 0.1 ikt sodium hydroxide and finally several times with water. Nitrobenzene was from Fisher Scientific Co. and obtained from Kistner Co., Sweclen, nitromethane nitroethane from Dr. Thcoclor Schuchardt Co., West Germany. These reagents were washecl twice with water. Sodium tctraphenylborate (NaTP13, Kalignost reagent) was obtainccl from Merck AG, West Germany. Doubly distilled water was used in all cases.

All the determinations were carriccl out in a thermostatted room at 25”. Stoppercd glass tubes (volume 50 ml) were used to equilibrate the organic and the aqueous phases, A given amount of one of the tracer solutions was placed in each tube followed by various amounts of the sodium tetraphenylborate solution. After aclclition of sodium perchlorate solution or water until the aqueous phase in each tube was x0.0 ml, 10.0 ml of one of the organic solvents was adcled. A small amount of cesium chloride was added as carrier in the. experiments with cesium. The initial concentration of potassium, rubidium or cesium in the aqueous phase was always 10-4 1M. The two phases were agitated vigorously for 3 min by a mechanical shaker and then separated by centrifugation. A 5-1111 portion was pipetted from each phase and transferred to a small polyethylene test tube. The y-radioactivity of the solution was measured with a well-type scintillation counter. Some measurements were carried out on the distribution of sodium ion with a Perkin-Elmer atomic absorption spectrometer, Model-303. A measured amount of sodium tetraphenylborate was dissolved in nitrobenzene ancl used as a standarcl and. each organic phase was diluted .with nitrobenzene until it contained 20-60 ,uM sodium ion before analysis. The sodium content in some organic layers was also checked gravimetrically, as follows. A s-ml portion was pipettecl from the organic phase which had been equilibrated with aqueous sodium tetraphenylborate solution and placed in a stoppered glass tube. The solvent was then shaken three times with 5 ml of I M hydrochloric acid to decompose tetraphenylboratc ion and to back-extract the sodium into the aqueous phase, the aqueous phase being subsequently evaporated in a platinum dish. Hydrochloric acid was added to the residue in the platinum dish and the solution was reevaporated. The residue was finally heated on a hot plate and the amount of sodium wcas determined as sodium chloride. The net distribution ratio of the metal ions was calculated as follows:

D=

[Ml

or#(,

tot

nl

totnl [Mla~,

=

y-count-rate y-count-rate

per ml org. phase per ml aq. phase

The net distribution ratio of sodium ion was cleterminecl absorption data or gravimetric data-as follows : D = [Nib/(

[Na]l,,itt,,l-

from

the atomic

[Na j&.,$

RESULTS

Two-phase As

distribalt@c of sodium tetra$huylborate tetraphexiylboric acid is unstable

~l,rnP.Chim.

Acta,

45 (IgGg)

433--446

in aqueous

solution,

this

reagent.. is

SOLVENT

EXTRACTlON

Ol? ALKALI

METALS

435

generally supplied as the sodium salt. In the present study, this salt was always added ini t i&y &0 Zh &q&MS &&?z ,o-~~~iC~~ ZW Z27iVXY~72~&&z?~~, z&iCXVAW ul CY?Sh!AT2 ion, and the tetraphenylborates of these heavy alkali metal ions were extracted into the organic phase. The distribution of sodium tetraphenylborate between water and nitrcbenzeee was, mareovcr, studied irr the absence of sa&Wt~ perchlorate in order to be able to determine the concentration of sodium tetraphenylborate in nitrobenzene in equilibrium with various initial concentrations of aqueous sodium tetraphenylborate. The results are given in Table I. It may be seen that about go(yO is extr;acte& into nitrobenzene.

:I tom ic

,_. ._. _ . ..____.... _-_-_~_-.--_.-__..

7.8 * 10-Z L}._5. J 0 I- ”

1.0.105.0’

10-z

10-Z x.0.10-~ 3.0. 10-3

3.0’

1.0.

lo-:’

3.0.

IO--.’ IO--”

1.0.

OIZ ALKALI

I’ERCI-ILORATIZ

SOLUTIONS

------

--

.._-

--

solucwt

Nitrobcnzcne

Nitromcthanc

Nitrocthnnc

-.--

-._-

‘L.f>.

10-y

g.r.10-3

()..I.

10-a

..__.-

[NnCZOOr~

0.1

IONS -

~XTWXILN - --..-

VARIOUS

ORChNIC

SOLVENTS

AND

Distvibrrtion vntio. ~__. Na+ I\‘-+ 63 44 29 ‘4

I.2

10.i ml-+

cs.+ 1500

4oo

o.o‘t

3.0 5.0

0.9s 1.4

0.1

1500

1.0

1200

5700 3800

3.0 5.0

1000

2200

g6oo 4600

830

I300

2100

380 350 250 100

1200

4000

0.1

3.0 5.0 0.5. 1.0

3.0 5.0

SODIUM

-.__--_.

-._--_

I.0

1.0

TM?

.#.‘t’lo-~

2.7.10-z

MRTAL

flM)

Hcxonc

c).o*10-2

2.7. x0-Z’ 8.3. IO_‘8 2.7. IO-.* 9.6.10-G

DISTR~l3UTION

Orgadc

C~aviw:t~y

n.!xorption

35 67 95 120

0.1

GIO

1.0

IGOO

3.0 5.0

1200

530

220

980

270

93 32

95 18000

770 -100 170

2100

870 230

39 82

46 67 65 49

z; 5x0

530 1500

I itoo

980 340

890 260

Rural.

Chim.

Acta,

45 (1969)

433-446

T. SEKINE,

436

D. DYRSSEN I’

Extraction oj’ alkali metaL $wrchZorates As sodium perchlorate solutions were used in most of the experiments, the extraction of the alkali metal perchlorates was studied in the absence of tetraphenylborate. Table II gives the net distribution ratios of. the perchlorates as a function of the sodium perchlorate concentration in the aqueous phase. Table II shows that these metal ions are extracted into the organic phase to some extent even in the absence of tetraphenylborate ion ; for example, more than 50’i/~ of cesium in 0.1 M sodium perchlorate solution is extracted into nitromethane by a single extraction. It was observed in the course of the experiments with nitromethane that at high sodium perchlorate concentrations the volume of the organic phase increased and that of the aqueous phase decreased after the two phases were equilibrated. Extraction of heavy alkali metaL io~ts in 0.1 M sodium $erchZorate z&h 0.01 M (initial) so&cm tetra$he?zyZborate Table III gives log D values and separation factors for the extraction of potassium, rubidium and cesium ions from 0.1 M sodium perchlorate solution, TABLE

III

DISTRIRUTION SOLUTION

OF

ALKALI

CONTAINING

MBTAL 0.1

&f

IONS

BETWRBN

SODIUM

(Initial conccntrntions in aqueous phase) ___. --.__--hcJ 1) SoCvlwl

0.888 0.339 0.516

Nitrobcnzcnc Nitrornctlmnc Nitrocthanc

--0.029

HCXOllC

--I.Oli

TBP

SALTING-IN SODIUM

I%l3Kl3CT

OIr

PERCHLORATE

Nitrobcnecnc

-o.Gog

ALKALI

METAL AQUl%OUS

THE

I .028

Pi-IASIE,

0.863 0.483

0.2

Hcxonc

Amtl. China. Ada,

SOLVENTS SODIUM

nc!s+lDIll-’ ----

5.2 3 *4 4.4

2 .o

CONTAINING

I *Go3 1.316

7.5 3 ’4 1.7 1.0

TIIE 0.01

2.400 2.179 I A58 I.130

0.086 -O.,~OI

2.0 3.0 4.0

-0.8~0 - 1.160 - I ..t36

-0.268 -0.614 -0.955

0.545 0.124 -0.237

0.1

0.2 0.5 1.0

-0.679 --0.95.5 - 1.388 - I .680

-0.642 --0.932 -1.371 - I.730

-0.377 -0.717 -1.188 - I .G38

2.0 3.0 4-o

-

- 1.996 -2.139 -2.038

- I.963 -2.132 -2.174

45 (1969) 433-446

0.84 I 0.324

AQUEOUS

D,,,,+/D&.

FOLLOWING

INITIALLY

AN

---.._-

0.5 1.0

1.889 = a35 1.917

AND

TBTR~~I’M~NYLl~ORAT~

I.0 1.0

TIZTRAI’I-IISNYLRORAT~~S

0.1

fi1

Sepnvnliou fucfor

2.476 1.274 I .c,gG -0.377 - I .025

I.GOS

TO

0.01

.cs+ _~---_-

0.865 l..lG.} -

ORGANIC

AND

_--.._-___

lS?i>+-

II”.

VARIOUS

I’ERCIILORATE

ADDITION n’f

NaTPI

OF

SOLVENT

EXI RACTION OF ALKALI

METALS

437

initially containing 0.01 M sodium tetraphenylborate. into five organic solvents. It is clear that both the distribution ratios and the separation factors depend strongly on the organic solvent. Nitrobenzene gives the highest values for D and the separation factor. * Effect

of sodium perchlorate OS the tetra$heuylborate extraction Table IV and Fig. I illustrate the extraction of potassium, rubidium ancl cesium from aqueous solutions initially containing 0.01 M tetraphenylboratc and various amounts of sodium perchlorate as a function of the initial concentration of

2-

l-

O-

n

l-

O-

.F -1 -

-2-2

-1 0

.2_____w___________.

-‘ 4

1

I

2 log& PQll”ltlal.aq

,

1

Fig. I. The decrease in the distribution of alkali mctnl tctraphcnylboratc following the addition of sodium pcrchloratc to the aqueous solutions initially containing 0.01 M NaTPB. The open, semi-closed and closed circles reprcscnt data for the oxtraction of I<+, I) NaTPB only (column A, Table V) ; (a) o. I M N&210.;+ NaTPB (column C, Table V); ( o) 0.1 M Na(TPB, Clod) (column B, Table V). For tctraphcnylborate concentrations less than 10-z M the results arc rcprcscntcd by the closed circles. The straight line through the closed circles is drawn with the slope + I. The dashccl line shows the distribution ratio for potassium when no sodium tctmphcnylboratc is aclcled to the system (0. I M NaC104 only),

sodium perchlorate in the aqueous phase. It can be seen that an addition of sodium perchlorate causes a remarkable decrease in the extraction of these ions as the tetraphenylborates (salting-in effect). For low concentrations of sodium perchlorate, the decrease in D is proportional to [NaClOA]. Again, it should be noted that the separation factor is larger for nitrobenzene than for hexone. The dependence of the extraction of potassizrm on the sodium tetrashenylborate concentration Table V and Fig. 2 illustrate the extraction of potassium into nitrobenzene as A,tral. Chiwa. Acta,

45 (1969) 433-446

‘.I’.SEICINE,

438

Z.J(>C 2

I.998

“.1.}3

1.7&

2.297 z.146 I.914 1 .!I70 1 .xql

I .‘tG‘t

I .aGo 0.903 .-_ _.-_ -_-. ---‘.2.200

-. -

DYRSSEN

I.794 I .Goo I *45G I .328

Z.IGI

.ogo

D.

0.998

o.8R8 0.358 --0.178 --. o.Ggo ..__I.166 --

2.200

Column (A) : log I> for potassiun~ when the nqucous phase initially contahs NaTIT only. Column (U): log D for I)ot:rssium when the nqucous phsc is a mixcturc of o. I ild Na~PU ;lnd 0.1 M NnCl0.1, i.c. o. I kf Nn(‘l’T?I3,Cl0.1). Column (C) : log 13 for potassium wlwn the nqucous phsc contains 0.1 &I NnClO.1 in addition to N;~TPB of various concentrations.

a function of the initial concentration of sodium tetraphenylborate in the aqueous phase. The semi-closed circles in Fig. 2 correspond to data obtained with an aqueous phase containing sodium tetraphenylborate and IO-4 M potassium ion only, while the


‘ChhS

CDTTESpDnh

tD

%DSe

DbtihJXb

Wjks:,

FUl LxQ’uf?DUS $XX%

bhb>dj~

CDJI-

taining 0.1 M Na (TPB, Cl0 4) and 10-d M potassium ion (i.e. the aqueous phase was, in this case, prepared by mixing various amounts of 0.1 M sodium perchlorate solution and 0.1 M sodium tetraphenylborate solution). The closed circles represent the results obtained with an aqueous phase initially containing 0.1 M sodium perchlarak in adctSan to various amounts af sac_Eum tetra.pl~ylf~~ate and ~0-4 M potassium ion. The data corresponding to the open and the closed circles coincide for tetraphenylborate concentrations less than 3 *x0 - 3 M, and the results in this region are therefore represented by closed circles only. The results for rubidium and cesium showed a similar tendency to those for potassium, but it was not possible to determine the distribution ratios of these ions accurately under the same conditions as potassium, owing to their high distribution ratios, especially when the aqueous phase contained no sodium perchlorate. Figure 3 shows the distribution ratios of these ions between nitrobenzene and 0.1 M sodium perchforate as a function of the initial sodium tetraphenylborate concentration in the aqueor&s ~+AWZ. XT-A s~m2&dL KTAe5 +A F?ig* 3 s+fsS +Si&. L3 is ~~OQWSWG?&~to alt’liougli t’lie ciiktx2ikfion ratio -rbr cesllum deviates somewliaf [N~~~~ji.it~a~,aq, from linearity. of the additio~z of $erchloric acid to the aqueous phase The extraction of alkali metal ions with tetraphenylborate is decreased by the addition of perchloric acid to the aqueous phase. The decrease in the distribution ratio of rubidium with the hydrogen ion concentration of the aqueous phase, initially containing 0.05 M sodium tetraphenylborate and 0.05 M (Na, H) Cl04 is exemplified in Table VI, and the graph log D vs. log [NaTPB]initial,nq[HCIOa)inttial,acl+ [H+] Effect

Amd.

Clriwt. AC&,

45 (my&)

433-446

SOLVENT

EXTRACTION

OF ALKALI

439

METALS

obtained from the data in Table VI is shown in Fig. 4. It may be seen from Fig. 4 that the distribution ratio is proportional to the remaining concentration of tetraphenylborate ion if it is supposed that phenylboric a&cl, CoHsB(OH)s is the main procluct of the acid hyclrolysisl. When an escess of perchloric acid is added the extraction becomes identical with tllat of rubidium perchlorate into nitrobenzene (cJ. Table II).

-4

c

2 MI @a-3 f’Qlinitiol,Og

Pig. 3. The distribution of potassium ( 0, o), rubidium (I, 0) and ccsium (A, a) tctraphcnylThe closccl symlx~ls intlicntc tkrta obtoinctl borate lxA.wccn nqucous solutions nncl nitrobcnzcne. The when the ~rqucous phnsc initially containctl 0.1 &I NaCl0.1 nncl various wnounts of NaTI’R. open symbols incliczrtc data obtainctl when the ~qucou~ phase WLS initinlly 0.1 fir Na(TP13, Cl0.1). The stmight lines m-e clrawn with slopes -k I. Fig. .I. Dccrcnsc in the cstroction of rubidium into nitrobcnzcnc ( 0) ant1 into hcxonc ( 0) cnuscd by the wlclition of pcrchloric acid. The aqueous phase initidly containctl 0.05 M (N;I,H)CIO.I nncl 0.05 M N:rTPB in all CELSCS. The term fi rcprcscnts the molarity of the pcrchloric acid atldccl, and [I-I+] is the molnr ion conccntrntion :Lt ccluilibrium. The straight lines arc clmwn with slopes f I.

DISCRISASE

NaTPI

IN ON

TIII5

TIIII

LPSTRACTION ADDITION

OF OF

RU13IDIULI

VARIOUS

Inif id cornposilion of the aqueous phase [Na 7’PB ] (M) 0.05

0.05 0.05 0.05 0.05 0.05 0.05 0.05

0

0.02 0.025

0.03 0.035 0.04 0.045

a The concentration

AN

AQUEOUS

OF NaCl0.1

PI-IASIZ AND

CONTAINING

PC1 0.05 0.04 0.03 0.025 0.02 0.015 0.010 0.005 of TPB-

at equilibrium

8.90

6.39 6.13 5*95 4.26 4.20 4.16 2.67

[TPB-]n

0.05

log D

-_- ____-_ _----I- --Methyl isohrtyl Jr&one -;&I

[TPB-1”

0.45 0.30

0.021

0.00

2.41

G.G‘}

240

0.02

2.OG

3.76 3.25 3.09 2.94

2.00

2.90

0.016

2.51 2.41

0.013 0.008g

is calculated

2.28

2.30

1.80 I.55

by cqn. Asal.

log 6

0.05 0.04 0.031 0.026

0.05 0.04 0.03 0.025 0.015 0.01 0.007

ill

HClOp

pl-i and [?‘PB-] at equilib~izrm --------Nitrobeme~re

[HC10‘1] (M)

0.01

FROM

nlIxruREs

0.21 0.1

-0.17 -0.24 -0.44

(24).

Cl&z.

Ada,

45 (1969) 433-446

I

T. SEKINE,

440

II. DYRSSEN

It was observed in the other experiments that an addition of sodium hydroxide to aqueous phses which had been acidified with perchloric acid as described in Table VI and Fig. 4 increased the PH, but left the distribution ratio unchanged. These results indicate that the addition of strong acid to the system destroys tetraphenylborate by irreversible acid hydrolysis and that phenylborate cannot replace tetraphenylboratc as an extracting agent for alkali metal ions. Other

observations The extraction of rubidium ion from 0.1 M sodium tetraphenylborate was performed with various organic solvents and an initial rubidium concentration of IO-‘1 M. In these experiments it was observed that the recovery of rubidium from the two pliascs was not quantitative when the organic phase was isopropylether,

Pig. 5. The extraction of rubidium into nitrobcnzcne (o ), into nitromcthane (a) and into hexonc (a) from mixtures of 0.1 M Na(TPJ3, C104) as a function of the initial concentration of tetraphenylborate. The dashed lines give the distribution ratio of rubidium pcrchlorate bctwcen these solvents and the aqueous phase in the absence of tctraphcnylborntc ion (- I .40 for nitrobcnzcne, -0.24 for nitrorncthanc and --=‘_.qr for hcxonc).

dichloroether, methyl isobutyl carbinol, benzene, chloroform, carbon tetrachloride or hexane, although rubidium tetraphenylborate was extracted to some extent into a number of these solvents. A4 quantitative recovery from the both phases was obtained only with the solvents listed in Table III. This is probably due to the low solubility of RbTPB. Figure 5 illustrates the distribution of rubidium between 0.1 M Na(TBP, C104) Anal.

Cirinr. Aclu, 45 (1969) 433-44G

SOLVENT

EXTRACTION

OF ALKALI

METALS

441

and the organic solvents nitrobenzene, nitromethane and hexone as a function of the initial tetraphenylborate concentration. It was, moreover, observed that the separation of the two phases was incomplete when solvents such as nitromethane, dichloroether or hexone were used; that is, even after a centrifugation at 2000 rev./min for 3 rnin, a turbidity was observed in the two phases or on the interface. The phase separation was very much improved when a small amount of sodium perchlorate was added to the aqueous phase. DISCUSSION

The extraction of a monovalent ion, M+, as an ion-pair anion, A-, can be described by the following equilibrium

with a monovalent

Mf + A- ---zMA (org)

(I)

The equilibrium constant is

Km=

[+]or,/[M+l

(2)

[A-l

The ion-pair in the organic phase may dissociate, especially in a solvent like nitrobenzene

MWw) Kiss= pJh?

+

M+(org) + A-(org)

[M+],,,[A-]orn/

COnStant

KnrAKciiss

M++.4-

+

=

I<

(3)

[MA],,,,

(4)

represents the equilibrium

M+(org) + A-(or&

(5)

I< being given by

K=

[M+]org[A-low/

(6)

[M+] [A-]

If it is assumed that M+ and .4- do not associate in the aqueous phase and that no polymerization of any species occurs, then the net distribution ratio of the metal ion may be expressed as D=

[M]t,t,~,.,,/[M]total,acl=(

[MA&,+

[M+lortz)/[M+l

(7)

Distribution of sodiwn tetrajhenylborate Although the data in Table I are somewhat scattered, it can be concluded that about 90% of sodium tetraphenylborate in water is extracted into the same volume of nitrobenzene, and the distribution ratio of sodium tetraphenylborate (Dmg) is independent of its concentration in both phases. Thus, the following relations can be introduced: [Naf]

Fronl

= [TPB-]

(8)

[Na+lorg = [TPB-]org

(9)

qns.

(2), (4), (7)) (8) and (g), the distribution ratio for sodium can be obtained:

D =KMA

[Na+]

(10)

+ 1/Knt~Kc~ss Amd.

Chint. Ada,

45 (rch)

433-446

T.

442

SBKINE,

D. DYRSSEN

According to the results in Table I, the distribution ratio of sodium is independent of the initial concentration of sodium tetraphenylborate in the aqueous phase. From eqn. (IO) it can then be concluded that the extracted ion-pair in the organic phase is almost completely dissociated into sodium and tetraphenylborate II ions i.e. ;Knl~ is small, and the distribution ratio can be written as

As= :s.a~!:~De!!.~~.~~

33 ~~~?-~~12~;~~~~~i~-~2*

2zu&ae

3hE.

:,3355Y~~~~~:~Pz~a~~,

the activity coefficients of the ionic species in each phase are altered, but since the total concentration of sodium tetraplienylborate is less than 0.1 M, eqn. (I I) slioulcl ancl .I< for sodium tetraphenylborate is approxibe valid as a first approximation, n1s~eQ

9z.

In Table II, the distribution ratio of sodium ion between nitrobenzene and soclium perchlorate is almost independent of the sodium perchlorate concentration, while the ratios of the other metal ions decrease with increasing sodium perchlorate concentration above 0.1 ill. The decrease is especially pronounced for cesium, not so pronounced for rubidium ancl still less for potassium. The same tendency is also observecl with nitromethane and nitroethane, but not with hexone and TUP, which are usually regardecl as solvating organic solvents. Table II also shows that the distribution ratio for the nitro compounds increases in the order Na-” < I<+ < Rb+ < Cs+, i.e. the orcler of decreasing hydration. This is not the case, however, for hexone and TBP. Further cliscussion should be postponed until the degree of dissociation of the ion-pairs MC104 in the organic phase has been ascertainecl.

The distribution inayl3esummarhA

ratio of potassium tetraphenylborate

as h3kws:

ii) if iwa+-J > ~lC+-j=x:O-Q

M,

in Table

V and ‘Fig. 2

the a'rstrhltmn

x-diiooi

potassium is almost independent of tile sodium tetraphenylborate concentration when no sodium perchlorate is added to the aqueous phase, (ii) the distribution ratio is proportional to the initial tetraphenylborate concentration in the aqueous phase when the aqueous phase always contains 0.1 M sodium perchlorate and (iii) when the aqueous phase initially contains 0.1 M Na(TPB, ClO.i), the slope of the plot log D VS. lo&YP3?~-~~~~~t~~~t, Rq is siinilar to &at given in silj in tk region 0%‘lower tetrapkny1berate cancentratians, but the slope increases s.omewha.t in tke regian af ki&er tetraphen$lborate concentrations, It can be concluded from the results discussed above that in these distribution systems: (i) about 90% of the tetraphenylborate in the system is distributed into the organic phase together with an equivalent amount of sodium ion, (ii) the metal ion-tetraphenylbora~e ion ian-pairs in tire organic phase are almost completely dissociated. This is in agreement with conductance measurements in anhydrous acetonitrik by ICAY et al. B. The dipicrylaminates also seem to be dissociated in nitrobenzeneD*lo. Accordingly the following equations can be introduced. R*rnl. CAim.

Acta,

45 (1969) 433-446

SOLVENT

EXTRACTION

OF

ALKALI

The net distribution

443

METALS

ratio of the metal ion is described by

D = [Me+Jorn/[M+ J

(12)

and the extraction of the metal balance of TPB is given by [NaTPB]

ions proceeds

~nil~a~, aci= [TI’B-]

according

to equilibrium

+ [TPB-I,,,

(13)

Since the amount of potassium is small in comparison to NaTPB experiments, sodium will determine the [TPB-I,,,/ [TPB-] ratio, according Na+ + TPB-

+

Na+(org)

in most to

+ TPB-(org)

(14)

Thus, if no sodium perchlorate is added, eqns. (S) and (9) are valid [TPB-] =9. K, defined by eqn. (G), will then be for potassium K=

(5). The

ancl [‘WI3-:I,,,,/

[K’]oPa9/I.I~‘]=9’Io”~“=1330

(15)

This ecluation, furthermore, says that the distribution ratio D = [I<+&,J [I<-+-] for amounts of potassium small in comparison with NaTPI will be constant. If, however, [Naf] is kept practically constant at 0.1 M witli sodium perchlorate, then cqn, (6) gives for sodium I<=

[N~~+]om[TI’B-30rw/o.x [TPB-]

or from eqn. (9), as sodium Table I I) : [TPB-]“o,,=8.1

perchlorate

=SI

(16)

in the organic layer

can be neglected

[TPB-]

(17)

As [TPB-JO,., 5 0.1 M, it can be seen from this equation that [TPB-],,,/[TPB-] and eqn. (13) may be approsimated to [NaTPB]mtlal. The distribution

,icI= [TPB-Ior,

ratio for potassium

D = IC [TPB-]/

(cj’.

2 SI (W

can then be derived from eqns. (6) and (12), i.e.

[TPB],,,

(19)

or, if eqns. (17) and (18) are used, “’ D =K

[NaTPB]lnltlai,

,,,-,/S.r

(20)

The straight line in Fig. 2 is drawn with slope +I and K/S.r = 102.aa or K=~IIo. Even if I< has a different value when no sodium perchlorate is present, eqn. (20) explains the data in Table V (C) and Fig. 2 (filled circles). Deviation from linearity occurs at low [NaTPB] ~,,ui~r,L1Q,where the amount of tetraphenylborate extracted with potassium cannot be neglected and the distribution of potassium perchlorate makes a contribution to D. The open circles connect the two cases discussed here, because in this case the sum of the initial concentrations of NaTPB and sodium perchlorate was kept constant at 0.1 M. The filled circles in Fig. 3 may be explained by eqn. (20) in the satne way as for potassium. The straight lines for rubidium and cesium in Fig. 3 are drawn with slopes + I and K values of 32400 and 128000 for rubidium and cesium, respectively. The open symbols in Fig. 3 are also explained as above for potassium. The increase Anal. Chim. Ada,

45 (1969)

433-446

T.

444

SEKINE,

D.

DYRSSEN

of log D above the straight lines for rubidium and cesium at initial TPB concentrations larger than o.01 IM may be explained by ion-pair formation, which has been neglected in eqn. (12). One possible reason could be that these ions are less hydrated and thus smaller. This would favour the formation of ion-pairs.

As can be seen in Fig. I and Table IV, the extraction of alkali metal ions in the presence of 0.01 M tetraphenylborate (initial concentration) is very much decreased by addition of sodium perchlorate to the aqueous phascj. The slope of the log D z)s. Iog[NaCK34j plot is aimost --I in the lower sodium perchlorate conccntration region, but the slope decreases as the perchlorate concentration’ increases. This is esplained by eqns. (6) and (9) which may be combined to give I< = CTPB-]%,.,/ [Na+] [TPl3-]

=Sr

(21)

Equation (IS) should bc valid as [Na-‘1 > 0.1 M and the distribution constant I< for the small amounts of potassium, rubiclium and cesium should therefore follow Ii’=DCT1?B-]or,/[TPB-]

(22)

Thus, from eqn. (21) we obtain K = I>81 [Na+]/o.or

(23a)

D = Ko.o1/81 [Nat-]

(231))

and The distribution ratio ought therefore to be inversely proportional to the sodium ion concentration. As, however, the ionic concentration in the aqueous phase alters considerably, changes in the activity coefficients of the chemical species may be large at high concentrations of sodium perchlorate. This may be one reason for the deviation of the plots from straight lines with slope -I. These lines have been drawn with the constants 5~x0, 36200 and 256000 for potassium, rubidium and cesium, respectively. Similarly, the data for hesone may be explained by ecln. (23b). Effect of the additiolt of $erchZoric acid If tetraphenylborate is hydrolyzed Na(CoHs)eB

+ HClO4

then the remaining [TPB-lors

+2&O

unhydrolyzed

+ [TPB-]

+

according

to

C~H~B(OH)Z+~COH~+N~C~O~

NaTPB

= [NaTPB]inltlal.

can be calculated

lrq - [H+]lnitlnl,

from

uq + [I-I+]

(24)

where [H-f] is the hydrogen ion concentration at equilibrium, determined by means of potentiometry. Thus, the distribution ratio should be proportional to the remaining TPB- as above (Figs. 2 and 3). The data in Table VI and Fig. 4 confirm this. It should be observed that the equilibrium concentration of sodium perchlorate is practically constant and equal to 0.05 M. Use of tetra#eny2borate for the analysis of heavy alkali metal ions From the results in Tables III-VI or those of Figs. 1-4, it may be concluded that (0 izhe e&rzt&iun is grea&st and the separation of tire heavy a,~izG~meta? ions Awnl. Chim. Acta, 45 (rgbg)

433-446

SOLVENT

EXTRACTION

OF ALKALI

h¶ETALS

445

most effective when nitrobenzene is used as the organic solvent, (ii) the extraction is greater when the concentration of the co-existing sodium ion in the aqueous phase is low, (iii) the estraction is greater when the concentration of tctrsphenylborate is high if the system contains a certain amount of sodium salt, and (iv) the extraction is decreased by the addition of a strong acid. This indicates how a satisfactory extraction or separation of the alkali metal ions may be achieved. A good separation of potassium, rubidium and cesium is effected by the use of nitrobenzene and the separation will not be affected very much by increases in the tetraphenylborate concentration or in the co-existing sodium ion concentration. The distribution ratio of these ions with tetraphenylborate is too high for the ions to be separated effectively in the absence of co-existing sodium salt (cz Fig. z or Table V) and the aqueous phase should therefore contain a certain amount of sodium salt to produce an effective separation of these metal ions. Too Ii_@ a concentration of sodium salt (> I M) may, however, decrease the separation factor (cj’. Fig. I). The concentrations of sodium salt and sodium tetraphenylborate should be chosen so that the distribution ratios for the ions to be separated are neither too high nor too low. As the solubilities of potassium, rubidium or cesium tctraphenylborate in water are very low and not very different from each otherl*ll~r2, it is not possible to separate these ions by precipitation of the tetraphenylborates, but the use of an extraction cycle under controlled experimental conditions seems to be quite feasible, provided that the solubility proclucts of the heavy alkali metal tetraphenylborates are not attained. However, as can be seen in Table I, an organic phase which has extracted these metal ions as tetraphenylborates always contains a large amount of sodium ions which will also be back-extracted (e.g. with acid) into the aqueous phase together with the heavy alkali metal ions. Thus, the separation of the latter from sodium must be achieved by precipitation with a carrier, preferably ammonium. This work has been partly supported by the Swedish dation. The English has been revised by Mrs. SUSAN JAGNBR,

Natural M.A.

Science Foun-

and

fil.lic.

SUM hIARY

The extraction of sodium and small amounts of potassium, rubidium and cesium from aqueous solutions containing sodium tetraphenylborate (TPB), sodium perchlorate and perchloric acid has been studied for nitromethane, nitroethane, nitrobenzene, methyl isobutyl ketone and tributyl phosphate. Nitromethane gives the highest distribution ratio (org/aq) for the perchlorates, and nitrobenzene gives the highest distribution ratio and separation factor for the tetraphenylborates. Most distribution data could be explained by assuming that the tetraphenylborates were fully dissociated in both phases. The equilibrium constants and distribution ratios increase in the order sodium -C potassium < rubidium < cesium. The distribution ratio of small amounts of the heavy alkali metal ions will thus not be appreciably influenced by the concentration of sodium tetraphenylborate, but will decrease on the addition of sodium chloride or perchlorate. The effect of perchloric acid can be explained by irreversible acid hydrolysis of the tetraphenylborate ion. Anal.

Chim. Ada,

45

(1969)

433-446

T.

446

SEKINE,

D.

DYRSSEN

L’extraction du sodium et de faibles teneurs de potassium, de rubidium et de du t&raph&nylborate de sodium, a CtC ccisium, en solutions aqueuses, renfermant &udibe pour les solvants suivants: nitrom&hane, nitro&hane, nitrobcn&ne, m&hylisobrutyW!cone et ‘tti~ntSr’lp’o~p?z&. 3.X Klitium&hane &Jnne 1es co&i~cients de partage lcs plus 6lev& (org/aq) pour les perchlorates; le nitrobenz&ne donne les meilleurs coefficient de partage et facteurs de sdparation pour les t&raphdnylborates. Les valeurs obtenues peuvent s’expliquer en admettant que les t&raph&ylborates sont totalement dissocMs dans les deux phases. Les constantes d’dquilibre et les coefficients de partage augmentent dans l’ordre sodium < potassium < rubidium x cesiun-i; L13303&%&3+3 & pa~&rgiZU&S3&+7iXiX&&+AZE~~iYiX&SA. SZi3&3 y”U2W&&+S m sont pas influenc& sensiblement par la concentration du tStraph6nylborate de sodium, mais 3s &minuen’c par a&Xtion de ch1orure & so&urn uu c3e per&>urate. L’influence de l’acide perchlorique peut s’expliquer par l’hydrolyse acide irrdversible de l’ion t6traph8nylborate.

Die Extraktion von Natrium und kleiner Mengen Kalium, Rubidium und (TPB), NatriuniC%sium aus wgssrigen Lasungen, die Natriurntetraphenylborat perchlorat und PerchIors8ure enthielten, wurde mit foIgenden 5 IXsungsmitteIn NitroSithan, Nitrobenzol, Methylisobutylketon und untersucliC : Nitromethan, fiir Tributylphosphat . Nitromethan ergibt den grijssten Verteilungskoeffizienten Perchlorate, Nitrobenzol gibt die giinstigsten VerteilungsverhAtnisse und Trennfaktoren ftir tie Tetrap3_reny%orate, 3% m&&en Vertej~~~s~oeffidenten konnten durch die Annahme erkltirt werden, dass das Tetraphenylborat in boiden Phaseri und Verteilungskoeffizienten vtillig dissoziiert ist. Die Gleichgewichtskonstanten von kleinen steigen vom Natrium zum C&ium an. Die Verteilungskoeffizienten Geh&czrr &Y s.21~~~~~~ kX&&z~&&>~~~~ WW&~ &&xz% kaui+zz&x& a& XOZZWZtration des Natriumtetraphenylborats beeinflusst ; aber sie verringern sich bei Zugabe von Natriumchlorid oder Perchlorat. Der Einfluss der Perchlors&ure kann mit einer irreversiblen S&urehydrolyse des Tetraphenylborations erklsrt werden. REFERENCES I z 3 4 5 6 7 8 g IO II 12

1~. FLASCH ICI\ ANn A. J . BARNARD, JR. , A dvnlrces ,in A mzlytical CImnistry und Imtvumantution, Vol. I, Interscicnce, New York, 1960. R. C. FIX, Thesis, Massachusetts institute of Technology, 1956, cited in ref. I. T. H. HANDLEY AND C. L. BURROS, Anal. Chem., 31 (1959) 332. I-K, L, FINSTON., privati c.~n~uw.~~~f_~~n c
Awal.

Chim.

Actn,

45 (1969)

433-44G

3925. North-Holland