Young, I. G., Ibid., 22, 1464 (1950).
I,., Campbell: M. E., Ibid., 25, 1588 (1963). (13) Manning, D. L., White, J. C., Ibid., 27, 1389 (1955).
(12) Luke, C.
(14) Mikula, J. J., Codell, M., 27, 729 (1955). (15) Ringbom, Chem. 1159 ,rnrln\ .' ( l Y S Y ).
*.,
Ibid., 332
G. F., hlccurdy, W. H., ANAL.CHERT. 24, 371 (1952).
(16) Smith,
RECEIVEDfor review April 3, 1 9 3 . Accepted January 14, 1957. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, P a , February 1956.
Spectrophotometric Determination of iron with 2-Fluorobenzoic Acid E. B. BUCHANAN, Jr.l, and WALTER WAGNER Department of Chemistry, University of Detroit, Detroit 2 7, Mich.
b The color of the water-soluble amethyst complex formed b y the reaction of ferric ions with 2-fluorobenzoic acid can be used as the basis of a spectrophotometric method for the determination of iron. The effect of pH, concentration of reagents, and the presence of diverse ions are discussed. Results similar to those obtained by the salicylic acid method for the determination of iron can be expected, with the advantage that only the ferric ion reacts.
100
80
" z Y
U c c
-
f
60
U Dz c c
z
40
V Y
= n. Y
20
I
T
between ferric ions and an organic compound containing a phenolic group generally results in the production of a colored complex. hfehlig has made use of the complex formed between salicylic acid and ferric ions for the quantitative determination of iron (2, 3). according to V7elcher, the reaction between sodium salicylate and iron in a n acetic acid solution is not restricted to the ferric ion; an identical color is also obtained with ferrous ion (4). I n a study of the hydrogen bonding within 2-fluorobenzoic acid, Whittaker noted that this reagent produced a n amethyst color with ferric ions similar to that produced by salicyclic acid (6). As this color is not affected by the presence of ferrous ions, an investigation was undertaken to determine the possible application of this reagent to the quantitative determination of iron. HE REACTION
APPARATUS AND REAGENTS
A Beckman Model DU spectrophotometer operating on a band width of 0.05 mp and utilizing 1-cm. cells was employed for all spectrophotometric measurements. A Coleman Model 3D DH meter was used for all determinations of pH. The 2-fluorobenzoic acid, prepared according t o Blatt ( I ) , was purified by 1 Present address, Iowa State College, Ames, Iowa.
754
ANALYTICAL CHEMISTRY
0
1 250
350
450
550
650 W A V E LENGTH
750
850
950
1000
Figure 1. Absorption spectra of 2-fluorobenzoic acid and its iron complex
steam distillation and recrystallization from water to give a product with a melting point of 126' C. A saturated solution was prepared by dissolving 7 grams of the reagent in 1 liter of warm water and cooling to room temperature. The standard iron solutions mere prepared by dissolving 100 mg. of electrolytic iron in hydrochloric acid. The iron was then oxidized to the trivalent state by the addition of bromine water, the excess of which was removed by boiling. The solution was then diluted with mater until the weight of the entire solution was 1 kg. The ammonium formate solution was prepared by dissolving 75 grams of the salt in 1 liter of water. METHOD FOR IRON
In Absence of Interfering Substances, Add 20 ml. of the reagent solution to the unknown solution which contains between 0.1 and 1 mg. of iron. Destroy the resultant color by the dropwise addition of concentrated hydrochloric acid, then add 5 ml. of the ammonium formate solution and dilute t o 50 ml. with mater. Measure the absorbance of the system
a t 526 n ~ pusing distilled water as a blank. Convert the absorbance reading to iron concentration by means of a previously prepared calibration curve. EXPERIMENTAL
The absorption spectra (Figure 1) indicate that the wave length of maximum absorption by the complex and of minimum absorption by the reagent is 525 mp. This JTave length mas used, therefore, for all absorbance measurements. Effect of pH on Degree of Complexation. Experiments showed t h a t the p H of the solution had a considerable effect upon the degree of complexation. To determine the p H which would give maximum absorption by the complex, several solutions containing identical quantities of iron were prepared, and 20 ml. of 2fluorobenzoic acid solution n-ere added t o each. The resulting color was just destroyed by the dropwise addition of concentrated hydrochloric acid, then restored by the addition of varying amounts of ammonium formate solu-
tion. Each solution was then diluted Table 1. Noncolored Interfering Ions to 50 ml. and the pH and absorbance were measured. Figure 2 indicates Ion Type of Interference that the p H range of 3.0 to 3.5 is most Ppt. suitable for quantitative determinations + Forms colorless complex of iron. SS’ithin this pH range and in ASO4--Forms colorless complex Bi’+T Ppt. the presence of an excess of reagent, the FForms colorless complex color of the complex \\as stable for 24 Hg?+ + Pot. hours. S b +E Pit. Beer’s Law. I n order t o deterPb++ Ppt. Pod--Forms colorless complex mine the applicability of Beer’s law t o sio4-4 Ppt. this complex, solutions of known iron Sn+4 Ppt. concentration were prepared from Ta +5 PDt. the standard iron solution in a series Th +4 Pit. Ti +4 Forms yello\+-complex uf 50-ml. volumetric flasks. The soluUOr Forms orange complex tions were treated as for the pH study, YO3 Forms yellow complex e-icept t h a t the color was restored by the addition of 5 ml. of the ammonium formate. At 525 m p maximum absorb:mce was obtaiucd when 20 nil. of the reagent. Although t h e absorbance of complexing reagent m s used. The colored ions in t h e solution can be complex follow Beer’s law for concencompensated for by using a blank trations from 0 to 20 p.p.m. of iron. solution containing this ion, their ,4n average value of 27.7 n-as obtained presence makes t h e adjustment of t h e for the absorptivity. On t h e basis of pH difficult. Table I lists such ions, the assumed formula, C21H1206F3Fe as well as noncolored interfering ions. (molecular weight, 473), a value of 13.100 was obtained for the molecular STRUCTURE OF COMPLEX evtinctioii cordTicient. A possible structure of the complex A a = was determined by assuming that i t C bc A was the 2-fluorobenzoate anion, which 0.0200 0,00062 32.25 complexed with the iron and not the 0.1500 0.00662 26.87 free acid. The degree of complexation 26 08 0 3010 0.01154 0,4179 0 OlGO6 26 02 would then be a function of the pH of 0 2000 26 00 0,5200 the solution and the equilibrium con0 0400 0,00129 28.77 stant for the reaction would be expressed Av. 27.66 as follom:
+&:-
+
+
27.7 X 473 = 13,100
rHBz Effect of Diverse Ions.
T o determine which of t h e more common ions interfere. t h e method described above was used, except t h a t t h e ion under investigation Eas added prior t o t h e
+ Fe+++
3.0
4.0
Figure 2. Effect of pH on absorbance of iron complex
yo Iron Sample Sheet brass Cast bronze
(3)
In a highly acid solution, where the changes in the molar concentration of
Found
0.076
0.075 0,076 0.074 0.033 0.032 0.033 0.24 0.25 0.25
0.032
52B
Ounce metal
0.26
134B
yo Iron(II1) Oxide Present Found Fluorspar
0.15
0.15 0.16 0.16 0.077 0.077 0.076
79D
+ log(K)(Fe+++j(HBz)” (2)
+C
Present
37D
=
log A , = -ZPH
Results of Analysis of NBS Samples
Table 11.
where x is the combining ratio and HBz represents the 2-fluorobenzoic acid. Upon rearranging the terms and taking the logarithms of each side, the equation reduces to the following:
5.0
PH
Several NBS samples were analyzed by this method. Samples of brass, glass, and fluorspar were taken to provide a variety. The glass mas prepared for the analysis by treatment with perchloric and hydrofluoric acids (6). The residue from the evaporation was moistened ivith hydrochloric acid, taken up in distilled ivater, transferred to a volumetric flas!;, and diluted t o volume. Aliquots of this solution were then pipetted into 50-ml. volumetric flasks and the color was developed. The fluorspar samples were treated in the same manner except that i t was not necessary to add hydrochloric acid. The brass samples were treated with nitric acid, and the tin was removed as metastnnnic acid. The filtrate v a s
+
If the conditions of the experiment are such t h a t the changes in the molar concentration of the iron and the 2-fluorobenzoic acid are small in comparison with their concentrations, then the second member of the right side of the equation becomes a constant and the equation reduces to that of a straight line. The slope of this line is then the combining ratio of the iron and the 2-fluorobenzoic acid. Furthermore, a s the absorbance values are proportional t o the molar concentration of the complex, absorbance can be substituted for the molar concentration. When these simplifications and substitutions are made, the equation then becomes: 2.0
RESULTS
zH+ complex K = -[H . + I z . complex (1) [ F e + + + ][HBz]’ . -f
log complex --zpH
I
the iron and the 2-fluorobenzoic acid are negligible, the curve obtained is a straight line with a slope of 2.9. This value is close enough to 3 to consider that 3 moles of the 2-fluorobenzoate ion are combined with 1 mole of the iron.
Glass
0.076
93
~
~~~
Table 111. Precision of Method b y Ten Determinations on Same Sample % Fez08 Dev. from Found Av.
2.56 2.51 2.53 2.54 2.57 2.57 2.55 2.54 2.55 2.53 Av. 2.55
Fe2Oa by titration, yo 2.57
$0.01 -0.04 -0.02 -0.01 +o ,02
+o ,02
0.00 -0.01 0.00 -0.02
__ __
VOL. 2 9 , NO. 5 , MAY 1 9 5 7
0
755
then treated with ammonium hydroxide and the resultant precipitate, containing the iron, was washed free of copper. The precipitate was dissolved from the paper and the solution evaporated almost to dryness to remove the excess acid. The cooled solution m-as then filtered into a volumetric flask and the color developed in the manner indicated earlier. The results of these analyses are listed in Table 11. PRECISION
The precision of the method was tested by analyzing 10 samples of a
cement by the method outlined above for the determination of iron in glass. The results of these analyses are presented in Table 111. A statistical analysis of the data indicates that this method is capable of results Tvith a maximum deviation from the mean of 0.04% and a n average deviation of 0.02%. Expressed in terms of the amount present, the average deviation is 0.870,.
Mehlig, J. P., IND.EKG. CHEM., ANAL.ED.9, 162-3 (1937). Ibid., 10, 136-9 (1938). Welcher. F. J., “Organic Analytical Reagents,” j‘an Nostrand, -New York, 1947.
Whittaker, M. H., hl. S. thesis, University of Detroit, 1952. Willard, H. J., Diehl, H., “Advanced Quantitative dnalgsls,” p. 277,, Van Sostrand, Kern York, 1943. RECEIVED for review July 14, 1954. Ac-
LITERATURE CITED
(1) Blatt, A. Hi: Gilman, H., “Organic Syntheses, Vol. 11, p. 299, Wiley,
New York, 1943.
cepted January 9, 1957. International Union of Pure and Applied Chemistry, XVth International Congress, Analytical Chemistry Section, Lisbon, Portugal, September 1956.
Spectrophotometric Determination of Uranium with Thiocyanate in Butyl Cellosolve-Methyl Isobutyl Ketone-Wa ter Medium OSCAR A. NIETZEL and MICHAEL A. De SESA Raw Materials Development laboratory, National lead Co., Inc., Winchester, Mass.
b A spectrophotometric method for the determination of uranium in ores and leach liquors consists of separation of uranium from interfering ions by extraction into methyl isobutyl ketone, using aluminum nitrate as a salting agent, followed by development of the color in a portion of the extract with a solution of ammonium thiocyanate in a butyl Cellosolve-water solvent. The yellow uranyl thiocyanate complex forms immediately, and the color is stable for a t least 48 hours. The optimum range of uranium concentration at 375 mp is from 0.4 to 2.0 mg. of uranium oxide (UsO8) in the sample aliquot. The coefficient of variation of absorbance measurements on standard solutions a t 375 mp was 0.34%. Titanium is the only serious interference, but procedures are given which make i t possible to analyze samples containing as much as 5 mg. of titanium in the sample aliquot.
V
ARIOCS modifications of the spectrophotometric determination of uranium n ith thiocyanate have been proposed. Currah and Beamish (4) first used thiocyanate as a colorimetric reagent for the determination of uranium. The yellow uranyl thiocyanate color was developed in an aqueous solution, and stannous chloride was recommended to prevent the interference of iron(II1). Selson and Hume (IW),who
756
ANALYTICAL CHEMISTRY
examined the procedure with respect to errors inherent in the analytical method and possible interferences, developed a more reproducible and sensitive method. Henicksman (11) made a thorough investigation of interferences and first recognized the serious interference of vanadium. Although the method may be applied to the analysis of certain materials, such as monazite concentrates (ZO), Tvithout preliminary separation of the uranium, most of the recent papers have been concerned with the removal of anionic and cationic interferences. Crouthamel and Johnson ( 2 ) found t h a t the stannous chloride reductant, in the presence of uranium and ammonium thiocyanate in aqueous solution, generated a n interference peak around 375 p, which became more serious as the thiocyanate solution aged. By deveIoping the color in an acetone-water solvent, Crouthamel and Johnson were able to inhibit this attack of thiocyanate by stannous chloride. The use of a n teetone-mater solvent eliminated the majority of the anionic interferences in the aqueous thiocyanate method, increased the sensitivity, enhanced the stability of color, and made the correct color development independent of p H in the acid region. However, several elements, such as vanadium and titanium, cannot be tolerated. The extraction of the colored uranyl thiocyanate complex into amyl alcohol
or ethyl ether in order to eliminate some of the interferences was recommended by Gerhold and Hecht ( 7 ) ,and dibutoxy tetraethylene glycol has recently been used in a similar manner (18). T h e ethgl acetate extraction of uranium from a nitrate solution, first recommended by Grimaldi and Levine (Q), has been utilized in several procedures which involve recovery of the uranium from the ethyl acetate phase and determination with thiocyanate in water ( 8 , I O ) or in a n acetone medium (6). A method (5, IS) for the analysis of uraniferous ores and leach liquors consists of extraction of the uranium from an aluminum nitrate solution into ethyl acetate, folloKed by color development in a portion of the ethyl acetate extract with a solution of ammonium thiocyanate and stannous chloride in an acetone-water solvent. This method offers several advantages over the other reported modifications of the determination of uranium with thiocyanate. Interferences are eliminated by the preliminary separation of uranium more efficiently than by extraction of uranyl thiocyanate after the color development. Less time is required than for other procedures involving a preliminary extraction separation, because the color is developed directly on the extract. Higher sensitivity is obtained in the acetone-ethyl acetatewater solvent. A slightly modified version of this method has been used a t
