Ibid., p. 182. Bauld, W., Engel, L. L., Goldzieher, J. W., Can. J . Biochem. and Physiol. to be published. Bernstein, S., Lenhard, R. H., J . Org. Chem. 18, 1146 (1953). Ibid., 19, 1269 (1954). Braunsberg, H., J . Endocrinol. 8, 11 (1952). Brsunsberg, H., Osborn, S. B., Anal. Chim. Acta 6. 84 11952). Diczfalusy, E., Act‘a Endoc;inol. Suppl. 12, 1 (1953). Finkelstein, M., Ibid., 10, 149 (1952). Goldzieher, J. T.,Endocrinology 53, 520 (1953). Goldzieher. J. W., Bodenchuk. J. M.. Kolan, P., A N ~ L CHEX. . 26, 853
ACKNOWLEDGMENT
The authors are greatly indebted to Kenneth Savard, University of Miami for the analyses of purity of the corticosteroids, to Merck, Sharp & Dohme, and Schering Corp. for their generous supplies of steroids, and to Sabri el Farra and Taylor Chandler, Jr., for technical assistance.
LITERATURE CITED
(1) Axelrod, L. R., J . A m . Chem. Soc. 75, 6301 (1953). (2) Bates, R. W.,Cohen, H., Endocrinology 47, 166 (1952).
(lCI54). \ - - - - I -
(13) Goldzieher, J. W.,Bodenchuk, J. M., Nolan, P., J . Biol. Chern. 199, 621 (1952).
(14) Kalant, H., Biochem J . 63, 101’ (1956). (15) Linford, J. H., Can. J . Biochem. and Physiol. 35, 299 (1957). (16) Sowaczynski, W. J., Steyermark, P. R., Arch. Biochern. BLophys. 58, 453 (1955). ( l i ) Kowaczynski, W. J., Steyermark, P. R., Can. J . Bzochem. a n d Physzol., 34, 592 (1956). (18) Sweat, M. L., ASAL. CHEX.26, 7i3 (1954): (19) Zaffaroni, A , J . Am. Chern. Soc. 72, 3828 (1950). ( 2 0 ) Zaffaroni, A., Recent Progr. Hormone Research 8 , 51 (1953).
RECEIVED for review September 13, 1957. Accepted December 30, 1957. Study supported by funds supplied by the School of Aviation Medicine, U. S. Air Force, under contract AF 18(600)-921.
Volumetric Determination of Aluminum in Presence of Iron, Titanium, Calcium, Silicon, and Other impurities H. L. WATTS Alcoa Research laboratories, Aluminum
Co. o f America, Easf Si. louis, 111.
b In highly basic solution, aluminum forms soluble sodium aluminate, and iron and titanium precipitate as hydroxides. When aluminum reacts with fluoride, the hydroxide which combines with the aluminum cam be released for titration with standard acid. The titration can b e performed in the presence of the precipitated hydroxides. Interference from calcium can b e eliminated b y precipitating i t as the oxalate. By limiting the sample size, aluminum can be determined in the presence o f silicon dioxide. The method i s fast, precise, and accurate.
T
CLASSICAL procedure for the determination of aluminum oxide (or aluminum) in bauxite involves the determination of R203 (the ignited residue obtained in the ammonium hydroside precipitation method), ferric oside, titanium dioxide, and phosphorus pentoside. The amount of aluminum oxide is calculated by subtracting the sum of the other oxides from the Rz03 value. This method is slow and accuracy of the aluminum value depends upon the accuracy of the R203and other determinations. Also, the accuracy is adversely affected by the presence in the R20n precipitate of small amounts of impurities that are not determined. Many authors have advocated a preliminary digestion with sodium hydroxide, follon-ed by filtration of the insoluble iron and titanium hydroxides. However, the highly basic solution necessary to convert all of the aluminum hydroside to soluble sodium alumiHE
nate is often difficult to filter, and the accuracy and precision are seldom acceptable. Killard and Diehl(6) briefly described a private communication from M. L. n’indle for the routine volumetric determination of aluminum in the presence of iron, cobalt, and nickel. The solution was adjusted to pH 7 , potassium fluoride was added, and the released hydroxide was titrated back t o p H 7 with standard acid. A volumetric method for the determination of aluminum in the presence of fluoride, zirconium, and uranium was developed by Paige, Elliott, and Rein (4). Paulson and Murphy (6) used an indicator method for the determination of aluminum after its separation from iron and chromium. In both methods, the reaction, though not quite stoichiometric, is represented by the equation : A102-
+ 2 H20 + 6 F-
+
AlFs---
40H-
+
(1)
The highest recommended aluminum concentration in either method is 11 mg., and the largest amount of potassium fluoride is 20 ml. of a 7% solution. A fast, accurate method for the determination of aluminum in bauxite and other aluminous materials was desired. Because bauxite often contains as much as 55% aluminum oxide, good precision would require a method with which amounts of aluminum larger than 11 mg. could be titrated. Also, it was desired to avoid the separation of iron and other impurities.
Busliey ( 1 ) showed a series of tunc's for the titration of sodium aluminate solutions with hydrochloric acid. In Figure 1, a curve for free sodium hydroxide shows that the equivalence point, a t vhich aluminum hydroxide starts to precipitate, occurs a t pH 9.9 for 0.11 gram of aluminum oside. Bushey found that the pH of the equivalence point decreases with lower concentrations of aluminum oside. Therefore, a t pH 10 and above, aluminum is present as soluble sodium aluminate. When the pH of a sample is adjusted to above 10, iron and titanium precipitate as the hydroxides and aluminum dissolves completely as sodium aluminate. This reaction is the basis of several methods, including those involving filtration of the hydroside precipitates (3). HoFT-ever, a separation is unnecessary for the iron and titanium concentrations present in bauxite. Aluminum can be determined volumetrically by titration of the hydroside released by the addition of fluoride (Equation 1). The pH values of 10.0, 10.5, and 11.1 were investigated as the start and finish points of the titration. As shown in Figure 1, the pH relationship permitted sharper end points a t pH 10.0 than a t any higher pH. Impurities-e.g., iron, fluoride, and phosphorus-have considerably less effect a t pH 11.1 than a t pH 10.0. However, on actual samples of bauxite, no significant difference in accuracy was obtained whether the start and finish points were a t p H 10.0, 10.5, or 11.1. Because the sharper VOL. 30, NO. 5, MAY 1958
967
end points seemed to present the greater advantage, pH 10.0 was selected. Calcium, when present as an impurity, causes high results. Apparently calcium is present as the hydroxide in basic solution, and the addition of fluoride causes formation of calcium fluoride with the release of titratable hydroxide. This problem was solved by use of potassium oxalate as a precipitant for calcium. The presence or absence of oxalate does not affect results on standard aluminuni solutions that contain no calcium. The maximum aluminum concentration was established at 0.11 gram of aluminum oxide because the factor for aluminum and the pH of hydrolysis change a t higher concentrations. The factor for grams of aluminum or aluniinum oxide per milliliter of standard hydrochloric acid is determined by titration of a standard solution of aluminum. The reaction is not stoichiometric; about 3.9 instead of 4.0 moles of hydroxide per mole of aluniinum are released and titrated with p H 10.0 as the start and finish of the titration. The factor is constant over the 0.004- to 0.11-gram aluminum ovide concentration range. Because of differences in commercial pH 10 buffers, a commercial pH 7 buffer was used for all adjustments of p H meters. This buffer checked that of another manufacturer and a laboratoryprepared pH 11.4 buffer. Accurate adjustment of the meter is necessary because pH 10.0 is so close to the pH 9.9 at which insoluble aluminum hydroxide starts to form when aluminum concentration is at the maximum. I n making actual determinations samples often do not contain the maximum aluminum concentration and the pH of hydrolysis decreases from p H 9.9 with decrease in aluminum concentration. With a solution which contains 0.11 gram of aluminum ovide and 0.010 gram or more of ferric oxide, incorrect adjustment of the meter towards a pH less than 10.0 will be indicated by a slow drift of the meter needle above the p H 10.0 reading after the initial adjustment to that reading. If such drifting occurs, the problem can be solved by use of either an accurate buffer or a higher pH setting, such as pH 10.5, for the start and finish of the titration. A Leeds &- Sorthrup line-operated pH meter n-a>used to obtain the results shown in this paper. It was not necessary to make continuous checks of the pH meter, and little or no adjustment was necessary from day to day. At present, a Beckman automatic titrator is in use. APPARATUS A N D REAGENTS
p H meter. The glass electrode should be of a type suitable for use 968
ANALYTICAL CHEMISTRY
,
,
,
1 2 - ,
,
,
,
,
,
NoOH
+ HCI --+
+
NaCI
/
l
,
I,
FREE SODIUM H Y D R O X I D E
I s
Hp0
I
,
,
I2
I
ALUMiNUM
'
No4102
+ 6KF+
2H20 -+K3AIF6
+NoOHt3KOH
lo
-
9 10
pH OF HYDROLYSIS FREE NoOH END POINT
'
1
8
II
I2
I?
(
14
2 '
15
1
8
16
17
M I L L 1 LITERS
Figure 1.
Table
a
18
0
36
37
38
39
40
41
42
9 43
STANDARD HYDROCHLORIC ACID
Titration curves for free sodium hydroxide and aluminum
Effect of Impurities on Results for Aluminum O x i d e
1.
ildded as
Impurity
SiOs
OF
K F ADDED
Na2SiO3 5HpO
MgO Fez03 Ti02
K2Ti0( C p 0 4 )2z H . 20
P206
&PO4
NaF
NaF
ZnO NiO Be0 CrzO3
coo
MnO SnO FeO
SrO
Ce203 Lip0
ZrOp
CUO CdO ("a)zS01
uo3
As206 As203
BaO V205 CrOa
PbO
KO2
B203 CaO
GazOa hfOO3 HC10, in alkaline solution. Standardize with buffer. Magnetic or mechanical stirrer. Sodium hydroxide solution, approximately 25%. Store in polyethylene. Potassium oxalate solution, 100 grams
Alp03 Concn., Gram 0.11 0.044 0.022 0.011 0,oo-L 0.11 0.011 0.11 0.044 0.011 0.004 0.11 0.011 0.11 0.011 0.11 0.011 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11
Appros. Bmt. (Gram) of Impurity That Produced 0.5 N g . of
AlpOaBias 0,0008 0,0017 0,0042 0,0165 0,028 0.006 0,002 0,008 0.021 0.12 0.31 0,017 0.08 0.055 0.138 0.09 1.0 0.0009 0.0011 0.0012 0.0016 0.0025 0,004 0.006 0.006 0.007 0.007 0,009 0.02 0.02 0.025 0,029 0,045 0,075 0.08 0.09 0.10 >o. 15 >o. 15 10.22 0.26 0.27 0.38 >1.0 12.0
of potassium oxalate monohydrate per liter. Store in polyethylene. Potassium fluoride solution, 307, anhydrous potassium fluoride by weight (or 50% potassium fluoride dihydrate). Store in polyethylene. Adjust the solu-
tion with potassium hydroxide solution and hydrochloric acid until 40 ml. of the solution, when added to about 10 ml. of the potassium oxalate solution and about 300 ml. of Tvater that has been adjusted to pH 10.00, will change the p H by less than the equivalent of one drop of 0.2K hydrochloric acid. Hydrochloric acid, about 0.2N. Standardize against a standard aluminum solution. Standard aluminum solution, 4.4 mg. of aluminum oxide per ml. Dissolve, by heating, about 4.65 grams of highpurity aluminum wire in 70 to 80 ml. of concentrated hydrochloric acid and about 150 ml. of water. Make the solution to volume in a 2-liter volumetric flask. Titrate, by the following procedure, aliquots that contain about 0.11 gram of aluminum oxide. Calculate the factor for grams of aluminum oxide (or aluminum) per milliliter of 0.2N hydrochloric acid. GENERAL PROCEDURE
Decomposition. Decompose a 1.0gram sample with a mixture of acids or by a fusion, depending upon the type of sample. Dehydrate the silica, filter and ignite the precipitate, evolve the silica by heating with hydrofluoric acid in the presence of several drops of sulfuric acid, fuse the residue and add it t o the filtrate, and dilute t o an appropriate volume in a volumetric flask. Selection of Aliquot Size. Pipet an aliquot containing 0.004 to 0.11 gram of aluminum oxide into a 600-ml. beaker. If the sample contains more than 0.02 gram of ferric oxide, limit the aluminum oxide concentration t o not more than 0.055 gram. Titration. Add 10 t o 15 ml. of potassium oxalate solution to the beaker containing the aliquot of sample, and dilute to a volume of 200 t o 250 ml. Place the beaker on the titration stand, insert the electrodes of the p H meter, and start the stirrer. Add 25% sodium hydroxide until the pH rises to a t least 11.5. Add hydrochloric acid (1 t o 1) dropwise until the p H falls to about 11. Wash down the electrodes and sides of the beaker to a volume of 300 to 350 ml. Add standard (0.2N) hydrochloric acid until one drop takes the meter needle just below p H 10.0. If too much acid has been added, repeat the addition of 25% sodium hydroxide and the p H adjustment to 10.0. Add 40 ml. of potassium fluoride solution. Titrate n-ith the standard hydrochloric acid until one drop takes the meter needle just below the p H 10.0 mark (to the exact starting pH). Record the volume of hydrochloric acid required for the titration. Calculation. ml. of standard HCl X factor % A1208 '(grams of AI,O~per ml.) x 100 gram of sample EFFECT OF INDIVIDUAL IMPURITIES
Individual tests of impurities were made in which a solution of the im-
purity was added to a standard solution of aluminum chloride. The samples were titrated by the regular procedure. Practical bias limits were arbitrarily set at 0.5 mg. of aluminum oxide.
centrations of aluminum oxide. In most cases, considerably more of the impurity could be tolerated a t the lower aluminum oxide concentrations than at the maximum concentration of 0.11 gram. The limits (Table I) are intended only as a guide.
*
The results of these tests are shown in Table I. Some of the most important impurities were tested a t several con-
ANALYSIS OF SYNTHETIC SOLUTIONS OF ALUMINUM CHLORIDE
Table II. Determination of Aluminum Oxide by pH 10 Method in Aluminum Chloride Solution A1203 Added, Gram 0.11034 0.01100 0.00440 Found 0.11009 0.01104 0.00428 0.11038 O.OllO1 0.00433 0.11038 0.01096 0.00428 0.11043 0.01101 0.00433 0.11038 0,01099 0 00438 0 11038 0 01101 0 00433 Av. 0 11034 0 01100 o 00432 Av. d 3 . 1 0 00000 -0 00008 dev*a Oo0l2 O0O03 0 00004 a
Std* dev* =
dz(x-
N - 1
z)
'I*,where
Solutions of aluminum chloride containing 0.11, 0.011, and 0.004 gram of aluminum oxide per aliquot n-ere prepared. Each solution was titrated six times. The titrations a t the 0.11-gram concentration were used to obtain the factor for grams of aluminum oxide per milliliter of hydrochloric acid. Therefore, the average bias a t this concen-
tration The vias results zero* are in Table 11. The largest difference was -0.25 mg. of aluminum oxide, and the largest standard deviation was 0.12 mg. These results indicated that the method would be applicable for a 25-fold concentration of aluminum
(X is deviation of each determination from average and N is total number of determinations.
OXIDE IN BAUXITE AND RELATED MATERIALS
Six samples of bauxite, Bayer mud,
Table 111.
Determination of Aluminum Oxide in Bauxite, Bayer Mud, Bayer Mud Sinter, and Sinter Mud
Sample No. SRC-6 SRC-1 NBS No.69.4 SRG2 SRC-3
Sample Type Bauxite Bauxite Bauxite Fine Bayer mud Bayer mud sinter (kiln feed) Sinter mud
BY ammonia method 46.9 51.1 55.0 26.8
ot
Average 46.59 50.99 55.14 26.58
Bias -0.31 -0.11 $0.14 -0.22
duplicates 0.19 0.07 0.07 0.06
15.7 15.84 + O . 14 4.31 4.2 $0.11 33.28 33.24 -0.04 Std. devea= 0.00013 gram of AlzOa, = 0.065% AlzOs,on 0.2-gram samples.
SRC-5
Std. dev.
=
-
dg,
where d = differencebetween duplicates and n
=
0.07
0.14 0.19
number of pairs
of duplicates.
Table IV.
Determination of Aluminum Oxide in Miscellaneous Materials AlzOa, % pH 10 Method Sample NBS Range Sample Sample Size, or of No. Type Gram USGSa Av. Bias duplicates 0.04 +o. 11 27b Sibley iron ore 0.70 0.8 0.59 0.20 $0.05 26 1.07 Crescent iron oreb 0 . 8 1.02 Argillaceous la 0.04 +0.17 limestoneb 4.33 0.2 4.16 Burnt (alumina) 78 0.18 -0.35 refractory 69.65 0.1 70.0 0.01 -0.06 Plastic clay 0.2 25.49 98 25.54 0.04 -0.02 99 Soda feldspar 19.04 0.2 19.06 0.08 -0.05 G-1 Granite 14.29 0.2 14.34 0.11 +0.23 15.40 0.2 w-1 Diabaseb 15.17 0.09 +0.01 18.75 18.74 a National Bureau of Standards or United States Geological Survey. * Used preliminary R203separation of aluminum from magnesium.
- -
VOL. 30, NO. 5, MAY 1958
969
Sample No. SRC-6 SRC-1
NBS No. 69A SRC-2 SRC-3 SRC-5 Av.
~
_
Table
V.
_
Sample Type Bauxite Bauxite Bauxite Fine Bayer mud Bayer mud sinter (kiln feed) Sinter mud
DETERMINATION OF ALUMINUM OXIDE IN MISCELLANEOUS MATERIALS
Six alumina-containing samples from the National Bureau of Standards and two from the United States Geological Survey were dissolved by various methods, each of which included the removal of silicon dioxide. In preliminary titrations on the Sibley iron ore, somewhat high results and badly buffered end points were obtained. This was not surprising, because unusually large samples (0.8 gram) were being used and the samples contained 165 times as much ferric oxide as alumi-
ANALYnCAL CHEMISTRY
~ ~
~
~~
_
~~
_ ~ _ _ _ _ _
Determination of Aluminum O x i d e in Presence of Silicon Dioxide
Bayer mud sinter, and sinter mud were selected. One of the samples was National Bureau of Standards bauxite No. 69A; the value for aluminum oxide on this sample is the average obtained by the contributing laboratories. The other five samples were analyzed in three different laboratories for R203, ferric oxide, titanium dioxide, phosphorus pentoxide, zirconium dioxide, vanadium pentoxide, and chromic oxide. The results for aluminum oxide were obtained by difference. For the titration of bauxite No. SRC-6, which contains 20.8% ferric oxide, 0.1-gram samples were used because of the high iron concentration. For the other five samples, aliquots containing 0.2 gram of sample were used. Duplicate aliquots of each sample were titrated. The aluminum oxide results are compared in Table I11 with average results obtained by the ammonia method. The titration results averaged 0.04% lower than those obtained by the ammonia method. The standard deviation was equivalent to 0.065% aluminum oxide on the 0.2-gram samples. The accuracy is comparable to that of the ammonia method, and the precision is better.
970
~
Sample Size, Gram 0.05 0.02
0.05 0.05 0.10 0.20
SiOz Removed pH 10 method, Ammonia av. method 46.59 46.9 50.99 51.1 55.14 55.0 26.58 26.8 15.7 15.84 4.31 4.2 33.24 33.28
num oxide. However, better results and end points were obtained after a preliminary digestion of the sample in a stainless steel beaker a t a light boil and in the presence of excess sodium hydroxide. The limestone, Crescent iron ore, and diabase contained magnesium, which interfered in the titration (see Table I). The aluminum was separated from the magnesium by a single ammonium hydroxide separation. Duplicate aliquots of each sample were titrated. In Table IV results are compared with the average results listed by the Bureau of Standards and by Fairbairn ( 2 ) . The average bias was +O.Ol% aluminum oxide. The largest bias was -0.35%; this was obtained on the alumina refractory. The procedure is not recommended for aluminum in iron ores because of the highly buffered end points. DETERMINATION OF ALUMINUM OXIDE IN PRESENCE OF SILICON DIOXIDE
-4s shown in Table I, limiting the aluminum concentration will permit titrations in the presence of considerable quantities of silicon dioxide. Fusions were made on the six samples of bauxite and,Bayer mud materials. I n each fusion, 6 grams of sodium carbonate and 0.5 gram of anhydrous sodium tetraborate were used for each gram of sample. The melts were dissolved in hydrochloric acid. This treatment appeared to leave all of the silicon in solution. In most cases the amount of sample to be titrated was reduced from 0.2 or 0.1 gram to 0.05 or 0.02 gram to prevent interference from the silicon dioxide, This reduction requires very careful titration and close readings of the buret to achieve accuracy and precision similar to those obtained on
Alz03, % SiOz Present, pH 10 Method Bias from Range R203 and of Av . difF. duplicates 47.19 +0.29 0.38 51.06 -0.04 0.27 55.19 $0.19 0.21 26.40 -0.40 0.11 15.60 -0.10 0.06 3.79 -0.41 0.07 33.20 -0.08 0.18
larger samples. Duplicate results are listed in Table V and compared with those by titration and by the ammonia method on samples from which the silicon dioxide had been separated. The average bias without the removal of silicon dioxide was 0.08% aluminum oxide below the average by ammonia method. The fusion method of sample preparation is much faster than the conventional decomposition, dehydration, filtration, ignition, hydrofluoric acid treatment, and fusion of the residue from the silicon dioxide precipitate. However, the value of this modification is limited by the necessity for smaller aliquots of sample. ACKNOWLEDGMENT
The author wishes to acknowledge the helpful suggestions and verification work contributed by D. W. Utley, L. R. Fortner, H. B. Hartman, L. J. Linder, and the analysts of the Alcoa Research Laboratories a t East St. Louis, Ill. LITERATURE CITED
(1) Bushev, A. H.. ANAL.CHEM.20, 169
(19Z8). Fairbairn, H. W., “Cooperative Investigation of Precision and Accuracy in Chemical, Spectrochemical and Modal Analysis of Silicate Rocks,” U. S. Government Printing Office, Washington, D. C., 1950-51. Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., ‘(Applied Inorganic Analysis,” 2nd ed., p. 84, Wiley,.New York, 1953. Paige, B. E., Elhott, M. C., Rein, J. E., U. S. Atomic Energy Comm. ReBearch and Develop. Rept. IDO14357, 9 (1985). Paulson, R. V., Murphy, J. F., AXAL. CHEM. 2 8 , 1182 (1956). Willard, H. H., Diehl, H., “Advanced Quantitative Analysis,” p. 153, Van Nostrand, New York, 1943. RECEIVEDfor review May 14, 1957. -4ccepted October 25, 1957.
