**4.2.1 Optimization of FI manifold**

Optimization of the flow system was performed to establish the best FIA variables. A fixed standard Fe (III) solution, 10 μg L*−*1 was injected into the flow system for the determination of optimum experimental conditions. The main variables influencing the intensity of the signal were: flow-rate, pH, and the concentration of salicylic acid. Therefore, optimization of the FIA system was carried out by changing these variables one by one.

The effect of salicylic acid in the carrier solution on the peak height was examined by changing the amount of salicylic acid in the range of 5*×*10*−*7–5*×*10*−*5 mol L*−*1 in buffer solution at pH 8.5, at the flow rate of 1.0 mL min*−*1. Peak heights were found maximum using a 2*×*10*−*6 mol L*−*1 salicylic acid solution for 10 μg L*−*1 iron(III) levels. Therefore, 2*×*10*<sup>−</sup>*<sup>6</sup> mol L*−*1 salicylic acid was chosen as the fluorescence reagent in the carrier solution.

The effect of flow-rate on the peak height of iron(III) was examined by varying the flow-rate from 0.5 mL min*−*1 to 1.5 mL min*−*1. Peak heights decreased at flow-rates above 1.2 mL min*−*1

Flow-Injection Spectrophotometric Analysis of Iron (II), Iron (III) and Total Iron 437

Fig. 7. Flow signal for iron(III) standard solutions by fluorescence quenching-FIA a) 100 μg L*−*1, b) 75 μg L*−*1, c) 50 μg L*−*1, d) 25 μg L*−*1, and e) 5 μg L*−*1 when using the optimized FIA

The effect of diverse ions on the detection of iron by the present system were examined using a solution containing 10 μg L*−*1 iron(III) and one of the other ions. The tolerable concentration of each diverse ion was taken as the highest concentration causing the error of

Over 50 000 Co(II), Cr(III), Al(III), Cu(II), Cd(II), Ni(II), Pb(II), Mn(II),

Table 6. Effect of foreign ions on the determination of 10 μg L-1 of iron(III) in solution

+

The proposed method was applied to the determination of iron in river, sea, and thermal spring water samples to evaluate its applicability. Iron(III) and total iron were determined according to the FIA procedure as described in the experimental section. Table 7 shows the analytical results of iron(III) and total iron. Atomic absorption measurements taken were

Cl-, PO43-, NH4

K(I), Na(I), Ag(I), Mg(II), Ba(II), Hg(II), CN-, NO3-, NO2-,

system.

**4.2.3 Interference study** 

*±* 5 %. The results are summarized in Table 6.

Tolerance limit (mg L-1) Foreign ion

Over 100 Fe(II)

**4.2.4 Analysis of water samples** 

No interfere CO32-, SCN-, Br-, SO42-, Ca2+, Zn2+

and below 0.8 mL min*−*1. Below 0.8 mL min*−*1 the peaks also broadened. Between the flowrates of 0.8–1.2 mL min*−*1, there were slight differences in the peak heights. Considering the stability of the pump, peak height, and sampling time, the flow-rate of the reagent carrier solution was adjusted to 1.0 mL min*−*1. This provided the sampling frequency of 60 h*−*1. pH of the carrier solution consisting of 2*×*10*−*6 mol L*−*1 salicylic acid was adjusted by an NH4+ /NH3 buffer solution to obtain the pH range of 8.0–10.0. The peak heights were found maximum at pH 8.5. Therefore, a 0.1 mol L*−*1 NH4 + /NH3 buffer solution (90 : 10) at pH 8.5 was used throughout the study.

The use of a mini-column in the flow-injection system provided an improvement in the sensitivity and selectivity due to on-line pre-concentration and fast interaction of metal ions with reagent molecules in the carrier solution (Isildak et al., 1999). A mini-column packed with strong cation-exchange resin was selected because metal ions are strongly bound by the resin so that low amounts of the resin can be used. Higher amounts of the resin minimized the use of higher flowrates due to an increase in the hydrodynamic pressure. Sampling time in the FIA system depends on the retention time in the cation exchange minicolumn and the residence time in the tubing in the flow-path. The effect of the column length was examined by changing the column length between 2 cm and 10 cm. From the results obtained, 6 cm column length brought the best results for the peak shape and sensitivity for iron for all concentration levels studied.

Also a mixing coil and a mini-column packet with silica and glass beads were inserted into the analytical path instead of the cation-exchange resin minicolumn. However, the observed peak height and sensitivity for iron(III) were lower and poorer, for all concentration levels studied. This result can originate from the short remaining time of iron(III) in each column, which means a narrow interacting zone of the sample. Finally, a mini-column packed with strong cation-exchange resin was used throughout the study for the determination of iron(III). Indeed, a significant improvement of the selectivity and sensitivity was observed.

### **4.2.2 Analytical performance characteristics**

Analytical performance characteristics of the method were evaluated under optimum conditions. Fig. 7 shows typical flow signals for iron(III) obtained by the proposed method. The reaction of iron(III) with salicylic acid resulted in negative peaks due to the fluorescence quenching of salicylic acid. Under the optimum working conditions, calibration graphs were prepared from the results of triplicate measurements of iron(III) standard solutions of increasing concentration. The calibration graph showed a good linearity from 5–100 μg L*<sup>−</sup>*<sup>1</sup> iron(III) with the linear regression equation: Y = 0.0353X + 0.0909, where Y is the peak height (cm) and X is the concentration of iron(III) in μg L*−*1. The correlation coefficient was *r*2 = 0.9963 and the relative Standard deviation (RSD) of the method based on five replicate measurements of 10 μg L*−*1 iron(III) was 1.25 % for a 20 μL injection volume. The limit of detection (determined as three times the standard deviation of the blank) was 0.3 μg L*−*1 and the sampling rate was 60 h*−*1. The limit of quantification (LOQ) was calculated as recommended (Currie, 1995); based on a ten fold standard deviation of ten consecutive injections of the blank, the value of 1.12 μg L*−*1 was obtained.

Fig. 7. Flow signal for iron(III) standard solutions by fluorescence quenching-FIA a) 100 μg L*−*1, b) 75 μg L*−*1, c) 50 μg L*−*1, d) 25 μg L*−*1, and e) 5 μg L*−*1 when using the optimized FIA system.
