Distribution Coefficients of Certain 8-Quinolinols and Their Copper Chelates JAMES FRESCO' and HENRY FREISER Department o f Chemistry, University o f Arizona, Tucson, Ariz.
b The distribution coefficients of 8 quinolinol, 2-methyl-8-quinolinol, and 4-methyl-8-quinolinol have been determined for the water-chloroform and water-carbon tetrachloride solvent systems. The distributiori coefficients for the copper(l1) chelates of these reagents were measured for the same solvent pairs. Differences among the distribution coefficienis are discussed in terms of variations in the structure of the compounds.
T
HE growing populurity of chelating agents in metal separations processes has been accompanied by a n increasing number of theoretical studies of the various factors affecting solvent extraction of metal cl-elates. lClost of the attention has been focused on shedding light on the formation of metal chelates, with very lit le work directed toward the systematic elucidation of the factors affecting the distribution of the metal chelates once they are formed. The work reported here, a part of a longrange program of investigation of analytically significar t properties of metal chelates, deals with the effect of methyl substitution 01' 8-quinolinol on the distribution constant of its copper (11) chelate between chloroform or carbon tetrachloride and m t e r .
EXPERIMENTAL
Materials. 8-Quinolinol and 2methyl-8-quinolinol were purified as described in preceding papers (6, 8). 4-iLIethyl-8-quinolini,l was prepared from methyl vinyl ket )ne and o-aminophenol by the Doebner-von Miller modification of the Skraup synthesis (12). After a vacuum distillation followed by two recrystallizations from 95% ethanol, the p r ~ d u c tmelted at 140.0-42.0°; reported, 141.0' (12). Nitrogen analysis: Calculated 8.80%, found 8.90%. Copper-64 was produced by neutron bombardment of copper foil as described in a preceding paper (6). After reaction with a minimum volume of concentrated nitric aciil, the copper was diluted to 1 liter. The copper concentration ranged from 3 X l O - 4 V to 7 x 10-451. Present address, Department of Chemistry, University of Nevada, Reno, Nevada.
Reagent grade chloroform and carbon tetrachloride were used throughout the study. Traces of alcohol were removed from the chloroform by estraction with water. Apparatus. Jacketed cylindrical separatory funnels with a capacity of 100 ml. and equipped with Teflon stopcocks a n d ground-glass stoppers were used. An inlet a n d outlet fused t o t h e outer wall of t h e extraction vessel permitted temperature-regulated water t o be circulated from a Wilkens-Anderson Co. Lo-Temp bath. The outside diameter of the vessel was sufficiently narrow t o permit embracement b y a Burrell wrist-action shaker. A Nuclear-Chicago Model DS5 Versatile scintillation counter coupled t o a Suclear-Chicago Model 183B Count-OMatic scaler was used for gamma counting. Corning 16 X 150 mm. screw-cap glass culture tubes mere employed as counting containers. Absorption spectra were recorded with a Cary Model 11 spectrophotometer. Final absorbance measurements were made using a Beckman Model DU spectrophotometer with a uniform spectral slit width of 0.05 mm. The cells used were the 1-em. silica type. A Beckman Model G p H meter was used for hydrogen ion measurements. The instrument mas standardized with a potassium acid phthalate buffer at p H 4.01. Distribution of Copper Complexes in Chloroform-Aqueous System and Carbon Tetrachloride-Aqueous System. Xliquots of t h e copper-64 stock solution were transferred t o separatory funnels containing sufficient buffer solution t o give after dilution aqueous volumes of 25 ml. with a n ionic strength of 0.1. T h e solution was shaken with a n equal volume of t h e nonaqueous solvent in which t h e ligand had been previously dissolved. I n all cases t h e analytical ligand-metal ratio was of t h e order of 100 t o 1. The shaking speed was approsimately 400 cycles per minute. The shaking amplitude and position of the funnel were adjusted, to bring the phases as thoroughly as possible in contact without emulsification. Three-hour shaking periods were employed. Although the minimum shaking time for attainment of equilibrium was not determined, beyond 1 hour significant differences were not detected. On completion of estraction, phase separation occurred rapidly. Samples were removed by dipping a 5-ml. pipet into the clear
aqueous phase. During this time a slight positive pressure was eserted through the empty pipet to prevent entrainment of chloroform droplets on the upper surface of the aqueous layer. Two successive 5-ml. aliquots were removed. The p H and temperature measurements made immediately after sampling were assumed to be equilibrium values. The 5-ml. samples of the aqueous phase were transferred to glass tubes and counted. All counting rates were extrapolated to the counting time of a CuS4 standard which had been prepared from a suitable dilution of the CuS4 stock solution. The maximum time difference between a sample and standard count was 4 hours and estrapolation was according to a 12.8-hour half life. The activity of the nonaqueous phase was measured to ensure the achievement of a proper mass balance. Distribution of the Ligands in Chloroform-Aqueous System and Carbon Tetrachloride-Aqueous System. Volumes of 25 ml. of chloroform or carbon tetrachloride containing known concentrations of 8-quinolinol, 2methyl-8-quinolinol, or 4-methyl-8quinolinol were shaken for 3-hour periods with equal volumes of constant ionic strength buffers. T h e tempwature and p H of t h e aqueous phase measured on completion of extraction were considered t o be equilibrium values. A 5-ml. sample of the aqueous phase was transferred to a 25-m1. volumetric flask. Sufficient 0.2.1.1 HClOj was added to attain a pH value between 1.4 and 1.6 after dilution. The absorbance of the solution was measured against 0.03.11 HC104 as a blank. At the analytical wavelengths chosen for 8quinolinol (251 mp), 2-methyl-8-quinolinol (254.5 mp), and 4-methyl-8quinolinol (249.5 mp), the molar absorptivities in liters per mole centimeter were determined as 4.39 X l o 4 =t 0.04 x 104, 4.59 x 104 =t 0.06 x 104, and 4.29 X l o 4 =k 0.06 X lo4, respectively. RESULTS
The dependence of the distribution ratio, D, of the 8-quinolinols on the hydrogen ion concentration is given by Morrison and Freiser ( 1 1 )
VOL. 36, NO. 3, MARCH 1964
631
Table
I.
Logarithm Values of Distribution Coefficients
T = 25.0" p = 0.1
8-Quinolinol 2-4Iethyl-8-quinolinol 4-Methyl-8-quinolinol Bis-8-quinolinolo-Cu(11) Bis-2-methyl-8-quinolinolo-Cu(II) - . .
Initial concn. CHCh/H20 0.03 2.64 f 0.Ola 0.09 3.22 f 0.01* 0.10 3.27 f 0.01 3.48 3Z 0.06 (3.50 f 0.02)c 4.45 f 0.10
Bis-4-methyl-8-quinolinolo-Cu( 11) a
(4.44 3Z 0 . l O ) C 4.56 f 0.11
Initial concn. 0.002 0.02 0.03
CC14/H20 2.06 & 0.02 2.64 f 0.01 2.73 f 0.01 2.05 f 0.10 3.50 f 0.18
*
(3.26 0 . i 3 ) c 3.29 & 0.03
2.59 according to (6).
* 3.4 according to ( 3 ) .
Values obtained at 2".
where K 1 ,Kz, and K D ,are the dissociation constants and distribution coefficient, respectively. I t may be readily shown that D is a maximum when p H = 1/2(pK1 pK2) and, when D and K D are ~ equal, p H = l/p(pK~ pK2). The converse of the latter argument does not necessarily follow, however. If pKl and pKz are sufficiently close, the maximum value of D does not reach KDR. Lacroix ( I O ) , Dyrssen (4, 6 ) , and Jankowski (7) have investigated the distribution of 8-quinolinol between water and chloroform as a function of pH. Proceeding from a low p H the distribution ratio increases, passes through a broad maximum, and decreases in the higher pH region. I n the case of 8-quinolino1, pK1 and pKz are sufficiently separated so that, within the precision of the measurements, the maximum is extended to a plateau on either side of the isoelectric point and the distribution ratio many be equated to the distribution coefficient. Although isoelectric points were not determined for the ligands in the present study, it was possible to predict from the published values ( I ) of the dissociation constants of these compounds that extraction maxima would most likely occur in the p H region 7.0 to 8.0. Over approximately 2 units of change in p H close to the neutral region, distribution ratios were constant and were considered to be measures of the reagent distribution constants, K D R . These data, averaged over a t least five separate determinations, are summarized in Table I. The precision of these measurements is +3% or better. I n the absence of hydrolysis and the formation of anionic complexes the distribution ratio for a metal ion in the presence of a suitable ligand will rise to maximum and remain constant with increasing pH. This maximum will equal the distribution coefficient of the The dis~ neutral chelates, K D (11). tribution ratios of the copper(I1) chelates of 8-quinolinol and its deriva-
+
632
ANALYTICAL CHEMISTRY
+
tives studied during this investigation remained constant over a range of 3 to 4 p H units near the neutral region (pH 5.4 to 9.2) and have therefore been considered to be the values of the distribution coefficients of the chelates, KDC. These data, averaged over at least six determinations, are summarized in Table I. A source of experimental difficulty in this p H range was the low buffer capacity of these solutions. Attempts to pre-adjust the p H with sodium hydroxide and perchloric acid were unsuccessful in achieving a spread of measurements over the range of the extraction maxima. Buffers possess the additional property of behaving like masking agents. Schweitzer ( I S , 14) has shown that the extraction of 8quinolinolates in the low p H region is influenced by the presence of simple bases such as acetate and formate ions. I n the higher pH regions the distribution ratio must be both independent of p H and nature of the aqueous phase solutes for the direct determination of the distribution coefficient of a complex. I n this study the constancy of the chelate distribution ratios mas maintained in the presence of various buffers, including phosphate, acetate, ammonia, and carbonate buffers. DISCUSSION
Methyl substitution in 8-quinolinol may be seen from the data in Table I to result in a fourfold increase in the K D Rvalue in both solvent pairs. The effect seems to be independent of the position of the substituent. S o t olily are the K D Rvalues of both the 2-methyl and 4-methyl compounds essentially the same, but an essentially identical value has been reported for the 5-methyl-8quinolinol (log K D R= 3.27 in CHCl,/ H 2 0 ) ( 3 ) . These findings are in accord with a relationship described by Dyrssen (2) for the change in K D R values in a homologous series of a wide-ranging
series of organic compounds. For saturated carboxylic acids, tropolones, tertiary amines, alkyl benzenes, and dialkylphosphoric acids the KDR (CHCI,/H?O) values increase by a factor of 4 per additional carbon atom. iilthough the data presented here provide only a limited basis for conclusion, it seems likely that this generalization applies to many organic solutes in CCL/HZO systems as well. Despite the doubling of the number of carbon atoms in the copper(I1) 8quinolinolate, the increase in its KD(CHC13/H20) value over that of the reagent itself is a mere seven times greater. Undoubtedly, the effect of solvation by water on the polar center of the chelate compensates for the great increase in the molecular weight. The K D Cvalues for the Cu(I1) chelates of the 8-quinolinols are larger than those of the Th(1V) chelates in the CHC13/ HzO system ( 3 ) , even though the latter contain twice as many carbon atoms. Since, in acetylacetone extractions higher K D Cvalues were observed for Th(1V) than for Cu(I1) (Q), it is obvious that attempts to systematize the effects of structural changes on K D values must take into account the nature of the ligand as well as that of the metal. I n the limited homologous series of copper(I1) 8-quinolinolates the effect of an increase in number of carbon atoms on the value of K Din CHCI3/H20 is seen to be lower than the fourfold increase observed with the reagents. As a result the quantity KDC,'KDR', which shows the effect of the distribution constants on the pH1 of the extraction of these Cu(I1) chelates ( I I ) , decreases somewhat with the increase in molecular weight. On this basis alone, 8-quinolinol would extract Cu(I1) a t a (slightly) lower p H region than would any of the methyl analogs. This advantage of 8quinolinol derived from the KDC/ KDR2 factor is stilI further increased from the contribution of KrKa? (where K r is the formation constant of the chelate and K , is the acid dissociation constant of the phenolic hydroxy group in the reagent) in the extraction equation (11). However, the methyl-& quinolinols are capable of extracting Cu(I1) with higher D values than is 8-quinolinol, since their K D Cvalues are greater. This would be of more significance in separation for decontamination of a radionuclide than for the usual analytical separation. Since higher K D c values are observed using CHCI, than Cc14, the former is a better solvent for decontamination and similar purposes. Comparison of the R', two solvents using K D C ~ K D however, shows that they are so closely matched that for ordinary analytical separations there is little to choose betneen them. I n CCI, the rise in KoC with the number of carbon atoms is higher than that
observed in CHC13 and reaches the factor of 4 per carbon ‘I t om. The slight change of K D values ~ obtained a t 2’ and 25“ C. is indicative of either the small values of the heats of solvation of the chelates in the two widely different solvents or of their close similarity. ACKNOWLEDGMENT
The authors gratefu 11y acknowledge the financial assistance of the U. S. .ltomic Energy Commission in this work.
LITERATURE CITED
(1) Bjerrum,
J., Schwarzenbach, G., Sill& L. G., “Stabi1,i:y Constants of Metal Ion Complexes, Parts I and 11, Chemical Society, London, 1957. 12) ~, Dvrssen. D.. Division of Analvtical Che“mistry, ACS, Summer Symp&ium, Tucson, Ariz., 1963. (3) Dyrssen, D., Rec. Trav. Chim. 75, 753 (1953). 14) Dvrssen. D.. Svensk Kem. Tidskr. 64,313 (1952).‘ (5) Dyrssen, I]., Dyrssen, >I., Johansson, E., Acta Chem. Scand. 10, 341 (1956). (6) Fresco, J., Freiser, H., ANAL.CHEM., 36, 372 (1964). ( 7 ) Jankowski, S.,Freiser, H., Ibid., 33, 776 (1961).
(8) Johnston, W. D., Freiser, H., AnaZ. Chim. Acta 11,201 (1954). (9) Krishen, A., Freiser, H., ASAL. CHEM.31,923 (1959). (10) Lacroix, S., Anal. Chim. Acta 1, 260 11947). (11) Morrison, G. H., Freiser, H., “Solvent Extraction in ilnalytical Chemistry,” Wiley, New York, 1957. (12) Phillips, J. P., Elbenger, R. L., Merritt, L. L., J . Am. Chem. Sac. 71, 3986 (1946). (13) Schweitzer, G. K., Bramlitt, E. T., Anal. Chim. Acta 23, 419 (1960). (14) Schweitzer, G. K., Coe, G. R., Zbid., 24,311 (1961).
RECEIVEDfor review October 8, 1963. Accepted Sovember 26, 1963.
A n Autorniatic Method for Measuring Slopes of Rate Curves Applied to Quantitative Determination of Cystine HARRY 1. PARDUE Department o f Chemisfry, Pardue University, lafayette, Ind.
b A new method i s described for the measurement of reaction rates. In the method, the slope of the rate curve i s determined automatic:ally near zero reaction time. The measurement i s accomplished b y matching the slope of the output signal froin an integrator circuit with that of the signal from the chemical system. A servo i s used to compare the two signals and to vary the integrator input until the signals are changing a t the !same rate. The slope of the integrator output and therefore of the rate c:urve a t balance i s proportional to the integrator input which i s indicated b y the position of the servo pen or measured with a meter. The method has been successfully applied to the quantitative determination of cystisle a t the partsper-million range. The measurement i s made completely alJtomatically and an integral multiple of concentration i s read directly from a dial. The procedure consists siniply of injecting reactants into the reaction vessel and reading the numerical answer from a meter. Total measurement times are in the range of 30 secclnds with relative standard deviation of 1 to 2y0.
R
there has been an increasing interest in the automatic measurement of reaction rates with application to rapid chemical analyses. One general approach has been to measure the time required for the reaction of interest to prcduce or consume a predetermined amount of product or renctant (3-5). X second approach has been to measure the e,tent of reaction over a conbtant time nterval ( 1 ) . I n ECESTLT
each case the measurement step is simple and rapid and the quantities measured are easily related to the concentration of sought-for constituent. In this work, a new method has been developed for the measurement of reaction rates. The new method automatically measures the slope of the rate curve near zero reaction time. The measurement is accomplished by matching the slope of the rate curve with the output from an electronic integrator. -1 servomechanism compares the signals from the chemical system and integrator and adjusts the integrator input until the slopes of the two are equal. ;it balance the input to the integrator is proportional to the slope of the rate curve. The method has been developed for the quantitative determination of cy+ tine based on its catalysis of the reduction of iodine by azide. The rate of decrease in iodine concentration is detected potentiometrically ( 5 ) . Under controlled conditions, the response curve is linear with a slope proportional to cystine Concentration. L-nder the condition of balance described above, the integrator input is proportional to cystine concentration. Some of the important favorable characteristics demonstrated by the method are as follows: The measurement equipment is easily constructed from commercially available components. The measurement step is rapid usually requiring less than 20 seconds per sample. Rate data (slopes) can be presented continuously by strip chart recording or can be read numerically from a meter.
The operational features of the system are very simple. The measurement step consists simply of adding reagents and sample to the reaction vessel and reading a multiple of the cystine concentration from a meter or chart. The reproducibility of the method is good. Relative standard deviations are less than 2%. The method is sensitive. Cystine is determined a t concentrations down to 0.25 p.p.m. in a total volume of 2 ml. PRINCIPLES
OF THE METHOD
C e l l Response. Conditions for the potentiometric determination of cystine based on its catalysis of the reduction of iodine by azide have been established ( 5 ) . L-nder the prescribed conditions and for cystine concentrations below 2.5 p.p.m., the rate of decrease in iodine concentration can be represented by Equation 1
where k 1 is a rate constant depending upon solution conditions, [Iz],is the iodine concentration a t time t , and C is the cystine concentration. For the potentiometric detection of the rate of change in iodine concentration the time dependent portion of the cell voltage E , is given in Equation 2. Et
=
k In [IZIt
(2)
where k is a temperature dependent constant from the Nernst Equation. The rate of change of potential with time is then given by Equation 3. (3) VOL. 36, NO. 3, MARCH 1964
633
