Double ammonium oxalates of the rare earths and yttrium

J. Inorg. NueL Chem., 1964, VoL 26, pp. 931 to 936. Pergamon Press Ltd. Printed in Northern Ireland

DOUBLE AMMONIUM OXALATES OF THE RARE EARTHS AND YTTRIUM M. F. BARRETT, T. R. R. MCDONALD and N. E. Tot'P National Chemical Laboratory, Teddington, Middlesex

(Received 30 July 1963; in revised form 22 October 1963) Almtraet--The rare earths and yttrium are precipitated by oxalic acid from acidic solutions, on the addition of ammonia, as double salts of the structure NI-I4LnOx~.yHzO,where y = 1 or 3. Precipitation is carded out from strongly acid solutions of the light earths, and less strongly acidic solutions of the heavy earths and yttrium. The normal and double oxalates have been examined by X-ray methods, and nine different powder patterns were obtained. The yttrium salt, NH4YOxeHaO has a monoclinic structure. Pyrolysis of the double oxalates has been studied; the salts all decompose at lower temperatures than the normal oxalates. OXALATE precipitation is commonly used for the quantitative recovery of the rare earth elements from aqueous solution. A classical method of obtaining the mean equivalent weight of complex rare earth materials is to determine the oxide-oxalate ratio of the precipitated material. I1~ This method was examined with rare earth concentrates derived from the minerals monazite, gadolinite and xenomite. Results of the expected magnitude were obtained with monazite, but consistent and unexpectedly low results were obtained with the other two materials, which contained much yttria and heavy earths. The oxalate precipitation of yttrium was examined in some detai/, tz~ Later the rare earth elements were examined similarly. The normal oxalates, LnzOxa, are precipitated from neutral solutions of the light earths and yttrium, by the addition of a m m o n i u m oxalate. However, with the heavy earths, the precipitate was a mixture of the normal oxalate with a smaller quantity of an oxalate, with the molar ratio L n : O x = 1:2. When precipitation w a s carried out with oxalic acid from acidic solutions, oxalates of molar composition Ln: Ox ----- 1 : 2 were obtained. These were formed more readily by the heavy earths and yttrium, strongly acidic solutions being required for the formation of the light earth salts. Analysis of these compounds showed them to be double salts of the composition NH4LnOx2nH~O. The normal and double oxalates have been examined by X-ray methods, and their thermogravimetric behaviour has been compared. EXPERIMENTAL

Materials

All of the rare earth oxides were >99.9 per cent pure, based on spectroscopic analysis.~3~ Neutral solutions were prepared by heating a small excess of oxide with nitric acid, and filtering off the neutral solution; these were standardised gravimetrically. Other reagents were of A.R. quality. Solutions of oxalic acid, ammonium oxalate, and potassium permanganate were standardised against sodium oxalate. c1~j. N. FRIEND,Textbook of Inorganic Chemistry, Vol. 4, p. 242. Griffin, London. t,~ Report of the National'Chemical Laboratory, p. 42 (1960). ~s~R. N. KINSELEY,V. A. FASSELet al., Spectrochim. dcta 12, 332 (1958) and earlier papers. 931

932

M . F . BARRETT, T. R. R. McDONALD and N. E. ToPP

Preparation and analysis of oxalates Standard method of precipitation. Oxalic acid or ammonium oxalate are the recommended reagents, since there is no reagent residue after ignition to the oxide. An excess of reagents is added to a dilute lanthanon solution ( < 5 g/l Ln~Os), the precipitate coarsened by heating for 1-2 hr, and cooled. It is then ignited to the oxide at 850--900°C, and weighed. If the pH of the solution, after precipitation of the oxalate, is adjusted within the range 1.5 to 3.0, a 99.9 ~o lanthanon recovery is obtained. This has been standardised at pH 2"0. Precipitation.from neutral solution. Neutral nitrate solutions of Y, Nd, Gd, and Er were precipitated with varying quantities of ammonium oxalate. An immediate precipitate was obtained in each case. The procedure outlined above was followed, duplicate samples being ignited and weighed as oxide while other samples were dissolved in 6 N sulphuric acid for oxalate estimation. A normal oxalate was obtained in all cases. TABLE I.---COMPOSITION OF OXALATES PRECIPITATED FROM NEUTRAL SOLUTION

Reactant ratio, mole Ox ~-:mole Ln 8÷

Precipitate composition, mole Ox~-/mole Ln 3+ Neodymium Gadolinium Erbium Yttrium

1.25

1.53

1.50

1"51

1.49

1.75

1.49

1.52

1-51

1.55

2.5

1.51

1.52

1.50

1"53

3.0

1.49

1"49

It was found subsequently that the heavy earth oxalate precipitates usually contained small quantities of the double oxalate. Small quantities of the latter component could be detected more readily by X-ray examination than by classical methods. Precipitationfrom acidic solution. Nitrate solutions were acidified with concentrated nitric acid, and a small excess of oxalic acid added. No precipitation was observed until the pH of the solution was increased by the addition of ammonia, and, after precipitation was complete, the pH was adjusted to 2"0 in the usual manner. As shown in Table 2, the molar ratio of the precipitate approximated to Ln: Ox = 1 : 2, except with neodymium precipitated from 3 N and 4 N acid solutions. TABLE 2.--EFFECT OF ACIDITY ON COMPOSITION OF PRECIPITATES; REACTANT RATIO 2"5 m o l e O x S - : ] m o l e L n s+

Solution acidity (mole/l)

Precipitate composition, mole Ox2-/mole Ln s+ Neodymium Gadolinium Erbium

I

1.97

2

2.02

3

1.51

4

1.64

1.96

5

1.95

1.99

1"96 1.94

Precipitation by varying quantities of oxalate anion was examined, selecting the initial acidity in the range expected to give the double oxalate. The same composition was obtained irrespective of whether oxalic acid or ammonium oxalate was used for precipitation. As shown in Table 3, this salt was obtained when an excess of oxalate anion was added, but if less than the stoicheiometric quantity of oxalate was added, mixtures of the two oxalates were obtained. Composition of oxalates. Analytical samples were stored at 75 % R.H. for one week before analysis. Normal oxalates of the lanthanum-gadolinium elements were decahydrates, while the

933

Double ammonium oxalates of the rare earths and yttrium TABLE 3.--CONDITIONS FOR PRECIPITATIONOF DOUBLE OXALbTES Reactant ratio, mole Ox ~- : mole Ln 3÷

Precipitate composition, mole Ox~-/mole Ln ~+ Neodymium Gadolinium Erbium Yttrium 6M 2M 1M 1"5 M

1.25 1.75 2.5 3.0

1.57 1'75 2'02 1.96

1.63 1.95 1.94

1.55 1.62 1.97 1.95

1.99 2.00 2.04

heavier earths had a smaller and variable hydration. (4-8 mole HzO/mole oxalate). Double oxalates of the La-Nd and Yb-Lu groups had the composition (NH4)LnOx~'3H~O, while the salts of yttrium and the remaining rare earths were monohydrates of composition NH4LnOx2"H20. Some analytical data is given below. TABLE 4.--ANALYSIS OF SOME DOUBLE OXALATES

Element

Hydrate

Pr

3

Dy

1

Lu

3

Y

1

Ln203, (%)

C204, (%)

NHo (%)

43'7 43'6 49"1 49"7 46'2 47"0 37"6 37-4

44"9 45'1 46'9 47'0 41 "0 40"6 58"3 59"3

4"53 4"62 4"78 4'81 4.23 4.26 5"90 5'98

Found Theory Found Theory Found Theory Found Theory

On washing the salts with dilute acid (0-1 N) they were converted to the normal oxalates with some loss of material. This unexpected observation was used for preparing pure samples of the heavy earth normal oxalates.

Physical examination of oxalates Crystal structures of the oxalates. X-ray diffraction photographs of the normal and double oxalates were taken using a Debye-Scherrer camera of radius 5"73 cm and Ni-filtered Cu Kct radiation. Nine different powder patterns were observed in all, the principal d-spacings of which are listed in Table 5. Only two of these patterns have so far been indexed, namely, the lanthanum oxalate decahydrate type (column 1), unit cell dimensions for which have been published by GILPrS and McCRON# 4), and the yttrium double oxalate type (column 5). The lanthanum oxalate decahydrate type of structure (column 1) was found for the normal oxalates of Y and of the elements La-Tb inclusive. Some differences were observed in the powder patterns, but these were minor and there is no doubt that these salts form a homogeneous structural group. The normal oxalates of the elements Dy to Lu, on the other hand, had a lower degree of hydration (4-8 mole H20/mole oxalate), and gave three different X-ray patterns (columns 2-4). The Dy oxalate structure appeared to be unique (column 2) while the Yb and Tm salts gave very similar patterns (column 3), as did the pair Er and Lu (column 4). The photographs corresponding to columns 3 and 4 had several strong lines in common, and the structures of the oxalates may be related. Double oxalates of Y and the elements in the group Sm-Tm, having the general formula NH4LnOx2"H20, formed another structural group (column 5). In the case of the Y salt, single crystals were obtained, and a detailed structure analysis is in progresL, The crystals are monoclinic with unit cell dimensions a = 9"20, b = 6-09, c = 7"88 A, fl = 90-1 °, and there are two formula weights per unit cell in the space group P2/m. ~4~V. GILPn~ and W. C. McCRor,~, Analyt. Chem. 24, 225 (1952).

w w w -

mw

mw

mw

mw

mw

mw

mw

w w w

mw

m

ms

mw

I

Y, L a - T b Dy Yb, T m Er, Lu

IO.27 7-~1 6"65 5.19 4"99 4-82 4.35 4-24 3-73 3"53 3"42 3.05 2.99 2-80 2"74 2-67 2.62 2"33 2.28 2.25

d

d

5.84 5-23 4-86 4.67 4.15 3-49 3.37 3"11 2.87 2-80 2.61 2.56 2-17 2.10 2.05 1"91 1"88

Dy

Ce

Type 1 Type2 Type 3 Type 4

mw

mw

mw

mw

w w

mw

mw

mw

w

mw

w w w

ms

m

mw

m w s

I

2

1

ms

m w

vw

mw(d)

row(d)

w(d)

ms

mw

w s w m

mw

I

3

Yb

N o r m a l oxalates

9"25 8"68 6-00 5.73 5.43 5-01 4"77 4-56 4"17 3"87 3-48 3"08 2"98 2-89

d

iv

w

w

mw

Iv

ms

mw

m

W

m

m

m

iv

mw

m

s

/)s

ms

m

w

I

Lu

4

9"13 7"19 5'89 5"72 4'77 4"61 4-38 4.11 3"59 3"53 3"46 3-18 3.08 2"97 2'86 2-81 2-77 2.60 2"37 2"30

d

s w rn

ms

m

mw

mw

s s w m w

ms

vs

mw

vs

mw

ms

ms

vvs

I 6-03 5"08 4"82 4-59 4'27 3"93 3"66 3-32 3"12 2-99 2"90 2"85 2-74 2"68 2"59 2"53 2"42 2-33 2"30 2"24

d

Type 5 Type 6 Type 7 Type8 Type 9

Y

5

d

6.42 6-22 5"78 5"23 5"03 4"72 3-89 3"65 3-53 3'31 2"99 2.81

7.26

10-62

Y, S m - T m La, Ce Pr, N d Yb Lu

m m

mw

w w

mw

vw

m

ms

w w

mw

ms

m

I

Ce

6

ms

m w w w m

mw

w

mw

m m m

ms

ms

mw

I

Pr

7

11"59 7"26 6"42 5-72 5-40 4"91 4-69 4"38 3"93 3"81 3"61 3"53 3-28 3"21 2"99

d

D o u b l e oxalates

TABLE 5 . - - d SPACINOS OF IREPIRESENTATIVENORMAL AND DOUBLE OXALAT~

II

I

I

Yb

8

7"09 5"67 4"43 3"95 3"49 2-91 2-82 2"63 2"53 2"36 2'30 2.26 2"20 2"18 2"10 2.00

d

ms

w

vw

mw

w w w

mw

s s m

mw

I

Lu

9

7-66 7"02 6"34 4"96 4"39 4"15 3.92 3-69 3"47 3-21 2.91 2"80

d

.rn

g~

O

Double ammonium oxalates of the rare earths and yttrium

935

The remaining double oxalates, of composition NH~LnOx~.3HzO had different structures. The La and Ce salts gave identical powder patterns (column 6), and Pr and Nd also formed a pair (column 7), while the patterns in columns 8 and 9 were found for the Yb and Lu salts. These last two patterns contain many common lines and possibly correspond to related structures. Thermogravimetrie behaviour. The pyrolysis of the normal and double oxalates was compared by means of a Stanton thermobalance, using a heating rate of 100°C/hr. Results on the normal oxalates were in good agreement with the data of Wendlandt.c~m The decomposition of the double oxalates took place with one intermediate step except in the eases of Yb and Lu, where two steps were found. The single steps corresponded to the loss of either one or three molecules of water from the salts. This dehydration took place in two stages with the Yb and Lu salts. The final decomposition temperatures were all appreciably lower than those of the corresponding normal oxalates (Table 6). The decomposition curve of the yttrium salt is shown in Fig. 1.

I00

L

I

I

I

I

I

Wtlght, eli o f initial 75

~0--

25 tO0

I

t

I

ZOO

500

400

I

500 600 Tcmp¢rat ur¢ ~eC

700

800

Fzo. 1. Decomposition curve of yttrium ammonium oxalate.

TABLE 6

Double oxalate

Element

La Ce Pr Nd Sm Eu Gd Tb Y

Normal oxalate Intermediate Final decompn. decompn, decompn, temp.(°C) 130 130 100 100 280 200 200 250 250

700 300 550 600 680 600 650 650 700

800 350 800 740 740 620 700 730 740

15)W. WENDLANDT,Analyt. Chem. 30, 58 (1958). 16)W. WENDLANDT,Analyt. Chem. 31, 408 (1959).

Double oxalate Element

Dy Ho Er Tm Yb Lu

Intermediate decompn, 200 225 175 250 150 200 130 200

Normal oxalate Final decompn. decompn, temp.(°C) 625 600 600 600 620 600

750 740 720 730

936

M.F. BARRETT.T. R. R. McDONALDand N. E. ToPP

DISCUSSION The nature of the lanthanon precipitate obtained by the addition of an oxalate solution will be determined by the solubility products of the two oxalates, and the concentration of H +, NH4+ and Ox 2- ions in the solution. The ratio of the solubility products of the two oxalates (SN and SD respectively) is: SN

(La3+)(Ox2-)

S~)

(NH4 +)

(1)

while the dependence of oxalate ionic species on hydrogen ion concentration is given by the equation (Ox 2-) K2 (nOx-)

(H ÷)

(2)

where K 2 is the second dissociation constant of oxalic acid (6.1 × 10-5). In neutral ammonium oxalate solutions, the preponderant oxalate species is Ox v-. Normal oxalates of the light earths are obtained in this case, so their solubility products are less than those of the double oxalates. These differences are smaller with the heavy earths, since co-precipitation was observed. This situation is reversed in strongly acidic solution, where the major oxalate ion is HOx-. In the region where precipitation was observed, N H , + >~ Ox 2- so the double oxalate was obtained. It seems reasonable to conclude that the product obtained by precipitating mixed lanthanons with oxalic acid in the presence of NHa + ions is usually a mixture of the normal and double oxalates. Some confirmation of this was obtained by precipitating a series of lanthanum-erbium mixtures from acidic solutions. X-ray powder photographs showed the presence of both normal and double oxalate structures, while the oxide-oxalate ratio corresponded to precipitation of lanthanum as a normal oxalate and erbium as a double oxalate. The structure analysis of YNHaOx2.H20 has proceeded far enough to establish the main features of the structure and to provide approximate values for the interatomic distances. The atomic co-ordinates are being refined by a three-dimensional least-squares method and full details will be published when this is complete. At this stage in the refinement, the bond lengths and angles of the oxalate ion appear to be in good agreement with the results of earlier investigations of oxalates and oxalic acid.CT,s,9~ There is also nine-fold co-ordination around the yttrium atoms, as found in Y(OH) 3 and YF3.~I°, 11~ The double oxalates of lanthanum and yttrium have been prepared by BRYZGELOVA and CHERNITAKAYAf112) They were also prepared by precipitation from strongly acid solutions. Acknowledgement.--We would like to thank F. H. Howm and J. C. OWENfor help in the experimental

work. This work formed part of the programme of the National Chemical Laboratory, and is published by permission of the Director. c7~E. G. Cox, M. W. DOUOILLand G. A. JEFFREY,J. Chem. Soc. 4854 (1957). ~s~G. A. JEFFREYand G. S. PARRY,J. Chem. Soc. 4864 (1952). ~9~F. R. AlliED and D. W. J. CRUICKSHANK,Acta Cryst. 6, 385 (1953). cl0~A. ZALKINand D. H. TEMPLETON,J. Amer. Chem. Soc. 757 2453 (1953). c11~R. WYCKOFF,Crystal Structures Vol. 2. Interscience, New York. I]2~ R. V. BRYZG'ELOVA and I. V. CHERNITSKAYA,Radiokhim, 3, 478 (1961).