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ANALYTICA CHIhf ICA ACTA 251

THE DETERMINATION OF THE STABILITY CONSTANTS OF THE

LANTHANIDE a-HYDROSYISOBUTYI2ATE AND LACTATE

COMPLEXES BY POTENTIOMETRIC TITRATION

I-I. DISELSTRA AND P. VERBEISK

Labordory for Atrulylicnl Chotrisfry. Ghctrt UtriversiLy, Ghcttf (13elgitcm)

(licccivad Jzmuary rst, 19G4)

The K-hydroxyisobutyrate and lactate ions arc known to form stable soluble com- plexes with the lanthanidcs. CHOPPIN ANI) CHOPOORIAN~ reported vulucs of the sta- bility constants for the first 3 complexes of 9 lanthanidc elements and yttrium. However, their, as well as other, investigations on the same subjectze3 provided eviclcnce that the isobutyratc ancl lactate anions form unincgative tctra-ligand com- plexes with the lanthanitles. It was also shown by EIXKHAUT cl al.4 that other cu-hydroxycarboxylatc lignnds as mcthylcthyl- and methylpropyl glycolatc form ana- logous complexes of the type ML4- (M = trivalent lanthanide ion; I, = organic ligand).

In the work described here, the determination of the 4 stability constants of the lanthanidcs and yttrium has been performed potentiometrically, using a glass-calo- me1 electrode system and the CN_VIN - WILSOS titration rncthod~, while CHOPPIN AND CHOPOORIAN used quinhydronc electrodes and FIIONAEUS’ titration tcchnique0.

The calculation of R, the average number of L- bound to MS+, has been carried out by a modification of the method used by CALVIN ANI) WILSON~ for MO+ and con~plcxes ML2. [L-) has been calculated from the apparent clissociation constant of the acid’.

The stability constants arc derived graphically from corresponding fl and [L-] values by B J ERRUM’S llalf-+I mctllodfl, FHoNAEus’~ and PouLsEN’s” extrapolation methods, and by means of a computer program, using the IBM 1620 digital compu- ter4* 10.

3>

Lactic acid, analytical grade from BDH, was dcclimcrizecl by completely neutra- lizing with soclium hydroxide and then removing the sodium ions with a DOWCX

50 column in the hydrogen form. The lactic acid used gave a negative dimer test. Other reagents were the same as in a previous invcstigation4.

A@arcttlrs and firocedwe

A Radiometer PH M-4C potentiometer with a G 202 33 glass electrode and a K IOO

f~rld. C/ri)rr. AC/cl. 31 (1964) 251-257

252 N. DEELSTRA, I:. VBRUEEK

saturated calomel electrode wcrc used. The glass clcctrodc was standardized against NBS standard buffers at 25”.

All titrations and PH measurements were made in a thermostated sample cell at 25.0 &- 0.1’. Rapid mixing was achieved by magnetic stirring which was stopped during the measurements.

l’itration series of the perchlorates of the lanthanidcs were performed once in q ml of o. I Mar-hydroxyisobutyric acid and z ml of 0.320 N pcrchloric acid (in the presence of 0.2 M sodium perchlorate) with r.gxo M sodium hydroxide solution. The metal- acid ratio was x/20. Other titration series were carried out in 25 ml of 0.3 Mcu-hydro- xyisobutyric acid (lactic acid) and 2 ml of I: .gGo M perchloric acid (also in tic presence of 0.2 M sodium perchlorate), with z.g8r M sodium hydroxide solution. The mctal- acid ratio was I/45.

Each titration series was at least duplicated.

RESULTS

FRONAOVS~~ has proved that when formation of complexes of the type MUL,& (y > I) occurs, the formation curve is dependent on the metal ion concct~tration, Preliminary experiments at different metal ion concentration for some lantllanidc-cu-hydroxy- isobutyrate compicx systems showed that polynuclear complexes are not present in me:tsurable amounts. This agrees with similar observations of CHc~xal~h’ ANn CnoPoo- RIAN l, and SONKSSON for tlrc lanthanidc-glycolate complexcsir.

Special attention was also paid to the effect of ionic strength. The results for yttrium and ytterbium, using increasing amounts of sotlium pcrchloratc, arc listed in Tables I and II. The total ionic stren@h (column 2) was calculated from the con-

I__ _ _.l__._.-

MO1 NaCiO.~ ItIM lng KI log K2

._ ---...-_._

$I 0,075 3.21 2.60 o.oH I 3.21 2.SQ.5

0” O*OQ I 3.23 2.m

0.056 O.lI() 3.13 2.55 0.051’ 0.320 3.13 2.55 0.X” 0.170 3.x I 2.50 0.1 c 0.179 3.13 2.48

0.2’ 0.252 3.12 2***7 0.21’ 0.2f-32 3*r4 2 . ,I 3 0.20 0.273 3*r3 .2.4’2 0.5” 0.520 3.00 2.49 0.5” 0.525 3.03 2.41

1.0* 0.0% 3ml l.‘lG I.00 0.973 3.oG 2.41 2.0” I.Htix I 3.x 2.43

0 CIl/CllL = f/ZO. b G&III, = t/ro. c) CntfCrlL - 115‘

STABILITY CONSTANTS OF LA&XHAXIDE COMPLEXES 253

centration of the different metal salts in solution at pw = pK,\ of the acid. By plotting the log & values against the square root. of the ionic strength, it can be seen that the stability constants remain approximately constant from an ionic strength of 0.2.

All other titrations were performed in 0.2 M soclium pcrchloratc.

nf0r SaClO., - _*---l”-----__L ----- - I_-_-_---__._._

0“ 0.079 34” ;r.H3 I.Lc, 1 *t;i, to.18 0.05” 0.1 I!, 3 . ‘2 G 2.7(J 2#;‘7 I.77 IO.00 0.05 ” o.t23 3.385 r*HI z.235 I .t.,(J 1o.q

Some titration curves of the s&C5 using 0.3 fir ~-11~droxyisoI,uCyric acid togcthcr with a blank titration in the absence of metal (curve? A) are given in Fig. x.

The stability constants derived from the experimental ,7 anti [I-:] values by mc’ikns

of the 4 procedures, arc represented in Table III. The results of the lnntlianidc- lactate complexes are given in ‘Table IV.

When allowance is made for the differences in titration technique, calculation methods and ionic strength, the results given in Tables III and IV agree well with those obtained by CHOPPIN END CHOPOORIAN~ for the first 3 constants. Also the fourth constant log & of the complex ML4 - is calculatccl with an acceptable dcvintion (co- lumn 6).

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25fi H, IXZELSTRA, F. VERI3EzEK

_-_- - . ..,.- - ____- __-.-__.--.- I_ .._..--_-- _ - .--..---- -__“- -..- -..._ ..__..“..._ I3e~?lcrlt log It’1 fog K2 fog I<3 cog I(4 log P4 -___ _ -_-__ .__-

IhI :t 2.83 I.94 i .oG O.l?C 6.00

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Sm :t 2.Hl I *Qt I *oq 0.13” 5.w

ta Z.sr> :kCJ.(>_j5 z.o.* -& 0.005 O.C)O f 0.12 0.4’ & 0.25 5.91 & 0.30

6

5 -L

Atomic number

The logarithms of the over-all stability constants of the 1,znthanicles withcu-hydroxy-

isobutyrntc and lactntc ligancls were plotted against the atomic number (Fig, 2).

The log &II, va1uc.s of the ctllyler~ec~inminctetr3ncct:tte-lcznthanide complexes found by ScxwAxzrwn4uzH and co-worlicrs~~~~~, are also given in Fig. 2. The catiot1-cxchangc elution clata give the same pattern 111, showing that the separation is mainly determined

by the ratio of tllc stability constants of the complcxcs involvecl.

STABILITY CONSTANTS OF LANTHANIDE COhlPLESES 257

The authors wish to express their thanks to Prof. Dr. J. HOSTE for his kind interest

in this work. We are also indebted to Mr. W. VANDERLEEN for his help with the IBM

computer, and to Mrs. I;. VAX DEB ABEELE for technical assistance. This investi- gation has partly beensponsored by the Interuniversity Institute for Nuclear Sciences (I.I.K.W.), Belgium.

2%~ stability constants of the lanthanidc and yttrium cornplcxcs with a-lrydroxyiuol,utyrntc at~d

lactate, have been dctcrmincd by potcntiomctric titration. Tllc average number of ligands bound per metal ion has been calculated by the method of CALVIX --WILSON and indicates the formation of unincgative tctra-ligand con~plcxcs of the form MI-J-. l’hc .) formation constants have been derived by 1 proccdurcs: BJ EKI~UXI’S half-ii-method. ~:RONAEUS and POULSIIN’S extrapolation methods, and by least squares calculation, using ail IMl I Gro digital computer.

IiBSU;\ll:1.

IAS autcurs ont ddtcrrnind par titrngc potcntiorndtriquc Its constantcs de stabilitd dc complcxcs de lanthanidcs et de I’yttrium (n-hvtlro?csisobut~r~Ltes ct lactates). i.cs calculs cIfcctu& par la mdthodc de CALVIN--WILSON incliqucnt la forliiutioli tic coniplcxcs tlu type Ml,4-.

Die Stabilit:itskotlstnrltcll von I<cmq~lcxcn dcr Lnnthanidc untl tics Ytztriums mit m.-Hytlroxyiso- lJUtyrat Ulld hchL~ WUrdcn nlittcls I’otcntioinctriscIl~r ‘L’itrihon bcstiiilnlt. I)iC Iilit~hXt! %ahl dcr

I,igandcn die pro Metallion gcbuntlcll wcr&m, wurtlc nach cior iVkthock von (TALVIN- WILSON bcrcchnct und diu Uildung von b1L.a’ fcstycstcllt. IXc ~3iItlungskollstantcll wurtlcn nnch .I vcr- schicdcncn \‘crfnlircn crmittult.

1 G. ii. CIIOPPIN AND J. h. CIIOPOOHIAN, J. Inorg. 6, A’ucC. Chrnr., 2.t (1961) 97. 2 A. ~ONISSSON. ,‘fCfa ChJJ. .‘klJMi., 13 (1959) l&)37. 3 1,. ~IOLM, G. I<. ~IIOIDI’IN AND L). bIOY, J. fPlot’&‘. L’& I\‘w[. C/IO~J., I() (rc>Gf) 251.

J I,. ~~cKHAUT, 1:. VRRBISIJK, 1.1. I~ISELSTRA AND J. kiOSTIS, .*1m/. c/JiPJJ. .41ch, 30 (lgc~.)) 369. 6 hl. CAI.VIN AND I<. WILSOS, J. Atn. Clwn~. Sot., 67 (10.45) 3003. a S. htOSAlZUS . , .4cla C/rem Scatad., r) (10,50) 72; 5 (1951) 139: G (195r) loo, 1200.

’ 1-t. I)ISISLS’I’RA ASD 1:. VE~IIEI:K, Bit/l. Sot. Chinl. iielgcs, 72 (19G3) (JI’L.

8 J. 13~ wzHuh1, Mrld n J?JwJiJJc ~70YwJutioJJ iJJ .d qJJcoJfs .!k~irlioJJ, 1’. I-Inasc, Copcnhngcn, I+) I,

Q K. G. I~OUISISN, J. I~JI~RIIu~I AND J. I’OULSISN, ,?cla Circnr. .SccJ#~d., H (19.54) 9r I. 1” kl. ~>IMLSTRA, \V. Vh~Di~Rl.Rl~N AND I;. \‘IZH131312K. Biill. Sot. Cliiw. I~elgcs, 72 (19G3) 632. 11 A. SONISSSON, /fcfa Clww. Sccrnd., 12 (1958) 105, 1937; 13 (1959) 99s. 12 G. %~WARZ~J~IACH, 13. Gbr AND G. ~NDICHI!GC, ~frh. ChirJJ. ActcJ, 77 (195.1) 937. 13 8. ~‘lIISIXWHIGIiT, 1:. i-1. SI%lmlNG AND G. ~CliWARiX~NI3ACil’,~ . Ant. i’/wr.:soc., 75 (1953) 4196.

1J 1-1. I>UI’.LsTRA. ‘I’/Jesis, Gllcnt, 1963.

.4riul. Cliiur. .4c!a, 31 (rgG,O 251-257