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
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.
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opyright © 2012 SciRes. AJAC

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
[
]
[
]
33
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
0.2
0.4
0.6
0.8
1
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
[
]
3
0.22 10±×
()
()
3
6
OH
yy
−
−
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].
()
3
6
AlF−Al F
[
]
[
]
13
6
2
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.
6
32
62
AlFresorcin blueAlresorcin blueF
eq
K−−


 
=



O
O
H OH
O
N
O
OH OH
OH
M
OH
O
N
O
OH O
Figure 2. Possible structure for aluminium resorcin blue 1:2
complex.
y = 0.3567x + 0.2773
R2 = 0.9933
0
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
9
R
2 = 0. 9982
0
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
[
]
3
0.07 10±×
()
2
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
()
3
6
AlF
−

−
3
6
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.
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