Ion-Selective Electrode for the Determination of Iron ( III ) in Vitamin Formulations

Construiu-se um eletrodo íon-seletivo de grafite-epoxi recoberto para Fe(III), baseado no par-iônico formado entre o ânion [Fe(citrato)2] 3e o cátion tricaprililmetilamônio (Aliquat 336) incorporado em uma matriz de PVC (cloreto de polivinila). Uma membrana fina foi preparada pela incorporação deste par-iônico, ftalato de dibutila (DBPh) em PVC e aplicados diretamente em um tubo Perspex® contendo o condutor de grafite-epoxi. A solução de recobrimento foi preparada pela dissolução de 30% (m/m) de PVC em 10 mL de tetraidrofurano seguido da adição de 65% (m/m) de plastificante DBPh e 5% (m/m) do par-iônico. Investigou-se o efeito do pH, concentração de citrato e de alguns cátions sobre a resposta do eletrodo. O eletrodo apresentou uma resposta linear para E(mV) vs. log[Fe(citrato)2] no intervalo de concentração de ferro(III) de 1,0 x 10 a 1,0 x 10 mol/L em meio de citrato 1,0 mol/L, uma inclinação de 19,3 ± 0,5 mV/década e um tempo de vida de pelos menos 6 meses (mais de 800 determinações por cada membrana polimérica usada). O limite de detecção foi 7,5 x 10 mol/L e o desvio padrão relativo foi menor de 3% para solução contendo 5,0 x 10 mol/L de Fe(III) (n = 10). A aplicação deste eletrodo na determinação de ferro em amostras de vitamina é descrito. Os resultados obtidos com esse procedimento estão em concordância com aqueles obtidos usando espectrofotometria de absorção atômica (r = 0,9999).


Introduction
Iron(II) is a constituent of hemoglobin which is essential for the normal transportation of oxygen to the tissues.
In the foods ingested during a day, approximately 10-15 mg of iron are present and studies indicate that normal subjects absorb ten per cent of iron in the food 1 .The absence of iron in the organism causes anemia, the result of a decreased red blood cell content.This deficiency is treated with iron salts via oral or intramuscular 2 ministration.The excess of iron during treatment with iron salts may produce severe poisoning, causing symptoms of gastric irritation, vomit, pallor and circulatory collapse 3 .
Liquid membrane ion-selective electrodes (ISE) in particular have much importance for the analysis of pharmaceutical products.The relatively low cost, high simplicity, selectivity and low analysis time are advantages of using ISE compared with tedious procedures suggested in the pharmacopeias 4,5 .
Recently, we developed a coated graphite-epoxy iron(III) electrode 6 .A graphite-epoxy conductor was coated with the ionic pair [Fe(oxalate)3][Aliquat 336]3 incorporated in a PVC matrix.This electrode showed a linear response for the concentration of iron(III) from 2.9 x 10 -6 mol/L to 1.0 x 10 -2 mol/L and a slope of 18.7 ± 0.5 mV/dec, in a pH working range of 2-8 and in 0.3 mol/L oxalate medium.Application of this electrode for iron(III) determination in biotonic samples (Brazilian tonic formula) was described.The results obtained with this procedure were compared with those from atomic absorption spectrophotometry.The same active material was used in a tubular electrode in a flow injection system 7 .In this flow injection procedure, an increase of reproducibility, precision and analysis rate were obtained.
The present work describes the construction and application of a coated graphite-epoxy ion-selective electrode of iron(III) in citric medium.The iron(III) electrode was constructed by a prior complexation of iron(III) with citrate anion, followed by the formation of an ionic pair with tricaprylylmethylammonium cation (Aliquat 336), [Fe(citrate)2][Aliquat 336]3.After convenient separation, this ionpair was incorporated in a PVC matrix onto a graphite-epoxy conductor substrate attached to the end of a Perspex R tube, using the same procedure previously described [6][7][8][9] .This electrode, which needs no internal reference solution or internal reference electrode, is easier and cheaper to make than the conventional PVC membranes 10,11 .The effect of pH, citrate concentration and some cations on the electrode response was investigated.

Apparatus
All potentiometric measurements were carried out at 25 ± 0.2 °C in a thermostatted glass cell with the iron(III) ion-selective electrode and a R684 model Analion Ag/AgCl double junction reference electrode, both connected to a EA 940 model Orion pH/ion meter(USA) with ±0.1 mV precision.
All pH measurements were made with the same pHmeter and a 10/402/3092 model Ingold glass membrane elec-trode(USA).Calibration curves were obtained by addition of an iron(III) standard solution in citrate medium at controlled pH, using a E274 model Metrohm microburette, to a citrate solution at the same pH and ionic strength.In the study of response time, electrode stability and lifetime, the signals were recorded on a two-channel strip-chart recorder (Cole Parmer, model 12020000-USA).A Varian model Gemini AA 12/1475 atomic absorption spectrophotometer was used for the determination of iron in the samples.

Reagents and solutions
All solutions were prepared using Millipore Milli-Q water.All chemicals were analytical reagent grade and were used without further purification.
The 0.1 mol/L iron(III) chloride stock solution was previously standardized 12 , and used to prepare iron(III) reference solutions in 0.5; 1.0; 1.5 mol/L sodium citrate medium, in the pH range from 4 to 8.

Construction of the ion selective electrode
A sensor solution of [Fe(citrate)2][Aliquat 336]3, dibutylphthalate and PVC was used for preparing the membranes.Initially, 50 mL of 0.05 mol/L iron(III) in 1.0 mol/L citrate solution was shaken with 75 mL of 0.1 mol/L tricaprylylmethylammonium chloride in chloroform during 10 min.The ionic pair [Fe(citrate)2][Aliquat 336S]3 extracted in the organic solvent was dried with sodium sulfate and chloroform was evaporated in a rotary evaporator.The membrane solutions were prepared by dissolving 30% (w/w) of PVC in 10 ml of tetrahydrofuran (THF) following the addition of 65% (w/w) DBPh and of 5% (w/w) of the ionic pair.
The coated graphite-epoxy conductor electrodes were constructed as described elsewhere 9 .A thin membrane film was prepared embedding the ion-pair, dibutylphthalate (DBPh) and PVC (compositions shown above) onto an electrically conductive graphite-epoxy support located inside a Perspex® tube with help of a dropper; then it was dried for 3-4 h.Before use, the electrode was conditioned for 1-2 h by immersion in a 0.1 mol/L Fe(III) in 1.0 mol/L citrate solution, at the working pH (2-8), and finally rinsed with water and 1.0 mol/L oxalate solution.

Sample preparation
For the potentiometric determination of iron(III) in two vitamin formulations (Brazilian tonic formulas), 150-300 µL of liquid samples were initially heated until dryness.
Then, the samples were dissolved in 1 mL of aqua-regia and heated to dryness to oxidize Fe(II) to Fe(III).After that, the samples were dissolved in a 1.0 mol/L sodium citrate solution and transferred to a 50 mL volumetric flask and this volume was completed with the same solution.The percentage content of iron in this sample was determined by the standard addition method and by atomic absorption spectrophotometry 13 at a wavelength of 248.3 nm.

Effect of pH
The effect of pH on the response of the iron(III) electrode was determined in solutions with 1.0 x 10 -2 mol/L Fe(III) in a 1.0 mol/L citrate solution.The pH of the solutions was altered by the addition of 2 mol/L HCl or 2 mol/L NaOH solutions.
The E vs. pH plot (Fig. 1) shows that there are no significant variations in the values of potential when the pH of the solution is varied between 2 and 7, which demonstrates that the iron(III) complex with citrate is sufficiently stable (Kf = 6.3 x 10 37 ) 14 .For pH values less than 2 there is a decrease in potential of the electrode due to the protonation of the citrate anion.At pH higher than 7 the [Fe(cit-rate)2] 3-complexis hydrolysed, resulting in a abrupt increase of the electrode potential.

Effect of citrate concentration
Figure 2 shows the potential response of the electrode at pH 6 and citrate concentrations of 0.5; 1.0 and 1.5 mol/L as a function of log[Fe(III)].Table 1 shows the linearity ranges, slopes (mV/dec.) and detection limits of this electrode in these media.The sensitivity of this electrode decreases with the decrease of citrate concentration (Fig. 2), due to the decreasing [Fe(citrate)2] 3-complex concentration, which can be demonstrated by the distribution of iron-citrate complexes as a function of citrate concentration (Fig. 3) 15 .

Potentiometric selectivity coefficients
Potentiometric selectivity coefficients were determined by the separate solutions method following IUPAC's recommendation 16,17 .In this method the concentration of Fe(III) and interfering ion were fixed at 5.0 x 10 -3 mol/L in a 1.0 mol/L citrate solution, at pH 6.The log K pot Fe, X values (Fig. 4) were calculated using the cell potential of the iron(III) ion selective electrode vs. reference electrode, obtained each of two separate solutions, one containing the iron(III) ion and other containing the interfering ion.These results show a good selectivity of the ion selective electrode for all anionic complex solutions tested.

General response
Stable potentials were achieved within a few seconds (less than 5 s) and the relative standard deviation of 10 identical measurements was less than 3% for a solution containing 5.0 x 10 -3 mol/L iron(III).No memory effect occurred for seven potential measurements in solutions containing 5.0 x 10 -3 mol/L and 5.0 x 10 -2 mol/L [Fe(citrate)2] 3-, alternatively.
This electrode presented a useful lifetime of at least six months (more than 800 determinations/polymeric membrane) without significant loss in sensitivity.

Analytical applications
Table 2 shows the results obtained for iron(III) determination in vitamin formulation samples using potentiometric (multiple-standard addition) 18 and atomic absorption spectrosphotometric methods.Figures 5 and 6 show typical plots obtained for multiple 1.0 x 10 -1 mol/L [Fe(citrate)2] 3-standard additive procedure for sulfato fer-roso® and rarical®, respectively.The results are in good agreement with those obtained by atomic absorption spec-     trophotometry and are within an acceptable range of error.The samples analyzed by the standard-addition method presented recoveries close to 99% (r = 0.9999), indicating that this electrode can be used for determination of iron(III) in these samples.

Conclusions
The iron(III) ion-selective electrode which has been developed is easy to make, has a low cost, a long lifetime and good selectivity.This electrode was successfully used for iron(III) determination in vitamin formulations, and was demonstrated to be rapid and precise.

Figure 1 .
Figure 1.Effect of pH on the response of the iron(III) ion-selective electrode for iron(III) concentration of 1.0 x 10 -2 mol/L in 1.0 mol/L citrate solution, at 25.0 °C.

Figure 6 .
Figure 6.Determination of iron(III) in 10.0 mL of Rarical® using the multiple-standard addition procedure.

Figure 5 .
Figure 5. Determination of iron(III) in 10.0 mL of Sulfato ferroso® using the multiple-standard addition procedure.

Table 2 .
Determination of iron(III) in vitamin formulations using iron(III) ion-selective electrode and atomic absorption spectrophotometric procedures.