5,7-dichloro-8-quinolinol complex

Analytica Chimica Acta, 184 (1986) 317-322 Elsevier Science Publishers B.V., Amsterdam -Printed

in The Netherlands

Short Communication

SPECTROPHOTOMETRIC DETERMINATION OF TIN( IV) BY EXTRACTION OF THE TERNARY TIN/IODIDE/5,7-DICHLORO-8QUINOLINOL COMPLEX

A. M. GUTIERREZ*, Departamento Complutense,

M. V. LAORDEN

de Q&mica Analitica, 28040-Madrid (Spain)

and A. SANZ-MEDELa

Facultad

Ciencias Qufmicas,

Universidad

J. L. NIETO Znstituto Estructura de la Materia, Consejo Superior Serrano 119, 28006-Madrid (Spain)

de Znvestigaciones

Cientificas,

(Received 12th November 1985)

Summary. The characteristics of the ternary complexes formed by fluoride, bromide, iodide and thiocyanate with tin(IV) and 5,7-dichloro-8-quinolinol (HCQ) have been studied. A spectrophotometric method is proposed for the determination of tin(IV) (l-20 pg ml-‘), based on the extraction into chloroform of the tin/iodide/HCQ complex. Aluminium, bismuth and lead do not interfere, but antimony, copper, iron(II1) and fluoride do. Copper and iron can be eliminated by preliminary extraction in the absence of iodide.

One of the major problems encountered in the spectrophotometric determination of tin is that preseparation of the tin is usually needed. Reagents commonly used are catechol violet [l-3] and quercetin [4], the tin first being extracted as tin(IV) iodide into toluene or cyclohexane [ 5,6] . Previous work [7,8] showed that tin(IV) can be extracted into chloroform by formation of a ternary complex with 5,7-dichloro-8-quinolinol (HCQ) and chloride ion. The extraction was from a strongly acidic medium, which ensured good selectivity. It was thought, however, that by using halide ions less electronegative than chloride, a ternary complex with increased molar absorptivity could be extracted, thus allowing a more sensitive spectrophotometric determination of tin whilst maintaining the selectivity obtained when chloride was used. For this reason, and in order to round off the previous studies [7, 81, an extractive-spectrophotometric study of the complexes formed by tin(IV) with HCQ and bromide, iodide and thiocyanate, as well as fluoride, was initiated. The iodide complex showed the highest molar absorptivity, and its stoichiometry and optimal conditions for its formation were examined.

*Present address: Departamento Oviedo, Oviedo, Spain. 0003-2670/86/$03.50

Qulmica Analftica,

Facultad

de Ciencias, Universidad

o 1986 Elsevier Science Publishers B.V.

318

As a result, an improved extractive-spectrophotometric proposed.

method for tin is

Experimental Appamtus and solutions. A Beckman DK-2A spectrophotometer with l-cm glass cells, a Metrohm E-516 pH meter with glass and saturated calomel reference electrodes, and loo-ml separating funnels were used. All chemicals were of analytical-reagent grade. Standard tin(IV) solutions were freshly prepared by dilution of a 1000 pg ml-’ stock solution obtained by dissolving tin metal in sulphuric acid [7]. The tin content was checked gravimetrically [8]. The HCQ solution, 1% (w/v) was in chloroform. All halide solutions were 1.0 M, except for the potassium iodide, which was 0.5 M. Procedure. Pipette a portion of the sample solution containing 25-200 pg of tin into a separating funnel, add sufficient perchloric acid (to ensure a 1 M final concentration in the aqueous phase) and 1 ml of the halide solution. Adjust the final volume to ca. 10 ml with distilled water. Add 10 ml of the HCQ solution with gentle manual shaking, allow to stand for 5-10 min, and extract the tin by shaking the funnel for 4 min. Allow the phases to separate, filter the chloroform solution through a dry Whatman No. 1 filter paper and measure the absorbance at 401 nm against a blank prepared in the same way but free from added tin( IV). Results and discussion Formation of the ternary complexes. Mixed ligand ternary complexes of tin(IV) with HCQ and bromide, iodide or thiocyanate were formed in acidic media; the ternary fluoride complex was not, probably because of the high stability of the hexafluorostannate(IV) ion. The spectral characteristics of the complexes are shown in Table 1. The molar absorptivity decreased in the order I- > Br- > SCN- > Cl-. The shapes of the spectra of all the complexes were very similar to that obtained with iodide (Fig. 1). The influence of the anion concentration on the concentration of complex extracted from 1 M perchloric acid is shown in Fig. 2. For iodide and bromide, the concentration extracted is nearly independent of the halide TABLE 1 Spectral characteristics

of tin(IV)/anion/HCQ

complexes

Anion

hmax (nm)

Molar absorptivity ( lo4 1 mol-’ cm-‘)

ClBrISCN-

403 402 401 400

0.47 0.61 0.83 0.51

319

‘1,fi, 350

coo

450

500

h.mn

Fig. 1. Absorption spectra: (A) tin(IV)/iodide/HCQ complex against reagent blank (10 rg Sn ml-‘); (B) reagent blank against chloroform as reference.

Ii/T --_I A 390

A LO1 nm

I

II If

-3

-2

-1

0

'

log 1x1

[HCI04]

M-

-pH

Fig. 2. Influence of anion concentration on the extraction of the tin(IV)/X/HCQ complex. Anion (X-): (I) Cl-; (II) SCN-; (III) Br-; (IV) I- (measured at h,, for each complex, 10 rg Sn ml-‘). Fig. 3. Effects of perchloric acid concentration and pH on the extraction of the tin(IV)/ iodide/HCQ complex (10 pg Sn ml-l). Reference solution: (0) chloroform; (0) reagent blank.

concentration over most of the range studied, but for thiocyanate and chloride, extraction is maximum only in the range 0.05-l M anion. These data confirm that the iodide complex should provide the most sensitive determination of tin, which would be least affected by variation of the anion concentration. This system was selected for further work. Effect of experimental conditions on the extraction. Four mineral acids (perchloric, sulphuric, nitric and hydrochloric) were used to acidify the aqueous phase. Hydrochloric acid was unsuitable because the blank absorbance was too high. This was because ion pairs of the type H,CQ+Cl- [9],

“In

320

extractable into chloroform, were formed. Nitric acid nitrates some of the HCQ, giving coloured species in both phases. The use of perchloric or sulphuric acids presented no special problems, the former being slightly advantageous because perchlorates are generally more soluble than sulphates, thus decreasing the risk of formation of precipitates when real samples are analyzed. The effect of perchloric acid concentration on the extraction of the tin/ iodide/HCQ complex was studied in the range 0.5-5 M. As shown by Fig. 3, final concentrations of 0.5-1.2 M perchloric acid in the aqueous phase produced constant absorbance so a perchloric acid concentration of 1 M is recommended. In >3 M perchloric acid, the constant absorbance observed is due to formation and extraction of iodine. As iodine is also formed in the blank, measurement of absorbance against the reagent blank (represented by the dotted line in Fig. 3) shows the effect of acidity on complex extraction which is similar to that observed for the ternary chloride complex [ 71. The pH of the aqueous phase was varied in the range 0.3-9.0. Its influence on the extent of extraction is also shown in Fig. 3. As the pH increases the absorption maximum shifts to shorter wavelengths (Table 2). Above pH 2, the absorption spectrum of the extracted species changes to the typical spectrum of the complex extracted in the absence of halide, i.e., SnO(CQ), HCQ [ 81, showing that with increasing pH, iodide ions are gradually replaced by hydroxide. Maximum absorbance was achieved with 1% (w/v) HCQ in the organic phase for 10 pg Sn(IV) ml-‘. A larger excess of HCQ increases the blank absorbance without increasing the net signal. A manual shaking time of 4 min was required to obtain maximum absorbance; the extract obtained exhibited constant absorbance for at least 24 h. The stoichiometry of the extracted species was studied by Job’s method of continuous variations extended to a two-phase system [lo] and the mole ratio method [ 111. Both gave tin/iodide/HCQ ratios of 1: 2: 2. Analytical performance. The extracted complex obeyed Beer’s law for l-20 pg Sn ml-’ in aqueous solution. The apparent molar absorptivity at 401 nm was (0.83 f 0.01) X lo4 1 mol-’ cm-‘. The relative standard deviation as evaluated from six determinations of 100 c(g of tin(IV) was 1.2%.

TABLE 2 Effect of pH on the absorption maximum and absorbance complex (10 fig Sn ml-’ in aqueous solution) PH

A,,

0.3 1.0 1.5 2.1 3.1

401 401 400 395 390

(nm)

of the tin(IV)/iodide/HCQ

Abs. at h,,

PH

hmax (nm)

Abs. at A,,

0.70 0.74 0.78 0.87 1.02

3.5 4.0 6.0 7.0

390 390 390 390

1.10 1.12 1.12 1.12

321 TABLE 3 Tolerance limits for various ions in the determination

of tin (10 pg ml-‘)

Ion added

Tolerance limit Ion/Sn(IV) (w/w)

Al”’ Pb2+ Bi3+ MnZ+,Mg2+ Znz+, Ni2+, Co’+ Sb(V), Sb(III), Fe3+, Cul+ Nitrate, sulphate Tartrate Fluoride, EDTA

100 5000s 500 20 7 Interfereb 5000s 20 Interfereb

sMaximum ratio tested. bIndicates significant interference at a 1 :l ratio.

Tin (10 pg ml-‘) was determined by the recommended procedure in the presence of possible interfering ions. The results are shown in Table 3. Aluminium, bismuth and lead are tolerated even if present in great excess. However, copper, antimony, iron(III), fluoride and EDTA seriously interfered. The copper and iron(II1) interferences could be eliminated by their preextraction with HCQ from 1 M perchloric acid in the absence of iodide, as described above. Once these cations had been removed, iodide was added and the tin was extracted into chloroform as before. The selectivity of the proposed method (Table 3) is similar to that of the less sensitive method based on chloride [7], except for the effect of antimony, which is greater in the present method, and the possibility of oxidation of iodide. The method was tested by analyzing a certified lead-based sample supplied by the Instituto de1 Hierro y el Acero (CENIM-CSIC, Spain). The sample was dissolved with a nitric/tart&c acid mixture [ 121, and tin content was determined by following the new procedure. The mean value of nine determinations of the tin content of the sample was 0.098% (2% relative standard deviation), which compares favourably with the certified value, 0.100%. REFERENCES 1 W. J. Ross and J. C. White, Anal. Chem., 33 (1961) 421. 2 Analytical Methods Committee, Analyst (London), 92 (1967) 320. 3 L. E. Coles, Pure Appl. Chem., 54 (1982) 1737. 4A. Engberg, Analyst (London), 98 (1973) 137. 5 E. J. Newman and P. D. Jones, Analyst (London), 91(1961) 406. 6 K. Tanaka and N. Takagi, Anal. Chim. Acta, 48 (1969) 357. 7 A. Sanz-Medel and A. M. Gutierrez, Analyst (London), 103 (1978) 1037. 8 A. M. Gutierrez, R. Gaiiego and A. Sanz-Medel, Anal. Chim. Acta, 149 (1983) 9 J. Duplessis, F. Nasr and R. Guiilaumont, Analusis, 6 (1978) 446.

259.

322 10 H. Irving and T. B. Pierce, J. Chem. Sot., (1959) 2665. 11 J. M. Yoe and A. L. Jones, Ind. Eng. Chem. Anal. Ed., 16 (1944) 111. 12 J. L. JimLnez, A. G6mez and M. T. Dorado, Rev. Metal. (Madrid), 5 (1969) 603.