Light Rare Earths with Arsenazo-DBS as a Chelator
Yuan Fuzhen
(Department of Chemistry, Yantai University Yantai 264005, shandong China)
Abstract A direct spectrophotometric method for the determination of the total light rare earths has been developed. In this method, arsenazo-DBS is used as a chelating agent with light rare earth elements in strong acidic medium (0.04-0.48 mol l-1 of acidity). The concentrations of total rare earths in 0-15 μg /(25 ml) range can be determined accurately by this method. An absorption maximum was observed at 630 nm at which a molar absorptivity of 1.14x105 l mol-1 cm-1 was determined. The method offers high selectivity and good sensitivity towards light rare earths and features simplicity and rapidity in operation. It has been applied to the determination of light rare earths in cast iron and Ni-Fe alloys.
Introduction
A literature search showed that several spectrophotometric methods [1-20] have been used for the determination of total amount of light rare earths and the concentration of single rare earth element. Most of the techniques require a long analytic cycle and involve complex preparations. As a result, the use of these methods is rather unproductive. From a screening test for complexing agents arsenazo-DBS was found to complex with light rare earth, forming a chromophore active in UV-visible spectral region. A direct spectrophotometric method was developed based on this finding. The method can be used to determine the total light rare earths, including La, Ce , Pr , Nd and Sm .
Arsenazo-DBS is a color reagent, with structural formula of 3-(2-arsonophenylazo)-6-(2.6-dibromo-4-sulphophenylazo)-4.5-dihydroxy-2.7-naphthalenedisulfonic acid [19]. In strong acidic medium (0.04-0.48 mol l-1 of hydrochloric or sulfuric acid), the color complex with light rare earths are highly stable and easy to quantify. The total light rare earths in high content Ni (8.0 mg) and Cr (3.0 mg) solutions can be determined directly by the present method. The satisfactory results are obtained with this method in determination of the total amount of light rare earths in Ni-Fe alloy and alkali resisting cast iron with high Ni content at 630 nm.
Experimental
Reagents
All reagents and chemicals used were of analytical grade and distilled water was used throughout this study, unless otherwise mentioned. Cerium (Ce4+) standard solution containing 1μg ml-1 Ce4+ was prepared by conventional method [3,12,19] using spectra grade CeO2. Analytical grade hydrochloric acid (4 mol l-1), oxalic acid (10% m/v), perchloric acid (11.6 mol l-1), aqua regia and 0.05% arsenazo-DBS aqueous solution were used as received.
Standard spectral curve
Absorption spectra were recorded and absorbance measured with a VIS-7230 digital spectrophotometer. A standard 1.0 cm path-length quartz cell was used. To measure the standard spectral curve, seven standard solutions of Ce4+ were prepared by adding 0.00, 0.50, 1.00, 2.00, 3.00, 4.00, 5.00 ml in seven 25 ml calibrated flasks respectively, then adding 2 ml of hydrochloric acid, 6 ml of oxalic acid and 1.5 ml of arsenazo-DBS solution successively, diluting these flasks to the 25 ml volume with distilled water. The standard solutions were measured at 630 nm against a reagent blank in 1.0 cm path-length cells.
Experimental method
Weighed exactly an alloy sample containing 10-50 μg of total amount of light rare earths (about 0.1-0.5 g of alloy sample), heated and dissolved it with 5 ml of aqua regia in a beaker, then added to 2.0 ml of HCIO4, until it smoked and boiled dry almost. After it cooled slightly, added to a little distilled water to dissolve salts, dropped in 1-2 drops of Na2SO3 for reducing Cr+6 to Cr+3, boiled about 1 minute, then cooled it again, removed it into a 50 ml flask, diluted it to volume with distilled water , shake and dry filtering were carried out. 5 ml of the filter aliquor was drawn into another 25 ml standard flask, then follow the procedure of standard spectral curve, to measure its absorbance.
Result and discussion
Absorption curve
To minimize the baseline signal of reagent blank, the absorbance at 630 nm was measured for quantitative analysis. As shown in Fig. 1, the absorbance increased linearly with Ce4+ concentration, i.e. Beer’s law was followed. This finding suggests that the quantitative analysis can be performed at this wavelength (630 nm). From the slope of the linear regression line, a molar absorptivity of 1.14x105 l mol-1 cm-1 was determined. The calibration equation is given by
A=0.0322C + 0.0010
where A is absorbance and the unit of concentration C is μg/(25 ml).
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| Fig.1 Standard absorbance spectrum of Ce4+ |
A typical absorbance spectrum of arsenazo-DBS complex in aqueous solution is shown in Fig. 2. One absorption maxium at λmax=630 nm was observed.
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| Fig.2 Absorption spectra of (Ⅰ) Ce-Arsenazo-DBS vs reagent blank; (Ⅱ) Reagent vs water blank |
Effect of acidity on absorbance A
In 0.04-0.48 mol l-1 of HCI or H2SO4 solutions, the absorbance A of the solution remained relatively constant as seen in Fig. 3. In the subsequent measurements, 2 ml hydrochloric acid (0.32 mol l-1 of colourated acidity) was added to 25 ml of sample solutions to minimize possible interference by diverse ions.
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| Fig.3 Absorbance A vs acidity |
Effect of arsenazo- DBS
The effect of arsenazo-DBS on the absorbance values were shown in Fig. 4. The results indicated that 2 ml of arsenazo-DBS solution is sufficient to complex 10 μg of Ce4+ with an absorbance stablized at 2 ml up to 4 ml of arsenazo- DBS addition. For the reliable measurement, therefore, a slight excess of arsenazo-DBS is beneficial.
Effect of oxalic acid
Oxalic acid is considered as a masking agent for metal ions other than light rare earths. Our measurements indicated that for a 25 ml solution (containing 50 mg of alloy sample), 4 ml of oxalic acid was sufficient to eliminate the interference of non-rare earth ions in the solution. The absorbance values were constant over a oxalic acid concentration range of 4-8 ml. Therefore, 6 ml of oxalic acid addition was recommended.
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| Fig.4 Effect of Arsenaze-DBS on absorance |
Effect of colouration time
The results in Table 1 show that the arsenazo-DBS-rare earth complex was stable over a two hour period. A higher sensitivity was observed at room temperatures.
Table 1 Effect of colouration time(added to 40 μg of Ce4+, 3 ml of arsenazo-DBS, 0.32 mol l-1 of HCI, λmax at 630 nm, 1.0 cm path-length cells, room temperature) |
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Effect of Ni and Cr ions
As shown in Table 2, the presence of Ni2+ and Cr3+ below 8.0 and 3.0 mg, respectively, does not interfere the determination of light rare earths. Cr6+, on the other hand, was found to oxidize arsenazo-DBS readily, interfering with the determination. It is therefore necessary to add, prior to arsenazo-DBS addition, dropped in 1-2 drops of Na2SO3 solutions (10%) for reducing Cr6+ to Cr3+.
Table 2 Effect of Ni and Cr ions |
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Applications
The present method has been applied to the determination of total amount of light rare earths in two different samples. Seven measurements were carried out for each sample. The average value (%), standard deviation SD, relative standard deviation RSD along with the value of Ni content indicated on the two samples of Ni-Fe alloy were summarized in Table 3.
Table 3 Two Determination of the total amount of light rare earths samples of Ni-Fe alloy |
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| * Mean of seven measurements |
The method using arsenazo-DBS as a colour reagent requires no extraction. It offers a rapid, sensitive and specific determination for total light rare earths.
Acknowledgment
The author is thankful to the Materials Laboratory of Physics-Chemistry Examination Center of Shandong Province for a generous supply of the alkali resisting cast iron with high Ni content and Ni-Fe alloys two samples.
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Received 9 June.1999，Revised 9 Sep.1999