Spectrophotometric Determination of Fluoride in Groundwater Using Resorcin Blue Complexes

doi:10.4236/ajac.2012.39085

Paper Menu >>

Journal Menu >>

American Journal of Analytical Chemistry, 2012, 3, 651-655

http://dx.doi.org/10.4236/ajac.2012.39085 Published Online September 2012 (http://www.SciRP.org/journal/ajac)

Spectrophotometric Determination of Fluoride in

Groundwater Using Resorcin Blue Complexes

Zaher Barghouthi1, Sameer Amereih2*

1National Agricultural Research Center (NARC), Jenin, Palestine

2Palestine Technical University-Kadoori, Tullkarm, Palestine

Email: *[email protected]

Received June 30, 2012; revised July 28, 2012; accepted August 15, 2012

ABSTRACT

New simple and sensitive spectrophotometric determination of fluoride in drinking groundwater has been developed

using aluminium-resorcin blue complex. The method is based on the reaction of fluoride with the coloured complex to

produce colourless aluminium fluoride complex and releasing of the free ligand. The relationship of the reaction of

fluoride with the complex is sixth-order polynomial function. The reaction reaches equilibrium at fluoride concentration

of 0.054 mM. The equilibrium constant (Keq) was calculated as 1.12 × 1014. Beer-Lambert law is obeyed in the range

0.0 - 0.024 mM of fluoride (0.0 - 1.0 mg·L−1). The molar absorptivity at 502 nm is 6.45 × 103 L·mol−1·cm−1. Fluoride

concentration higher than 1.0 mg·L−1 can be measured after proper dilution. The sensitivity, detection limit, quantitation

limit, and the percentage recovery of 0.75 mg·L−1 fluoride for the method were found to be 0.357 μg·ml−1, 0.07 mg·L−1,

0.2 mg·L−1, and 101.1 respectively.

Keywords: Fluoride; Groundwater; Spectrophotometric Method; Aluminium Resorcin Blue Complex

1. Introduction

WHO (2006) [1] has considered fluoride as one of the

very few chemicals that have been shown to cause signifi-

cant effects in people. There is a narrow margin between

the desired and harmful doses of fluoride [2]. Low con-

centration of fluoride in drinking water have been con-

sidered beneficial to prevent dental carries [3], but excessive

exposure to fluoride can give rise to a number of adverse

effects such as causing fluorosis [1,4,5]. WHO has set a

limit value of 1.5 mg·L−1 for fluoride in drinking water

[6]. This necessitates an accurate, simple, rapid and cost

effective analytical method is of high importance.

Spectrophotometric methods, which are widely used in

the determination of fluoride, are based on the reaction of

fluoride with coloured metal chelate complexes, produc-

ing either a mixed-ligand ternary complex or replacement

of the ligand by fluoride to give a colourless metal-fluo-

ride complex and the free ligand with a colour different

of the metal-ligand complex [7].

Resorcin blue is used in literature as pH indicator for

mineral acids, strong bases and alkaloids where the vis-

ual-transition interval is red at pH 4.4 to blue at pH 6.4

[8]. It is also used as a redox indicator in the titration of

Fe(II), As(III), Sb(III), U(IV), Mo(V), hydroquinone, and

oxalic acid with ammonium hexanitratocerate (IV) in

HClO4 medium [9]. Resorcin blue is used in the deter-

mination of Cr(VI) [10], and for staining cellulose in plants

[11].

The present study aimed to develop spectrophotomet-

ric method for determination of fluoride in drinking wa-

ter using aluminium resorcin blue complex with fluoride

ions.

2. Experimental

2.1. Apparatus

Beckman DU-7500 single beam spectrophotometer with

1.0 cm quartz cells was used for wavelength scanning and

for spectral studies. Hitachi U-1500 UV/V is single beam

spectrophotometer with 1.0 cm quartz cells was used for

the absorbance measurements at fixed wavelength. Orion’s

Portable 210 A pH Meter with Orion Triode electrode was

employed for the pH measurements.

2.2. Reagents

Resorcin blue provided by Acros (339290050), and alu-

minum chloride hexahydrate (Purum p. a, 06232) provided

by Fluka were used without any further purification. All

the chemicals were of analytical reagent grade except where

stated otherwise. Solutions were prepared using double

distilled water. Resorcin blue ligand solution and the alu-

minum complex solution were prepared using ethanol from

*Corresponding author.

Z. BARGHOUTHI, S. AMEREIH

652

Merck (reagent 96%, 159010). Standard fluoride stock sol-

ution was prepared by dissolving 0.1382 g of sodium fluo-

ride provided by Merck (ACS reagent, 106449) in 250 ml

water. The stock solution was further diluted as needed.

2.3. Preparing of the Metal Complexes Solutions

Job’s method of continuous variation was adopted for de-

termination of the composition of the coloured complex

[12,13]. Aluminium to ligand ratio was also studied by

making comparison between the spectra of complexes of

different metal to ligand ratios such as 1:1, 1:2, 1:3, 2:1,

3:1, 2:3, and 3:2. The blank was prepared by the same

procedure using the solvent instead of the aluminum ion

solution. Aluminum to ligand ratio was found to be 1:2.

The complex solutions for the spectrophotometric mea-

surements were prepared by mixing aluminum 1:2 resor-

cin blue ratio of 5 × 10−3 M of aluminum and 5 × 10−3 M

of resorcin blue in ethanol solution, which was then di-

luted to (2 × 10−4 M) that is suitable for the spectropho-

tometric measurements. The stability of the complex in

ethanol solution was examined for two weeks, and the com-

plex is stable.

2.4. Reaction of Fluoride with the Prepared

Complexes Solutions

Various amounts of fluoride were added in the range 0 -

2 mg·L−1 to 25 ml volumetric flask containing aluminum

1:2 complex solution of resorcin blue in ethanol (2 × 10−4

M, 24.5 ml). The solution was completed to volume by

water. The absorbance was measured at the wavelength

of the maximum difference between the absorption spec-

tra of the complex and the ligand which was 624 nm.

2.5. Determination of Fluoride in Real Water

Samples

The method under investigation was tested using a real

drinking water sample which had been collected and ana-

lysed by the Central Public Health Laboratory belonging

to Ministry of Health and responsible for controlling wa-

ter quality. The sample was collected in June 2011 from

a groundwater well in Tubas District (Aqaba well). Fluo-

ride was analysed colourimetrically using SPADNS as

fluoride reagent and Hack-DR/2010 as spectrophotome-

ter. Nitrate, sulfate, chloride, and other characteristic data

of the sample are given in Table 1.

Table 1. Analytical data of Aqaba groundwater sample ana-

lysed by ministry of health laboratories.

pH Conductivity

µS·cm−1

Fluoride

mg·L −1

Nitrate

mg·L −1

Chloride

mg·L −1

Sulfate

mg·L −1

TDS

mg·L −1

7.17 826.00 0.68 0.33 90.33 87.00413.00

Fluoride was measured in the sample using aluminium

resorcin blue 1:2 complexes and the obtained results were

compared with that reported by the Central Public Health

Laboratory using SPADNS method (Table 1 and Table

2). The method under investigation was tested using a

real drinking water sample which had been collected and

analysed by the Central Public Health Laboratory belong-

ing to Ministry of Health and responsible for controlling

water quality. The sample was collected in June 2011 from

a groundwater well in Tubas District (Aqaba well). Fluo-

ride was analysed colourimetrically using SPADNS as

fluoride reagent and Hack-DR/2010 as spectrophotome-

ter. Nitrate, sulfate, chloride, and other characteristic data

of the sample are given in Table 1. Fluoride was meas-

ured in the sample using aluminium resorcin blue 1:2 com-

plexes and the obtained results were compared with that

reported by the Central Public Health Laboratory using

SPADNS method.

3. Results and Discussion:

3.1. Resorcin Blue and its Aluminium Complexes

Resorcin blue is soluble in ethanol (20 mg·mL−1), metha-

nol, acetic acid and acetone, and slightly soluble in ether.

Its solubility in water is 30 mg·mL−1 [8]. Resorcin blue

exhibits blue colour in ethanol solution and displays two

bands in the visible region at 505 and 617 nm (Figure 1).

The molar absorptivity at these two wavelengths is

[

]

[

]

5.980.1210and 4.910.0910±× ±× L·mol−1·cm−1 res-

pectively.

Table 2. Sensitivity, detection limit, quantification limit, and

recovery of the method.

Parameters Values at 624 nm

Sensitivity [μg·mL−1] 0.357 ± 0.005

Detection limit [mg·L−1] 0.07

Quantification limit [mg·L−1] 0.2

Recovery of real water sample % 99.1 ± 4.4

Recovery of 0.75 mg·L−1 % 101.1 ± 3.9

Recovery of 1.5 mg·L−1 % 98.3 ± 4.3

Recovery of 2.0 mg·L−1 % 96.1 ± 3.7

0.2

0.4

0.6

0.8

340 440 540 640740

Wavelength in nm

ABS

Dye

Co mpl ex

Figure 1. Electronic spectra of resorcin blue and its alumin-

ium 1:2 complex in ethanol at 2 × 10−4 M.

Z. BARGHOUTHI, S. AMEREIH 653

The obtained results from applying of Job’s method of

continuous variation indicated that aluminium to resorcin

blue complex ratio is 1:2. The ratio was also determined

spectrophotometrically by comparing the spectra of alu-

minum resorcin blue complexes of different ratios with

each others. A possible structure for the complex is given

in Figure 2.

The complex exhibits brown colour in ethanol solution

and has one band in the visible region at 502 nm (Figure

1) where the molar absorptivity is

[

]

0.22 10±×

()

−

6.45

L·mol−1·cm−1. Thus, there is hypsochromic shift (decrease

in the wavelength) of about 115 nm after complexation

with aluminum. This is resulting in a change in colour from

that of the ligand, blue to the colour of the complex, brown.

Resorcin blue displays two bands in the visible region

at 496 and 591 nm while its aluminium complex exhibits

one band at 483 nm in water solutions. Therefore, using

of water instead of ethanol as a solvent is resulting in a

hypsochromic shift in the absorption spectra of the resor-

cin blue ligand and its aluminium complex of about 26

and 19 nm, respectively. Thus, the solvent has a negative

solvatochromism effect on the spectra of both of the ligand

and it aluminium complex where increasing of the polar-

ity of the solvent, as we move from ethanol to water, leads

to a hypsochromic shift (a decrease in the wavelength)

[14,15]. The difference in the absorption spectra between

the complex and the ligand is bigger in ethanol than in

water solution. This is due to the polarity of water and its

ability to form hydrogen bonds in comparison with etha-

nol. This leads to destabilize the excited state which is

expected be less polar than the ground state [15]. There-

fore the effect of fluoride on the absorption spectra of the

complex was examined in ethanol solution.

3.2. Reaction of Fluoride with the Resorcin Blue

Complex

Fluoride reacts with the brown aluminium resorcin blue

1:2 complex to produce a colourless aluminium fluoride

complex by replacement of the resorcin blue by fluoride

and liberating of the free ligand. This is resulting in a

change in the colour from that of the complex, brown to

the colour of the free ligand according to the equation

below. Aluminium reacts with fluoride to give compounds

of the nature of or [16].

()

AlF−Al F

[

]

[

]

Alresorcin blue6FAlF2

Brown Colorless

−−

+→ +resorcin blue

Blue

()

0.3567 0.2773yx=+

()

[]

()

Fluoride interacts with complex under investigation to

cause an increase in absorbance at 624 nm due to the re-

leasing of the free ligand. The absorbance of the released

free ligand is related linearly at 624 nm to the concentra-

tion of fluoride in the range 0.0 to 0.024 mM (0.0 to 1.0

mg· L–1) (Figure 3).

The squared correlation coefficient R2, is 0.993, and the

equation of the linear calibration curve is

. The relationship of the reaction

of fluoride with aluminum resorcin blue 1:2 complex was

best described by a sixth-order polynomial function (Figure

4) where the squared correlation coefficient R2, is 0.9982.

The reaction reaches equilibrium at fluoride concentration

of 0.054 mM (≈2.27 mg·L−1). The equilibrium constant

(Keq) was calculated from the equilibrium equation below

as 1.12 × 1034. The large value for Keq indicates that the

equilibrium lies far to the right.

AlFresorcin blueAlresorcin blueF

K−−









=







OH OH

OH O

Figure 2. Possible structure for aluminium resorcin blue 1:2

complex.

y = 0.3567x + 0.2773

R2 = 0.9933

0.1

0.2

0.3

0.4

0.5

0.6

0.7

00.2 0.4 0.6 0.811.2

Fluoride mg L

-1

Absorbance

Fluoride mg·L

–1

Figure 3. Calibration curve for determination of fluoride in

the range (0.0 - 1.0) mg·L−1 at 624 nm by aluminum resor-

cin blue complex of 2.0 × 10−4 M.

y = 3E-05x6

- 0.0009x5 + 0.0105x4 - 0.0546x3 + 0.1002x2 + 0.1132x + 0.279

2 = 0. 9982

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

024681012

Fluoride (mM x102)

Abso r banc e

Figure 4. Absorbance of aluminum resorcin blue 1:2 com-

plex of 2.0 × 10−4 M versus fluoride concentration in the

range 0.0 - 0.119 mM at 624 nm.

Z. BARGHOUTHI, S. AMEREIH

654

where: [resorcin blue] is measured using the molar ab-

sorptivity for resorcin blue, which was found to be

[

]

0.07 10±×

()

in blueis





5.67 L·mol−1·cm−1 at 624 nm, and the ab-

sorbance at the equilibrium;

Al resorc

()

[

]

2initial

Al resorcin blue12r



−



esorcin blue;

()

[]

esorcin blue

()

AlF

−



−

AlFis 12r

−



 ;

and

––

initial

FisF



.

Figures 3 and 4 show that aluminium resorcin blue

complex is suitable for determination of fluoride in the

range 0.0 to 1.0 mg·L−1. The sensitivity, detection limit,

limit of quantification, percentage recovery of fluoride in

real water sample, and the percentage recovery of 0.75,

1.5, and 2.0 mg·L−1 fluoride of the aluminium resorcin

blue complex for the spectrophotometric determination of

fluoride at 624 nm are given in Table 2.

The sensitivity was taken as the average of the slope of

the calibration curve for five replicates. The detection limit

and the limit of quantification were calculated as (3.3 σ/S)

and (10 σ/S) respectively, where σ is the standard devia-

tion of response and S is the slope of the calibration curve.

The recovery was measured as the average of 10 rep-

licate. The recover of high fluoride concentration such as

1.5, and 2.0 mg·L−1 fluoride was calculated by measuring

the absorbance for the diluted solutions (1 to 3).

The interference studies were done by measuring the

influence of the anions such as chloride, nitrate, and sul-

phate in such concentration commonly found in the natural

water on the determination of 1.0 mg·L−1 fluoride. Chloride

and nitrate which were added in the range of 100 - 500

and 5 - 100 mg·L−1 respectively do not interfere with the

determination of fluoride. Sulphate interferes with the most

visual and photometric methods for determination of fluo-

ride by its competition with fluoride to form a complex

with the metal and therefore it results in higher concen-

trations [17,18]. In the present work, sulphate up to 100

mg· L−1 does not interfere with the determination of fluo-

ride. However, at higher concentration, sulphate interferes

with determination of fluoride by causing a positive error

of about 25%. This error can be overcome by precipitat-

ing sulphate in the cold by the addition of aqueous bar-

ium chloride solution and aqueous agar-agar solution, then

to separate the precipitate by filtration [19].

The proposed spectrophotometric method can be ap-

plied without any previous preparations, such that were

necessary to separate fluoride ions, to control fluoride in

countries with low fluoride content water resources.

4. Conclusions

The relationship of the reaction of fluoride with alumin-

ium-resorcin blue complex is related linearly at 624 nm

to the concentration of fluoride in the range 0.0 - 1.0

mg· L −1. However, at higher fluoride concentration, the

relationship was best described by a sixth-order polyno-

mial function. The reaction reaches equilibrium at fluo-

ride concentration of 0.054 mM, and the equilibrium con-

stant (Keq) was found to be 1.12 × 1034.

Aluminium-resorcin blue complex was used success-

fully as new spectrophotometric reagent for determina-

tion of fluoride in water in the range 0.0 to 1.0 mg·L−1.

Due to its simplicity and high sensitivity, the method can

be recommended as new spectrophotometric reagent for

determination of fluoride in drinking water at low con-

centration. However, determination of fluoride at higher

concentration is possible by diluting of the water sample

to fit the requirements of the proposed method.

5. Acknowledgements

The authors thank Professor Walter Kosmus from the Ana-

lytical Chemistry Institute in the University of Graz-Aus-

tria for his advices, suggestions, ideas, comments, helpful

discussions, and for his supervision through the Ph. D

research. The Director (Mr. Ibrahim Salem) and the staff

of the Central Public Health Laboratory—Ministry of Hea-

lth are highly acknowledged for their cooperation and for

sharing their data.

REFERENCES

[1] WHO, “Fluoride in Drinking-Water,” IWA Publishing,

London, 2006, pp. 1-3, 83-95.

[2] W. Czarnowski, K. Wrzesniowska and J. Krechniak,

“Fluoride in Drinking Water and Human Urine in North-

ern and Central Poland,” Science of the Total Environ-

ment, Vol. 191, No. 1-2, 1996, pp. 177-184.

doi:10.1016/0048-9697(96)05259-X

[3] S. M. Maliyekkal, S. Shukla, L. Philip and I. M. Nambie,

“Enhanced Fluoride Removal from Drinking Water by

Magnesia-Amended Activated Alumina Granules,” Che-

mical Engineering Journal, Vol. 140, No. 1-3, 2008, pp.

183-192. doi:10.1016/j.cej.2007.09.049

[4] Y.-N. Wang, K. Q. Xiao, J. L. Liu, G. Dallner and Z. Z.

Guan, “Effect of Long Time Fluoride Exposure on Lipid

Composition in Rat Liver,” Toxicology, Vol. 146, No. 2-5,

2000, pp. 161-169. doi:10.1016/S0300-483X(00)00167-0

[5] M. A. Armienta and N. Segovia, “Arsenic in Fluoride in

Groundwater of Maxico,” Environmental Geochemistry

and Health, Vol. 30, No. 4, 2008, pp. 345-353.

doi:10.1007/s10653-008-9167-8

[6] WHO, “Guidelines for Drinking-Water Quality. Recom-

mendation,” 2nd Edition, Vol. 1, World Health Organiza-

tion, Geneva, 2004, pp. 375-377.

[7] H. Einaga and I. Iwasaki, “Spectrophotometric Study of

the Reaction of Zirconium with Chrome Azurol S and

Fluoride,” Talanta, Vol. 28, No. 12, 1981, pp. 889-900.

Z. BARGHOUTHI, S. AMEREIH

655

doi:10.1016/0039-9140(81)80224-X

[8] F. J. Green, “The Sigma—Aldrich Handbook of Stains,

Dyes and Indicators,” Aldrich Chemical Company, Mil-

waukee, 1990, p. 624.

[9] N. V. Rao and K. R. Murty, Journal of the Indian

Chemical Society, Vol. 56, 1979, pp. 641-642.

[10] H.-Z. Wu, Z.-R. Wu, J. Lin and Z.-S. Zheng, “Catalytic

Spectrophotometric Determination of Trace Chromium

(Ⅵ) with Sensitization Effect of Surfactant,” Fenxi

Shiyanshi, Vol. 18, 1999, pp. 26-29.

[11] M. Ulrychova, E. Petru and Z. Pazourkova, “Permanent

Staining of Callose in Plant Material by Ponceau,” Stain

Technol, Vol. 51, No. 5, 1976, pp. 272-275.

[12] L. Werner and D. F. Boltz, “Theory of Contineous Varia-

tions Plots and a New Method for Spectrophotometric

Determination of Extraction and Formation Constant,”

Analytical Chemistry, Vol. 43, No. 10, 1971, pp. 1265-

1272. doi:10.1021/ac60304a006

[13] L. Werner and D. F. Boltz, “Spectrophotometric Deter-

mination of Extraction Constants of Certain Metal 1-Pyr-

rolidinecarbodithioates,” Analytical Chemistry, Vol. 43,

No. 10, 1971, pp. 1273-1277.

[14] P. F. Gordon and P. Gregory, “Organic Chemistry in

Colour,” Springer-Verlag, Berlin, 1983, p. 303.

[15] H. Zollinger, “Colour Chemistry: Syntheses, Properties,

and Applications of Organic Dyes and Pigments,” Verlag

Helvetica Chimica Acta, Zuerich, 2003, p. 42.

[16] B. J. Macnulty, C. J. Hunter and D. G. Barrett, “The De-

termination of Microgram Quantities of Fluoride: The

Use of Aluminium-Eriochromecyanin Complex,” Ana-

lytica Chimica Acta, Vol. 14, pp. 1956, 368-380.

[17] M. J. Price and O. J. Walker, “Determination of Fluoride

in Water,” Analytical Chemistry, Vol. 24, No. 10, 1952,

pp. 1593-1595. doi:10.1021/ac60070a018

[18] J. A. Ruzicka, H. Jakschova and L. Marklas, “Determina-

tion of Fluorine in Bones and Teeth with Xylenol Or-

ange,” Talanta, Vol. 13, No. 9, 1966, pp. 1341-1344.

doi:10.1016/0039-9140(66)80224-2

[19] E. J. Dixon, “Determination of Micro Amounts of Fluo-

ride with Zirconium and Solochrome Cyanine R.,” Ana-

lyst, Vol. 95, No. 1128, 1970, pp. 272-277.