338
ANALYTICAL CHEMISTRY
Table I. Temperature of Drying
c.
S o t dried
105 150 200
The surface of pastes should be scraped aside and a sample taken from deep in the center of the mass.
Drying of Aluminum Powder ,
Drying Mzn.
...
60
1,080 60 1,080 30 60 70 160
Metallic Aluminum 7c
91 3 91.9 92.6 93.1 94.1 94.4 94.4 94.0 93.1
sults checked with those obtained with a normal sample run a t the same time. The common impurities (3) in aluminum met>al-aluminum oxide and silicon-do not interfere in this method, and the most probable adulterant, mica, is also without effect. Iron and other impurities, such as zinc and copper, are usually present only in quantities Tvell within the experimental error of the method, and may therefore be disregarded. Various concentrations of ferric sulfate were tried and it was found necessary t o use 25 to 30 grams of ferric sulfate per 100 ml. of solution to ensure a quantitative reduction of ferric sulfate equivalent to the amount of metallic aluminum present (Table 11). The acid concentration is not critical up to 10 ml. per 100 ml. of solution, but for higher concentrations, IOK results are obtained oning t o the evolution of hydrogen. The reaction vias carried out in an atmosphere of carbon dioxide in order to prevent the reosidation of the ferrous sulfate formed in the hot solution. Table I1 shows the inconsistency of results obtained without the carbon dioxide atmosphere as compared with the relatively constant values obtained with it. After the solution had been cooled, the amount of reoxidation \vas negligible
(4. The method proved entirely satisfactory with aluminum pastes, as the initial drying procedure removed the volatile solvent. The residual powder was then treated in the normal way. REAGENTS
Ferric Sulfate Solution. Dissolve 330 grams of reagent grade ferric sulfate, Fe2(S04)1.9H20,in 750 ml. of water and 75 ml. of C . P . sulfuric acid (specific gravit’y 1.84). The ferric sulfate dissolves very slowly, even with agitation and heating, and 1 or 2 days should be alloived for its solution. When it has dissolved, make up the volume t o 1 liter. Sodium Bicarbonate Solution. h saturated solution of reagent grade sodium bicarbonate (approximately 10%). Phosphoric acid, C.P. 85% orthophosphoric acid. Potassium permanganate, 0.5 N solution.
Table 11.
Effect of Concentration of Ferric Sulfate and the Use of Carbon Dioxide
Ferric Sulfate G./lGG ml. 30 30 27.5 20 15
SI etallic Aluminum without COn
Metallic Aluminum with CO?
R
70
93.8 (94.5 193.3 92.9 93.7 91.2
94.4 94.4 94.4 93.7 92.4
SAMPLIIVG
As aluminum povidere or pastes stored for some length of time may have a tendency to oxidize on the surface, the following precautions should be taken in obtaining samples from them: Containers of, polvder should be shaken or rolled to ensure thorough mixing of the contents. After mixing, some time should be allowed for the povder to settle before the container is opened.
PROCEDURE
Weigh roughly a 0.2-gram sample into a tared 3- to 4-ml. weighing bottle with a outside stopper. Dry for 45 minutes a t 200” C. Place the top on the weighing bottle before removing the bottle from the oven. Weigh accurately. Remove the stopper from the weighing bottle and place the lower portion, containing the sample, in a 500-ml. wide-necked flask. Add 100 ml. of the acid ferric sulfate solution, and stopper the flask tightly with a rubber stopper fitted with a 25-ml. separatory funnel and an outlet tube (Figure 1). Immerse the end of the outlet tubeiii sodium bicarbonate solution contained in a small flask. Kithout agitation add as quickly as possible 50 t o 75 ml. of sodium bicarbonate solution through the separatory funnel and then shake the flask gently. Place the flask on a hot plate, bring t o the boil, and boil 5 minutes. Cool contents of the flask t o 10” t o 15” C., taking care that the rubber outlet tube is kept in the bicarbonate solution. Remove the stopper, add 15 ml. of phosphoric acid, and titrate with potassium permanganate.
% metallic aluminum
=
ml. of K X n 0 4 X N 0.02697 Xweight of sample 3
The precision of the method is *O.lyoaluminum, and the time required for the deterniination, exclusive of the drying, is approsimntely 1 hour. NOTES ON PROCEDURE
To prevent immed i a t e s o r p t i o n of moisture, the stopper is p l a c e d o n t h e weighing bottle before the bottle is removed from the oven. C o n t r a r y t o \That might be expected, the stopper is not difficult to remove later. Khen t h e f e r r i c s u l f a t e solution is added, care must be taken not to pour it directly on the powder, which may Figure 1. Reaction Apparatus be blown out of the flask. The bicarbonate solution is added to evolve carbon dioxide, which removes the air from the flask and creates an inert atmosphere. It requires 50 to 75 ml. to achieve this purpose, and the right condition is reached vhen a thick persistent orange foam is formed before shaking the flask. The addition of phosphoric acid prior to titration is necessary to produce a colorless acid phosphate complex, which allows the end point to be readily seen. LITERATURE CITED (1) Aluminum Co. of America. “Chemical Analysis of Aluminum”, p . 110, Sew Kensington, Pa., Aluminum Research Laboratories, 1941. Boner, J. E., Helv. Chim. Acta, 28, 352-61 (1945). Edwards, J. D., “Aluminum Paint and Powder”, pp. 29-31, Sew York, Reinhold Publishing Corp., 1936.
Hildebrand, W. F., and Lundell, G. E. F., “Applied Inorganic .halysis”, p. 300, New York, John Wley & Sons, 1929. Kohn-Abrest, E., C‘ompt. rend., 147, 1293-6 (1908). Kray, R. H., IND. ENG.CHEM.,ANAL.ED.,6, 250 (1934). Scott. “Standard Methods of Chemical Analysis”, p. 50, New York, D. Van Nostrand Co., 1939. (8) Steinhauser, K., Al~cmintim,24, 176-8 (1942). ISSUEDas N.R.C. No. 1432.
