The extraction of lithium chloride by tri-n-butyl phosphate

J. Inorg. Nucl. Chem., 1963,Vol.25, pp. 291 to 301. PergamonPressLtd. Printedin Northern Ireland

THE

EXTRACTION OF LITHIUM CHLORIDE BY TRI-N-BUTYL PHOSPHATE D. F. C. MORRIS and E. L. SHORT Department of Chemistry, Brunel College, London W.3

(Receit, ed 23 May 1962; in revised form 23 July 1962) Abstract--The extraction of lithium chloride from aqueous solutions by tri-n-butyl phosphate (TBP) has been investigated. Lithium chloride in the organic phase behaves as an associated electrolyte with a dissociation constant ~ 5 × 10-6. It has been shown that up to ca. 0.96 M LiCI in the organic phase there is no strong TBP-salt interaction but there appears to be a definite tendency for the extracted lithium chloride to possess a primary hydration number of four. At higher concentrations. TBP molecules begin to substitute for H20 molecules in the solvation shell of the extracted LiC1 but a total solvation number of four seems to be maintained. Physical properties suggest the possibility of aggregation in the most concentrated extracts. BALDWIN et al.
Reagents. A concentrated solution of laboratory grade lithium chloride was purified by filtration and by passage through a column of the anion-exchange resin Deacidite-FF in the C1--form. Calculated amounts of the solution were taken for the preparation of a number of stock solutions of various concentrations. These were standardized by Volhard's method involving the addition of a little nitrobenzene. TBP was purified by the method of ALCOCK et al. (" The dried compound was finally distilled under reduced pressure (b.p. 142 at 2-3 mm pressure). Benzene used in some experiments was material from BDH "for molecular weight determinations". Equilibrhm~ and vvhtme change measurements. The swelling of the organic phase, due to transfer of aqueous lithium chloride, was measured in 10 ml graduated (0.1 ml) centrifuge tubes: these had been thoroughly cleaned to ensure a complete separation of the phases. Five ml of pure TBP were carefully added from a 10 ml microburette to 5 ml of standardized lithium chloride solution. The positions of the liquid liquid interface and of the liquid -air meniscus were immediately noted. In each case the stoppered tube was then shaken mechanically for 15 min. After centrifuging, the positions of the menisci were read to within &0.025 ml. The average was taken of two determinations for each standard solution of lithium chloride. The ambient temperature was 20': ± 1° during these measurements. Measurements ° f the distribution o/'lithium chloride. Two ml aliquots of each layer in the tubes from the equilibrium measurements were separated by means of transfer pipeties. Each aliquot (after dilution where necessary) was analysed for chloride content by the Volhard method. All analyses were preformed in duplicate. The estimated total chloride content of both organic and aqueous phases after equilibration alv,ays corresponded with the initial total chloride to within 2 per cent. Measurements of density. A pyknometer, calibrated according to specification ASTM D 941, was used to determine the densities of the freshly equilibrated organic layers at 20" z:_ 0.1 ~. The values of density obtained were used in the calculation of values of the viscosity in centipoises. Measurements of viscosity. The viscosities of the organic phases were determined by means of an Ubbelhode (suspended level) No. 1 viscometer (B.S.S. 188) at 20 ~ 5 0.1:. The viscometer was calibrated by use of a 40 per cent sucrose solution. The time of flow between calibration marks was measured to within 0.1 sec with a stop-watch. J~leasurements ofconducticit),. Resistance measurements were carried out using a bridge network, t~ W. H. BALDWIN, C. E. HIGGINS and B. A. SOLDAYO,J. Phys. Chem. 63, 118 (1959). c2, T. V. HEALVand P. E. BROWN, Report, AERE C/R 1970 (1956). ~3~A. S. KER'rES,J. Inor(. Nucl. Chem. 14, 104 (1960). (.1~ K. ALCOCK,S. S. GRIMLEY,T. V. HEALY,J. KENNEDYand H. A. C. McKAv, Trans. Faraday Soe. 52, 39 (1956). 291

292

D. F. C. MORRIS and E. L. SHORT

oscillator and amplifier, similar to those described by DIPPY and HUGHES.Is) A six decade resistance box (max. resistance 111,111 f2) was used in series with a single decade megohm box. The capped conductivity cell was constructed of Pyrex glass and had a capacity of ca. 10 ml. The stout, greyedplatinum electrodes were mounted rigidly by means of small Pyrex struts at a distance of 2 mm apart. No solvent correction was necessary, since the conductance of TBP phases was due solely to extracted hydrated electrolyte. The cell constant was determined by use of a standard solution of potassium chloride, as described by DIPPY et al. ce~ All measurements were carried out in a thermostatic water bath at 20 ° ::[-0.02°. Water determinations. The water content of the TBP layers was found by means of the Karl Fischer reagent, using the dead-stop endpoint method as determined with a Fischer Titrimeter. The reagent was standardized against a known "Fischer Scientific Reagents" solution of water in methanol. In addition, the reagent was checked by titration against TBP equilibrated with water. Measurements of refractive index. Refractive indices for the sodium D doublet were measured using an Abb6 refractometer. All measurements were performed at 20° ~ 1°. Measurements with TBP diluted with benzene. Some experiments were performed with solutions of TBP in benzene as the organic phase. In these cases the lithium chloride content of an equilibrium organic layer was determined using an EEL flame photometer, the salt being washed into water before measurement. Degradation of TBP. It has been shown by various workers ~7-1°~ that aqueous solutions of chlorides can promote decomposition of TBP. To investigate the possibility of degradation of TBP by aqueous lithium chloride, potentiometric, gas chromatographic, density and viscosity measurements were performed on organic phases at various times after equilibration. Potentiometric titrations were carried out using an ElL direct reading pH-meter, model 23A. A very high sensitivity 40ft-nylon capillary Argon Chromatograph loaded with a thin film of dinonyl phthalate and with a micro Lovelock detector was used to detect and determine more volatile decomposition products. RESULTS T o p e r m i t a r e a d y c o m p a r i s o n to be m a d e b e t w e e n properties o f the systems T B P - L i C I - H z O a n d T B P - H C 1 - H 2 0 , results are p r e s e n t e d in a similar m a n n e r to t h a t e m p l o y e d b y KERTES/3) Partition

of the

lithium chloride

T h e m o l a r c o n c e n t r a t i o n s o f l i t h i u m c h l o r i d e in the initial a q u e o u s s o l u t i o n s a n d in b o t h the e q u i l i b r i u m o r g a n i c a n d a q u e o u s layers are listed in T a b l e 1. C o l u m n (6) shows the calculated d i s t r i b u t i o n coefficients D, where D = [LiCI]0/[LiCI ] In c o l u m n (7) are given the calculated mass d i s t r i b u t i o n coefficients,/~, where total a m o u n t o f LiCI in the o r g a n i c layer /~ = total a m o u n t o f LiC1 in the a q u e o u s layer T h e r e l a t i o n s h i p b e t w e e n the two d i s t r i b u t i o n ratios is given by

V0

/z = -~-. D where V0 a n d V r e p r e s e n t the v o l u m e s o f the c q u i l i b r i u m o r g a n i c a n d e q u i l i b r i u m a q u e o u s layers, respectively. T h e increases in v o l u m e o f the o r g a n i c phases are given in c o l u m n (5) o f T a b l e l, a n d values o f the ratio V o / V a r e listed in T a b l e 2, c o l u m n (3). (5~ j. F. J. DIPPY and S. R. C. HUGHES,J. Chem. Soc. 953 (1954). (6, j. F. J. DIeev, S. R. C. HUGHm and J. W. LAXTON,J. Chem. Soc., 1470 (1954). (:~ A. J. MOFFATand R. D. TgOMPSON,J. Inorg. Nuel. Chem. 16, 363 (1961). (8) A. S. KERI~ and M. HALPERN,J. Inorg. NueL Chem. 20, 117 (1961). (9) A. S. KERTESand M. HALPERN,J. Inorg. Nuel. Chem. 16, 308 (1961). (io~ A. S. KERTm and M. HALPERN,J. Inorg. Nucl. Chem. 19, 359 (1961).

The extraction of lithium chloride by tri-n-butyl phosphate

293

TABLE 1 Molarity of LiCI

_

In the initial No. aqueous of soln. soln. [LiCI]' I

In the equil, organic phase [LiCI],,

increase of the organic phase (ml)

D

tt

3

4

5

6

7

0'30 0"30 0-30 0'30 0"30 0'275 0-275 0'275 0"25 0"21) 0"15 0"15 0"125 0" 175 0'225 0"30 0'375 0"475 0"50 0"50 0-50

0"002 0"002 0"003 0"002 0'003 0'0047 0"0056 0"0060 0-0113 0'0142 0-0184 0'0253 0"0294 0"0430 0'0612 0"0841 (~'1091 0"1123 0'1185 0"1509 0-1560

0'002 0"002 0"003 0"003 0"004 0-005 0.006 0'007 0"013 0'015 0"020 0-027 0'031 0"046 0"066 0-095 0'127 0'136 0"145 0"184 1"190

2

1 2 3 4 5 6 7 8 9 I0 11 12 13 14 15 16 17 18 19 20 21

Vol.

In the equil, aqueous phase [LiCI]

0'4796 0'5084 0"9734 1"040 1"463 1'545 1-952 2'060 2"412 2"556 2"942 3'005 3'330 3"490 3"773 3'995 4"723 4"919 5"230 5'432 5'751 5'911 6"235 6"329 6"578 6'665 6"993 7"094 7"590 7"460 8"290 8"100 9"186 8"775 10'05 9-783 11"14 10"95 12"24 I 1-67 13"40 12.77

0-001 0"002 0'004 0"005 0"008 0'014 0'0195 0"0240 0"0555 0-0770 0"1089 0"1599 0"1960 0-30-19 0'4433 0"6811 0.9577 1-0987 1.298 1"530 1"836

LiCl in the organic phase, mmole* 8

0"005 0"011 0"021 0'027 0"042 0"074 0'103 0"127 0"291 0'400 0"561 0"824 1"005 1-578 2"305 3"610 5"148 6"015 7"139 8"950 10"098

LiCI/TBP ratio 9

0"0003 0-0006 0"0012 0"0015 0'0023 0'0040 0"0056 0"0069 0'0159 0"0219 0"0306 0.0450 0"0549 0"0862 0"126(I 0"1973 0.2813 I)'3287 0"3901 0"5296 0-5518

H20 in the organic H20/ phase, TBP mmole* ratio 10

11

18'2 18"5 18"1 18"1 15"7 14"8 14'7 13"3 12-2 11"2 10-5 9"7 10"8 11-4 13-2 16"2 19.7 22"5 23"0 22"6 22"6

0"99 1"01 0"99 0'99 0"86 0"81 0'80 0'73 0'67 0"61 0"57 0"53 0"59 0"62 0"72 0"89 1'08 1-23 1"26 1"23 1"23

* The number of mmoles of a substance extracted is the number of mmolcs present in the whole organic phase at equilibrium, resulting from the agitation of 5 ml of TBP with 5 ml of aqueous solution. T~BLli 2

No. of soln.

p Density at 20 ° (g/cm s)

Ratio V,,/V of the equilib. phases

nz~ 20" Refractive index

Viscosity at 20 ° (centipoise)

1

2

3

4

5

6

7

8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0"9799 0'9793 0"9839 0"9837 0"9839 0"9839 0"9841 0"9841 0-9843 0-9843 0'9860 0'9871 0"9895 0"9918 0'9951 1"003 1"011 1"018 1'025 1'030 1"034

1"13 1"13 1"13 1-13 1" ! 3 1"12 1"12 1"12 1'11 1-08 1"06 I '05 1"05 1"07 1'0'~ 1-13 1"16 1"21 1"22 I "22 1"22

1"419 1'419 1"419 1"419 1"419 1"419 1'419 1"419 1'419 1"421 1"422 1-422 1-422 1-423 1-426 1"427 1"42~q 1"429 1'430 1"431 1"432

4"687 4.603 4"972 4"975 4"910 4.956 4"930 4-952 5-043 5" 156 5-327 5-735 6-241 7"108 8'723 13-22 20"82 34-04 51"54 89.87 127"2 J,

0-0720 0-1245 0-1727 0"2138 0"2468 0"3024 0'2962 0"3878 0"4643 0"5595 0"6622 0"8686 1"3072 1"5157 1"6606 2"3188 2"2914 2'1594 1"6928 1"232~ 0"8450

0"7294 0"6223 0'4319 0"4275 0-3085 0-2160 0"1519 0"1616 0"0837 0'0727 0'0608 0'0543 0"0667 0-0497 0'0374 0"0341 0"0239 0"0197 0"0130 0"0081 0-0046

0"057 0-048 0"036 0"035 0"025 0-018 0"013 0"013 0"0070 0"0062 0'0054 0"0031 0-0069 0"0059 0'0054 0'0075 0-0083 0"011 0"011 0-0217 0-059

~/

10"K

A

Specific Equivalent conductance conductance at 20 ~, at 20 °, (f2 1 cm-~) (f~-I cm--")

Degree of dissociation

294

D. F. C. MORRIS and E. L. SHORT

The concentrations of lithium chloride in the equilibrium organic and aqueous phases are plotted in Fig. i. The distribution isotherm indicates that the extraction of lithium chloride from water into TBP is not uniform. The break in the curve corresponds with that in Fig.2 in which the percentage of lithium chloride extracted is plotted against the concentration of the initial aqueous solutions. ~'~

/

1.5

o

I.~

0.$

I

o

:'

4

6

s

~o

tz

f4

[LiC0,M FIG. 1.--Concentration of lithium chloride in the equilibrium organic phase against concentration in the equilibrium aqueous phase.

Swelling of the TBP phase. The volume increases of the organic phases in ml. are plotted against mmoles of lithium chloride extracted in Fig. 3. Three breaks in the curve are clearly distinguishable. A plot of the ratio Vo/V against mmoles of LiCI extracted shows three breaks at corresponding positions. Density of the TBP phase. Experimental values for the densities of the organic phase at equilibrium are quoted in column 2 of Table 2. These results are plotted against the amount of lithium chloride extracted in Fig. 4. The curve shows two breaks which correspond with breaks in the previous plots. Viscosity of the TBP phase. The viscosities in centipoises of the series of organic extracts are tabulated in column (5) of Table 2. The plot of r/against mmoles of LiCI extracted (Fig. 5) shows one intersection corresponding to 5.1 mmole of LiCI in the organic phase. Conductivity of the TBPphase. The measured specific conductances and the equivalent conductances of the equilibrium organic phases are listed in columns (6) and (7)

The extraction of lithium chloride by tri-n-butyl phosphate

295

15

"~

fO

t. .4-

I -4

5

0

g

4

6

8

I0

12.

I 14

[Lic,]' FIG. l - - D e p e n d e n c e of percentage of lithium chloride extracted on molarity of the initial aqueous solution.

0"~

-k .o

0.3

.~ o.g

O'll

I

to

5

0 m

mol~s

LiCI

~-xtr'ac-'f'e..d

FIG. 3.--Increase in volume of the organic phase corresponding to amount of LiCI extracted.

D. F. C. MORRIS and E. L. SHORT

296

t03

f-OZ

1.01

% S.O0

0-~ o.~

I

I

I

I

I

I

I

I

I

5

I

t0

LiCI ~ x f r a ~ f a d

mmol~,s

FX~. 4.--Density of the equilibrium organic phase versus mmole of LiCI extracted. rio

t2o 11o

~o |o

.! ~. r0

5O

o. Io

0

I

I

I

I

I

I

I

I

I

!

2.

3

4

6

6

7

8

P

I

I0

nmol~n LiCI ~xfraofdr~l

Fio. 5.--Viscosity of the equilibrium organic phase against mmole of LiCI extracted.

297

The extraction of lithium chloride by tri-n-butyl-phosphate

1"0

•~

0.5

)

0

f'O

0.5

fl'5

J[L, Cl}o FIG. 6.--Equivalent conductance of lithium chloride in the equilibrium organic phases.

/ --.J

Q ~

O---

2o

t5 -1-

f5

1 I

I b

f0

l

J

a

;

]

o

d

~oI~s

LICI ~ x ~ r a c ~ d

FIG. 7.--HsO extracted into the organic phase against LiCI extracted.

I

IO

298

D. F. C. MORRIS and E. L. SHORT

of Table 2. Fig. 6 shows a Kohlrausch plot of conductivity data. The form of the curve is typical for solutions of an associated electrolyte. Water content of the TBP phase. Numerical data for the water content of the organic phases are given in columns (10) and (11) of Table 1. The results are plotted in Fig. 7. TBP saturated with water contains one mole of H20 per mole of TBP and there appears to be formed a definite TBP monohydrate.(4, u) The negative slope of the first part of the curve in Fig. 7 can be attributed to the lowering by lithium chloride of the water activity in the aqueous phase, leading to a resultant decrease in the partition of (non salt hydrate) water. (12) After 0.9 mmole of LiCI have been extracted such an effect appears to be offset by increasing extraction of hydrated salt into the organic layer, Following the procedure of YATES et al. (la) application of Henry's law to water extraction in the region of small lithium chloride extraction yields a constant value for (aw),,/aw of 3-5 (a w -- water activity, subscript o denotes organic phase). The slope of the second part of the curve in Fig. 7 is 2-3, indicating that each lithium chloride molecule is accompanied by 2.3 water molecules on passing from the aqueous phase into TBP. After 5-1 mmoles of LiCI have been extracted there is an increase in

IO-~ a

.0~ I'L,Ct1. 9.87~

-/

"~

so'°'6s

so 2.o /

,a $

so= o

/

01 -, ~o-

3

EL,a] =8.3H

.i.iCc]=,3

K3

3

S

IO

"/. TBP IN B~NZENE

3

,s

IO

FIG. 8 . - - T h e extraction of LiCI into TBP diluted with benzene. Variation o f the distribution ratio with concentration of TBP in the organic phase.

the slope of the curve to 3.3, and after 6.1 mmole of LiC1 have entered the organic phase further salt is extracted without accompanying water.

Solutions o f TBP h7 benzene as the organic phase To gain information on the average number of molecules g of TBP directly associated with extracted LiCI, results of experiments with dilute solutions of TBP in benzene as the organic phase were used. For constant aqueous phase conditions and low concentrations of TBP, activity coefficients in the organic phase can be neglected and the limiting law D oc [TBP] ~ (6 log D/6 log [TBP]) = g should apply. Using this method over the range 1-12 per cent (volume fraction) TBP in benzene, plots on logarithmic scales of D vs. per cent TBP in benzene yielded ~u) D. J. TUCK, J. Chem. Soc. 2783 (1958). ~l°'J L. 1. KATZIN and J. C. SULLIVAN,J. Phys. Colloid Chem. 55, 346 (1951). ¢1s) p. C. YATES, R. LARAN, R. E. WILLIAMS and T. E. MOORE, J. Amer. Chem. Soc. 75, 2212 (1953)

The extraction of lithium chloride by tri-n-butyl phosphate

299

straight lines. These corresponded to values for ~ of 0, 0"65 and 2 for respective values of [LiC1] of 8.30, 9.87 and 13.0 M (Fig. 8).

Chemical stability of TBP All the results reported above were obtained within 4 hr of equilibration, except the conductivity results which were determined within eight hours after equilibration. Since it has been shown that TBP is prone to degradation in the presence of aqueous solutions of various chlorides, 17-I°1 the chemical stability of the phosphate under the present experimental conditions was investigated. Over a period of up to one day after equilibration, potentiometric titrations of organic extracts with standard alkali in a 3 : 1 ethanol-water mixture showed no evidence of hydrolysis which would have led to the formation of butyl phosphoric acids. However. it has been shown~:, s) that the degradation of TBP promoted by hydrochloric acid involves fission of the Bu-O bond rather than true hydrolysis. Our gas chromatographic studies of the vapours of organic extracts showed no butyl chloride over a time of 20 hr after equilibration in the case of organic extracts with [LiCl]o = 0-002 or 0.15 M. The sensitivity of the procedure was such that 5 × 10-a mmole of butyl chloride could be detected in 5 ml of organic phase. An organic extract with [LiCl]o --: 1"36 M was found to contain 0.01 mmole of butyl chloride 4.5 hr after equilibration and 0.025 mmole of the alkyl chloride 6.5 hr after equilibration. After a period of 19 hr the phase contained 0.04 mmole of butyl chloride; this corresponds to only 0"22 per cent of the original TBP having been converted into dibutyl phosphoric acid. Maximum variations in densities and viscosities of organic phases at 20 ~ noted over a period of one day after equilibration were as follows: [LiCl]o (M)

At,, (o/)

A,/(!'~i)

0"002 0" 15 1"36

-:~0-01 0"03 2k0"01

1-2 0"6 2"8

Tile observations reported in this section suggest that decomposition of TBP by lithium chloride and water does not pertinently affect physical chemical measurements on organic phases made within eight hours of equilibration. It is interesting to note that an unstable hydration phenomenon, similar to that reported by K~:RT~:S~ul, was observed in the case of the [LiCI], : 1.36 M solution about eight days after equilibration. DISCUSSION

From the conductivity results an approximate measure of the degree of dissociation :¢ of lithium chloride in the organic layers has been estimated from the formula ~ls) A~/60. The calculated values, presented in Table 2, column (8), show that the lithium chloride exists to a large extent as ion-pairs. The Ostwald dilution law explains the results over a considerable range and leads to a dissociation constant of ca. 5 :-: 10-6. ~lt~ A. S. KERTES, J. Inorq. Nucl. Chem. 12, 377 (1960). ca~) H. A. C. MCKAY and A. R. MATIIIESON, Trans. Faraday Soc. 47, 428 (1951).

300

D. F. C. MORRISand E. L. SHORT

In many cases of the extraction of salts MX into TBP c2) the activity coefficient of MX in the organic phase may be taken as unity for concentrations [MX]o up to ca. 0.1 M. However, in the present system, the expression (1 -- ~) [LiCl]0/~,2m'-' is not constant for values of [LiCl]0 from 10-3 to 10 1 M (m is the molality of lithium chloride in the equilibrium aqueous phase and y is the mean activity coefficient of LiCI in aqueous solution). This "abnormal behaviour", which was first pointed out by HEALY and BRowrq t2~, could possibly be due to the fact that lithium chloride is more complexed than other 1:1 salts in either the aqueous or organic phase/16~ On the other hand, the result is probably due merely to changes in the activity coefficient of lithium chloride in the organic layer as a result of the wide variation in water content of the TBP phases., For concentrations of lithium chloride in the organic phase below ca. 0.96 M there seems to be no strong TBP-electrolyte interaction, as occurs for example in the extraction of nitric acid by TBP. 11,4,11,17~ The discontinuities in the plots of physical chemical properties which arise when 5"1 mmole of lithium chloride have been extracted ([LiCl]0 ~.0.96 M) correspond to a H20/LiCI mole ratio in the organic layer of four. It appears, therefore, that lithium chloride in the organic phase has a tendency to maintain a primary hydration number of four. This is borne out by the fact that after 5-1 mmole of LiC1 have been extracted there is an increased tendency for water to be extracted relative to lithium chloride, until 6.1 mmole of LiCI have entered the organic phase. When more than 5.1 mmole of LiCl are extracted, TBP molecules begin to substitute for water molecules in the solvation shell of the extracted alkali chloride. However, a solvation number of four appears to be maintained and in the almost saturated solution, when 10.098 mmole of LiC1 have entered the organic phase, the ratio (TBP -- H,,O)/LiCI is equal to four. The stage at which lithium chloride begins to enter the organic layer without accompanying water molecules commences when the H20/LiCI mole ratio in the equilibrium aqueous phase is four. This suggests that in concentrated aqueous solution lithium chloride tends to maintain a hydration number of four. There is additional evidence for the latter conclusion. GLUECKAt;Ft~s~has obtained a value of 4.3 for the hydration number of LiC1 by application of his equation for activity coefficients in concentrated electrolyte solutions to tabulated experimental data.rig, ~°' Moreover, application of the BRUNAUER, EMMETI"and TELLER adsorption isotherm ~21~to very concentrated aqueous solutions of lithium chloride indicates that the electrolyte has four sites for the "adsorption" of water molecules t~). Also, from a comparison of viscosity and apparent molar volume data STOKESt~a) concludes that in concentrated aqueous solutions of lithium chloride the number of " b o u n d " water molecules is 3-4. CHIEN-HANCHU, Univ. Michigan, Microf. diss., 1943, No. 1169, abstr. IX, 2, 1949. c~7~E. HESFORDand H. A. C. McKAY,J. inorg. Nucl. Chem. 13, 156 (1960). tls~ E. GLUECKAUF,Trans. Faraday Soc. 51, 1235 (1955). 119~R. A. ROBINSONand R. H. STOKES,Trans. Faraday Soc. 45, 612 (1949). t2o~R. H. STOKES,Trans. Faraday Soc. 44, 295 (1948). c161 j .

121~S. BRUNAUER, P. H. EMMETr, and E. TELLER, J. Amer. Chem. Soc. 70, 1870 (1948).

t~2~R. A. ROBINSONand R. H. STOKES,Electrolyte Solations (2nd Ed) Chap. 3. Butterworths, London (1959). ~s, R. H. SlOKES, in W. J. I-IAMER(Ed.), The Structure of Electrolytic Solutions, p. 298, Wiley, New York; Chapman Hall, London (1959).

The extraction of lithium chloride by tri-n-butyl phosphate

301

The extremely high viscosities of TBP extracts from very strong aqueous solutions of lithium chloride may be due to the formation of aggregations (of. BEt.io and BELI,O(21,25)) such as OBu BuO

OBu

I

I

-P--OBu

BuO- .-P--OBu

',

0 ! .-

HmO

I_i"

o

-

....

I

H

I

O.

H...CI-...H--O

.....

I

I

H

H

I

Li ~

.....

O--H...CI-...

: I

o

o

BuO--P--OBu

BuO--P--OBu

I

OBu

I

I

OBu

In these formulae, dots may be regarded as indicating hydrogen bonds, and dashes ion-dipole interactions. The extracts show mixing lines on shaking which is compatible with the formation of such aggregates. Cases of lithium salts forming solvates with dipolar aprotic solvents and yielding extremely viscous solutions have been reported e l s e w h e r e . ~26~

Acknowledgements--The conductivity measurements described in the present paper were performed at Chelsea College of Science and Technology. Grateful acknowledgement is made to Dr. J. F. J. DJppv, Dr. S. R. C. HUGHES and Mr. S. WHVrE for helpful advice and for putting apparatus at the disposal of the authors. Mr. S. TItORBURN is to be thanked for valuable help with the gas chromatographic measurements. ~ J. BELLO and H. R. BELLO, Nature Lond. 190, 440 (1961). t-.s~ j. B~LLO and H. R. BELLO, Nature Lond. 194, 681 (1962). ~6~ A. J. PARKER, Quart. Revs. 16, 163 (1962).