Solvent Extraction of Aluminum Oxinate TOMIHITO KAMBARA and HlROSHl HASHlTANl Department of Chemistry, Ritsumeikan University, Kyoto, Japan
FTaking into consideration the amphoteric dissociation of 8-quinolinol and the formation of aluminate ion in alkaline medium, a theory for the extraction of 8-quinolinol and aluminum oxinate by organic solvent i s developed. The similarity between the polarographic current-voltage curve and the extractability-pH curve is shown.
D’ =-[HOs] [HOx]Eo
(3)
where D’ is the partition coefficient, in which activity is approximately replaced by concentration. When v, liter of organic solvent is shaken with V , liter of aqueous solution, the total amount of 8-quinolinol is given by Go, = Y[HOx],
the pioneering ~ o r l rof Berg (1) on the analytical application of 8-quinolinol(8-hydroxyquinoline, oxine) many monographs (2, 6, 16, 17) have treated this subject. The most fundamental studies mere performed by Got6 (5), Fleck and Ward (S), hfoeller (11, 12), AIorrison and Freiser ( I S ) , Lacroix (IO), Irving (8, Q), and others, mho elucidated the effect of p H on the precipitation of metal oxinates or their extractability by an organic solvent immiscible. with water. Hon-ever, the close similarity between the extractability-pH curve and the familiar dissociation curve and the rational method of analyzing the experimental extractability-pH curve have not been successfully explained. I n this work, a theory of aluminum oxinate extraction is developed and the theoretical prediction is confirmed by experimental investigation, so that the extraction colorimetry of aluminuni is on a theoretically more concrete basis.
Theoretical Treatment. According to Irving and Williams (9) cationic, anionic, and molecular species of 8quinolinol in aqueous phase will be shown by HzOx+, Ox-, and HOx, respectively. The amphoteric dissociation can thus be represented by
where square brackets represent the concentration and the subscript w refers to the aqueous phase. ( H + ) is the hydrogen ion activity and K1‘ and K,’ are the Bronsted acidity constants. Partition of molecular 8-quinolinol between aqueous solution and the immiscible organic solvent-chloroform, denoted by suffix 0-is given by
(5)
Next, let g be the concentration ratio of molecular form in aqueous phase to the total amount of 8-quinolinol in the same phase. Then it can be found that
I n acidic medium, where -log K1’ = pKz’ >> pH, and neglecting the term K2‘/{H+) in the denominator of Equation 6, there is the following approximate expression
1. i l + tanh 2 303 (pH - pKi’)[ 2 /
2 pH = pKi‘
+ 1 0 g iY- q
3
(7)
Such an expression containing hyperbolic function was first introduced into the study of extraction by Irving and Williams (9). I n the same manner, in alkaline medium, where p H >> pK‘,, it is deduced
Extractability of 8quinolinol by chloroform denoted by z can now be obtained in an analogous manner. Defining B by
one obtains the following equation in an acid medium z = - - [HOxIoVo -
-
pH =
(12)
(pHl,?)r.ncidio
+ logZ-
(13)
1 - 2
1
2.303 I + tanh 7 (pH - PIG’ + B ) ; (10)
where shown by
x- 11 --
2
(14)
where the pH value of half-extraction is shown by (pHl/:)c-alraline
= p&’
+B
(15)
Likewist! the distribution of 8-quinolinol as ca tionic, anionic, and molecular species in r,queous phase can bc deduced, and because it has a similar form, is omitted. Experimental Verification of the Theory. hfoeller and Pundsack (13) accur:itely determined the distribution of 8-quinolinol in various forms between c iloroform and aqueous solution a t 2:)” C. with considerable dependence on the p H value of the latter phase. Their result is used here to verify thct theory developed above. The numerical values shown by them were recalculated and tabulated in a different nianner by Hollingshead ( 7 ) . I n Figure 1 are drawn the theoretical y-pH and z-pH curves and the points found experimentally are indicated by open circle 3. To find :orrectly the p H values corresponding to the half-extraction or half-dissociation, the method of socalled logarithmic plot that has been widely applied to the analysis of polarographic current-voltage curve is used. Figure 2 demonstrates the logarithmic plots, in which the values of pK,’, ~ K z ’ , and pHl/zc in be easily determined from experiment11 data by drawing straight lines, the slope of which clearly shows the theoretical value-unity. Further it is found f .om the logarithmic plot that pKi’
Gox
h2
-B
pKi’
whence
=
21 =
=
I n the case of alkaline solution one derives
INCE
EXTRACTION OF OXlNE
(pH,/P)r-neidio
(4)
where
S
is approximately equal to unity. The p H value of half-extraction is shon-n by
-
( p I r 1 / 2 ) z acidic
= 5.05
2.45 = 2.60
(pHi/~),nlknline - pKz‘
= 12.35
-
9.86
=
2.50
indicating ihe essential validity of the VOL. 31, NO. 4, APRIL 1959
567
above consideration. Experimental values for pKl' and pK,' thus obtained from the data of Moeller and Pundsack are also in good agreement with those reported by other authors such as Lacroix ( l a ) .
p'i
P i(2'
M
EXTRACTION O F ALUMINUM OXINATE
Theoretical Treatment. When concentration as the first-order approximation is replaced by activity, the partition coefficient of aluminum oxinate between organic solvent and aqueous solution is given by
I
and the dissociation constant of the chelate compound shown by
Thus there is [AlOz-]( H f )
l
11
Distribution of 8-quinolinol in various forms
[A13+]
A portion of 8-quinolinol in the whole system exists now as aluminum oxinate; therefore the total amount of the reagent is shown by Gox = 3a[AlOs,]w where a =PT,
+
OX-]
+ v,
The extractability oi' aluminum oxinate is now defined by
(19) (20)
Y{H+)/K*' (21) I n the same manner the total amount of aluminum is given by a' =
-where
from which the tb3oretical expression for the extraction w w e as a function of acidity could be derived. I n acidic solution, in which the concentration of anionic 8-quinolinol is vanishingly small but practically no hydrolysis of aluminum ion takes place ( p K'VJ, Equation 25 can be approximately transformed into
Elimination of [AIOxs], from Equations 19 and 22 gives
+ ~(3G.41- Go,) [ O S - ] $ + a'fl[Os-] - 3 Gox = 0 (24)
[Ox-]
+
GoxlCz'
-~ H +) D 'V,
aa'[Ox-]'
Now it can be seen that
The solution of this quadruplicate equation is complicated, but it can be rationally solved. From this distribution in varying forms of 8-quinolinol, it can be reasonably considered that in acidic medium the concentration of anionic 8-quinolinol is vanishingly small, so that the first two terms in the lefthand side of Equation 24 can be neglected, whereas in alkaline medium the anionic species becomes predominating, so that the last two terms will now drop out, Thus one obtains, assuming that an excess of 8-quinolinol is used, the following
x
568
9
/
y-pH curve showing concentration ratio of molecular species to total omornt of 8-quinolinol in aqueous phase [cf, Equations 6, 7, 8 ) B. Conccmtration ratio of cationlc species to total amount in aqueous phase C. Conc?ntration ratio of anionic 8-qulnolinol to total amount in aqueous i111ose A'. r - p t l curve showing extracted fraction of total amount of 8-quinolinol in both phases (cf. Equations 10 and 13) 6'. Fraction of 8-quinolinol in cationic form C'. FratlEon o f 8-quinolinol in anionic form 0. Meailired values o f Moeller and Pundsack (72) in whose experiment VO =- V , = 25 ml. and temperature i s 25' C. Theoretical curves, drawn a f w values of pK'1, pK'z, and pHm's, are determined by logarithmic plol shown In Figure 2
+ 2 H20 7;! -4102- + 4 H +
ICE' =
7
h
A.
Furthermore the formation of soluble aluminate ion is shown by Ala+
5
3 Figure 'I.
/
ANALYTICAL CHEMISTRY
=
4
where
[l
+t
2.303 X 3 a n h 2- L
The greater the value of K,' the more easily the aluminate ion is formed and the lower becomes the pH value of halfextraction. This feature of oxinate extraction was not explained by Lacroix (10) and Irving and Williams (9). Experimental. Extraction of aluminum oxinate with chloroform and other solvents has been experimentally investigated by Moeller ( I I ) , Gentry and Sherrington (4), Lacroix (IO), Sudo ( I C ) , and Motojima (14). In the first three publications, the formation of aluminum hydroxide was sometimes observed to result in a decrease in the extractability of aluminum oxinate from neutral solution, which is clearly due to the use of a chloroform solution of 8quinolinol. To avoid this difficulty, the last two authors employed the method, in which, after the addition of 8-quinolinol solution to the sample solution cont,aining aluminum, the acidity is regulated as in the gravimetric procedure, and the resulting aqueous solution is shaken with chloroform. I n this manner the precipitation of
1.0
2 0
0
-2
Figure 2. log
Y vs. 1 -Y
Logarithmic plots
p H and log
I vs. p H curves 1-2
Data of Moeller and Pundsack (12). From logarithmic plot it it found that pK1‘ = 5.05; pKn’ = 9.85; ( p H ~ / & . ~ ~ =~ 2.45 d ~ ~ and (PHi/d.-dkaiine = 12.35. i Isoelectric point 0.
aluminum hydroxide can be prevented, because once formed, it converts only slowly to oxinate.
PROCEDURE. To 10 nil. of solution
containing 25.0 y of aluminum (GAI = 0.926 x 10-6 mole) is added 3.0 ml. of 8-quinolinol-acetic acid solution containing 1 gram of 8-quinolinol in 100 ml. (Go, = 207 x 10-6 mole), and the p H of the aliquot is regulated by adding suitable amounts of acetic acid, ammonium acetate, ammonium hydroxide, and sodium hydroside solutions. The aqueous solution thus obtained is diluted to 50 ml. with water and shaken with 10.0 ml. of chloroform a t room temperature. The aluminum oxinate concentration in the chloroform phase is determined spectrophotometrically a t the wave length 390 mp. The hydrogen ion activity of the aqueous phase is measured using a glass electrode p H meter. The result is tabulated in Table I and from the measured values the 2 us. pH and log x/(1 - z) 2)s. pH curves are plotted as illustrated in Figure 3. As shown by the logarithmic plot, the slope of extraction wave in acidic and alkaline region was three and four, respectively
.
No special attention was paid to maintaining the ionic strength constant; nevertheless the experimental result clearly shows the essential validity of this theory. DISCUSSION
The expression “pH range of complete extraction” has often been used in the literature. According to this investigation, the term “pH value of half-extraction” is more reasonable, because the analogous terminology “half-wave potential” is so frequently used in polarography for a clear understanding of the depolarization potential characteristic of each redox system. According to Equations 29 and 31, the value for
Figure 3. Effect of pH on extracttrbility of aluminum as oxinate from aqueous solution irito chloroform From logarithmic plots, p H valuer for haif-edroction a r e 4.05 and
1 1.27 in acidic and alkaline wave, respectively
99.90% extraction in acidic and alkaline region, respectively, is shown by PH
99.90%
=
Table
I.
Extradion of Aluminum Oxinate
(PHl/t)o-acidie$. 1
~ H Q P . Q=O( % ~ H I / =-alkaline z)
3
- -4
These could more accurately indicate the analytical procedure required. ACKNOWLEDGMENT
The authors express their thanks to
M. Ishibashi, Kyoto University, for his advice. LITERATURE CITED
(1) Berg, Richard, “Die analytische Verwendung von o-Oxychinolin (Oxin) und seiner Derivate,” 2nd ed., Enke, Stuttgart, 1938. (2) Flagg, J. F., “Organic Reagents,” Interscience, New York, 1948. (3) Fleck, H. R., Ward, A. M., Analyst 58,388 (1933); 62,378 (1937). (4) Gentry, C. H. R., Sherrington, L. G., Anal@ 71,432 (1946). (5) Gotd, Hidehiro, Nippon Kagaku Zassi 54, 725 (1933); 56, 314 (1935); Sci. Repts. TGhoku Univ. 26, 391 (1937-38); 26,418 (1938). (6) Hollingshead, R. G. W., ‘‘Oxine and Its Derivatives,” Butterworths, London, 1954. (7) Ibicl., Vol. 1, p. 52. (8) Irving, H. M., Quart. Rev. (London) 5 . 200 (1951)(9) ’Irving, H.‘ M., Williams, R. J. P., J. Chem. SOC.1949, 1841. (10) Lacroix, S., Anal. Chhim. Acta 1, 260 (1947). (11) hIoeller, Therald, IND.ENG.CHEU., ANAL.ED. 15, 346 (1943). (12) Moeller, Therald, Pundsack, F. L., J. Am. Chem. SOC.75,2258 (1953). (13) Morrison, G. H., Ffeiser, Henry, “Solvent Extraction in Analytical Chemistry,” Wiley New York, 1957. (14) Motojima, Kekji, Nippon Kagaku Zasshi 76,903 (1955). (15) Sandell, E. B., “Colorimet;,ic Determination of Traces of Metals, 2nd ed., Interscience, New York, 1950. (16) Sudo, Emiko, Nippon Kagaku Zasshi
PH 3.40 3.68 3.87
Observed Absorbance at 300 R l p Aci’licWave 0.033 0.105 0.185 0.236
3.88 3.90 3.95 4.00 4.12 4.22 4.35 4.49 4.72 0.648 Complete Extractiono 4.75 0.659 0.650 4.9e 0.677 5.12 5.20 6.42 8.09 9.07 0.36 9.70 9.71 Alkaline Wave 0.658 10.00 0.652 10.74 0.641 10.90 0.630 11.01 0.590 11.08 0.344 11.21 0.422 11.21 0.208 11.34 0.030 11.70 0.018 12.20 6 Mean value of absorbance at complete extractions (pH range 4.75 - 10.00) was 0.661 5 f 0.0819 6.
72, 718 (19511; Sci. Repts. TBhoku Univ. Ser. A. 4, 268 (1952). (17) Welch:;, F. J., “Organic Analytical Reagents, Var. Nostrand, New York, 1947. RECEIVED for review March 10, 1958. Accepted Novemker 10,1958. VOL. 31, NO. 4, APRIL 1959
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