samples-e.g., A1 and Cr in high purity uranium, Al, Cr, and F e in nonferrous alloys, Be, Al, Ga, In in aqueous solutions, A1 and Cr in steel, etc. (3, 18-23). Mass spectrometric studies of several other fluorocarbon fl-diketone complexes are presently in progress. Several other studies of the application of &diketone complexes to ultra-trace quantitative analysis are presently under way. One study of particular interest is the possible analysis of complex samples. The combined resolution of mass spectrometry with the quantitative reactions of such chelating agents should permit rapid, accurate analysis of many types of samples having elements varying over several orders of magnitude concentration without separation or preconcentration steps.
__ (18) C. Genty, C. Houin, and R. Schott, 7th Intl. Symp. on Gas
Chromatography, Copenhagen, June 1968. (19) J. Savory, P. Musak, N. 0. Roszel, and F. W. Sunderman, Jr., Federation Proceedings (Biochem.) Abstracts 52nd. Mtg., Atlantic City, N. J., April 1968,p 777. (20)R. W.Moshier and J. E. Schwarberg, Tulunru, 13,445 (1966). (21) G. P. Morie and T. R. Sweet, Anal. Chim. A m . , 34, 314 (1966). 37,1552 (1965). (22) G.P.Morie and T. R. Sweet, ANAL.CHEM., (23)W. G. Scribner, M. T. Borchers, and W. J. Treat, ibid., 38, 1779 (1966),and references therein.
It should also be emphasized that metal isotope analyses can be conducted with conventional instrumentation by using volatile chelates. It is expected that this will be particularly helpful in studies of the origin and history of samples obtained from the surface of the moon and other extraterrestrial sources. This new method should also facilitate investigations of the introduction, function, transport, and excretion of biologically important metal ions. By intxoducing either a stable or radioactive tracer of a given metal isotope and analyzing minute samples of fluids or tissues to determine isotope dilution, the distribution can be easily quantitated and followed. ACKNOWLEDGMENT
The authors gratefully acknowledge the advice and suggestions of A. L. Crittenden. We are also grateful for samples provided by C. S. Springer, Jr., J. W. Connolly, and W. G. Scribner. D. C. Eckert assisted in the experimental work. RECEIVED for review January 10, 1969. Accepted April 29, 1969.
Metal Complexing Properties and Proton Magnetic Resonance Spectra of 5-Halo-8-Quin01 inoIs R. G . Beimer and Quintus Fernando Department of Chemistry, University of Arizona, Tucson, Ariz.
A series of 5-halo-8-quinolinols and 5-halo-2-methyl-8quinolinols have been synthesized from the corresponding 5-nitroso and 5-amino compounds. The proton magnetic resonance spectra of these compounds have been obtained in dimethyl sulfoxide (DE) and the proton chemical shifts have been measured. The acid dissociation constants of these ligands and their chelate formation constants with several transition metal ions have been measured in 75% v/v dioxane-water. The electronic effects caused by the introduction of halogen substituents in the 5-position of the 8-quinolinol ring system have been eva Iuated. THEINTRODUCTION of a halogen substituent in the 5-position in the 8-quinolinol molecule will result in a redistribution of electron density in the molecule and will influence the availability of electrons at the oxygen and nitrogen donor atoms for metal chelate formation. This electron redistribution results in the shielding or deshielding of the protons in the quinoline ring system. It should be possible, therefore, to evaluate these electronic effects from a measurement of the proton chemical shifts in the ligands. The enthalpies of formation of the metal chelates of the 5-substituted 8-quinolinols will also be influenced by an electron redistribution in the ligand. In the absence of steric effects, the entropies involved when chelate formation occurs between a given metal ion and a series of 5-halogen substituted ligands can be assumed to be constant. Consequently, the free energies of chelate formation or the values of the metal chelate formation constants can be used as an index of the availability of electrons on the bonding atoms in the ligands. Although it is well known that the thiocyano group is a
pseudo halogen, there is little quantitative information on the electronic effects of the thiocyano group, especially in ligands that participate in metal complex formation. For this reason the ligands, 5-thiocyano-8-quinolinoland 5-thiocyano-2-methyl-8-quinolinolhave been included in this study of 5-halo-8-quinolinols. EXPERIMENTAL
Synthesis of 5-Substituted Derivatives of 8-Quinolinol and 8-Quin2-Methyl-8-QuinolinoI. ~-FLUORO-~-QUINOLINOL. olinol was first converted into the 5-nitroso derivative, reduced to the 5-amino-8-quinolino1, and finally converted into the 5-fluor0 compound by the Schiemann reaction. The method described by Urbanski ( I ) was used in the synthesis of the 5-nitroso derivative and its reduction to the 5-amino8-quinolinol. The Schiemann reaction as described by Hollingshead ( 2 ) was used to convert the 5-amino-8-quinolinol sulfate into the 5-fluoro derivative which was recrystallized from ethanol and purified further by a vacuum sublimation. Found C, 65.61; H,3.61; F, 11.55; C a l c d ( z ) : C, 66.26; H, 3.71; F, 11.64; (mp 111-112 "C). ~ - B R ~ M ~ - ~ - Q u I NA~ L Sandmeyer ~ N ~ L . reaction was used to convert the 5-amino-8-quinolinol sulfate into the 5-bromo derivative. The method used was an adaptation of the method described by Vogt and Jeske (3). The compound C, 47.28; was purified by vacuum sublimation. Found H,2.72; Br, 35.31; Calcd C,48.24; H, 2.70; Br, 35.66; (mp 124.5 "C).
(z):
(z):
(z):
(1) T. Urbanski, Rocz. Chem., 25,297(1951). (2) R.G. W.Hollingshead, Chem. bid. (London), 344(1954). (3) H.Vogt and P. Jeske, Arch. Phur., 291,168 (1958). VOL. 41,
NO. 8,JULY 1969
1003
Table I. Proton Chemical Shifts for the 5-Substituted 8-Quinolinols and 2-Methyl-8-Quinolinols (TMS References) Proton Chemical Shifts PPM ( T ) 5-Substituent in 8-quinolinol (2)a (3) (4) (6) (7) 1.92 3.10 3.32 1.40 2.74 F 1.88 2.73 3.18 1.32 2.63 c1 1.87 2.50 3.15 1.32 2.58 Br 1.95 2.45 3.07 1.42 2.70 I 3.10 1.32 2.52 1.68 2.32 SCN 5-Substituent
Proton Chemical Shifts PPM ( T ) (3) (4) (6) (7) 7.57 2.06 2.80 3.21 2.85 c1 7.52 2.77 2.05 2.58 3.20 Br 3.01 7.46 2.74 1.98 2.44 I 3.07 2.60 1.75 2.33 SCN 7.47 a The numbers in parentheses refer to the position of the proton in the heterocyclic ring system. in 2-Methyl-8-
quinolinol
(2)
~-CHLORO-~-QUINOLINOL. The Sandmeyer reaction was also used to convert the 5-amino-8-quinolinol sulfate into the C, 59.99; H, 3.47; C1, 5-chloro derivative. Found C, 60.18; H, 3.37; C1, 19.74; (mp 19.70; Calcd. 125.5-126.5 "C). ~ - T H I ~ c Y A N O - ~ - Q ~ I N O L I This N O L . compound was also C, 59.92; prepared by the Sandmeyer reaction. Found H, 3.14; N, 13.69; Calcd (Z): C, 59.39; H, 2.99; N, 13.85; (mp 146-147 "C). ~-Iow-~-QuINoLINoL.The method described by Pirrone and Cherubino (4) was used for the synthesis of 5-iodo-8quinolinol. The product was purified by vacuum sublimaC, 40.00; H, 2.11; I, 46.97; Calcd tion. Found C, 39.88; H, 2.33; I, 46.82; (mp 129-130 "C). The syntheses of the 5-halo derivatives of 2-methyl-8quinolinol were carried out in a manner identical with the synthesis of the corresponding 5-halo-8-quinolinol. ~-CHLQRO-~-METHYL-~-QUINOLINOL. Found ( %) : C, C, 62.03; H, 4.16; 62.42; H, 4.10; C1, 18.38; Calcd C1,18.31; (mp 60-61 "C). ~ - B R ~ M ~ - ~ - M E T H Y L - ~ - ~ U I NFound O L I N(O%) L .: C, 49.88 ; H, 3.33; Br, 33.24; Calcd (Z): C, 50.44; H, 3.39; Br, 33.56; (mp 63-64 "C). ~-THIOCYANO-~-METHYL-~-QUINOLINOL. Found ( : c, 60.53; H, 3.57; N, 12.63; S, 14.55; Calcd C, 61.09; H, 3.73; N, 12.95; S, 14.83; (mp96-98 "C). ~ - I ~ D ~ - ~ - M E T H Y L - ~ - Q U I N O LThe INOL 5-iodo-2-methyl-8. quinolinol decomposed when sublimed under vacuum. The compound was therefore purified by recrystallization from ethanol. Found C, 41.99; H, 2.80; I, 44.71; Calcd C, 42.13; H, 2.83; I, 44.53; (mp 151-3 "C). All elemental analyses were carried out by Huffman Laboratories Inc., Wheatridge, Colorado. Attempts to synthesize 5-fluoro-2-methyl-8-quinolinol gave an oil which could not be purified further. Mass Spectra. The 80 eV mass spectrum of each 8-quinolinol that was synthesized was recorded on a Hitachi-Perkin Elmer RMU 6E double focussing mass spectrometer. No peaks were observed with a mass higher than that of the parent region in any of the spectra. Thus there were no high molecular weight impurities in any of the compounds and all the compounds were free from any 5,7-disubstituted derivatives. From the calculated and observed isotopic abundances it was found that all the compounds were of an acceptable purity (97-99 %I.
(x):
(x):
(x):
(x):
(x):
(x):
(x):
(x):
x)
(x):
(4) F. Pirrone and A. Cherubino, Gazz. Chim. Ztal., 65, 743 (1935). 1004
ANALYTICAL CHEMISTRY
Proton Nuclear Magnetic Resonance Spectra. The 60 MHZ NMR spectra of all the 8-quinolinol derivatives were obtained with a Varian Associates Model A-60 spectrometer. Tetramethylsilane was used as an external standard because solubility problems arose when it was added to the solution containing the sample. The concentration of the 8-quinolinol was 25% by weight in every case, and the solvent used was deuterium enriched (99.575) dimethyl sulfoxide obtained from Stohler Isotopic Chemicals. The proton chemical shifts and their assignments in the 5-substituted-8-quinolinols and -2-methyl-8-quinolinols are given in Table I. Potentiometric Determination of Acid Dissociation Constants in 75% v/v Dioxane-Water at 25 "C. REAGENTS. A 0.1M solution of NaOH was prepared from carbonatefree 50% NaOH solution by dilution with boiled deionized water. The solution was stored in a polyethylene bottle and the air was displaced with nitrogen. The NaOH solution was fed into a self-filling buret under nitrogen pressure and was standardized against potassium hydrogen phthalate (N.B.S. primary standard). A 0.1M solution of HC104 was prepared by dilution of 73 HClO4 with boiled deionized water and standardized against the NaOH solution. The 1,4-dioxane (Mallinckrodt) used in this work was purified by the method described by Fieser (5). Three liters of dioxane were refluxed with 300 ml of water and 40 ml of concd HC1 for 12 hours while a stream of nitrogen gas was passed through the system. The solution was then treated with KOH, the dioxane decanted and refluxed with sodium metal for 3 days. The entire distillation system was swept with nitrogen. The dioxane was collected at 98 "C by distillation through a Widmer concentric band column which was 3 ft long. The dioxane was stored in the dark under nitrogen and was repurified if stored for more than a week. The 1,4-dioxane contained no oxidizing agents (negative KI test), and did not consume any base on titration. Its NMR spectrum and gas chromatogram failed to show the presence of any impurities. Apparatus. The titration apparatus consisted of a waterjacketed 250-ml beaker fitted with a rubber stopper with holes for the NaOH and dioxane burets, the glass and saturated calomel electrodes, and the nitrogen bubbler. The nitrogen gas which was used to maintain a carbon dioxide free atmosphere in the titration vessel was presaturated with a 7575 vjv dioxane-water mixture at 25 "C. Water from a constant temperature bath was circulated through the jacketed beaker and its contents were maintained at 25 "C. The solution was stirred continuously with a magnetic stirrer which was turned off before each pH meter reading. All the potentiometric measurements were made with a Beckman Model G pH meter with a glass-saturated calomel electrode pair calibrated with at least two N.B.S. buffer solutions. The entire system was grounded to obtain a reproducible response in the pH meter. Titration Procedure. A weighed amount of the 8-quinolinol was introduced into the jacketed beaker and dissolved in 75 ml of dioxane and 25 ml of a standard acid solution. The solution was stirred for at least 15 minutes before the first increment of NaOH was added. NaN08 was used to maintain a constant ionic strength of 0.1 in the titration vessel. For each increment of base added, three times the volume of dioxane was added to maintain a 7 5 x v/v dioxane-water medium in the titration vessel. The contraction in volume when dioxane and water are mixed was taken into account in the 7 5 x vjv dioxane-water medium. The contraction factor for this medium was found experimentally to be equal to 0.984.
(5) L. F. Fieser, "Experiments in Organic Chemistry," D. C. Heath Co., 1941, p 369.
pH-Meter Correction. The hydrogen ion concentration in 75% vjv dioxane-water medium was calculated from the pH-meter reading by the use of a conversion factor which was obtained by the titration of a known quantity of acid with standard base in the two media. The difference between the pH-meter reading and the calculated pH (from the calculated hydrogen ion concentration on the assumption of complete dissociation) in the acid buffer region was found to be (0.20 + 0.03) in the 75 % vjv dioxane-water medium. This value agrees with the value reported recently by Irving and Mahnot (6). Potentiometric Determination of Metal Chelate Formation Constants in a 75% vjv Dioxane-Water Medium at 25 “C. Most of the metal chelates precipitated in a 50% v/v dioxanewater medium, long before any useful potentiometric data could be obtained. Hence, all metal chelate formation constants were determined in 75% vjv dioxane-water. The solutions containing the metal ions were made by dilution of perchlorate salts (G. T. Smith Chemical Co.) with deionized water, and standardized with EDTA which was previously standardized with a zinc solution made from 99.999z pure zinc. Six titrations of each metal ion solution were carried out. The titration procedures have been described by Flaschka (7). The titration procedure was the same as that described above for the determination of acid dissociation constants. A solution containing the ligand, metal perchlorate, and perchloric acid was titrated with standard NaOH solution; the 75% vjv dioxane-water medium was maintained at an ionic strength of 0.1.
+
RESULTS
8 Gc“l
42
The following equations were used to calculate the successive acid dissociation constants of the 8-quinolinol from the potentiometric titration data in the appropriate buffer regions.
+
W
3
KI =
+
[H+I (CL- [Clod-] - [OH-] [H+l [Na+l) (1) ([Clod-] [OH-] - [H+l - [Na+l)
+
K? =
+
[H+l (“1 [H+l - [OH-]) (CL - [Na+l- [H+l [OH-1)
+
(2)
where all the concentration terms are expressed in mole$. and CL is the molar analytical concentration of the 8-quinolinol. All the calculations were carried out with a CDC-6400 computer and the values reported in Table I1 have been obtained by a least squares method. Values of the Bjerrum formation function, ii, and L, the molar concentration of the ligand anion, were calculated from the potentiometric titration data. The successive chelate formation constants Pi and PZ/Piwere calculated with the aid of a computer program that has been developed in this laboratory (8). Briefly, the method consisted of generating the surface, S(&, P2/P1), in which the minimum was located by a simple search technique.
Values of the successive metal chelate formation constants are collected in Table 111. (6) H. M. N. H. Irving and U. S. Mahnot, J I m r g . Nucl. Chem., 30, 1215 (1968). (7) H. A. Flaschka, “EDTA Titrations,” Pergamon Press Inc., New York, 1959. (8) E. A. Unwin, R. G. Beimer, and Q. Fernando, Aizal. Chim. Acta, 39, 95 (1967). VOL. 41, NO. 8, JULY 1969
1005
Table 111. Metal Chelate Formation Constants of 5-Substituted-8-Quinolinolsand -2-Methyl-8-Quinolinols in 75% vjv Dioxane-Water at 25 "C and Ionic Strength 0.1
Metal ion Mn(l1)
Co(I1) Ni(I1) Cu(I1) Zn(I1) a
b
log PI 6.67 8.67 9.31
. . .c
8.62
F
log
5-Substituent in 8-Quinolinol c1 Br I log log log log log log
log
SCN
(8.71)
log (PZlP1) 6.06 8.14 (6.85)b
8.51
7.86
(PZlPd
81
(PZ/PI)
81
(PZIPI)
PI
(PZlPI)
PI
6.15 8.10 8.76
6.55 8.53 9.40
6.03 8.01 8.78
6.60 8.53 9.19
6.15 8.14 8.70
6.65 8.66 9.62 . . .c 8.70
6.08 8.08 8.97
6.41 8.20
...e
8.19
e
8:45
c
7:96
...c
8.52
. . .c
8.17
. . .c
8.33
...
5-Substituent in 2-Methyl-8-Quinolinol~ c1 Br I SCN log log log log Pz
11.80 15.68 15.41 (19.67)* 16.04
P2
P2
P2
11.85 15.60
12.02 16.10 15.86
12.90 16.20 16.32
15.33
(19.19)* C 15.97 16135
C
16:29
Values of PI are unreliable because insufficient experimental data were obtained before precipitation occurred. These values are minimum values. Precipitation at low pH prevented the determination of these constants.
DISCUSSION A typical NMR spectrum in DMSO(D6) of a 5-halo-8quinolinol is shown in Figure 1 , and that of a 5-halo-2methyl-8-quinolinol in Figure 2. Hydrogen bond formation between the solvent (DMSO) and the solute (8-quinolinol) molecules shifts the phenolic proton resonance downfield and also broadens it. It was only in the 5-fluoro-8-quinolinol that the phenolic proton resonance could be located as a very broad band. The spectra of the 5-halo-8-quinolinols consist of five proton resonances with well defined splittings that correspond to the five protons in the ring system (Table I). The assignments of the resonances to specific protons are readily made and agree with those that have been reported recently (9, IO). Approximate values of the coupling constants for 5-chloro-8-quinolinol are: .Iz3= 4.0 Hz, Jza= 1.5 Hz, J34 = 8.8 Hz, and J67 = 8.1 Hz. The coupling constants for the rest of the 5-substituted 8-quinolinols are of the same order of magnitude. In the 5-halo-2-methyl-8-quinolinols the resonance signals arising from the methyl group and the four protons in the ring system are clearly resolved (Figure 2), and the chemical shifts are summarized in Table 11. The halogen atom withdraws electron density inductively and donates electron density by resonance-Le., it is a uacceptor and a r-donor. In all cases however, the inductive effect is the larger of the two, and the overall effect is to deactivate the ring system by removal of electron density. It can be deduced from the Hammett reaction constants (c,,, and cg) that the inductive effects of the halogen atoms are essentially the same whereas resonance participation is greatest for fluorine and least for iodine. Although no a,,,reaction constant has been reported for the thiocyano group, it may be inferred from the large positive u value that resonance participation of the thiocyano group is at least as great as that of the iodo substituent. Resonance donation of electron density by the fluorine would tend to destabilize a partial negative charge formed on the phenolic oxygen atom in the para position and lessen the participation of the phenolic proton in hydrogen bonding. This is probably why the phenolic proton resonance is found only in the 5-fluoro-8-quinolinol spectrum. Halogen substituents in the 5-position of 8-quinolinol influence the chemical shifts of the protons in the 6-position
(ortho to the halogen substituents) to the greatest extent. The proton in the 6-position is deshielded least by the fluor0 and most by the thiocyano group. This indicates that the resonance participation of the thiocyano is less than that of the iodo group. The thiocyano group withdraws electron density inductively in much the same way as the other halogen atoms. The chemical shift of the proton in the 7-position in the thiocyano derivative indicates that the extent of inductive electron withdrawal is about the same as that for the iodo derivative. The chemical shifts of the 2-, 3-, and 4-protons in the pyridine ring should not be affected to any great extent by a halogen substituted in the phenol ring. The only exception is the unusual deshielding of the 4-proton by the thiocyano group. This can be attributed to the magnetic interaction that arises from the presence of multiple bonding in the thiocyano group. A similar interaction may be responsible, at least partially, for the large deshielding effect observed for the 6-proton in the 5-thiocyano derivative. On the basis of the proton magnetic resonance spectra of the 5-halo-8-quinolinols and 5-halo-2-methyl-8-quinolinols, the thiocyano group appears to be a pseudo-halogen with electronic effects about the same as that of aniodo substituent. The successive acid dissociation constants of the 5-substituted 8-quinolinols and 2-methyl-8-quinolinols increase in the order: F, C1, Br, I, SCN (Table 11). This is the order that was predicted from the proton magnetic resonance spectra, and is a net result of electron withdrawal from the ring system. The acid dissociation constants therefore, indicate that the thiocyano group has electron withdrawing properties that are somewhat greater than the iodo substituent. Because all the halogen substituents including the thiocyano group decrease the basicity of the oxygen and nitrogen donor atoms, it is expected that this will result in a decrease in the stability of the metal chelates of the 5-substituted 8-quinolinols and 2-methyl-8-quinolinoIs relative to the unsubstituted 8-quinolinol and 2-methyl-8-quinolinol (IZ). The results summarized in Table I1 confirm this expectation. It is more meaningful to compare the proton displacement constants ( K p . D , ) rather than the metal chelate formation constants of the 5-substituted-8-quinolinols and 2-methyl-8quinolinols. Equation 4 shows the relation between k p . ~ . p2, the overall chelate formation constant, and Kz the second
(9) B. C. Baker and D. T. Sawyer, ANAL.CHEM., 40,1945 (1968). (10) R. F. Hirsch and J. A. Sbarbaro, Analyt. Letters, 1 (6), 385 (1968).
(11) J. G. Jones, J. B. Poole, J. C. Tomkinson, and R. J. P. Williams, J . Chem. Soc., 2001 (1958).
1006
ANALYTICAL CHEMISTRY
,
k 1.0
L 2.0
l 1
1
I
I
i I
1
3.0
,
i I
I
*
--+-lwlLiu ,
L . L L
E
;
T
PPM
;
T
i
f
;
:
r
6.0
5.0
4.0
-
-
7
7.0
-
-
A
8.0
(7)
Figure 1. NMR Spectrum of 5-Chloro-8-Quinolinol in DMSO (de).
(TMS reference)
CI
acid dissociation constant. Values of pKP.D. calculated from Equation 4 are shown in Table 11. The errors (4) in the proton displacement constants are minimized because the reliability of the overall chelate formation constant is much greater than either of the successive formation constants, especially in the systems encountered in this study in which the insolubility of many of the metal chelates caused serious problems. The potentiometric determination of acid dissociation constants which are less than involves relatively large errors ( K l in Table 11). However, the second acid
dissociation constant, K2,which appears in Equation 4 can be accurately determined. The proton displacement constants in Table I1 show definite trends. All the p K p . ~ .values, except the pKp.D. values of the 5-thiocyano-8-quinolinol-nickel(II) complexes, decrease in the substituent order: F, C1, Br, I, SCN. This confirms the conclusion that was reached above from a study of the acid dissociation constants and the proton magnetic resonance spectra. It is possible that the log p2 value of the bis(5-thiocyano-8-quinolinolato)nickel(II) chelate is too low and may have been caused by incipient precipitation during the potentiometric titration. This is quite evident if the p K p . D . values are plotted against the corresponding chemical VOL.
41, NO. 8,JULY 1969
1007
shifts of the ligand protons-e.g., the 6-proton chemical shifts. A set of curves of similar shape will be obtained when p K p . ~ for . a given metal ion is plotted against the 7 value that corresponds to the proton resonance of the 5substituted ligand (Table I). Further confirmation that the p K p. D . of the nickel(I1) chelate of 5-thiocyano-8-quinolinol is too low comes from an examination of the p K p . ~ values . of the nickel(I1) chelates of 2-methyl-8-quinolinols and the corresponding P K P . D . values of the 8-quinolinols. The steric effect of the 2-methyl group lowers the stability of the 2methyl-8-quinolinol chelates relative to the corresponding 8-quinolinol chelates ; hence the p K p . D . values of the 8-quinolinol chelates should be less than those of the 2-methyl-8quinolinol chelates, and p K p . ~ of , bis(5-thiocyano-8-quinolinolato)nickel(II) should be ((2.52 (Table 11). In this study of a series of chelating agents the effect of
halogen substitution on the following parameters was measured. Chemical shifts of the ring protons; the successive acid dissociation constants and metal chelate formation constants from which proton displacement constants were calculated. The relationships between any two of these parameters are nonlinear. This nonlinearity is especially marked when the parameters for the unsubstituted ligands as well as those for the pseudo halogen substituted compounds are examined. This conclusion agrees with previous work (11) that has shown that, in general, linear free energy relationships should not be expected, and that the relationships between the measured parameters can be quite complex. RECEIVED for review January 8, 1969. Accepted April 28, 1969. This work was supported by the U. S. Atomic Energy Commission.
Solvent Extraction in the Presence of Emulsion-Forming Residues Application to the Atomic Absorption Determinationof Gold in Low Grade Ores Stephen L. Law and Thomas E. Green U.S . Department of the Interior, College Park Metallurgy Research Center, College Park, Md. Extraction in the presence of insoluble residues caused a serious emulsion problem in the development of an aqua regia, methyl isobutyl ketone extraction, atomic absorption method for determining microgram quantities of gold in ore samples weighing up to 500 grams. A study of factors affecting the change in volume of ketone during the extraction showed that, under proper conditions, atomic absorption analysis of the small quantity of ketone which separated as a clean phase provided quantitative results. The use of solvent extraction in the presence of insoluble residues which cause the formation of large emulsions has not been previously reported. This technique should have general application in many other extraction methods of analysis.
SOLVENT extraction techniques provide an excellent means for separating small quantities of metals from interfering elements and for concentrating the desired metals in a small volume of extractant. Such techniques are especially useful in atomic absorption spectrophotometry if the extractant is suitable for aspiration into the flame. However, solvent extraction techniques are generally used under conditions which provide a clean separation between the aqueous phase and the organic extractant. A serious emulsion problem was encountered in our attempt to develop a simple aqua regia dissolution-methyl isobutyl ketone (MIBK) extraction-atomic absorption method for determining gold in large samples of low grade ores. Use of samples weighing several hundred grams was necessary to minimize the sampling errors caused by the occurrence of gold in discrete particles. Fire assay, which Beamish and Chow (1) considered to be the best method for determining gold in ores, is not generally applied to samples larger than 60 grams. Alternate methods employ such techniques as cyanide dissolution of the gold, treatment of silicate residues with hydrofluoric acid or filtration to remove insoluble -~
(1) A. Chow and F. E. Beamish, Talanta,14,219 (1967).
1008
0
ANALYTICAL CHEMISTRY
residues. These approaches are impractical for the routine rapid analysis of 500-gram samples. Two other methods have been reported for determining gold in large samples: an atomic absorption method using 100-gram samples by Van Sickle and Lakin ( 2 ) and a colorimetric method using 500-gram samples by Shima (3). Both methods employ mixtures of bromine and ethyl ether to dissolve the gold. This dissolution process is reported to be quite slow for large particles of gold (2). Shima reported results lower than those obtained by fire assay and Van Sickle and Lakin reported results with standard deviations of 16 to 39W. Neither method was considered to meet the Bureau of Mines requirements. EMULSION PROBLEM
When extraction of the aqua-regia-dissolved gold was attempted in the presence of the insoluble matter from 500gram samples, nearly all of the MIBK was held in an emulsion as shown in Figure 1. The insoluble residues were found to be the cause of the emulsion. No emulsions occurred when the residues were removed by filtration before the extraction. Emulsions did form when the well-washed residues were suspended in dilute hydrochloric acid and shaken with MIBK. Filtration of the sample solutions prior to extraction would therefore have eliminated the emulsion. However, filtering and thorough washing of the large residues were time consuming and negated the desired simplicity of the method. Attempts to break the emulsions or prevent their formation by chemical additives were unsuccessful. The same emulsion problem was encountered with other extractants reported to be suitable for aspiration into the atomic absorption flame. The authors therefore investigated the use of the small portion of MIBK which separated as the clean upper phase. (2) G. H. Van Sickle and H. W. Lakin, Geological Survey Circular
561 (1968). (3) M. Shima, Japan Analyst, 2,96 (1953); CA 47,7938~ (1953).
