1.1 Abstract Chromium (VI) which is one of a heavy metal is
classified as a strong oxidation agent and it poses a great deal of
toxicity to humans and animals as well due to its carcinogenetic
even at low aqueous solutions concentration. Chromium ions are
found naturally in rivers, lakes, and streams. This health-risky
situation has led to many studies and investigations on determining
the presence and quantity of chromium in biological and
environmental samples. In the present experiment, an absorption
spectroscopy method was used for the determination of chromium (VI)
concentration and the method is operated by tracing the presence of
chromium (VI). As a result for this experiment, the concentration
of Chromium (VI) in the lake water sample tested was 75 p.p.m. The
experiment is completed and successfully conducted.1.2
Introduction
1.3 AimsThe objectives of conducting this experiment are :
1)To determine the Chromium (VI) content which present in
simulated lake water sample using a spectrophotometer.
2)To demonstrate the proper method of diluting solution to
prepare a series of standard solutions in the range of 5 to 100
parts per million (p.p.m) to be used in the calibration of the
spectrophotometer.
3)To analyze whether the stimulated lake water sample tested is
suitable for drinking water and agriculture purposes.
1.4 Theory Chromium is a metal found in natural deposits as ores
containing other elements. The greatest use of chromium is in metal
alloys such as stainless steel; protective coatings on metal;
magnetic tapes; and pigments for paints, cement, paper, rubber,
composition floor covering and other materials. Its soluble forms
are used in wood preservatives. One source of chromium
contamination in natural water is automobiles. Chromium that is
deposited on roads derived from brake dusts and exhaust of
automobile engine can unfavorably permeate into water streams
through flowing rainwater. The most susceptible areas to be
contaminated by such chromium are lakes and water streams which is
nearer a congested with automobiles area.
A number of federal environment agencies are doing researches to
determine its presence in many environmental samples. This is done
due to the cancer-risk character and toxicity. Therefore, in this
experiment absorption spectroscopy is being used to detect low
level concentrations of Chromium (VI) in a lake water sample.
Colored aqueous solutions have chemical species contents which
absorb significant wavelengths of light. Similarly, heavy metals
can also be determined by absorption of wavelengths of light. In
addition, the amount of light absorbed is linearly proportional to
the concentration of the solution metal ions The amount of light
absorbed by the chemical species in the sample is equivalent to the
difference in the amount of light before it enters the sample and
after it exits the sample. The light must be set to a specific
wavelength in order to allow the light to be absorbed by the
chemical species. Every chemical species absorbs distinct
wavelengths of light. In absorption spectroscopy, the wavelength of
light absorbed by a metal in solution are detected. In this
experiment, standard solutions will be prepared by diluting 300
parts per million of Chromium (VI) standard solution. Standard
solutions need to be prepared from the concentrated Chromium (VI)
standard solution. The solution dilution formula can be used to
calculate the dilute solutions volume:
(M1) (V1) = (M2) (V2)
Quantitative analysis using spectrophotometer is based on
Beer-Lambert Law:
A = LC
A= absorbance value = molar absorbance (L/mol.com)L = path
length of the cuvette in which the sample is contained (cm)C =
concentration of the compound in solution (mol/L)
According to Beer-Lambert Law, the absorbance value depends on
the total quantity of the absorbing compound in the light path
through the cuvette. Thus we will get a straight line passing
through the origin (0,0) if we plotted the graph of absorbance
versus concentration of the compound solution. The molar
absorbance, , is a constant for a particular substance, therefore
if the concentration of the solution is halved, so is the
absorbance value. A compound with a high molar absorbance is very
effective at absorbing light (of the appropriate wavelength), and
hence low concentrations of a compound with a high molar absorbance
can be easily detected.
1.5 Apparatus Distilled water Spectrophotometer 10 millilitres
square cuvette bottle Pipette 5 set of 10 milliliters of 5, 20, 30,
60, 100 p.p.m of diluted solution of Chromium (VI) 5 set of 300
p.p.m of standard Chromium (VI) solution Lake water sample
Figure 1 : Spectrophotometer
Figure 2 : Spectrophotometer
1.6 MethodologyIn this experiment, the apparatus was set up to
carry out the experiment to determine the concentration of chromium
(VI) by using absorption spectroscopy. Firstly, the 10 milliliters
of square cuvette bottle was filled up with distilled water and
placed on the spectrophotometer slot. A zeroing step was done to
ensure that all the data obtained later was accurate. It followed
by preparing 5 series of diluted chromium (VI) solution by using
the diluted method with distilled water with the range
concentration of 1 until 100 parts per million (p.p.m) solutions.
Thus, for this experiment it used 5 p.p.m, 20 p.p.m, 40 p.p.m, 60
p.p.m, 80 p.p.m and 100 p.p.m solutions which were 300 p.p.m of
standard Chromium (VI) solution of 0.17, 0.67, 1.33, 2.00, 2.67 and
3.33 milliliters. Each series of diluted solution was poured into
square cuvette bottle and placed on the spectrophotometer slot to
check the absorbance value by using the spectrophotometer. Then,
all the data obtained was recorded first. This experiment continued
by preparing 10 milliliters of lake water sample and poured it into
the square cuvette bottle and spectrophotometer was used to
determine the Chromium (VI) concentration contained in the water
sample collected. Lastly, all the value was analyzed for the
observation and the data obtained were recorded. The graph was
plotted to figure out the value of chromium (VI) concentration.
1.7 Results
Concentration of Chromium (VI) solution (p.p.m)Volume of
chromium (VI) solution (mL)Absorbance value
50.30.001
201.30.011
402.70.014
604.00.026
805.30.034
1006.70.043
Figure 1.7.1.1 Table of concentration of Chromium (VI) solution
and their absorbance value
Absorbance of lake water sample = 0.031
Graph 1.7.1.2 Graph of Absorbance Value versus Concentration of
Chromium (VI) in unitsof p.p.m
1.8 CalculationsTo dilute 20 milliliters standard solution of 5
p.p.m, the solution dilution formula is used in order to calculate
the volume of Chromium (VI) solution.(M1)(V1) = (M2)(V2) (300)V1 =
(5)(20) V1 = 0.3 mL
To dilute 20 milliliters standard solution of 20 p.p.m, the
solution dilution formula is used in order to calculate the volume
of Chromium (VI) solution.(M1)(V1) = (M2)(V2)(300)V1 = (20)(20) V1
= 1.3 mL
To dilute 20 milliliters standard solution of 40 p.p.m, the
solution dilution formula is used in order to calculate the volume
of Chromium (VI) solution.(M1)(V1) = (M2)(V2)(300)V1 = (40)(20) V1
= 2.7 mL
To dilute 20 milliliters standard solution of 60 p.p.m, the
solution dilution formula is used in order to calculate the volume
of Chromium (VI) solution.(M1)(V1) = (M2)(V2)(300)V1 = (60)(20) V1
= 4.0 mL
To dilute 20 mililiters standard solution of 80 p.p.m, the
solution dilution formula is used in order to calculate the volume
of Chromium (VI) solution.(M1)(V1) = (M2)(V2)(300)V1 = (80)(20) V1
= 5.3 mL
To dilute 20 mililiters standard solution of 100 p.p.m, the
solution dilution formula is used in order to calculate the volume
of Chromium (VI) solution.(M1)(V1) = (M2)(V2)(300)V1 = (100)(20) V1
= 6.7 mL
Based on the graph, the value of the gradient is :m = = =
0.0004By interpolating graph, the equation of the linear line was
given by as :Y = mX + CThe y-intercept in the graph is 0.001, so,
the value of C = 0.001. by substituting the value of , the equation
is given as :Y = 0.0004X + 0.001X =
By substituting the value of Y with the value of the absorbance
of water sample, which is 0.031 abs, the equation is given as :X =
X = 75X is equivalent to the concentration of Chromium (VI)
contains in the water sample, which is 75 parts per million
(p.p.m).
1.9 Discussion The experiment was conducted to achieve a few of
main objective in this experiment such as to determine the Chromium
(VI) content which present in the water sample by using the
spectrometer, to demonstrate the proper method of diluting
solution, to prepare of standard solution in the range 5 to 100
p.p.m and lastly to analyze whether the water sample is suitable
for drinking water and agriculture purpose. In chemistry,
spectrophotometry is the quantitative measurement of the reflection
or transmission properties of a material as a function of
wavelength and the water sample. The water sample that have been
used to examine the presence of chromium is assumed to be
contaminant, the source of the water sample is the source of water
sample located near a roadway where road users with automobiles use
it every day. This is due to rivers, lakes or any water streams
which are near to congested areas have the highest risk or most
susceptible to contain heavy metals such as chromium itself and is
classified as contaminated water source. The first step in this
experiment is to prepare six standard solution Chromium (VI)
standard solutions using serial method as mentioned in the theory
section. The concentrations for standard 1, standard 2,standard 3,
standard 4, standard 5 and standard 6 is 100 ppm, 80 ppm, 60 ppm,
40 ppm, 20 ppm and 5 ppm respectively. The absorbance for each six
standard solutions and the lake water sample is taken by using the
spectrophotometer. A graph of the absorbance is plotted against the
concentration of standard solutions. Linear regression analysis is
performed in order to determine the linear best fit for the
absorbance versus concentration data. Graph 1.7.1.2 in results show
a graph of absorbance versus concentration of six standard
solutions. The line determined from the regression analysis will be
in the form of y = mx + b, where y is the absorbance value and x is
the concentration of the solution. Algebraic substitution of the
absorbance value (y) for the unknown metal solution into the linear
regression equation for the line permits the determination of the
concentration (x) of the unknown solution. The value of absorbance
of lake water sample is 0.031. From the graph, the concentration of
chromium (IV) in water sample is apparently calculated to contain
75 parts per million.Based on the consideration of state and
federal regulatory agencies, natural waters are regarded to be
toxic if the concentration of Chromium (VI) is any higher than 0.05
parts per million. Natural waters whose chromium (VI)
concentrations exceed 0.05 ppm may not be used for drinking water
or for agricultural purposes. After doing some calculations based
on the graph, the concentration of the lake water sample is
determined to be at 75 ppm which is obviously not suitable for
drinking water and for agricultural purposes. The lake water sample
tested is then concluded to be unsafe and unhygienic for drinking
water or even agricultural purposes. However, a few steps might be
done erroneously which results in such an outrageous value of
chromium (VI) content. First and foremost, the techniques applied
to measure such a small volume of diluting substance might not be
suitable. This might lead to difficulties when measuring the volume
of chromium (VI) needed to dilute the standard solutions.Besides
that, wrong way of holding the cuvette bottle used to contain the
diluted solutions might also lead to errors. Any fingerprints or
smudge imprinted on the outer surface of the cuvette can become an
unfavorably interference for the solutions to absorb wavelengths of
light in the spectrophotometer. Thus, the reading of absorbance by
the instruments may not be accurate.In addition, the experiment
conducted to determine the absorbance value of diluted solutions
that contain chromium (VI) is done only once. Thus, no average
value can be obtained from the results, and so the accuracy of the
value taken is not much convincing.Others, the square cuvette
bottle might not be cleansed properly with distilled water each
time before it is used for another attempt. This will lead to
inaccurate contents of either standard solutions or the chromium
solution.
1.10 ConclusionBased on the experiment, it can be concluded that
chromium (VI) are present in the sample water. The concentration of
Chromium (VI) in the lake water sample tested is calculated to be
75 parts per million. Since it is above the level of standard
allowed by the Malaysian Environmental Quality (Sewage and
Industrial Effluents) Regulations of 1979 which is 0.05 ppm thus,
the water is concluded as contaminated and does not suitable for
drinking or agricultural purposes.
1.11 Recommendations1.Avoid leaving a fingerprints or any kind
of smudge on the outer surface which will corrupt the reading of
absorbance value of spectrophotometer.2.Ensure the square cuvette
bottle clean thoroughly and properly so that the accurate reading
can figure out.3.Use a specific wavelength where the component
absorbs substantially to achieve the accurate value.4.Ensure there
are no air bubbles in the nozzle of the stopper.5.Measure the
volume of diluted chromium accurately and make sure read the level
of solution by noting the lower level of the meniscus at eye to
avoid parallax error.6.Ensure the square cuvette bottle is filled
up with distilled water first and placed on the spectrophotometer
slot to ensure the zeroing step is done so that all the data
obtained later is accurate.
1.12 Reference/Appendix
