Summer Research Fellowship Programme of India's Science Academies 2017
Estimation of fluoride and nitrate in
water samples using
spectrophotometer
Shubham Jain
Under the guidance of
Prof. K. Kesava Rao
Department of Chemical Engineering
Indian Institute of Science
Bangalore 560 012 (India)
July 2017
Contents
Acknowledgement 2
1 Estimation of fluoride in water samples 3
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Origin and Occurence . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Materials and Method . . . . . . . . . . . . . . . . . . . . . . 5
1.5.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5.2 Reagents and Apparatus . . . . . . . . . . . . . . . . . 6
1.5.3 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Estimation of Nitrate in Groundwater 12
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Origin and Occurence . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5 Materials and Method . . . . . . . . . . . . . . . . . . . . . . 14
2.5.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5.2 Reagents and Apparatus . . . . . . . . . . . . . . . . . 15
2.5.3 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.6 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 Packed bed modelling for purification of groundwater 18
Appendix 20
References 24
1
Acknowledgement
I am thankful to Prof. K. Kesava Rao, whose encouragement, guid-
ance and support from the initial to the final level enabled me to
develop an understanding of the subject and for his constant guid-
ance throughout my period of work here. It was very interesting and
enjoyable learning process for me throughout my association with
him. I would also like to thank the Indian Academy of Sciences for
providing me this wonderful opportunity.
2
Chapter 1
Estimation of fluoride in
water samples
1.1 Introduction
Groundwater is a very important source of drinking water in India
and many other countries Unfortunately,in many places the water so
obtained is polluted by fluoride. The presence of excess fluoride in
drinking water causes various diseases that are collectively referred
to as fluorosis. The total number of people affected is not kown, but
a conservative estimate would be tens of millions[1]. The earth’s up-
per crust has minerals containing fluoride, which release the fluoride
ion F
into the groundwater. This ion can enter the human body
through drinking water and other sources such as food, toothpaste
and air. A minimum amount of fluoride is required in drinking water
to prevent tooth decay[2]. However a regular intake of water con-
taining excess fluoride can cause severe diseases[3].
As fluorosis has no cure, it is essential that people drink water with
fluoride within a definite amount. According to the WHO guide-
lines([4], p.376) this limit in drinking water is 1.5 mg/L, whereas the
Indian Standard recommends 1 mg/L[5].
3
1.2 Origin and Occurence
Naturally occurring fluorides in groundwater are a result of the dis-
solution of fluoride-containing rock minerals in water [6]. The major
minerals of fluoride found in hardrock are fluorite, appatite, cryolite
etc. It is not only the presence of minerals, but also the hydrogeolog-
ical conditions contribute more towards the mobilization of fluoride
in groundwater. Alkaline groundwaters generally tend to solublize
fluoride minerals. Fluoride rich groundwaters generally have more
Mg than Ca. Salinity buildup due to extensive irrigation also con-
tribute to fluoride in ground water e.g. Haryana, Punjab. States
such as Andhra Pradesh, Tamil nadu, karnataka, Uttar Pradesh, the
occurance of high fluoride is due to prevaliing hydrogeological con-
ditions. Anthropogenic sources contributing to fluoride are mainly
mining activities, phosphate fertilizer effluents. Globally, fluoride in
groundwater is mostly due to geogenic in nature.
1.3 Effects
Ingestion of low levels of fluoride compounds is beneficial to the body
and prevents dental caries. But long term ingestion of excess fluoride
can be harmful to the body and cause a condition known as fluorosis
that affects teeth and bones. Moderate amounts of fluoride ingestion
can cause dental fluorosis, which is characterized by staining and
pitting of the teeth. In more severe cases all the enamel may be
damaged.
Chronic high-level exposure to fluoride can lead to skeletal flu-
orosis. In skeletal fluorosis, fluoride accumulates in the bone pro-
gressively over many years. The early symptoms of skeletal fluorosis
include stiffness and pain in the joints. In severe cases, the bone
4
structure may change and ligaments may calcify, with resulting im-
pairment of muscles and pain.
There is no medicine for fluorosis, but treatment systems that can
regulate the amount of fluoride in water are available. The control
of drinking-water quality is therefore critical in preventing fluorosis.
In all fluoride affected areas it is advised that rainwater harvesting
is done to recharge the groundwater source that shows high fluoride
levels.
1.4 Objective
The objective of this project is to perform experiments to determine
the concentrations of fluoride ion in groundwater samples. For this
SPADNS method was used. After making the known solutions cal-
ibration curves were made using the absorbance readings of these
solutions obtained with the help of UV-Visible spectrophotometer.
The concentration of the unknown samples were obtained with the
help of these curves. Owing to the limitations of time, only synthetic
water samples were tested.
1.5 Materials and Method
1.5.1 Principle
The SPADNS colorimetric method is based on the reaction between
fluoride and a zirconium-dye lake. Fluoride reacts with the dye lake,
dissociating a portion of it into a colorless complex anion (ZrF
2
6
);
and the dye. As the amount of fluoride increases, the color produced
becomes progressively lighter[7].
5
The reaction rate between fluoride and zirconium ions is influenced
greatly by the acidity of the reaction mixture. If the proportion of
acid in the reagent is increased, the reaction can be made almost
instantaneous.
1.5.2 Reagents and Apparatus
Stock fluoride solution: 221.0 mg anhydrous of sodium fluo-
ride, NaF, was dissolved in deionised water and diluted to 1000 mL,
thus 1.00 mL = 100 µg F
.
Standard fluoride solution: 100 mL of stock fluoride solution
was diluted to 1000 mL with deionised water, thus 1.00 mL = 10 µg
F
.
SPADNS solution: 95.8 mg of SPADNS, sodium 2-(parasulfophenylazo)-
1,8-dihydroxy-3,6-napthalene disulfonate was dissolved in deionised
water and diluted to 50 mL.
Zirconyl-acid reagent: 13.3 mg of zirconyl chloride octahydrate,
ZrOCl
2
.8H
2
O, was dissolved in about 2.5 ml of deionised water and
35 mL of HCl (35%) were added. The solution was made up to 50
mL by adding deionised water.
Acid zirconyl-SPADNS reagent: The SPADNS and zirconyl
chloride solutions were mixed in equal volumes to produce a SPADNS-
ZrOCl
2
complex, henceforth referred to as reagent S. This reagent is
stable for more than 2 years if stored away from light.
A UV-Visible spectrophotometer for use at 570 nm.
6
1.5.3 Procedure
Fluoride standards in the range of 0 to 1.4 mg F
/L were prepared
by diluting appropriate quantities of standard fluoride solution to 50
mL. 10 mL of the reagent S were added to each of the standards
and mixed well. The standard which has 0 mg F
/L was taken
as the reference solution and the spectrophotometer was set to zero
absorbance. The absorbance readings of other standards were taken
with respect to the reference solution and were used to construct the
calibration curve (Fig. 1.1). The 95% confidence limits, shown by
the dashed lines were also calculated and drawn (Fig. 1.1). Polyno-
mial regression was also done to find the best equation that fits the
graph (Table 1.1).
A slightly modified version of the above mentioned procedure was
used to measure fluoride concentrations greater than 1.4 mg/L. Through
this method we can find fluoride concentration up to 5 mg/L in wa-
ter[8]. Fluoride standards in the range of 0 to 5 mg F
/ L were
prepared by diluting appropriate quantities of standard fluoride so-
lution to 50 mL. 5 mL of each of these standards were pipetted out
in separate beakers and 5 mL of the reagent S were added to each of
these beakers and the volume was made up to 30 mL using deionised
water in each beaker. A solution prepared by adding 5 mL of reagent
S to 25 mL of deionised water was used as the reference solution. The
absorbance readings of other standards were taken with respect to the
reference solution and were used to construct the calibration curve
(Fig. 1.2). The 95% confidence limits, shown by the dashed lines
were also calculated and drawn (Fig. 1.2)(Appendix 1).
Polynomial regression was also done to find the best equation that
fits the graph (Table 1.2). Since the samples in this case were diluted
with deionised water by a factor of five relative to previous proce-
7
dure; hence a larger concentration of F
in the undiluted sample can
be measured as compared to previous one. However the confidence
limits are larger in the latter case as can be seen in the figure.
1.6 Observations
y = 0.294x
R² = 0.9989
-0.03
0
0.03
0.06
0.09
0.12
0.15
0.18
0.21
0.24
0.27
0.3
0.33
0 0.2 0.4 0.6 0.8 1 1.2
Absorbance
Concentration(mg/L)
Figure 1.1: Absorbance v/s Concentration curve for first
experiment.
1
1
The observations table is in Appendix 2
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