Synergism in the solvent extraction of di, tri and tetravalent metal ions—IV

J. Inotg. NucL Chem., 1962, Voi. 24, pp. 1449 to 1461. lNtrllamonPrma Ltd. Printed in BUlfland

SYNERGISM IN THE SOLVENT EXTRACTION OF DI, TRI AND TETRAVALENT METAL IONS--IV ABSORPTION SPECTRAL STUDIES OF THE SYNERGISTIC COMPLEXES* T. V. ~ ¥ t

and J. R. F ~ d ~ o ~ :

(Received 17 January 1962) Abetraet--In previous work with radioactive tracers in this series, formulae have been assigned to the metal complexes synergistically extracted from aqueous solution by an organic phase containing thenoyl trifluoracetone (I-ITTA) and neutral organophosphorous compounds. Stable solid complexes have now been isolated and are shown to have the same formulae. These formulae have been confirmed by ultra-violet and visible absorption spectrophotometric measurements of dilute solutions of uranyl, thorium and neodymium complotes. I s earlier work in this series,O) formulae were derived for the metal species synergistically extracted from aqueous solution into organic phases containing thenoyl trifluoracetone (HTTA) and neutral organophosphorous esters (S). This enhanced synergic effect has been defined as a definite enhancement of metal extraction by a mixture of an acid and a neutral additive, the resulting mixture giving a better metal extraction than either the acid or the neutral additive alone. Among the formulae suggested for the extracted species on the basis of radioactive tracer work were UO2(TTA)2S1, UO2(TTA)2S3, Th(TTA)4S1, and Pm(TTA)3S2. A number of crystalline solid complexes have now been isolated in the uranyl and thorium series and these have been examined both analytically and by absorption spectrophotometry. The rare earth complexes that were prepared remained as oils and did not solidify. Absorption spectral measurements were carried out in the systems H20/Mn+/HTFA/S and it was found that by keeping all constituents constant except HTTA or except S a maximum spectral peak height could be obtained. This enabled formulae for the extracted species to be ascertained at concentrations much greater than those used in tracer work. The studies indicated that the same formulae for the species were found at tracer levels, at nfillimolar levels and in the solids or oils. In this work, S was usuaUy a neutral organophosphorous ester as in the tracer studies.O, 2~ N.n.butyl acetanilide (BAA), shown to give large synergic effects in combination with HTI"A¢S), was also used in these studies. Isolation and properties of metal/TTA/S synergic complexes Uranyl complexes Two volumes of an aqueous solution of 0.025 molar uranyl nitrate were added to one volume of a hexane solution of 0.10 molar HTTA and 0-05 molar neutral organophosphorous ester. The mixture was shaken vigorously for about 20 win, * Based on work performed under the auspices of the United States Atomic Enerly Commission. t Harwell Exchange Fellow at Argonne National Laboratory. Argonne National Laboratory. (1) T. V. HEALY,Part I of this series, J. Inorg. Nucl. Chem. 19, 321 (1961). ta) T. V. HILaLY,Part II of this series, Y. Inorg. Nucl. Chem. 19, 328 (1961). ~3)T. V. H~LY, Part HI of this series, Y. Inorg. Nucl. Chem. 24, 1429 (1962). 1449

1450

T.V. HEALYand J. R. ~ o

the aqueous phase removed and the hexane solution allowed to evaporate. Yellow microcrystalline material gradually settled out during the evaporation. When about nine-tenths of the hexane had evaporated the precipitate was filtered off, washed with a little water and a little hexane. Further purification was brought about by recrystallization from one of many organic solvents, such as hexane, cyclohexane or benzene. In this way, solid crystalline components of the formula UO2(TTA)2S were obtained, where S is TBP, TOPO or BAA. UO2(TI'A)2(TOPO)3 was also isolated as a solid by using three to four times the usual concentration of TOPO. Thorium complexes These salts were obtained in a manner similar to the above uranyl complexes except that the reagent mole ratio for Th :HTTA:S was 1:4:1. Cream coloured solids were obtained in these thorium salts where S is TBP, TOPO, or BAA and the general formula is T h ( T T A ) 4 S . Neodymium complexes These were also o b t a i n e d similarly using the reagent ratio for Nd:HTFA:S of 1:3:2. It was found necessary to raise the pH of the aqueous phase in order to effect complete extraction of Nd into the organic phase. Complexes made with TOPO and BAA were oils at room temperatures. All the metalfic solid complexes isolated were anhydrous. The simple compounds M(TTA)x, where x is the valency of the metal, can be made in exactly the same manner as the M/TTA/S complexes, the only difference being that S is omitted from the preparation. Alternatively, they can be made by the same methods used for preparing the corresponding acetylacetonates¢4). Table 1 gives the complexes prepared their melting points and analytical data. TABLE 1.~DATA ON MErAL/TrA/S coMPI,IeXES

Analysis Complex UOz(TrAh uO~rTA)2TBP UO~(TTA)2TOPO

UO2(TrA)2(TOPO)3 UO~'ITA)2BAA

Th(TFA)4 Th(TTA)4TBP Th(TrA)4TOPO ThfTrA)~BAA

NdOWA)3 Nd(TI'A)3(TOPO)2 Nd(TI'A)3(BAA)2

M.P. (°C)

•Theory Metal (~)

212 112

. 24.3

57

.

37 107 226 178 110 Oil 155-165

.

Oil Oil

P (~) .

. 3.17

.

.

.

.

.

.

.

.

.

16-7 15.5 .

Metal ( ~ )

. .

.

.

.

23.8

2.7

16.6 164)

2.3 1.9

.

.

2.26 2.07 .

Experimental P (~)

.

.

.

.

.

. .

. .

. .

. .

It can be seen from the above table that introduction of a mole of TBP into a metal/TTA complex lowers the melting point about 50°C, while introduction of a mole of TOPO lowers it about 100°C. The formation of UO2(TTA)2(TOPO)3 is demonstrated by lowering of the mixed melting point, by tracer studies(2) and by infra-red studies¢5). (4) W. C. Fm~Nm.JUS,Editor, Organic Syntheses Vol. 2, p. 15. McGraw Hill, New York 0946). ¢5)j. R. F~RR^ROand T. V. H~ALY,Part V of this series. J. Inorg. Nucl. Chem. 24, 1463 (1962).

S y n e ~ m in the solvent ¢gtraction of di, tri and tetravalent metal ions--IV

1451

UO2(TTA)2 itself, which is anhydrous, has the same m.p. as that given in the literature(e) (212°C). Analyses for uranhzm and thorium were carried out by standard methods after the metal was wazhed out of the benzene solution of the complex by strong aqueous acid. Phosphorus was estimated also by standard methods after evaporation of the metal-free organic solvent. VISIBLE AND ULTRA-VIOLET SPECTRAL RESULTS A Cary 14 automatic recording spectrophotometer was used throughout this work. Uranyl solid complexes The absorption spectra of UO2(TTA)2 in o r ~ n i c solvents, such as benzene, are well known in the literature.(6) Two peaks are observed at about 330 and 380 mp, the first being somewhat more intense than the second. The absorption spectra of the complexes UO2(TTA)2TBPand UOz(TTA)2TOPO in benzene are reproduced in Fig. 1 and are almost identical with that of UO2(TTA)2 in the same solvent. In Fig. 1, the spectrum of UO2(TTA)2TBP in normal hexane is shown, which is also very similar. UO2/TrNS Spectra

U02/1TNS Spectra 2 x 10-4M, Uranium I cm Ceil

2x 10"4 M. Uranium 0.l cm Cell A = U02/I~A/TBP in Benzene B-U02/TTNTOP0 in Benzene C • UO2/TTNTBP in Hexane

~

0"6--

0"4 (.3

X,,o'/ ~

OL 270

" 't~

'~

i 300

/ l'

-~t

k

I

I 340

|

X

W "I" Z

I

| | ~

N Z UJ m

laJ W N Z hJ

t--

|

~

"~

"-.

•

W

Z

I I 380 420 WAVELENGTH (mp.)

Fzo. 1.--Absorption spectra of solid uranyl comple~s dissolved in organic dilmmts. The big difference between the spectra of UO2(TTA)2TBP in benzene and hexane is at the wavelength above 400 rap. There are well defined peaks at 427, 441 and 456 nap in hexane whereas, in benzene, these are only humps in the spectra, and are practically invisible in UO2(TTA)2 alone. This point is illustrated at the'fight hand side of Fig. 1 (e) E. L. Zem~os~ T.I.D. 1098 (1947).

1452

T.V. I'IeALYand J. R. Fmuu~to

where the heights of the spectra are increased by a factor of ten compared with the left hand side. These peaks beyond 400 m/t are characteristic of the uranyl grouping and are present in both aqueous and organic solutions of uranyl nitrate. However, the extinction coefficient in the UO2(TTA)2TBP complex at 426 m/t is more than a hundred times greater than it is in most other uranyl salts. The only other relatively high uranyl extinction coefficient is that of the trinitratouranyl nitrate, but even those peak heights are less than one tenth the height of those in the M/TTA/TBP complexes. These high extinction coefficients at above 400 m/t no doubt account for the increase in intensity of the yellow colour of UO2(TTA)2 solutions upon addition of the synergic agent TBP. Because of these more clearly defined spectra in hexane at above 400 m/t compared with benzene, much of the subsequent ultra-violet work was done using hexane as the organic medium. Although UO2(TTA)2 is very soluble in benzene, it is almost insoluble in dry hexane. Addition of water to a hexane suspension of UO2(TTA)2 changes the spectrum of the hexane solution, the spectrum obtained being the normal "enol" spectrum of HTTA. This indicates hydrolysis of the UO2 (TTA)2 in wet hexane, a phenomenon which does not occur in wet benzene. Extinction coefficients (E) for the uranyl complexes are listed in Table 2. TABLE2.--ExTr~CTIOlq coemcn~crs oe ULT]U~-WOLeTPEAKSIN THE UP.A~,'LCOM~.ZXeS Peak Peak Peak Peak Peak Complex (m/t) E (m/t) E (m/t) E (m/t) E (m/t) E

UO~'TA)2 in benzene

338 26,400 384 21,200

UO2(TrA)2TBP in benzene

332 27,500 380 22,000 430

1,450 AA.A. 950

458

500

330 27,000 377 24,000 427

1,400 441

950

454

50O

328 26,000 375 21,000 427

1,350 441

950

456

550

UO2(TTA)2TOPO in benzene

UO2(TTA)2TBP in hexane

ii

Uranyl complexes extracted into the organicphase The above spectra were obtained from the dissolved crystalline compounds of known composition. In extremely dilute solutions, that is, solutions of uranyl salts of < 10-5 molar, the composition was found by tracer work to be the same, namely, UO2CITA)2S1 where S is a neutral additive such as TBP, DBBP or TOPO. Two species were, however, obtained using the last additive, TOPO, namely UO2(TrA)2 (TOPO)I and UO2(TTA)2(TOPO)3. An attempt was made, using absorption spectra, to ascertain the formulae of the species in dilute organic solutions of uranium (10-3 molar) by using 10-3 to 10-1 molar H T r A . Owing to the very high extinction coefficients of HTTA and its salts, it proved impossible to compare solution spectra using a diluent such as hexaue as a reference in the spectrophotometer. Hexane solutions were therefore made up of known concentrations of HTTA and TBP, and divided into two equal portions, one to be shaken with an aqueous uranium solution and the other with a similar aqueous solution containing no uranium. After equilibrium, the organic phase which had extracted the uranium would be compared spectrophotometrically with the other half of the HTTA-TBP mixtures as a reference.

Synersism in the solvcmt e~traction of di, tri and tetravalent metal ious--IV

1453

In this way, absorption spectra could be obtained using HTTA as high as 0.2 molar in solution. In order to find the ratio of U to HTTA or to TBP in the extracted species, two of these would be kept constant, of which one would be present in excess. The third constituent is then gradually increased in concentration until a maximum is obtained in an absorption peak. In all these experiments, uranium was kept constant at 10- 3 molar. By keeping excess TBP in solution and varying HTTA it was observed that the spectral peaks rose to a maximum at a HTI'A concentration of 2 × 10-3 molar indicating that there were 2 TTA moles per uranyl mole in the species Uo~rrA)2TBP. These spectra are illustrated in Fig. 2. In these experiments, the E 2.0

A • 0,0005M. HTTAIHexane B • 0,00]0 M. HTTAIHexane C • 0,001,5M. HTTAIHexane D • 0,0020M. HTTAIHexane E - O,OlO0M. HTTAI Hexane

b_l~

io, ,

Jr |~.

l

0.1 cm Cell +ve Ceil

i /P ! /

[~

-ve Ceil

0"IM.TBP 0"1M.TBP Varying HTTA VaryingHTTA 0,001M. UO~+

q

I'

>. I-

ciil

/

(n

z

u_

:i/:

b.l

.J <[ to. 0

i.o

\.//,, I

I

•

i i t

*

:I

/

I

I

I

500

330

380

'~t 420

WAVELENGTH (m/z)

I~o. Z--Effect of added HTYA or abeorpt/on spectral peak heishts below 400 m/~ in the UO2/HTTA/TBP system. uranium was increasingly extracted out of the aqueous phase as the HTTA concentration of the organic phase increased. In a similar manner, ~ n s I0- 3 molar aqueous uranium plus an excess of HTTA in the hexane phase, a precipitate of UO2(TTA)2 was produced. By gradual addition of T]SP, the precipitate dissolved and the

1454

T.V. He~Y and J. R. Fmut~.o

absorption peaks increased to a maximum, then remained constant. This occurred when 10-3 molar TBP had been added, indicating formation of the species UO2(TTA)2 (TBP). The following tables show the gradual increase to the maximum peak heights. T,~.e 3.---OrnCAL v ~ s r ~ ~

HTTA molarity

FOXu/m'rx MOLERATIOSIN U/HTTA/TBP

U "= 0"001 molar

TBP =. 0.1 molar

0.1 can path length

1 cm path length

D321m~

D37Im/z

D42sm//

D442m/t

D456mg

HTTA/U mole ratio

0~005

0.32

0.55

0.26

0.17

0.09

0.5:1

0.0010 0.0015 0~020 0.0100

0.72 0.91 1.2 1.2

1.I 1.57 2.1 2.15

0.55 0.80 1.01 1.05

0.36 0.57 0.71 0.72

0.17 0.25 0.36 0.38

1:1 1.5"1 2:1 10:1

It iS interesting to note that the extinction coefficient of the 328 m/t peak is higher than that of the 378m/t peak in the absorption spectral data given in Table 2 for the solid complexes dissolved in hexane, whereas the reverse is true in Table 3. The latter table, however, gives the spectral data for a comparison of U/TTA/TBP against H T r A ~ B P in hexane. An examination of the spectra of HTTA/TBP in hexane against hexane alone shows a high peak at 330 ra/t and no peak at 380. It is the production of this 330 m/z peak in HTI'A/TBP solutions which accounts for the apparent reversal or lowering of the 330 peak relative to the 380 m/t peak in Table 3. An examination of Table 4 not only shows the reversal relative to Table 2, but shows a large shift in these peaks to longer wavelengths. This is because a relatively large constant concentration of HTTA (0.1 molar)is used in the series in Table 4, the shift increasing further to the fight as the I-ITTA molarity is increased. This is undoubtedly due to the poor light transmission of the more concentrated HTTA solutions. TAaLe 4.---OFncAL DeNsrrYMAX~.AFORU/TaP MOLeXATIOS U = 0"001 molar

HTTA am0"1 molar

0.1 ¢m path length

1 ¢rn path length

TBP molarity

D3semp

0~0025 0.00050 04)010 0~020

0.3 0.6 1.1 1.1

D39em# 0.55 1.1 2.1 2.1

D425mfl D442m~t D456m# 0.26 0.55 1.04 1~6

0.17 0.36 0.71 0.72

0.09 0.17 0.34 0.36

TBP/U mole ratio 0.25"1 0.5"1 1:1 2; 1

In a similar procedure to that given for the UOz/TrA/TBP systems, the UOz/TTA/ TOPO systems have been examined and it has been ascertained that one mole of uranium complexes with two moles of HTTA and one mole of TOPO to give the complex UO2(TTA)2(TOPO). Production of a complex containing more than one mole of TOPO, namely UO2(TTA)2(TOPO)3 does not appear to cause any effect on

Synergism in the solvent extraction of di, tri and tetravalent metal ions--IV

1455

absorption spectrum and yet there is strong evidence for its production both from

tracer work and from mixed melting points of the solid complexes UO2(TTA)2, UO2(TTA)2TOPO and UO2(TTA)z(TOPO)3. Fig. 3 and Tables 5 and 6 illustrate formation of the species UO2(TTA)2(TOPO)I. 426 :'~

J4!

i::

1.0

4;)6

'

!

i'i !! ""

•

+

i?

!

44~

iii:o!tP , ;::- I ! I! : :ll |

>I-z _1

~F

0.5

J

456

0

r:

v

I

...

•

~|

!c

/

B

A

I II

.

Cdl

*re Cell

• -re Cell

o-ol to o,ooo~ M loPO C-~IOM u ~ w O . ~ M HTIA

o+o

0

~;

ii.o=.ki; iom

_

•

"

. • •

O.Ol io o.1~0~ i~ iopo oloo~ HnA ....

X ~f

'* •

".

...~

_.+_

I

I

......

I I I WAVELENGTH ( m p )

I

FIG. 3.--Effect of added TOPO on absorption spectral peak heights above 600 m/~ in the system U O 2 ~ r T A ] T O P O . T ~ L e 5.---OrncxL VE~m~" tO.,'Cm~ FOrt u/m'rx MOLE~TtOS n~ U/HTTX/TOPOSgS~MS U = 0.001 molar TOPO is 0.10 molar 0.1 cm cell 1 cm cell path length HTFA

molarity 04)005 0"0010 0"0015 0-0020 0"0100

D3~m/~ 0.40 0"70 1"0 1-3 1"4

D37sm/1 D426m~ 0"56 1 "1 1"5 2-1 2.2

0"25 0"47 0"69 0.94 0"97

D~om/~

D+56m/~

0"18 0"34 0-46 0"70 0.72

0"10 0"18 0"29 0"35 0"37

T. V. I-IeALYand J. R. FeRRARO

1456

TAmE 6.--OPTXCALDENSITY MEASUREMENTS FOR U/TOPO vlm'rAIToPo SYSTEMS U is 0.001 M

1.0 cm path length

TOPO molarity

D4um/~

0.00025 0-00060 0.00090 0.0010 0.0020 0.010

0.27 0.63 0.95 1.02 1~)2 1.04

MOLE RATIC6 IN

H T r A is 0.004 M

D 4 4 0 m p D45em~t 0.20 0.45 0.66 0.74 0.74 0.74

0.10 0.23 0.33 0.36 0.36 0.38

Among the amides which exert a strong synergism on the metal.TTA system, n-butyl acetanilide (BAA) is shown here as an example. Again, it is clear from these spectrophotometric measurements in Table 7 which confirms tracer and macrowork that the synergistic species is UO2(TTA)2(BAA)I. TABLE 7.----OPTXCALDENSITY MAXIMA FOR U/HTTA A N D U/BAA MOLE RATIOS IN

U/m'rA/BAA SYSTEMS

U is 0.001 M

U is 0.001 M 1.0 c m cellpath length

B A A is 0"I molar

HTTA molst'iVy 0.0005 0~010 0.0015 0~020 0.0100

H T r A is 0"I molar

D428mfl D442m/t 0.26 0.49 0.77 0.89 1.06

BAA molarity

0.17 0.34 0.50 0.6.5 0.72

D428mfl D442m~t

0.00025 0.00050 0.00075 0.00100 0~)050

0.28 0.54 0.81 1.06 1.07

0.19 0.37 0.57 0.73 0.74

Thorium solid complexes The absorption spectra of Th(TTA)4 in organic solvents such as benzene are well known(6) and are presumed to consist of the normal " e n o r ' form o f chelate. Fig. 4 shows these absorption spectra plus those of Th(TTA)4TBP and Th(TTA)4 TOPO. The spectra consist mainly of one large peak at 345-350 m/z with an extinction coemcient of 85,000 and in all three instances are more or less identical. Th/TBP Th/H'ITAffBP SYSTEMS

TABLE 8.----OFHcAL DENSITY MAXIMA FOR

MOLE RATIO8 IN THE

System contains 5 x 10-4 M Th and 2-5 x 10-3 M H T r A (0.1 cm cell path length) TBP molarity Optical density (D35om/~

1 x 10-4 0.5

2.5 X 10 - 4 1.1

5 X 10 - 4

50 × 10 - 4

2.2

2.3

Table 9 and Fig. 5 show the ratio of Th to HTTA is 1 to 4 in the system Th/I-ITrAfrOPO and Table 10 shows the ratio of Th to BAA is 1 to 1 in the system Th/I-ITrA/BAA.

Synergism in the solvent extraction of di, tri and tctravalent metal ions--IV I'00-

1457

--2.00

2-5 x lO"5 M. Th(ITAI4 Cocaplexes 1 crn Cell Path Length

0"75-

- 1"75

A = Th(TTAI4 8 - Th(TI'A)4 TBP C = Th(TTA)4 TOPO

:... C :. /

>,..

.\

t-.-

i ,.A

Z LU .j

i/i,

0.50-

o

0,25-

0

--I.25

I 260

ill 320

290

i i~t "~.~,,,,,.~ 350 380 410 W A V E L E N G T H (m # )

FIO. 4.--Absorption spectra of solid thorium complexes dissolved in benzene. Th/HTTA Th/HTTA/TOPO SYST~

TABLE 9 . - - O P T I C A L DENSITY MAXIMA FOR

MOLE RATIO IN THE

System contains 5 × 10-4 molar Th 4 x 10-z molar TOPO in hexane (0.1 cm path length) HTTA molarity x 104 Optical density (Ds4smp)

5 0.58

10 1.1

15 1.58

20 2.08

50 2-11

Thorium complexes extracted into the organic phase Both tracer work in solution and macro analysis of the solid show the formula of the TBP extract to be Th(TTA)4(TBP)I. Spectrophotometric evidence confirms that there is one TBP mole per mole of Th in the complex. The method used is similar to that carried out with the uranium complexes. At a constant concentration of thorium and HTTA, gradual addition of TBP causes an increase in the height of the 350 mp peak up to a maximum at a Th/TBP ratio of 1 to 1. It should be noted that the peak wavelength in Table 10 (388 rap) is much higher than in the previous tables. In this case, a higher HTTA concentration had to be used and this caused the shift in wavelength. This is undoubtedly due to the poor light transmission of the more concentrated HTTA solutions. As an example of this

1458

T.V. ~ Y

and J. R. FmmARo

behaviour, a hexane solution containing 5 x 10-4 molar Th, 10-3 molar TOPO and 2.5×10 -3 molar HTTA, using 10-3 molar TOPO and 2.5× 10-3 molar HTTA as reference, has an optical density of 2.1 at 345 m/~ when viewed through a 0.1 cm cell. E 2'0 - -

Ij//~.\~ ~' I~

5 x IO'4M • Th O.04M.TOPO Vs O.04M.TOPO Varyincj HTTA Varyincj HTTA Hexane+re 0 1 cm .CelIHexane-ve

!\ ~!

A • 5 x lO"4 M. HTTA

11344 iX m,: ~ I

B-IO'3M.HTTA

C- l'SxIO-3M.HTTA D" 2 x lO-3 M. HTTA

i

>.

Z Ul .J

<

I-O - -

b-

,#I g !', "++ ',i!/

I::I/ \ ]I:i/ 0

;500 320 340 360 WAVELENGTH

,i!',

\ ',',?,! 380 4 0 0 (mp,)

FIO. 5.--Effect of added HTTA on absorption spectral peak heights in the Th/HTTA/TOPO system. T~m~

10.----OPTICAt,DBNSITY MAXIMA ]FOR T h ~ A A

MOL]E RATIO m TB[B

T h / H T T A / B A A s~'r~ System contains 5 x I0-4 molar Th, 10-z molar H T r A in hexane (I c m path length) B A A molarity x 104 Optical density(D3ssm~

5 0.55

I0 0.98

20 1.91

50 1.95

According to Beers Law, this solution, viewed through a 1 cm cell, should have an optical density of twenty-one. Instead, the optical density is only 2.3 but the wavelength of the peak has increased to 384 m/~.

Synergism in the solvent egtraction of di, tri and tetravalent metal ions--IV

1439

Neodymium complexes extracted into organicphase Aqueous neodymium nitrate has a visible spectrum with the ~ peaks at 576 (E = 6.90) and 522 (E = 4.20). Similarly, the organic solution of neodymium nitrate in TOPO/hexane has peaks at 584 (E = 48) and 528 (E = 8-3). The extinction coefficient at the higher wavelength is about seven times greater for the organic .solution than the aqueous solution. Nd(TFA)3TOPO has practically the same visible spectrum as Nd(NOa)3 and Nd (TTA)z in benzene. By using the same spectrophotometric technique as that used above for both uranium and thorium/ H T T A complexes it is shown that Nd/HTI'A/TOPO exist in the complex as Nd(TTA)3 (TOPO)2 which is in agreement with tracer work.U) (See Fig. 6 and Table 11). Table 12 indicates formula Nd(TTA)z(BAA)2. I'O

A contains B contains C contains D contains E contains

(TOt M. HTTA 0.0~ M .HTTA 0.04 M. HTTA 0.05 M .HTTA 0.]0M .HTTA

10 cm Cell D

E 584

[584 I

J

II

¢

j 0,5

I

,

II

i 1[ =

i

or

L/

::

i!~ I i! " i,~i i !?:

:

!I

i

528

::

II

! . 572::

I

o IQ. 0

'

' ':"

I ::::

I

,

~----[.-)

l

B 584 "

Jl

.

,,

i 1

'

ii

I

'"

.

p,

I

[

! i

,' I

I .I' "J

Hexane -ve

584

i! 572i~1

i!

Vary HTTA 0.] M.TOP0

vs

Hexane solution +ve

i

572,.,

Z w a

Vary HTTA 0.l M. TOP0 • 0.015 M. Ncl

"~" ]

i

I.

',

A 584

~l 2A ,57

:. z .... .']

l.J

I

WAVELENGTH(mp)

FIG. 6.--Effect of added HTTA on absorption spectral peak heights in the Nd/HTTA/TOPO system.

T. V. He.~Y and J. R. Fegs,~o

1460

TAmm ll.---OP'rICALDENsrrY MAXIMA FORNd/HTTA ANDNd/TOPO IN THE Nd/HTTA/TOPO SYSTEM Nd is 0.015 molar HTTA is 0.20 molar TOPO molarity

Nd is 0"015 molar TOPO is 0.10 molar

1 cm cell path length

Dss4m/L

HTTA molarity

Dss4m#

0"005 0.010 0"020 0"030

0.13 0.25 0.50 0.72

0.010 0.020 0.040 0.045

0.17 0.33 0"63 0.72

0"045 0"10

0"72 0"72

0.050 0"20

0.72 0"72

Similarly, it can be shown (Table 12) that the mole ratio of Nd to BAA is 1 to 2. TABtJ~ 12.---OPTICALDENSITYMAXIMAFORNd/BAA IN THESYSTEMN d / H T T A / B A A System contains 0.015 molar Nd and 0.2 molar HTTA in Hexane B A A molarity Dss4m/~

0.005 0.12

0.010 0.24

0.015 0.35

0.020 0.47

0.030 0.71

0.050 0.72

0.I00 0.72

DISCUSSION The absorption spectral work discussed does not shed any light on the structural differences between Mx(TTA)y and Mx(TTA)yS. However, it does substantiate the formulae obtained for the complexes of U, Th and Nd by both tracer work and work with the solid complexes. This indicates that at all concentrations there seems to be the same species present in solution. There is also no differentiation observable spectrophotometrically in the species UO2(TTA)2TOPO and UO2(TTA)2(TOPO)3. The spectrum of Mx(TTA)y has. been discussed in the literature(6) and is considered to be the normal enol chelate form. The synergistic enhancement of the extraction of metals by HTI'A plus neutral additives which changes the chelate group into a monodentate ligand(5) does not appear to affect the visible spectra of the ]V[x(TTA)y. The only observable difference is the large enhancement of uranyl peaks at 426 to 454 m/t which do indicate an effect on the structure even more marked than the change from uranyl [UO2(NO3)z] to trinitrato uranyl nitrate [HUO2(NO3)3] species.(7) An attempt was made to observe the formation of UO2(TTA) (NO3)S or UO2(TTA) (OH)S in the presence of insufficient HTTA and an excess of NaNO3 or NaOH. NaOH caused precipitation of U from some solutions otherwise the spectra was normal. On a number of solutions with insufficient HTTA, saturated aqueous NaNO3 did cause a general lowering of the spectrum by a factor of about two, but this was probably due to the salting in of UO2(NO3)2"2S giving a mixture of this complex with the usual UO2(TTA)2S. The same situation was true for the Th(TI'A)4.TBP complex, there being no evidence for substitution of TTA by nitrate group. The use of this spectrophotometric technique for ascertaining the formula of the species in solution is really based on the fact that the Mx(TTA)y complex (1) has a high extinction coefficient and (2) it has a low solubility in hexane or cyclohexane. (7) L. K~LAN, R. A. HILD~RANDTand M. ADER,J. lnorg. Nucl. Chem. 2, 153 (1956).

Synergism in the solvent extraction of di, tri and tetravaleat metal ions--IV

1461

Addition of the neutral donor solvent very greatly increases the solubility of the metal by forming the synergistic complex M~(TTA)ySz. Some of these complexes have solubilities in the organic phase greater than 0"1 molar and where the complexes do not form solids, as in most of the rare earths, the solubilities are appreciably higher. Use could be made of this spectrophotometric technique in analysing solutions for many metals such as thorium, uranium, rare earths or transuranics. The unknown aqueous concentration could be brought to a pH between 1 and 6, then extracted by an equal volume of a mixture of HTTA plus TBP in hexane. The organic extract could be examined spectrophotometrically using as reference solution, par~ of the original extractant previously wetted. The effect of various aqueous complexing agents and other interfering metals would have to be ascertained. The method has good possibilities for estimating many metals including for example, micrograms of thorium and uranium. The authors, however, do not propose to pursue this analytical method any further.