Effect of pH. The extraction of copper was studied at p H 1 t o 7 (Table I). The results showed that extraction is quantitative between p H 2 t o 5. However, the extraction is incomplete beyond this p H ; hence, the optimum p H for quantitative extraction of copper is 2.0 to 5.0. Calibration Curve. Different amounts of copper(I1) were taken and extracted at p H 4.0 and measured at various wavelengths, as shown in Figure 2. The Cu(I1)-STTA system conforms to Beer’s law over the concentration range of 1.23 to 12.35 pg of copper per ml at 490 mp only. Furthermore, at this wavelength there is a maximum difference in absorbance between the complex and the reagent blank. Stability of the Color. As per the general procedure, the absorbance of the Cu(I1)-STTA complex was measured at elapsed intervals of 0, 1,24,48,72, and 118hours. The absorbance was stable t o 72 hours. Hence, the complex should be measured within 72 hours of the extraction. Reagent Concentration. The extraction of copper was carried out with varying concentrations of the reagent (Table 11). The results show that a single extraction with 10 ml of 0.001M reagent is adequate for quantitative extraction. There is insignificant enhancement in the extraction of copper with a greater volume of reagent at this concentration and, with dilute solutions, the extraction is incomplete.
Effect of Salting-Out Agent. The sulfates of ammonium, lithium, sodium, and magnesium (1 to 3M) were used as salting-out agents to study their effects on the extraction of copper with 0.001M STTA at p H 4.0. The results revealed that they d o not enhance the extraction. Diverse Ions. Results in Table 111 show the effect of various ions on the process of extraction. The tolerance limit was set at the amount required to cause =t2,0Z error in the copper recovery, Thallium, lead, antimony, bismuth, tin, platinum, chromium, beryllium, calcium, strontium, molybdate, tungstate, acetate, and selenite can be tolerated in the ratio of 1 t o 20, but ions such as cadmium, ruthenium, iron, nickel, and zirconium are tolerable in the ratio of 1 t o 1. Gold, mercury, oxalate, cyanide, and thiocyanate show serious interference. From eight determinations with 49.4 pg of copper, the absorbance was found to be 0.470 0.010. Thus, the relative standard deviation is approximately f1.02z. The total operation requires about 30 minutes.
*
RECEIVED for review February 29,1968. Accepted September 13, 1968. This project was sponsored by the Council of Scientific and Industrial Research (India), which awarded a Junior Research Fellowship t o one of the authors (V.M.S.).
Spectrophotometric Determination of Copper in Alkali Metals and Hydroxides with 4,4‘-Dihydroxy2,2’-Biquinoline Alfred A. Schilt and William C . H o y l e Department of Chemistry, Northern Illinois University, Dekalb, Ill. 60115 Thirteen new symmetrically substituted derivatives of 2,2’-biquinoline were evaluated as chromogenic reagents. One of these, the 4,4’-dihydroxy derivative, is capable of complexing copper(1) directly in strongly alkaline solutions. A detailed study of the complex and its application for the determination of copper in sodium and potassium metals of hydroxides are reported. Simplicity is achieved in the determination since neutralization of base and pH adjustment are unnecessary. Sensitivity is favored by extractability of the complex. In common with other cuproine reagents, 4,4’-dihydroxy-2,2‘-biquinoline is a specific chromogenic reagent for copper(1).
stituted ferroin and cuproine reagents initiated by Professors Smith and Case (3, 4). The present investigation was undertaken to evaluate the chromogenic properties of the 13 newly synthesized biquinolines with regard to metal ion chelation. A detailed study was subsequently made of the reaction between copper(1) and the most promising of these, the 4,4’-dihydroxy derivative, from which practical procedures for the determination of copper in alkali metals and in concentrated alkaline solutions were established.
THEPREPARATION of 13 different 4,4’-disubstituted 2,2‘-biquinolines was recently described by Case and Lesser ( I ) . They report that the 4,4‘-dihydroxy derivative can complex copper(1) in concentrated alkaline solutions. This remarkable property was not unexpected, for an earlier study had revealed that hydroxy substituents in the 4,7-positions in 1, 10phenanthroline will impart greatly improved alkaline stability to the iron(I1) phenanthroline complex (2). It is fair to assert that 4,4’-dihydroxy-2,2‘-biquinoline was literally custom made by Case and Lesser for the purpose of chelating copper in strongly alkaline solutions. The appropriate design for its construction evolved through the systematic studies of sub-
Apparatus. A Cary Model 14 recording spectrophotometer was used for the absorbance and spectral measurements. A Corning Model 7 p H meter, with saturated calomel-glass electrode system, was used for the p H measurements. Reagents. Preparative and analytical details of the substituted 2,2’-biquinolines have been reported (1). Solutions, 0.004M, were prepared by adding a slight excess of concentrated hydrochloric acid to weighed amounts of the biquinolines, followed by measured volumes of ethanol. The 0.004M solution of 4,4‘-dihydroxy was prepared by adding 3 drops of 6 M sodium hydroxide to 0.115 gram of compound, followed by 100 ml of 9 5 z ethanol. A 10% solution of hydroxylamine hydrochloride, for use as a reductant for
(1) F. H. Case and J. M. Lesser, J . Heterocycl. Chem., 3, 170 (1966). (2) A. A. Schilt, G. F. Smith, and A. Heimbuch, ANAL.CHEM., 28, 809 (1956).
(3) G. F. Smith, ANAL.CHEM.,26, 1534 (1954). (4) F. H. Case, “A Review of Syntheses of Organic Compounds Containing the Ferroin Group,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1960.
EXPERIMENTAL
344
ANALYTICAL CHEMISTRY
copper(II), was prepared by dissolving 100 grams of the salt in 900 ml of distilled water. Standard copper sulfate solution, containing 0.1596 mg Cu per gram of solution, was prepared by dissolving the pure metal in nitric acid, followed by addition of sulfuric acid, evaporation to near dryness to drive off the nitric acid, and dilution to a known concentration with distilled water. The isoamyl alcohol was Baker and Adamson, purified grade. The metallic potassium was J. T. Baker, purified grade. The metallic sodium and all other chemicals were ACS reagent grade. Spectral Characteristics. Solutions of the various copper(1) biquinoline complexes were prepared for spectrophotometric examination as follows : weighed quantities of the standard copper sulfate solution were delivered into 60-1111 separatory funnels; 1 ml of 10% hydroxylamine hydrochloride, 5 ml of 1 M ammonium acetate, and 2 ml of the 0.004M biquinoline solution were added in that order; each mixture was extracted twice with 4 ml portions of isoamyl alcohol, and the extracts were combined in a 10-ml volumetric flask and diluted to volume with 95 % ethanol. For solutions of the copper(1) complex of 4,4 ’-dihydroxy-2,2’-biquinoline, 5-ml portions of sodium hydroxide solutions of various concentrations were used in place of the ammonium acetate solution. The spectral characteristics of the copper(1) complex of 4,4 ’-dihydroxy-2,2’-biquinoline were also examined in aqueous solutions of various sodium hydroxide concentrations. Weighed quantities of the standard copper sulfate solution were delivered into 10-ml volumetric flasks; measured amounts of sodium hydroxide, 1 ml of 10% hydroxylamine hydrochloride, and 1 ml of 0.004M biquinoline were added; and the contents were diluted to volume with distilled water. Mole Ratio Study. The method of Yoe and Jones (5) was applied to aqueous solutions of 6 M sodium hydroxide containing 4,4 ‘-dihydroxy-2,2‘-biquinoline at a fixed concentration of 9.57 x lO-bM, varied amounts of standard copper sulfate, and 0.1 gram/liter hydroxylamine hydrochloride. Absorbances were measured at 528 mp. Acid Dissociation Constants. Ultraviolet spectra of aqueous solutions of 4,4 ’-dihydroxy-2,2’-biquinoline at a fixed concentration of 1.55 x lOP5Mbut of different pH were recorded at room temperature. The pH was varied from 0 to 14 using appropriate amounts of 1 M solutions of hydrochloric acid, sodium hydroxide, and ammonium acetate, either individually or in combination so that the ionic strength remained essentially constant. Solution pH was measured just prior to recording spectra. Procedure. DETERMINATION OF COPPER IN STRONGLY BASICSUBSTANCES. The weight of sample taken for analysis should be such that the hydroxide concentration of the final solution is 7-12M. Pipet a sample (7-12M in hydroxide) of sufficient size to contain 5-100 pg of copper into a separatory funnel. Add 2 ml of 10 hydroxylamine hydrochloride and 2 ml of 0.004M 4,4’-dihydroxy-2,2’-biquinoline.Extract once with 5 ml and again with 1 ml of isoamyl alcohol. Combine the extracts in a 10-ml volumetric flask, add 1 ml of 10 hydroxylamine hydrochloride in 1 :1 water-ethanol solution, dilute to volume with 95% ethanol, and measure the absorbance of the solution us. a similarly prepared reagent blank at 525 mp. Make use of a suitably prepared calibration curve or empirical calculation to convert absorbance to concentration. An alternative method, that omits extraction at the expense of sensitivity, may be preferred for its simplicity: Pipet a sample (6-8M in hydroxide) containing 5-100 pg of copper into a 10-ml volumetric flask. Add 1 ml of 10% hydroxylamine hydrochloride solution, 2 ml of 0.004M 4,4 ‘-dihydroxy2,2 ’-biquinoline solution, and dilute to volume with 6 M sodium hydroxide. Measure the absorbance at 528 mp us. a similarly prepared reagent blank. ~
~
~~
(5) J. H. Y o e and A. L. Jones, IND. ENG.CHEM., ANAL.ED.,16, 111 (1944).
I
I
4 50
I 5 00
I
I
550
600
6 50
WAVELENGTH, m y Figure 1. Absorption spectra of isoamyl alcohol-ethanol solutions of the copper(1) complex of 4,4’-dihydroxy-2,2’biquinoline obtained following the recommended analytical procedure DETERMINATION OF COPPER IN SODIUM METAL. Wash the sodium metal with anhydrous ether, and carefully slice off the oxide layer. Accurately weigh a clean 9-gram sample, and carefully dissolve it in 100 ml of methanol. Evaporate the solution on a hot plate to a thick paste, carefully and slowly add 25 ml of distilled water, and heat to boiling to drive off the remaining alcohol. Transfer the solution quantitatively to a 50-ml volumetric flask, and dilute to volume with distilled water. Complete the determination using an aliquot of suitable size and the above recommended procedure. DETERMINATION OF COPPER IN POTASSIUM METAL. Follow the above procedure as for determining copper in sodium metal, except with the following important modification: dissolve an accurately weighed 16- to 17-gram sample of the potassium in 100 ml of methanol, kept at 0 “C in an ice bath, by carefully adding pea sized chunks one at a time to the cold methanoi. RESULTS AND DISCUSSION
Chromogenic Properties of the Substituted Biquinolines. Of the 13 compounds studied, only five proved sufficiently soluble in common solvents to enable color tests to be made with various metal ions. Copper(1) was the only metal ion that gave colored products with these. The five compounds and the spectral characteristics of their copper(1) complexes are listed in Table I. The 4,4 ’-symmetrically disubstituted 2,2’-biquinolines that failed to dissolve or to give colored products with copper(1) were those with chloro, bromo, p-chlorophenoxy, p-bromophenoxy, phenoxy, naphthoxy, thiophenoxy, or p-fluorophenoxy groups. VOL. 41,NO. 2, FEBRUARY 1969
0
345
Table I. Spectral Characteristics of Copper(1) Chelates in Various Solutions 2,2’-Biquinoline derivative 4,4’-Dipyridino4,4’-Dipyrollidino4,4 ’-Dimethoxy4,4 ’-Diethoxy4,4’-Dihydroxy-
a
Cu(1) color
Solvent Isoamyl alcohol Isoamyl alcohol Isoamyl alcohol Isoamyl alcohol Isoamyl alcohol. 1M NaOH 2M NaOH 3M NaOH 4M NaOH 5M NaOH 6M NaOH 7 M NaOH 8M NaOH
Violet Purple Purple Purple Magenta
+ + +
Table 111. Effect of Foreign Ions4
”4’
Bez+ VOa~ 1 3 +
Zn2+ Pb2+ Biz+ As 3+
Cr 3+ Ni2f coz+ Fez+ c1Br-
IOAcNO$clodClOaSOaz~
Relative error,
Concentration, PPm lo00 1000 lo00 lo00
Ion Li+ K+
0
3
-
SCNCN-
Source LitS04 KC1 NH4Cl Be(N03)~.3II& lo00 NaV03 1000 Al(N08)a lo00 ZnClz 1000 Pb(N0a)z 1000 NaBi03 lo00 ASzOa 100 Cr(N03)3‘6HzO 100 NiClz.6Hz0 100 Co(N03)z. 6Hz0 10 Fe(NH&(S0&.6HtO 100 (+-lo00ppm Tartrate) lo00 NH4CI 1000 NaBr lo00 NaI lo00 NaOAc lo00 NaN03 1000 NaC104 1000 KClOa lo00 NasS04 lo00 Na3P04 lo00 NaSCN 100 10 KCN 1
C4H40e2-
NatC4HaOa
z
0.3
0.3 0.3 2.7 0.0 -0.5 0.0 0.0
1.3 1.1 0.0 0.0 0.0
8.0 0.8 0.0 0.0 0.0 0.0 0.0
0.3 0.0 0.0
0.3 -23. 0.8 -100. -50. 0.0
4 All solutions contained 3.19 pg Cu per ml and were 12M in sodium hydroxide; 5-ml samples were taken for analysis.
346
552 536 538 538 525 517 521 523 525 527 528 528 528
Molar absorptivity 10,100 9,600 7,000 7,300 6,900 5,900 6,100 6,300 6,700 7,000 7,100 7,200 7,100
Extracted from 7-12M NaOH solutions into isoamyl alcohol.
Table 11. Acid Dissociation Constants of 4,4’-Dihydroxy-Z.Z’-Biquinoline(HzB) in Aqueous Solution as Determined by the Effect of pH on Ultraviolet Spectra Isosbestic points Apparent A, mp pH range Species and equilibria pK. 0-4 272; 327 H 3 B f S H+ HzB -2 4-10 268; 334 HzB e H+ HB7.8 10-14 295 HB- s H+ B*-12
~
Am,, ( m d
ANALYTICAL CHEMISTRY
The colored complexes precipitate from aqueous solutions of p H 3-9 but dissolve readily in isoamyl alcohol permitting their extraction. Absorbances follow Beer’s law in all cases. The most promising copper chromogen of the group appears t o be 4,4‘-dihydroxy-2,2’-biquinoline. Because it complexes copper(1) in concentrated hydroxide solutions, it can be employed for the direct determination of copper in strongly alkaline substances without the necessity of neutralizing the alkalinity or adjusting the pH. The other four biquinoline derivatives do not appear to afford any advantages over other commonly used copper chromogens (6). Spectral Characteristics. Figure 1 shows the absorption spectra of a series of isoamyl alcohol-ethanol solutions containing different amounts of copper(1). These were prepared by applying the recommended analytical procedure to the determination of copper in reagent grade sodium hydroxide solutions containing known amounts of added standard copper sulfate solution. Spectra were recorded using a 1 .OOcm cell against air as a blank. The lowest curve is that of the reagent blank to which no copper was added. Spectra of the copper(1) complex of 4,4’-dihydroxy-2,2’biquinoline exhibit some dependence on solvent and pH, as evidenced by the data in Table I. In aqueous solution, a small bathochromic shift and increased absorptivity accompany an increase in hydroxide ion concentration from 1 to 5M. From 5 to 8M the spectral characteristics remain essentially constant. The effect of sodium hydroxide concentrations above 8M was not studied because the ethanol, added with the chromogenic reagent, was no longer miscible and extracted the complex. If extracted into isoamyl alcohol from 7 to 12M sodium hydroxide solutions, the copper(1) complex exhibits a wavelength of maximum absorbance of 525 mp and a molar absorptivity of 6900. Above a concentration of 12M sodium hydroxide, the copper(1) complex is extracted by isoamyl alcohol but fades rapidly because of air oxidation. Neither hydroxylamine nor ascorbic acid is effective in preventing this. Nature of Complex. The mole ratio study indicated a 2-to-I mole reaction of 4,4’-dihydroxy-2,2‘-biquinoline with copper(1). Deviations of the measured absorbances from the extrapolated absorbances in the region of the stoichiometric point permitted calculation of the conditional forma(6) H. Diehl and G. F. Smith, “The Copper Reagents: Cuproine, Neocuproine, Bathocuproine,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1958.
tion constant (7), defined as follows. For the reaction (ignoring charges on the species)
cu
+ 2L
CUL2
the conditional formation constant is given by
K E
[CUL21 - [CUI [LIZ
[CuLd (CC“0- [CUL21) (CLO- 2[CULSl)2
where CcUoand CLoare analytical or initial molar concentrations of copper(1) and 4,4‘-dihydroxy-2,2’-biquinoline,respectively. The value found for the conditional formation constant in 6Msodium hydroxide is 2.0 X 10“. The charge on the copper(1) complex depends on the charge on the ligand which, in turn, is determined by the pH. Examination of the ultraviolet spectra of the ligand over a p H range represented by 1 M HC1 to 10M NaOH revealed three distinct sets of consecutively formed isosbestic points. This is convincing evidence for the existence of three distinct equilibrium stages in the ionization of 4,4’-dihydroxy-2,2’-biquinoline, involving four different species in addition to the proton: HsB+, H2B, HB-, and B2- (where H2Brepresents the unionized molecule). The pH range over which a particular equilibrium stage predominates is indicated in Table 11, together with an estimate of the apparent pK, value. In acid solutions the free ligand is protonated through the biquinoline nitrogen atoms and thus positively charged. In basic solutions of p H greater than 12, both hydroxy groups of the ligand are ionized, and the free ligand is dinegatively charged. Hence the copper (I) complex exists as a trivalent anion, [CuL2I3-,in strong base. The species extracted into isoamyl alcohol is presumably M3 [CuL?],where M is either Na or K. No appreciable change in extractability is apparent in the presence of large concentrations of lithium or tetrabutylammonium ions. Ion exchange studies to identify charge type were inconclusive due to decomposition of the complex. Effect of Variables on Determination. Beer’s law is followed over the concentration range studied. The optimum concentration range for measurement at 1.00 cm optical path is 1 to 9 ppm of copper. Extraction of the complex into isoamyl alcohol enables determination of much lower concentrations, provided sufficient sample is available. Extraction of the copper(1’ complex is quantitative in one equilibration step from 6-1211f sodium hydroxide solutions using as little as 5 ml of isoamyl alcohol for 50 ml of aqueous phase. Rate of color development is rapid. Temperature and order of mixing are not critical. If an excess of hydroxylamine hydrochloride reductant is added to protect the solutions from atmospheric oxidation, no change in absorbance occurs on exposure to the air for one hour. In stoppered flasks the color is stable for several weeks. The effect of foreign ions on the determination of copper in 12M sodium hydroxide, containing 3.19 ppm of copper, is shown in Table 111. Certain ions precipitate from concentrated sodium hydroxide and do not interfere; these include manganese(II), barium, strontium, oxalate, and fluoride ions. Cyanide ions interfere seriously even at 1 ppm. Up to 100 (7) G. H. Ayres, “Quantitative Chemical Analysis,” 2nd ed., Harper and Row, New York, N. Y. 1968, pp 474-6.
Table 1V. Determination of Copper in Sodium Metal Micrograms Cu per gram Na Present Added Found % Recocery X 0.06
0 0.63
0.06
0.87
0.06
2.29
0.06
4.61
0.06
8.43
0.06 0.67 0.69 0.94 0.91 2.33 2.34 4.69 4.71 8.54 8.54
97.1 100.0 101.1 97.8 99.1 99.6 99.6 100.9 99.4 99.4
Table V. Determination of Copper in Potassium Metal Micrograms Cu per gram K Present Added Found % Recovery 0.01
0 0.60
0.01
1.27
0.01
2.66
0.01
5.29
0.01
7.47
X
0.01 0.54 0.59 1.25 1.25 2.68 2.66 5.34 5.33 7.53 7.53
... S8.5 96.6 97.8 97.8 100.4 99.6 100.8 100.6 100.7 100.7
ppm of thiocyanate is tolerated. Sodium tartrate does not interfere and can be used as a masking reagent for iron and other heavy met a 1 ions. ’ The precision of the recommended analytical procedure was tested by 14 replicate determinations of a 12M sodium hydroxide solution containing 3.19 ppm of copper. The relative standard deviation was 1.08 %. Determination of Copper in Sodium or Potassium Metal. The method was applied to the analysis of sodium and potassium metals because of their importance as heat transfer media in nuclear reactors. Results are given in Tables IV and V. Because suitable standard alkali metal samples were not available, synthetic standards were prepared by adding weighed amounts of standard copper solution to the methanol used to dissolve the metal samples. The results indicate that recovery of copper is quantitative within the precision of the measurement. A significant advantage of the method over existing methods is that neutralization of the alkalinity and pH adjustment are eliminated. ACKNOWLEDGMENT
Samples of the substituted 2,2’-biquinolines were kindly provided by Professor Francis H. Case of Temple University. RECEIVED for review September 18, 1968. Accepted November 22, 1968. Financial support provided by the G. Frederick Smith Chemical Company. Taken in part from a dissertation submitted by W. c. Hoyle to the Graduate School of Northern Illinois University in partial fulfillment of the requirements for the M.S. degree, August 1968.
VOL. 41, NO. 2, FEBRUARY 1969
347
