ANALYSIS OF NITRATE ION CONCENTRATION IN WATER
Increasing energy
Light can interact with molecules/matter by:
• Being absorbed
Causing the molecule to go to
an excited or higher energy
level
hυ
• Being emitted
As a molecule loses energy and
goes to a lower or ground state.
UV radiation is absorbed by nitrate ions.
source
Measuring at Absmax
polariser
Reduces instrument
(monochromator) error
diffraction grating
Best signal strength for
concentration
A
Differs from molecule to
molecule
sample
nm
detector
spectrum
Schematic of a Spectrophotometer. Experimental setup can be
varied depending on the analyte.
Radiation sources
Deuterium lamp (160-375nm)
Tungsten/halogen lamp (350-800nm)
Types of cells
Quartz (~200-400nm UV)
Glass (~450-700nm Visible)
Plastic (~400-780nm Visible)
Eutrophication
Nitrate ions and phosphorus ions are together known as nutrients
when found in water bodies. These two species if present in sufficient
quantities allow excessive plant growth and eventual stagnation of the
water. This process is known as eutrophication.
This has recently occurred in the Hunter river in 2013. Flood waters
washed fertilisers off farmlands upstream (high in phosphates and
nitrates) into the river. This resulted in oxygen levels to significantly
decrease which lead to mass suffocation and death to all marine
organisms in the river. The Hunter prawn industry was greatly affected.
Calibration Curves
The Beer-Lambert law shows that the absorbance of any solution is
directly proportional to its concentration. To put it simply, the higher
the concentration, the higher the absorbance. The relationship between
concentration and absorbance is shown in the formula:
A=ɛcb
Where A = absorbance, ɛ = molar absorptivity, c = concentration, and
d = pathlength which is usually 1cm. Which simplifies the equation to:
A=ɛc
Plotting a graph of absorbance vs. concentration using solutions of
known concentration, yields a linear calibration curve. Using the
equation of this line, the absorbance of an unknown solution can be
converted to a concentration.
2.5
Absorbance
signal
processing
A=ɛc
2
y = 0.1458x
1.5
1
0.5
0
0
5
10
15
Nitrate concentration (mg/L)
20
Example Calculation using the absorbance of a water sample = 1.214
Rearranging equation to solve for x
x = y / 0.1458
= 1.214 / 0.1458
= 8.32 mg/L (diluted water sample 20mL into 100mL)
http://www.theherald.com.au/story/1363057/fish-kill-stench-of-death-hangs-over-river/
Hunter river near Stockton bridge after 2013 flood event
= 41.6 mg/L (taking into account 5 fold dilution)