Studies on hydrolytic polymerization of rare-earth metal ions—V Hydrolytic polymerization of Dy3+

Talatm, Vol 37, No 3, pp 357-360, 1990 Prrnted m Great Bntain Ail nghts reserved

Copyright 0

ANALYTICAL

0039.9140/90 $3 00 + 0 00 1990 Pcrgamon Press plc

DATA

STUDIES ON HYDROLYTIC POLYMERIZATION RARE-EARTH METAL IONS-V HYDROLYTIC

POLYMERIZATION

QINHUILuo*, MFNGCHANG SHEN,YI DING,XINLU Coordmatlon

OF

OF Dy3+ BAO

and ANBANG DAI

Chemistry Institute, Nanjing University, NanJmg, People’s Repubhc of Chma

(Recezued 20 March I989 Rewed 23 Augi*st 1989 Accepted 29 September 1989) Summary-The hydrolytic polymerlzatlon of Dy’+ was determmed by the eqmhbrmm-pH method The concentration of Dy3+ was varxd from 0 1to 0 6M The cornposItion and hydrolysis constants of the Dy’+ hydrolysis products were obtained by a graph& method and then refined by computer fit&g and w analyszs The results show that the speczes m the Dy’+ solutron are py(OH)12+, [Dyz(OH),y+ and [Dy,(OH),]*+, but the last of these 1s a minor species The behavlour of Dy’+ IS the same as that of Er’+ and Yb3+ but different from that of the medmm lanthamde ions Sm’+, Euj+ and Gd3+

Hydrolyttc polymerization of the lanthamde tons 1s closely related to the separation and determmatlon of the lanthamde elements, treatment of nuclear fuel and environmental protectlon, and has been extensively studted.‘-‘s The lanthamde tons have not yet all been studied, so the regularity of their hydrolysis IS still not clear The light lanthamde tons have been studied more than the heavy ones, and although Dy3+ IS an Important lanthamde ion, no detailed studies on Its state In solution have been reported. We have tned to study the regularity of hydrolysis of the lanthamde tons and have made systematic studies of Pr3+,16 Sm3+,” Eu3+,‘* Gd3+,” and Yb3+ and Er3+ I9 We discovered that the hydrolytic polymertzatron of lanthamde ions is not an instantaneous process, so we studied It by the equihbrmm-pH method In the present paper, we first estimated the hydrolysis model of Dy3+ by a graphical method and then used computer fitting and pq analysis*’ to screen possible species one by one, and thus studied the state m solutton m detail and obtained the hydrolysis constants. The results showed that the mam hydrolysis products of Dy3+ may be expressed as [Dy(OH)]$+, with n = 1, 2, 3. These are different from the hydrolysis products of the medium lanthamde ions Sm3+, Eu3+ and Gd3+ which we have studied, but the same as ~. *Author for correspondence 351

those of Er3+ and Yb3+, perhaps because the 4>’ orbitals of the heavy lanthamde ions are all at least smgly occupted. EXPERIMENTAL

Reagents Dysprosmm sesquroxtde, purity 99 95%, Yaolung Chemtcals Factory, sodium nitrate, analytical grade, recrystalhzed twice from redistllled water, sodmm hydroxide, analytical grade, treated by the method of Powell et a/.*’ All other chemicals used were analyttcal grade.

A Cornmg pH-meter, equipped with a Corning glass-Ag/AgCl combmatlon electrode, prectslon +O 1 mV Data were calculated by the program LEMITZ2 23on a Honeywell DPS 8/49 computer. Procedure A known wetght of Dy2O3 was dtssolved m mtnc acid, and the Dy determined by EDTA titration The excess of nitric acid m the solution was determmed by titration with CO,-free sodium hydroxide solution. Solutions with 0.100, 0.200, 0.400 and 0.600M Dy(NO,)3 concentration were prepared, enough sodnun nitrate was added to make the nitrate concentration of all solutions 2.0&f, and the pH

ANALYTICAL

358 Table B = 0 200M, V NaOHv ml 0 100 0 200 0 400 0 700 1000 1 400 1 900 2 400 2 800 3 200

1 Typlcal

data

DATA

for log h and Z (NaOH

concentration

0 03325M)

V = 50 00 ml, H,, = 2 85 x IO-‘A4

B=O6OOM,

E, mV

10’ x Z

VNaOH, ml

E, mV

0 223 0 542 1 20 2 19 3 19 4 52 6 18 7 84 9 17 1050

0 160 0 320 0600 1 100 1600 2 200 2 900 3600 4 300 5 100

137 2 1308 119 1 1106 1044 97 5 95 1 92 0 904 87 7

131 6 1184 107 5 94 8 89 8 84 4 80 0 78 2 76 7 75 2

-log

h

5 180 5404 5 588 5 803 5 887 5 978 6 053 6 083 6 109 6 134

V=25OOml,

H,=855x

lo-‘M 10’ x z

-1ogh

0 226 0 578 1 19 2 30 341 4 74 6 29 7 84 9 39 11 16

5 086 5 194 5 392 5 535 5640 5 757 5 797 5 850 5 877 5 923

was then adjusted by adding standard sodmm Typical data are shown m Table 1. Figure 1 hydroxide until precipitation Just began The shows a plot of Z US.-log h for soluttons with solutions were then diluted to a preselected different Dy3+ concentrations. volume and stored m polythene bottles at 25” in The family of curves m Fig. 1 Implies that a nitrogen atmosphere The potential, E, of the Dy3+ IS hydrolysed and partly polymerized at a soluttons (m a nitrogen atmosphere) was mea- given Dy3+ concentratron and acidity according sured at 25 _t 0 01” at intervals of several hours to the equation until eqmhbrmm was thought to have been qDy3+ +pH,O z$ [Dy,(OH)P](3q-P)+ +pH+ (3) established, z.e , when the successrve measurements of E did not doffer by more than 0 5 mV The equrhbrmm constant 1s The relattonshrp between the equrhbrium potenBP,= Py,(OW#W’ (4) tial E (m mV) and the hydrogen-ion molar concentratton h can be written m the form24 where b denotes the equrhbrmm concentration of Dy3+, the charges of species are omitted for E=E0+E,+59.1510gh simplicity. where E, 1s a constant, different for each type of From Fig. 1 it IS seen that the Z values are cell, and E, is the liquid-Junction potential, small m the experimental pH region but the which IS an approximately linear function of h. total concentratton of Dy3+ IS comparatrvely To determine E, + E,, a 2M sodmm nitrate high, so we have solutton with a known concentratron of mtrtc BZ = c 1 p& BqIhP = c pK,JhP (5) acid was prepared, and titrated with sodium P 4 P hydroxide solutton, and the measured potential where E was plotted against the hydrogen-ion concentration calculated from the volume of tttrant Kp=CBpqB” 4 added and the rntrrc acid concentratton For our system, we obtained the emptncal equation K, 1s a homo-hgand constant concerned with E=43808+59

16logh

(1)

12

Determinatron of hydrolysrs products by the graphrcal method”” 8

The average number (Z) of hydroxyl groups bound to each Dy3+ ton was calculated by means of equation (2) with h obtained by use of equation (1) Z = (h + MOH- HJB

s x 4

6 4

(2)

where MO,, denotes the molar concentratton of the added sodmm hydroxrde, H,, denotes the nitric acid concentratton m the Dy(NO,X solution before addltron of the sodmm hydroxide, and B denotes the total concentratron of Dy3+.

0

50

55

60

-log Fig

1 Plotofzus

h

-1ogh B 1,01~,2,02~,3,04~, 4,06M

65

ANALYTICAL

4

5-

54+22 I-6961

40+60 l-13 12)

36+04 (-19 16)

38+50 t-2520)

40+00 (-31 21)

4-

43+67 l-7 211

26+36 (-13 351

24+52 (-19 41)

24+16 l-25 441

25+77 f-31 46)

29+90 l-7451

12+70 l-13 59)

i-19

12+60 I-25721

3 l+60 l-31 60)

14+10 t-7711

0+44 (-13671

15+70 l-20 01)

67+10 t-26 231

133+50 l-32 63)

20+00 tt-6 05)

56+50 l-14 361

124+00 l-20 821

167+00 (-27 36)

m4’00 (-33 90)

3-

201

02

04

06 1

8

I

-2,_(b) B 5

6+60 66)

I

I

I

1

2

3

0

-2oA -19-

s

A

0

0

A

A. .

.

.

’ 01

4

5

Fig 3 U and log b values for [Dyq(OH),](‘~-P’+

. -16

I

P

0

b I L” 0

359

DATA

I

I 04

02

I 06

e

Fig 2 Plot of (log K,, - q log B) us B (a) p = 2, (b) p = 3, 0,4=2,A,q=3,O.q=4

the concentratron of the metal ion and the values of p. Experimental curves were obtained by plotting BZ us. -log h values m Table 1 and were fitted with normalized curves for assumed numbers of hydroxyl groups. Species with p = 2 existed over the widest pH region, so [Dy,(OH),](3q-2)+ species are predominant. Although species withp = 1 andp = 3 existed m a narrower pH region than those with p = 2, they also fitted well. Existence of species wrth p = 5 could not be confirmed because there were not many fitting points. K, values were obtained by fitting. Only one kmd of species exrsted m certain pH regions, because of the low degree of hydrolysis of Dy3+, so we have log KP - q log B = log &,

(7)

When (log KP - q log B) IS plotted vs. B for various q values, only the line with the correct q value will be parallel to the absctssa, and thus correct q and &, values can be obtained. Very good fitting was obtained for p = q = 1, p = q = 2 and p = q = 3, but it was drfficult to establish a fit for p = 5, q = 3, so the computer Table 2 Hydrolysis

was used to screen possible specres. Some of the fitting results are presented m Fig 2. Because only one kmd of specres predomrnates m solutron, we have Z = pfiwBq- ‘/hp, so by plotting Zus. l/P and extrapolatron, rS, 1s also obtained The /?, values obtained by the two methods are summarized in Table 2. Computer JittmgzO*“”

Each expenmental pomt should satisfy the following general equations G, = ? p,B,,q,b:+rP + 4

(8)

/=I

TM,=

“c” q,Bp,q,WW + h

,=,

”

(9)

, N) denotes the ordinal where i (I = 1,2, points, J of the expenmental number , NK) denotes the ordinal number (I = 132,. of the complexes, TH, and TM, denote respectively the total acidity and total Dy3+ concentration correspondmg to the zth expenmental point. First, estimates of the fl values, the hydrolysis constants of the complexes, are input, then equations (8) and (9) are solved rteraNewton-Raphson successive trvely by approxrmatton to obtain h, and b,, and next the TH, values are calculated by equation (8) to grve T$. The error-square sum, U, for a grven /? value IS calculated by using Z”$ and the corresponding experimental value T;;P *

(10)

constants of Dy’+, at 25°C (u = standard dewatlon)

m 2M NO< medmm

Method

log@,, f 30)

log&, f. 30)

log@,, f 30)

Homo-hgand constant Extrapolation Computer-fitting

-8.55&O 11 -8.48 *O 17 - 8.63 f 0 24

-1388*008 -1390&007 -1388&005

-1960*035 -1954kO38 -1953*045

360

ANALYTICAL

05r

I

0 50

42

54

56

58

60

-log I, Rg 4 Dlstrlbutlon plot (B = 0 4M) for 1, [DY~(OH),]~+,2, [DY(OH)I*+, 3, [Dy@H),16 ’

DATA

m previous papers ‘o-‘3it is seen that the three representatives of heavy lanthamde ions behave m the same way and may be expressed as IWOW?+, Ln = Dy3+, Er3+, Yb’+, n = 1, 2. 3 Earlier papersa*9 reported that the major hydrolytic products of Sm3+, Eu3+ and Gd3+ are [Ln(OH)]‘+ and (Ln[Ln(OH),],)(3+“)+, n = 1, 2, 3, which are different from those of the heavy lanthamde ions. This may be due to the fact that the 4f orbitals of Dy3+, Er3+ and Yb3+, being more than half-full, decrease the iomc radu still further, so the hydroxylation number and degree of polymerization also decrease. The values reported are conditional constants, since they will be affected by any formation of nitrate complexes of Dy3+ REFERENCES

First it is assumed that only one kmd of complex exists m the solution, and the U values are calculated by equation (lo), with p and q changed one by one, and the calculated U and log/l values are displayed as a function of p and q as shown m Fig. 3, where the values of lo6 x U and log j? (m brackets) corresponding to given p and q values are displayed for [Dy,(OH),]“4-P’+ From Fig. 3 it is seen that U is least for @, q) equal to (2,2) and (3,3) Combining these species one by one with species (l,l), (2,3), (4,3) and others with rather small U values gives log U = -6 6606 for the species combination (l,l), (2,2) and (3,3), and this is the lowest U value When (5,3) is added to the combinatton system or substituted for (3,3), divergence is obtained, so (5,3) is eliminated and the combination of (l,l), (2,2), (3,3) is the most probable model It also has the least Hamilton R-factor (0.422 x 10e3), which again implies that it is the best model. The log@ f 30) values obtained are presented m Table 2 An F-test showed that the results obtained by the three methods are not significantly different (90% confidence hmits) and are all acceptable. From Table 2 it is seen that log & has the lowest value, and from the graphical fitting that species (2,2) exists m the widest acidity region and is therefore the major species. Species (3,3) exists only m a comparatively high basicity region and m small amounts, so is the minor species. A distribution plot of the species m solution as a function of -log h is shown m Fig. 4. By comparmg the values for Dy3+ species in Table 2 and those for Er3+ and Yb3+ reported

G Bledermann and L Clavatta, Acta Chem Stand, 1961, 15, 1347 G Bledermann and L Newmann, Arkrv Kemr, 1964,22, 303

K A Burkov, E A Bus’ko and I V Pxhugma, Russ J Inorg

Chem

, 1982, 27, 643

K A Burkov, L S LIhch and Nguyen Dm Ngo, lbrd , 1973, 18, 1513

K A Burkov, E A Bus’ko, L S Llhch and Nguyen Dm Ngo, Izvesr Vyssh Ucheb Zaved SSSR, Khlm I Khrm

Tekhnol, 1983, 26, 771

6 K A Burkov, L S Llhch and Nguyen Dm Ngo, lbfd, 1975, 18, 181 I Nguyen Dm Ngo and K A Burkov, Russ J Inorg Chem

, 1974, 19, 1249

8 L N Usherenko and N A Skonk, rbrd, 1972,17,2918 9 T Amaya, H Kaklhana and M Maeda, Bull Chem Sot Japan, 1973, 46, 1720

10 J Kragten and L G Decnop-Weever, Talanta, 1979, 26, 1105 11 Idem, lbrd, 1980, 27, 1047 12 Zdem, rbrd, 1983, 30, 131 13 Idem, Ibid, 1982, 29, 219 14 L Clavatta, M M Iuhano and R Porto, Polyhedron, 1987, 6, 1283

15 N A Kostromma and Yu B Badaev, Ukr Khlm Zh , 1986, 52, 793

16 Qmhm Luo, Menchang Shen and YI Dmg, J NanJmg Umverslfy (Nut Scr ), 1983, 429 17 Qmhul Luo, Mengchang Shen, YI Dmg and Zhongdm Chen, Acfa Chum Sowa, 1983, 41, 1115 18 Qmhul Luo, YI Dmg and Mengchang Shen, Chem J Chinese Umversrrtes, 1985, 6, 201 19 Qmhul Luo, YI Dmg, Mengchang Shen. J Chrnese Rare Earrh Sot , 1988, 6, 73

20 L Petterson, I Andersson, L Lyhamn and N Ingn, m E Hogfeldt (ed ), Coordmarron Chemlsrry tn Solution, p 116 Berhngska Bostryckenet, Lund, 1972 21 J E Powell and M A Hlller, J Chem Educ, 1957,34, 330

22 Qmhul Luo, Mengchang Shen, Y1 Ding, Qmgyun Tu and Anbang Dal, Acta Chum Swca, 1986, 44, 568 23 Idem, rbrd, (English edition), 1987, No 2, 144 24 S Hletanen, Acta Chem Stand, 1954, 8, 1628