Fluorometric Determination of Aluminum Use of 8-Quinolinol EDW 4RD GOOX, JOSEPH E. PETLEYI, WARREN H. M c l l U L L E N 2 , ~ N STEPHES D E. W-IBERLEY Rensselaer Polytechnic Institute, Troy, S. Y . Solution B, 2 % 8-Quinolinol in 1 N Acetic Acid. Twenty grams of (hlallinckrodt, reagent grade) 8-quinolinol were dissolved in a total volume of 1 liter of 1 A' acetic acid (60 ml. of glacial acetic acid diluted t o 1 liter). Solution C, Standard Aluminum Solution. Reagent grade aluminum nitrate [AI( XO,)3.9H?OIr 13.9 grams, was dissolved in water, made up t o 1 liter, and standardized by ammonia precipitation and ignition to the oxide. Solution*contained 1.000 mg. of aluminum-per milliliter. Solution D. One hundred milliliters of t8he above solution were pipetted into a 1-liter volumetric flask and diluted to the mark. An aliquot of 100 ml. of this solution ivas then taken and diluted to the mark in another 1-liter flask. An aliquot of 200 ml. of this second dilution was taken and diluted to 1 liter to yield a final solution containing 2 micrograms of aluminum per milliliter. Solution E, Standard Quinine Sulfate Solution. Quinine sulfate (O.lOOO,gram,C.S.P. 3Iallinckrodt) was dissolved and diluted t o 1 liter with 1 S sulfuric acid. One hundred milliliters of this solution were pipetted into a 1-liter volunietric flask and diluted to the mark with 1 S sulfuric acid. This procedure was repeated to yield a solution containing 1 microgram of quinine sulfate per milliliter. Chloroform, Reagent Grade. Klett Fluorimeter Model 2070. This instrument was eauiooed with glase cuvettes (approximately 14 by 54 by 57 mA.j.* A Corning filter S o . 5970 (which transmits approximately 8 0 5 of the 365 mp mercury triplet) was used to isolate the ultraviolet radiation, and a combination glass filter, which cuts off a t 410 m p and transmits over 80% in the visible region above 450 mp, was used to isolate the green fluorescence produced by the aluminum complex in chloroform. This filter combination n as selected by measuring the approximate wave length of the fluorescent band using a Hilger quartz spectrograph and panchromatic plates. Beckman pH Meter, Model M.
The chloroform solution of aluminum %hydroxyquinolate has been successfully used for the colorimetric determination of aluminum. As the aluminum complex also fluoresces, this investigation was undertaken to see if the fluorescence was reproducible enough for a quantitative method. .A sensitive, quantitative fluorometric method for aluminum was successfully developed. The fluorescence does not vary critically with time, as is the case with other fluorescent methods for aluminum, nor with the amount of the reagent used. This method has advantages over a similar colorimetric method in that a smaller aliquot may be used, simplifying the separation procedure, and that in amounts equal to the aluminum concentration, iron, titanium, and vanadium do not seriously interfere.
T
H E quantitative determination of aluminum by fluorescence
m x s initially accomplished by K h i t e and Lowe (9) with
the reagent, morin. Davydov and Devekki ( 1 ) found this method unsuitable because the reaction proceeds within narrow pH limits and depends greatly upon the amount of coloring agent added. These authors developed a fluorescent method with the aid of quercetin. Weissler and K h i t e ( 7 ) then shelved that the dyestuff Pontachrome Blue Black R was superior to quercetin in sensitivity, accuracy, and freedom from interferences. The fluorescence of the aluminum salt of 8-quinolinol (8hydroxyquinoline) in chloroform for the quantitative determination of aluminum has been suggested by several investigators (3, 8). Tullo et al. (6) have developed a quantitative fluorometric method for aluminum in beer. Grimaldi and Levine (4)have developed a quantitative fluorometric method for aluminum in phosphate rock, Killard and Horton (11) reported that the fluorescence of the extracts of aluminum 8-quinolinolate followed a modified Beer's lax. Sandell ( 5 ) has shown that the 8-quinolinol complexes of gallium and indium mav be evtractcd into chloroform for a quantitative fluorometric determination. Feigl ( 2 ) in his study of the chemical behavior of some metalloquinolinolates demonstrated that a fluorescent test can be used to detect qualitatively the presence of aluminum in paper, and also investigated the cause of quenching of metal quinolinolates in chloroform by nonfluorescent metal quinolinolates. .4 recent paper ( I O ) by one of the authors was concerned mith the colorimetric determination of aluminum in steel using the aluminum salt of 8-quinolinol in chloroform solution. In this method both titanium and vanadium 8-quinolinolates yield absorption bands in chloroform solution which overlap the band of the aluminum complex and seriously interfere. A4qualitative test showed that only the aluminum complex fluoresced, and the following investigation m-as undertaken to see whether this fluorescence could be the basis of a quantitative method for the determination of aluminum in steel.
PROCEDURE FOR DETERVIN.ATION OF 4LUMINU.11
Preparation of Calibration Curve. T-sing 250-ml. separatory funnels, samples containing 0, 2, 4,6, 8, 10, 12, 14,and 16 micrograms of aluminum, respectively, in a volume of approximately 100 ml. were treated with 2 ml. of solution B (2% 8-quinolinol), 2 ml. of buffer solution A, adjusted t o a pH of 8.0 Z'C 1.5, and extracted with two 15 t o 20-ml. portions of chloroform. The chloroform layers were drawn off through 9-em. S o . 40 Khatman filter paper into 50-ml. borosilicate glass volumetric flasks and diluted to the mark with chloroform. The Klett fluorimeter v a s standardized against a quinine sulfate solution containing 1 microgram of quinine sulfate per milliliter by setting the potentiometer scale a t 200 and then turning the slit adjustment knob until the galvanometer T a s at a convenient null point. The chloroiorm
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$ 160 0 a W
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REAGENTS Ah-D EQUIPMENT
Solution A, Buffer Solution. Two hundred grams of reagent grade ammonium acetate and 70 ml. of concentrated ammonium hydroxide (approximately 15 M) were dissolved in a total volume of 1 liter. 1
*
-.
0
4
MICROGRAMS
Present address, General Electric Corp., Pittsrield, Mass. Present address, Singer Sewing Machine Co., Xewark, N . J.
8
OF ALUMINUM CHC L,
12
PER 50
16
ML
Figure 1. Calibration Curve for Aluminum Determination
608
609
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 solutions were then measured by bringing the galvanometer to the .game null point with the potentiometer dial and reading the potentiometer scale. Figure 1 shows the values obtained corrected for the blank. The plot is linear to 12 micrograms with a slight tapering off a t higher aluminum concentrations. I n the linear portion of the graph, 1microgram of aluminum in 50 ml. of chloroform gives a scale change of 20 divisions. The minimum concentration which can be determined reliably and distinguished from a blank is 0.2 microgram of aluminum in 50 ml. of chloroform. The sensitivity of this method is therefore equal to but no better than that of the Pontachrome Blue Black R method developed by Weissler and White ( 7 ) .
MICROGRAMS OF INTERFERING E L E M E N T PER 5 0 M L CHCL,
Figure 2.
Table I.
Effect of Interferences on Aluminum Recovered
..inalysis of Bureau of Standards Samples
Sample Designation
BS-63 (phosphor bronze) BS-125 (high-silicon steel) BS-lO6A(chromium-niolyhdenum-aluminum steel)
Individual results 0.046, 0.260, 0.246, 0.280. 0,260,
0.051, 0.271, 0.275, 0 269. 0.259
Aluminum. % Average result 0.041 0.267
0.235
Accepted value
0.046 0 259
0 0.5 0 261
1.09
1 07
0.272
1.07, 1.16, 1.09 1.05, 1.06, 1.09
Application of Samples. The sample of steel or bronze is weighed to 1 gram to the nearest milligram and dissolved in an acid mixture of 10 ml. of water, 10 ml. of concentrated nit,ric acid, and 20 ml. of 72% perchloric acid using 400-ml. Vycor beakers. The sample is heated until dense fumes of perchloric acid appear; it is then cooled, diluted to 200 nil., and filtered into 250-ml. or 2000-ml. borosilicate glass volumetric flasks depending upon the suspected aluminum content. A suitable aliquot containing 2 to 10 micrograms of aluminum is taken, diluted if necessary with 0.5 S sulfuric acid to approximately 50 ml., and electrolyzed in a water-jacketed mercury cathode cell according to the procedure described in reference ( I O ) . -4fter electrolysis, the solution is removed from the cell, treated x i t h 2 ml. of solutions B and -4,respectively, adjusted to a pH of 8.0 + 1.5 with 6 -\i ammonium hydroxide, transferred to a 250-1x11. separatory funnel, and estracted and measured according to the procedure described under preparation of t,he calibrat,ion curve. A blank should br carried through the entire procedure n i t h each duplicate determination. Analytical Results. Two Bureau of Standards steels of high aluminum content, and one bronze of lo^ aluminum content were analyzed. The results are tabulated in Table I. The total time required for analysis of duplicate samples is about, 2 hours, n-hich is faster than the conventional gravimetric procedure. The results shown in Table I give an indication of
the accuracy of the method. These results compare favorably with those obtained with the fluorometric method of Weissler and White ( 7 ) . EFFECT OF VARIABLES
Effect of pH. A series of nine solutions each containing 10 micrograms of aluminum in about 100 ml. of water was treated with 2 ml. of 2% 8-quinolinol (solution B), 2 ml. of buffer solution A, and sufficient acetic acid or ammonium hydroxide to fall within a p H range of 2 to 10 a t pH intervals of approximately 1.0. These solutions were extracted with two 15- to 20-ml. portions of chloroform diluted in volumetric flasks to 50 ml. and measured on the fluorimeter. The extraction was complete over the pH range from 6.5 to 10.0. If the pH range is controlled a t 8.0 f. 1.5, there is still allowance for small errors in pH adjustment without affecting the complete recovery of the aluminum. Effect of Standing. A solution containing 20 micrograms of aluminum R as extracted according to the standard procedure previously described except that the final dilution was made to 100 ml. This solution was divided into four 25-ml. portions. One mas measured as rapidly as possible, and the other three after standing 4, 8, and 24 hours, respectively. The scale readings obtained were 195, 197, 194, and 194 showing that the maximum fluorescence is immediately developed and does not increase or decrease with standing over a 24hour period. However, if the same solution is left in the fluorimeter cell and readings are made over a period of 1 hour with continuous exposure to ultraviolet light, there is a gradual decrease in fluorescence as shown by the data in Table 11. Since the time required for a measurement is less than 1 minute, this effect is of little concern in an analytical procedure. Evidently the aluminum complex of 8-quinolinol in chloroform is stable to visible light but unstable to intense ultraviolet radiation. Effect of Amount of Reagent. Solutions containing 10 micrograms of aluminum, varying amounts of 8-quinolinol, and 2 ml. of buffer solution -4were extracted a t a pH of 8.0 f. 1.5 and the fluorescence m*as measured. The intensity of the fluorescenre varied inversely with the amount of reagent added. Since the greater sensitivity was obtained using 2 ml. of 0.2% 8-quinolinol, this amount of reagent \\as tested by having six different investigators totally inexperienced with the procedure measure solutions containing 2, 4, and 8 micrograms of aluminum. The percentage reliability of the mean for these determinations was 8.8, 6.7, and 8.0%, respectively. Two milliliters of 2% 8quinolinol were also tested in this same manner since this amount of reagent had yielded more reproducible results in the colorimetric procedure ( I O ) . The percentage reliability of the mean of sivteen determinations of solutions containing 4, 8, and 10 micrograms of aluminum was 2.1, 2.1, and l.O%, respectively. Although the sensitivity x a s approaimately 25% less using this amount of reagent, the greater precision ohtained xvarranted itadoption in the standard procedure. Effect of Temperature. -4solution containing 20 micrograms
Table 11. Effect of Ultraviolet Light on a Chloroforni Solution of the Aluminum Salt of 8-Quinolinol Time, llinutes
Fluorimeter Scale Reading
Time, Minute,
Fluorimeter Scale Reading
610
ANALYTICAL CHEMISTRY
of aluminum was carried through the standard extraction procedure, but not through the mercury cathode electrolysis, except that the final dilution was made to 100 ml. The solution was divided into four 25-m1. portions. The fluorescence of the solutions was measured a t temperatures ranging from 10’ to 40’ C. There was a slight decrease in fluorescence as the temperature increased but when a correction was applied for the volume change of the chloroform solution, the decrease was insignificant. Effect of Anions. Solutions containing 0.5 gram of anion as the ammonium salt and 10 micrograms of aluminum were carried through the standard extraction procedure. The anions studied were the acetate, chloride, citrate, nitrate, perchlorate, sulfate, and tartrate. Only the citrate and tartrate caused interference, the recovery of aluminum being only 25% in the presence of the citrate and 82% in the presence of tartrate. I n the presence of the other anions the recovery of aluminum varied from 93 to 100%. Effect of Cations. Iron, titanium, and vanadium were investigated since these cations are most likely to be involved in steel analysis. Iion is quantitatively removed from aluminum using a mercury cathode cell, but the amount that can be tolerated without serious error is of interest since iron is the main constituent in steel. Titanium and vanadium both remain with the aluminum and hence constitute a more serious problem. Three separate series containing 10 micrograms of aluminum and 0, 10, 20, 40, and 60 micrograms, respectively, of iron, titanium, or vanadium were carried through the standard extraction procedure but not through the mercury cathode electrolysis. The
results are shown in Figure 2. 111 of the cations mentioned quench the fluorescence. For a 1 to 1 weight ratio of titanium, vanadium, or iron t o aluminum, respectively, 100, 89, and 83% recovery of aluminum is obtained. For a 6 to 1 weight ratio of interfering element to aluminum, about 65% recovery of aluminum waa found in all three cases. Since the amounts of titanium and vanadium in steel often do not exceed the aluminurn concentration, the interference from these elements is lees serious than that encountered in the colorimetric method (IO). LITERATURE CITED
(1) Davydov, A. L., and Devekki, V. S., Zauodskaya Lab., 10, 134-8 (1941). (2) Fiegl, F., and Heisig, G. B., Anal. Chim. Acta, 3, 561-6 (1949). (3) Gentry, C. H. R., and Sherrington, L. G,, Analyst, 71, 432-8 (1946). (4) Grimaldi, F. S., and Levine, H., U. 8. Geol. Survey, Trace Elements Investigations, Rept. 60 (1950). ( 6 ) Sandell, E. B., IND.ENG.CHEM.,ANAL.ED., 13, 844-5 (1941). (6) Tullo, J. W., Stringer, W.J., and Harrison, G. A. F., Analyst, 74, 296 (1949). (7) TVeissler, A . , and White, C. E., IND.ENG.CHEM.,AN.AL.ED.,18, 530-4 (1946). (8) Welcher, F. J., “Organic .hialytical Reagents,” Vol. 1, p. 270, New York. D. Van Xostrand Co.. 1948. (9) White, C. E., and Lowe, C. S., IND.ENG.CHEW, AXAL.E D , 12, 229-31 (1940). (10) Wiberley, S. E., and Bassett, L. G , ANAL.CHEM.,21, 609-12 (1949). (11) Willard, H. H., and Horton, C. .1.,Ibid., 24, 862-5 (1952) RECEIVED for review February 26, 1951. -4ccepted J a n u a r y 15, 1953
Gravimetric Methods for Zinc and Its Separation from Certain Elements Use of a Radioisotope in Evaluation of Analytical Procedures JOHN E. VANCE
AND
RICHARD E. BORUP, R‘ew York University, iVew York, N. Y .
I
T IS useful in the evaluation of a gravimetric procedure to
have information on the amount of an element remaining in solution in addition to the usual comparison between the weight of a precipitate obtained and the known amount of an element taken in any experiment. In the past, the amount of an element escaping precipitation has been estimated directly only infrequently because of the tedium involved. The use of radioisotopes makes the determination of the unprecipitated element a relatively simple problem. Apart from the additional information which may be obtained through their use, the simplification which radioisotopes bring to a systematic study of analytical methods makes more attractive than formerly the critical comparison of gravimetric procedures on which the choice of standard or referee procedures can be based. Similarly, useful studies can be made on separations and on coprecipitation phenomena. Zinc was chosen for an initial study of this type because of its general importance. The Zne6 isotope has decay characteristics which make its determination straightforward since it has a half life of about 250 days and since gamma rays of 1.11m.e.v. energy are a part of its decav scheme (15). The classical phosphate and sulfide precipitations, three precipitations with organic reagents and several separations of zinc from elements with which it is commonly encountered, were examined. MATERIALS AND METHODS
The method of studying each procedure was the same in outline. -4stork solution of zinc sulfate, containing a quantity of
zinc-65 together with stable zinc, was prepared and analyzed gravimetrically in several ways and the activity of the solution was determined; thus there \\-as obtained a relation between counts per minute and total weight of zinc. I n the course of the experiments, two such standard solutions were used; the initial activity of each corresponded to about 1000 counts per minute per milligram. The activity of the solution in use was measured repeatedly and the current value of the specific activity was applied to the experiments in progress a t that particular time. The stable zinc was a distilled product of the Xew Jersey Zinc Co. reported to have a purity of 99.999%. Two sources of zinc-65 were used, one obtained from the Oak Ridge National Laboratory in the form of dust (from which possible traces of members of the hydrogen sulfide group n-ere removed) and the other, a sample of the distilled zinc which was irradiated a t Oak Ridge. Samples of the standardized stock solution were measured from a weight buret and treated according to the procedure being investigated. It was estimated that less than 5 microcuries of zinc-65 were present in all but a few of the experiments. I n most cases, the precipitates were filtered on porcelain (Selas) filter crucibles of about 25-1111. rapacity and were heated to constant weight. The filtrates and washings were collected directly in volumetric flasks and after dilution to volume were set aside for the counting measurements. A conventional scaler was used with an RCL Mark 1, Model 30.4 gamma counter tube to measure the zinc-65, and therefore the total zinc, in the filtrates. Several counter tubes were used
