Arrnine D. Paul and John A. Gibson, Jr. West Virginia University Morgantown
I
I
Qualitative Analysis for Aluminum
For a number of years, we and our colleagues in this and other institutions have experienced difficulty in grading qualitative analysis unknowns with respect to Al+3. Using the classical procedures, students report Al+a in almost all unnknowns, even though no appreciable concentration of this ion was initially present in the samples. I n the work reported here, the amount of A1+3 ktroduced in the procedure has been determined by quantitative methods, and a method for testing for AIC3 on the original solution is presented. In most qualitative analysis schemes (I-&), Al+=is precipitated in the (NH&S group. After treatment of the precipitate with hydrochloric acid, the Cr+3, and Zn+2 are separated from Mn+2and Fe+3by addition of N&Oa or Hz02to a solution strongly alkaline with NaOH. Most of the procedures then specify that the reaction mixture be boiled in a test tube or casserole for varying periods of time. Separation of A1(OH)s-S and Zn(OH),-2from Cr0,-2is accomplished by neutralization of the alkaline solution and precipitation of AI+a with NHIOH. Confirmation of Al+3isbased on the formation of a red lake with alnminon in buffered solution. Qualitative tests indicated that the chief source of introduction of Al+3in the above procedure occurred in the step which involved boiling the alkaline solution with N%Oz or H2O2. Quantitative experiments were therefore carried out according to the following pro--?I ~tiUUlC.
A 6 M solution of NaOH was prepared by dissolving the solid in distilled water. Samples (25 ml) of this solution were placed in Coors porcelain casseroles or Pyrex beakers, brought to the boiling point, and boiled for ten min. The solutions were then neutralized with HCI using litmus, and the Al+$ precipitated with 8-hydroxyquinoline (5). After filtering and washing, the precipitate was dissolved in 2 M HC1 and titrated with 0.1 M KBr03 in the presence of KBr (6). The experiments were repeated using NaOH to which had been added !mown quantities of NazOz or H20z. In some experiments the solutions were boiled to dryness, which required about a 15-min boiling period. Each experiment was performed in a t least two different me~ nr n n l . ~ ~ l n iVn ~ P ~ S -PJ--.. -.r" In all experiments, considerable SiOz was extracted. This was not removed before precipitation with 8-hydroxyquinoline. A separate experiment, in which NazSi03 was dissolved in water, acidified, boiled, treated with 8-hydroxyquinoline, filtered, washed, and titrated, showed that adsorption of &hydroxyquinoliue by SiOpwas negligible. The results of these studies are shown in the table. ~h~ amount of A1+3obtained is indeed appreciable, and far X lo-' M , which, to Hammett and Sottery (6), is the minimum amount of AlC3 .uu--..A
398
/ lourno1 o f Chemical Educafion
which will react with aluminon. The data show that more Al+ais extracted from porcelain than from Pyrex, and that boiling to dryness greatly increases the amount extracted. Additional experiments not included in the table indicate that the amount extracted in the first five min of boiling is only about 10% of that obtained after boiline for 10 miu.
-
Moles per Liter of Alf3 Extracted from Porcelain and Pyrex Vessels -Al +~moles/liter)--. Porcelain
25 m l 6 M NaOH boiled 10 rnin 2.5 g 25 m l 6 M NaOH Nan09 boiled 10 min 25 m l 6 M NaOH 3 ml30% H20. bailed 10 min 25 m16 M NaOH boiled to dryness 2.5 g 25 m16 M NaOH N%02boiled to dryness
+ + +
Pyrex
0.0034.014 0.002940031 0.001t0.011 0.00150.W21 0.014-0.015
0.0005-O.Wl
0.0260.029
0.0094.01
0.02
0.00874.0093
As might be expected, the reproducibility of results was poor. The amount of Al+a obtained varied from container to container, and depended on whether the vessel was new or used. The time required to reach the boiling point was not controlled, and the actual boiling time probably varied by 10%. Because of these variables, it is difficultto interpret the effect of addition of Na202or HzOzon the amount of Al+3extracted. These ex~erimentshave shown the necessitv for testing for Al+3on the original sample. Three reagents were studied from this standpoint: (1) Aluminon (7, 8); (2) Alizarin Red S or sodium alizarin sulfonate which also forms a red lake with Al(OH), (9-11); (3) Morin (3,5,7,2',4'-pentahydroxy flavanol), whose alcohol solution reacts with aluminum salts to give a green fluorescence (9, 12-14). The morin test was found to be the most satisfactory and gave the most reliable results in student hands. Aluminon and Alizarin Red S were far too sensitive. The acetic acid required in the test with Alizarin Red S often contained sufficient AI+3from storage in soft glass bottles to give some red color. Both aluminon and Alizarin Red S seemed to give as good or better tests with 10W3M A P 3 as with 0.1 M A1+S,which makes interpretation difficult. The following procedure for testing for Al+3 using moriu has been used by the 250 students who have taken our sophomore course in qualitative analysis during the past two years; it has been found satisfactory. Treat 5 drops of the original sample with 8 drops of 3 M NaOH and 2 drops of 1 M KgCOa. Centrifuge and discard the precipitate. Moisten a piece of filter pitper with an alcoholic solution of morin. Acidify a drop of the above decantate with HCrHsO?and place on the paper. Observe the spot under ultraviolet light. A yellow green fluorescence, which,beeomes more.
blue-green on moistening with dilute HCI, confirms the presence of ALf< If Alt3 is absent, any original fluorescence will be quenched on treatment with HC1.
A saturated solution of morin in ethanol is used, which requires about 1.5 g morin per liter of ethanol. The reagent can be purchased from chemical supply houses for $3.40 per kg. The fluorescence is reportedly due to formation of a colloidally dispersed inner complex aluminum salt or to an adsorption compound of morin with aluminum (9). The fluorescence cannot be satisfactorily observed in daylight. However an inexpensive ultraviolet source can be obtained by replacing the fluorescent tubes in a desk lamp with black light tubes. If the lamp is partially enclosed in a cardboard box and placed in a dark corner of the laboratory, satisfactory results are obtained. Treatment with KzCOa and NaOH removes Mn+2, Ag+, Fe+3, and Cu+=which interfere by oxidizing the reagent. Pb+?, Be+l, Zn+2, and Sn+2will fluoresce in basic solution but not in acid. Zr+4,Gaf3, InCS,S C + ~ , Y+: Cef4, Sb+8, and Th+&are reported ( I t ) to fluoresce in acid solution, hut these ions can all be removed by the NaOH-K&03 treatment. The anions P04c3,As04-3, and F- are reported (IS) to decrease the fluorescence. The NaOH should be kept in a polyethylene container.
The above procedure will detect A1+3 when present in concentrations greater than 10W3M. The method has the further advantage of introducing beginning students to a fluorescence method of analysis. Literature Cited (1) EVANS,W. L., GARRETT,A. B., A N D SISLER,H. H., "Semimicro Qualitative Analpis," Ginn & Co., Boston, 1957, pp. 71, 72. (2) SORUM,C. H., "Introduction to Semimicro Qualitative Andlwis," Prentice-Hall, Inc., Englewood Cliffs, N. J., 1953; pp. 102-11. (3) KING, E. J., "Quditstive Analysis and Electrolytic Solutions," Hareourt, Brace & Co., New York, 1959, p. 462. (4) NORDMAN, J., "Qualitative Testing and Inorganic Chemistry," John Wiley & Sons, Inc., New York, 1957, pp. 301, 302. (5) KOLTHOFF, I. M., AND SANDELL, E. B., "Textbook of Quantitative Inorganic Analysis," The Maemillan Co., New York, 1948, pp. 327, 638. (6) . . HAMMEIT. . L... AND SOTTERY,C., J. Am. Chem. Soe., 47, 142 (1925). G., Anal. Chirn. Aeta, 1,221 (1947). (7) CHARLOT, ( 8 ) YOE,J. M., AND HILL, W. L.,J. Am. Chem. Soc., 49,2395 (1927). (9) FEIGL, F., "Spot Tests in Inorganic Analysis," 5th ed., Ekevier Publishing Co., Amsterdam, 1958, pp. 182-88. (10) ATACK,F. W., J . Sot. Chem. Ind., 34, 936 (1935). P., Mikrochemie, 7,221 (1928). (11) HELLER,K., AND KRUMHOLZ, G., Anal. Chim. Acta, 1,231 (1947). (12) CHARLOT, (13) WHITE,C. E., AND LOWE,C. S., Anal. C h a . , 9,430 (19373. E. B., Anal. Chem., 12, 762 (1940). (14) SANDELL,
Volume 39, Number 8, August 1962
/
399
