Mixed Ligand Complex Formation in the Extraction of Zinc in the Presence of 8-Quinolinol and 1,lO-Phenanthrolines Colin Woodward' and Henry Freiser Department of Chemistry, Unioersity of Arizona, Tucson, Ariz. 85721 Equilibrium distribution studies have been made of the extraction into chloroform of zinc(l1) with 1,lOphenanthroline, mixtures of 1,lO-phenanthroline and 8-quinolino1, and related systems. A synergic effect is noted in the extraction from mixed ligand systems, which arises from the formation of ZnOx,.phen and Znphen20x,+C104-. The results have been used to construct a three dimensional "phase" diagram for the zinc-8-quinolinol-phenanthroline-perchlorate system. 4,7-Dimethyl-l,lO-phenanthroline behaves similarly to 1,lO-phenanthroline.
RECENTLY, a considerable amount of attention has been devoted to the solvent extraction of metal ions with mixtures of ligands. In most of these systems, the second ligand is monodentate and an adduct is formed from which none of the chelating ligand is replaced. For example, Ca and Sr have been extracted with thenoyltrifluoroacetone and tri-n-butylphosphate ( I ) and Th and Am have been extracted with mixtures of TTA and trioctylamine ( 2 , 3 ) . Examples of mixed chelate formation in extraction are less common. In one study, the formation of both adducts and mixed chelate complexes of Cu(I1) and Zn(I1) with TTA and P-isopropyltropolone has been demonstrated by distribution measurements ( 4 , 5 ) . The extraction of rare earths with these two ligands has also been investigated (6). Yoshida (7) has examined the extraction of UOs2+and Eu3+ with TTA and 8quinolinol and found a synergic effect in the presence of both ligands. The formation of a mixed ligand complex of Cu(I1) with 1,IO-phenanthroline and 8-quinolinol-5-sulfonic acid has been demonstrated potentiometrically (8). In a previous communication from this laboratory (9), the results of experiments which indicated mixed ligand complex formation of zinc with 8-quinolinol and 1,lO-phenanthroline were reported. A more thorough study of this system and that with 4,7-dimethylphenanthroline made to establish the nature of the mixed complexes formed and to evaluate pertinent equilibrium constants is reported here. EXPERIMENTAL
Apparatus. Samples were prepared in 45-ml cylindrical tubes fitted with polyethylene stoppers and plastic screw caps. The samples were then shaken in an Eberbach re-
' Present address, Department of Chemistry, Louisiana State University, Baton Rouge, La. 70803.
ciprocating shaker at the high speed setting. Water at 25' =t0.2" C from a Wilkens-Anderson Co. Lo-Temp bath was circulated through the jacketed shaker tray. Radioisotope counting was carried out with a NuclearChicago Model DS-55 well-type scintillation detector connected to a Model 192 A scaler. Kimax lipless culture tubes (15 X 125 mm), covered with Parafilm, were used as counting containers. A Beckman Model G pH meter with a glass-calomel electrode pair was calibrated with Beckman buffer solutions at pH 4.00 and 7.00and used for all pH measurements. Absorption spectra and quantitative spectrophotometric measurements were obtained with the aid of a Beckman DB spectrophotometer. Reagents. A.R. Grade 8-quinolinol (Mallinckrodt) was recrystallized from absolute ethanol to give a product melting at 72.5-73.5 O C. 1,IO-Phenanthroline monohydrate (G. F. Smith Chemical Co.), 2,9-dimethyl-1,lo-phenanthrolinemonohydrate (Eastman Organic), and 4,7-dimethyl-1,lO-phenanthroline hemihydrate (G. F. Smith Chemical Co.) were used without further purification. Chloroform was shaken three times with an equal amount of water to remove possible traces of alcohol. Carrier-free 65Zn radiotracer solutions (approximately 10-6M) (New England Nuclear Corp.) were used in all extraction experiments. All other reagents used in this study were A.R. Grade. Buffer solutions, pH 4.85, were prepared by adding sodium hydroxide solution to acetic acid solutions until the required pH was reached. To obtain meaningful results from comparative experiments carried out under different experimental conditions, a very close control of pH was necessary. This was ensured by preparing a large volume of pH 4.85 sodium acetate buffer, dividing this into two parts and adding the small required amount of sodium perchlorate to one portion. The pH values of the two buffer solutions used were, therefore, virtually identical. Procedure. In a typical experiment, 10 ml of chloroform containing known concentrations of 8-quinolinol and 1,lophenanthroline were equilibrated with an equal volume of aqueous buffer (pH 4.85) containing 65Zn. A shaking time of 30 minutes was found to be adequate for the attainment of equilibrium and was used throughout the study. The solutions were then allowed to stand for at least 1 hour to accomplish phase separation. The final volume of each phase was unchanged after the completion of extraction. After phase separation, 5-ml aliquots of both phases were removed using a pipet and were counted separately. RESULTS AND DISCUSSION
(1) 'T.Sekine and D. Dyrssen, Anal. Chim. Acta, 37,217 (1967). (2) L.Newman and P. Klotz, J . Phys. Chem., 67,205(1963). (3) L.Newman and P. Klotz, Inorg. Chem., 5,461 (1966). (4) T. Sekine and D. Dyrssen, J . Inorg. Nucl. Chem., 26, 1727 (1964). ( 5 ) Ibid., p. 2013. (6) T. Taketatsu and C. V. Banks, ANAL.CHEM. 38,1524(1966). (7) H.Yoshida, Bull. Chem. SOC.Japan, 39,1810(1966). (8)J. P. Scharff and M. R. Paris, C . R . Acad. Sci. Paris, 263, 935 (1966). (9) F. C. Chou and H. Freiser, ANAL.CHEM., 38, 1925 (1966).
When a metal ion forms more than one extractable complex, the experimentally observed distribution ratio can be expressed as the sum of the distribution ratio values of each of the complexes involved if it can be assumed that none of the extracted species will influence the extent of extraction of any of the others. In order to use equilibrium extraction data for proper assessment of the role of each of the metal complexes in the extraction system and to evaluate all of the equilibrium constants involved, it is necessary to identify those regions where the exVOL. 40, NO. 2, FEBRUARY 1968
* 345
3t
Table I. Dissociation and Distribution Equilibria of Some 1,lO-Phenanthrolines (CHCI3/water)at 25" C and fi = 0.10 PK. log KDR 1,1bPhenanthroline 5.05 f 0.10 3.05f 0.15 4,7-Dimethyl-l,IO-phenanthroline 6.04& 0.14 3.70 f 0.15 2,9-Dimethyl-l,lO-phenanthroline 5.97f 0.10 3.80 3= 0.10
traction of a single complex predominates. If the appropriate extraction equation for such a complex can be determined, its contribution to the total metal extraction under other experimental conditions (i.e,, other reagent concentrations) can be evaluated. If an experimentally significant extent of extraction remains after such contributions are subtracted from the total, these residual extraction values can be used to detect the presence of, and the quantitative evaluation of, other component complexes. Proceeding in this fashion in the zinc(II)-8-quinolinol-l,lOphenanthroline-(phen)-perchlorate system, four extractable complexes were found to be necessary and sufficient for a quantitative description of the observed extractions over a wide variety of experimental conditions. These are ZnOxz . HOx, Znphen32+.2C104-, and the mixed ligand complexes ZnOxz.phen and ZnOxpheniC .C104-. When acetate buffers are employed, Znphen(OAc)2is also extracted. In the extraction of Zn as Z n ( ~ h e n ) ~2C104-, +~
LOG
1
I
I
I
-
Dd
=
[Zn(~hen)3+~. 2C104-],
cz.
(1)
Then, taking KIPas the constant for the reaction
+
KIP
Z n ( ~ h e n ) ~ ~ 2c104+ $ Zn(phen)3+zs2C104-
(2)
Kf as the overall formation constant of Zn(~hen)3~+, (3)
and KDC=
[ Z n ( ~ h e n ) ~2C104-], +~ [Zn(phen) 3+ 2 . 2C104-]
phenanthroline for the zinc coordination positions and reacting as a coordinating ligand rather than as an ion-pair forming species. Thus, in the absence of perchlorate, the slope of log D us. log [phen], curves at [OAc-] = 10-lMwas 1.1 which was compatible with the formation of Znphen (OAC)~ as the extractable species. It was noteworthy that when extractions were carried out over the same range of [phen],, but at [OAc-] = 10-zM, log D us log [phen], curves showed a slope of 1.8-i.e., zinc complexation with phenanthroline was now more favored. It follows that for this species,
e
(4)
De
=
K,,,[phenl[OAcI*
Pz.
(6)
where K,,, is the overall extraction constant of Znphen (OAC)~.Wherever necessary, extraction values were corrected for the contribution of this specie. A similar analysis of the extraction behavior of the two substituted phenanthrolines was carried out. To obtain the K,,, ~C10~-lz[~henlo3P~. (5) needed K D R values, the values of their acid dissociation conwhere K,,, is the overall extraction constant of Z n ~ h e n ~ ~ + . stants, Ka, determined spectrophotometrically (IO),and their distribution ratios over a range of pH values were measured 2C104- and p, the fraction of zinc in the aqueous phase in the (Table I). form Zn2+,is introduced to account for complex formation in The measurements of distribution ratios for the three the aqueous phase. The use of P is necessary in those conligands had relative standard deviations of 17 %, 7 x ,and 9%, centration ranges where there is sufficient phen in the aqueous respectively, equivalent to 0.07, 0.03,and 0.04 log units in the phase to form Znphen2+and Znphenz+z. results for KDR. The major source of inaccuracy was, thereThe zinc extraction curves obtained in this portion of the fore, in the spectrophotometric determination of the pK, study, at pH 4.85 in the presence of perchlorate with chlorovalues and this was caused by the similarity between the ultraform solutions of phen, are shown in Figure 1. These curves violet absorption spectra of the neutral and protonated forms have linear portions whose slopes vary from 0.9 to 3. After of the ligand in all three instances. application of the p correction, however, these slopes approach The extraction of zinc with 4,7-dimethylphenanthroline three except at low perchlorate and relatively high acetate concentrations. Comparison of the formation constants of Znphenz2+,Znphen32+,ZnOAcf, and ZnOAc2 suggested that, (10) S. Takamoto, Q. Fernando, and H. Freiser, [bid., 37, 1249 under these conditions, OAc- could be competing with (1965). it can be shown that
346
ANALYTICAL CHEMISTRY
Table 11. Summary of Extraction Constants Extracted species phen 4,7-DMP ZnOx, HOx Log Kexo - 5 . 2 f 0 . 4 - 5 . 2 rt 0 . 4 ZnOx,. phen Log K e x , -2.2 f0.3 -1.4 f0.7 8.7 f 1 . 1 Log K,,, 6.8 f0 . 6 Znphen20x+.Clod13.7 f0 . 4 18.5 f 1 . 5 Znphen3*+.2C104- Log K,,, 4.3 f0 . 2 5 . 5 rt 0 . 2 Log K,, Znphen(OAc)s 9
-Ii
C
(DMP) in the presence of perchlorate is quite similar to that with phen. A comparison of the extraction constants, K,,, (Table 11), demonstrates the greater extent of extraction achieved with DMP under comparable conditions. Because the Zn-DMP-C104- system has an extraction constant of 5 to 6 orders of magnitude greater than the Zn-phen-C104- system, it is possible to extract zinc quantitatively using approximately one hundredth as much ligand or one thousandth as much perchlorate. This extraction system then, offers clear advantages over many existing zinc extraction procedures. The change in K,,, may be attributed largely to an increase in KipKDC because the changes in KDR (given in Table I) and K, (log Kr = 17.1 for phen, ca. 18.5 for 4,7-DMP (11) are mutually compensating. For the extraction of zinc as ZnOx2. HOx, it has been shown previously (12) that D, =
[ZnOx2.HOx],K’[OX]2[HO~],Pz, - KIKZK’DC (7) CZn KDR [HOxl, where KDR= __ [HOxI K’DC=
-
[ZnOxz HOx], [ZnOxz.HOx]
(9)
and K,, KZare the stepwise formation constants of ZnOxn and K‘ the adduct (Kn0xZS HOx) formation constant. From this, it follows that
li
-2 -2
I
I
I -2
-3
-1
/*
l
l
I
-5
-4
LDG (ox ne),
I
I
-3
-2
LOG G phOPb
Figure 2. A. Extraction of zinc with 8-quinolinol, pH 4.85; no perchlorate, no 1,lO-phenanthroline B. and C. Extraction of zinc with 1,lO-phenanthroline, pH 4.85; B, no perchlorate; C, log[perchloratel = -3.0. Dotted lines are after B correction
-
plexes. Thus, in order to show the existence of the ZnOx2. phen complex, the extent of extraction of ZnOx2.HOx at pH 4.85 in perchlorate-free conditions was first established (see Figure 2 4 . Let the distribution ratio of the zinc complex at a given 8-quinolinol concentration be Dl(=DQ). The extraction behavior of zinc with 1,lO-phenanthroline alone was then investigated under similar perchlorate-free conditions (see Figure 28). The slight extraction noted was caused by formation of Znphen2+ (OAc-)p as discussed above. Let the distribution ratio at a given phenanthroline concentration here be Dn(=D,). Extractions were then carried out under the same conditions but with both 8-quinolinol and phenanthroline present, distribution ratio Ds(=D,+ ob+ De)(see Figure 3A). The difference between the distribution ratio observed using both 8-quinolinol and phenanthroline together ( D 3 )and the sum of the distribution ratios obtained when each ligand is used alone under the same conditions (Dl D2)must be attributed to the extraction of mixed ligand complexes (Le., &). Thus, by plotting log [ D 3 - (Dl Dz)] us. log [phen], (see Figure 3B) the contribution of the mixed complex ZnOxz.phen can be isolated. In this figure, the dependence of the extraction on phenanthroline concentration after p correction is unity. This confirms that the extracted species has one phenanthroline molecule per zinc atom. Furthermore, the dependence upon 8-quinolinol concentration, given by the vertical distance between the two curves in Figure 38, is approximately 1.6 (calculated = 2.0). The quantitative description of the extraction of Zn as ZnOx2Sphen depends upon the equilibria discussed above and also upon the formation of the adduct:
+
+
where K, is the acid dissociation constant of 8-quinolinol and K,,, is the overall extraction constant for ZnOxz.HOx. The BZn term, which was unnecessary in the previous work on the extraction of zinc with oxine alone is included here to correct any complexation of zinc in the aqueous phase by phen during extractions from mixed ligand systems. Thus, extraction curves (log D us. log [HOx],) for this system show a dependence on oxine concentration of three; the dependence on pH is two. In order to investigate the nature of the mixed complexes formed, extractions of zinc were carried out from systems containing both 8-quinolinol and 1,IO-phenanthroline. After correction of the observed distribution ratios for the known contributions of the single ligand complexes under these conditions, the residual extraction could be attributed to the presence of one or more extractable mixed ligand com-
KAD
ZnOxs
+ phen $ ZnOxpphen
and its distribution between the two phases,
KDC” = (1 1) L. G . Sillen and A. E. Martell, “Stability Constants of Metal-
Ion Complexes,” Chem. SOC., London, 1964. (12) F. C. Chou, Q. Fernando, and H. Freiser, ANAL. CHEM.,37, 361 (1965).
(11)
[ZnOxzphen], [ZnOx2phen]
It can then be shown that for this system, the distribution ratio of ZnOx:, phen is VOL 40, NO. 2, FEBRUARY 1968
347
I
d
I
-1
-1
I -5
I
I
-3 ioG(4.7-tNP). -4
I -2
I -5
I
I
-3 LcG(4.7-DMP), -4
I -2
Figure 4. Extraction of zinc with 4,7-dimethylphenanthroline, pH 4.85; curves 0 , log[oxine], = -1.3, curves x, log[oxine], = -2.3; A, no perchlorate; B, Log[perchlorate] = -3.0. Dotted lines are after fl correction
I
I
I
I
I
I
Figure 3. Extraction of zinc with l,l&phenanthroline, pH 4.85; curves 0 , log(oxine), = -1.3; curves x, log (oxine), = -2.3. In A and B, no perchlorate; in C and D , log[perchlorate] = -3.0. Dotted lines are after fl correction
where K,, is the overall extraction constant of ZnOxz.phen. A series of experiments similar to those described above was also carried out on the extraction of zinc with 4,7-dimethylphenanthroline and 8-quinolinol. Figure 4A shows the results of plotting log (D3 - B D1,2)us. log [4,7-DMP], for the zinc-4,7-DMP-8-quinolinol system in perchlorate-free conditions. These results provide good evidence for the formation of the species ZnOxs .(4,7-DMP). To test for the presence of other mixed ligand species, the extraction due to ZnOxz.HOx, and ZnOxz.phen (called D3 above) was combined with the extraction of zinc with phenanthroline alone at the same pH and in the presence of a known concentration of perchlorate, called D 4 ,(see Figure 2C) which is attributable to Znphen32f.2C104-and Znphen (OAc)'. The sum of these two extractions, Z D 3 , 4 ,was then compared with the extraction in the presence of both 8-quinolinol and phenanthroline and at the same perchlorate concentration, called D s (see Figure 3C). Any difference found between D6 and BD3,4,measured under similar conditions, must be attributed to the formation and extraction of another complex containing both 1,IO-phenanthroline and 8-quinolinol (i.e., 0,). The results of plotting log ( D s - B D 3 , 4 )us. log [phen], are shown in Figure 3 D . In order to obtain a reasonable estimate of the validity of these results, it was assumed that the maximum possible error 348
ANALYTICAL CHEMISTRY
in the measurement of a given D value was &12% for 0.01 < D < 100 and * 2 5 % outside of these limits. These estimates are equivalent to errors of 10.05and 20.10 log units, respectively, in log 0. The results of these calculations of the possible errors in the values of log ( D - ED) are indicated in Figures 3B and 3 0 . Although the relative deviation in Figure 3 0 is larger than might be desired, it will be seen that, within the limits of experimental accuracy, these results indicate the formation of the mixed ligand species Znphen20x+. C104--i.e., the dependence of extraction upon phenanthroline concentration after @ correction is two (1.7 & 0.3); the dependence upon 8-quinolinol is unity (0.9 f 0.3). Similarly, Figure 4B shows the results of plotting log ( D s - BD3,4) us. log [4,7-DMP], for the zinc-8-quinolinol4,7-dimethylphenanthrolinesystem. As before, although the relative inaccuracy of many of the points is high, it is felt that the results are sufficiently valid to be interpreted as evidence for the formation of Zn(4,7-DMP)20x+,C104-. A major source of the inaccuracy in these experiments arose from the necessity of measuring log D values >2.5, in which instances the very low activity in the aqueous phase necessitated great care to avoid contamination. For the formation of the mixed ligand complex Znphenz Ox+ .C104-, the following equilibria must also be considered: KIPKZP
+ 2 phen $ Zn(~hen)~'+ KM Zn(phen)zz++ Ox- $ ZnphenzOx+ Zn(phen)20x+ + C104- S Znphen20x+.C104ZnZ+
(14) (15 (16)
and
It then follows that the distribution ratio of this species,
D, =
KIP.K2p.K,w.K ' , P . K ~ K D c ~ [ C ~ O ~ - ] [ ~ ~-~ ~ ] ~ ' [ H O X ( K D R ~ )* 'KDR[H+]
where K,,, is the overall extraction constant of Znphenl ox+ . clod-. The complexes discussed above were sufficient to account
Table 111. Summary of Equilibrium Constants 4,7-DimethylPhenanthroline phenanthroline 10.4 & 0.7 Log K A D K D C ~ 9.0 f 0 . 2 4.8 zk 0.7 Log K D C ~ 4.0 f 0 . 2 Log K ’ I ~ K D c ~ ~ 5.4 & 0.6 7 . 8 f 1.0 5.8 f 0.4 11.1 1.5 Log KIPKDC
*
quantitatively for the total extraction observed with this mixed ligand system under a wide variety of experimental conditions. To obtain values for the extraction and equilibrium constants of each of the complexes involved, the data from these experiments were treated as indicated above. Using Equations 5,10, 13, and 18 and the experimental results, it was possible to obtain values for the overall extraction constant for each complex. These are given in Table 11. By substituting all known values for the various K’s which comprise these extraction constants and by making logical assumptions for others, approximate values for some equilibrium constants were also computed. The known values for oxine used were: Log KIKz = 17.06; log K , = -9.94 (11); log KDR = 2.64 ( 1 3 ) ; for phen (4,7-DMP), log K D R , = 3.05 (3.70); Log K l p K Z p= 12.1 (14) (assumed = 12.9). It was assumed that the value of KAD would be the same as K3P,the third stepwise formation constant of Zn~hen~~+-i.e., K A D= 5.0 (5.6 for 4,7-DMP). Similarly, it was assumed that K,, for the addition of Ox- to Znphenp2+,would be the same as K2, the second stepwise formation constant of Zn+2 and oxine-Le., log K.v = 8.0. Although Kjv for the corresponding 4,7-DMP complex is not expected to be the same (perhaps somewhat lower because 4,7-DMP is a stronger primary ligand), the value of log K.,, = 8.0 was assumed in this instance also. Of course, mixed ligand complexes may be more stable. The results of these calculations are summarized in Table 111.
These values of log K& for both ZnOx2.phen and ZnOxz. (4,7-DMP) are of the expected order of magnitude. As noted above, Z I I ( ~ , ~ - D M P )2C104~ ~ + . is extracted more readily than Znphena2+.2c104-, and this is reflected in the results for log K I P K D Cfor these two complexes. Furthermore, the result for log K’lpKDC’vis also considerably higher for the substituted reagent-Le., for Zn(4,7-DMP)z0x+.C104cf. Znphen20x .Clod-. Although one cannot directly compare K , , values of different dimension, it can be said that Znphen32f.2C104- will extract to a greater extent than will any of the other phen-containing species or even the ZnOxz.HOx. The values of K,, compare the effect of ligand substitution in an otherwise identical species. Thus, the effect of replacing phen by 4,7-DMP is an increase in K,, of approximately one log unit per mole. Of the various component constants involved in the overall value of K,,, it can be seen from Tables I1 and I11 that the change in the product of the formation constant of the adduct, or ion pair complex, and its distribution constant quantitatively accounts for the change in the extraction constant due to methyl substitution in the phenanthroline. The extraction equilibrium data were used to construct a three-dimensional (oxine-phenanthroline-perchlorate) (13) J. Fresco and H. Freiser, ANAL.CHEM., 36,631 (1964). (14) F. C. Chou, University of Arizona, Tucson, Ariz., private
communication, 1967.
-8
-7
-6
-5
-3 l o g [phen],
-4
-’,
-2
A
Figure 5. Projection of species dstribution diagram for Zn8-quinolinol-phenanthroline system at pH 4.85 “phase” diagram to describe the nature of the predominating extracted species under given conditions. Such a diagram is extremely helpful in designing experiments for optimizing total extraction, for isolating particular intermediate complexes as well as improving understanding of the overall extraction system. A projection of this diagram is shown in Figure 5. The planes represent conditions under which equal amounts of two species are extracted. Equations for these planes were derived by using the observed K,, values (Table 11) in Equations 5,10, 13, and 18 and equating D i = D j for appropriate pairs of equations. In the diagram, the region to the left of ABC-C’B’A’ is that in which the principal extracting species is ZnOx2.HOx; within the triangular prism BCD-D’B’C’, the main species is ZnOxp .phen; the region bounded by DBAE - E’D’B’A’ is that in which ZnOxphen2+.c104is most important; finally, to the right of AA‘E‘E, the principal extracted complex is Z n ~ h e n ~2c104 + ~ -. A striking feature of the phase diagram is the important role played by perchlorate in determining the number of mixed complexes of significance. Whereas at low Clod- concentration, e.g., [CIO,-] = lO-3M, there are regions in which each of the two mixed complexes predominates, at [Clod-] lo-’. ‘ M (line BB’), one of the species, ZnOxz phen, drops in importance leaving ZnOxphenz+ C104- as the only important mixed ligand species. Although not experimentally attainable in this system, all mixed ligand species are relegated to a minor role when [C104-] 2 103.3M(1ineAA’). One of the ways in which this diagram has proved useful is in examination of sections through it made at constant D values. Unfortunately, although easily constructed on the three dimensional diagram, such sections cannot be displayed clearly on a projection. When questions of completeness of extraction are of primary importance, a second three-dimensional diagram, comprising constant D surfaces, can be useful. By referring to the appropriate “phase” diagram, it was possible to select extraction conditions in the analogous Zn-oxine-4,7-DMP system to obtain ZnOxz.(4,7-DMP) as the predominant species in the organic phase. The spectrum of this extract was run, using as the reference solution the extract
>
-
e
VOL. 40, NO. 2, FEBRUARY 1968
349
obtained by omitting 4,7-DMP from the organic phase in the extraction. Because neither 4,7-DMP nor Zn(4.7-DMP)3+2.2C104- absorbs above 360 mp, the appearance of an absorption band maximum at 400 mp was attributed to the effect of the 4,7-DMP ligand on the electron density in the coordinated 8-quinolinol ligand. The observed bathochromic shift of 20-30 mp from the maximum for the simple chelate, ZnOxz, is similar to that observed when other nitrogen adducts are formed (15). Some preliminary work has also been performed on the extraction of zinc with 2,9-dimethyl-l,lO-phenanthrolinealone. This has been sufficient to show that the behavior of this re(15) Fa-Chun Chou, University of Arizona, Tucson, Ariz. unpublished results, 1967.
agent differs considerably from that of 1,lo-phenanthroline and the 4,7-dimethyl analog. The first difference is that zinc is extracted by 2,9-DMP to an appreciable extent in the presence of acetate ion but not too well with perchlorate (cf. Figure 2 8 which shows that zinc extraction is very low with 1,lO-phenanthroline in the presence of acetate alone). The preliminary indications are that the complex formed is Zn(2,9-DMP)2+, 20Ac-, but more work remains to be done on this, on the possible formation of mixed species of zinc with 2,9-DMP and 8-quinolinol, and related systems. RECEIVED for review June 9, 1967. Accepted November 15, 1967. The authors gratefully acknowledge the financial assistance of the U. S. Atomic Energy Commission.
Equilibrium between Sulfuric and Acetylsulfuric Acids in Acetic Acid-Acetic Anhydride Leo J. Tanghe and Richard J. Brewer Polymer Technology Division, Eastman Kodak Co., Rochester, N . Y .
A method is presented for determining the degree of conversion of sulfuric acid to acetylsulfuric acid in mixtures of acetic acid and anhydride. The reaction mixture is neutralized with an excess of sodium acetate, the solvent completely evaporated in vacuum, and the degree of conversion is determined from the weight of the residue. Conversion of sulfuric to acetylsulfuric acid is favored by high percentage of acetic anhydride, high ratio of solvent to sulfuric acid, and low temperature. Equilibrium was established from both directions in a few minutes at Oo C. The role of acetylsulfuric acid in the acetylation of cellulose is briefly discussed.
ACETICANHYDRIDE and sulfuric acid react to give acetylsulfuric acid, which can rearrange to sulfoacetic acid:
+ AczO AcOH + AcO-SO~OH
HzS04
--+
+
acetylsulfuric acid
HOSOz-CH2-COOH
(1)
sulfoacetic acid
While sulfoacetic acid is a well characterized compound (1) crystallizing as the monohydrate and melting at 84'436" C, acetylsulfuric acid has never been characterized as a pure compound. On attempted isolation from acetic acid by evaporation of the solvent, it rearranges to sulfoacetic acid or decomposes to give acetic anhydride and pyrosulfuric acid (2). In the presence of water it is hydrolyzed immediately to give sulfuric and acetic acids. Sodium acetylsulfate, however, is a well defined compound. In 1921 Van Peski ( 2 )summarized his work on this compound and described various ways of preparing it. By reacting a mixture of acetic anhydride and acetic acid (60:40) with sulfuric acid at -5' to - 10" C and neutralizing the acetylsulfuric acid with sodium acetate, he obtained the sodium salt in nearly quantitative yield. The weight of the product and the (1) 0. Stillich,J. Prakr. Chem., 73, 538 (1906). (2) A. J. Van Peski, Rec. Trm. Chim.,40, 103 (1921).
350
elemental analysis corresponded to the formula AcO-SOn-ONa. Sodium acetylsulfate hydrolyzes instantly in water to give sodium acid sulfate and acetic acid, whereas the isomeric monosodium sulfoacetate is stable in water solution. Van Peski dissolved sodium acetylsulfate in water; the resulting aqueous solution required two equivalents of base for neutralization and yielded one equivalent of acidity by steam distillation. The hot, dilute solution also gave one mole of barium sulfate when barium chloride was added. In contrast, an aqueous solution of monosodium sulfoacetate requires only one equivalent of base for neutralization, yields no acidity on steam distillation, and gives no precipitate with barium chloride in hot, dilute solution. Elliott ( 3 ) prepared acetylsulfuric acid from nitrosylsulfuric acid and acetyl chloride.
0
ANALYTICAL CHEMISTRY
HO-SO~-ONO
+ AcCl+
NOCl
+ AcO-SOzOH
(2)
Nitrosyl chloride was evolved, leaving a syrup, which when dissolved in water had the amount of titratable acidity and precipitatable sulfate expected for acetylsulfuric acid. Sulfuric acid is the catalyst commonly used in the acetylation of cellulose with acetic anhydride. The present work was therefore undertaken to obtain some information on the extent, speed, and reversibility of the reaction between sulfuric acid and acetic anhydride to form acetylsulfuric acid. EXPERIMENTAL
Procedure. We proposed simply to neutralize a reaction mixture of acetic anhydride and sulfuric acid in acetic acid solution with sodium acetate, evaporate the solution to dryness, and weigh the residue. One mole (98 grams) of sulfuric acid would give as little as 120 grams of sodium bisulfate if none of the sulfuric acid had been converted to acetylsulfuric acid, or as much as 162 grams of sodium acetylsulfate if all the sulfuric acid had been converted. (3) G. A. Elliott, J . Chem. Soc., 1926, 1929.
