Spectrophotometric Studies of Chelates of 8-QuinoIinoI in Some Water-Miscible Organic Solvents Photometric Titrations with 8-Quinolinol WALTER G. BOYLE, Jr.,I and REX J. ROBINSON University o f Washington, Seattle
5, Wash.
b The titrations of copper(ll), zinc(ll), cadmium(ll), nickel(ll), lead(ll), and uranyl ion have been investigated photometrically in dioxane-n-propyl alcohol and in dimethylformamide, with 8-quinolinol as titrant. Copper and zinc could be titrated with a precision to about 1 or 2 parts per 1000, as little as 0.03 mmole of cation; titrations of the other cations were unsatisfactory. A high absorbance reference procedure was used. Evidence was obtained of a nickel chelate with a 3 to 1 ratio of oxine to nickel. Cadmium, lead, and uranyl also reacted with more than two molecules of oxine. The investigation suggests that photometric titrations with other organic reactants in nonaqueous media can be made.
T
HE use of chelating agents as titrants in volumetric analysis has been greatly extended by the technique of photometrically locating the equivalence point. Much of the work has been on water-soluble systems, with aniinopolycarboxylic acids such as ethylenediaminetetraacetic acid [(ethylene dinitri1o)tetraacetic acid] and nitrilotriacetic acid. A number of the organic precipitating agents commonly used in gravimetric analysis form chelate compounds, many of which are soluble in organic solvents. I t was of interest to determine whether these reactions occurred in organic solvent systems and would be suitable for volumetric application by the photometric titration technique. Bobtelskj-, Bihler, and K e h a r t (2, 3) investigated a number of reactions of this type by measuring the relative absorbance of the suspension of the prrcipitate in aqueous solution as the prccipitating agent was added in successive steps. Brunimet and Hollweg (4) investigated reactions of some organic prccipitatiiig agents in organic solvents by means of potentiometric titrations. No previous work has been done to deter1 Present address, University of California Radiation Laboratory, Livermore,
Calif.
958
ANALYTICAL CHEMISTRY
mine whether formation of this type of soluble chelate can be followed photometrically to yield satisfactory analytical results. 8-Quinolinol (8-hydroxyquinoline, oxine), as one of the most widely used organic precipitants, was chosen as the chelating agent because it has the correct solubility properties, it chelates many cations, its optical properties and metal chelates are suitable for photometric investigation, and the stability constants for its cationic chelates are very high ( 5 ) . TKO solvent systems, N,n'-dimethylforniamide as one and a mixture of equal volumes of n-propyl alcohol and 1.4-dio~aneas the other, were found suitable on the basis of their solvating power for both inorganic cations and the organic reagent. Reactions of oxine with copper(II), zinc(II), cadmium(II), nickel(II), lead (11), and uranyl ions were investigatcd. REAGENTS
Standard solutions of @.lJI copper, zinc, cadmium, and nickel were prepared by solution of Baker's reagent grade metal, (conforming to ACS specifications) in a minimum amount of nitric acid, followed by dilution to volume. Lead and uranyl solutions, also O.liM, were made by dissolving the C.P. nitrate salts and were diluted to prepare 0.OlM solutions. 8-Quinolinol, Baker's analyzed reagent. Concentrations ranging from 0.015M to 0.15M in each of the two solvent media were prepared to result in volumes of from 3 to 5 nil. of titrant per titration. 1,4-Dioxane (Matheson, Coleman and Bell, melting point, 10-11' C.), was distilled from a n all-glass system and the last 20% by volume discarded. en-Butylamine, Matheson, Coleman and Bell, boiling range 76-78' C. Triethylamine, Eastman White Label. N.A~-Diineth~dformamide. Iliatheson, Coleman and" Bell, boiling range 152-4' C. All other reagents conformed to -4CS specifications. APPARATUS
A Beckman Model DUspectrophotonieter was used for the optical measurements. The spectra iyere taken with
pure solvent as reference, matched 1cm. silica cells, and the usual operating procedure. The titration cell was a 50-ml. low-form borosilicate glass beaker seated in a rubber ring secured to the bottom of a microcolorimetric cell compartment. A Lucite cover to the beaker minimized evaporation of the solvent. The titrant was measured with a calibrated 10-mI. microburet. Both the buret and motor-driven glass stirring rod passed through holes in the 1%-oodencell compartment cover and Lucite beaker cover. The holes in the wooden cover yere protected from light mith felt washers. The exposed portion of the stirring rod and the lower ungraduated portion of the buret n ere covered with black plastic tape. T o maintain the stability of the spectrophotometer during the titration, the battery F a s charged with a Aiallory battery charger, Type 6-AC-6, connected to a constant power supply (110 volts, alternating current, =tl%). Experiments with various colored titrants indicated the instrument could reproduce the absorbance reading of equal increments of titrant to *@.001 absorbance unit. I n most titrations the sensitiyity knob was turned one revolution from its extreme counterclockwise position. This allowed use of minimum slit widths, with the sensitivity still slightly greater than the precision of the transmittance dial reading. The buret was calibrated a t 1-nil. intervals with the two solvent media used in this work. For this, the densities of the solvent media covering the temperature range of calibration were needed. These were obtained by using 10-ml. pycnometers and a constant temperature bath controlled to A0.1" C. Table I lists the densities and cocfficients of cubical expansion for dimethylformamide and a 1 to 1 mixture of dioxane and n-propyl alcohol. The chief difficulties in reproducing the buret calibrations were improper drainage of dimethylfornianiide and evaporation of the dioxane-n-propyl alcohol. As the photometric titrations require a comparatively long time for completion, long drainage times had to be used in the buret calibrations. The flow time of 0.02 nil. per second al-
~
0.70
0.50
wuz
a 0.30 (0 0
4\
m
a
0.10
355
3 75
395
415
WAVELENGTH. mY 355
375
415
395
Figure 2 . Spectra of 2 X methylformamide
WAVELENGTH.my
Figure 1. Spectra of 4 n-propyl alcohol
I *n_l
X 1O-3M oxine in dioxane-
I. 2.
3.
1.
No base
2.
370 concentrated ammonia, 10% triethylamine
7' 91
Figure 3. Spectra of 1 O-3M copper oxinate and 1O-3M copper(l1) ion in dimethylformamide
3\ \2
1.
2. 3.
1
350
Copper(l1) nitrate, 5% n-butylamine, 10% water Copper oxinate, 5% n-butylamine Copper oxinate, 5% n-butylamine, 10% water
10-3M oxine in di-
10% water 10% n-butylamine 10% water, 10% n-butylamine
Table I. Densities and Coefficients of Cubical Expansion for Dioxane, nPropyl Alcohol, and Dimethylformamide
Dioxane-n-Propyl Alcohol Density, g./ml.
Dimethylformamide Density,
Temp., g./ml. (i0.0005) "C. (iO.0005) 0 9175 0.9161 23.6 0.9454 0.9150 0 9142 25.4 0.9437 0 9129 29.0 0.9405 Coefficient of Cubical Expansion 0.0011/0c. 0.0001 0.0009/0c. 0.0001 T'Ep'' 21 7 23.0 24.2 24.9 26.2
*
I
390
I
430
I
470
510
WAVELENGTH, my
lowed almost complete drainage. K h e n dioxane-n-propyl alcohol was used, appreciable evaporation occurred while the buret was draining into the open weighing flask; this was determined to be about 0.9 mg. per minute. Such a correction was applied to the buret calibrations using this solvent mixture. Although evaporation is not an important factor in the calibration with dimethylformamide, drainage was extremely difficult to reproduce, unless the buret \vas cleaned frequently with warm chromic acid solution. ABSORPTION SPECTRA
Prior to performing the photometric titrations, information was needed regarding the absorption characteristics of the solutions of oxine and the various metallic oxinates in the two solvent media, so that a wave-length range could be selected to permit titration of the cation without interference from oxine and with the desired sensitivity. Investigation of the spectrum of osine in dioxane-n-propyl alcohol (Figure 1)
showed that an absorption peak was present a t about 350 mp and was shifted slightly to longer wave lengths when the solvent was made basic with ammonia or triethylamine. I n dimethylformamide (Figure 2) the 3.50-mp peak was also shifted slightly to longer wave lengths in the presence of n-butylamine. When water was added to dimethylformamide, the absorption peak due to oxine broadened greatly, so that the type of curve was different (Figure 2). This phenomenon was not encountered with the dioxane-n-propyl alcohol system, as dioxane seems to remove water up to certain proportions (1). K i t h the type of photometric titration curve obtained it n-as necessary to eliminate absorbance due to the osine past the equivalence point. Evaporating the water from the aqueous solution of cation to be determined effectively reduced the absorbance of oxine in dimethylformamide, even with considerable base present in the solvent. The spectra of copper oxinate in dimethylformamide are shown in Figure 3. I n a neutral or slightly acidic solu-
tion there is only a weak maximuni in absorbance a t about 390 mp. K h e n the solvent is made basic with n-butylamine, the peak increases in height and shifts to longer wave lengths. This shift has been correlated with increasing strength of chelation by Sone ( 7 ) . The spectra of copper oxinate in dioxane-n-propyl alcohol were, in general, similar to those in dimethylformamide. However, when 12-butylamine \vas present, absorption in this solvent increased with time. It was ascertained that this was due to the formation of peroxide in the dioxane, which then oxidized the nbutylamine t o colored products in the presence of cations. KOoxidation occurred when the solvent was made basic with triethylamine. However, ammonium hydroxide has to be added to the solution to maintain solubility by compleaing the ammine-forming metals. Spectra of zinc, cadmium. nickel, lead, and uranyl oxinates were also obtained in these solvents. I n general, they \$-ere very similar to copper oxinate. The peak absorption appeared around 400 mh when the solvents had been made basic. I n the case of lead and uranyl oxinates, the solvents could be made VOL. 30, NO. 5, MAY 1958
959
0.401
W 0
1
a
m K
In 0 m
a W
1
s W I K
0.lOl
IB
0.001 3.9
I
4.7
I
5.5 ML. OF OXINE
I
6.3
I
7.I
1 2 .o
Figure 4. Photometric titration of copper(l1) ion in dimethylformamide 12% n-butylamine, 0.04 mmole copper(ll), 0.01 5M oxine. length 470 m p
Wave
3.0
4.0 ML. OF OXINE
5.0
Figure 5. Photometric titration of nickel(l1) ion in dimethylformamide 12% n-butylamine, 0.3 mmole nickel, 0.1 5 M oxine.
somewhat basic by first complexing these cations with acetate before adding base. PHOTOMETRIC TITRATIONS
A highly sensitive method of titration was used, sensitivity being defined as the change in absorbance per 0.1 ml. of titrant added. This method is somewhat analogous to Hiskey’s (6) reference solution method for colorimetry. After most of the cation had been titrated, the instrument was rebalanced to zero and the titration continued by taking points at 0.1- to 0.2-ml. intervals until well past the equivalence point. This permitted measurement of the absorbances near the point of equivalency in the region of maximum sensitivity of the instrument. Dilution corrections were made to obtain an accurate plot of the titration, and particularly in the case of the more dilute solutions to improve the precision of locating the end point. The normal procedure would be to multiply the volume of titrant a t each point by (T’ X ) / V , but in this method of rebalancing an added absorbance is being diluted. When this dilution is taken into account, the formula becomes:
+
where
A,
corrected absorbance reading to be plotted A , = final observed absorbance reading A , = absorbance reading before rebalancing X = milliliters of titrant added after rebalancing I’, = vo!ume of solution a t rebalanc1ng This formula calculates the correction for the dilution of the highly absorbing solution, A,, and adds this correction to 960
=
ANALYTICAL CHEMISTRY
6.0
the observed absorbance, A,, and then the usual dilution correction is applied to this corrected absorbance. The values for A , and V , need not be accurately known, although, as the sensitivity and V , increase, A , increases and the range of allowable error decreases. The ability of the spectrophotometer to give an accurate value for A , also decreases with increase in A,; hence the process is limited in application. Because of the greater accuracy resulting from the use of this equation, titrations may be made with smaller volumes and smaller absolute quantities of cations. TITRATION PROCEDURE
A solution of oxine was made up to a concentration to yield a total volume of titrant a t t h r equivalence point of from 3 to 5 ml., and \vas standardized with a standard solution of the cation to be determined. With dioxane-n-propyl alcohol as the titration medium the procedure was as follows: A standard aqueous solution of the cation to be titrated was measured from a 10-ml. microburet into the 50ml. titration cell. About 15 ml. of dioxane and an equal volume of n-propyl alcohol were added and mixed thoroughly. One milliliter of concentrated ammonium hydroxide was added with stirring, then 2 to 5 ml. of triethylamine. The outside of the beaker was carefully wiped with lens tissue, and the beaker was seated in its holder and positioned in front of the slit. The beaker and the cell compartment covers were adjusted in place. A wave length was chosen which would give a sensitivity of about 0.020 absorbance unit per 0.1 ml. of titrant, rather than utilize the region of maximum absorbance. This was determined by a trial titration. Mechanical stirring was begun and after a few moments the instrument was balanced to zero. After the instrument sholyed no drift from its previously balanced position (usually about 5 minutes), the buret was placed in position so that
Wave length 498 m p
the buret tip was barely immersed in the solvent in the cell. The titrant was added slowly and the instrument was rebalanced, after the -4, reading had been noted a t a volume of titrant which would allow a t least five points to be taken on the straight-line portion of the curre before the end point of the titration. Successive small increments of titrant liere then added until past the equiralence point and the nhsorbances were noted, The procedure with dimethylforniamide as solvent was essentially the same. After measurement of the cation solution into the beaker, the water was evaporated by using a vacuum desiccator. Thirty-five milliliters of dimethylformamide was added, and then 5 ml. of n-butylamine, with constant stirring to prevent surface tension from forcing liquid up the walls of the beaker. The absorbance readings were corrected for dilution by using the previously drrived formula. The results were plotted on millimeter graph paper; 1 mm. on the abscissa represented 0.005 ml. of titrant; 1 mm. on the ordinate represented 0,001 or 0.002 ahsorhance unit. When there was uncertainty in the linearity of the two lines to be extrapolated, the average position was token RESULTS AND DISCUSSION
Preliminary titrations were made in each case to establish the correct wave length, concentrations of reagent and mctallic ion, and conditions of reaction most suitable for completeness of reaction. Conformance to Beer’s law n-as also examined. Figure 4 shows a typical titration curve for copper(I1) in dimethylforniamide. The character of the curve n a s essentially the same in diosane-n-propyl alcohol. It was determined that the presence of a base in the solvent enhanced the completeness of the reaction considerably. An organic base was chosen because it could be added as a
separate solvent and in sufficient amounts t o maintain the solution a t a high basic strength during the chelation reaction. On the other hand, the oxine spectrum is shifted to higher wave lengths in highly basic solutions, so that only a limited basicity may be used. A concentration of 5 t o 10% by volume of triethylamine was determined to be suitable n it11 dioxane-n-propyl alcohol as the solvent medium; approximately 12% n-butylamine was used with dimethylformamide. The titration curves for zinc(I1) ion were very similar to those for copper with these solvents. The equilibrium near the equivalence point is less favorable for zinc than for the copper titrations. To attain an end point comparable with copper it was necessary to work n-ith zinc concentrations about ten times those for copper. Past the end point there was a slight drop below the horizontal in the slope of the titration curve. The reason for this is unknown, but possibly i t was due to failure to obey Beer’s larv throughout the entire range of titration. From the location of the end point for the titration and the amount of copper or zinc present it was calculated that oxine was reacting with the metallic ion in the ratio of 2 to 1. The spectra of cadmium oxinate are very similar t o those of zinc oxinate. Cadmium may be titrated photometrically in dimethylformamide but with less satisfactory results than for zinc (Table 11). The end point at the 2 to 1 ratio of oxine to cation could be reproduced to =k lyo. The continued rise of the titration curve past the end point, though at a lewer inclination, is probably due to further addition of oxine to the cadmium complex. I n dioxane-n-propyl alcohol the end point is not reproducible because of slow hydrolysis of the cadmium ion, causing a turbid solution. Photometric titrations of nickel(I1) (Figure 5 ) did not indicate breaks in the curve suitable for analytical determinations. The slope of the curve a t a molar ratio of two oxine to one nickel began to increase rather than decrease as in the case of copper, zinc, and cadmium. This may be taken as evidence of the reaction of oxine with nickel at a ratio greater than 2 to 1. Approaching a 3 to 1 ratio the slope of the curve decreased but with considerable curvature. Although nickel oxinate is precipitated from water solution as a 2 to 1 chelate, evidence of the addition of a third oxine was obtained by Brummet (4) in the poteritionietric titration of nickeI in a methanol-benzene medium. Solutions of lead(I1) and uranyl in dioxane-n-propyl alcohol could be made basic n ith triethylamine without precipitation, by first complexing these cations with acetic acid. Under these conditions photometric titrations showed considerable sensitivity. However,
Table II.
Analytical Results
Oxine Alole/L. Dimethylformamide Titration of Copper(I1) Ion at 470 m p , 5 RI1. n-Butylamine, 35 111. Solvent Copper,
hlmole
311.
0 03390
4 4 5 5
0 03955
529 524 282 280
0 0 0 0
01496 01499 01498 01498
Zinc, Oxine Mmole All. Mole/L. as Solvent Titration of Zinc(I1) Ion at 500 mp, 5 MI. n-Butjlamine, 35 Ml. Solvent 0 03000 0 03500
4 4 4 4
140 173 115 788
0 0 0 0
01449 01438 01458 01449
5.437
Average Standard deviation
0.01498 0.000011 0,3000 0.3500 0.4000
3,913 3.925 4.580 4.570 5.206
0.01458 0.00014 0.1533 0,1529 0.1528 0.1532 0.1537 0.1532 0 00036
Average Standard deviation Dioxane-n-Propyl Alcohol as Solvent Titration of Zinc(I1) Ion at 478 mp, Titration of Copper(I1) Ion at 470 mp, 1 M1. 15N Ammonia, 2 All. Triethylamine, 1 All. 15N Ammonia, 2 All. Triethyl32 MI. Solvent amine, 30 111. Solvent 0,04520
4.597 4.609 4.615 4.602 4.613 4.604 4.604 4.607
Average Standard deviation
0.01966 0.01961 0,01959 0,01964 0.01960 0.01964 0.01964 0.01962 0.01963 0~000024
there was no detectable break in the curve at a 2 to 1 molar ratio of oxine to cation. The systems followed a Beer’s law relationship until somewhat past a 1 to 1 ratio of oxine to cation, when the slope increased and continued beyond the 2 to 1 equivalence point. From a consideration of the results with nickel and cadmium it is reasonable to conclude that a third molecule of oxine was interacting with these cations. Of the photometric titrations performed, only those for copper, zinc, and cadmium had quantitative significance. Results for copper and zinc are given in Table 11, with the calculated standard deviations for these titrations. Considerable precision was attained for the titrations of copper and zinc, with less for cadmium. When 0.3 mmole of cadmium was titrated under similar conditions, the results of three determinations had a standard deviation of 2.8%. These results indicate that successful titrations involving chelating agents, whose reaction products are insoluble in water, are feasible using organic solvents to solubilize the chelates. These titrations, in general, are comparable in accuracy and sensitivity but are not so versatile as the titrations in water
0.30OO 0.3500
4.122 4.790 4 . 779
0.1455 0.1461 0.1465
0,1461 0.00051
medium n-ith aniinopolycarboxylic acids. Investigations of this type help to elucidate the chemistry of the commonly used organic reagents. The large number of organic precipitating agents and organic solvents allow a wide range of possibilities for investigation. ACKNOWLEDGMENT
The authors wish to express their thanks to the Procter and Gamble Co. for a predoctoral fellowship to Walter G. Boyle, Jr., for the year 1985-1956. LITERATURE CITED
Azume H., Sci. Repts., Tohoku Cniv. 1st idw. 35, 95-102 (1951).
Bobtelsky, M . , Bihler, L., Anal. Chim. Acta 10,’260 (1954). Bobtelskv. M . . Kelwart. Y.. Ibid.,
9, 281:’374 ’(1953); 10, 151, 156 (1954). Brummet, B. D., Hollweg, R. AI., AXAL.CHELI.28, 448 (1956). Freiser, H., Analyst 77, 830 (1952). Hiskey, C. F., h s a ~ .CHEM. . 21, 1440 (1949). ( 7 ) Sone, I
