Sensitive Spectrophotometric Determination of Lanthanum 2 - J-Stage

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National University of San Luis, P. ... The reagent 2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol (3 .... 4 Stoichiometry of the complex in the La(III)-3,5-.
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Sensitive Spectrophotometric Determination of Lanthanum 2-(3,5-Dichloro-2-pyridylazo)-5-dimethylaminophenol Liliana

FERNANDEZ

and

Roberto

with

OLSINA

Department of Analytical Chemistry, Faculty of Chemistry, Biochemistry and Pharmacy, National Universityof San Luis, P. 0. Box 375, 5700-San Luis, Argentina

The reagent 2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol (3,5-diC1DMPAP) has been synthesized and its analytical properties investigated. This reagent can be used for the spectrophotometric determination of La(III) in concentrations ranging from 0.045 to 0.72 ppm. The reaction takes place at a pH between 9 and 10.8. In the presence of Triton X-100 this complex is soluble in water (~=1.45X1051 moM cm-1). The maximun tolerances for cations as well as for any anions were determined. In order to overcome difficulties caused by the presence of other lanthanides, an ion exchange chromatographic technique was used. Keywords

Lanthanum,

spectrophotometric

determination

Our interest in the analytical evaluation of azo compounds and their applications to the quantitative evaluation of ions belonging to the rare earths is basically due to the increasing economic importance to these elements and the need for safe and rapid methods for their separation and determination. Though at present there are some azo compounds that can be used in the spectrophotometric determination of rare earths'-9, the sensitivities so far reported, in general, are not sufficiently high when samples of geochemical interest must be analyzed. The reagent used in this work has already been used in the determination of cobalt in steel samples10, where it was necessary to introduce liquid-liquid type extraction in order to avoid interference. In the present work, we propose the use of 2-(3,5dichloro-2-pyridylazo)-5-dimethylaminophenol (3,5-diClDMPAR, RH) for the spectrophotometric determination of La(III).

, 2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol

A solution of the purified reagent (3X10-3mol 1-') was prepared and then dissolved in ethanol. From this, solutions of lower concentration were prepared. La(III) standard solution. A 1 mg ml-' solution of La(III) was prepared from La(N03)3 of certified purity, by dissolving it in redistilled water. Normalization was carried out as described in ref. 11. Triton X-100 solution. A 5%(v/v) surfactant solution in redistilled water was prepared. Buffer solution. A 0.1 mol 1-1 sodium tetraborate solution was prepared, obtaining the desired pH by addition of dilute NaOH solution. All reagents used were of analytical grade. Results and Discussion

Apparatus A Varian spectrophotometer (model 634 UV-Visible) with 10 mm-optical path glass cells was used to perform the absorptiometric measurements. The pH values were measured with an Orton 701-A pH-meter equipped with a glass combined electrode. The X-ray fluorescence spectra were taken in a Philips PW 1400 spectrometer.

Acid base behavior of the reagent The values of the constants corresponding to the protonation of the pyridine ring, the protonation of the aminotertiary group and the dissociation of the phenolic hydroxyl group were determined by an absorptiometric method.12 They were pKa,=-1.19, pKa2=1.67 and pKafl 1.50 (to 25°C, ,u=0.1 mol l-', 15% ethanol). In order to complete this study, the constants were evaluated in different surfactant concentrations.13 The binding constant between the 3,5-diC1DMPAP and the surfactant Triton X-100 was 6.1X1031mol-' at 25° C, according to the model of Berezin.14

Reagents 3,5-diCIDMPAP standard solution. The reagent 3,5diC1DMPAP was prepared in our laboratory following the common techniques of synthesis and purification.

Formation of the La(III) complex Different solvents were tried so as to select the one producing the best result regarding solubility. An increase in the organic solvent percentage has always

Experimental

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Fig.

3

Stoichiometry

diC1DMPAP

system.

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La(III)-3,5pH=9.8; 2%

acetone. Fig.

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9.2X106

Fig.

2

Influence

of the

surfactant

mol L'; CRH=2.76

Influence

of the pH.

concentration.

CLa(lll)-

mol 1-'; pH=10.0.

Cu,(lil)=9.2X 10-6 mol 1-'; C R H-

1. lox 10-4 mol 1-' ; 0.4% surfactant.

produced a loss of sensitivity. It was therefore agreed to introduce a surfactant into the reaction medium in order to solve the solubility difficulties. The surfactant, Triton X-100, proved to be most suitable for our system, in a proportion of 0.4%(v/ v). Figure 1 shows the influence of the surfactant percentage upon the absorbance of the La(III)-3,5-diClDMPAP complex. Our experiments enable us to locate the optimal pH range for complex formation. The results obtained are shown in Fig. 2. The value of log Kf was also calculated, being 3.07 for the La(III)-3,5-diC1DMPAP-Triton X-100 system. In these experiments the ionic strength remained constant at 0.1 mol 1-' and the reagent excess was 12:1 with respect to the metal ion. Complex stoichiometry A) The stoichiometric study was carried out in an La(III)-3,5-diC1DMPAP system without any addition

of Triton X-100. The experiments were performed in a 2% acetone-water medium, at pH=9.8, adjusted with a borax buffer; the ratio La(III)/ 3,5-diC1DMPAP was varied from 0 to 1.5. In all cases, the absorbance reading was performed in the reagent absorbance maximum after centrifuging for 10 min in order to separate any precipitate formed. Figure 3 shows the results obtained, from which it can be inferred that the complex under study presents a stoichiometry of 2 :1 (3,5-diC1DMPAP : La(III)). This result is supported by the fact that under normal conditions the complex can only be partially extracted by organic solvents. Verification of the complex charge. In order to confirm that the La(HI) complex has a positive charge, electrophoresis was performed. The potential difference used was 200 V, the solvent was 50% t-butylalcohol, the pH was 9.8 (sodium tetraborate buffer), and the excess of the reagent with respect to La(III) was 20:1. After 30 min it was observed that the complex had migrated approximately 2.5 cm from the origin line of the negative pole, while the spot corresponding to the reagent, alone, remained unaltered. It could thus be verified that the complex has a positive charge, and that its possible formula can be interpreted in the following way: [(3,5-diClDMPAP)2Laj. B) The stoichiometry in the La(III)-diC1DMPAPTriton X-100 complex was determined. This was achieved by using the Yoe-Jones methods in the presence of a borax buffer of pH=9.8 and 0.4% surfactant. The concentration of La(III) remained at 1.84X10-6mol 1-I and the reagent concentration was varied from 0.7X 10-6to 21.6X10-6mol 1-I. Figure 4 shows the results obtained. The stoichiometry in this case is 3:1 (3,5-diC1DMPAP-La(III)), which is in agreement with results reported by other authorsls"6, who attribute this difference to a modification of the capacity for coordination of the metal ion in a micellar medium. Reagent excess effect Trials were carried out in order to determine the

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Table 1 Experimental conditions for determination thanum(III) with 3,5-diC1DMPAP

Fig. 4 Stoichiometry of the complex in the La(III)-3,5diC1DMPAP-Triton X-100 system. CLa(111)=1.84X10-6mol 1-'; pH=9.8; 0.4% surfactant.

Table

2

elements

Comparative with

chart

of determination

of lan-

of rare-earth

azo-compounds

I, Arsenazo M; II, 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-

Fig. 5 Reagent surfactant.

excess

effect,

pH=9.8;

µ=0.1

mol 1-'; 0.4%

optimal reagent-metal ion relation, while keeping the protonic concentration at a fixed value by the addition of sodium tetraborate (pH=9.8). The surfactant concentration remained constant at 0.4%(v/v), as well as the ionic strength, which remained at 0.1 mol l-1. Results are presented in Fig. 5. Beer's law of the La(III)-3,5-diCIDMPAP Triton X-100 complex Table 1 summarizes the optimal experimental conditions for the quantitative evaluation of the La(III) ion with reagent 3,5-diC1DMPAP. A comparative chart of the different spectrophotometric methods used in the determination of various rare earths is also presented (Table 2). Interferences The tolerance of the method developed was determined with respect to a group of anions. Those ions that

phenol; III, l-(2-pyridylazo)-2-naphtol; IV, Carboxyarsenazo & Chlorophosphonazo; V, p-Cetylarsenazo derivated; VI, Arsenazo III. a. Spectrophotometry. b. Extraction-spectrophotometry. c. Spectrophotometry with surfactants.

Table

3

Interference

a. Interferent

of anions

: La(III).

could be present after a sample dissolution were especially considered (Table 3). The anions C2042-, P043-, F- and EDTA produce serious interferences because they form very stable

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complexes with La(III). With respect to the cations, it can be said that, considering an error lower than 2%, the ions Na(I), Mg(II), Ca(II), Sr(II) and K(I) can be present in a relation 50:1 with respect to La(III). The ions Be(II), Ba(II), B(III), Ce(IV), As(III) and Cr(III) have a maximum tolerance of 10:1. The ions Fe(III), Mn(II), Mo(IV), Ni(II) and Co(II) interfere seriously. Most of the elements belonging to the group of rare earths react with 3,5-diC1DMPAP, resulting (with this method) in serious interference with the determination of La(III); this makes it indispensable to use a separative technique beforehand. Separation of rare earth mixtures In order to apply the method developed for the determination of La(III) to a mixture of rare earths, ion exchange chromatography was used to obtain, separation of the constituents. From a bibliography on the subject, the technique proposed by Edge27 to separate mixtures of yttrium, neodymium and lanthanum was considered to be highly convenient. In practice, though, synthetic samples of the ions mentioned of different composition were used. In the first phase, HN03 0.08 mol L'-80% (v/v) ethanol was used as an eluent. In this way Y(III) and Nd(III) could be eluted from the column. Later, H2O was used to elute La(III). The elution of the three different ions was checked by X-ray fluorescence, using a preconcentration technique. The fraction containing La(III) was preconcentrated and then taken to a final volume of 50 ml. The spectrophotometric determination was carried out on an aliquot of this fraction with the reagent 3,5-diC1DMPAP using the above-mentioned method. The results were highly satisfactory, obtaining in all cases an error less than 1%, as can be seen in Table 4. In conclusion, tiometric method

Table

4

among the advantages of the absorpdeveloped here, it should be mention-

Determination

of rare-earths

s, standard

deviation.

CL, confidence

level.

of lanthanum(III)

of ternary mixtures

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ed that the reagent used is relatively simple to synthesize, easy to purify and of high stability in solution. It is also important to note that when the chelation reaction takes place, the bathochromic effect is marked. Finally, the determinative method proposed is rapid and of excellent sensitivity. The authors wish to thank for their financial support.

CONICET,

UNSL and SECyT

References

1. S. Sangal, Microchem. J, 8, 304 (1964); 9, 9 (1970). 2. N. Sentyurina, Zh. Anal. Khim., 17, 442 (1962). 3. S. Savvin, R. Propistsova and R. Strel'nikova, Zh. Anal. Khim., 24, 31 (1969). 4. B. Budesinsky and K. Haas, Anal. Chem., 210, 263 (1965). 5. S. Savvin, T. Petrova and P. Romanov, Talanta, 19, 1437 (1972). 6. N. Perrisic, A. Muk and V. Canic, Anal. Chem., 45, 798 (1973). 7. J. 0' Laughlin and D. Jensen, Talanta, 17, 329 (1970). 8. T. Taketatsu, M. Kaneko and N. Kono, Talanta, 21, 87 (1974). 9. B. Budesinsky and B. Menclova, Talanta, 14, 688 (1967). 10. M. Nakamura, Y. Sakanashi, H. Chikushi, F. Kai, S. Sato, T. Sato and S. Uchikawa, Talanta, 34, 369 (1987). 11. K. Marczenko, "Spectrophotometric Determination of Elements", John Wiley & Sons, New York, 1976. 12. E. King, "Equilibrium Properties of Electrolyte Solutions", Vol. 4, Pergamon Press, Oxford, 1965. 13. L. Fernandez and R. Olsina, in preparation. 14. "Ordered Media in Chemical Separations", ed. W. Hinze and D. Amstrong, ACS Symposium Series, Washington, D. C., 1987. 15. J. M. Rao and D. Satyanaraya, J. Indian Chem. Soc., 57, 1 132 (1980). 16. J. M. Rao, D. Satyansaya and A. Umesh, Bull. Chem. Soc. Jpn., 52, 588 (1979). 17. X. Wang, Huaxue Shiji, 23, 362 (1982). 18. B. Kuznik, J. Inorg. Nucl. Chem., 43, 3363 (1981). 19. Vm. Sivanova and N. Batasheva, Chem. Abstr., 101, 32502k. 20. X. Yu, R. Cai and H. Liang, Wuhan Daxue Xuebao, Ziran Kexueban,1, 105 (1984). 21. S. Savvin, Talanta, 8, 673 (1961). 22. L. Budanova and S. Pinaeva, Zh. Anal. Khim., 20, 320 (1965). 23. A. Cherkesova and N. Alykov, Zh. Anal. Khim., 20, 1312 (1965). 24. P. Spitsvn and V. Sharev, Zh. Anal. Khim., 25, 1503 (1970). 25. S. Shibata, Anal. Chim. Acta, 28, 388 (1963). 26. J. Hernandez Mendez, B. Moreno Cordero and L. Perez Pavon, Talanta, 35, 293 (1988). 27. R. Edge, J. Chromatogr., 5, 526 (1961). (Received September 18, 1989) (Accepted November 27, 1989)