V O L U M E 2 7 , NO. 2, F E B R U A R Y 1 9 5 5 Falk, H. L., Steiner, P. E., Goldfein, S..Rreslow, A., and Hykcs, R., Cancer HesParch, 11, 318 (1951). Fieser, L. F., and Campbell, W.P., J . Am. Chem. SOC.,60, 1142 (1938). Goulden, E . and Tipler, ll.,Brit.J . Cancer, 3, 157 (1949). Hartwell, J. I,., “Survey of Compounds Which Have Been Tested for Carcinogenic Activity,” Public Health Service, Pub. 149, 2nd ed., pp. 197-232, 1951. Hieger, I., J. Chent. Soc., 1933, 398. Jones, I{. S . . J . .4m.Chem. Soc., 6 7 , 2127 (194.5). llulliken, R. S., Ibid.. 72, 600 (1950)
253 (13) Ihid., 74, 511 (1952). (14) Pestemer, AI., and Freiber, E., Ber., 74, 964 (1941). ASAL. C m x , 19, 29s (15) Seyfried, W. D., and Hastings, S. H., (1947). (16) Steiner, P. E., Cancer Research, 14, 103 (1954). (17) Waller, R. E., Brit. J . Cancer, 6 , 8 (1952). (18) Wedgwood, P., and Cooper, R., Analyst, 78, 170 (1953). (19) Windaus, A., and Raichle, K., Ann., 537, 157 (1939). (20) F r i g h t , AI., IND.ESG. CHEM.,ASAL.ED.,13, 1 (1941). for review May 10, 1964. Accepted October 23, 1964. T h e majority of this work was carried out as part of t h e American Petroleum Institute Research Project MC-1. RECEIVED
Copper( I)-2,2’-biquinoline~ Complex in Aqueous Dimethylformamide RONALD T. PFLAUM, ALEXANDER I. POPOV, and NEIL C. GOODSPEED Department of Chemistry, State University of /owe, /owa City, lowa
.
The copper(I)-2,2’-biquinoline complex was investigated in order to determine the nature of the absorbing species in a water-miscible solvent and the feasibility of utilizing this solvent in determining copper. The bis2,2’-biquinoline-~opper(I)ion was found to be responsible for the characteristic purple color developed in the reaction of the reagent with cuprous copper. A mixture of equal parts by volume of dimethylformamide and water w-asa satisfactory medium for this chelation reaction. Such a solvent mixture functions in the role of a reducing agent in the reduction of copper(II) to copper( I ) ion. 4 s a consequence, it was unnecessary to add a reductant to the s>stem. Reducible ions and those that enter into precipitation reactions interfere in the detelopnlent and measurement of color. The results obtained on the determination of copper in selected samples indicate the feasibility of employing the described s>stem for such determinations. An unreported maximum in the absorption curve of the complex was found and described. The ultraviolet absorption curves for the reagent in dimethylformamide and in isoamyl alcohol were obtained.
T
HE color reaction between the organic base, 2,2’-biquinoline, and copper(1) ion was first observed by Breckenridge, Lewis, antl Quick in 1039 (1). Since t h a t time, several investig:rtors have *tiitlied the colored system from the standpoint of t h e selectivity of the reaction (3,5, 6). Breckenridge and coworkers (1)utilizetl g1:tciitl acetic acid as a solvent, in their studies whereas Hoste antl coworkers (3, 6, 6) have restricted themselves t o the use of .rvater-ininiiscil,le alcohols. These latter investigators have tlctermined that the reagent is specific for copper and have proposed t h a t a bis-2,2‘-hiquinoline-copper( I ) species existed in these nonaqueous media. The specificity of this reaction has led t o some interesting proposals for. the steric requirements of the reagent in t h e chelation process ( 2 ) und lends great analytical significance t o its use. Applications of t h e colored complex t o the determination of copper in a variety of samples have been described (4,7 , 8). All the determinations have been carried out in amyl or isoamyl alcohol with the use of hydroxylamine hydrochloride for the reduction of copper(I1) t o copper(1) ion. T h e standard analytical procedure requires a change in the oxidation st,ate of the metal ion and an extraction of the colored species out of aqueous solution into a nonaqueous phase. I n view of the specificity of this cuproirie reagent,, it seemed desirable to carry out a rigorous determination of the formula of t h e colored species involved in t h e reaction. This investigation
is concerned with such a study, together with the application of new conditions in the determination of copper. These permit the ube of a simplified procedure in the analysis of certain selected samples. APPhRATUS AND REhGEYTS
All spectrophotometric measurements \sere made with a Cary recording spectrophotometer. One-centimeter matched silica cells %ereused in every measurement. A Beckman Model C, pH
m
~ ~ e ~ from the ~ Rohm and ~ Haas Co. Purification was effected by treatment with barium oxide for a period of 24 hours with subsequent rectification in a n all-glass system. T h e fraction boiling a t 151” =t 1” C. was utilized a s the purified solvent. T h e 2,2’-biquinoline, obtained from the G. Frederick Smith Chemical Co., was used a s received. A standard solution of copper(I1) ion was prepared b y dissolution of pure copper n i r e in nitric acid and conductivity water. T h e p H of the final solution was adjusted to 5 with sodium hydroside’ All other chemicals used were of reagent grade quality. EXPERIlIENTA L
Investigation of Reagent. An investigation of the reagent was undertaken initially in order t o ascertain its solubility and absorption characteristics. It was found that 2,2’-biquinoline is sparingly soluble in ethyl alcohol, dioxane, acetonit,rile, and isoamyl alcohol, ‘but very soluble in diniethylformamicle. Dissolution in dimethylformamide is not accompanied by a n y color change nor is color developed on standing for a t least 2 months. The reagent easily retains its chelative powers for this period of time. Thus, dimethylforninmide appears t o be a n ideal solvent for 2,2’-biquinoline. Ahsorption curves for 2,2’-biquinoline in the ultraviolet region of the spectrum are shown in Figure 1. Curves for the reagent in isoamyl alcohol and in dimethylformamide are presented. The same absorption characteristic? were ohserved in both solvents. S o variations were apparent for fresh and aged solutions. Recr:-stallization of the reagent from aqueous alcoholic mixtures did not result in changes in the absorption curve. Effect of Solvent on Color Reaction. T h e addition of solid 2,2’-biquinoline to a dimethylformamide solution containing 1 X 1 0 - 3 J I copper(I1) ion praduced a faint purple color. The subsequent addition of 0.05 gram of hydroxylamine hydrochloride completely removed an>- purple color formed, and resulted in a colorless solution. T h e addition of water t o a solution containing the reagent and copper(I1) ion in dimethylformamide or t o a dimethylformamide solution containing the reagent, copper ion, and hydroxylamine hydrochloride resulted in the formation of a n intense purple color. T h e maximum color in the reartion was developed in the absence of hydroxylamine hydro-
~
254
ANALYTICAL CHEMISTRY
chloride or a similar reductant. Thus, the addition of a reducing agent t o a n aqueous dimethylformamide mixture is unnecessary. The observations described above point t o an interdependence of water and the organic solvent and t o the function of such a mixture as a reductant in the reduction of copper(I1) to copper(1) ion in the presence of 2,2’-biquinoline. It was found t h a t maximum color developed in solutions containing a definite proportion of water. Solutions containing 40 to 60% of water by volume yhowed maximum color formation. Color in these systems was \table for a t least 3 weeks. As a consequence, studies were carried out in 50% dimethylformamide with the solvent acting in a dual role-i.e., as the reductant and as a medium for the chelation reaction.
I I
I I I
a
1
0.4
1
-
I I
0.2
I t
I
I
x
300 320 340 360 380 WAVELENGTH (mg) Figure 1. Absorption spectra of 2,2’-biquinoline 1. 4.25 X IO-5.11 2,2’-biquinoline (recrystallized) in diniethylformamide 5.70 X 10-6.M 2,2’-biquinoline in isoamyl alcohol 7.0 X 10-6M 2,2’-biquinoline in dimethylformamide ( 5 weeks old) 4. 1.0 X l O - 4 M 2,2’-biquinolinr in dimethylformamide (fresh)
2. 3.
Effect of Reagent Concentration. .4 continuous variations study was carried out according t o the method of Job (9) in order t o determine the formula of the complex and the effect of reagent concentration. Stock solutions of 1 X 10-alW copper(I1) ion (aqueous) and 1 X 1O-36f 2,2’-biquinoline (in dimethylforniamide) were used for this purpose. Solutions containing varying ratios of reactants were prepared in the usual manner. The resulting mixtures of 10-ml. volume were diluted t o 25 ml. with water and dimethylformamide, so t h a t final solutions contained 5OY0 by volume of dimethylformamide. Absorbance measurements were made a t 500, 545, and 575 mp for the series of eight solutions. Continuous variations plots a t 500 and 545 mp are shown in Figure 2. Corrected absorbance values are plotted against the ratio of reagent t o copper ion. It is readily apparent t h a t the characteristic purple color in the reaction is due t o a 2 t o 1 absorbing species. The sharpness of the maxims indicates a high degree of stability in the complex. I t is therefore unnecessary t o have excess reagent (greater than a 2 to 1 ratio of reagent t o copper ion) for maximum color formation. The absorbance values obtained in this study show t h a t excess reagent does not interfere in color development or measurement.
1
Effect of Copper Concentration. Solutions containing varying concentrations of copper and the reagent were prepared and measured in order t o determine the conformance t o Beer’s law and t o make calculations of the molar absorptivity of the complex. 3.50 X lO-4M copper(I1) Solutions containing 1.50 X ion and a threefold concentration of reagent were st.udied. The ahsorption curves for fsur such aqueous dimethylformamitie solutions are shown in Figure 3. Absorption maxima occur at 358 and 545 mp. Molar ahsorptivities, based upon concentration of copper(I1) ion in moles pt’r liter, are 52,000 and 6450 respectively. The former value is approximate inasmuch as escess reagent masks the absorpt,ion maximum. This maximum has been unreported heretofore. The great sensitivity exhibitctl shows promise for the utilization of the reagent in detecting micro amounts of copper. At 545 mp, the system conforms t o Beer’s law in the concen- 2.75 X 10-4.1f (1.1 to 17.4 p.p.m. tration range 1.72 X of copper). Effect of pH. The sensitivity of the system to conditione of acidity wa8 overcome by controlling the p H of the standard copper solution or of the solution containing the copper sample. The adjustment, of p H was made hy the addition of dilute sodium hydroxide to these aqueous solutions. With the p H adjusted t o 5.0 to 7.0, the color of the complex in the resulting 50% aqueous riimethylformsmide medium devrloped satisfactorily. Reproducible results were obtained when the p H of the aqueous solution was adjusted to 5.0 t o 6.0. Effect of Diverse Ions. Although the color reaction is specific for copper, interferences due t o certain diverse ions were encountered in the system described. These occur in the presence of colored metallic ions and ions t h a t either cause precipitation reactions or introduce undesirable oxidative effects. Advantage of specificity q-as taken in the study of interferences such t,hat the particular ion in question was added t o the blank and t o the colored chelate solution. Chloride and nitrate salts, readily soluble in 5070 aqueous dimethylformamide, were added in appropriate amounts t o solutions containing 13 p.p.m. (2.04 X 10-4M) copper(I1) ion. Interferences in the clear solutions were determined spectrophotometrically.
-
280
1
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5 A list of the common ions studied is presented in Table I. The effects of the colored metal ions were cancelled out by use of a proper blank solution. Silver ion is partially reduced to metallic d v e r and is precipitated as such. Aluminum, iron, and lead may interfere through precipitation. Lead can be separated in the preparation of the sample and the addition of tartaric acid will prevent the precipitation of aluminum and iron. Ferric ion and similar oxidants interfered in the development of color. Addition of masking agents such as tartrate, citrate, and fluoride did not prove entirely satisfactory in eliminating the undesirable oxidative effects of these ions. Relatively large amounts of iron can be present in the lower oxidation state. The addition of a reductant, hydroxylamine hydrochloride, eliminated the interferences due to iron. . . ..
Table I.
Effect of Diverse Ions
Permissible Amt..
P.P.hI.
Ion CnHsOaAI+++ Cd++
Cr+++ CO++
I’e
A
T’b
+
Permissible Amt., P.P.M.
2000 800 500 2000 500 500 300 300 10 10
c1F-
Ion
c
vet+* +
255 Table 11.
Results of Determinations Copper Present,
Sample NBS 67 NBS 86 S-51 s-101 s-151 5-202
LIanganese metal Aluminum alloy Co, Ni, Cu Co, 51, Fe, Cu Zn, Cd, htg, AI. Cir Cr. h l n , Cu
Copper Found,
%
%
0 16 7 66 0 25
0 17 7 64 0 25 1 23 0 45 0 63
1 25 0 44 0 67
_ ~ -_
‘
~
trate to an appropriate volume in a volumetric flask. Add 1 gram of tartaric acid to the solution and adjust the pH to 5.0 to 6.0 with dilute sodium hydroxide. Add 5 ml. of the aqueous copper solution to 12.5 ml. of 1 X 10-3M reagent in dimethylformamide. Dilute the mixture to 25 ml. with distilled water. Prepare a blank by adding 5 ml. of the sample solution to 12.5 ml. of dimethylformamide and diluting to 25 ml. with water. hleasure absorbance a t 545 m r spectrophotometrically. Determine copper concentration from the absorbance value and a prepared calibration curve. RESULTS OF DETERMIYATIORS ON SELECTED SAMPLE
S U G G E S T E n iMETHOD FOR DETERMIhING COPPER
Add 20 ml. of 6N hydrochloric acid to 0.5 gram of the sample in a 250-ml. beaker, and heat to boiling. Cool, add 5 ml. each of concentrated nitric and sulfuric acids, a n d evaporate to fumes of sulfuric acid. After cooling, add 30 ml. of water, boil to dissolve the soluble salts, and filter off‘ the insoluble sulfates and siliceous material. Wash the residue with hot water and dilute the fil-
The results obtained with this method are shown in Table 11. Synthetic samples were prepared from a mixture of soluble nitrate and perchlorate salts. Sational Bureau of Standards samples were chosen for very low concentrations of iron. Results indicate t h a t the method i.; applicable to determination of copper in samples low in iron content. CONC LU SIOK s
The results of this investigation prove that the specific reaction between copper ion and 2,2’-biquinoline can be carried out and studied in a new solvent medium. The use of this solvent, 50% aqueous dimethylformamide, eliminates the necessity for reduction and extraction in the preparation of the colored complex. The color of the reaction was found to be due t o the bis2,2’-biquinoline-~opper(I) ion. Absorption spectra of thii chelate in the dimethylformamide medium show maxima at 358 and 545 mp exhibiting molar absorptivities of 52,000 and 6450, respectively. Absorption spectra of the reagent in the pure solvent arid in isoamyl alcohol show maxima at 315,326,and 338 mP. The feasibilit? of employing 2,2’-biquinoline in dimethylformamide for the determination of copper was demonstrated. The system described obeys Beer’s law, is independent of reagent concentration, and exhibits a very high degree of stability. The method proposed is sensitive arid is noteworthy for it? simplicity. The results on several samplcs \how that arcurate determinations can be obtained. LITERATURE CITED
400
450 500 WAVELENGTH
550
600
(mAl
(1) nreckenridge. J. G., Lewis, R. W ,J.. and Quick, L. A , , C a n . J . Research, 17B, 258 (1939). (2) Gillis, J., Anal. Chim. Acta, 8 , 97 (1953). (3) Gillis, J., Hoste, J., arid Fernandem-Caldas, E., Anales eda/oZ. 21 fisol. tegetal ( M a d r i d ) , 9, 586 (1950). (4) Guest, R . J . , h . % I . . C H E M , , 25, 1484 (1953). (5) Hoste, J., Anal. Chim. Acta. 4 , 23 (1950). (6) Hoste, J., Research, 1, 713 (1948). (7) Hoste, J., Eckhout, J., and Gillis. J.,A n a l . Chim. Acta, 9, 263
(1953).
Figure 3. Absorption curves for copper(I)-2,2’biquinoline complex in 50% aqueous dimethylformamide
(8) Hoste, ,J., Heiremans, .4.,and Gillis, ,J., M i k r o c l i P m i e v e r . Mikrochim. Acta, 36, 349 (1951). (9) Job. P., Compt. rend., 180, 928 (1925).
1. 1.72 X lo-5.V copper 2. 2.5 X 10-5.11 copper (isoainrl alcohol) 3. 7.02 X 10-5.U couuer 4. 2.02 x io-4.v copper 5. 2.75 X 10-4.V copper
RECEIVED f o r review July 23, 1954. Accepted October 25, 1954. Presented before the Division of Analytical Chemistry a t t h e 12Zth Meeting of the SOCIETY, Kansa.s City, hlo. AMERICASCHEVICAL
