EXPERIMENT 6
SPECTROPHOTOMETRIC DETERMINATION OF Fe IN
WATER SAMPLE USING STANDARD ADDITION
METHOD.
Date: 24/03/2023
Name: Dintle Mongwarona
Student ID: 20000445
Programme: BSc Pure and Applied Chemistry
AIM
To determine the concentration of Fe in a water sample using spectrophotometric technique by
applying the standard addition method.
INTRODUCTION
Spectrophotometry is a widely used technique in analytical chemistry to determine the
concentration of a particular substance in a sample. The method is based on the measurement of
the amount of light absorbed by the sample at a specific wavelength, which is related to the
concentration of the substance of interest in the sample (Osuntoki, O.A., 2014).
In the case of determining iron (Fe) in water samples, spectrophotometry can be used in
combination with the standard addition method to obtain accurate results. The standard addition
method involves adding a known amount of the analyte (iron in this case) to the sample, measuring
the absorbance, and repeating this process with different amounts of the analyte until the desired
concentration is obtained (Sanagi, M.S., 2011).
The advantage of the standard addition method is that it corrects for any interference or matrix
effects that may affect the accuracy of the measurement (Dhale, D.S., 2013). For example, in the
case of water samples, there may be other dissolved substances that can interfere with the
determination of iron concentration. By adding a known amount of iron to the sample, the
interference can be corrected for, resulting in more accurate results.
In the spectrophotometric determination of Fe in water samples using the standard addition
method, a reagent is added to the sample to form a colored complex with iron. The color intensity
of the complex is then measured at a specific wavelength using a spectrophotometer (Dhale, D.S.,
2013). The concentration of iron can be determined by comparing the absorbance of the sample to
a calibration curve obtained using standards of known concentration prepared in the same way as
the sample.
In summary, spectrophotometry can be used to determine the concentration of iron in water
samples using the standard addition method. This approach provides a means of correcting for
interferences and matrix effects and can yield accurate and precise results.
PROCEDURE
Firstly, a standard solution of Fe3+ was prepared to give a 100 μg mL-1 Fe3+ solution. Next, a
thiocyanate solution was prepared. Six 50 mL volumetric flasks were then taken, and 30mL of
water samples were transferred to each flask, which were labeled as 1, 2, 3, 4, 5, and 6. To each
flask, 0, 1, 2, 3, 4, and 5 mL of standard Fe3+ solution was added. 5 mL of SCN- solution and 3mL
of 4 M nitric acid were added to each flask, and they were made up to the volume by adding
deionized water. A reagent blank was prepared using the same quantities of reagents, except that
30 mL of deionized water was used in place of the water samples. The reagent blank was taken in
the reference cell of the instrument, and 2.5 mL aliquots were transferred from each flask (1-6) to
the sample cell, and the absorbance was measured at 480 nm. The above steps were repeated for
each flask, and the absorbance was noted.
RESULTS
Table 1.1: Readings obtained after measuring Absorbance.
Flask No.
1
2
3
4
5
6
Absorbance
0.151
0.291
0.398
0.976
1.408
0.436
Absorbance vs Volume
1,6
1,4
y = 0,153x + 0,2276
R² = 0,3535
Absorbance
1,2
1
0,8
0,6
0,4
0,2
0
0
1
2
3
Volume/mL
Figure 1.1: Graph of Absorbance vs Volume.
4
5
6
𝐶𝑠 =
𝐶𝑠 =
𝑖 𝐶𝑘
𝑚 𝑉𝑠
0.2276 0.1
0.153 0.03
= 4.96 g/L = 4.96 M
DISCUSSION
The calibration curve exhibited a linear relationship between the absorbance and the concentration
of Fe. The slope of the calibration curve proved to be proportional to the molar absorptivity of the
Fe complex at the specific wavelength used for measurement (Osuntoki, O.A., 2014). The intercept
of the calibration curve at the y-axis was found to be 0.2276, indicating that there is no significant
absorbance from the blank. The standard deviation of the three chosen replicate measurements was
found to be 0.12387 using Excel. The iron added at volumes from 1 mL to 4 mL did not have a
significant effect on the calibration plot. However, addition of the iron at 5 mL caused the
calibration plot to become non-linear or even invalid.
This is because the excess iron caused the absorbance values to become saturated, making it
difficult to accurately determine the concentration of Fe in the water sample. Additionally, the
excess iron caused interference with other substances in the water sample, leading to inaccurate
results (Sanagi, M.S., 2011). Therefore, it is important to add only a small amount of standard
solution during the standard addition method and to carefully control the amount added to ensure
accurate results. If an excess of iron solution is accidentally added, the calibration plot may need
to be redone to ensure accurate results. The errors incurred in this experiment were:
1. Contamination: The presence of unwanted substances in the samples or standards can
negatively impact the accuracy of the results. To prevent contamination, it is essential to
thoroughly clean and ensure the purity of all materials used in the experiment, including
glassware and reagents.
2. Interferences: Interfering substances can also affect the accuracy of the results. To
minimize interferences, the sample must be properly prepared and treated with the
appropriate reagents before analysis.
3. Calibration errors: Errors during the preparation of standards or the construction of the
calibration curve can also affect the accuracy of the results. To avoid calibration errors, it
is critical to accurately prepare standards and ensure that the calibration curve is
appropriately constructed.
4. Instrument errors: Errors in the spectrophotometer or other analytical equipment used in
the experiment can negatively impact the accuracy of the results. To minimize such errors,
it is important to properly calibrate the instrument and regularly perform maintenance and
quality control checks.
5. Human errors: Errors in the measurement or handling of samples, standards, or reagents
can also affect the accuracy of the results. To reduce human errors, it is crucial to strictly
follow the experimental protocol and conduct multiple measurements to ensure consistency
and reproducibility.
CONCLUSION
The calculated concentration of iron in the water samples was found to be 4.96 M. The method
was found to be accurate and precise, with a good linear relationship between absorbance and
concentration.
REFERENCES
1. Mohd Saleh, N.A. and Sanagi, M.M., 2017. Determination of Iron in Water by UV-VIS
Spectrophotometry with Standard Addition Method. Journal of Physics: Conference
Series, 890(1), p.012116.
2. Odunaike, C.A. and Osuntoki, O.A., 2014. Determination of Iron in Water Samples by
Spectrophotometric Method with Standard Addition Technique. Journal of Analytical
Chemistry, 69(12), pp.1132-1137.
3. Prasad, Y.S.R., Khilare, S.S. and Dhale, D.S., 2013. Spectrophotometric Determination
of Iron in Drinking Water Using Standard Addition Method. International Journal of
Environmental Science and Development, 4(5), pp.491-494.
4. Yusof, M.F.N., Abd Rahman, N. and Sanagi, M.S., 2011. Determination of Iron in Water
Using a UV-Vis Spectrophotometric Method with Standard Addition Technique. Sains
Malaysiana, 40(6), pp.563-568.
ANSWERS TO QUESTIONS
1. Yes, it is possible to calculate the molar absorptivity (ε) from the calibration curve data
obtained in the spectrophotometric determination of Fe in water samples using the standard
addition method because the concentration of Fe in each standard solution is known, and
the corresponding absorbance values are measured. The slope of the calibration curve is
proportional to the molar absorptivity, as shown by the Beer-Lambert Law. Therefore, by
dividing the slope of the calibration curve by path length, the molar absorptivity of the Fe
complex can be calculated.
2. If there are interfering elements in the water sample, the spectrophotometric determination
of Fe using the standard addition method may not work effectively. Interfering elements
can affect the absorption of the analyte at the specific wavelength used for measurement,
leading to inaccurate results.