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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.