Cinnamo- and 2-Furohydroxamic Acid Chelates Ronald Rowland’ and Clifton E. Meloan Department of Chemistry, Kansas State University, Manhattan, Kan.
HYDROXAMIC ACIDS have received considerable attention in the last few decades as analytical reagents (1-16). Many transition metal ions chelate with hydroxamic acids to form soluble, colored species or insoluble precipitates. Monohydroxamic OH
II I
acids, R-C-N-OH, are bidentate chelating agents which form OH
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5-membered rings. The tautomeric hydroxamic acid, R-C-NOH, has been shown to exist (17) but is much less stable and converts to the hydroxamic acid form. The object of the work undertaken here was to study the reactions of 2-furohydroxamic acid, to see what chelates
Q
g-r-H
would form, and to see how an oxygen heterocyclic affects the extraction of any chelates that might form. In addition, cinnamohydroxamic acid (C6Hs CHCHCONHOH) was considered to be of interest because of the extended conjugation in the acid. Compounds with extended conjugation are known to be chromophoric and the longer the conjugated system the more intense the color. Some previous work with the Fe(II1) cinnamohydroxamic acid system has been done (18J9). This started as a spot test for pharmaceuticals and was then developed into a method for iron. Present address, Kansas City Junior College, Kansas City, Mo.
(1) I. P. Alimarin and S. A. Hamid, Zh. Anal. Khim., 18, 1332-4 (1963). (2) A. S. Bhaduri and P. Roy, Sci. CUI. 18,97-8 (1952). (3) A. H. Blatt, “Organic Synthesis,” Vol. 11, Wiley, New York, 1944, p 67. (4) W. W. Brandt, Rec. Chem. Progr., 21, 159-177 (1960). ( 5 ) A. K. Das Gupta and M. M. Singh, J. Sci. I d . Res. 11B, 268-73 (1952). (6) S. K. Dhar and A. K. Das Gupta, ibid., pp 500-1. (7) R . L. Dutta, J. Indian Chem. SOC.,34, 311-16 (1957). (8) R. F. Goddu, N. F. Leblanc, and C. M. Wright, ANAL.CHEM., 27, 1251-55 (1955). (9) V. R. Kaimal and S. C. Shome, Anal. Chim. Acta, 29, 286-8 (1963). (10) A. K. Majumdar and A. K. Mukherjee, ibid., pp 286-8. (11) C. E. Meloan, P. Holkeboer, and W. W. Brandt, ANAL. CHEM.32, 719-23 (1960). (12) A. T. Pilipenko, E. A. Shpak, and P. P. Ruban, Ukr. Khim. Zh., 29, 1 2 0 9 4 4 (1963). (13) S. M. Poddar, J. N. Adhya, and N. R. Sengupta, Sci. Cult. - _ 29 (5), 253 (1963). (14) U. Privadorshini and S. G. Tandon. Analvst. 86. 544-7 ‘ (1961). (15) A. K. Tanaka and N. Takogi, Bunseki Kaguku, 12, 1175-8 (1963). (16) W. M. Wise and W. W. Brandt, ANAL.CHEM., 27, 1392-5 (1955). (17) L. W. Jones and L. F. Werner, J . Amer. Chem. SOC.,48, 169 (1926). (18) V. Springer and I. Benedikov, Chem. Zuesti, 19, 481 (1965). (19) V. Springer, J. Majer, and R. Karluck, Cesk. Farm., 12, 4 (1963). r
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EXPERIMENTAL Chemicals. All chemicals were reagent grade unless specified otherwise. Potassium 2-furohydroxamate was prepared from methyl-2-furoate according to the method of Blatt (3). Cinnamohydroxamic acid was prepared from ethyl cinnamate in the same manner. Equipment. Spectrophotometer measurements were made with a Beckman Model DB spectrophotometer and pH measurements were made using either a Beckman zeromatic or a Leeds and Northrup model 7401 pH meter. RESULTS
As a general observation, the chelates of potassium 2-furohydroxamate appeared to be more soluble in water and to possess a less intense color than the corresponding complexes of cinnamohydroxamic acid. The furohydroxamates of uranium, iron, osmium, titanium, and vanadium were considered to be of interest for further study. Absorption Spectra and the Effect of pH on Transmittance. Solutions of each metal chelate were prepared at each pH where any signs of color formation were observed. Water was used as the reference solvent. The results are shown in Figures 1 to 5. Potassium 2-furohydroxamate itself appears at wavelengths shorter than 350 nm. Conformity to Beer’s Law. A summation of the results shows that: the Ti chelate follows Beer’s law in the concentration range 1.2 to 10 X at 370 nm and pH 9. The E was 7.17 X lo3. Vanadium at pH 8.0 and 540 nm had a E 2.99 X IO3 and followed Beer’s law from 0.2 to 3.2 X 10F4M. At pH 7.0 and 380 nm the uranyl chelate had an E 2.17 X lo3 and followed Beer’s law from 0.4 to 6.0 X 10-4. Beer’s law was followed by the iron system from 0.4 to 4.0 lO-4Mat pH 5.0 and 450 nm. The E was 1.86 X lo3. Structure Studies. Job’s method of continuous variations was applied to the furohydroxamates of Ti(IV), Os(VIII), Fe(III), and V(V) to determine the composition of the chelates. Studies were made at two wavelengths to ensure that only one system was present. A composition study on Ti(1V) showed that a 4 : l ligand-to-metal ratio existed at pH 9.0. The absorbance was measured at 410 and 460 nm. Osmium(VI11) at pH 6.0 showed that the osmium furohydroxamate consists of two ligand groups per metal ion. Measurements were made at 380 nm and 450 nm. The investigation of the red-orange system at pH of 2.0 showed an apparent equilibrium mixture of the 1 :1 and 2 :1 ligand-to-metal complexes to be present. Vanadium formed what appeared to be a 4 : l chelate at pH 8.0. It must be mentioned that the metal ion absorbed in the region where the chelate absorbance was measured; and because of the high blanks, the results are probably not as reliable as the other systems. Solubility of KFHA Chelates in Organic Solvents. The iron, uranium, titanium, and vanadium chelates appeared to behave similarly. See Table I. The vanadium system was investigated more closely. Wise and Brandt (16) use benxohydroxamic acid to determine vanadium in crude oils and one of the major disadvantages is that the iron must be removed by a Hg cathode. If either iron or
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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Figure 1. Spectrum of potassium 2-furohydroxamate and titanium(1V) furohydroxamate
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Spectrum 1. Potassium 2-furohydroxamate solution O / ~ T (2 X lO-'M) at pH 9.0 Spectrum 2. Titanium(1V) 2-furohydroxamate sohtion (2 = lO-'M) at pH 9.0 Inset shows Mest of pH on transmission of chelate at 370 nm
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Table I. Extraction Study of Furohydroxamate Chelates Solvent Extractability Benzene No No CCl, No CHCla No Cyclohexane Yes Cyclohexanone No Diisobutyl ketone No Ethyl acetate Yes 1-Hexanol Yes Isoamyl alcohol Yes 1-Octanol No Xylene
vanadium could be separated by an extraction, this would simplify the procedure. It was found that more than three extractions would be necessary to separate the vanadium from the iron so no real advantage is gained over the present procedure. However, the experiments did show that I-octanol quantitatively extracts vanadium furohydroxamate at pH 3.0 from aqueous solution. The alcoholic solution of chelate followed Beer's law from 1.0 to 2 X mole of chelate/liter of I-octanol and had a molar absorptivity of 2.60 X IO3.
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Figure 2. Spectrum of uranyl(I1) furohydroxamate (2 X 10-4M) solution at pH 8.0
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Inset shows effect of pH on transmission of chelate at 38onm
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Cinnamohydroxamic Acid. Ions tested which did not differ from the metal blank were Tl+, Tea+, Te4+,Se6+, Se4+, Re'+, Cr6+, Cd2+,Be2+,As5+,and As3+. Of the forementioned, those chelates of Vb+,Fe3+,Ti4+,and U02+were studied further. Effect of pH on Transmittance. Cinnamohydroxamic acid absorbs very little in the visible region as shown in Figure 6 Absorbance measurements on its chelate must be made at pH's more acidic than 6.2 and at wavelengths longer than 370 nm to avoid interference. ADHERENCE TO BEER'S LAW FOR 1-OCTANOL EXTRACTS
For the uranyl(I1) extract, it was necessary to prepare a blank by extracting the ligand alone from an aqueous solution of pH 9.0. High absorbance values would be obtained for the uranium chelate if the blank wasn't prepared in this manner, as the ligand is extracted and absorbs in basic solution. I-Octanol was used as the blank for the vanadium complex extraction. Uranyl(I1). Uranium cinnamohydroxamate of varying concentrations was extracted at pH 9.0 from aqueous solutions and the absorbance of the extract was measured at 380 nm. The chelate appears to follow Beer's law up to 1 X IO- 4
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8ot Figure 3. Spectrum of vanadium(V) furohydroxamate (2 X 10-4M) at pH 8.0 Inset shows effect of pH on transmission of chelate at 540 nm
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Figure 4. Spectrum of osmium(VII1) furohydroxamate (2 X l O - 4 M ) solution of pH 12.0 %T lriset shows effect of pH on transmission of chelate at 40. 390 nm
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Figure 5. Spectra of iron(II1) furohydroxamate (2 X 10-4M) at (1) pH 0, (2) pH 2.0, and (3) pH 5.0 Inset shows effect of pH on transmission of chelate at 450 nm
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300 M (in 1-octanol) and has a molar absorptivity in 1-octanol of 6.7 x 103. Vanadium(V1. Vanadium(V) cinnamohydroxamate of varying concentrations was extracted at pH 2.0 with I-octanol and the absorbance was measured at 460 nm. Beer's law was followed up to 1 X 10-4M(in 1-octanol) and the complex had
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a molar absorptivity of 7.0 X lo3. The molar absorptivity of the vanadium cinnamohydroxamic acid complex is exactly double that of the corresponding benzohydroxamic acid complex and this reagent should be substituted for benzohydroxamic acid in the determination of vanadium in steel and crude oil in order to increase the sensitivity ofthe determination.
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Figure 6. Spectra of cinnamohydroxamic acid (approximately 2.0 X lOaM solution at various pH values) Spectrum 1. Spectrum 2. Spectrum 3. Spectrum 4. Spectrum 5. STRUCTURE STUDY
Solution at pH 2.0. Solution at pH 6.2. Solution at pH 7.0. Solution at pH 8.0. Solution at pH 9.0.
PROCEDURE FOR DETERMINATION OF VANADIUM(V) WITH CHA
CHA (10 ml of 0.01M) into the beaker and add distilled water so that the total volume is approximately 25 ml. Adjust the pH of the solution to 2.0 using a pH meter to measure the pH, and stir the solution for five to ten minutes with a magnetic stirrer. Remove the electrodes from the solution and wash them with distilled water, adding the wash solution to the beaker containing the vanadium(V) cinnamohydroxamate; 1-octanol, (10 ml) which had previously been distilled, saturated with water, and stored in a brown bottle is pipeted into the beaker. Stir the two-phase system for five minutes and transfer the contents to a 125-ml separatory funnel. Shake the funnel several times and allow to stand until the organic and aqueous phases separate. Drain the aqueous phase through the stopcock and pour the I-octanol solution out through the top of the funnel. Centrifuge the extract for ten minutes to remove any remaining water from the organic extract, and read the per cent transmission of the extract on a spectrophotometer at a wavelength of 450-m~using 1-octanol as the blank. From a previously prepared calibration chart, the transmission reading taken is used to find the concentration of vanadiumv) in the original aliquot of vanadium solution.
Pipet an aliquot of vanadium(V) solution containing 1.O to 10.0 x lo7 moles of vanadium into a 100-ml beaker. Pipet
RECEIVED for review April 15, 1968. Resubmitted May 15, 1970. Accepted May 26,1970.
Iron(II1). At pH 2.0, iron forms its most intense color with cinnamohydroxamic acid when the concentrations of ligand and metal are identical. At this pH then, the complex apparently has a ligand to metal ratio of 1 :1. The wavelength selected for the continuous variations study was 510 nm. Uranyl(I1). Continuous variations studies were made on uranium cinnamohydroxamate at pH 4.0 and 5.0, measuring the absorbance of the solutions at 380 nm. At pH values of 4.0 and 5.0, the highest absorbance value of the aqueous chelate solutions occurred at a mole fraction of ligand of 0.6. This indicates the presence of a mixture of 1 :1 and 2 :1 ligand to metal chelates. The results of the ligand to metal ratio study are quite normal. Absorbance values increased almost in a linear fashion with reagent up to ratios of 8 :1. After this, the slope of the curve tapers off rapidly until at ligand-to-metal ratio of 15 :1, after which point increases in ligand-to-metal ratios will not bring about any increase in the absorbance of the complex.
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