The observed heat of the replacement complexation reaction was -6.0 Kcal/mole and is somewhat higher than the calculated value of -5.1 Kcal/mole obtained by adding the reported heats of reactions ( 2 ) shown by Equations 11 and 12. In order to verify the substitution reaction technique Mg EDTA2- was reacted with Zn(I1) and a heat of reaction of -10.6 Kcal/mole was obtained. The calculated heat of reaction using AH values reported by Jordan and Alleman (I) was -10.0 Kcal/mole. This agreement was within the experimental error of the thermometric method used. A brief study of metal-EDTA complexes was initiated to see if the deprotonation-complexation could be separated from the complexation reactions as was the case with metalNTA complexes. Equations 13 and 14 show the reactions studied.
+
EDTA*M2+ e MEDTA2MZf e MEDTAZH+ HEDTAa-
(1 3) (14) Calcium(I1) was selected for the EDTA study because the equilibrium constants for reactions 13 and 14 were of the same order of magnitude as the same reactions of Zn-NTA. The results of thermometric titrations of EDTA with Ca(I1) at Variods pH values were similar to those obtained for metal-NTA complexes (Figure 2). However, because the deprotonation-complexation reaction is exothermic for Ca-EDTA, only a change of slope in the titration curve was noted (Figure 4). Zn(I1) and Ni(I1) complexes with EDTA also appeared to give similar results.
+
+
RECEIVED for review June 13, 1968. Accepted September 27, 1968.
Aluminum Materials Raymond S. Lambert Hercules Incorporated, Bacchus Works, Magna, Utah 84044 Metallic aluminum in aluminum powder or aluminum staples is determined by dissolution of the sample in a sulfuric acid solution of iron(ll1) and cerium(lV). The cerium(lV) that remains after oxidation of the metallic aluminum is titrated with arsenic(lll) using osmium tetroxide catalyst. The end point is detected by use of Ferroin indicator or by a potentiometric system. The effects of acid and iron(ll1) concentrations are discussed.
METALLIC ALUMINUM, as either powder or staples, is an important ingredient in many solid propellants. However, metallic aluminum readily acquires an oxidized coating on the surface when it is exposed to moisture and air. Therefore, laboratories are required to determine the metallic content of incoming shipments of aluminum powder and aluminum staples. Several procedures have been used for the determination of metallic aluminum in the presence of oxides of aluminum. One of these procedures requires the measurement of the volume of hydrogen evolved upon solution of the aluminum in sodium hydroxide solution ( I ) . In another method the hydrogen is converted to water and weighed ( I ) . These procedures have poor reproducibility and require the use of cumbersome and awkward equipment. Light and Russel published a procedure ( 2 ) for use on pigments and pastes and our laboratory has used a somewhat similar procedure on aluminum powder. In both of these procedures an acidified solution of iron(II1) sulfate is reduced by the aluminum metal and the iron(I1) is titrated with potassium permanganate solution. However, there is a loss of hydrogen gas during the dissolution of the aluminum metal and the end point of the titration is difficult to detect. The combination of these factors yields results with poor precision. A modification of the procedure (1) F. J.
Welcher, Editor, “Standard Methods of Analysis,”
Sixth Edition, Volume Two, Part B, D. Van Nostrand Company, Inc., Princeton, N. J., p 1306 (1963). (2) A. K. Light and L. W. Russell, ANALCHEM., 19, 337 (1947).
followed in our laboratory was developed. This modification uses a different shaped container during the reduction step to increase the recovery of any hydrogen gas that might be formed, and the iron(I1) is titrated with cerium(1V) sulfate using Ferroin indicator. The latter titration has a much better end point but the results did not demonstrate satisfactory precision. Evidently there was still some loss of hydrogen. In addition, these procedures required an inert atmosphere both during the dissolution of the aluminum metal and the titration of the reduced iron. A simple and d k c t route for the determination of metallic aluminum would appear to be oxidation of the aluminum in an acid sdution of cerium(1V) sulfate. The stronger oxidation potential of cerium(1V) should minimize or possibly eliminate the formation and loss of gaseous hydrogen. Also, the excess cerium(1V) sulfate might be titrated with excellent precision by the use of sodium arsenite solution (3). In addition, an acid solution of cerium(1V) sulfate is stable in air and boiling its solution does not cause any loss of its oxidizing value (4). EXPERIMENTAL
Apparatus. All weighings of aluminum samples were with small weighing dishes made from short sections of 1-cm 0.d. glass tubing that had one end sealed off and the other end flared out. A weighing dish had a volume of about 0.5 ml. Titrations were carried out with a buret or a Metrohm Potentiograph, Model E336. This automatic titrimeter was equipped with a platinum wire electrode, Fisher Scientific Co. No. 9-313-210, and a double junction calomel electrode, Beckman No. 40452. The outer compartment of the calomel electrode was filled with saturated ammonium sulfate solution. (3) A. J. Zielen, ANALCHEM., 40,139 (1968). (4) I. M. Kolthoff and R. Belcher, “Volumetric Analysis-111,” Interscience Publishers, Inc., New York, p. 129 (1957). VOL 41, NO.
1, JANUARY 1969
57
Table I. Effect of Iron(II1) Sulfate Content on Recovery of Metallic Aluminum Fez(SOd3. x H20
Added, g 0.0
6.6 13.2 20.0 a Less than
Added 157.8 157.8 157.8 157.8
Al”, mg Recovered 134.1~ 145.6 157.2 157.8
Recovery, 85.0 92.3 99.6 100.0
Reagents. A 0.1000N solution of sodium arsenite was prepared by dissolving 9.892 g of dried primary grade Asz03 in 120 ml of 1N NaOH diluted with 50 ml of distilled water. The cooled solution was neutralized with 1N sulfuric acid, plus 20 rnl added in excess, and then diluted to 2 liters with distilled water. A 0.2N solution of cerium(1V) sulfate was prepared by weighing 138 g of Ce(NH4)4(S04)4 into a liter beaker and adding 40 ml of concentrated sulfuric acid and 850 ml of distilled water. The solution was heated to boiling to dissolve the salt, cooled, and diluted to 1 liter with distilled water. An iron(II1) sulfate solution was prepared by dissolving 200 g of Fe2(S0& . xHzO in 800 ml of distilled water and 10 ml of 18N sulfuric acid with heating, cooling, and diluting to 1 liter with distilled water. Ferroin indicator [l,lO-phenanthroline-iron(I1) sulfate], 0.025M solution, and osmium tetroxide (osmic acid), 0.01M solution, obtained from G. F. Smith Chemical Co., were used as received. The samples of high purity aluminum were an “atomized” sample (K-101) and a granular sample (SAA-1) from Aluminum Corporation of America and a sample of aluminum “staples” (K-102) from Reynolds Aluminum Co. Procedures. Standardization of Cerium(1V) Sulfate Solution. Carefully pipette 20.00 ml of cerium(1V) sulfate solution into a 400-ml beaker. Add 15 ml of 18N sulfuric acid and dilute to 250 ml with distilled water. Add six drops of osmium tetroxide and three drops of Ferroin indicator. Titrate with standard sodium arsenite solution from a yellow color through a pale blue to the first permanent change of the solution to a reddish-orange color. Recommended Procedure for Metallic Aluminum Determination. Mix the aluminum powder and place a sample (about 2.0 g) into a weighing bottle. Dry the unstoppered bottle and contents for 1 hour at 105 “C. After drying, stopper the bottle and cool in a desiccator. Accordingly weigh 0.157G1620 g of the dry powdered aluminum into an accurately weighed small weighing bottle. Place weighing bottle containing the weighed aluminum into a 600-ml beaker. Carefully pipet 100.00 ml of cerium(1V) sulfate solution into the beaker. Carefully swirl the beaker to wash the sample out of the weighing bottle. Dilute 100 ml of iron(II1) sulfate solution with 250 ml of distilled water. Carefully pour this solution into the reaction beaker. Cover the beaker with a watch glass and set the beaker on a cold hotplate. Switch on the hotplate and gently warm the solution until the aluminum is dissolved (three to four hours). Then increase the heat of the hotplate and slowly boil the solution for a few minutes. (CAUTION: Do not boil the solution before the aluminum is visually dissolved or some of the reducing power of the aluminum will be lost in the form of escaping hydrogen gas.) Place the beaker in a cold water bath and cool the solution to room temperature. Remove the beaker from the water bath, add 20 ml of 18N sulfuric acid, 10 drops of osmium tetroxide, and four drops of Ferroin indicator. Titrate with standard sodium arsenite solution. The end point is the first permanent change of the solution to a reddish-orange color. For the determination of a reagent blank, pipet 20.00 ml of cerium(1V) sulfate solution into a 600-ml beaker. Care58
ANALYTICAL CHEMISTRY
fully add a mixture of 80 ml of 1.5N siilfuric acid solution, 100 ml of iron(II1) sulfate solution, and 250 ml of distilled water. Heat the solution to boiling and cool in a water bath. Titrate with standard sodium arsenite solution after adding 20 ml of 18N sulfuric acid, 10 drops osmium tetroxide, and four drops Ferroin indicator. Titrate to the same color end point as used for unknowns.
RESULTS AND DISCUSSION At the beginning of this investigation, attempts were made to use only an acidified cerium(1V) solution for oxidation of the aluminum. Unfortunately, the cerium(1V) was not reduced efficiently by the powdered aluminum metal. In this combination of chemicals, only about 80 per cent recovery of the metallic aluminum was attained. Therefore, various techniques for improvement of the recovery were tried. Included in the modifications were addition of catalysts, various concentrations of iron(II1) sulfate, and various acid concentrations. Several materials were tried as catalysts to produce a satisfactory reduction of cerium(1V) sulfate by metallic aluminum. Of these, osmium tetroxide, manganese(I1) sulfate, mercury(I1) acetate, and phosphoric acid inhibited the reduction of cerium (IV) by aluminum metal. Perchloric acid in volumes of 5 ml per determination improved the recovery from 83 per cent to about 84 per cent; in volumes of 10-15 ml, it retarded the reduction. Small quantities-about 0.3-0.5 g of ammonium molybdate in combination with 5 g of iron(II1) sulfate increased the recovery of the metallic aluminum to about 96 per cent. However, the addition of a gram or more of ammonium molybdate caused unsatisfactory results. When a solution that contained the higher concentration of molybdate was heated to dissolve the metal, molybdic acid precipitated. This precipitate masked the end point in the titration of the excess cerium(1V) such that a greater than 100 per cent recovery of the metal was sometimes indicated. The proper concentration range of molybdate, if there was one, was too narrow for a good procedure. Preliminary testing indicated that the addition of iron(II1) sulfate increased the efficiency of the reduction of cerium(1V) by metallic aluminum. Therefore, a series of aluminum samples with 0.0 to 20.0 grams of iron(II1) sulfate added was prepared. Each beaker contained about 17.5 milliequivalent (meq) of powdered metallic aluminum, 100.00 ml of standardized 0.2N cerium(1V) sulfate in 1.44N sulfuric acid, the desired volume of iron(II1) sulfate solution, and sufficient distilled water for a final volume of 450 ml. The aluminum was dissolved and the titration with standard arsenite solution using osmium tetroxide catalyst and Ferroin indicator was performed as in the Recommended Procedure. No inert atmosphere was required either during the dissolution of the metal or for the titration of the excess cerium(1V) sulfate. The results of these tests are listed in Table I. The optimum recovery of the metallic aluminum was obtained when 20.0 g of iron(II1) sulfate was dissolved in the solution. Smaller amounts of iron(II1) sulfate gave lower results. The optimum concentration of iron(II1) in the solution was too high for use as a catalyst to be the major function. The major role was as a scavenger. The iron(II1) supplied sufficient additional reducible ions around the particles of metallic aluminum to prevent loss of reducing ability of the aluminum in the formation of gaseous hydrogen. The belief during the early phase of this investigation was that the acidity of the solution for the dissolution of the metal would have quite an effect on the recovery. Nevertheless, this was found to be not so. If 450 ml of solution in which
0
0
Po tent iome t r Point
-
1
0
9
I
I
-
P4
P6
95
P6
0.1OOON NazHAs03, mi
21
I 98
Figure 1. Curve of potentiometric titration of Ce(1V) with AS(II1) Using recommended procedure B. With 4 drops of 0.025MFerroin added
A.
was dissolved 20 meq of cerium(1V) and 20.0 g of iron(II1) sulfate was added to 17.5 meq of aluminum, satisfactory recovery was obtained when the acidity ranged from 0.36N to 0.96N in sulfuric acid. However, low results were obtained if the solution was heated to boiling before all of the metallic aluminum had dissolved, regardless of the acid concentration. Arbitrarily, a solution of 0.36N sulfuric acid was selected. Zielen (3) obtained excellent results with potentiometric titration of acidified solutions of microequivalent amounts of cerium(1V) sulfate with sodium arsenite. Equally satisfactory results were obtained in the Bacchus Laboratory from potentiometric titration with sodium arsenite of acid solutions that contained 20.0 g of iron(II1) sulfate in addition to 2-4 meq of cerium(1V) sulfate. An automatic titrimeter was used with a platinum wire and a double junction calomel electrode. Four samples of aluminum powder were dissolved as in the Recommended Procedure. After cooling, 10 drops of osmium tetroxide were added to each sample. Then four drops of Ferroin indicator were added to two of the determinations before titration and the other two were titrated without indicator. The potentiometric curves of the titrations are shown in Figure 1. The break in the curves extended for about 400 millivolts. When no indicator was present, the point of inflection as determined by rectangulation of the sigmoid curve was the end point. When indicator was present, there was a hump in the curve corresponding to reduc-
tion of the indicator. This was corrected by projecting the ends of the curve to produce an intersection that was interpreted as the end point. Table I1 lists the results. There was excellent precision of the results and very good agreement between titrations, with and without indicator. Also, when indicator was present there was good correlation between the potentiometric end point and the visually observed end point. The quantity of oxidant used in an indirect titrimetric procedure such as this may be varied while keeping a high degree of precision in the titration of the excess oxidizing agent. Consequently, there may be a wide latitude in sample size. In this work 100 ml of 0.2N cerium(1V) sulfate solution was believed to be adequate. With this quantity of oxidizer, a sample size of 17.5 meq of aluminum metal left sufficient cerium(1V) after dissolution of the metal to ensure complete-
Table 11. Results Obtained by Potentiometric Titration with and without Indicator in Recommended Procedure Metallic aluminum found, No indicator added Using 4 drops Ferroin 98,52 98.49 98.50 98.51
VOL. 41, NO. 1 , JANUARY 1969
59
Table 111. Results of Metallic Aluminum Determination on Check Samples of Aluminum Powder and Aluminum Staples Metallic aluminum, K-101 K-102 SAA-1 Results 1st Set 1st Set 97.63 98,61 99.12 97.66 98.66 99.11 97.72 98.16 99.14 97.60 98.63 99.20 97.69
Average Standard deviation Table IV.
2nd Set 98.57 98.61 98.56 98.61 98.62
2nd Set
0.027
0.067
98.94 99.09 99.05 99.20 99.11
97.73 97.72
97.68 0.050
Analysis of High Purity Aluminum Samples K-101 K- 102 SAA-1 98. 56a 98.93n 97.20a 0.02 0.40 0.004 0.12 0.38 0.43 0.28 0.40 0.24 0.02 0.08 0.01
Aluminum Copper Iron Silicon Nickel NDb ND 0.08 Tin 0.02 0.07 Manganese 0.01 0.03 0.20 0.003 Zinc 0.02 0.08 0.01 Lead 0.003 0.04 Titanium 0.006 0.004 0.06 Magnesium 0.003 0.508 0.980 Oxygen (as A1203) 0.665 TOTAL 99.631 100.215 loo.02 The results for the aluminum determinations from Table I11 were corrected for interfering elements. * None detected.
num will produce a positive error in the aluminum determination. If the corrected value for the metallic aluminum is wanted, the error due to these metals must be calculated and subtracted from the gross result. The precision of the Recommended Procedure is illustrated by the results listed in Table 111. The data were obtained on check samples of aluminum powder No. K-101 and No. SAA-1 and aluminum staples No. K-102. The two different sets of results on samples No. K-101 and K-102 were obtained five days apart. New solutions of sodium arsenite, cerium(1V) sulfate, and iron(II1) sulfate were prepared and used t o obtain the second set of results. The standard deviation for the present Hercules procedure was 0.18 per cent. The pooled standard deviation for all three samples analyzed by the Recommended Procedure was 0.051 per cent. There were no published figures for the amounts of metallic aluminum in these check samples. Therefore, to check the accuracy of the procedure, a complete analysis of the metals was attempted in order to obtain a closure to 100 per cent. The percentage of the trace elements other than oxygen was determined by X-ray emission spectrometry and atomic absorption spectrometry. Oxygen was determined by neutron activation analysis. The aluminum result obtained by the Recominended Procedure was then corrected for the interfering elements. These results are listed in Table IV. Excellent closure, indicating good accuracy, was obtained on sample SAA-1. The closure for sample K-102 was slightly high, possibly because the correction figure for the oxygen may be based on the wrong compound. The closure for sample K-101 was low. However, the atomized material in this sample may have been coated with an organic material that was not determined. CONCLUSION
(1
ness of the oxidation. This is nearly twice the sample size used in the present procedure. An appreciable reagent blank was observed. This was due largely t o iron(I1) in the iron(II1) sulfate. Therefore, the amount of this blank varied with different lots of iron(II1) sulfate as well as the quantities of reagents added. When 20.0 g of iron(II1) sulfate, 10 drops of osmic acid, and four drops of Ferroin were used, the reagent blank ranged from 0.10 to 0.21 ml of 0.2 N cerium(1V) sulfate solution. During the determination of the reagent blank, the color of a weakly acidified solution of cerium(1V) sulfate and iron(II1) sulfate noticeably became darker upon being heated to a boil. Even after cooling, the color of the reagent blank remained darker than actual sample solutions that contained dissolved aluminum. However, the color of the solution became lighter as sodium arsenite was added during the titration so that an end point color change similar to the titration of an aluminum determination was obtained. The interferences in this procedure include any metal that, when dissolved in acid, liberates hydrogen or is oxidized by cerium(1V). Interfering metals include iron, zinc, manganese, and copper. The presence of these metals in metallic alumi-
60
ANALYTICAL CHEMISTRY
The use of a weakly acidic solution of cerium(1V) sulfate and iron(II1) sulfate permitted a satisfactory recovery and assay of metallic aluminum. Titration of excess cerium(1V) with sodium arsenite and osmium tetroxide catalyst was performed by using either Ferroin indicator or a potentiometric technique. A distinct end point was obtained by both methods and agreement was very good. The results obtained by following the Recommended Procedure have shown very satisfactory precision and accuracy. The 95 per cent confidence limit on high purity aluminum was hO.11 per cent. Several advantages over methods presently used were observed in the Recommended Procedure. Included in these were: (a) A wide range in sample size could be used without affecting the precision of the titration, (b) an inert atmosphere was not required either during oxidation of the metal or the titration of excess ceric sulfate, (c) no loss of hydrogen was observed during dissolution of the metallic aluminum, and (d) the end point was distinct and easily reproducible. ACKNOWLEDGMENT
The author expresses his appreciation and thanks t o R. J. DuBois for encouragement during this work and assistance in preparing the manuscript.
RECEIVED for review August 12, 1968. Accepted September 25, 1968.
