Gravimetric Determination of Aluminum in Magnesium Alloys Benzoate-Oxine Method V. A. STENGER, W. R. KRAMER, AND A. W. BESHGETOOR The Dow Chemical Company, Midland, Nlich.
K
Any residue of co per or carbon should be allowed to settle out, but undissolved sifica, which might contain some occluded aluminum, should be kept in suspension. Pipet a 50-ml. aliquot into a 400-ml. beaker and dilute with 50 ml. of water. Add 1 to 1 ammonium hydroxide drop by drop with stirring until the precipitate which forms as each drop strikes finally redissolves only very slowly-that is, until nearly all the free hydrochloric acid is neutralized without permanent precipitation of aluminum hydroxide. Add 1 ml. of glacial acetic acid, about 1 gram of ammonium chloride, and 20 ml. of ammonium benzoate solution. Heat to boiling m-hile stirring well, keep at gentle boiling for 5 minutes, then filter with the aid of suction on a previously weighed glass crucible, Gooch type, with fritted disk of fine porosity. T a s h eight to ten times with hot benzoate wash solution, making no effort t o transfer all precipitate to the crucible. Return the precipitate to the beaker, transferring most of it with the aid of the stirring rod and washing the remainder back with water. Dissolve a small part remaining in the crucible by washing with five 10-ml. portions of hot ammoniacal tartrate solution and combine with the precipitate in the beaker. Heat to 70" to 90" C., add 25 ml. of oxine solution, and digest for 30 minutes without boiling. Filter on the same crucible and wash eight times with water. transferring all Drecioitate. Dr for 1.5 ro 2 hours at 120" to' 130" C., cGl, and wAgh as A~&~H~oN)~.
APID routine analyses of magnesium alloys for alu-
minum are generally carried out by potentiometric (1) or spectrographic (5) methods. There is a need, however, for a more accurate procedure suited to referee work and other special purposes. The difficulties encountered in separating aluminum from much magnesium and also from zinc are well known ( 3 ) . I n the usual hydroxide precipitation method several precipitations must be made, and care is required to avoid errors in weighing the hygroscopic aluminum oxide. I n addition, removal of or correction for silica is necessary in many cases. The 8-hydroxyquinoline (oxine) reagent ( 2 ) is useful for determining aluminum in magnesium alloys low in manganese, zinc, and other interfering elements, but since most commercial alloys contain one or more of these, preliminary separations are required. To precipitate the aluminum as oxyquinolate from ammoniacal tartrate solution, which in the authors' experience yields somewhat more precise results than precipitating from a n acetic acid-acetate buffer, involves also the prior removal of magnesium. Addition of ammonium benzoate to a weak acid solution of the metals has been shown (4) to give a virtually complete separation of aluminum from most of the divalent elements, in one step. The benzoate precipitate is readily soluble in warm ammoniacal tartrate solution, from which aluminum may be precipitated with oxine. This combination of methods has been in use in laboratories of The Dovv Chemical Company for nearly 5 years. I n view of the present great practical importance of the magnesium alloys, and since the procedure is currently being considered for adoption as a tentative standard method of the American Society for Testing Materials, it seems advisable that a description, together v i t h results on known mixtures, should be published at this time.
yo A1
=
Al(CsH60N)s X 0.05873 X 100 grams of sample in aliquot
Tests on Known Solutions The magnesium alloys to which this method applies ordinarily contain more than 88 per cent magnesium, up to 10 per cent aluminum, up to 3 per cent zinc, and 0.1 to 0.3 per cent manganese. There are also individual alloys which contain 1.5 per cent manganese in one case, 3.5 per cent cadmium in another, and 0.5 per cent silicon in a third. Generally copper and iron will be below 0.04 per cent, as will silicon also except in the case just noted. Special treatment would be necessary for certain less common alloys high in copper, tin, or silver. The effects of some of these elements
Reagents Ammonium benzoate solution. Dissolve 100 grams of pure ammonium benzoate (preferably recrystallized) in 1 liter of warm water and add 1 mg. of thymol as a preservative. Benzoate wash solution. To 100 ml. of the ammonium benzoate solution add 900 ml. of warm water and 20 ml. of glacial acetic acid. Oxine solution. Dissolve 50 grams of 8-hydroxyquinoline in 1 liter of water containing 120 ml. of glacial acetic acid. Filter if necessary and keep in a dark bottle. Ammoniacal tartrate solution. Dissolve 30 grams of ammonium tartrate in 1 liter of water containing 120ml. of concentrated ammonium hydroxide. Ammonium hydroxide, 1 to 1. Hydrochloric acid, c. P., concentrated. Acetic acid, glacial. The magnesium and aluminum chloride solutions used in testing the method were prepared from neighed amounts of the pure metals dissolved in hydrochloric acid and made to known volume. Other elements were added as solutions of the c. P. chlorides.
TABLE I. ANALYSISOF KKOWNMIXTURES Aluminum Taken
Additions
Slurninurn Found
Mg.
Mg.
MQ.
%
24.92 24.98 50.08 9.99 25.05 49.86 25.09 9.96 10.01 24.96 25.04 10.02 10.06 24.93 2;7,01 24.93 24.96 24.92 49.92 24.88 50.36
-0.32 -0.08 + O . 16 -0.10 +0.20 -0.28 +0.36 -0.40
25.00 25.00 50.00 10.00 25.00 50.00 25.00 10.00 10.00 26.00 25.00 10.00 10.00 25.00 25.00 25.00 25.00 25.00 50.00 25.00 50,OO
Procedure Keigh out a sample calculated to contain 0.2 to 0.5 gram of aluminum, place in a 250-ml. beaker with 25 ml. of water, and dissolve by adding in small portions a total of 10 ml. of hydrochloric acid for each gram of sample. T h e n dissolved, cool to room temperature and dilute to 500 ml. in a volumetric flask.
50.00
25.00 25,oo 25 00 25.00
797
50.60
25.25 25.30 25.19 25.24
Error
+0.10
-0.16 + O . 16 +0.20 4-0.60 -0.28 $0.04
-0.28 -0.16 -0.32 -0.16 -0.48 +0.72 +1.2 +l.O
+1.2 +0
i6
4-0.96
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
798
on the determination of aluminum have been tested, with results as shown in Table I. I n general the deviations fall within acceptable limits except in the presence of considerable copper, tin, or iron, which is not likely to be encountered. Copper would largely remain insoluble in hydrochloric acid, the amount going into solution being too small to cause serious interference. Tin is preferably precipitated with hydrogen sulfide, the filtrate then being boiled and cooled before neutralization and precipitation with benzoate. The small amounts of iron usually present as an impurity have negligible influence on the aluminum determination. Larger quantities present as the result of some special condition may be precipitated by passing hydrogen sulfide through the ammoniacal tartrate solution of the benzoate precipitate. The filtrate is then warmed and treated a t once with oxine reagent, without removal of excess sulfide. Enough ammonia should be present to keep any free sulfur in solution as polysulfide, yet an excess should be avoided as it may cause precipitation of the oxine. Behavior of Silicon Since one of the advantages of the method is that silicon does not have to be removed, it is of interest to consider what happens to silicon in the analysis. The alloy having the highest content of silicon, 0.5 per cent, contains 10 per cent aluminum. Even if all of the silicon stayed with the aluminum and were weighed as silicon dioxide, the ratio of silicon dioxide to the total weight of the precipitate would amount to only 0.0063 because of the large molecular weight of aluminum oxyquinolate. This would correspond to an error percentage of 0.63 based on the aluminum; in other words, 10.063 per cent aluminum might be reported instead of 10.00 per cent.
Vol. 14, No. 10
I n practice, up to about 0.15 per cent silicon dissolves in the dilute hydrochloric acid. This soluble silica does not interfere. Known solutions of aluminum and magnesium chlorides have been analyzed with and without the addition of small amounts of sodium silicate, no significant differences being observed in the results. The remainder of the silicon evidently dissolves but reprecipitates as a flocculent hydrous oxide. This again can partly dissolve when the benzoate precipitate is taken up in ammoniacal tartrate. The final percentage of error on the aluminum can thus drop to about 0.3, which is not too serious even in this unfavorable case. Since the majority of the alloys contain much less silicon, the error generally introduced in the aluminum figure is negligible even for referee purposes. Summary A fairly rapid gravimetric procedure, suitable for referee analyses, is presented for the determination of aluminum in magnesium-base alloys. Aluminum is precipitated with ammonium benzoate, dissolved in ammoniacal tartrate solution, and reprecipitated with 8-hydroxyquinoline. None of the common alloy constituents interferes, and no separation of silica is necessary. Literature Cited (1) Dow Chemical Co., Midland, Mich., “Dowmetal Laboratory
Methods”, Bull. DM 41,p. 12,1941. (2) Hahn, F. L.,and Vieweg, K., 2. anal. Chem., 71, 122 (1927). (3) Hillebrand, W. F., and Lundell, G . E. F., “Applied Inorganic Analysis”, pp. 390-8, New York, John Wiley & Sons, 1929. (4) Kolthoff, I. M., Stenger, V. A,, and Moskovitz, B., J. Am. Chem. SOC.,56, 812 (1934). (5) Owens, J. S., IND.ENQ.CHEM.,ANAL.ED., 10,64 (1938). PRESENTED in part before the Ohio-Michigan Regional meeting
Studies on the Carotenoids Spectrophotometric Determination of the Carotenoids of Yellow Corn Grain JONATH4N W. WHITE, JR.’, ARTHUR M. BRUNSON, AND F. P. ZSCHEILE, Purdue University Agricultural Experiment Station, Lafayette, Ind.
I
(4)presented a method for determining the carotenoids of corn [Zea Mays L.] but did not separate the carotene from cryptoxanthol. Consideration of cryptoxanthol in analytical methods for corn pigments is important because of its provitamin A activity. These workers reported the concentration of “xanthophyll” in several corn varieties, but the standard absorption coefficients which they used for this determination were much lower than those reported by others, leading to erroneous results for “xanthophyll” concentration. Later, Buxton (2) described a method for the determination of “carotene and/or cryptoxanthin” in yellow corn. He used adsorption on calcium carbonate for separation of carotene from cryptoxanthol in his final solution. Both workers used 90 per cent methanol for the separation of zeaxanthol from the provitamin pigments. Fraps and Kemmerer (5) recently reported an adsorption method for the determination of the pigments of corn. They separated the pigments epiphasic to 90 per cent methanol into five compounds: beta-carotene, alphacarotene, K carotene, cryptoxanthol, and neocryptoxanthol. An abridged method for routine determination vas developed in which the pigments were divided into two groups, caroN 1937, Clark and Gring
1 Present address, Eastern Regional Research Laboratory, .Wyndmoor. Penna.
tenes and cryptoxanthol plus neocryptoxanthol, by adsorption on magnesium carbonate. Calculation of the provitamin A potency of these two groups was based on their average composition as found in the preliminary study. This method is probably the best of the three, though losses from incomplete recovery of adsorbed pigments must not be overlooked. It is also necessary to test and standardize each new lot of adsorbent. White, Zscheile, and Brunson (11) demonstrated the presence of luteol and gamma-carotene in corn, and noted the occurrence of unnamed carotene 1 which may be identical with the K carotene reported by Fraps and Kemmerer ( 5 ) . I n this paper it is shown that large amounts of neo isomers are present in corn extracts and must be considered in analysis. The solvents suggested in a previous paper (10) for the separation of the pigments into three groups-carotene, cryptoxanthol, and carotenol-have been applied to the separation of corn-grain pigments. Where possible, account is taken of the presence of pigments other than beta-carotene, cryptoxanthol, and zeaxanthol. Experimental For immature corn grain, the sample (20 t o 40 grams, de ending on water content) was extracted 5 minutes in a &ring
