764
Anal. Chem. 1991, 63,764-766
Coated-Wire Ion-Selective Electrode for the Determination of Antimony(V) Concepcidn Sinchez-Pedreiio,* Joaquin A. Ortuiio, a n d Joaquin Alvarez Department of Analytical Chemistry, Faculty of Sciences, University of Murcia, 30071 Murcia, Spain
The construction, performance characteristics, and applications of a coated-wire antimony( V) Ion-selectlve electrode, based on the Ion palr between hexachloroantlmonate(V) and the 1,2,4,6-tetraphenylpyrldlnlum cation In a poly(vinyl chloride) matrix, are described. The influence of membrane compostllon, hydrochlorlc acid concentration, and forelgn Ions was lnvestlgated. I n the selected condltlons, the electrode shows a nearHernstian response over the antlmony(V) concentratlon range 5 X 10-7-10-3 M, wRh good preclsion and excellent selectivity. Appllcations to the potentlometric titration of 0.03-3.0 mg of antlmony(V) and to the direct potentlometrlc determlnatlon of antknony In sphalerltes, bronzes, and Iron oxlde plgments are reported.
Table I. Preparation of the Membranes coating solutions" mass 70 of electroactive material in extractb vol/pL the extract
membrane
PVC/mg
A B
102.6
100
98.3 100.0
100 100 200 100
C D E F G H
102.1 201.2 100.0 100.0 200.0
200 100
100
0.8 3.3
6.7 3.3 3.3 6.0 6.0 6.0
"Dissolved in 3 mL of tetrahydrofuran. *In NPOE for membranes A-E and in DBP for F-H. INTRODUCTION Since the development of coated-wire ion-selective electrodes (CWEs) by Cattrall and Freiser ( I ) , several CWEs, containing as electroactive materials ion-association compounds formed between negatively charged halide complexes of the metal ions t o be determined and strong hydrophobic cations, have been reported. Iron (Z),copper (3),mercury ( 4 ) , zinc (51, bismuth (6),cobalt (3, gold (8),and thallium (9)have been determined as their chloro complexes and cobalt also as its tetrathiocyanatocobaltate(I1)complex (10). Aliquat 336 (2-3, 1,2,4,6-tetraphenylpyridinium(8,9),and benzalkonium (10) have been used as counterions. The behavior of ion-selective electrodes based on ion exchangers depends on the extraction characteristics of the ion pairs involved (11). It seems surprising then t h a t since hexachloroantimonate(V) is a n excellent anion for the formation of extractable ion-association compounds, the references to ion-selective electrodes for the determination of Sb(V) are so scarce (12). In this unique paper, Fogg et al. incorporated the hexachloroantimonate(V) and tetrachlorothallate(II1)salts of Sevron Red GL, Flavinduline 0, and Phenazinduline, in lightly cross-linked natural rubber membranes for the construction of liquid-state ion-selective electrodes to determine antimony and thallium. No mention of poly(viny1 chloride) (PVC) matrix electrodes, with or without internal solution, for the determination of Sb(V) has been found. A possible explanation could be the problems associated with the hydrolysis of hexachloroantimonate(V) ion. However, this reaction has been well studied and is known t o be slow under certain conditions (13, 14). The CWE described in this paper, using 1,2,4,6-tetraphenylpyridinium hexachloroantimonate(V)in a PVC membrane, allows the determination of antimony in a wide concentration range with higher selectivity than previously reported (12). EXPERIMENTAL SECTION Apparatus. Potentials at 25 f 0.1 "C were measured with a Philips PW9415 ion-selective meter and recorded with an OmniScribe D500 chart recorder. A R442-SD1 calomel doublejunction reference electrode containing 1M KCl solution in the outer compartment was used. Automatic titrations were per-
formed with a Radiometer ABUl3 autoburet. Reagents. 2-Nitrophenyl octyl ether (NPOE) and poly(viny1 chloride) of high relative molecular mass were purchased from Fluka. Dibutyl phthalate (DBP) and antimony(V) chloride were purchased from Merck. Tetrahydrofuran (THF), reagent grade from Merck, was used as received. 1,2,4,6-Tetraphenylpyridiniumacetate (TPPA) solution, 0.1 M, was prepared by the method of Chadwick (15) and standardized gravimetrically with perchlorate. Working solutions were prepared by dilution with doubly distilled water. Hexachloroantimonate(V) standard solution, 0.25 M, was prepared by dissolving antimony(V) chloride in 12 M hydrochloric acid. Working solutions were prepared by dilution with 12 M hydrochloric acid. These solutions were stored in a refrigerator. Electroactive Material. 1,2,4,6-Tetraphenylpyridinium hexachloroantimonate (TPP+SbC16-)was precipitated from 2.5 X lo-* SbC&- standard solution by adding 0.1 M TPPA solution. The final hydrochloric acid concentration was 6 M. The precipitate formed was extracted quantitatively by shaking the mixture for 1 min with the solvent chosen as plastizicer (NPOE or DBP). This operation must be made in an ice bath to prevent the formation of some possibly hydrolyzed products. The volumes of reactants used depend on the concentration of electroactive material desired in the plasticizer. In all cases, about 20 mol % excess SbC16- with respect to TPPA was used to prevent the extraction of any other ion pair with TPP+. The ratio by volume of aqueous phase to organic phase was 1O:l. Construction of t h e Electrodes. Powdered PVC and the extract of the electroactive material in the plasticizer were dissolved in tetrahydrofuran; coating solutions are shown in Table I. A platinum wire, about 2 cm long and 1.0 mm in diameter, sealed into the end of a glass tube and soldered onto a shielded cable, was dipped into this solution 25 times and the solvent evaporated with an air gun each time. A membrane was formed on the platinum surface and the electrode allowed to set 30 min. The coatings solutions are stable for several weeks if kept in a refrigerator and can be used for the construction of new membranes. Conditioning and Calibration of the Electrodes. The electrodes were conditioned by soaking with constant stirring in 25 mL of M SbC16-solution with the selected hydrochloric acid concentration until the electrodes gave a constant potential. The same procedure was then followed with a solution containing only hydrochloric acid of the same concentration. Small volumes
0003-2700/91/0363-0764$02.50/00 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,NO. 8, APRIL 15, 1991
r D
I
I
I
1
I
I
-5 -4 -3 'q'Sb(V) Flgure 1. Calibration graphs for membranes A-H in 0.1 M hydrochloric acid. -0
-7
Time
I
-6
of the hexachloroantimonate(V)standard solutions were injected with micropipets to obtain final concentrations in the range 10-8-10-3 M. The electrodes were stored in a solution of the same composition as that used for conditioning and, before use, were conditioned in a fresh solution since the storing solution would have been hydrolyzed. The first conditioning times were in all cases less than 60 min and then were only about 10 min for successive uses. Potentiometric Titrations. An aliquot of the sample solution containing 0.03-3.0 mg of antimony, as SbCb-, was pipetted into a 100-mLbeaker and the HCl concentration adjusted to 1 M. This was titrated automatically with 1.5 X 10-4-1.5 X M TPPA solution. Titration rates were held constant at 0.3 mL min-'. Potentials were then monitored with the amtimony(V) CWE. Determination of Antimony in Sphalerites, Bronzes, and Iron Oxide Pigments. The samples were dissolved in aqua regia and boiled nearly to dryness. Then 8 mL of concentrated hydrochloric acid and 0.5 g of (NH,),Ce(NO,), were added and set aside for 1 min. The solution was transferred into a 100-mL calibrated flask, diluted with distilled water, and rapidly measured with the CWE. The antimony(V) concentration was determined by using the standard addition technique by adding increasing concentrations of SbC&-standard solutions. If the concentration of antimony is sufficiently high, the sample solution can be diluted with 1 M HCl and measured in different regions of the calibration graph.
RESULTS AND DISCUSSION Composition of the Membrane. I t has previously been found (8, 9) that the plasticizer to PVC ratio may have an important effect on the response of coated-wire electrodes. In the present paper, the study of the composition of the membrane has been extended to include the effect of the concentration of electroactive material in the membrane and two different plasticizers (membranes A-H in Table I). The calibration graphs obtained with these membranes are shown in Figure 1. Membranes A-C correspond to different concentrations of electroactive material, using an approximate ratio 1:l of NPOE to PVC. This ratio offered the best results for previously developed CWE (8,9). In this study, the three membranes showed similar responses to each other with a slight super-Nernstian behavior in the lower concentration range that increased as the concentration of electroactive
$
765
2min H
J
E . )
Flgure 2. Responses of membranes D and E to different antimony(\/) concentrations in 0.1 M hydrochloric acid. 1.0 X lo-' (1 and 7),2.0 X (8),3.0 X (9),1.1 X 10" (2), 1.3 X 10-'(10), 1.1 X (3and ll), 1.1 X (4and 12), 6.1 X (5 and 13),and 1.1 X M (6 and 14).
material increased. In an attempt to suppress this phenomenon, the NPOE to PVC ratio was altered to 2:l and 1:2 (membranes D and E). I t can be seen that super-Nernstian response increased for membrane D and disappeared for membrane E. Figure 2 corresponds to the response potential of these membranes versus time and shows that the response was slow for those concentrations of SbC&- where membrane D showed super-Nernstian behavior, while the response of membrane E was very fast in the whole calibration range. Another plasticizer was tested to clarify whether these types of responses depend on the nature of the plasticizer. Membranes F, G, and H correspond to dibutyl phthalate to PVC ratios of 2:1,1:1, and 1:2. As can be seen in Figure 1,the same effect is observed even more clearly. The super-Nernstian behavior disappears again when the ratio is reduced to 1:2. Taking all these results into account, membrane E was selected for further work. Effect of Hydrochloric Acid Concentration and Response Range. The response of the electrode in the hexachloroantimonate(V) concentration range 10-8-10-3 M was studied a t 0.1,0.5, and 1M hydrochloric acid concentrations. The calibration graphs obtained present the same range of linearity (5 X 10-'-10-3 M) and near-Nernstian slopes (-58.6, -57.7, and -58.4 mV/decade concentration, respectively). Since the stronger acidic medium is more convenient for the direct determination of antimony in real samples, 1 M HC1 was used for further work. Response Time, Detection Limit, and Reproducibility. Response times of the electrode were tg5%within 10 s and tB, within 1min for 5 x 10-7-10-3M antimony(V). The detection limit of the electrode, considered as the hexachloroantimonate(V) concentration a t which the potential deviates by 18 mV from the extrapolation of the linear portion, is
M. The reproducibility and stability of the electrode were evaluated by replicate calibration graphs (n = 10) over a period of 6 weeks. Although the absolute potential of the electrode changed, as is common with CWE, the calibration slope remained constant over this period (mean & standard deviation, -58.40 f 0.69).
766
ANALYTICAL CHEMISTRY, VOL.
63,NO. 8,
APRIL 15, 1991
a:,
T a b l e 11. P o t e n t i o m e t r i c S e l e c t i v i t y Coefficient, for t h e Coated-Wire A n t i m o n y ( V ) Ion-Selective Electrode
T a b l e 111. D e t e r m i n a t i o n o f A n t i m o n y
sample
ion
Ag(I), Zn(II), Cd(II), "I), Cu(II),"Co(II), Pb(II), Mn(II), Hg(II), Sb(III), Fe(III), In(III), Bi(III), Cr(III), Ca(III),Ce(III), Al(III), Sn(IV), As(V), ClO,-, NO,, C1-, SO?
