reactions
+ 2Br- + 2H+ HzOz + 21- + 2H+
HzOz
+ -+
+ 2Hz0 Iz + 2Hz0
Brz
The result of this experiment show a possibility of a convenient determination of three halides in their mixtures by use of the following steps : Titrate potentiometrically an aliquot with silver nitrate, Titrate another aliquot with silver nitrate to determine the C1- ion, using oxine to eliminate the interference of iodide and bromide ions, Determine iodide iodometrically in a third aliquot, Calculate the concentration of bromide ion by difference.
and bromination of oxine takes place according to the equation
+ 2Br-
C9H70N
-+
+ 2Br-
C9HsBr20N
The bromide formed in this way is oxidized again and again until all is consumed by the oxine. The mechanism of iodine reaction with oxine is not known, but the fact that after its reaction with oxine, AgN03 forms no precipitate, allows us to assume that the iodine may react similarly to bromine. The above data of titrations show that 8-hydroxyquinoline quantitatively eliminates interference of iodide and bromide in titration of chloride in acetone mixtures of halides. The fact that titration of C1- in these mixtures produces smaller A E / A V ratio than in the absence of Br- and I- ions, cannot be attributed to incomplete elimination of ionic bromide and iodide. That these ions are completely eliminated by oxine can be easily proved by the fact that AgN03 in the absence of C1- in a mixture of halides with oxine and H202 forms no precipitate of AgI or AgBr. The decrease of potential break can be attributed only to the presence of reagents in the mixtures of halides. The above data of titrations also show that it is possible to determine C1- even in the case when iodide and bromide ions exceed the amount of chloride tenfold. ~
In a case of bromide-chloride mixture, Titrate an aliquot with silver nitrate, Titrate another aliquot to determine chloride, using oxine to eliminate the interference of bromide, and calculate the concentration of bromide by difference. Halogens in organic iodo-bromo-chloro-compounds and in bromo-chloro-compounds, after conversion to ionizable form by closed flask combustion (11, 12) or by other methods (12), also can conveniently be determined by this method.
RECEIVED for review April 1, 1970. Accepted May 26, 1970. (11) W. Schoniger, Mikrochim. Acta 1955, 123; 1956, 869. (12) T. S. Ma, Quantitative Microchemical Analysis, in “Standard Methods of Chemical Analysis,” F. J. Welcher, Ed., 6th ed., Vol. I1 A, Van Nostrand, Princeton, N.J., 1963, p 357.
~~
Spectrophotometric Studies of Complexes of 2-Benzoyl pyridine-2-Pyr idylhydrazone John E. Going1 and Ronald T. Pflaum Department of Chemistry, University of Iowa, Iowa C i t y , Iowa THECOLORIMETRIC REAGENTS that are among the most widely known are the ferroin and terroin chromagens with the functionalities, -N=C-C=N-, and -N=C-C==N-C-C= N--. The examples, beginning with bipyridine, appear to be endless. Recently some new reagents have become available which can be considered as slightly modified terroin chromagens. These include certain of the substituted hydrazones, -N=C-NH-N=C-C=N-( I, 2). The secondary amine proton in this grouping has been shown to be quite labile when the ligand undergoes coordination with a metal ion. The complexes formed upon elimination of this proton are intensely colored and are amenable to analytical application. The purpose of this paper is to introduce 2-benzoylpyridine2-pyridylhydrazone (BPPH) as a useful and extremely sensitive reagent for certain metal ions. A detailed study of the iron(II), cobalt(III), nickel(II), copper(II), and zinc(I1) complexes has been undertaken. A procedure for the determination of cobalt in the presence of large amounts of iron, copper, and palladium, based upon a simple extraction, will be disPresent address, Department of Chemistry, University of Wisconsin, Milwaukee, Wis. (1) J. F. Geldard and F. Lions, Inorg. Clzem., 2, 270 (1963). (2) R. W. Green, P. S. Hallman, and F. Lions, ibid., 3,376 (1964). 1098
cussed. In addition, a study of the determination of zinc in a phosphor bronze and in the presence of a large excess of aluminum will be presented. EXPERIMENTAL Apparatus. All absorption measurements were made with a Cary Model 14 recording spectrophotometer at room temperature in 10-mm matched silica cells. Conventional ion exchange columns were used for the isolation of zinc from the NBS sample. A 1.3- X 15-cm column containing 50-100 mesh Dowex 1-X8 anion exchange resin was used for the separation. Reagents. The reagent, 2-benzoylpyridine-2-pyridylhydrazone was prepared in this laboratory from commercially available materials, Stoichiometric amounts of the reactants, 2-benzoylpyridine and 2-pyridylhydrazone (0.22 mole), were dissolved in 250 ml of ethanol. The mixture was heated under reflux from 8 hours. Upon cooling at dry ice temperatures, the hydrazone crystallized out of solution. The light yellow product, recrystallized from ethanol, yielded 40 grams (67%), mp 113-114 “C. A reagent solution 2.00 X 10-2M was prepared by dissolving 0.55 gram of BPPH in a minimum amount of hydrochloric acid with heating and diluting to 100 ml. Solutions of metal ions approximately 0.05M were prepared
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
from their salts and standardized where appropriate. Working solutions were prepared by dilution. All other chemicals were of reagent grade quality. Procedures. Dissolve a sample containing the desired metal ion by appropriate means. Add 3 ml of 2 X 10-2M reagent solution to a suitable aliquot of the sample. Add sufficient ethanol to yield 50 ml of test solution containing 50z alcohol by volume. Adjust the pH with sodium hydroxide to the value indicated in Table I. Measure the absorbance us. a blank solution at the specified wavelength. Calculate the metal ion concentration from a calibration curve constructed from data obtained on solutions prepared in the same manner.
Table I. Summary of Analytical Procedures
x Measurement, nm
Metal ion Sample, a PH 15-200 8.5-12.8 465 Fe(I1) 3.8-12.6 478 Co(I1) 10-150 >11.5 460 Ni(I1) 6-100 6-100 > 14” 485 CU(I1) Zn(I1) 6-100 >11.0 455 >ll* 455 Zn in A1 6-100 a Sufficient NaOH added to yield final concentration of 0.5M. b Sufficient NaOH added to dissolve hydrous aluminum oxide.
RESULTS AND DISCUSSION
Table 11. Absorptimetric Data on Metal Ion Complexes in Basic Solution E Concn ( M x 106)” Metal ion X,, nm Fe(1I) 603 10,700 8.00-96.0
The reagent, 2-benzoylpyridine-2-pyridylhydrazone, shown below, is a light yellow crystalline material soluble in most organic solvents. It is nearly insoluble in water but quite soluble in dilute acid solution. BPPH exists as syn and anti stereoisomers; the anti form is shown. It has been shown that the method of synthesis and dissolution ensures a predominance of the anti isomer (3).
465 19.000 4.02-52.3 478 29;300 3.97-31.8 460 47,000 458 45,000 3.88-31.0 2 .OO-36.0 Zn(I1) 455 51,000 Concentration range of conformance to Beer’s law.
Co(I1) Ni(I1) CU(I1)
a
H
I
+
The reagent forms intensely colored complexes with a number of transition metal ions. In these complexes, BPPH acts as a tridentate ligand. Complexation is accompanied by elimination of the labile proton of the ligand in most of the chelate systems. The characteristics of the individual complexes are discussed below. Metal Ion Complexes. Continuous variations and mole ratio studies showed that BPPH forms 2:l 1igand:metal ion complexes with the transition metal ions. Stable, water soluble, cationic species are formed in acidic solution with iron and cobalt. Under basic conditions, all metal ions, with the exception of cobalt, form difficultly soluble neutral complexes with the anionic form of the ligand. Complex formation is rapid and the complexes are stable for at least 24 hours. Elemental analysis on the isolated solid complexes confirmed the stoichiometry in each metal ion system. The absorptiometric characteristics of the bis complexes in basic ethanolic solution (50 % ETOH-HeO) are given in Table 11. The structural similarity of BPPH to tridentate ligands such as terpyridine suggests that BPPH would coordinate with iron(I1) in slightly acidic solution to form a bis cationic species. Deprotonation of this chelate would then result in a bis neutral complex. Geldard and Lions (1) have isolated both types of iron complexes containing ligands similar in structure to BPPH. In the present study at pH 3-4, iron(I1) forms a soluble red complex with a Amax at 535 nm, E 12,300. In the pH range 8.5-12.8, the neutral complex, soluble in 50% ETOH-H20, exhibits a sharp maximum at 603 nm and a shoulder at 465 nm. Cobalt(I1) ion reacts with BPPH in the pH range 3.85-12.6 to form a bis cationic water soluble species. The metal ion (3) J. E. Going, “Analytical Chemistry of Chelates of 2-Benzoylpyridine-2-Pyridylhydrazone,”Doctoral Dissertation, University of Iowa, Iowa City, Iowa, 1968. Univ. Microfilms., Ann Arbor Michigan 69-8738. Diss. Abstr. B 1969, 29 (11) 4069.
in the complex has been shown to be in the 3 oxidation state ( I , 4 ) . Oxidation of the metal ion is due to dissolved oxygen in the system. In the presence of perchlorate ion, the cobalt complex can be extracted into benzene, carbon tetrachloride, chloroform, ethyl acetate, isoamyl alcohol, nitrobenzene, and toluene. The extraction out of water at pH 7-11 into chloroform is quantitative. A chloroform extract of the complex exhibited a, , ,A at 502 nm with a molar absorptivity of 34,000. Nickel, copper, and zinc do not react with the reagent in acidic solution. Under basic conditions, nickel forms an orange precipitate with the ligand. This neutral complex is readily soluble in a mixed ethanol-water medium. In 50% ETOH-H20, maximum color formation occurred in solutions ofpH > 11.5. In basic solution, copper forms both a 1 : 1 and a 2 : 1 complex with the ligand. The neutral (deprotonated) 2 : 1 species predominates in a large excess of ligand and at high pH (0.5Mhydroxide). Zinc ion forms an intensely colored complex with the ligand in basic solution. At pH > 10.5, a complex exhibiting a wavelength of maximum absorption at 455 nm and a molar absorptivity of 51,000 is formed quantitatively in the presence of a small excess of ligand. This species is readily adaptable to the determination of trace amounts of zinc. Analytical Applications of BPPH. The reagent under consideration is extremely sensitive but only reasonably selective. In addition to the transition metal ions already discussed, cadmium and palladium form stable reaction products with the reagent. Thus, except as described below, at least seven metal ions are mutually interfering. Diverse ion studies carried out on four metal ion systems showed that practical analytical procedures were feasible. One part-per-million concentrations of cobalt, copper, nickel, or zinc can be measured without interference in the presence of 200 ppm of A13+, As3+, ASS+, Ga3+, In3+, Li+ Mo5+,W042-, (4) M.L. Heit and D. E. Ryan, Anal. Chim. Acta., 32, 448 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO, 9, AUGUST 1970
1099
As expected, iron, cobalt, copper, zinc, cadmium, and palladium offer serious interference in the determination of nickel. A simple passage of the sample solution in 12M HC1 through a Dowex 1 anionic exchange column permits the recovery of nickel apart from these interferences. In a similar manner, copper and zinc can be separated from their respective interferences by anionic ion exchange chromatography using standard techniques, The results of the determination of zinc in selected samples are presented in Table 111. The NBS sample of a phosphor bronze bearing metal, containing 78.05 % Cu, 0.027 % Fe, and 0 . 4 8 z Zn, was analyzed for zinc after passage of a sample solution through a Dowex 1 anion exchange column. CH3COO-,Br-,F-,I-,NOzd,N03-,PO~3-,S04-2,SCN-, or Zinc in the presence of large amounts of aluminum can be Sz032-. The interferences due to the formation of insoluble determined with good accuracy. Solutions containing 150 alkaline earth hydroxides or hydrous lead oxide can be elimimg of aluminum and varying amounts of zinc were prepared nated by filtration or centrifugation. Large amounts of ciwith sufficient reagent, sodium hydroxide, and ethanol to give trate ion are beneficial in preventing the formation of lead clear homogenous solutions. The results obtained show that precipitates. it should be possible to determine 0.01 zinc in aluminum The interference of nickel in the measurement of cobalt directly by this method. In this specific application, BPPH was masked by the addition of 2000 ppm of citrate ion. Five compares well with dithizone in sensitivity and is simpler to milligrams of iron, 5 mg of copper, and 0.5 mg of palladium, use as no extractions are required (6, 7). were extracted from 58.9 pg of cobalt at pH 4.1 using 2- to 10ml portions of 50z acetylacetone in chloroform (5). Analysis of the remaining cobalt was accurate within 1% relative RECEIVED for review April 7,1969. Accepted May 25,1970. error. Table III. Determination of Zinc in Selected Samples Zinc. UP Sample Present Found NBS-63 35.3 35.3 s-la 12.7 13.0 s-20 25.4 25.5 s-34 50.8 50.4 s-4” 76.2 76.2 Synthetic samples contain 150 mg of aluminum.
z
(5) J. Stary, “The Solvent Extraction of Metal Chelates,” Pergarnon Press, New York, N. Y., 1964, p 56-58.
(6) D. F. Phillips and L. J. Holton, Metallurgia, 38,237 (1948). (7) M. Jean, Anal. Chim, Acta., 7, 338 (1952).
Determination of Cobalt Using Cobalt40 and Paper Chromatography Robert S. Morse and George A. Welford Health and Safety Laboratory, U.S . Atomic Energy Commission, New York, N . Y. 10014 A RADIOREAGENT TECHNIQUE for the determination of cobalt in steels has been developed and applied to the assay of cobalt added to solutions of NBS steel standards. The technique is based on the equilibration of a constant amount of W o with the cobalt in the dissolved steel sample. The cobalt is isolated by paper chromatography and the density of the tracer in the cobalt area on the paper is measured by beta scintillation counting. These results are compared to those obtained by optical density measurements of radioautographs prepared from the chromatogram. In paper chromatography the area of the element of interest depends on the velocity of the compound in the solvent system, solubility of the compound in the developing solvent, and the concentration of the compound. Usually, for a solvent system, the spread of the desired element is directly proportional to the concentration. If this is not so, a careful calibration of the system will provide a measure of the spread which is relative to the concentration. Several authors have described quantitative analyses of chromatograms by measurement of the optical density of the element area after development of the chromatogram with suitable reagents. Takitani, Fukuoka, and Mitsuzawa ( I )
measured the density of colored compounds obtained by thinlayer chromatography of metal ions with a variety of reagents. Frei and Ryan (2) used diffused reflectant spectrometry to determine traces of metal ions with rubionic acid, and many other references to similar work exist in the literature. The present technique is probably more specific than any colorimetric method of measurement and can be readily applied to systems where suitable radionuclides are available. Chromatography, after equilibration of the element to be determined with a radioactive tracer, provides two alternate means of measuring the density. A plastic phosphor (beta detector), small in relation to the total area occupied by the element after development, can be placed on the area and the observed beta activity compared to standards chromatographed simultaneously. Alternately, radioautographs can be made of the chromatogram and optical density measurements compared with standards chromatographed simultaneously. EXPERIMENTAL
Materials. All chemicals were reagent grade, used without further treatment. The %o tracer was obtained from the New England Nuclear Corporation.
(1) S . Takitani, N. Fukuoka, and Y. Mitsuzawa, Japan Analyst, 15, 840 (1966).
1100
e
(2) R. W. Frei and D. E. Ryan, Anal. Chim.Acta, 37,187 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
