Determination of Aluminum in Thorium Compounds by Linear-Sweep Oscillographic Polarography T. M. FLORENCE Australian Atomic Energy Commission Research Establishment, h a s Heights, N.S. W., Australia
b A method is presented for the determination of traces of aluminum in thorium compounds b y oscillographic polarography using the dye Superchrome Garnet Y (5-sulfo-2’,4’,2trihydroxyazobenzene). A separation of aluminum is avoided by masking thorium with acetate, and of 35 ions investigated, only Fe+’, Fe+31 Ni+’, Pb+2, and V+s caused significant interference. After a rapid mercury-cathode electrolysis to remove many heavy metals, the method is highly selective for aluminum. The limit of detection is 1 pg. of aluminum per gram of thorium with a relative standard deviation of *370 a t an aluminum concentration of 400 p.p.m., Results and =t7% at 10 p.p.m. obtained on various thorium compounds showed good agreement between the polarographic method and a spectrophotometric method using 8quinolinol.
T
HE DETERMISATIOK Of traces O f aluminum in thorium compounds is usually carried out spectrographically or spectrophotometrically \\-ith S-quinolinol. Spectrographic procedures using a carrier distillation technique are sensitive to only 5 to 10 p.p.m of aluminum, and the precision a t this level is very poor ( I ) . lIargerum, Sprain, and Banks (11) extracted aluminum quinolinate with chloroform from a 6M ammonium acetate solution at p H 4.7 and measured the absorbance of the extract at 385 mp Thorium is complexed by acetate and is not extracted. The interference of iron is eliminated by complexing u ith 1,lO-phmanthroline, and Cu, Zn, Co and Ni may be removed by u aqhing the chloroform extract with an alkaline cyanide solution. Howei er, zirromum, frequently present in thorium compounds, is not masked by the acetate buffer and causes errors. The alkaline CJ ariide solutio11 is unstable and must be prepared daily h more recent 8-quiiiolinol method (IO) utilizes a preliminary separation of thorium, zirconium, and other metals by extraction with thenoyltrifluoroacetone (T.T.A.)in hexone Aluminum is then determined in the aqueous phase by a procedure similar to that of Mar-
496
ANALYTICAL CHEMISTRY
gerum, Sprain, and Banks. -4lthough this method is selective for aluminum, i t is too time consuming to be considered for routine use. The polarographic determination of aluminum by di-o-hydroxyazo dyes was reported by Willard and Dean (12) in 1950, but did not receive the attention it deserves. Recent work in these laboratories has established a theoretical basis for the behavior of several di-ohydroxyazo dyes a t the dropping mercury electrode (6, 8, 9). I n analytical methods using these dyes linears n eep oscillographic (cathode ray) polarography possesses several unique advantages over conventional polarography (7’). I n contrast to the oscillographic currrnts of metal ions, those of di-o-hydroxyazo dye complexes are enhanced by an “adsorption-reduction” process ( 6 ) and have a sharply-peaked naveforni (Figure l ) , which is maintained even a t extreme dilutions, greatly facilitating measurement and resolution. The ultimate sensitivity for aluminum is normally limited only by base electrolyte impurity, and under optimum conditions, 0.2 p.p.b. of aluminum in solution may be detected, and 10 p.p.b. may be measured with
a precision of *57, ( 5 ) . This paper describes a simple method for estimating traces of aluminum in thorium compounds using the azo dye, Superchrome Garnet Y (5-sulfo-2‘,4’,2-trihydroxyazobenzene, C.I. 168) in a n acetate buffer of pH 5.75. S o separation of thorium is necessary, but heavy metal impurities are removed by a rapid mercury-cathode electrolysis after which the method is virtually specific for aluminum. Although an overnight standing period is necessary, actual operator time is less than 2 hours for duplicate determinations. EXPERIMENTAL
Apparatus. A Southern Instruments Computer Division (Surrey, England) KlOOO linear-seep cathode ray polarograph ( 3 ) is used. To allow full use of the inherent sensitivity of this instrument by reducing vibration, a n Ag/AgCl wire anode n ound around the dropping mercury electrode (DALE.) is preferable to a mercury pool. An antivibration mounting for the electrode stand may also be necessary. Unless othermise stated, all measurements were made a t 25.0’ =t 0.2’ C., and the D.M.E. characteristics
Solution composition: 4 X 1 O-6M S.G.Y., 0.1 5 2 Gg. of AI per ml., 4.00 mg. of Th per ml., 0.32M ammonium acetate, p H 5.75 The wave a t - 0.80 volt is due to iron impurity in the base electrolyte
were ~n = 1.14 mg. per second, t = 6.50 seconds (-0.6 volt us. Ag/AgCl). The small constant-current mercury cathode cell has been described previously (4). A11 dassware should be acid-rinsed before-use, and a laboratory with a filtered air supply is preferable. Reagents. The sodium salt of Superchroxne Garnet Y (S.G.Y.) was prepared according to Cooncy ( 2 ) , and purified by three crystallizations from ethyl alcohol. Elemental analyses and potentiometric titration with XaOH, indicated that the purity was 9 i =k as CI2H9Y2O6SNa.2HzO (11.lV. = 368). The ammonium acetate reagent (4.OOJf) was prepnred by dissolving 57.1 grams of amnionium acetate and 0.85 gram of SH4C1 in water and diluting to 250 nil. The batch of ammonium acrtate u s d should be specially selected for a low aluminum rontent. Further purification may be achieved, if necessary, by s l o ~ l ypassing the reagent solution through a 5-ml. column of DowexA1 chelating resin, rvhich reduces aluminum to less than 0.05 pg, per ml. Recommended Procedure. Keigh a sample (0.2 to 15 p g . of Al) containing no more than 200 mg. of thorium into a small platinum dish. Add 2 ml. of 407, H2F2,5 ml. of HSOs, and 2 ml. of Z?YGHClO,. Evaporate just t o dryness under an infrared lamp. If the sample is a highly-sintered thorium oxide, a second treatment 17 ith HZFz niay be necessary to complete dissolution, Dissolve the residue in a little water, add 2 nil. of I-IC104, and again evaporatc t o dryness t o ensure removal of fluoride. Allotv the dish to cool, dissolve the residue in 10.00 nil. of 0.2V &SO4> added from a pipet. Immediately transfer the solution to the dry mercury-cathode electrolysis cell and c~lectrolyze a t 0.5 amp. (9 volts) for 20 to 30 minutes. Remove the air condenser. and with current still floming, \vithdraw about 7 nil. of t h e solution. Transfer to a small, dry, stoppwed flask and cool to room tcmperature. Pipet a 5-nil. aliquot into a 50-ml. beaker, add 2.00 ml. of the ammonium acetate reagent and 5.00 ml. of 2.00 x 10-4X S.G.Y. Dilute to about 20 ml. and adjust the p H to Dilute 5.75 =t0.05 using 23' ",OH. to 25 nil. in a volumetric flask and allow to stand a t room temperature for a t least 5 hours, but preferably overnight. Transfer about 5 ml. of the solution to a polarographic cell, deaerate for 5 t o 10 minuteq and measure the height of the peak a t -0.63 volt ZJ.S. Ag'AgC1 using a start potential of -0.50 volt. The peak height varies with the position of the trace on the cathode ray screen, and the start potential should be adjusted so that the peak occurs a t 0.130 i 0.005 volt on the screen, which is the optimum position for maximum sensitivity and resolution from the free dye wave. A blank determination must be carried through the whole procedure, and its value must be subtracted from that of the sample. Correct the net peak current for thorium depression,
050
025j
0 16
o
032 TOTAL
KETATE
48
COKEMRATICU
I om
064 (M)
Figure 2. Effect of acetate concentration on the aluminum S.G.Y. peak current Solution composition:
4 X 1 O-5M S.G.Y., 0.1 4 5 pg. of AI per ml., p H
5.75
0
Th-free
0
0.80 mg. of Th/ml.
and using a calibration factor, calculate t h e aluminum content of the sample. Choice of Dye and Optimum ConIn addition t o Supercentration. chrome Garnet Y , t h e following dyes were investigated : Pontachrome Violet SIT (5-sulfo-2-hydroxybenzeneazo-2-naphthol, C.I. 169), Eriochrome Black P V (5-sulfo-2-hydroxybenzene-
azo-4,8-dihydrouynaphthalene, C.I. 170), and Solochrome Dark Blue (4sulfo 2,2' - dihydroxyazonaphthalene, C.I. 202). However, Pontachrome Violet S'w was subject to more interferences than S.G.Y., and the other tn o dyes were less sensitive. The concentration of S.G.Y. required was rather critical. With a final dye concentration of only 2 X 10-5M, the aluminum-peak current calibration curve was nonlinear in the presence of thorium, b u t with 8 X 10-5N dye, the residual thorium wave a t -0.68 volt became inconveniently large. A S.G.Y. concentration of 4 X 10-5df gave calibration curves linear to 0.30 pg, of A1 per ml. in the final solution, and the thorium wave was small enough not to interfere (Figure 1). Masking of Thorium. In acetatefree solutions, thorium reacts with S.G.Y. n i t h a sensitivity similar t o t h a t of aluminum a n d produces a n oscillographic peak a t -0.68 volt (pH 6). This wave is readily removed by t h e addition of acetate, 0.32M ammonium acetate being t h e minimum concentration necessary t o mask 100 mg. of thorium. Increasing amounts of acetate tend t o reduce t h e
-
A A
2 . 4 0 mg. of Th/ml. 4 . 0 0 mg. of Th/mI.
height of t h e aluminum peak also, particularly a t relatively high thorium concentrations (Figure 2 ) . Optimum pH. The pH-peak current curves all show a well defined maximum at pH 5.75 (Figure 3). It is apparent t h a t the p H must be carefully controlled, especially in solutions containing higher amounts of thorium. Equilibration Rate and Effect of Temperature. At 25' C., S.G.Y. reacts almost immediately with alumi-
I
I
450
5 00
5 5J
62
2 0
pn
Figure 3. Effect of pH on the aluminum4.G.Y. peak current Solution composition: 4 X 10-W S.G.Y., 0.1 45 p g . Al/ml. 0.32M ammonium acetate
0 Th-free
0.80 mg. of Th/ml.
A 2 . 4 0 mg. of Th/ml. A 4.00 mg. of Th/ml.
VOL 34, NO, 4, APRIL
1962
497
num in thorium-free solutions, b u t the reaction rate is decreased in the presence of thorium (Figure 4). Equilibrium may be attained by heating the final solutions t o 60' C. for 10 minutes, b u t a t temperatures above 35' C., thorium begins t o precipitate. Although this represents a precipitation from homogeneous solution, some aluminum was found to be co-precipitated, leading to low results. Solutions allowed to stand a t room temperature (20' to 25" C.) remained clear for a t least 24 hours. The temperature coefficient of the aluminum wave was +1.5% per O C. in the range of 20" to 30' C. Calibration Curves and Sensitivity. Aluminum concentration-peak current calibration curves were linear to 0.30 pg. of A1 per ml. in the final solutions with a S.G.Y. concentration of 4 X 10-5M and thorium no greater than 4 mg. per ml. Higher amounts of thorium led to nonlinear calibration curves. Figures 2 and 3 show that thorium causes a slight depression of the aluminum peak current. A calibration curve for thorium-free solutions had a slope of 7.40 pa. per pg. of A1 per ml., compared a-ith slopes of 7.35, 7.20, and 7.05 pa. per pg. of A1 per ml. for solutions containing 0.80, 2.40, and 4.00 mg. of Th per ml., respectively. As mentioned previously, the ultimate sensitivity attainable is limited mainly by the reagent blank aluminum
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figure. By carefully selecting batches of acids and ammonium acetate with low aluminum contents and using conductivity water throughout, a blank of 0.29 =t0.03 pg. of A1 was obtained using the recommended procedure. This places the limit of detection of the method a t about 1 p.p.m. of A1 in Th, using a 200-mg. sample. Study of Interferences. T h e effect of diverse ions was studied both in thorium-free solutions and in solutions containing 20 mg. and 100 mg. of thorium (Table I). Cobalt, iron, and nickel gave sharp peaks a t -0.75,
Per Cent Error in Solutions Containing 20-100 mg. Th/25 ml.
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