CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
Title:
Determination of Nitrate in Drinking Water by UV Spectrophotometry.
Aim:
To Determine Nitrate in Drinking Water by UV Spectrophotometry.
Theory:
The UV spectrophotometer can perform electronic analysis on the solutions (calibration standards, unknowns and control) and in so doing yield quantitative information pertaining absorbance, that is, the amount of a specific wavelength of light/ electromagnetic radiation that an analyte absorbs. The UV spectrophotometer has a few basic compo nents. These include the source, a device for wavelength selection, the sample compartment and detector. Considering this study, the source would most likely be a Deuterium lamp since this lamp is capable of producing a continuous c ontinuous UV emission with wavelengths ranging from 185 nm to about 375 3 75 nm which is satisfactory for most UV analysis. Next is wavelength selection, this may be achieved via absorption filters and a monochromator. From here a specific wavelength of light, like 340 nm will be produced. This specific wavelength is then feed to the sample compartment which holds the sample. The analyte will absorb an amount of this light that is proportional to the concentration of the an alyte. The unabsorbed light goes on to the detector where the signal is amplified and can then be quantified. From the beer lambert law we know that absorption is proportional to the concentration of the sample and the shape/size of the sample container. This is why a stand ard sized and shaped cuvette is used as the sample container. Anothe r thing to consider Page 1 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
is that components of the matrix may also absorb l ight. In our case this is corrected by using a double-beam spectrophotometer. In this spectrophotometer, the light coming from the device that selects the wavelength which came from the source is split via beam splitter or light chopper so that two daughter beams are produced. One of the daughter beams goes on to the matrix while the other goes on the sample. As such any absorbance by the matrix can be subtracted from that of the sample to get the absorbance of the analyte only. In this case, the analyte is nitrate. Nitrate has λ max at the wavelengths of electromagnetic radiation 220 nm and 275 nm. However, because of the probable presence of the interference, dissolved organic matter, in the solution, a correct is made and the absorbance is taken at 340 nm and 395 nm instead of 220 nm and 275 nm. By making solutions in which the concentration of Nitrate is known to a high level of accuracy, that is, 0.5, 1.0, 3.0, 5.0 and 10.0 ppm Nitrate and then taking the absorbance of these five systems, one can construct a standard curve. From the standard curve, a straight line would be obtained. This line will have an equation given by = + (equation 1). In this equation, m and c will be known constants (calculated via MS word 2013), y represents the absorbance and x represents the co ncentration of Nitrate in ppm in the water. Therefore by measuring the absorbance of the unknown solutions and the control solution, the concentration of Nitrate in ppm in these solutions can be
Page 2 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
calculated simply via the use of a derivative of equation 1 which is given by equation 2; =
−
.
According to the Environmental Protection Agency (2015 ), the maximum contaminant level for nitrate that is allowed in drinking water is 10 ppm. Moreover, Saunders (2014) verifies that lead nitrate is an ingredient of paint. Diagram:
The spectrophotometer that utilized in this laboratory Exercise is shown in figure 1.
Figure 1: Diagram of the UV-Vis Spectrophotometer Method:
As per Laboratory Handout barring the following amendments; 1. Absorbance for each solution was read at 340 nm and 395 nm instead of 220 nm and 275 nm. 2. 50 mL beakers were used as an alternate to overcome the shortage of 50 mL volumetric flasks. Page 3 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
Data:
Please see Appendix 1.
Graph:
Please see Appendix 1.
Treatment of Data:
Please see Appendix 1.
Results:
The results obtained from Laboratory Exercise 1 for group 1 is tabulated in table 1. Table 1: Results obtained from Laboratory Exercise 1 for group 1. ≈ Concentration of N itrate
Solutions
Unknowns
ppm 1
44.0
2
66.0
3
83.0 0.0
Control
Discussion:
in solutions /
A perfect standard curve was not obtained from the absorbance of the five calibration standards. The e xperimenter believes that this could be due to a few reasons. Such as;
The intensity of the light varying for each run for the calibration standards. Even if one calibrating standard (5 ppm) has more nitrate than another calibrating standard (3 ppm) and the intensity of the light source is lower in the for the 5ppm run than the 3ppm, then the expected higher absorbance for the 5 ppm solution because it has more analyte to absorb more light, would not be observed. This is not unreasonable as the UV-Vis spectrophotometer is quite old and would be prone to such faultiness. Page 4 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
The introduction of interferences into the sample by the experimenters since they did not wear gloves for the entire experiment. As such, even if one calibrating standard (3 ppm) has less nitrate than another calibrating standard (5 ppm) and nitrate, especially from the hands of the experiments enter the sample then, the expected lower absorbance for the 3 ppm solution because it should has less analyte to absorb less light, would be reversed and the expected outcome would not be not be observed.
The use of an old spectrophotometer that may be very well be defective. Suppose the detector of the instrument has lost it functionality over the years, then the absorbance readings would not be accurate and therefore anomalies appear in the study like this one. With samples having more analyte (5 ppm) absorbing less than samples with less analyte (3 ppm). All three of the unknown solutions had nitrate
concentrations above the maximum contaminant level of the EPA (10 ppm). This was expected since the unknown solution in question was taken from a site that is very close to a facility that does painting. Paints contain lead nitrate as confirmed via Saunders (20 14). It would not be a stretch of the imitation to conclude that paint or its vapour that contain the aforementioned nitrate gets into the water via diffusion.
Page 5 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
On average the drinking water (the unknown solution in question) that was obtained from next to a facility that does painting had a nitrate content o f ≈ 64 ppm. This exceeds the EPA limit by over 500 % , therefore it is unfit for human consumption. As it relates to why the same unknown had variances of roughly 20 ppm nitrate for each run, the experimenters believes the reasons given below would be able to explain;
The use of an old spectrophotometer that may be very well be defective. o
Suppose the intensity of the light varying for each run for. Even if all solutions have the same concentration but the intensity of the light source is lower in one run (like unknown 1) than another run (like unknown 2) then unknown 1 would absorb less light and therefore have a smaller absorbance than unknown 2.
o
Suppose the detector of the instrument has lost it functionality over the years, then the absorbance readings would not be accurate and therefore anomalies appear in the study like this one. With samples having the same concentration of analyte have different absorbance.
The introduction of interferences into the samples of each run by the experimenters since they did not Page 6 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson. wear gloves for the entire experiment. As such, even if all the samples should have the same concentration and absorb the same amount of light and have the same absorbance, each sample has vastly different absorbance because each sample
was exposed to different amounts of interferences. The control solution would not be useful in these cases because it does not contain the interferences from the experimenters, thus, these interferences cannot absorb any light, contribute to the control’s absorbance which would be subtracted from that of the sample. All it has is the matrix from the sample. The reason for adding HCl to all of the systems studied was for it to act as a solvent in this study. It can be concluded that the drinking water
Conclusion:
obtained from next to the facility that does painting has a Nitrate content of ≈ 64 ppm on average. Therefore it is unfit for human consumption via EPA standards. References:
1. Kenkel, J. (2014). UV-Vis and IR Molecular Spectrometry. In Analytical chemistry for technicians (4th ed.). Boca Raton: Lewis. 2. Clark, J. (2006). Double beam UV-visible absorption spectrometer. Retrieved January 24, 2016, from http://www.chemguide.co.uk/analysis/uvvisible/spectrometer.html#top
Page 7 of 8
CHM 4101 Lab Report 1.
GOKHUL, Raja.
October 30, 2015.
1015485.
Group 1: Alicia Sukhdeo, Leanna Simon and Shivraj Balkisson.
3. Unite States Environmental Protection Agency. (2015, December 8). Table of Regulated Drinking Water Contaminants. Retrieved January 24, 2016, from http://www.epa.gov/your-drinking-water/table-regulated-drinking-watercontaminants#Organic 4. Saunders, N. (n.d.). Making Paints. Retrieved Janu ary 24, 2016, from http://www.creative-chemistry.org.uk/activities/paints.htm 5. Thermo Spectronic [PDF]. (n.d.). Retrieved January 24, 2016 , from http://www.molecularinfo.com/MTM/UV.pdf
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