378 ANALYTICA CHIMICA

‘I-HE STAl3ILITY CONSTANTS OF THE ALKALINE EARTH LACTATE a-HYI)ICOIYYISORUTYRATE COMPLEXES

ACTA

AND

The USC of ammonium lactate and a-hydroxyisobutyratc solutions for alkaline earth separations by cation-exchange chromatography has been discussed by several invcstigatorsl-0. BAT:RC AND BARTHOLOMDW~ and WISH” dcvelopcd a procedure for the separation of calcium, strontium and barium from the rare earths and from each other by clution with ammonium a-hydroxyisobutyrate solutions. I’OLLAI<D et al. separated all 4 alkaline earth metals, magnesium included, using lactate6 and a- hyclroxyisobutyratc”. It was stated that the elution was in the order magnesium, calcium, strontium and barium.

This phenomenon can bc explained by the decrease of the stability constants of the complexes involved with increasing atomic number, but only the K1 values for the lactate-alkaline earth complexes arc reported in the literature7 and these values indicate a decrease of the stability in the order: calcium, magnesium, strontium and barium. To explain this clisaqcement between the elution order and stability of calcium and magnesium, and also to complete previous work on the stability of lactate and &lydroxyisobutyrate complexes with the lanthanide9, in this investiga- tion the determination of the stability constants of both ligancls with the alkaline earth metals is described.

A Kcithlcy 6Go guarded differential voltmctcr was used. Potentials were measured to 0.01 mV.

Other apparatus was as previously describeclD.

Alkalim earth fierchlorate solutiom. The solutions were prepared by dissolving the allcalinc earth carbonates (Merck, analytical grade) in pcrchloric acid. The small amount of acid in excess (2-G mM) eras determined by potentiomctric titrations in a nitrogen atmosphere. The concentratiolz of the alkaline earths (ca. 50 mN) were determined by EDTA titration.

Ovgn&c acid buffer solutions. These solutions were prepared as beforeo. The buffer ratio (r lactate

=: CCHLo/C~o was respectively : dFILO = 0.971 M, CI,O = 1.003 M; 6 = o.c$S

a-hydroxyisobutyratc : CHL” = 1.011 M. CLO = 1.000 M; S = I.OII

R?lcll. Chinr. A cfn, 33 (I&i) 378-383

STABILITY CONSTANTS OF ALKALINE EARTH COMPLEXES 379

Procedure Some modifications were introduced in the procedure previously described.

Instead of CHOPPIN’S arrangement10 with 3 half-cell compartments in series, FRO-

NAEUS’ technique11 with z compartments was used. The potential differences, Enr, and EAC, between the.quinhydrone electrodes in the reference pcrchloric solution in compartment A (concentration HA) and rcspcctively the metal-ligand solution in compartment B, and the I M sodium perchlorate solution in compartment C were determined by z separate titrations. Every titration series was at least duplicated; the potentials were stable and could usually he reproduced within 0.05 mV.

E,UJ potentials were corrected for change in ionic strength (dl) during the titration, due to complex formation in compartment B.

Ei\u’ = EAI1 + dL;AII where AEAI, = !z Al

where /2 is a constant depending on the nature of the ligand. For M2+, d1 is given by

(1)

AI = 0.5C~fr’i(s - 9i) (2)

/c was determined by adding known amounts of sodium pcrchlorate to cell B and measuring the potential differences involved’“.

I; -AU, EAC, k, HA, 6, CLO, ChlO, CIIU (concentration of pcrchloric acid in com- partment B) and volume data were supplied to an Il3M rGzo digital computer which gave tile corresponding [L] and pi va.lucs. For the calculation of [L] tllc full equation was used

-

1 [ *+

Cu- [.Hlu _---

[L) = EC bcr, + Cl1 - [If])1 + r_H]c Cl. 1

-

- [Hjc ,‘i__-

CL 1 (3)

Although calculation of [I-] according to cqn. (3) dots not require the nume-rical value of the dissociation constant of the organic acid, KA values were calculated from the E*c potenti&

KA CL + IH1t-z

= [W err,, _ [Hlc (4)

Other conditions (t=zg.ofo.~“; I= I), and the calculations of the stability constants by means of an IBM xG20 computer were as before”.

RESULTS ANI) DISCUSSION

Table I presents an example of the cxperimcntal data and the calculated values of AE,,\o, KA, [IL] and 6 for the calcium-lactate complex system as a function of added buffer solution. All other,titrations were performed in the same manner. The KA value of cu-hydroxyisobutyric acid varied from I -73 - x0-4 to I .3g l x0-4 during the titration.

The formation curves, re;,resenting fi as a function of pL( = -lo&L-]) of the alkaline earth complexes with lactate and a-hydroxyisobutyrate are reproduced in

Aanl. Chirti. Ada, 33 (rgGg) 378-383

1:. VERBEEK, H. THUN

initial y~li~J*c’ := Jo ml) _.

I~‘oluw2c of lilrrurt (Id) _---..- _.__. .

0.10

0.20

0.3lo

O.,)O

0.5”

. I:Al,(111 If) IS a,:( HI I,‘)

0.007 JOZ.r),5 O.OJ!j JOz.oc) 0.02 I 101.82 o.or7 JO1 .‘q 0.032 IOJ.CJj

_..... __ 2.2m

2.297 2.305 2.3oH 2.309

6.5 I 0.05G I~.61 O.JZI 22.78 0.17’) 30.9 1 0.232 3~.00 0.282

o.bo 0.7” o.Ho 0.90 I .oo

0.037 0.o.t I 0 .o.t .t 0.0.tX 0.05 I

1OJ.C10 JOJ.pJ 101.58 JOJ.!j’) IOJ.CJO

0.329 a.373 0.414 0.453 0.489

I .20 97.21) 0.0.56 lOl.CJJ 2.zgH q3.12 o.ssG I.40 97.(‘1, 0.000 JOJ.c,j 2.2q5 JV7.87 0.01 I 1 .Cio 97.96 OdJj IO1 .(iIJ 2.2<)2 122.10 o.Gfq I .Ho 9H.23 O.Ol)fJ 101.7” 2.2HH 13!‘.08 0.7x 2 2.00 (JH.47 O.dlH JoJ.7‘I 2.28.3 149.G3 0.755

2.25 2.50 3 * 00

3.50 ‘1.00

4. SO 5.00 5*SJ G.oo 7.00

H.OO q.00

1O.OO 12.00 I .t .OO

1 cJ.00 J 8.00 ‘LO.00

98.73 0.070 101.79 2.278 160.02 O.&A 98.97 0.072 IO1 .q 2.273 18J.c)J 0.848 99.37 o.cl7.t IO1 .cj.) 2.2G.t 212.00 0.92G ‘)9*7J 0.075 102.03 2.2s(i 240.20 0.985

100.00 0.07fP 102.12 2.2.t7 1CJCL52 I-037

JOO.ZG 0.075 IO2.21 2.239 2c--1+1g I .080 J 00.49 0.075 102.31 2.230 31.t.rG 1.120 I oo.cJrJ 0.074 X02.39 2.223 335.HJ x.161 lOO.H8 0,073 102.47 z:.? 16 35G.zG 1.187 101.20 0.07 J ro2.O1 2.20.) 393.04 I.233

JoJ.‘t7 101.70 101.g2 102.25 102.5

J02.80 X02.98

JO.3.17

0.Oh-J 0.ofi7 0.0fi.t o.ocN~ cJ.c~_50

x02.75 102.87 102.99 JlJ*~.J8 103.34

‘I.JC&? 2.JHJ 2.171 2.1.55 2.1.t1

,t 2G.80 ,tgLGr .t83.GG

530.19 5G9. J 7

0.a5 I o.O‘t!, o.o.tcJ

1o3.so 103.ht “‘3.77

2.JZ8 2.116 2.105

.

bO;?.(J9 030.72

GS5d3

I .2&t I.323 I.345 I.407

‘*450

I ..)OS I .SOO x.51*+

_..... - .

STAJ~IJ.~T~ cow4T~wrs 01’ TJJJc ALJCAJ.JNJC JCARTJJ-J~ACT,~TJ~ cohJJc~:x sys~~chls AT 25.0~ . ..-.- . . .__. ._ - _. _ _ mcwztrl f3jcrrir tit f:rorlrlrrl.~

r-i&f n Nrclllorl Conrprrtcr

. _..

.._-

.- -..-.__.__._. -___ hb’ fj, log fh

..--- _I--.._-. .-_.. ..-

log 01 log pa log fh log p* --_---- . ..-.-.--.... ..-- -.. -----.---.-.. .-..- ..-.. -- ._...._.__--. .- _ ._.. .._. _ ._.. _____. ._ .._-... ._.._. _____ _.___ ______ nn 0.49 0.49” 0.3-t 0.4.) 0.34 r!: O.OJ5 0.42 f Sl- 0.68

0.025 0.7‘)”

CJb 0.54 0.71 0.53 rk 0.01.5 o.G9 -+ 0.035

r.00 1.29 0.9.5 I.IG 0.90 f 0.01 m

1.2.) f 0.015

1.00 I.31 0.76 1.20 0.73 z-k 0.03 1.30 f o.oG w-------e _- --..-_ ______. --_-_.-___

* Extrnpoliltctl vnluc.

A~rrtl. Clriur. Ada. 33 (19G5) 378-383

STABILITY CONSTAKTS OF ALKALINE EARTH COhIPLESES 381

Figs. I and 2. The G values exceeding I, except for barium, indicate the formation of ML+ and ML2 complexes in the investigated pL range.

Approximate values of the stability constants were obtained directly ftom the formation curves, using BJERRUM’S half G method 13: the logarithms of the stepwise

0 05 10 15 20 25

formation constants Z<l and Z<z rue ;~pproztimatcly given by the 1% values at E=o.g and x.5 respectively. The corresponding log p values, together with the constants and standard deviations calculated by the computer ;Lrc listed in Tables II and III. ‘These deviations WCIC in general about 2’;: on K1 and about so/& on ZCz, except for the magnesium complexes, where deviations of rcspcctivcly so/:, and 7% wcrc found. To clleck the computer values, the stability constants wet-c dctcrmined by FRONAISUS’

graphical method*.

.

ISIC~~J~VJ~ ~jEYY1JttJ I:YCttJCZClJS ~0ttJ~lJt0

&df fi t?JCtlJVCf ___.___-. -. - --..-_ .-.-.. -._-

log PL [OK pa log p1 log p:, log fl, hi5’ fh _ . _ _.. ..- . . .-. _ . ., .__ ..__ -... ._ --.. -.-. --.-.--. - r&r 0.5t O.S3O 0.38 0.52 0.36 & 0.01 0,5x f 0.02 .

:r, o.7r o*77B 0.58 0.74 0.55 & 0.01 O-73 3: 0.025 1.11 I **I.* o.gG 1.38 o.gz _L 0.015 1.4’2 f 0.025

XIg 1.10 I.52 0.83 x.43 0.81 & 0.02 I.47 f 0.04

a Extr;~polntctl value.

The solubility of the neutral magnesium-a-liydroxyisobutyrate complex (MgLz) was rather small because it precipitated at G=I, when an initial metal concentration of 24 mMo1 was used. In a second titration series at a metal concentra- tion of 12 mMo1 the precipitation of MgLz began at fi.= 1.2.

,dtJd. Ckitn. Ada. 33 (rgG5) 378-383

382 I’. VEHBEEK, H. THUN

For the computer calculation only the z values smaller than 1.20 of the second series were used. PZ was obtained with good precision. The formation curves, being exactly the same for It < I at both metal concentrations, indicated that no polynuclear complexes were formed in solution. Under the same conditions the neutral magnesium- lactate complex was soluble.

‘I’hc 1<1 or PI values of the magnesium lactate and a-hydroxyisobutyrate arc smaller than the corresponding constants of the calcium complexes, whereas the l<z values and also the overall stability constants & are greater for magnesium. This plienomcnon corresponds with the literature data for KI of tlic calcium and magne- sium lactate complcxcs, and also with the elution experiments of PoLLAlzn el aL.GsO.

The log & values plotted in the order of decreasing atomic number increase more for the a-llydroxyisobutyrate complexes than for the lactate complexes. This shows t11at, as for the rare earths, m-hydroxyisobutyrate is a better elucnt for ion- exchange separation of the alkaline earths, which conforms to the elution experiments of POLLARI) cl al.“*“.

In the literature the only stability constants reported at the same temperature (25”) and it compnrablc ionic stt-cngth (o.IG) are the I<1 values of the strontium and calcium lactate complcxcs determined by S~HUIXHT ANO LINI)ENBAU~I~~, using cation-exchange methods. Thcsc authors found values of 0.5 and 0.8 respcctivcly; this is in fzkir agrccmcnt with the constants in Table II, as is tllc 1<t value of 0.82 for tllc calcium lactate complex dctcrmincd by JOSEPHSON, from EM17 measurements.

‘The formation of ML2 complcxcs is not in disagrecmcnt with Sc~uI3awr’s conclusion that only x :I spccics would exist at lactate concentrations ranging from 0.04 to 0.08 (1’L = I ,4-I. I). As can be seen from the formation curves (Fig. I) at pL = I.1 the value of Z is 0.23 for strontium and 0.50 for calcium, indicating that almost only ML+ complexes are in solution.

‘I’hc authors wisll to cxprcss their thanks to Prof. Dr. J. HOSTE for his lcind intcrcst in this work, to Mr. W. VANI>ERJ.EEN for llis help with the IBM computer, and to Mrs. VAN I)EN Auaet~s for technical assistance.

SUhlhlAl<\

l’11c stability constants of the lactate and a-hyclroxyisobutyratc complexes of the all~aline earths were determined by potentiomctric titration. The average ligand number exceeds a value of one, indicating the formation of ML+ and ML2 complexes. a-Hydroxyisobutyratc forms stronger complexes than lactatc.Thc stability constants p:! incrcnsc as a function of decreasing atomic number for both ligands: 13a-c Sr-z Ca< Mg, although the constant PI is greater for Mg than for Ca.

On a cl6tcrmin6 lcs constantcs de stabilitc de complcxcs: lactates ct a-hydroxy- isobutyratcs alcalino-tcrrcux (du type ML+ ct MLz), par titrage potentiomdtrique. Les a-hydroxyisobutyrates sont clcs comlkxes plus forts clue lcs lactates. Les con- stantes dc stabilitd augmcntent lorsquc le nombre atomique clu mdtal diminue (Bae Sr<Ca< Mg).

ArbuC. Ciriru. .4ckc, 33 (xcy5g) 378-383

STABILITY CONSTANTS OF ALKALINE EARTH COXII’LESES 383

ZUSAhlXlENFASSUNG

Durcb potentiometrische Titration wurden die Stabilitiitskonstanten der Lactat- und or-Hydroxyisobutyrat-Komplexe der Erdalkalien bestimmt. Die durch- schnittliche Ligandcnzahl iiberstieg den Wert von I, was auf die Bildung van ML+- und MLz-Komplexen hinweist. Die cu-Hydroxyisobutyrate hilden stlirkere Komplexe als die Lactate Die Stabilitatskonstanten /?z steigen bei beiden Liganden mit ab- nellmender Ordnungszahl : I3a c Sr e Ca c Mg, obwohl die Konstante ,631 bcim Magnesium grosser ist als beim Calcium.

I E-1. hUNEK ANI> \v. I(;ELIAN, /f1Acll. ChlWl.. 26 (;95.() 610. 2 G. M. MXLTON AND W. II. ~I~U$lhl1TT, CUM. J. Cimtr.. 35 (1957) 541, 3 A. I-‘. l~Al;HC AS:) I<. 11. 13AllT110L0hlE\C’, CUtJ. J. Che?N., 35 (1957) 080. 4 I.. \VIS;;, rlrrd. Clrem., 33 (1961) 53; U.S. Naval Radiological Jhfcnse Ln6. iCcpt.. US A’RDL-

Tit!-$#r , 195’h 5 1:. I-i. ~‘OL;.AHD, (;. NICKLKSS AND I>. SPINCKR. ./. Ch?‘V,tlckJg., IO (19ti3) 215: II (;96_3) 542. <> 1:. I-1. ~‘OLLARD. (;. NICKLESS AND 1). SPINC~R, J. C/WOWikJ&‘., I.3 (X904) 224. 7 L. (;. S;;,l.~s AND h. 1:. hlAKTY.LL, Stctbilify &,l.Shl,l~S Of I%ldU[ fW ~O#~J,dl!.t’~S, Sl’CC. J’ublic. RIO.

17, Chcrn. Sot., ISurlingtorl Mouse, London, 196.1. 8 H. I>I:.ISI.STIIA ANI> I;. VILHIJEEK. .4nul. Chinz. .-Ida, 31 (1904) 25;. 9 I,. 1<;XKllAUT, 1;. VEHI1lEI:.K, bi. ~~ISRLSTRA ANIB J. iiOSTlc, /1 t~nl. Claim. Aclu, 30 (r 964) 3G9.

IO c. I<. CIIOI*I*IN AND J. ,\. ~1101’00KXhN. 1. f MM’&,‘. & .~Itl*l. ChJlr., 22 (1961) 97. 11 s. b-KOSAKUS, . . Iclu Chm;. Scami., 4 (1950) 72; 5 (1951) 139; Cj (1952) 100, 1200. 12 A. SONILSSON. .dc’td Ch,tl. .~CUHd., I2 (1958) Xh.5, 19.37; 13 (1959) 9913. 13 J. 13~ I’.Itstunl, ~IrIrrl .d wt?li#te ~ormnlion it8 .d quecws .%ltltioll. P. 1-I ;~ILSC, Copcnhagcn, x 94 I. 14 J. !%IIUIJ;:RT ASI) r\. I,INDI~P*‘IJAU~;. J. AOJ. Cher~t. SW., 74 (195~) 35~0. 15 S. I<. JOS;SVI;, J. f~iui.C/~ot~.. rGq (194’1) 5~.

A #rd. CJrim. AcIN, 33 (x965) 378-383