Photometric Determination of Aluminum in Manganese Bronze, Zinc Die Casting Alloys, and Magnesium Alloys A n Aluminon Method C. L. LUKE, Bell Telephone Laboratories, Inc., Murray Hill, N. J., AND I(. C. BHAUN, American Smelting a n d Refining Co., Barber, N. J . The work was undertaken i n an effort to produce a rapid reliable method for the determination of aluminum appearing as a major constituent i n copper, zinc, and magnesium alloys. The work shows that the photometric aluminum method described by Craft and Makepeace is very satisfactory and that by employing thioglycolic acid as a complexing agent i t is possible to simplify the usual photometric methods for the determination of aluminum in nonferrous alloys. The paper contains experimental material that will aid future workers i n the application of this method to other materials.
CRAFT
and Makepeace ( 2 ) have developed a very satisfactory photometric aluminon method for the determination of aluminum. Chenery ( 1 ) has shown that the interference of iron can be eliminated by the use of thioglycolic acid. (While reviewing this paper 34. L. Moss suggested that the objectionable odor of thioglycolic acid could be eliminated by using its ammonium salt.) With the aid of this valuable information, the authors have developed a rapid direct photometric method, somewhat similar to that described by Sherman (3),for the determination of aluminum in manganese bronze, zinc base die casting alloys, and magnesium alloys. APPARATUS
Photoelectric Photometer. An Evelyn photometer with n 515mp filter was used in the present investigation and the procedure was written with this instrument in mind.
vigorously boiling water in 400-ml. beakers, and allow to remain exactly 5 minutes. Remove t o the bench for a minute or so and then place in a cold water bath. Cool to room temperature, dilute to the mark with distilled water, and mix well. Transfer about 30 ml. of the solutions to absorption cells, allow to stand 1 or 2 minutes, and read the per cent transmittancy at approximatelj, 525 mp, using distilled water as the reference solution. Prepare a calibration curve. Procedure. Transfer 0.200 gram of the sample to a 125-mI. chemically resistant conical flask. Add 5 ml. of hydrochloric acid and 5 mi. of hydrogen peroxide (30%) and cover immediately. Cool if the reaction is too violent, When solution is complete, boil down to about 2 ml. to destroy the excess hydrogen peroxide. Transfer to a 200-ml. volumetric flask, dilute to the mark, and mix. Transfer 10 ml. of this solution to a 100-ml. volumetric flask. Add 2.0 ml. of 4 to 96 thioglycolic acid solution and then 15.0 ml. of aluminon-buffer composite solution and proceed as in the preparation of calibration curve. With the aid of the calibration curve determine the amount of aluminum present in the portion taken for analysis.
REAGENTS
Copper Chloride Solution ( 5 mg. of copper per ml.). Dissolve 1.35 grams of cupric chloride dihydrate in 100 ml. of distilled water. Thioglycolic Acid Solution (4 to 96). Dilute 10.0 ml. of thioglycolic acid to 250 ml. with distilled water in a volumetric flask and mix well. Keep stoppered when not in use. Prepare fresh each week as required. Standard Aluminum Solution (0.01 mg. of aluminum per ml.). Dissolve 20.0 mg. of pure aluminum in 20 ml. of hydrochloric acid Dilute to 2 liters in a volumetric flask. by heating gent1 Aluminon-BuftYer Composite Solution. Transfer 500 grams of ammonium acetate to 1 liter of distilled water in a 2-liter beaker. Add 80 ml. of glacial acetic acid from a graduate and stir to dissolve the ammonium acetate. Filter if necessary. Dissolve 1.OOO gram of a suitable grade of aluminon-Le., Eastman's aurin tricarboxylic acid-ammonium salt-in 50 ml. of distilled water and transfer to the buffer solution. Dissolve 2 grams of benzoic acid in 20 ml. of methanol and pour into the buffer solution while stirring. Dilute the mixture to 2 liters. Transfer 10 rams of Knox gelatin to 250 ml. of distilled water in a 400-ml. %eaker. Place the beaker in a boiling water bath and allow to remain, with occasional stirring, until the gelatin has dissolved completely. Pour the warm gelatin solution into 500 ml. of distilled water while stirring. Cool to room temperature, dilute to 1 liter, and mix. Transfer the aluminon and gelatin solutions to a Cliter chemically resistant glass-stoppered bottle, shake to miu well, and keep in a dark place when not in use.
.
DETERMINATION OF 0.1 T O 1% OF ALUMINUM I N MANGAhESE BRONZE
Preparation of Calibration Curve. Transfer 0-, 1-, 2-, 3-, 4-,
6-,8-, and 10-ml. portions of standard aluminum solution (0.01 mg. of aluminum per ml.) to eight 100-ml. volumetric flasks. Add 1 ml. of cop er chloride solution ( 5 mg. of copper per ml.) to each flask and d i k t e to 10 ml. with distilled water. Add 2.0 ml. of 4 to 96 thioglycolic acid solution and then 15.0 ml. of aluminonbuffer composite solution. Swirl, place each flask in 300 ml. of
DETERMIVAT103 OF 4 TO 12% OF ALUMINUM IN ZINC DIE CASTIYG ALLOYS A V D MAGVESIUM ALLOYS
Preparation of Calibration Curve. Transfer 0-, I-, 2-, 3-, 4-,5-, 6-, and 7-ml. portions of standard aluminum solution (0.01 mg. of aluminum per nil.) to eight 100-ml. volumetric flasks. Dilute each sample to 10 ml. with distilled water. Add 15.0 ml. of aluminon-buffer composite solution to each. Swirl, place in 300 ml. of vigorously boiling water for exactly 5 minutes, cool to room temperature, dilute to the mark, mix, and transfer about 30 ml. of the solutions to absorption cells. Allow to stand 1 or 2 minutes. Read the per cent transmittancy of the solutions at approximately 525 mp, using distilled water as the reference solution. Prepare a calibration curve. Procedure. Transfer 0.1000 gram of the allov to a 125-ml. chemically resistant conical flask. Add 5 ml. of'l to 1 hvdrochloric acid, cover, and warm to dissolve most of the sample. Add 1 or 2 drops of hydrogen peroxide (30%) and heat to complete the solution of the sample. Boil d o m to a volume of about 2 ml. Cool, transfer to a 500-ml. volumetric flask, dilute to the mark, and mix. Transfer an aliquot portion of the zinc- or magnesium-base sample, containing 0.02 to 0.06 mg. of aluminum, to a 100-ml. volumetric flask and dilute to 10 ml. Add 15.0 ml. of aluminon-buffer composite solution and proceed as in the preparation of calibration curve. DISCUSSION
Thioglycolic Acid as a Complexing Agent. Aluminum and 5evera1 other metal ions form red colored lakes when they are heated with aluminon-buffer composite solution. The intensity of the color of the lakes varies with the metal ion. When thioglycolic acid is present a t the time of color development, the intensity of color produced is diminished in all cases. The bleaching action is somewhat proportional to the amount of thioglycolic acid present. Fortunately, the strength of the complexes formed varies markedly. Because of this, it is possible to mask the in1120
1121
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 terference due to a number of metal ions which would normally cause trouble in an aluminum determination. In the method for manganese bronze described above, thioglycolic acid is added t o euppress interference due to copper and iron. As the amounts of interfering metal ions present in zinc and magnesium alloys are negligible, there is no need to add the complexing agent when analyzing these alloys. 100 90 80
70
g
60
“ 50 W
E W
z
40
2> 30 2
t
z
2 20 E
15
10
-20
0
20
40
60
80
100
120
ALUMINUM IN MICROGRAMS
Figure 1. Comparison of Different Samples of Aluminon A. B.
Matlieson Chemical Co., currently available product Eastman Kodak Co., currently available product
C. Eastman Kodak Co., old product
With a given set of conditions the ability of the thioglycolic acid to suppress the interference of other lake-forming ions in the aluminum determination will depend upon the total amount of complexable non-lake-forming metal ions present,. For example, it has been found that in the analysis of a brass, not much more than about 10 mg. of zinc can be present a t the color development; otherwise the complexing power of the thioglycolic acid will be so attenuated as to permit interference due to copper and iron. An addit,ional source of error may result from the enhancement of the intensity of the aluminum lake as a result of the weakening of the complexing action of the thioglycolic acid by the zinc. Behavior of Iron with Thioglycolic Acid. When aluminon-buffer composite solution is added to a dilute acid solution of ferric or ferrous iron, a stable purple-red color is formed. If sufficient thioglycolic acid is added before the addition of the aluminon solution, ferric iron is reduced and then complexed so strongly as virtually to prevcnt the formation of the purple-rcd color. If an appreciable amount of iron is present during the aluminum color development, the purple-red color due to iron may appear when the sample is diluted to volume. On standing, the color fades out completely, which suggests that, the ferrous iron is subject to temporary air oxidation. Because of this it is necessary, when appreciable quantities of iron are prepent, t o allow the sample to remain undisturbed long enough to permit complete reduction to the colorless ferrous state before the photometric reading is made. As much as 50 mg. of ferrous iron can be present a t the time of the aluminum color development without causing high resultsproviding the solution is allowed to remain undisturbed for 15 minutes before the photometric reading is taken. When, however, the iron is present in the ferric state, i t is reduced a t the expense of the thioglycolic acid added and the results for alun~inum are usually somewhat high. I n view of this, it is necessary that the iron, when present in amounts greater than a few tenths of a milligram, be in the reduced condition when the thioglycolic acid is added.
Behavior of Heavy Metals with Thioglycolic Acid. Traces of heavy metals can be tolerated in the aluminon-thioglycolic acid method. However, if more than about 0.1 mg. of lead or bismuth is present a t the time of color development, a colloidal precipitate will be formed and high results for aluminum will be obtained. Tin and antimony tend to hydrolyze a t the p H required for color development. I n the determination of aluminum in manganese bronze alloys, separation of the heavy metals is not usually required, since the aluminum content is high and the lead, tin, and antimony content is low. When thioglycolic acid is added dropwise to an acid solution of cupric ion, a cloudy precipitate appears and the solution turns dark blue in color. When, however, a sufficient excess of the complexing acid has been added, the color changes to yellow. If aluminon-buffer composite solution is now added, the precipitate in the solution dissolves. The copper is so strongly complexed by the thioglycolic acid that very little interference occurs in an aluminum determination such as that described above for manganese bronze. The small amount of interference can be canceled out by putting an equivalent amount of copper in the standards and in the blank solution. Quality of Aluminon. Commercially available aluminon varies widely in its behavior, as can be seen in Figure 1. The currently available Eastman product was used throughout the present investigation because it provides a “high sensitivity” calibration curve and because the composite solutions prepared from this m e terial do not become cloudy on standing. I t is true that the Eastman product does not yield a straight-line calibration curve, but this does not preclude the use of this material, as the curves can be accurately reproduced. The acid form of aluminon prepared by Smith, Sager, and Siewers ( 4 ) yields a calibration curve which is very similar to that obtained with the currently available Eastman product. Some of the discrepancies in the literature can probably be ascribed to the use of aluminon of different behavior and it is to be hoped that in the not too distant future some standardization of a suitable commercially available product can be obtained. Such a product should preferably provide high sensitivity and yield a straight-line calibration curve. Craft and Makepeace note some initial instability in their aluminon-buffer composite solutions. When the composite is prepared from the Eastman product as described in the present method, it ran be used immediately and does not deteriorate over a period of several months.
Table 1. Determination of Aluminum in Manganese Bronze, Zinc-Base Die Casting Alloys, and MagnesiumBase Alloys NO. 1 2 3
NBS ‘Sample 62-b 62-b 62-b
Aluminum Present, 0.97
.4luminum Found, % 0.97 0.98 0.97
4 5 6
94-a 94-a 94-a
3.90
3.92 3.95 3.97
7 8 9
171 171 171
2.97
2.96 2.94 2.95
Control of Variables. The intensity of the color produced in an aluminum determination is influenced by the time and temperature of heating during the color development. I n order t o ensure uniform temperature conditions, it is desirable to place the volumetric flasks in vigorously boiling water on a high temperature hot plate which is capable of supplying enough heat to return the temperature of the baths quickly t o the boiling point after insertion of the volumetric flasks. The intensity of the color produced is also influenced by the quantity and quality of acids and salts present. Tartrate and large amounts of sulfate seriously in-
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ANALYTICAL CHEMISTRY
terfere by preventing complete color development. Close control of the p H of the solution a t the time of color development is required if reproducible results are to be obtained. Measurementa show that when 15 ml. of aluminon-buffer composite solution is diluted to 25 ml. with distilled water, the mixture has a pH of 5.3. During the heating to develop the aluminum lake, some of the gelatin deposits on the walls of the volumetric flasks. Before these flasks are used again, they must be thoroughly cleaned with cleaning solution. EXPERIMENTAL
In order t o compare'samples of aluminon obtained from different sources, calibration curves were obtained using aluminon-buffer composite solutions prepared from various samples of alumiDon. The curves were obtained as directed in the method for the
analysis of zinc die casting and magnesium alloys. The results are recorded in Figure 1. Kational Bureau of Standards samples of manganese bronze No. 62-b, of zinc base die casting alloy Xo. 94-a, and of magnesium base alloy No. 171 were analyzed by the appropriate method described above. The results were recorded in Table I. LITERATURE CITED
Cheneru. E. AI.. Amlust. 7 3 ,. 5 0 1 (1948)~ , (2) Craft, C. H., and Makepeace, G. R., ISD. ENG.CHEM.,A N ~ L . ED.,17, 206 (1945). (3) Sherman, hl., paper presented before Division of Analytlcal SOCIETY, Chemistry at the 118th Meeting,AMERICANCEIEMICAL Chicago, Ill. (4) Smith, H., Sager, E. E., and Siewers, I. J., ANAL. CHEII.,21,
(1)
w.
1334 (1949). RECEIVED for review April 28,1951. Accepted May 13,1952.
Photometric Determination of Aluminum in lead, Antimony, and Tin and Their Alloys A n Aluminon Method C. L. LUKE, Bell Telephone Laboratories, Inc., Murray Hill, N . J . The work was undertaken because of a need for a reliable method for the determination of traces of aluminum in lead, antimony, and tin and their alloys. As a preparatory step toward the development of such a method, a thorough study of the specificity of the aluminon-thioglycolic acid and the oxine-cyanideperoxide photometric aluminum methods was made. As a result, an accurate specific method for aluminum has been developed. This method is applicable to the analysis of lead, antimony, and tin and their alloys and can also be adapted for use in the analysis of a wide variety of other ferrous and nonferrous alloys.
I
Ili THIS country the aluminon and the oxine methods are most
often used for the photometric determination of aluminum in lead, antimonv, and tin and their alloys. Because neither method is very specific it is necessary to isolate the aluminum rather completely before the photometric determination is attempted. I t is customary to rcmove lead as sulfate and antimony and tin by volatilization as bromide. Following this, a mercury cath6de separation is made, which removes a wide variety of metal ions but may not completely free the sample solutions of interference. In the first place, it is often difficult to obtain complete deposition of certain interfering metal ions, Moreover, several interfering metal ions are not capable of being removed a t the mercury cathode, and it is usually necessary to resort to more complete isolation of the aluminum or to the use of complexing agents to mask interference. Precipitation or extraction with eupferron has been used for the removal of traces of iron that may have escaped the electrolysis. On the other hand Chenery (1)has been able t o eliminate the interference of iron in the aluminon method by complexing with thioglycolic acid. Luke and Braun ( 4 )have used this acid to suppress the interference of copper and iron in the photometric determination of aluminum in manganese bronze using aluminon. Gentry and Sherrington (2) have markedly increased the specificity of the oxine method by complexing several metal ions with cyanide, and Kassner and Oaier (a) have further increased specificity by the use of hydrogen peroxide to complex several other metal ions. As a preliminary step toward the development of a suitable aluminum method, the author has undertaken a thorough investigation of the specificity of the oxine-cyanide-peroxide method
and of the aluminon-thioglycolic acid method. From this work a very specific method for the determination of aluminum in lead, antimony, and tin and their alloys has been developed. In this new method the solution from the mercury cathode separation is freed of such interfering metal ions as titanium, zirconium, and hafnium, by a cupferron-chloroform extraction. The solution is then neutralized to p H 5 and the aluminum is separated from such metal ions as beryllium and scandium by an oxine-chloroform extraction. Following this, the chloroform and oxine are removed by evaporation and the aluminum is determined by the aluminonthioglycolic acid method. APPARATUS
Photoelectric Photometer. An Evelyn photometer with a 515mp filter was used in the present investigation and the procedure was written with this instrument in mind. Glassware. Unless otherwise specified, all the glassware used in the method should be made of Pyrex glass No. 7740 or its equivalent. REAGENTS
Standard Aluminum Solution (0.01 mg. of aluminum per ml.). Dissolve 20.0 mg. of pure aluminum in 20 ml. of hydrochloric acid by heating gently, Dilute t o 2 liters in a volumetric flask. Cupferron Solution. Dissolve 1 gram of cupferron in 100 ml. of distilled water. Prepare fresh daily as needed. Chloroform. Redistill the commercial product to free it from traces of metalion impurities. m-Cresol Purple Indicator Solution. Dissolve 0.1 gram of mcresol purple in 10 ml. of distilled water containing 1 pellet of sodium hydroxide. Cool and dilute to 100 ml. with distilled water.
