Coated Wire Ion Selective Electrode for the Determination of lron(ll1) R. W. Cattrall and Pui Chin-Poh Department of Inorganic and Analytical Chemistry, La Trobe University, Bundmra 3083, Victoria, Australia
A method is described for the determination of iron(ll1) by ion selective potentiometry using a coated wire electrode In which the electroactlve membrane is composed of the tetrachloroferrate(ll1) salt of the quaternary ammonium compound, aliquat 3368, combined with poly(vlny1 chloride). A linear near-Nernstian response for this electrode is obtained over the total iron(111) concentration range of to 10-'M for solutions which contain a controlled total chloride ion concentration of 6M. The interference of other Ions on the electrode response is reported and the results show that the electrode is very selective towards the tetrachloroferrate(1ll) ion. Results are reported for the analysis of three iron ores and these are compared with the results obtained using conventional methods. Freiser et al. ( I ) have recently reported a series of anion responsive coated wire ion selective electrodes based on ion association extraction systems using various salts of a l i q u a t 3368 and poly(viny1 chloride) as t h e m e m b r a n e material. T h e s e electrodes showed near N e r n s t i a n response over a concentration range of about 10-3 t o 10-lM. Scibona et al. ( 2 ) have s t u d i e d highly selective liquid m e m b r a n e electrodes responsive t o t h e tetrachlorozincate(I1) and t e t r a chloropalladate(I1) complex anions which a r e based on salts of these anions with certain q u a t e r n a r y a m m o n i u m ions dissolved in benzene and chloroform. T h e work of Scibona et al. ( 2 ) posed the very interesting question as to whether their a p p r o a c h could be applied t o t h e development of an ion selective electrode for t h e analysis of a multivalent cation s u c h as Fe3+ which m a d e use of a complexed species of charge lower than three. This, of course, would give greater sensitivity to the analysis because of t h e larger potential change per decade change in activity. We wish t o r e p o r t the development of a coated wire elect r o d e which is highly selective for the tetrachloroferrate(II1) ion and which c a n b e used, a f t e r suitable a d j u s t m e n t of solution parameters, for t h e analysis of iron(II1).
EXPERIMENTAL Materials. Aliquat 3368 (tricaprylylmethylammonium chloride) was obtained from General Mills Chemicals, Inc. Poly(viny1 chloride) powder was obtained from Monsanto Co. Tetrahydrofuran (May and Baker reagent grade) and Chloroform (Analar) were used as received. Ferric chloride hexahydrate (Analar), anhydrous lithium chloride (May and Baker reagent grade), and hydrochloric acid (Analar) were used for the preparation of iron so!utions. Anhydrous lithium metaborate (BDH reagent grade) was used for the fusion of the mineral samples. Platinum wire (0.028-in. diameter) was used without elaborate preparation of the metal surface. All other chemicals were of reagent grade. Conversion of Aliquat 3368 to t h e Tetrachloroferrate(II1) Form. Twenty-five cm3 of a 10% (V/V) solution of aliquat 3368 in chloroform was shaken with 250 cm3 of a 2M ferric chloride solution which contained 8M HC1. The phases were allowed to separate overnight, and the oranic layer was removed and filtered. Chloroform was evaporated on a steam bath and the aliquat tetrachloroferrate(II1)salt was obtained as a dark brown viscous oil. Construction of Electrodes. Coated platinum wire electrodes were constructed using the technique as reported previously ( I , 3 ).
This technique consisted of dissolving weighed amounts of the aliquat 336s salt and PVC in the minimum amount of solvent and repeatedly dipping a bead formed on the end of a platinum wire into the mixture until a uniform coating was obtained. The bead was air dried overnight. The aliquat salt behaved as a plasticizer for poly(viny1 chloride) and so no secondary plasticizer was necessary. Two solvents for PVC were used, tetrahydrofuran and cyclohexanone, and these both produced electrodes with similar behaviors, except that films made using tetrahydrofuran as solvent tended to be cloudy initially, possibly because of the presence of water. Films made using cyclohexanone as solvent were dark brown in color but clear and homogeneous. The films were flexible and had excellent mechanical characteristics for electrode construction. Several membrane compositions were investigated varying from 90% of the aliquat tetrachloroferrate(II1) salt and 10%PVC (W/W) to 30% of the aliquat salt and 70% PVC (W/W). The composition which gave the best electrodes with regard to response time, stability, and reproducibility was 80% of the aliquat salt and 20% PVC. Electrodes obtained using this composition had the following characteristics. A stable potential, which was held for at least 6 min, was obtained in 25 min or less in solutions which were 10-4M total Fe(II1) and the response was almost instantaneous in solutions which were 10-lM total Fe(II1). A drift of no more than 5 mV was found during the course of one day in the stronger solutions. One electrode was found to be working with unchanged response characteristics four months after the initial preparation. This electrode had been stored in air. In addition to the coated wire electrodes, electrodes were constructed of the same membrane composition using the technique of Moody et al. ( 4 ) which contained an internal liquid reference system. This consisted of a calomel electrode dipping into a 1O-'M iron(II1) solution containing 5M LiCl and 1M HCI. All electrodes were conditioned before use by soaking for 1 hour in a 10-*M Fe(II1) solution 6M chloride (5M LiCl and 1M HC1). Reference Electrode. It was necessary to use a triple junction reference electrode for all measurements because of problems associated with the diffusion of hydrochloric acid into the calomel electrode. This consisted of an outer compartment with a very low leak asbestos fiber junction which was filled with saturated KC1. The remainder of the reference electrode consisted of a normal double junction calomel electrode with the outer compartment filled with saturated KCl. Iron(II1) Solutions. The formation of the tetrachloroferrate(II1) species in aqueous chloride solutions is not particularly favorable even a t very high chloride ion concentrations. Consequently, in any iron(II1) solution containing chloride, the actual concentration of the tetrachloroferrate species is considerably lower than the total iron concentration. Because of this, it is necessary to carefully control (to a predetermined value) the total chloride ion concentration in both the standard solutions and in the unknown solutions. It was most convenient to use a constant chloride concentration of 6M for all solutions. This was achieved by addition of known volumes of standard lithium chloride and hydrochloric acid solutions until the final solutions were 6M Lick and 1M HCl. This procedure was more convenient than using strong hydrochloric acid solutions. It should be emphasizd that a t a high constant chloride ion concentration (which is not appreciably altered by the addition of small amounts of iron("), the concentration of the tetrachloroferrate(II1) species is directly proportional to the total iron(II1) concentration. For the study of the Nernst behavior of the electrodes, solutions of total iron(II1) concentration in the range to 10-'M were obtained by dilution of a standard ferric chloride solution. For the analysis work, it was found to be more difficult t o control the total chloride concentration of the solutions within the precise limits required when using diluted ferric chloride solutions and, hence, for this work, ferric ammonium sulfate was used for
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
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Figure 1. Interference curves for the coated wire electrode (1) Pure Fe(lll) solution containing 6M chloride. (2) 10-2M ZnC12. (3) 10-'M HgC12. (4) 10e2M FeCI2. (5) 10d3M SnCI2. (6) 10-'M AIC13. (7) 10-'M CuC12. (8) 10-'M LiN03. (9) 10-'M NaF. (10) 10-'M
Figure 2. Titration of 10 cm3 of a 0.025M total iron(ll1) solution containing 6 M LiCl at pH 1.3 with a 0.05M EDTA solution also containing 6M LiCl
to dryness, the residue was dissolved in the 6M chloride solution, filtered to remove silica, and made to volume with the chloride solution. The potential of the solutions was measured with the ion selective electrode. All potential measurements were made with an Orion Model 801 digital pH meter.
Li2SO4
preparing the standard solutions. Initially a 10-1M iron(II1) solution was prepared by dissolving ferric ammonium sulfate in a solution which contained a total chloride ion concentration of 6M (5M in LiCl and 1M in HCI). This stock iron(II1) solution was then diluted with the 6M chloride solution to give standards which matched the total iron(II1) concentration of the sample solution quite closely. It was found most convenient to work in the total iron(II1) concentration range of to 10-3M. A calibration curve of potential us. the total iron(II1) concentration was prepared, and this was used to obtain the total iron(II1) concentration in the sample solution. This was, of course, possible because the total chloride ion concentration in the sample solution was made the same as that in the standard solutions. For the potentiometric titration studies, iron(II1) solutions were obtained by dilution of an acidified (HCI) stock ferric ammonium sulfate solution containing 6M LiCl with a solution containing 6M LiCI. The pH of the solutions was adjusted to about 1.3 where necessary. The EDTA solution was obtained by dissolving the disodium salt in a solution containing 6M LiCl. Interference Studies. These were carried out according to the graphical procedure described by Moody and Thomas ( 5 ) . The concentration of the interfering ion was generally fixed a t 10-2M while the total concentration of iron(II1) was varied between to 10-'M by dilution of a standard ferric chloride solution. The chloride salts of the interfering cations and the lithium salts of the interfering anions were used, and the solutions were made 6M in total chloride concentration by addition of appropriate amounts of LiCl and HCl as before. In cases where the interference was large, a 10-3M solution of the interefering ion was used (e.g., Sn2+),and for cases (Cu2+,A13+,NOz-, F-, S042-)where the interference was very small, a 10-1M solution of the interfering ion was used. Analysis of Hocks a n d Minerals. Iron pyrites was analyzed by wet ashiiig a weighed sample (to give a final solution containing a total iron(II1) concentration in the range 10-3 to 10-*M) with a concentrated H2SO.+/HNOs mixture. The solution was evaporated to dryness and fumed twice with concentrated nitric acid to ensure that all the iron was present as iron(II1). The solution was again evaporated to dryness and the residue was dissolved overnight in the solution containing 6M total chloride. After making to volume with the 6M chloride solution, the potential was measured with the ion selective electrode. The sulfate ion concentration in this SOlution should not he greater than 10-'M. Iron pyrites was also analyzed using a known method (6 ). Two silicate rock samples containing between 10 and 15% iron were analyzed by fusing weighed powdered samples with approximately six times as much lithium metaborate. The fusion was carried out by heating the well mixed material in a high purity graphite crucible to 900 OC in a muffle furnace for 10-15 min. The melt was poured into dilute nitric acid and stirred until dissolved. The solution was evaporated to near dryness on a hot plate and treated with concentrated nitric acid to oxidize the iron. After evaporation 94
RESULTS AND DISCUSSION Potential Responses. The potential responses of three electrodes, for which the best membrane composition of 80% aliquat tetrachloroferrate(II1) salt and 2U% PVC was used, were studied as a function of the total iron(II1) concentration in solutions which contained a constant chloride ion concentration of 6M. Two of these electrodes were of the coated platinum wire type (made using tetrahydrofuran and cyclohexanone, respectively, as the solvents for PVC) and the third was of the type reported by Moody, Oke, and Thomas ( 4 ) which contained an internal aqueous reference system. The responses of these electrodes were virtually identical showing a near linear region between to 10-'M total iron(II1) with a negative slope of 55 mV per concentration decade in each case (refer to Figure 2 which shows a representative calibration plot). This suggests that the eiectrodes are extremely sensitive to the tetrachloroferrate(111) species which would only be present in approximately the concentration range of to 10-6M in these solutions. The response characteristics of these electrodes are reported in the Experimental. Interference Studies. The interference on 'the electrodes of the addition of the following ions to the iron(II1) solutions was investigated, Ni2+, ZnZ+, Mg", Coif, Mn2+, Ca2+, Sn2+,Al3+, Hg2+, Fez+. The only ions which dhowed appreciable interference were Sn2+, Hg", Fez+, and Zn2+. For the remainder, there was no observable change in the measured potentials over the total iron(II1) concentration region of to 10-lM in the presence of 10-2M of the interfering ion and, in the presence of IO-lM of the interfering ion, the interference only became appreciable below a total iron(II1) concentration of 10-4M. Thus, in the absence of ions such as Sn2+,Hg*+, Fez+, and Zn2+,the electrodes appear to be highly selective for the tetrachloroferrate(II1) ion. Representative experimental interference curves for the coated wire electrode prepared using tetrahydrofuran as the solvent for PVC are shown in Figure 1 where the relative potential is plotted us. the logarithm of the total iron(111) concentration for a constant concentration of each of the interfering ions. Results are shown for the four strongly
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
interfering ions and also for solutions containing IO-lM of Cu2+ and A13+. The interference curves for the other two electrodes were similar. Figure 1 shows the order of increasing interference for the strongly interfering ions as Zn2+ < Hg2+ < Fez+ < Sn2+. For Zn2+ and Hg2+ the interference is, no doubt, due to the formation of anionic chloro complexes of these ions, whereas for Sn2+ and Fez+, the effect is almost certainly due to a redox reaction and the subsequent interference from species such as SnC15-1 in the case of tin. The interference curves for the simple anions NO3-, F-, and s04’- are also included in Figure 1 for a constant concentration of the interfering ion of 10-lM. It can be seen that these ions do not interfere strongly; however, very high concentrations of these ions (>lO-lM) should be avoided. The order of increasing interference is S042- < F-
