**4.2.3 Interference study**

436 Macro to Nano Spectroscopy

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

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

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

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

was used throughout the study.

sensitivity was observed.

sensitivity for iron for all concentration levels studied.

**4.2.2 Analytical performance characteristics** 

injections of the blank, the value of 1.12 μg L*−*1 was obtained.

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


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

#### **4.2.4 Analysis of water samples**

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

Flow-Injection Spectrophotometric Analysis of Iron (II), Iron (III) and Total Iron 439

water and did not absorb in the visible region of the spectrum, therefore, it might be well suited for flow-injection analysis of iron(III) and total iron. An addition of copper sulphate (1*×*10*−*4 mol L*−*1) into the reagent carrier solution resulted in baseline absorbance, and possible interfering ions were eliminated without a significant decrease in the sensitivity of the method. The method was successfully applied in the determination of iron (III) and total iron in water and ore samples. The method was verified by analysing a certified reference

All chemicals used were of analytical reagent grade, and solutions were prepared from double deionised water. Standard iron(II) and iron(III) stock solutions were prepared by dissolving 278.02 mg of iron(II) and 489.96 mg of iron(III) sulphate (Merck; Darmstadt, Germany) in 100 mL of 0.01 mol L*−*1 hydrochloric acid to give 0.01 mol L*−*1 stock solution of iron(II) and iron(III). Iron(II) and iron(III) working standard solutions were prepared daily by suitable dilution of the stock solutions with double deionised water. Standard reference material consisting of 0.085 % Fe (Zn/Al/Cu 43XZ3F) was provided from MBH Analytical Ltd. (UK). Hydrogen peroxide solution of 30 vol. % was obtained from Merck. AcSHA was synthesised according to the procedure described previously (Asan et al., 2003). A stock solution of AcSHA (0.01 mol L*−*1) was prepared by dissolving 0.095 g of AcSHA in 100 mL of aqueous methanol (2 vol. %). For the spectrophotometric study, AcSHA complex solutions of various metals were prepared by mixing 1 mL of 1×10−4 mol L−1standard solution of each metal in double deionised water with the suitable volume of 1×10−4 mol L−1 AcSHA stock solution. Reagent carrier solution was composed of AcSHA in a 2 % methanol solution and 1×10−4 mol L−1 CuSO4 in 0.001 mol L−1 HCl 98 % (pH 2.85). UV-VIS spectra of metal-AcSHA complexes were taken with a Unicam spectrophotometer (GBC Cintra 20, Australia). A Jenway 3040 Model digital pH-meter was used for the pH measurements. In the FIA system, a peristaltic pump (ISMATEC; IPC, Switzerland) 0.50 mm i.d. PTFE tubing was used to propel the samples and reagent solutions. Samples were injected into the carrier stream by a 7125 model stainless steel high pressure Rheodyne injection valve provided with a 20 μL loop. Absorbance of the coloured complex formed was measured with a UV-VIS spectrophotometer equipped with a flowthrough micro cell (Spectra SYSTEM UV 3000 HR,Thermo Separation Products, USA), and connected to a computer incorporated with a PC1000 software programme. A UNICAM 929 model (Shimadzu AA-68006) flame atomic absorption spectrophotometer with a deuterium-lamp background correction was used for the determination of iron in reference to the FIA method. The measuring conditions were as follows: UNICAM hollow cathode lamp, 10 cm 1-slot burner, air-acetylene flame (fuel gas flow-rate 1.50 L min−1), 0.2 nm spectral bandwidth, and 7 mm burner height. The wavelength

material Zn/Al/Cu 43XZ3F and also by the AAS method.

and the lamp current of iron were 248 nm and 5 mA, respectively.

was used in all measurements. Five replicate injections per sample were made.

The FIA system used, similar to that proposed in our previous works (Asan et al., 2003), is quite simple. The sample solution was introduced into the reagent carrier solution by the Rhodyne injection valve. A water-soluble complex (*λ*max = 475 nm) was then formed on the passage of the reagent carrier solution in the mixing coil. As a mixing coil, PTFE tubing (50 cm long) was attached before the flow-through detection cell. The absorbance of the coloured complex was selectively monitored in the cell at 475 nm. The transient signal was recorded as a peak, the height of which was proportional to the iron(III) concentration in the sample, and it

**5.1 Experimental** 

also given for comparison. The results obtained with the standard addition and the calibration curve methods, and the AAS measurements were in good agreement with each other.


1. Samples were collected at Samsun, Turkey.

2. Values in parantheses are the relative standard deviations for *n* =5 with confidence level of 95 %.

3. Calibration curve method.

4. Standard addition method.

Table 7. Determination of total iron in water samples1

Accuracy of the proposed method was also tested by analyzing a certified metal alloy solution (MBH Zn/Al/Cu 43XZ3F). Three replicates of the solution using the sampling volume of 20 μL were analyzed. The certified and the obtained values were 0.085 % and (0.084 *±* 0.006) % of iron, respectively. An excellent agreement between the found and the certified values was obtained for the certified metal alloy solution. The obtained results show that the proposed method can be applied to the determination of iron(III) and total iron content in water samples without a pre-concentration process.
