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ANALYTICAL SCIENCES AUGUST 1995, VOL. 11
689
Notes
Solid-Liquid Extraction with an Ammoniacal EDTA
the Separation of Traces of Copper from Aluminum
Solution for
Masataka HIRAIDE,Yasushi MIKUNI nd Hiroshi KAWAGUCHI
Department of Materials Science and Engineering, Nagoya University,
Nagoya 464, Japan
Keywords Extraction, copper, aluminum, EDTA, graphite-furnace atomic absorption spectrometry
Although modern instrumental determination
methods are highly sensitive and selective, preliminary
separation techniques are often required to improve the
precision and accuracy of analytical results. Solid-
liquid extraction is a useful separation technique: the
desired trace elements are quantitatively extracted from a
solid matrix with a simple 1 3 A metal sam-
ple is first dissolved in a solvent and then evaporated to
dryness to redistribute the trace elements on the surfaces
or in the interstitial spaces of agglomerates of pure matrix
crystals. The trace elements are selectively extracted
into an appropriate solvent for the determination by
instrumental analytical methods. Extraction solvents
used heretofore were mainly organic solvents containing
small amounts of acids.
In a previous work4, we proposed the use of diluted
nitric acid for the multielement extraction of impurities
in high-purity silver metal samples. Extraction with
such aqueous solvents allows the direct combination with
different determination methods, including inductively
coupled plasma-atomic emission spectrometry (ICP-
AES), ICP-mass spectrometry and different electro-
chemical techniques. The present communication de-
scribes the scope of solid-liquid extraction with aqueous
solvents, by taking the separation of copper(II) from an
aluminum matrix as an example. The aluminum was
converted into its hydroxide, through which traces of
copper were extracted with an ammoniacal EDTA
solution. The proposed separation technique has been
successfully applied to the determination of traces of
copper in high-purity aluminum.
Experimental
Apparatus
A Seiko I & E SAS-760 atomic absorption spectro-
meter equipped with an SAS-715 graphite furnace
atomizer was used for the determination of copper. The
graphite tube was gradually heated to 150° C, held for
10 s and then heated during 5 s to 400° C and held for 15 s.
The tube was further heated to the atomization tem-
perature of 2400° C for 2 s for measuring the absorbance
at 324.8 nm.
A Seiko SPS 1100H ICP-atomic emission spec-
trometer was employed for the determination of
aluminum under the following operating conditions:
wavelength 309.27 nm, RF power 1.2 kW; argon flow
rates (dm3 min 1)16, 0.7 and 0.6 for outer, intermediate
and carrier, respectively.
The evaporation apparatus consisted of a Yamato HF
41 heater and an aluminum heating block (12 holes,
25 mm diam.X65 mm depth), where Pyrex glass test
tubes (20 mm inside diam., 24 mm outside diam.,100 mm
height) were inserted to evaporate solvents.
A Tokyo Rikakikai AU-60C ultrasonic cleaning bath
(28 kHz, 210 W) was used for the extraction of copper
from the aluminum matrix. A Hitachi ECV-843 BY
clean bench was used for separation procedures.
Reagents
An aluminum solution (20 mg cm-3) was prepared by
dissolving aluminum chloride hexahydrate in 0.1
mol dm 3hydrochloric acid and purifying the solution by
extraction with APDC and chloroform.5,6 The purifi-
cation was effective because no copper was detected, as
described below.
A standard copper(II) solution (1 µg cm 3, in 0.1 mol
dm-3 hydrochloric acid) was prepared from a commercial
standard solution and diluted with 0.1 mol dm 3 hydro-
chloric acid to appropriate concentrations immediately
before use.
An ammoniacal EDTA solution (0.01 mol dm 3) was
prepared by dissolving EDTA (disodium salt dihydrate)
in 1 mol dm-3 aqueous ammonia.
Water was purified by distillation and ion exchange,
and then passed through a Millipore Milli-Q purification
system. All reagents used were of reagent grade and
were employed without further purification, unless
otherwise stated.
Procedure
A synthetic sample solution (containing 20 mg of
aluminum and nanogram amounts of copper), 2 cm3, was
placed in a Pyrex glass test tube and sealed with a
silicone-rubber stopper bearing two glass tubes. The
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ANALYTICAL SCIENCES AUGUST 1995, VOL. 11
solution was evaporated to dryness by heating at 150°C
for 20 - 30 min. During the evaporation, nitrogen gas
(previously filtered through a 0.1-µm membrane filter)
was introduced into the test tube at a flow rate of
0.5 dm3 min 1 to provide a clean atmosphere and to
sweep out the solvent vapor. The residue was cooled to
room temperature and pulverized with a Teflon rod.
After adding 2 cm3 of ammoniacal EDTA solution, the
test tube was irradiated with ultrasound for 10 min to
extract the copper from the aluminum matrix. The
extraction solvent was separated by centrifugation at
1000g for 15 min and a 10-mm3 aliquot (after dilution, if
necessary) was injected to the graphite cuvette for the
determination of copper by AAS. The measurement
was repeated three times and the absorption readings
were averaged. A calibration graph was prepared by
using ammoniacal EDTA solutions containing nano-
gram quantities of copper.
Results and Discussion
Selection of extraction solvent
Generally, extraction solvents are selected from the
viewpoints of (1) sufficient solubility of the desired trace
elements, (2) minimum solubility of the matrix element,
and (3) no interference in the subsequent determination
step.
EDTA reacts with many elements to form stable and
water-soluble chelate compounds. Large amounts of
aluminum ions, however, generate bulky and amorphous
hydroxide precipitates from moderate alkaline solutions,
even in the presence of EDTA. Therefore, aqueous
ammonia containing EDTA was examined for the
extraction of copper from aluminum.
A sample solution (containing 20 mg of aluminum and
100 ng of copper) was evaporated to dryness and then the
residue was treated with the extraction solvent. With
0.01 mol dm-3 EDTA in 1 mol dm 3 aqueous ammonia,
the copper was quantitatively extracted with a recovery
of 94 -100%, while the dissolved aluminum was less than
4%.
The EDTA was essential for the complete extraction.
With aqueous ammonia alone, no copper was extracted
because the copper was strongly trapped in the flocculent
aluminum hydroxide.
Effect of ultrasonic irradiation on extraction
Because the application of a sound field accelerated the
extraction rates2'3, he effect of ultrasound was studied by
changing the irradiation time. As shown in Table 1,
almost constant and complete recoveries were obtained
by applying ultrasound for 3 - 30 min. This indicates
that the ammoniacal EDTA solution is a powerful
solvent, because the irradiation of 30 - 60 min was
usually required in the previous studies.2,3
Separation and determination of copper in aluminum solutions
Different amounts of copper ions were added to 1 cm3
of aluminum solution, and evaporation, extraction and
determination were carried out. The copper added was
nearly completely recovered, as shown in Fig. 1.
The aluminum accompanying the copper was
determined by ICP-AES and found to be 600 - 700 µg;
this amount did not interfere in the subsequent
determination of copper.
Analysis ofhigh purity aluminum metal
The proposed method was applied to the analysis of
high-purity aluminum metal. A sample of commercial
metal was dissolved in aqua regia and diluted to 20 cm3
with water. An aliquot of the solution (containing
20 mg of aluminum) was placed in a test tube; the
separation and determination were carried out as de-
scribed in Procedure.
Table 2 summarizes the results obtained for two
samples: No. 1 (99.999% purity, 3 mm diam.X10 mm
long rods, Mitsuwa Pure Chemicals) and No. 2 (99.99%
purity, l OX2OX mm chips, Nakarai Chemicals). Cop-
per added previously to the sample was quantitatively
recovered and successfully determined. The relative
standard deviations were 4.9% (for No. 1) and 0.8% (for
No. 2). Blank values through the whole procedure were
less than the detection limits (
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(e.g., Co, Ni,
copper.
Zn, Cd) are expected to behave similarly to
References
1. A. Mizuike, Enrichment Techniques for Inorganic Trace
Analysis , p. 52, Springer, Berlin, 1983.
2. A. Mizuike, K. Fukuda and Y. Ochiai, Talanta, 19, 527
(1972).
3. M. Hiraide, Y. Mikuni and H. Kawaguchi, Analyst
[London], 119, 1451 (1994).
4. M. Hiraide, Y. Mikuni and H. Kawaguchi, Fresenius' J.
Anal. Chem., in press.
5. E. B. Sandell and H. Onishi, Photometric Determination
of Traces of Metals: General Aspects , p. 529, Wiley, New
York, 1978.
6. 0. G. Koch and G. A. Koch-Dedic, Handbuch der
Spurenanalyse (Tell 1) , p. 308, Springer, Berlin, 1974.
(Received April 7, 1995)
(Accepted May 24, 1995)
Table 2
metal
Determination of copper in high-purity aluminum
a.
b.
50 ng of Cu was added.
100 ng of Cu was added.