A Fluorescence Technique to Determine Low Concentrations of

Chapter 9

A Fluorescence Technique to Determine Low Concentrations of Ferrate(VI)

Downloaded by COLUMBIA UNIV on June 14, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch009

Determination of Micromolar Fe(VI) Concentrations for Laboratory Investigations NadineN.Noorhasan, Virender K. Sharma*, and J. Clayton Baum Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901 *Corresponding author: [email protected]

A fluorescence technique to determine low concentrations of aqueous ferrate(VI), [Fe O ], in water was developed over a wide pH range using the reaction of ferrate(VI) with scopoletin reagent. The rates of the reaction of ferrate(VI) with scopoletin as a function of pH at 25°C were determined using the stopped-flow technique to demonstrate the reaction is rapid (< 1 min). Spectral measurements on scopoletin at different pH showed that the maximum in absorption varies with the pH while the emission maximum is independent of pH. The absorbance measurements were used to determine the acid dissociation constant, K = 1.55 ± 0.01 x 10 (pK = 8.81 ± 0.05) for scopoletin. The intensity of fluorescence for scopoletin decreases linearly with increase in the concentration of ferrate(VI), which suggests the suitability of the method. Moreover, a relatively large decrease in intensity per micromolar ferrate(VI) concentration was observed, especially at low pH, which makes fluorescence a sensitive technique to determine low ferrate(VI) concentrations. VI

2-

4

-9

a

a

© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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146

Introduction In recent years, there has been tremendous interest in the innovative use of ferrate(VI), which has the molecular formula Fe 0 " where iron exists in the +6 oxidation state to which four oxygen atoms are bonded covalently to give a tetrahedral structure (/). In the "super-iron" battery, ferrate(VI) replaces the usual manganese dioxide cathode since ferrate(VI) can gain more electrons than manganese dioxide. Additionally, the "super-iron" battery does not produce toxic compounds in contrast to the manganese cathode (2). Ferrate(VI) has also been proposed as a green chemistry oxidant for organic synthesis (3). Moreover, ferrate(VI) has the highest redox potential (+2.2V in acid) of any oxidant used in water and wastewater treatment (4,5). The most common treatment method is chlorination, but it produces known toxic by-products (6-8). In comparison, ferrate(VI) has been shown to destroy pollutants and bacterial species in seconds to minutes without producing harmful by-products (9). Ferrate(VI) decomposition produces Fe(III), which itself is an excellent coagulant for removal of metals and radionuclidesfromcontaminated water (10). Studies of ferrate(VI) include its production, stability, oxidation, and magnetic properties, all of which require accurate knowledge of the ferrate(VI) concentration in dilute solutions. The concentration of Fe 0 " in an aqueous sample of potassium ferrate (K Fe0 ) can be determined by titrating it with chromium(III)(//)(eq. 1): VI

2


4

Y1

2

4

2

4

2

Cr(OH) - + Fe0 * + 3H 0->Fe(OH) (H 0) + C r 0 4

4

2

3

2

3

2_ 4

+ OH"

(1)

The resulting chromate(VI) solution is then acidified as dichromate and titrated with a standard solution of ferrous ions. A similar titration procedure has also been used in the reaction of ferrate(VI) with arsenic(III) (12). Both methods determine concentrations only at the sub-molar to molar level of ferrate(VI). In addition, the titration steps are time consuming and use toxic heavy metals. Another method is the use of cyclic voltammetry to determine low concentrations of ferrate, but this method is inconvenient to use (13). To determine low concentrations in micromolar to millimolar levels of aqueous ferrate(VI), one could resort to the use of simple and convenient UV-Vis spectrophotometry where the absorbance at 510 nm is measured to determine the aqueous ferrate(VI) concentration. However, the molar absorption coefficient of ferrate(VI) at 510 nm (e o m) is not only low, but also varies with pH (1150 M - W at pH 9.1 to 520 N f W at pH 6.20) (14). Moreover, the absence of a strong chelating agent (e.g. phosphate) in solution for complexation of Fe(III), produced from the self decomposition of ferrate(VI), causes significant errors in optical monitoring of the solution (15). The study of the kinetics of ferrate(VI) reactions with various substrates is presently restricted to 51

1

n

1


147 the basic region due to fast disproportionation of ferrate(VI) at neutral and acidic pH range (15). The self-decomposition follows second-order kinetics and can be minimized by using lower Fe(VI) concentration. Recently, a method was developed to determine low concentrations of aqueous Fe(VI) in acidic medium. This method uses the reaction of Fe(VI) with 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) (16). ABTS reacts with oxidants via a single-electron transfer to give ABTS* , a stable and intense green colored radical that absorbs in the visible region (16). However, this method may not be suitable if products formed from the reaction of Fe(VI) with a substrate also absorb at similar absorption wavelengths (17). Under these conditions, a fluorescence method would be better to study the reactions of Fe(VI) with substrates. Thus, the present study offers an alternative method to determine concentrations at the low |iM range in acidic solutions. In this chapter, a new fluorimetric technique is proposed to determine low concentrations of ferrate(VI) in water. Fluorescence analysis in general is more sensitive than UV-Vis absorption analysis, so fluorescence should give better measurements at concentrations of ferrate(VI). Scopoletin (7-hydroxy-6methoxy coumarin) (Figure 5.1), a knownfluorescenceagent, was chosen for this method. Scopoletin has been used to determine hydrogen peroxide using a peroxidase catalyzed oxidation method in natural waters (18). Although species present in natural waters were found to interfere in determining the concentrations of hydrogen peroxide, the goal of the proposed method is to determine the concentration of Fe(VI) in distilled deionized (DD) water with no interferences before adding the sample to natural waters, if at all. To demonstrate that this technique is sufficiently rapid to be useful over a wide pH range, a kinetic study of ferrate(VI) reaction with scopoletin was first examined at pH values where the reaction can still be detected at > 0.005seconds. Detailed absorption andfluorescencespectral studies of scopoletin at different pH were carried out to choose appropriate wavelengths forfluorometricmeasurements. In addition, calibration curves as a function of pH were constructed to demonstrate a linear decrease in thefluorescenceof scopoletin with increasing concentrations of ferrate(VI) in DD water.


+

Figure 1. The molecular structure of scopoletin


148

Experimental Materials All the chemicals were purchased from Sigma-Aldrich, reagent grade or better, and were used without further purification. Solutions were prepared with water that had been distilled and passed through an 18MQ Milli-Q water purification system. Scopoletin was prepared in 0.01M Na HP0 and it was placed in a dark bottle to prevent decomposition due to visible light (19). Likewise, solutions containing scopoletin were minimally exposed to light. Potassium ferrate (K Fe0 ) of high purity (98% plus) was prepared by the method of Thompson et al. (20). The ferrate(VI) solutions were prepared by the addition of solid samples of K Fe0 to deoxygenated 0.005M Na HPO /0.001M borate at pH 9.0. Phosphate was used in the buffer for complexing Fe(III), which would otherwise precipitate as a hydroxide to interfere with the optical monitoring of the solution (14). The concentrations of ferrate(VI) were determined by measuring the absorbance at 510 nm and using the molar absorption coefficient e i m = 1150 M" cm" at pH 9.0 (21). 2


2

4

4

2

4

2

1

5

4

1

0n

Apparatus A stopped-flow spectrophotometer (SX.18 MV, Applied Photophysics, UK) equipped with a photomultiplier detector was used to make the kinetic measurements. The kinetic curves were analyzed using a non-linear leastsquares algorithm within the SX.18 MV software. The temperature of this system was 25 ± 0.1 °C, which was controlled by a Fischer Scientific Isotemp 3016 circulating water bath. The rate constants obtained represent the average value of six kinetic runs. An Orion 71 OA ion selective electrode system equipped with a glass pH electrode was used for all pH measurements. Standard buffers of pH 4.0, 7.0, and 10.0 were used to calibrate the electrode and to determine the pH of the mixed solutions. An HP8453 UV/Vis spectrophotometer was used for spectral studies. A 1 cm quartz cuvette was used to carry out the measurements at 25 °C. A Spex FluoroMax-3 fluorimeter was used to perform fluorescence measurements at 25 °C. The excitation and emission wavelengths were 335 nm and 460 nm, respectively. Slit widths were set at 2 nm band pass. The shutter was kept closed until the measurement was made (>3 min) in order to exclude incident radiation that can cause photobleaching (19).


149

Results and Discussion


Kinetic Experiments In these experiments, equal volumes of 200 jaM ferrate(VI) and 2000 jiM scopoletin were mixed at different pH values. The reaction was followed by monitoring ferrate(VI) absorbance at 510 nm as a function of time. Excess scopoletin ensured that reactions were measured under pseudo-order conditions. The absorbance versus time profile for ferrate(VI) gave a single-exponential decay curve, indicating the reaction wasfirst-orderwith respect to ferrate(VI). Reactions of ferrate(VI) with several similar compounds have also shown a firstorder rate with respect to the compound (5,22); thus, the rate expression for the reaction of ferrate(VI) with scopoletin is assumed to be: -d[Fe(VI)]/dt = klFeCVOftSC]

1

(2)

where [Fe(VI)] and [SC] are the concentrations of ferrate(VI) and scopoletin, respectively, and k is the overall reaction rate constant. The values of k were determined for different basic solutions and are given in Table 1. The rate constant of the reaction increases with a decrease in pH. At pH—i—i—i—'—i—«—r~ 20

40

60

80

100

[Ferrate(VI)], » 1 pH=4.95

n

I 20

1

I 40

100

[Ferrate(VI)], juM Figure 4. Emission intensity of scopoletin versus [Ferrate] at pH 10.10 and at 4.95.


153 Fig. 4 represent the average value of four measurements. Fig. 4 shows a linear decrease in intensity with the increase in concentration of ferrate(VI) up to 80 JLIM. The following slopes (±0.0004) were obtained: 0.0073 at pH 10.10, 0.0077


at pH 9.10, 0.0100 at pH 8.88, 0.0120 at pH 5.94, and 0.0120 at pH 4.95.

The

slope is the change in relative intensity divided by the change in concentration of Fe(VI). At the lower pH, the higher fluorescence intensity results in a greater change in intensity producing a higher slope. The steeper slope allows low concentrations of Fe(VI) to be determined even more precisely at the lower pH values, where other methods are ineffective. The linearity of the curves at low Fe(VI) concentrations clearly demonstrates the ability of the fluorimetric technique to determine low concentrations of ferrate(VI) over a wide pH range. Moreover, correlation coefficients > 0.98 for the plots show that possible interfering variables do not influence the results significantly in this fluorometric method. Additional experiments were performed at pH 9.0 to confirm that Fe(lII), produced from Fe(VI) does not interfere with the fluorescence measurements. In these experiments, the fluorescence intensity was measured for solutions having 50 uM scopoletin and Fe(III) at concentrations ranging from 10 (xM to 80 \iM. The concentration range of Fe(III) is similar to what it would be after Fe(VI) reaction with scopoletin. The fluorescence intensity of scopoletin was unaffected by the addition of Fe(III) within experimental error. Thus, the presence of Fe(III) does not interfere with the fluorescence determination of Fe(VI) using scopoletin.

Applicability of the Method With this technique, it is possible to determine low Fe(VI) concentrations over a wide pH range in DD water. This is important for laboratory studies involving Fe(VI), such as kinetics experiments, and for adding known concentrations of Fe(VI) to natural waters, as in water treatment. Experiments in this study showed that low concentrations of Fe(III) do not interfere in the fluorescence measurements. This fluorimetric technique to determine low Fe(VI) concentrations has several advantages. Most importantly, this fluorimetric technique is applicable over a wide pH range including acidic solutions; it is possible to measure Fe(VI) concentrations at low pH values especially below pH 6 if the absorption spectroscopy method fails due to the interferences. Finally, the simplicity of the method is that excitation and emission wavelengths can be fixed at 335 nm and 460 nm, respectively, independent of the pH and concentration in determining the standardization curves and unknown concentrations of Fe(VI).


154 Acknowledgment The authors wish to thank Dr. Yunho Lee for useful comments on this manuscript.

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A Fluorescence Technique to Determine Low Concentrations of

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