Determination of Aluminum in Presence of Iron Spectrophotometric Method Using Ferron W. H. DAVENPORT, JR., Oak Ridge National Laboratory, Oak Ridge, Tenn. Aluminum and ferric iron in a buffered acetate-acetic acid solution form complexes with the reagent ferron, 8-hydroxy-7-iodo-5-quinolinesulfonicacid. The absorption maxima of the two complexes are suficiently far apart to permit spectrophotometric measurements of the ferric-ferron complex at 600 mp and the ferric-ferron complex plus the aluminum-ferron complex at 370 mp, The complexes are stable for a reasonably long period of time. The method is suitable for the accurate determination of 0 to 50 micrograms of aluminum in the presence of 0 to 100 micrograms of iron.
F
ERRON was first proposed as a reagent for iron by Yoe ( 5 ) . Later Swank and R;Iellon (4)reported that aluminum in large concentrations (2 mg. of aluminum to 0.1 mg. of iron in 100 ml.) interfered in the ferron method for iron, although no visible colored complex was formed. The author has found that this aluminum-ferron complex has a maximum absorption at 370 mp and that at this wave length, using small concentrations of aluminum a t a constant pH, the complex obeys Beer’s law. The iron-ferron complex also absorbed a t this wave length but the amount of absorption due to the iron complex can be accurately determined. One method using hematoxylin (1-3) makes use of the mixed colors obtained from the aluminum and iron lakes. However, the absorption curves for the two lakes overlap each other, which means that the optical density a t each absorption maximum is dependent upon the concentration of both cations. The color of the aluminumhematoxylin lake changes rapidly on standing.
360
500 WAVE LENGTH, Mp
400
Figure 1.
600
Effect of pH on Spectral Absorption of Aluminum-Ferron Complex
In acetic acid-acetate solution. 207 of aluminum plus 2 ml. of ferron (0.2 %) i n 25 ml.
A . pH 5.5 B . pH 5.0
C. pH 4.0 D . pH 3.0
SOLUTIONS AND METHODS
Redistilled water w m used in the preparation of all solutions. Standard aluminum solution, 1.000 mg. per ml., was prepared by dissolving 1.OOO gram of aluminum metal turnings (J. T. Baker Chemical Co.) in 100 ml. of 1 to 9 hydrochloric acid and diluting to 1 liter. The final solution was standardized gravimetrically using ammonium hydroxide. A solution of 10.0 micrograms per ml. was prepared by dilution of an aliquot of the standard. Standard iron solution, 1.000 mg. per ml., was repared by dissolving 0.100 gram of iron wire (J. T. Baker d e m i c a l Co. No. 36 iron wire for standardizing) in 50 ml. of 1 to 20 hydrochloric acid and diluting to 100 ml. A solution of 10.0 micrograms per ml. was prepared by diluting an aliquot of the standard. Ferron, 0.274, obtained from Eastman Kodak Company was used without further purification. An aqueous solution was repared by dissolving 1.OOO gram of reagent in 500 ml. of redistjled water. Ammonium acetate, 10%. Fifty grams of reagent grade salt were dissolved in 400 ml. of redistilled water and dlluted to 500 ml . Hydrochloric acid, 1to 9, C.P. analytical grade. Nitric acid, 1to 9, C.P. analytical grade. Buffers. The desired pH values were obt+ned by varying the concentration of either hydrochloric acid or nitric acid pnor to the addition of 5 ml. of ammonium acetate. This procedure was followed in order to ensure complete solution of aluminum and iron. In every cme 5 ml. of the ammonium acetate solution and 2 ml. of the ferron solution were added. Solutions were made up to a final volume of 25 ml. and read in a Beckman spectrophotometer using 1-cm. cells.
aluminum-ferron complex transmits more light than the blank between 410 and 540 mp. Maximum color development for each of the complexes was found at approximately pH 5. Absorption by the aluminum-ferron complex decreases rapidly at pH values higher than 5.5. At pH 6.4 the absorption is less than that shown by the blank.
Table I.
Effect of Common Cations on Aluminum-Ferron Complex (20.0 y of aluminum in 25 ml.)
Ion
E:++++++ cu++ ++a
ILlg + +
Mn + +
::OE
;h,:’,+;
Zn”
++++ +
zr++++ 0
Present in 25 Ml., y 20.0 20.0 20.0 20.0 20.0 20.0 20.0
Increase in Absorption a t 370 mp. % 0 14 100 0 5 24
14.5
20.0 20.0 20.0
100 50
19 65 35
As much as 1 mg. of calcium in 25 ml. did not affect absorption.
SPECTRAL ABSORPTION AND EFFECT O F p H
VALIDITY O F BEER’S LAW
It can be seen from Figures 1 and 2 that absorption maxima for the iron and aluminum complexes are obtained at a wave length of about 370 mp and that a t a wave length of about 600 mp only the colored complex of iron shows absorption. The
Calibration data for varying iron concentration us. extinction a t 370 and 600 mp and for varying aluminum concentration us. extinction at 370 mp give the curves obtained in Figures 3 and 4. The aluminum-ferron complex does not obey Beer’s law at con710
V O L U M E 21, NO. 6, J U N E 1 9 4 9
71 1
centrations higher than 40 micrograms per 25 ml. but the curve may be used to at least a Concentration of 60 micrograms per 25 ml. The plot of iron concentration against extinction data a t 600 mp gives a straight line under the above conditions, whereas Swank and Mellon ( 8 ) reported that Beer's law was not obeyed under their conditions (pH 2 to 3 ) when the ferron concentration was kept constant.
Table 11. Analysis of Aluminum-Iron Solutions by Ferron Method Present Sample NO. 1 2 , 3 4 5 6 7
EFFECT O F FOREIGX IONS
The effect of anions was not investigated. A number of cations give positive interference. Uranium, thorium, copper, nickel, chromium, molybdenum, manganese, zirconium, and zinc interfered when present in a 1 to 1 ratio with aluminum. The degree of interference encountered when these cations are present appears in Table I. In general, cations that form hydroxyquinolates in the pH range of the colorimetric method will interfere. Thus, magnesium, which completely precipitates with 8-hydroxyquinoline in the pH range 9.4 t o 12.7, does not interfere when present in moderate amounts. Thorium, which precipitates at pH 4.4 to 8.8, gives a definite positive interference in the colorimetric method. PROCEDURE
Calibration curves for aluminum and iron should be prepared as in Figures 3 and 4. Transfer to a 25-ml. volumetric 5ask 5 to 10 ml. of a nearly neutral sample solution containing aluminum, iron, or both, having a total concentration of 10 to 60 micrograms. Add, in the following order, 1 ml. of hydrochloric acid (1 to 9), 1 ml. of nitric acid (I to 9) ( t o convert all iron to the ferric state), 5 ml. of ammonium acetate (lo%), and 2 ml. of ferron (0.2%). Dilute to
.
\ \ II
-
I /B
I
I
/, 360
Figure 2.
400
/
C
I
600
500 WAVE LENGTH, MP
Effect of pH on Spectral Absorption of IronFerron Complex
I n acetic acid-acetate solution.
7/25 ml.
9" loa
10.0 20.0 30.0 20.0 20.0 20.0 10.0 10.0 40.0 20.0
Found
Fe,
AI,
7/25 ml. 20.0 20.0
7 / 2 5 ml.
20.0 10.0 20.0 30.0
29.0 19.2 20.0 19.2 9.6 9.2 39.0 19.5
10.0 80.0 10.0 40.0
9.5 20.0
Fe,
7 / 2 5 ml.
19.5 20.5 20.5 10.0 20.0 30.5 10.0 78.0 10.0 40.0
llbsorption did not change after standing 24 hours.
25 ml. and read the extinction in Beckman 1-cm. cells a t wave lengths 600 and 370 mp. Determine the total iron present from curve A , Figure 4, the calibration curve for iron a t 600 mp. The contribution of this concentration of iron to the total extinction measurement at 370 mp may be determined from curve B , Figure 4. Subtract this value from the total extinction. CaIculatc from Figure 3 the concentration of aluminum present which is responsible for the remainder of the total extinction at 370 mp. RESULTS
A number of mixed solutions containing known amounts of aluminum and iron in varying ratios were prepared and analyzed by the ferron method (Table 11). SUMMARY
Aluminum in a buffered acetate solution a t a pH 5 forms a complex with ferron which obeys Beer's law for amounts of aluminum in the range 0 to 40 micrograms in 25 ml. The interference due to iron may be accurately determined by spectrophotometric measurements of the same solution a t another wave length, The complexes of aluminum and iron are stable for a t least 24 hours. Cations which form hydroxyquinolates in slightly acid media give positive interference in the colorimetric method. I n the presence of these interfering cations, a preliminary separation of aluminum and iron is necessary. The method is applicable to the direct determination of traces of aluminum in water solutions and soil extracts.
207 of iron p l u s 2 ml. of f e r r o n (0.2
%) in 25 ml.
A. p H 5 . 0 t o 5 . 5
AI,
B. p H 4 . 0
ACKNOWLEDGMENTS C. p H 3 . 0
The author wishes to thank H. L. Hemphill and P. F. Thommon of this laboratory for constructive criticisms. 0.6 LITERATURE CITED
-d
0.5
(1) Hatfield, W. D.,Ind. Eng. Chem., 16,233 (1924). (2) Knudson, H. W.,Meloche, V. W., and Juday, C., IWD.ENQ.CHEM., ANAL.ED.,12, 715 (1940). (3) Steenkamp, J. L., J . S. African Chem. Znst., 13, 64 (1930). (4) Swank and Mellon. IND. ENQ. CHEM.,ANAL.ED.,9, 406 (1937). (5) Yoe, J., Am. Chem. Soc., 54, 4139 (1932).
"0.4 Y ~
0.3
-A0
OS!
0.1
y IN 2 5 ML. Figure 3. Aluminum Calibration Curve
370 mw,pH 5.0.
2 ml. of f e r r o n (0.2%)
7
Figure 4. pH 5.0.
A.
IN 25
ML.
Iron Calibration Curve 2 ml. of f e r r o n (0.2 Z) 6 0 0 m p B. 3 7 0 m r
RECEIVEDAugust 13, 1948. Based on work performed under Contract Number W-7405 eng-26 for the Atomic Energy Project a t the Oak Ridge National La noratory.
