Extraction Equilibria for the System Toluene-3,4-Dithiol and Zinc Hobart G. Hamilton* and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
The extraction of zinc by toluene3,4-dithiol in conjunction with either quaternary n-hexylammoniurn iodide or a phenanthroline (parent, 4,7-dimethylphen, 3,4,7,8tetramethylphen) into CHCI3 has been investigated and the results used to evaluate the component equilibrium constants. Zinc is well extracted as either an ion association complex or as a mixed ligand complex. A number of other divalent metal ions have also been shown to extract with toluenedithiol under the described conditions.
TOLUENE-3,4-dithi01 (H2T)(dithiol) has been frequently used in solvent extraction methods for a number of the higher valent metal ions such as molybdenum, tungsten, and rhenium (1-3) because the doubly charged dithiolate anion (T2-) forms neutral chelate species with these metal ions that are "coordination-saturated." Although dithiol has enjoyed some use as a qualitative reagent because it forms colored complexes with many metal ions (4), it has not been employed in extraction procedures for metals of lower valence because of the difficulty of forming a suitable complex. With zinc, for example, the 2 :1 complex, ZnT2-* is charged and therefore, water-soluble. Although the 1:1 ZnT complex is neutral, it is not extractable because Zn is 4-coordinate giving an organic-insoluble dihydrate, ZnT.2H20. If it is desired to incorporate such complexes into extraction processes, two courses are open. If conditions are adjusted to favor the formation of the anionic chelate, ZnTZ2-,then addition of a suitable cation such as a quaternary ammonium ion might be expected to result in an extractable ion association complex. Alternatively, replacement of the water of hydration from the neutral ZnT complex by neutral bases (adducting agents) might also be effective in producing an extractable species. In order to explore the feasibility of these two approaches to enlarging the scope of dithiol as an analytical extraction agent, it was decided to study the quantitative effect of tetrahexylammonium iodide (THAI) in promoting extraction of an ion association complex and of phenanthroline (phen) in effecting formation of an extractable mixed ligand complex using zinc as a typical divalent metal ion. The success of both these approaches with zinc prompted our qualitative examination of the behavior of H2T-THAI and H2T-phen systems with other metal ions which is also reported here. EXPERIMENTAL Reagents. Toluene-3,4-dithiol (HzT), in sealed ampoules (Eastman Organic Chemicals), tetra-n-hexylammonium iodide (THAI) (Eastman Organic), 1,lO-phenanthroline (phen) (Mallinckrodt Chemicals) and 4,7-dimethyl-l,lO-phenanthroline (DMP) (G. Frederick Smith Co.) were used as ob3,4,7,8-Tetramethyl-I,lO-phenanthroline (TMP), tained. which had been recrystallized from benzene and from meth-
Present address, Department of Chemistry, Stanislaus State College, Turlock, California.
(1) J. H. Hamence, Analyst, 65,152(1940). (2) C.C. Miller, J. Chem. Soc., 1941,792. (3) C. C. Miller, Analyst, 69, 109 (1944). (4) R.E.D. Clark, ibid., 83,396(1958). 1310
ANALYTICAL CHEMISTRY
anol (mp 282-284 "C) was provided by K. R. Turnbull of this laboratory. All other reagents used in this study were of analytical reagent grade. Carrier-free 65Zn solutions (approximately 10-6M) (New England Nuclear Corp.) were used for the zinc distribution studies. Distribution studies for THAI made use of carrierfree lalI (New England Nuclear Corp) in the form of NaI. Chloroacetate, acetate, phosphate, and borate buffers were employed over the pH range 2.5 to 13, with Na2S04being added to these solutions to maintain ionic strength at 0.10. Apparatus. Extractions were performed using 45-ml cylindrical glass vials fitted with polyethylene stoppers and plastic caps. Samples were shaken in an Eberbach reciprocating shaker at the high speed setting, with temperature control being maintained at 25 f 0.2 "C by circulating water from a Wilkens-Anderson Co. Lo-Temp bath through the jacketed shaker tray. A Nuclear-Chicago Model 186 scalar in conjunction with a Nuclear-Chicago Model DS-55 well-type scintillation detector was employed for radioisotype counting. Kimax lipless culture tubes (15 X 125 mm), covered with Parafilm, were used as counting containers. Spectra were obtained with a Beckman DB recording spectrophotometer equipped with 1-cm and 4-cm silica cells. Spectral measurements for the determination of the acid dissociation constants of dithiol were made with a Beckman DU spectrophotometer. A Beckman Model G pH meter, which had been calibrated with pH 4, pH 7, and pH 10 standard Beckman buffer solutions was used for pH measurements. Atomic absorbance measurements were made with a Perkin Elmer Model 303 atomic absorption spectrophotometer equipped with an Atomic Spectral (Melbourne, Australia) zinc lamp. Extraction Procedures. In a typical extraction, 10 ml of a buffer solution to whichB5Znhad been added, was equilibrated by shaking with an equal volume of CHCl3 containing appropriate concentrations of H2T and either THAI or phen. Solutions of dithiol were prepared daily to eliminate the effects of oxidation of this reagent. Following 30 minutes of shaking, a time found to be adequate for the establishment of equilibrium in all systems studied, the phases were allowed to separate and 5-ml aliquots of the organic and aqueous layers were pipetted into separate counting tubes. The same procedure was employed in the determination of the distribution equilibrium for THAI, except that l a l I was substituted for 65Znand H2Twas omitted. For the determination of high distribution ratios (D > 1000) in the H2T-phen systems 30 ml of aqueous buffer and 3 ml of a CHC13 solution of phen, DMP or TMP and H2T were used. After equilibration, 1 ml of the organic phase was removed and diluted to a volume of 5 ml with CHCla to maintain constant counting geometry, while 20 ml of the aqueous was removed and back-extracted into 5 ml of the CHC13-phen-H2T solution. The activity of the back-extracted aqueous phase was then determined in the usual manner. The ratio of the activities of both phases was combined with the phase volume ratio to obtain the distribution ratio. The distribution of dithiol between CHC13 and aqueous buffer solutions was measured over the pH range 5.94 to 8.92. As with the zinc distribution studies, 10 ml of buffer solution and 10 ml of a HzT-CHCI, solution were shaken for 30 minutes. After phase separation was complete, 5 ml of the aqueous layer was added to 5 ml of 0.19N HC104 and
10 ml of CHCl,. The sample was purged with Nz, shaken for 30 minutes, and the concentration of the HzT backextracted into CHCl, was determined by measurement of the absorbance of the solution' at 296 mp. The distribution of TMP between CHC13 and aqueous buffer solutions was measured over the pH range 3.78 and 5.61. Samples containing 10 ml of a CHCl3-TMP solution and 10 ml of aqueous buffer were shaken for 30 minutes, after which an aliquot was removed and its pH was adjusted to 2 by addition of HC104. Spectral measurements were taken at 280 mp and from the value = 4.30 X lo4 1 mole-', the aqueous concentration of TMP was calculated. Spectrophotometric Estimation of pK, values. The ionization constants of HzT were determined spectrophotometrically. A 200-1 aliquot of a HzT in 95 ethanol solution was diluted to 10 ml with a buffer solution through which NZ has been bubbled. Spectral measurements were then made within one minute after mixing. The absorbances at 300 mp of solutions of pH 3.0 or below were constant and taken as the value corresponding to that of the neutral species, HzT. Similarly, in the pH range 7.7 to 8.3 constant absorbances were observed at 300 mp and this value was ascribed to the absorbance of the species HT-. At pH values in the vicinity of 13, a third region of constant absorbance was observed at 300 mp and was attributed to T2-. The value of the pK was calculated from these measurements in the usual way. A similar technique was employed in the determination of the dissociation constant of TMP. For these measurements, 200-1 of a 1.00 X 10-3M TMP dissolved in 0.1N HCl was diluted to 25 ml with an appropriate buffer solution of p = 0.10. Below pH 3.6, absorbances at 280 mp were constant and were taken as the value corresponding to the protonated species, while absorbance values at pH 9.5 were used as a measure of the neutral form of TMP. A series of spectral measurements were made at 280 mp over the pH range 5.11 to 7.38 and the pK was obtained from these data in the usual way. Qualitative Extraction Tests (HzT-THAI System). In all the extraction tests, metal ion solutions were 5 X 10-4M. HzT and THAI were dissolved in CHCl, to give concentrations of 5 X 10-3M each. The metals extracted at pH 4.1 included : Bi(III), orange; Co(II), blue; Cu(II), light green; Mo(VI), green ; Pd(II), reddish-brown ; Sn(II), yellow; W(VI), yellow. At pH 4.1, Fe(II1) and V(V) extracted as purple complexes while Ni(I1) was colorless but at pH 6.3, Fe(II1) yielded a pink extract, V(V), a blue-green extract, and Ni(II), a light green extract. Other metals which do not yield colored complexes under these conditions, but which are found to extract, include In(III), Cd(II), Hg(II), Zn(II), and Zr(1V). The extraction of U(V1) proceeds slowly with a dark brown coloration appearing in the organic phase after 24 hours at either pH 4.1 or 6.3. The metals tested for which no apparent extraction is observed include Cr(III), Ru(III), and La(II1). Qualitative Extraction Tests HzT-phen System. Metal ion concentrations of 5 X 10-4M and [HzTIoand [Phenlo of 1 X 10-2M were employed in making these qualitative extraction tests. At pH = 4.5, Ni(II), violet; Pd(II), violet; Cd(II), yellow; Tl(III), pale yellow; Sn(II), yellow; U308, pink; Co(II), Mo(VI), green; In(III), yellow; Ga(III), yellow; Bi(III), orange; and Fe(III), pale red, form extractable complexes of the colors indicated. At pH 6.0, Cu(I1) forms a red-orange extractable complex, and the organic phase color observed for Fe(II1) is pale blue rather than the red color at pH 4.5. Mn(I1) yields a very pale green coloration in the organic phase at pH 6.0 and 8.9, which is accompanied by some precipitation. Preparation of Mixed Ligand Complexes. The mixed ZnT .phen ligand complexes were prepared using a modification of the method reported by Wallenfels and Sund ( 5 )
x
( 5 ) K. Wallenfels and H. Sund, Biochenz. Z . , 329,41 (1957).
Table I. Equilibrium Extraction Data at 25 "C for Toluene3,4-Dithiol between CHCI, and Aqueous Solution of Ionic Strength = 0.1
PH 5.97 6.29 6.60 6.94 7.11 7.43 7.75 8.11 8.46 8.92
Log D 3.27 3.07 2.80 2.53 2.36 1.98 1.73 1.41 1.07 0.60
Log KDR 4.08 4.15 4.15 4.12 4.12 4.05 4.12 4.16 4.17 4.16 K(s = 0.025)
for the preparation of ZnTephen. Phen (0.40 gram, 2 mmole), HzT (0.31 gram, 2 mmole) and Zn(C104)z.6H~0 (0.74 gram, 2 mmole), in separate 10-ml beakers, were dissolved in dimethylformamide (1 ml) and the three solutions were then combined with constant stirring. After standing for 1 hr, the resulting solution was cooled to 0 "C, and the yellow material which precipitated was collected by filtration, washed with petroleum ether, and dried in vacuo at 40 "C. This procedure was also followed in the preparation of ZnT.DMP and ZnT.TMP. Weighed samples of the three complexes were dissolved in a small volume of concentrated "0, and 3 0 x H202, heated to fumes, cooled, and then diluted with water to a total volume of 100 ml. Atomic absorbance measurements of these solutions were used to determine zinc concentrations with the aid of a zinc absorbance working curve. Calcd. for ZnT.phen-Zn, 16.38; Found 16.3 Calcd. for ZnTeDMP-Zn, 15.30; Found 15.9 Calcd. for ZnT.TMP-Zn, 14.36; Found 13.4 RESULTS AND DISCUSSION
Toluene-3,4-dithiol is a diprotic acid which could be expected to be stronger than, but similar to, catechol. The spectrophotometric evaluation of the acid dissociation equilibria gave pK., = 5.34 f 0.06, [(Lit. 5.4 (6)] and pK,, = 11.0 f 0.1. The pK,, value may be less reliable than indicated because of the possible oxidation of dithiol in alkaline solutions despite the precautions taken to eliminate or minimize such effects. In this regard, however, measurements carried out without the specified precautions resulted in pK. value not much different from 11.0. The pK values of dithiol are 4.0 and 1.8 units lower than the corresponding values of catechol (7). The difference in pK,, is expected and is similar to that observed with other phenol-mercaptan pairs, but the difference between the first and second pK, values of the dithiol(5.6 units) is inexplicably large. The corresponding change in catechol (3.3 units) is much less despite the necessarily greater influence of H-bonding in this case. This might be rationalized on the basis of the contribution of structures of the following sort (in which an increased electron density at the #3-carbon decreases the acid strength) to the ground state of HT-:
(6) T. W. Gilbert and E. B. Sandell, J . Amer. Chern. Soc., 82, 1087 (1960). (7) L. G. Sillen and A. E. Martell, Stability Constants, Special Public. No. 17, The Chemical Society (London),(1964). VOL. 41, NO. 10,AUGUST 1969
1311
Table 11. Equilibrium Extraction Data for Tetra-n-HexylAmmonium Iodide at 25 "C between CHCl3 and Aqueous Media Pure water [THAI10 D 1311 Log KDRKIP 9.90 x 1.96 x 2.91 x 3.85 x 6.98 X 9.09 x
241.4 367.6 383.9 421 .O 452.9 470.0
10-4~ 1 0 - 3 ~
10-3~ 10-3~ 10-3M 10-3~
7.77 7.85 7.70 7.66 7.47 7.39 7.64 (S = 0.18)
Table 111. Extraction of Zinc with Toluene-3,4-Dithiol in the Presence of Tetra-n-Hexylammonium Iodide, pH = 4.50, temp = 25 "CyI.( = 0.10, [OAc-1 = 2.55 X 10-3M A. Variation of Log D with B. Variation of Log D with -Log [H2TIoat [THAI] = -Log [THAI] at [H2T10= 8.21
x
lO-aM
-Log [HzTIo
3.28
Log D
x
10-3~
-Log[THAI]o
Log D
3.33 3.03 2.57 2.35 2.19 2.07
-0.02 0.40 1.23 1.53 1.89 2.14
3.74 -0.19 3.44 0.44 3.15 1.00 2.98 1.34 2.86 1.56 2.77 1.73 D a [ H 2 ~ o Z2J0 . 1
D a[THAI]1.9 2
oA PH
Figure 1. Extraction of zinc as 2R4Nf, ZnT22in CHCl, at 25 O C and p = 0.10 [THAI10
1.0
The distribution of dithiol between aqueous buffer solutions and CHC13is given by Equation 1,
where KDRis the distribution constant of HzT between water and CHCla, and Kl and K2 are the acid dissociation constants of H2T. Using Equation 1 it is possible to determine KDR from the variation in Log D with pH. Such distribution data are presented in Table I, from which Log KDRis found to be 4.13 (s = 0.025). The distribution of THAI between CHC1, and water is described by D =
[THAI]
+ [THA+]
where THA = (CBH1&+
(2)
Considering the equilibria THA+
+ I-
KIP
KDR
THAI
8.30 X lo-' M , [HzTJo = 3.08 X
where COis the known [THAI10 and CAis the aqueous THAI concentration. Substituting these values in Equation 5 yields
Using Equation 7, the product of the constants KDRKIp was found to be in the THAI concentration range shown in Table 11. The distribution of zinc between CHCl, and aqueous buffer solutions in the presence of TDT and THAI is given by Dzn =
[2 THAf, ZnT22-Io -
Czn where Czn is the total zinc concentration in the aqueous phase can be expressed as [Znz+]jPo,where POis the fraction of zinc in the aqueous phase present as uncomplexed ZnZ+. A quantitative description of the extraction of zinc as the complex(2 THA+, ZnTJ requires consideration of the two ionassociation reactions KIPC
eTHAI(0)
(3)
it can be shown that (4) and because electroneutrality is preserved, FHA+] so that
=
10-4 M
=
+ ZnT, = d 2 THA+, ZnT2 KIPQX THA+ + X THA+, X-
2 THA+
(9)
(10)
and the following distribution equilibria
[I-],
[2 THA+, ZnTz2-Io
Further, the activities in the aqueous and organic phases, A and A . respectively, may be related to THAI concentrations through A0
1312
=
KCo and AjK
ANALYTICAL CHEMISTRY
=
CA
(6)
KDc = [2 THA+, AnTz2-]
as well as the aqueous chelate formation equilibrium [ZnTzz-) KJ = [Zn z+][T2-]
(13)
LOG [CIO;]
Figure 2. Effect of clod- an extraction of zinc as 2R4N+, ZnTz*- in chloroform at 25" C, p = 0.1 THAI10 = 9.10 X [HzT], = 2.23 X pH = 4.50
10-3
M
M
I
1
3
4
I
5
I
I
6
7
\ O l
Figure 3. Extraction of Zn(Phen)(T) into CHCl, at 25 "C; [HzT]~= 1.0 X lO-4M [Phenh o = 1.0 x 10-4 M = 1.0 X 10-8 M A = 1.0 X 0 = 5.0 X
From these equilibria the overall extraction expression is found to be D z = ~
K j K a 2 K a 2 2 ( K r ~ ~ *[HzTlo2 K ~ c ) [THAXIO~ Po K DR T 2+ (KORQX '. KIPQX2,
from which pK, and KDRwere evaluated. For the extraction of zinc as the complex ZnT .phen, the distribution is given as [ZnT.phenIo CZn
p =
0.10
M
M
From Equation 16, the following expression describing the extraction of ZnTSphen may be written,
(15)
The validity of this expression in describing the extraction of zinc with H2T and THAI is evident from the following distribution studies. As shown in Figure 1, a plot of log Dznus. pH displays increasing extraction with a slope of 4, and is in accord with the predicted release of four protons upon formation of the ZnTz2- complex. Further, logarithmic plots of extraction data obtained at constant pH us. [H2Tj'0and show slopes of 2.0 and 1.9, respectively, in agreement with Equation 15. These data are presented in Table 111. For the conditions employed in this part of the study, it was assumed that the major zinc species in the aqueous phase was the uncomplexed zinc ion, so that POwas taken as unity. Evaluation of the relevant equilibrium constants as described below permitted a check on the validity of this assumption. A fourth extraction variable included in Equation 15 and denoted as [X-1, results from the competition of anions in the aqueous phase for the quaternary ammonium ion. As shown in Figure 2, the effect of Clod-, a readily extractable anion, is quantitatively in accord with what would be predicted from Equation 15. The distribution data for TMP obeyed the relationship
D =
8
PH
Where K , is the formation constant of ZnT.phen, Kal and Kaz are the ionization constants of HzT, KDCis the distribution constant of the mixed ligand complex, KDRTand K n ~ are p the distribution ratios of the ligands H2T and phen, respectively. It has been assumed that the distributions of H2T and phen are unaffected by each other's presence. Extraction studies were carried out over the pH range 3.75 to 8.92. The results of these investigations are shown in Figure 3, where it should be noted that log Dznvalues increase with a slope of 2.0 =t0.1 at the low pH range, then pass through a maximum or plateau and decrease with a slope of -1.8 i 0.1 atthehigherpHrange. Each of these regions of the extraction curve may be quantitively attributed to changes in the value of 00as various zinc containing species predominate under different conditions. Thus, in the low pH region the predominant zinc species is the hydrated ion so that = 1 , and log Dzn us. pH will have a slope of 2. In this range, an interesting effect is observed when the phenanthroline concentration is increased. At first the extraction is improved as might be expected but upon further phen increase, the extraction curve shifts to the right. This effect arises from the formation of higher zinc-phen complexes (such as ZnphenZ2+and Znphena2+). When the presence of these higher complexes are quantitatively accounted for in the calculation by means of the known stability constants, the various portions of the lower pH region of the extraction curves coalesce to give a single linear portion of slope 2 (Figure 4). In the region in which ZnTphen is predominant, it can be shown that VOL. 41, NO. 10,AUGUST 1969
1313
I
Table IV. Extraction of Zinc with Toluene-3,4-Dithiol and 1,lO-Phenanthroline, pH = 4.25, at 25 "C and p = 0.10 A. Variation of Log D with B. Variation of Log D with -Log [phen]~ when [HzTIo= -Log [HzTIoat [phenlo = 1
x
10-M
2.4
-Log [ H Z ~ O Log D 4.30 3.78 3.50 3.32 3.10 2.96 2.82
x
10-4~
-Log [phenIo
1.32 1.72 1.99 2.18 2.34 2.60 2.66
Log D
5.30 -0.10 5.00 0.30 4.71 0.60 4.54 0.79 4.42 0.90 4.30 1.02 4.01 1.30 A Log D = 1.0 A Log [phenlo
Table V. Extraction of Zinc with Toluene-3,4-Dithiol and 4,7-Dimethylphenanthroline at 25 "Cand p = 0.10 A. Variation B. Variation of Log of Log C. Variation D with pH for D with [4,7-DMP]o of Log [4,7-DMP]o D with [H2TIofor for [H2TIo= ~ ~ 1 X~ lOPMand 1x 1 0 - 4 ~ [ = [4,7-DMP] = 10-5 1 x 10-4~ pH = 4.76 and pH = 3.83 -Log -Log pH Log D [4,7-DMP]o Log D [HiTIo Log D 3.75 3.99 4.24 4.45 4.78 5.19 5.62 6.28 6.78 7.22 7.46 7.80
1.23 1.99 2.05 2.37 2.68 3.48 3.58 3.09 2.41 1.66 1.17 0.54
5.00 4.71 4.54 4.42 4.32 4.18 4.04
0.77 1.11 1.26 1.37 1.48 1.60 1.67
5.00 4.71 4.54 4.42 4.32 4.13 4.04
0.07 0.26 0.39 0.49 0.64 0.91 1.07
I
Figure 5. Effect of ZnT22complexation upon the extraction of Zn(phen) (T); temp = 25 "C, = 0.10, [phen]~ = 1.0 x 10-4 M , PH = 6.78
which, when incorporated in Equation 18, predicts that, in this region, the slope of log D with p H will be zero (log D = log KDC). Finally, at higher p H values, the predominant species in the aqueous phase is ZnTzZ-from which it can be seen that
Upon substitution in Equation 18, one finds that in this high p H range the slope of the extraction curve will be -2. A decrease in log D with log [HzT], in this region should occur with a slope of -1 (Figure 5 ) . Representative extraction data are contained in Table IV, in which log Dzn values are noted as a function of log [HzTIo and log [phenIo. The log Dz,, values show a dependence of unity with respect to each of the latter variables, which is also in complete accord with Equation 18. Extraction studies which parallel those just discussed were conducted with TDT and the 4,7-dimethyl- and 3,4,7,8-
Table VI. Extraction of Zinc with Toluene-3,4-Dithiol and 3,4,7,8-Tetramethylphenanthroline at 25 "C and p = 0.10 A. Variation of Log D B. Variation C. Variation of Log D of Log D with pH for [TMPIo = 1 X with [TMPIo for with [HZTIofor and [HZT]0= [HzTIo = 10-5M [TMPIo = 10-sM and pH = 3.80 and pH = 3.80 1 x 10-5 -Log -Log pH Log D [TMPIo Log D [TDVo Log D I
3
I
I
4
I
1
PH Figure 4. Effect of aqueous Zn-Phen complexing on the extraction of Zn(PhenXT); temp = 25 "C, = 0.10, [HzTIo = 1.0 X M tphenlo
o = 1.0x 0 = 1.0x
10-4 M 10-3 M D = 5.0 X 10-8 M A = 1.0 X M 1314
ANALYTICAL CHEMISTRY
3.78 3.99 4.22 4.45 4.82 5.09 5.31 5.61 6.34 6.97 7.23 7.46 7.80
0.02
0.32 0.92 1.34 2.25 2.38 2.79 2.98 3.07 3.02 2.65 2.36 1.68
6.00 5.71 5.42 5.32 5.18 5.04
-1.30 -0.94 -0.68 -0.62 -0.46 -0.34
6.00 5.71 5.54 5.42 5.32 0.18 5.04
-1.62 -1.21 -0.99 -0.89 -0.74 -0.71 -0.30
Table VII. Dissociation and Distribution Constants of Some l,l0-Phenanthrolines and Formation Constants of Their Zn(Phen)T Complexes between CHCla and Aqueous Media at 25 "C and Ionic Strength 0.10 PKll Log K f Log KDR Log KDC 3.60 rt 0.20 5.05 i 0.10 (10) 20.3 3.05 i 0.15 (10) 1,lo-Phenanthroline 21.9 3.70 f 0.15 (10) 3.95 f 0.20 4,7-Dimethyl-l,IO-phenanthroline 6.04 rt 0.14 (IO) 4.50 f 0.30 3,4,7,8-Tetramethyl-l,IO-phenanthroline 6.42 f 0.11 22.3 4.76 rt 0.23
tetramethylphenanthroline ligands, abbreviated as DMP and TMP, respectively. Extraction data obtained as a function of pH and also as a function of ligand concentration for these two systems are presented in Tables V and VI. These data provide verification that the extracted species are ZnT. D M P and ZnT.TMP. With the help of Equations 15 and 18, it is possible to evaluate various component equilibrium constants using the extraction data. In Table VII, are listed the formation and distribution constants for the mixed ligand chelates formed by toluenedithiol and either phen or one of its methylated analogs. It is interesting to note that although the formation constants (log K,) increase about as rapidly as the acid dissociation constants of the phenanthrolines decrease-i.e., pK, increase), the extraction constant shows only the beneficial effect of the increasing log Kf. This results from a high log KDRvalue-Le., the retention of the bulk of the phenanthroline in the CHC18 phase even at pH values significantly below the pK, and consequently, an almost pH-independent concentration of phenanthroline in the aqueous phase. An additional advantage in using the methylated phenanthrolines arises from the increase in K, so that higher maximum Dz.values can be achieved. The increase in K, is matched by the increase in KDE so that this factor does not have any appreciable effect on the extraction constant. As mentioned earlier, at higher pH values, the predominant Zn species in the aqueous phase becomes ZnTzZ-making it possible to use the HzT-phen extraction data to evaluate the formation constant of this complex. Values of log Kr (ZnT?) of 25.9, 25.7, and 25.4 were obtained using the data from the phen, dimethyl-, and tetramethyl-phen systems, respectively. This complex is among the most stable of known zinc complexes [log Kf for Zn(0xinate)z is 17.1 (8); for ZnEDTA is 16.4 (7); for ZnLz when L is cysteine 18.2 (7); when L is P-mercaptopropionic acid, 12.80 (8)l. With the help of the values of log Kf(ZnTz2-) obtained from the ZnT-phen data as well as the acid dissociation and distribution constants of HzT, it is possible to evaluate the other component constants in Equation 15. Using a value of log (KIQKD&X for THA.C104 of 8.04 (9) and the data shown in (8) F. Chou and Henry Freiser, ANAL.CHEM., 40,34 (1968). (9) G . Carmack, H. G . Hamilton, and H. Freiser, Unpublished
data, University of Arizona, 1968. (10) C . Woodward and Henry Freiser, ANAL. CHEM.,40, 345 (1968).
Figure 2, the value of the combined ion-association and distribution constants of the complex 2R4N+, ZnTz2- may be calculated as 19.2, a very high value indeed, considering that simple divalent anions, such as sulfate cannot be extracted by CHCh to any significant extent using tetrahexylammonium cations with which to pair. In conclusion, it would appear that toluene-3,4-dithiol, used in conjunction with either large cations or neutral bidentate ligands, forms highly extractable complexes with zinc and other divalent metal ions that can be of great analytical significance. ACKNOWLEDGMENT
The authors acknowledge the assistance of Gary Carmack with some of the experimental aspects of this work.
RECEIVED for review March 6, 1969. Accepted May 19, 1969. Work was supported by a grant from the U. S. Atomic Energy Commission.
Correction Improvements in Preparation and Operation of Electrodeiess Discharge Lamps as High Intensity Sources in Atomic Fluorescence Flame Spectrometry In this article by K. E. Zacha, M. P. Bratzel, Jr., J. D. Winefordner, and J. M. Mansfield, Jr., [ANAL.GEM., 40, 1733 (1968)l certain elements reported to fluoresce in the flames reported were found not to do so in further study. Additional experimental results have shown that Be, Hf, Mo, Ti, U, and Zr do not atomize sufficiently in Hz/Ar/Entrained Air or Hz/Air flames to produce atomic fluorescence signals. The observed signals in the above paper were a result of scattered source radiation. However, the sources prepared for the above elements were intense and possibly useful for atomic fluorescence. Since the above manuscript was published, atomic fluorescence signals for Be in premixed CzHz/NzO flame has been observed. Atomic fluorescence signals for Hf, Mo, Ti, Zr and possibly U should be observable in premixed GHz/NzOor GH1/OZflames.
VOL. 41,
NO. 10, AUGUST 1969
1315
