observed: Kel-F-8114 = Kel-F-683 > CMTPA = D T P > MCMPA > PPA > TTP. Various reasons can be cited for the failure of the Hammett acidity to describe accurately the acid-base reactions in solvents such as toluene. The apparent strength of acids in toluene varies with the indicator used as a reference point and with acid concentration; aggregate formation is another complicating factor. An important
requirement for acidity functions is that the quantity f B / j B R + must be independent of the base B. Apparently such is not the case in these systems. These and other factors have been discussed in detail previously (10-12).
RECEIVED for review May 4, 1967. Accepted July 14, 1967.
Spectrophotometric Determination of Iron Using Ethyl 4,6-Dihydroxy-5-Nitrosonicotinate Curtis W. McDonald Department of Chemistry, Alabutna State College, Montgomery, Ala.
John H.Bedenbaugh Depurtment o j Cheniistrj,, Unicersitj, of Southern Missiuippi, Hattiesburg, Miss.
NUMEROUS METHODS: for the determination of iron have been reported. In fact, over 100 colorimetric methods alone have been suggested for its determination (1, 2). With the great abundance and wide distribution of iron in nature, very sensitive and selective methods for its detection and determination are of great interest to analysts. This paper describes a spectrophotometric method for the determination of iron based on the blue color formed when ethyl 4,6-dihydroxy-5-nitrosonicotinate(hereafter referred to as E D H N N for brevity) is added to a slightly acidic solution of iron(I1). Ayres and Roach (3) reported a spectrophotometric method for the determination of iron with quinisatin oxime which, in terms of the functional groups involved in chelation, can be considered the quinoline series analog of EDHNN. Ayres and Briggs ( 4 ) reported a spectrophotometric method for the determination of osmium also using quinisatin oxime. Quinisatin oxime showed great promise as an analytical reagent, but was highly insoluble, requiring N,N-dimethylformamide to dissolve it. E D H N N retains the basic chelating features of quinisatin oxime, yet is much more soluble in common solvents. EXPERIMENT.4L
Apparatus. All absorbance measurements were recorded with a Beckman DU spectrophotometer. Matched silica cells of 1.00-cm optical path were used in the instrument. A Beckman Model H-2 meter was used to make pH measurements. Reagents. STANDARDIRON SOLUTION.A 1.000-gram sample of electrolytic iron (obtained from the G. Frederick Smith Co., Columbus, Ohio), was dissolved in a mixture of 25 ml of dilute hydrochloric acid and 1 ml of nitric acid; the resulting solution was then diluted to the desired volume with distilled water. E D H N N COLORREAGENT. Ethyl 4,6-dihydroxy-5-nitrosonicotinate, which has not been reported in the literature, was prepared by nitrosating ethyl 4,6-dihydroxynicotinate. (1) K. Kodama, “Methods of Quantitative Inorganic Analysis,”
John Wesley, Ed., Wiley, New York, 1964, p. 177. (2) D. F. Boltz and M. G. Melion, ANAL.C ~ t h i . 36, , 256R (1964). (3) G. H. Ayres and M. K . Roach, Ai7ui. Chin?. Actci. 26, 332 (1962). (4) G. H. Ayres and T. C. Briggs, Ibid., p. 340.
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ANALYTICAL CHEMISTRY
I
0.71
WAVELENGTH IN Mu
Figure 1. Spectral curves
+
A . 2.56 p.p.m. iron (11) excess reagent B. Reagent blank cs. solvent
Theoretical percentages for C8H8NZ05:C, 45.29; H , 3.80; N, 13.20. Percentages found: C, 45.64; H, 4.09; N, 13.01. (This chemical may be obtained from Aus-Tex Chemical Co., 7201 South Congress Ave., Austin, Texas 78745). A 0.1 % (w/v) ethanolic solution was prepared for use in the recommended procedure. BUFFER. Sodium acetate-acetic acid buffer of pH 3.5 was prepared by using 9 parts 1.OM acetic acid and 1 part 1.OM sodium acetate. OTHERREAGENTS.Reagent grade chemicals were used in the interference studies. Cations were used in the form of chlorides and nitrates. Recommended Procedure. Into a 2 5 m l volumetric flask, transfer 15 ml or less of the iron solution. Reduce the iron to iron(I1) with a 1 % hydroquinone solution which has been buffered t o pH 3.5 with sodium acetate-acetic acid buffer. If the pH of the resulting solution is not in the
range of 2.2 to 4.1, the solution may be preneutralized with sodium hydroxide or hydrochloric acid. Then add 2 m l of 0.1 % (wiv) E D H N N and dilute to volume with distilled water. Mix thoroughly and measure absorbance after 15 minutes a t 653 mp against a blank containing all components except iron. On the graph showing the spectral curve of the product, Figure 1, curve A is characterized by an absorption maximum at 653 mp. The system conforms to Beer’s law over the concentration range studied (up to 4.8 ppm). Reproducibility and Sensitivity. In the test for reproducibility (precision) of the method, 17 samples, each containing 2.40 ppm iron, were prepared according t o the recommended procedure and measured a t 653 mp. The standard deviation of absorbance was found to be 0.005 absorbance unit or about 1 .0%. The optimum concentration range for measurement at 1.00-cm optical path is about 0.8 to 3.5 ppm of iron. The method has a molar absorptivity of 1.3 X 104. STUDY OF VARIABLES
Tests in each study were made on solutions containing a fixed amount of iron which was color developed by the recommended procedure except for the variable being studied. Stability of the Product. Full color development was obtained in about 10 minutes. The color was stable for several days. After one week, turbidity was observed and a blue deposit was formed on the bottom of the flask. Stability of EDHNN Reagent Solution. The 0.1 (w/v) ethanolic solution of E D H N N was stable for a t least 2 months. N o further time study was obtained. Although E D H N N is fairly soluble in water, aqueous solutions of the reagent were not as stable as the ethanolic solutions and decomposed after 3 days. The instability was even more pronounced in the more dilute aqueous solutions (in the order of l O - 3 M ) . Effect of Reagent Concentration. Full color development is obtained with a mole ratio of reagent to iron of about 6 to 1. Higher reagent-iron ratios caused no change in absorbance at 653 mp, but solutions with very high ratios appeared green instead of the usual blue. The green color is due to the excess yellow reagent. Effect of pH. The optimum pH range for color formation was 2.2 to 4.1. No color was formed in highly acidic solutions. Above a pH of 4.2, there is a gradual decrease in absorbance. A sodium acetate-acetic acid buffer of p H 3.5 was used in the recommended procedure. Effect of Foreign Ions. Varying amounts of the foreign ion were taken with a fixed amount of iron and the color developed and measured the usual way. The tolerance for the foreign ion was taken as the largest amount that could be present and give an absorbance differing by no more than 0.01 from that produced by iron alone. To!erances for various foreign ions are shown in Table I. DETERMINATION OF REACTION RATIO Mole Ratio Method. For application of the mole ratio method of Yoe and Jones (3, a series of 13 solutions was prepared containing a constant amount of iron(I1) (4.0 X 10-jM) and varying amounts of E D H N N reagent. A plot of absorbance of 653 mp against the mole ratio of reagent t o iron showed an indication of a 4 to 1 complex (reagent to iron). For confirmation, another series of solutions was prepared in which the reagent concentration was held con(5) J. H. Yoe and A. L. Jones, IND. ENG.CHEM., ANAL.ED., 16, (1 944).
Table I. Tolerance for Foreign Ions (Iron concentration, 2.40 ppm) Foreign ion Tolerance, ppm, at 635 Cobalt(11) 1 Copper(I1) 2 Chromium(II1) 16 >200 Manganese(I1) 2 Molybdate Nickel(11) 3 Vanadium(IV) 20 >200 Zinc(11) Sulfate >200 Chloride >200
rnp
stant and the iron(I1) varied; the plot of absorbance against mole ratio of iron t o reagent showed definite evidence of a 0.25 to 1 iron(I1) to reagent mole ratio. Continuous Variations Methods. The method of continuous variations (6, 7) was applied using a series of solutions which was prepared from equimolar concentrations of iron(I1) and E D H N N reagent. The sum of the concentrations of iron(I1) and E D H N N was constant (4.0 X 10-4M), while their ratio varied. A plot of absorbance against mole fraction of iron for measurements a t 653 mp indicates a maximum of about 0.2 mole fraction iron giving further evidence of a 4 to 1 reagent to iron complex. DISCUSSION
The molar absorptivity of iron, measured as the E D H N N complex is 1.3 X lo4. Its sensitivity compares favorably with other spectrophotometric methods for the determination of iron. Probably the most common spectrophotometric method for iron is with 1,lO-phenathroline (8). It has a molar sensitivity of 1.1 X lo4. The E D H N N method is more sensitive than that of 1,lO-phenathroline and is very easily performed. One outstanding advantage of the E D H N N method is the very high solubility of the reagent in water compared to many other reagents recommended for the spectrophotometric determination of iron. The structural formula for E D H N N may be shown as
?
OH
I
Considering this structure, it is seen that several possibilities exist for chelation and coordination. The number of possibilities becomes greater when the several tautomeric forms of E D H N N are considered. Any proposed structure for the EDHNN-iron complex from the available data would be somewhat speculative. Although several ions usually found with iron in nature d o interfere, they can be easily removed before analysis by conventional analytical separations. A method as simple and sensitive as this has definite possibilities for the development of clinical procedures for the determination of iron in biological materials. There are numerous other possible applications of the method. RECEIVED for review June 26, 1967. Accepted July 20, 1967. ( 6 ) P. Job, A u 7 . Chim. (Puris),9 (lo), 13 (1928). (7) W. C. Vosburg and G. R. Cooper. J . Am. Cliem. Soc., 63, 437
(1941). (8) E. €3. Sandell, “Colorimetric Determination of Traces of Metals,” Interscience, New York, 1959. VOL. 39, NO. 12, OCTOBER 1967
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