- Email: [email protected]
Liquid—liquid extraction of alkaline earth metal ions with a non-ionic surfactant having a poly(oxyethylene) chain (Triton X-100)
65
Analyirca Chrmlca Acta, 264 (1992) 65-70
Elsevler Science Pubhshers B V , Amsterdam
Liquid-liquid extraction of alkaline earth metal ions with a non-ionic surfactant having a poly(oxyethylene) chain (Triton X-100) Yolchl K&uchl, Toshlo Suzulu and Kyoshl Sawada Laboratory
of Analytrcal Chemutry, Faculty of Scrence, Nugata Unrversuy, Nugata 950-21 (Japan)
(Received 25th November 1991)
The Ion-pair extractlon of poly(oxyethylene) alkylphenyl ether (Tnton X-100, S) complexes of alkahne earth metal Ions CM’+) wth plcrate Ion (Ps-) mto 1,Zdlchloroethane (1,ZDCE) was studled The extractlon eqmhbrla are interpreted by the extractlon of the ion pair, [SM’+ (Plc-I,], and the dtssoclatlon of the ton pair mto [SMZf (Plc-)] and PIG- m 1,2-DCE Thermodynarmc constants of the extraction and the dissociation of the ion pairs were evaluated The extractlon constant increases m the order Ca”< Sr*+ < Ba*+ Thus order IS explained by the magmtudes of the formatlon constants of the alkahne earth metal Ion complexes with Trlton X-100 m the aqueous phase The dlssoclatlon constant of the Ion pair m 1,2-DCE IS almost constant for the alkaline earth metal ions Keywords Alkaline earth metals, Extraction, Surfactants
Non-lomc surfactants havmg a non-cyclic poly(oxyethylene) chain form catlomc complexes with alkaline earth and alkali metal Ions [l-5] The catlomc complexes are extracted mto organic solvents by formmg ion palrs with llpophlllc anions [6-81 Although these extraction systems have been used for the determmatlon of trace metals [g-12] and applied to an ion-selective electrode sensor [13,14], details of the extractlon equlhbrla are still not well understood In a previous study, the extraction of alkali metal plcrates with a poly(oxyethylene) alkylphenyl ether (Tnton X-100) mto l,Zdlchloroethane (1,2-DCE) was exammed using hquldliquid extraction and vapour pressure osmometry [15] These studies revealed that the extraction equlhbrmm can be explamed by the extraction of the ion pair of the alkali metal ion complex of Correspondence to Dr K Sawada, Department of Chemistry, Faculty of Science, Nngata University, Nugata 950-21 (Japan)
Trlton X-100 with plcrate ion and the dlssoclatlon of the ion pair m 1,ZDCE The effects of the number of oxyethylene CEO) umts on the extractability of alkah metal ions and on the dasoclatlon of the extracted ion pairs have also been mvestlgated by usmg monodlsperse poly(oxyethylene) monododecyl ethers [16] In this work, the extraction of alkaline earth metal plcrates mto 1,ZDCE with Trlton X-100 and the extraction and dlssoclatlon equlhbrla of the Ion pair of the alkalme earth metal ion complex of Trlton X-100 with plcrate ion were studled EXPERIMENTAL
Reagents Poly(oxyethylene) alkylphenyl ether (Tnton X-100) was purchased from Wako The average number of EO umts was determined as 9 6 Plcnc acid (Wake) was recrystalhzed ice from dls-
0003~2670/92/%05 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved
Y Kikuchr et al /Anal
66
tilled water 1,2-Dlchloroethane (Wake) was washed three times with distilled water Other chemicals were of analytical reagent grade
Chm Acta 264 (1992) 65-70
X-100 (S) slmdarly to the alkali metal picrate extraction system [15], the extraction equ111br1um of the 1on pair [SM*+ (Plc-),I (Plc-= picrate ion> IS expressed by
Extraction procedure
Alkalme earth metal picrates were extracted with Tr1ton X-100 by a method described elsewhere [15] A portion of an aqueous solution (20 ml) containing p1cr1cacid and alkaline earth metal chloride, with the pH adlusted to 8-10 with an alkaline earth metal hydroxide, was shaken with an equal volume of a 1,2-DCE solution contam1ng Tr1ton X-100 1n a centrifuge tube (50 ml) in a thermostated water-bath for 30 mm at 25 f 0 1°C After centnfugatlon, the concentrations of picrate 1n both phases were determined by spectrophotometry at 356 nm No d1strlbutlon of picrate 1on into the organic phase 1n the absence of Tr1ton X-100 1n this pH range was confirmed by prelmunary experiments Three types of experiments under the condltlons (a)-(c) below were performed for the extraction of cahum, strontium and barium picrate The extraction of magnesium picrate 1s inadequate for the quantitative analysis of the extraction equ111br1um (a) CP*c,*= 2 x lo-‘-5 x 10e4 M (for Sr and Ba system), 1 X 10e4-2 X 10e3 M (for Ca system), C,,=OOl M, C,,l=Ol M, = 1 x lop4 M (Sr, Ba system), 1 X (b) CPX1 lo-’ M (Ca system), C,,l = 1 x 1O-3-O 05 M, C,,, = 0 1 M, (c) CPIC,= 1 x 10m4 M (Sr, Ba system), 1 X lo-’ M (Ca system), Cs,, = 0 01 M, C,,, = 1 x 10e4-0 02 M (Sr, Ba system), 2 X 10e3-0 05 M (Ca system), where Cplc,, and C,,l are the m1t1al concentrations of picrate and alkaline earth metal ion 1n the aqueous solution, respectively, and C,,, is the initial concentration of Tr1ton X-100 1n the organic solution RESULTS
Extractron equddmun
If we assume the formation of a 1 1 complex of the alkaline earth metal 1on (M*‘) with Tr1ton
M++ 2P1c-+ SorgtY[ - SM*+
(Pl~-)2]~~~
(1)
where the subscript org refers to the organic phase The extraction constant, K,,, 1s defined as = w*+
K a
(PlC-)210rg
[M*+][Plc-]*[S]org
If we do not consider the dlssoclatlon of the 1on pair 1n l,ZDCE, the dlstrlbutlon ratio of picrate ion, D, 1s given by D=C =
PlC,O&PlC
aq
2[SM*+ (PIG-)2]0rg/[P1~-]
(3)
where Cp,c,Organd CPs,aq refer to the total concentration of picrate 1n the organic and aqueous phase, respectively From Eqns 2 and 3, we obtain log D = +(log K, + log[M*+] + log[S],,, +
hi? cP,c,org- log 2)
(4)
The plots of log D vs log CRc,_ for expenment (a) are shown 1n F1g 1 The transfer of Tr1ton X-100 into the aqueous phase 1s neghgble and the concentration of picrate ion 1s much smaller than that of other components under the conditions of experunent (a) Hence the free concentrations of alkaline earth metal 1on 1n the aqueous phase, [M*‘], and Tr1ton X-100 1n the organic phase, IS]_, can be approximated by C,,, and Cs,,, respectively, and can be taken as constant Consequently, as can be seen from Eqn 4, the plots 1n F1g 1 should be straight lines having a slope of 05 However, they are not straight 11nes and the slopes are much less than 0 5 These facts suggest that this extraction system cannot be explained only by the equ111bnum Eqn 1, and predict a partial dissociation of the extracted 1on pair 1n 1,ZDCE If we take into conslderatlon the first step of the dlssoclatlon of the extracted 1on pair, [SM*+
Y Kikuchc et al /Anal
67
Chrm Acta 264 (1992) 65-70
Substltutlon of Eqns 2 and 6 into Eqn 7 leads to
’
_opo’op/
D = K:,/2[M2+]1’2[S]::;(2[SM2+
(8)
+ 2%*.&g)
O- /
The total concentration of picrate ion in l,ZDCE, C P,c,org,1s expressed by Eqn 9 by using Eqn 6
0 :
(P~c-),I;;;
-l-
= 2[SM2+ (PIc-)~],,~~ C P1c,org + [SM2+ (PK-)I,,,
-2 -
+ [Plc-I,,
= 2[SM2+ (P~c-)~]~~~ + 2K”;i,*[SM*+ (P~c-)~];;;f,f,
I
I
1
-L
-5
-6
By solving the quadratlc (Plc-),lg
-3
‘og CPc,org
Fig 1 Plots of log D vs log Cplcorg for expenment (a) 0, Ca’+,
A,
Sr’+,
0,
Ba*+
[SM*+ (P~c-)~]:/,‘= +(%,~;2,,
(PIc-)J,~~, the extraction of alkaline earth metal picrate wth Tr1ton X-100 1s expressed by
+
(6)
wheref *,org
IS the mean activity coefficrent of [SM2+ (Plc-)I,, and P1c& 1n the 1,ZDCE phase, and was evaluated by using an extended Debye-Huckel equation The distnbutlon ratio of picrate ion, D, IS given by Plc,org/CPlc,aq
+
(PIc-)~]~~~ + [SM2+ (Plc-)]_
wlc&[plc-l
[ -K"~;'2f&g
+ 2cPlc,org )
l’*1/2
(10)
%xPlw, - logf+,org+ logF
(11)
where the function F IS given by
SM*+ (P~c-)] erg + Plc,,
The thermodynamic constant of the first dlssoclatlon, K&, 1s defined as
= (2[SM*+
9 for [SM*+
log D = ;log K,, + $log KS, + +log[M2+]
(5)
D=C
Eqn
Substltutlon of Eqn 10 into Eqn 8 leads to
M++ 2P1c-+ Sorg5- [ SM*+ (PIc-)~],_ 3
(9)
I
(7)
f= 1 + (1 +
2K~,,f;~~,gcP,~,org)1'2
(12)
As the concentrations [M2’] and [S],, can be taken as constant under the conditions of expenment (a), the change 1n log D can be attributed solely to the log F term Thus, Eqn 11 can be normalized to a function given by
Y=log[l+(1+2x)“*] x=
log x
(13)
where x corresponds to K&f;~o,Cp,c,o, The plots 1n F1g 1 show a good frttlng with the normahzed curve (Eqn 13) and the constants K,, and Kd,s are estimated as a first approxlmatlon The mean activity coefficient 1n l,%-DCE, f*,org, depends on the concentration of ionic species, 1 e , on the values of K& The curve fitting was repeated by using the newly obtamed value of &I, Three iterations gave the converged values
68
Y Kkuchr et al /Anal
TABLE 1 Logarlthmlc extractlon constants and dlssoclatlon constants Constant
Ca*+
Sr2+
Ba2+
h
5 24 -462
700 -488
8 28 -446
K:,
Log G,
of K,, and K”,,, Finally, the values of K,, and K& were refined by means of successive approxlmatlon with a non&near regression using a mlcrocomputer The values of Kz,, thus obtained are given m Table 1 and the calculated curves obtained from these constants are shown as solid lines m Fig 1 These curves reproduce the experimental results very well, and suggest the validity of Eqn 5 for the extraction of alkaline earth metal plcrates with Trlton X-100 By using BJerrUm’S theoretical equation [17], the constant of the second dlssoclatlon of the ion pair [SM ’ + (Plc- )I,,, + [SM*+],, + Plc& 1s estimated to be of the order of 10m9 M from the value of the first-step dlssoclatlon constant, KS,, Hence the theoretical predlctlon also corroborates that the second dlssoclatlon of the ion pair is negligible m this system By using the values of K, and K;,, thus obtained, the results of the Trlton X-100 concentration dependence [experiment (b)] were analysed As the free concentration of alkaline earth
Chum Acta 264 (1992) 65-70
metal ion m the aqueous phase, [M*‘l, can be taken as constant under the conditions of expenment (b), the plots of log D + log f+ erg - log F as a function of log[S],, should give straight lmes with a slope of 0 5 (Eqn 11) As shown m Fig 2, the experimental results agree well with straight lines with a slope of 0 5 This confirms that there 1s one Trlton X-100 molecule m the ion pair For the purpose of the strict analysis of the equlhbrla m the orgamc phase, the lomc strength m the aqueous phase was kept constant at a higher concentration m expernnent (a) In order to obtain the thermodynamic constant of extraction, K&,, the effect of the concentration of alkalme earth metal ion was studied under the condltlons of relatively low alkaline earth metal ion concentration [expernnent (c)l The thermodynamic constant of extractlon, K’&, 1s defined as [SM*+
Kzx =
WC-),I,,,
[M*+][P1c-]*[S]orgf+ff
=Kex(f+fy
(14) where f, and f_ are the activity coefficients of M*+ and Plc-, respectively, m the aqueous phase Thus, Eqn 11 1s rewritten as log D = ilog K:, + ;log K& + $og[M*+] + ~log[Sl0, + log fY’f--
log f*,org
+ log F
(15)
1 t
I . -L
-3
-2 log
Fig 2 Plots of log D + log f* oTg-log F vs log[Sl,,s for experiment (b) Symbols as m Rg 1 Solid lines are straight hnes with a slope of 0 5
-1
IM”l
Fag 3 Plots of log D -log f:“f_ -log f+ ors -log F vs log[M2+ I for experrment (c) Symbols as m Fig 1 Solid lmes are straight hnes with a slope of 0 5
Y fikuchr
et al /Anal
Chm
69
Acta 264 (1992) 6.5-70
equation [19], the values of the ion-pair formation constant m the aqueous phase, K,p,aq, can be assumed to be the same between the metal ions Consequently, the term K,p,aqKd,,pK~~ m Eqn 16 can be assumed to be constant, and the dlfference m the extractlon constants between alkalme earth metals, Alog K&, 1s attnbuted to the dlfference m the S-M2+ complex formatlon constant m the aqueous phase, A log Km,,,, As seen from Table 1, the thermodynamic extraction constants increase m the order Ca2+< Sr2+ < Ba’+, 1 e , the formation constant of the S-M2+ complex m the aqueous phase increases with increase m the ionic radms of the central metal ion This order of log K& agrees with the order of the formation constants of l&crown-6 complexes m aqueous solution, and the dlfferences m the log K’& values between the metal ions are comparable to those of the forrnatlon constant of l&crown-6 complexes (log Kcom,,aq of l&crown-6 0 5 for Ca2+, 2 72 for Sr2+ and 3 87 for Ba2+ 1201) The poly(oxyethylene) compound 1s considered to form complexes with the structure of a “turban” [15,21-231, 1e , the metal ion 1s surrounded by the EO cham m a hehcal conformation In a previous study of the etiractlon of alkali metal plcrates with monodisperse POE compounds [161, it was suggested that the cavity size of the helical conformation of the poly(oxyethylene) compounds is almost the same irrespective of the number of EO umts, and its 1s comparable to that of 18-crown-6 Hence it 1s reasonable that the coordination behavlour of Trlton X-100 1s similar to that of l&crown-6 The effect of the lomc radms, r, on the extraction constant of alkaline earth metals 1s much
Consequently, the plots of log D - log fi’“f_+ log f *,org - log F as a function of lo~M*‘l should give straight lmes with a slope of 0 5 These plots obtamed from expenment (c) are shown m Fig 3, where the values of f, and f_ were evaluated by usmg the Davies equation [181 The experimental resuls agree well with straight lines with a slope of 05 Hence the number of metal ions m the complex ion was confumed as one The thermodynamic extraction constants, Kz,, thus obtained are given m Table 1
DISCUSSION
Extractwn constant The extraction equlhbrmm
of the ion parr 1s written by elementary processes as m Scheme 1, 1e , the dlstrrbutlon of Trlton X-100 mto the aqueous phase (Ki,t) formation of the alkaline earth metal complex with Trlton X-100, S-M2+ ion-pan formation of the S-M2+ com(L,,%J plex vvlth plcrate ion U&J m the aqueous phase and the dlstrlbutlon of the ion pair mto 1,2-DCE (&,,,) The extraction constant of the ion pair, Kz,, 1s expressed by K& = K,,,,,K,~,~,K,,,,K~,~
(16)
Kd,, 1s common for any systems of alkaline earth metal ions The value of K,,,p can be assumed to be the same lrrespectlve of the kmd of the alkaline earth metal Ion, because the molar volume of the ion pan 1s scarcely changed on changing the central metal ion As will be mentioned below, the mterlomc distance Ff all ion pairs studied here 1s longer than 2 A even m 1,ZDCE Thus, as deduced from the Fouss’s
m2+ org aq
.2+
w=-1210,g
phase %iP
phase
+
Scheme 1
2pic-
+
+
2pic-
Ki ,P,
a
K 19!42+ (Pic-)21
-
70
Y Kkuchr et al /Anal Chm Acta 264 (1992) 65-70
larger than that for alkali metals, 1e , the dlfference m log K& between Ca*+ (r = 0 99 A> and Ba*+ (r = 136 A) 1s A log K& = 3 04, whereas Alog K&=13! between Na+ (r=O97 A) and K+ (r = 133 A) The extractablhty (log D) for the Ba2+ system 1s less than that of the K+ system under the present experimental condotions Interwnrc dntance of the ion paw m 1,2-DCE The mterlomc distance, a, between Ions of the
Ion pair can be estimated from the dissociation constant of the ion pair usmg @errurn’s theoretlcal equation [17] If Ion-pair formation by 1 1 charge interaction between [SM (Plc)]+ and PIG1s assumeOd, the a value was estimated to be about 4 A Irrespective of the kmd of alkaline earth metal ion The a value was obtained as 6 w for the alkali metal ion-Tnton X-100 system [15] If we take mto conslderatlon the structure of the ion pair of the “turban” complex m which the metal ion IS surrounded by the oxyethylene chain of Trlton X-100, and the plcrate ion 1s situated :utslde the oxyethylene chain, the value of a = 6 A for the alkah metal complex 1s reasonable That is, the a value for the alkalme earth metal complex seems to be too small Assummg a 2 1 interaction between the local positive charge of [SM2+ (Plc-)I, [SM*‘] and Plc- as an extreme case, we obtain an unreasonable value 12 A for a These results may mdlcate that the dlssoclatlon constant of the ion pan of the alkaline earth metal complex IS lowered by the effect of the double charge of the metal complex ion
REFERENCES 1 W Rabhofer, W M Muller and F Vogtle, Chem Ber , 112 (1979) 2095 2 F Votle and E Weber, Angew Chem , Int Ed Engl, 18 (1979) 753 3 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn ,51 (1978) 1294 4 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn, 51 (1978) 3111 5 DC W Slew, R P Cooney, M J Taylor and A J Easteal, J Chem Sot , Faraday Trans ,86 (1990) 1109 6 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn , 50 (1977) 1386 7 AMY Jaber, G J Moody and J D R Thomas, J Inorg Nucl Chem , 39 (1977) 1689 8 Y Sakw, H Nakamura, M Takagl and K Ueno, Bull Chem Sot Jpn ,59 (1986) 381 9 T Sotobayashl, T Suzuki and S Tonouch], Chem Lett , (1976) 585 10 T Sotobayashl, T Suzuki and K Yamada, Chem Lett , (1976) 77 11 T Sotobayashl, T Suzulu and H Kudo, J RadIoanal Chem , 36 (1977) 145 12 T Suzuki, N Murakaml and T Sotobayashl, Bull Chem Sot Jpn ,53 (1980) 1453 13 D L Jones, G J Moody and J D R Thomas, Analyst, 106 (1981) 439 14 R J Levms, Anal Chem , 43 (1971) 1045 15 Y Kdmchl, N Takahashl, T Suzulu and K. Sawada, Anal Chum Acta, 256 (1992) 311 16 Y Qkuchl, Y NoJlma, H IOta, T Suzuki and K Sawada, Bull Chem Sot Jpn, m press 17 N BJerrum, Dan Vldensk Selsk Mat Fys Medd, 7(9) (1926) 1 18 C W Davies, J Chem Sot, (1938) 2093 19 R M Fouss, J Am Chem Sot, 80 (1958) 5059 20 R M Izatt, R E Terry, B L Haymore, L D Hansen, N K Dailey, AG Avondet and J J Chrlstensen, J Am Chem Sac , 98 (1976) 7620 21 W Saenger, I H Suh and G Weber, Isr J Chem, 18 (1979) 253 22 G Weber, W Saenger, F Vogtle and H Sleger, Angew Chem , Int Ed Engl , 18 (1979) 226 23 M D Adams, P W Wade and R D Hancock, Talanta, 37 (1990) 875
Analyirca Chrmlca Acta, 264 (1992) 65-70
Elsevler Science Pubhshers B V , Amsterdam
Liquid-liquid extraction of alkaline earth metal ions with a non-ionic surfactant having a poly(oxyethylene) chain (Triton X-100) Yolchl K&uchl, Toshlo Suzulu and Kyoshl Sawada Laboratory
of Analytrcal Chemutry, Faculty of Scrence, Nugata Unrversuy, Nugata 950-21 (Japan)
(Received 25th November 1991)
The Ion-pair extractlon of poly(oxyethylene) alkylphenyl ether (Tnton X-100, S) complexes of alkahne earth metal Ions CM’+) wth plcrate Ion (Ps-) mto 1,Zdlchloroethane (1,ZDCE) was studled The extractlon eqmhbrla are interpreted by the extractlon of the ion pair, [SM’+ (Plc-I,], and the dtssoclatlon of the ton pair mto [SMZf (Plc-)] and PIG- m 1,2-DCE Thermodynarmc constants of the extraction and the dissociation of the ion pairs were evaluated The extractlon constant increases m the order Ca”< Sr*+ < Ba*+ Thus order IS explained by the magmtudes of the formatlon constants of the alkahne earth metal Ion complexes with Trlton X-100 m the aqueous phase The dlssoclatlon constant of the Ion pair m 1,2-DCE IS almost constant for the alkaline earth metal ions Keywords Alkaline earth metals, Extraction, Surfactants
Non-lomc surfactants havmg a non-cyclic poly(oxyethylene) chain form catlomc complexes with alkaline earth and alkali metal Ions [l-5] The catlomc complexes are extracted mto organic solvents by formmg ion palrs with llpophlllc anions [6-81 Although these extraction systems have been used for the determmatlon of trace metals [g-12] and applied to an ion-selective electrode sensor [13,14], details of the extractlon equlhbrla are still not well understood In a previous study, the extraction of alkali metal plcrates with a poly(oxyethylene) alkylphenyl ether (Tnton X-100) mto l,Zdlchloroethane (1,2-DCE) was exammed using hquldliquid extraction and vapour pressure osmometry [15] These studies revealed that the extraction equlhbrmm can be explamed by the extraction of the ion pair of the alkali metal ion complex of Correspondence to Dr K Sawada, Department of Chemistry, Faculty of Science, Nngata University, Nugata 950-21 (Japan)
Trlton X-100 with plcrate ion and the dlssoclatlon of the ion pair m 1,ZDCE The effects of the number of oxyethylene CEO) umts on the extractability of alkah metal ions and on the dasoclatlon of the extracted ion pairs have also been mvestlgated by usmg monodlsperse poly(oxyethylene) monododecyl ethers [16] In this work, the extraction of alkaline earth metal plcrates mto 1,ZDCE with Trlton X-100 and the extraction and dlssoclatlon equlhbrla of the Ion pair of the alkalme earth metal ion complex of Trlton X-100 with plcrate ion were studled EXPERIMENTAL
Reagents Poly(oxyethylene) alkylphenyl ether (Tnton X-100) was purchased from Wako The average number of EO umts was determined as 9 6 Plcnc acid (Wake) was recrystalhzed ice from dls-
0003~2670/92/%05 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved
Y Kikuchr et al /Anal
66
tilled water 1,2-Dlchloroethane (Wake) was washed three times with distilled water Other chemicals were of analytical reagent grade
Chm Acta 264 (1992) 65-70
X-100 (S) slmdarly to the alkali metal picrate extraction system [15], the extraction equ111br1um of the 1on pair [SM*+ (Plc-),I (Plc-= picrate ion> IS expressed by
Extraction procedure
Alkalme earth metal picrates were extracted with Tr1ton X-100 by a method described elsewhere [15] A portion of an aqueous solution (20 ml) containing p1cr1cacid and alkaline earth metal chloride, with the pH adlusted to 8-10 with an alkaline earth metal hydroxide, was shaken with an equal volume of a 1,2-DCE solution contam1ng Tr1ton X-100 1n a centrifuge tube (50 ml) in a thermostated water-bath for 30 mm at 25 f 0 1°C After centnfugatlon, the concentrations of picrate 1n both phases were determined by spectrophotometry at 356 nm No d1strlbutlon of picrate 1on into the organic phase 1n the absence of Tr1ton X-100 1n this pH range was confirmed by prelmunary experiments Three types of experiments under the condltlons (a)-(c) below were performed for the extraction of cahum, strontium and barium picrate The extraction of magnesium picrate 1s inadequate for the quantitative analysis of the extraction equ111br1um (a) CP*c,*= 2 x lo-‘-5 x 10e4 M (for Sr and Ba system), 1 X 10e4-2 X 10e3 M (for Ca system), C,,=OOl M, C,,l=Ol M, = 1 x lop4 M (Sr, Ba system), 1 X (b) CPX1 lo-’ M (Ca system), C,,l = 1 x 1O-3-O 05 M, C,,, = 0 1 M, (c) CPIC,= 1 x 10m4 M (Sr, Ba system), 1 X lo-’ M (Ca system), Cs,, = 0 01 M, C,,, = 1 x 10e4-0 02 M (Sr, Ba system), 2 X 10e3-0 05 M (Ca system), where Cplc,, and C,,l are the m1t1al concentrations of picrate and alkaline earth metal ion 1n the aqueous solution, respectively, and C,,, is the initial concentration of Tr1ton X-100 1n the organic solution RESULTS
Extractron equddmun
If we assume the formation of a 1 1 complex of the alkaline earth metal 1on (M*‘) with Tr1ton
M++ 2P1c-+ SorgtY[ - SM*+
(Pl~-)2]~~~
(1)
where the subscript org refers to the organic phase The extraction constant, K,,, 1s defined as = w*+
K a
(PlC-)210rg
[M*+][Plc-]*[S]org
If we do not consider the dlssoclatlon of the 1on pair 1n l,ZDCE, the dlstrlbutlon ratio of picrate ion, D, 1s given by D=C =
PlC,O&PlC
aq
2[SM*+ (PIG-)2]0rg/[P1~-]
(3)
where Cp,c,Organd CPs,aq refer to the total concentration of picrate 1n the organic and aqueous phase, respectively From Eqns 2 and 3, we obtain log D = +(log K, + log[M*+] + log[S],,, +
hi? cP,c,org- log 2)
(4)
The plots of log D vs log CRc,_ for expenment (a) are shown 1n F1g 1 The transfer of Tr1ton X-100 into the aqueous phase 1s neghgble and the concentration of picrate ion 1s much smaller than that of other components under the conditions of experunent (a) Hence the free concentrations of alkaline earth metal 1on 1n the aqueous phase, [M*‘], and Tr1ton X-100 1n the organic phase, IS]_, can be approximated by C,,, and Cs,,, respectively, and can be taken as constant Consequently, as can be seen from Eqn 4, the plots 1n F1g 1 should be straight lines having a slope of 05 However, they are not straight 11nes and the slopes are much less than 0 5 These facts suggest that this extraction system cannot be explained only by the equ111bnum Eqn 1, and predict a partial dissociation of the extracted 1on pair 1n 1,ZDCE If we take into conslderatlon the first step of the dlssoclatlon of the extracted 1on pair, [SM*+
Y Kikuchc et al /Anal
67
Chrm Acta 264 (1992) 65-70
Substltutlon of Eqns 2 and 6 into Eqn 7 leads to
’
_opo’op/
D = K:,/2[M2+]1’2[S]::;(2[SM2+
(8)
+ 2%*.&g)
O- /
The total concentration of picrate ion in l,ZDCE, C P,c,org,1s expressed by Eqn 9 by using Eqn 6
0 :
(P~c-),I;;;
-l-
= 2[SM2+ (PIc-)~],,~~ C P1c,org + [SM2+ (PK-)I,,,
-2 -
+ [Plc-I,,
= 2[SM2+ (P~c-)~]~~~ + 2K”;i,*[SM*+ (P~c-)~];;;f,f,
I
I
1
-L
-5
-6
By solving the quadratlc (Plc-),lg
-3
‘og CPc,org
Fig 1 Plots of log D vs log Cplcorg for expenment (a) 0, Ca’+,
A,
Sr’+,
0,
Ba*+
[SM*+ (P~c-)~]:/,‘= +(%,~;2,,
(PIc-)J,~~, the extraction of alkaline earth metal picrate wth Tr1ton X-100 1s expressed by
+
(6)
wheref *,org
IS the mean activity coefficrent of [SM2+ (Plc-)I,, and P1c& 1n the 1,ZDCE phase, and was evaluated by using an extended Debye-Huckel equation The distnbutlon ratio of picrate ion, D, IS given by Plc,org/CPlc,aq
+
(PIc-)~]~~~ + [SM2+ (Plc-)]_
wlc&[plc-l
[ -K"~;'2f&g
+ 2cPlc,org )
l’*1/2
(10)
%xPlw, - logf+,org+ logF
(11)
where the function F IS given by
SM*+ (P~c-)] erg + Plc,,
The thermodynamic constant of the first dlssoclatlon, K&, 1s defined as
= (2[SM*+
9 for [SM*+
log D = ;log K,, + $log KS, + +log[M2+]
(5)
D=C
Eqn
Substltutlon of Eqn 10 into Eqn 8 leads to
M++ 2P1c-+ Sorg5- [ SM*+ (PIc-)~],_ 3
(9)
I
(7)
f= 1 + (1 +
2K~,,f;~~,gcP,~,org)1'2
(12)
As the concentrations [M2’] and [S],, can be taken as constant under the conditions of expenment (a), the change 1n log D can be attributed solely to the log F term Thus, Eqn 11 can be normalized to a function given by
Y=log[l+(1+2x)“*] x=
log x
(13)
where x corresponds to K&f;~o,Cp,c,o, The plots 1n F1g 1 show a good frttlng with the normahzed curve (Eqn 13) and the constants K,, and Kd,s are estimated as a first approxlmatlon The mean activity coefficient 1n l,%-DCE, f*,org, depends on the concentration of ionic species, 1 e , on the values of K& The curve fitting was repeated by using the newly obtamed value of &I, Three iterations gave the converged values
68
Y Kkuchr et al /Anal
TABLE 1 Logarlthmlc extractlon constants and dlssoclatlon constants Constant
Ca*+
Sr2+
Ba2+
h
5 24 -462
700 -488
8 28 -446
K:,
Log G,
of K,, and K”,,, Finally, the values of K,, and K& were refined by means of successive approxlmatlon with a non&near regression using a mlcrocomputer The values of Kz,, thus obtained are given m Table 1 and the calculated curves obtained from these constants are shown as solid lines m Fig 1 These curves reproduce the experimental results very well, and suggest the validity of Eqn 5 for the extraction of alkaline earth metal plcrates with Trlton X-100 By using BJerrUm’S theoretical equation [17], the constant of the second dlssoclatlon of the ion pair [SM ’ + (Plc- )I,,, + [SM*+],, + Plc& 1s estimated to be of the order of 10m9 M from the value of the first-step dlssoclatlon constant, KS,, Hence the theoretical predlctlon also corroborates that the second dlssoclatlon of the ion pair is negligible m this system By using the values of K, and K;,, thus obtained, the results of the Trlton X-100 concentration dependence [experiment (b)] were analysed As the free concentration of alkaline earth
Chum Acta 264 (1992) 65-70
metal ion m the aqueous phase, [M*‘l, can be taken as constant under the conditions of expenment (b), the plots of log D + log f+ erg - log F as a function of log[S],, should give straight lmes with a slope of 0 5 (Eqn 11) As shown m Fig 2, the experimental results agree well with straight lines with a slope of 0 5 This confirms that there 1s one Trlton X-100 molecule m the ion pair For the purpose of the strict analysis of the equlhbrla m the orgamc phase, the lomc strength m the aqueous phase was kept constant at a higher concentration m expernnent (a) In order to obtain the thermodynamic constant of extraction, K&,, the effect of the concentration of alkalme earth metal ion was studied under the condltlons of relatively low alkaline earth metal ion concentration [expernnent (c)l The thermodynamic constant of extractlon, K’&, 1s defined as [SM*+
Kzx =
WC-),I,,,
[M*+][P1c-]*[S]orgf+ff
=Kex(f+fy
(14) where f, and f_ are the activity coefficients of M*+ and Plc-, respectively, m the aqueous phase Thus, Eqn 11 1s rewritten as log D = ilog K:, + ;log K& + $og[M*+] + ~log[Sl0, + log fY’f--
log f*,org
+ log F
(15)
1 t
I . -L
-3
-2 log
Fig 2 Plots of log D + log f* oTg-log F vs log[Sl,,s for experiment (b) Symbols as m Rg 1 Solid lines are straight hnes with a slope of 0 5
-1
IM”l
Fag 3 Plots of log D -log f:“f_ -log f+ ors -log F vs log[M2+ I for experrment (c) Symbols as m Fig 1 Solid lmes are straight hnes with a slope of 0 5
Y fikuchr
et al /Anal
Chm
69
Acta 264 (1992) 6.5-70
equation [19], the values of the ion-pair formation constant m the aqueous phase, K,p,aq, can be assumed to be the same between the metal ions Consequently, the term K,p,aqKd,,pK~~ m Eqn 16 can be assumed to be constant, and the dlfference m the extractlon constants between alkalme earth metals, Alog K&, 1s attnbuted to the dlfference m the S-M2+ complex formatlon constant m the aqueous phase, A log Km,,,, As seen from Table 1, the thermodynamic extraction constants increase m the order Ca2+< Sr2+ < Ba’+, 1 e , the formation constant of the S-M2+ complex m the aqueous phase increases with increase m the ionic radms of the central metal ion This order of log K& agrees with the order of the formation constants of l&crown-6 complexes m aqueous solution, and the dlfferences m the log K’& values between the metal ions are comparable to those of the forrnatlon constant of l&crown-6 complexes (log Kcom,,aq of l&crown-6 0 5 for Ca2+, 2 72 for Sr2+ and 3 87 for Ba2+ 1201) The poly(oxyethylene) compound 1s considered to form complexes with the structure of a “turban” [15,21-231, 1e , the metal ion 1s surrounded by the EO cham m a hehcal conformation In a previous study of the etiractlon of alkali metal plcrates with monodisperse POE compounds [161, it was suggested that the cavity size of the helical conformation of the poly(oxyethylene) compounds is almost the same irrespective of the number of EO umts, and its 1s comparable to that of 18-crown-6 Hence it 1s reasonable that the coordination behavlour of Trlton X-100 1s similar to that of l&crown-6 The effect of the lomc radms, r, on the extraction constant of alkaline earth metals 1s much
Consequently, the plots of log D - log fi’“f_+ log f *,org - log F as a function of lo~M*‘l should give straight lmes with a slope of 0 5 These plots obtamed from expenment (c) are shown m Fig 3, where the values of f, and f_ were evaluated by usmg the Davies equation [181 The experimental resuls agree well with straight lines with a slope of 05 Hence the number of metal ions m the complex ion was confumed as one The thermodynamic extraction constants, Kz,, thus obtained are given m Table 1
DISCUSSION
Extractwn constant The extraction equlhbrmm
of the ion parr 1s written by elementary processes as m Scheme 1, 1e , the dlstrrbutlon of Trlton X-100 mto the aqueous phase (Ki,t) formation of the alkaline earth metal complex with Trlton X-100, S-M2+ ion-pan formation of the S-M2+ com(L,,%J plex vvlth plcrate ion U&J m the aqueous phase and the dlstrlbutlon of the ion pair mto 1,2-DCE (&,,,) The extraction constant of the ion pair, Kz,, 1s expressed by K& = K,,,,,K,~,~,K,,,,K~,~
(16)
Kd,, 1s common for any systems of alkaline earth metal ions The value of K,,,p can be assumed to be the same lrrespectlve of the kmd of the alkaline earth metal Ion, because the molar volume of the ion pan 1s scarcely changed on changing the central metal ion As will be mentioned below, the mterlomc distance Ff all ion pairs studied here 1s longer than 2 A even m 1,ZDCE Thus, as deduced from the Fouss’s
m2+ org aq
.2+
w=-1210,g
phase %iP
phase
+
Scheme 1
2pic-
+
+
2pic-
Ki ,P,
a
K 19!42+ (Pic-)21
-
70
Y Kkuchr et al /Anal Chm Acta 264 (1992) 65-70
larger than that for alkali metals, 1e , the dlfference m log K& between Ca*+ (r = 0 99 A> and Ba*+ (r = 136 A) 1s A log K& = 3 04, whereas Alog K&=13! between Na+ (r=O97 A) and K+ (r = 133 A) The extractablhty (log D) for the Ba2+ system 1s less than that of the K+ system under the present experimental condotions Interwnrc dntance of the ion paw m 1,2-DCE The mterlomc distance, a, between Ions of the
Ion pair can be estimated from the dissociation constant of the ion pair usmg @errurn’s theoretlcal equation [17] If Ion-pair formation by 1 1 charge interaction between [SM (Plc)]+ and PIG1s assumeOd, the a value was estimated to be about 4 A Irrespective of the kmd of alkaline earth metal ion The a value was obtained as 6 w for the alkali metal ion-Tnton X-100 system [15] If we take mto conslderatlon the structure of the ion pair of the “turban” complex m which the metal ion IS surrounded by the oxyethylene chain of Trlton X-100, and the plcrate ion 1s situated :utslde the oxyethylene chain, the value of a = 6 A for the alkah metal complex 1s reasonable That is, the a value for the alkalme earth metal complex seems to be too small Assummg a 2 1 interaction between the local positive charge of [SM2+ (Plc-)I, [SM*‘] and Plc- as an extreme case, we obtain an unreasonable value 12 A for a These results may mdlcate that the dlssoclatlon constant of the ion pan of the alkaline earth metal complex IS lowered by the effect of the double charge of the metal complex ion
REFERENCES 1 W Rabhofer, W M Muller and F Vogtle, Chem Ber , 112 (1979) 2095 2 F Votle and E Weber, Angew Chem , Int Ed Engl, 18 (1979) 753 3 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn ,51 (1978) 1294 4 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn, 51 (1978) 3111 5 DC W Slew, R P Cooney, M J Taylor and A J Easteal, J Chem Sot , Faraday Trans ,86 (1990) 1109 6 S Yanaglda, K Takahashl and M Okahara, Bull Chem Sot Jpn , 50 (1977) 1386 7 AMY Jaber, G J Moody and J D R Thomas, J Inorg Nucl Chem , 39 (1977) 1689 8 Y Sakw, H Nakamura, M Takagl and K Ueno, Bull Chem Sot Jpn ,59 (1986) 381 9 T Sotobayashl, T Suzuki and S Tonouch], Chem Lett , (1976) 585 10 T Sotobayashl, T Suzuki and K Yamada, Chem Lett , (1976) 77 11 T Sotobayashl, T Suzulu and H Kudo, J RadIoanal Chem , 36 (1977) 145 12 T Suzuki, N Murakaml and T Sotobayashl, Bull Chem Sot Jpn ,53 (1980) 1453 13 D L Jones, G J Moody and J D R Thomas, Analyst, 106 (1981) 439 14 R J Levms, Anal Chem , 43 (1971) 1045 15 Y Kdmchl, N Takahashl, T Suzulu and K. Sawada, Anal Chum Acta, 256 (1992) 311 16 Y Qkuchl, Y NoJlma, H IOta, T Suzuki and K Sawada, Bull Chem Sot Jpn, m press 17 N BJerrum, Dan Vldensk Selsk Mat Fys Medd, 7(9) (1926) 1 18 C W Davies, J Chem Sot, (1938) 2093 19 R M Fouss, J Am Chem Sot, 80 (1958) 5059 20 R M Izatt, R E Terry, B L Haymore, L D Hansen, N K Dailey, AG Avondet and J J Chrlstensen, J Am Chem Sac , 98 (1976) 7620 21 W Saenger, I H Suh and G Weber, Isr J Chem, 18 (1979) 253 22 G Weber, W Saenger, F Vogtle and H Sleger, Angew Chem , Int Ed Engl , 18 (1979) 226 23 M D Adams, P W Wade and R D Hancock, Talanta, 37 (1990) 875