Phosphate concentration in Shoewalter Fountain water by spectrophotometry

Phosphate is a major contributor to the environmental balance in marine ecosystems. If there is too much phosphate in water, algae can use the excess as a nutrient and grow at uncontrollable rates. This uncontrollable growth is called eutrophication which results in severely depleted oxygen levels in the water. These low levels of oxygen cause fish and other marine animals to die from lack of oxygen1. In the Gulf of Mexico, run-off from a variety of sources including fertilizer has created one of the largest areas of virtually no life in the world with a size of 9421 square miles in 2011.

The Mississippi River is one of the main pathways that phosphate gets into the Gulf2. Geographically speaking Indiana’s run-off should eventually get into the Mississippi River and thus into the Gulf, therefore it is very important to monitor water sources in Indiana for high levels of phosphate in case the source needs remediation.

Normal levels of phosphate vary greatly depending on if the water source is a river, lake, or ocean and how fast the water moves in the source.

However, some general values have been found3, in a large data set the mean phosphate concentration was 0.055ppm which can be considered “normal” for this experiment. The concentration for negative effects on the ecosystem was found to be 0.17ppm3.

The goal of this experiment is to determine if and to what extent the water in Showalter Fountain has high levels of phosphate in it. This can be found using absorbance which is defined as where A is absorbance and T is transmittance which is defined as where I0 is the intensity of the light entering a sample and I is the intensity of light measured after going through a sample.

Get quality help now

Marrie pro writer

Verified

Proficient in: Environmental Chemistry

5 (204)

“ She followed all my directions. It was really easy to contact her and respond very fast as well. ”

+84 relevant experts are online

Hire writer

To find an unknown concentration the Beer’s Law equation can be used which is and this can be related to a linear equation y = MX + b. A spectrophotometer works by using a wavelength of light that is absorbed by the substance being measured. When a known intensity of light passes through a water sample, some light will be absorbed, this loss of light intensity is what is measured. The two intensity values are placed in the transmittance equation and then converted to absorbance using the logarithmic equation. The wavelength of light that is absorbed is correlated with the color of a sample. Color that is observed with the naked eye is the wavelength of light that a substance does not absorb. When light is absorbed certain wavelengths are absorbed more than others, the wavelength that is absorbed is dependent on the electronic structure of an atom and can be shown using line spectra which have been previously shown4.

Methods

The materials used in the experiment were: 77.5mL of 20ppm Phosphate ion solution diluted to solutions of 0.5ppm, 1ppm, 2ppm, 3ppm, 4ppm, and 5ppm; 5mL of ammonium molybdate solution, 2mL of tin (II) chloride, 500mL distilled water, 100mL volumetric flask +/- 0.08mL, 20mL volumetric pipette +/- 0.03mL, 1mL volumetric pipette, 150mL Erlenmeyer flask, 5 100mL Beakers, 5 cuvettes, and a Spectronic 200 Spectrophotometer5.

To find the concentration of phosphate in the Showalter Fountain water sample, an absorbance standardization curve had to be created. This was done by first taking the 20ppm standard phosphate solution and diluting 2.5mL, 5mL, 10mL, 15mL, 20mL, and 25mL to 100mL in a volumetric flask. These solutions were transferred to beakers after being made. Next, in order of most dilute to least dilute phosphate solutions, 20mL of phosphate solution was added to an Erlenmeyer flask using a 20mL volumetric pipette. Then, using a 1mL volumetric pipette, 1mL of ammonium molybdate solution was added to the solution. Next, 2 drops of tin(II) chloride were added to the solution and the absorbance was measured at 650nm after 6 minutes. Excluding the dilution portions, these steps were repeated 3 times for the Showalter Fountain samples5.

Discussion

The calibration curve in Figure 1 is necessary to determine the unknown concentration of any sample that could be tested. If a publicly available curve on the internet was used, it would not reflect the accuracy of the Spec-200 used in this particular experiment and would create unnecessary and highly preventable sources of error. To understand fully how the spectrophotometric techniques work, the chemistry must be understood. On the molecular level, ammonium molybdate reacts with phosphate to form phophomolybdenum. Then tin(II) chloride acts as an indicator when added to phosphomolybdenum which forms the resulting blue color4. This solution absorbs a wavelength of 650nm and when passing through the solution, the blue absorbs some of this light. The higher the concentration of phosphate, the darker the solution will become, and so more light will be absorbed increasing the absorbance value. The calibration curve had an R2 value of 0.9787 which shows that while it is not a perfectly linear curve, it is very close. If the R2 value was closer to 0 that would show that there would be no statistical correlation in the curve, and it would therefore be useless in determining unknown concentrations. Since that is not the case, the calibration curve for this experiment can be trusted to give very close approximate results. After 3 trials the average concentration of phosphate in Showalter Fountain of 1.39ppm is greater than the reported3 environmentally damaging concentration of 0.17ppm which suggests that the water in Showalter Fountain would have negative effects on the environment if it were to get into run-off water. An explanation for this high value would be that water in the Showalter Fountain could be in a closed system where the water is continuously reused. Rain with high amounts of phosphate pollution may also cause elevated levels of phosphate compared to large bodies of water like lakes or oceans where the effect of rain is more negligible in comparison simply due to the difference in size. There may also be a groundwater protection system in place where even if the water leaks from the closed system there is still little chance of it reaching groundwater. The protection could be via filtration or some method of sealing the ground area directly under and around the fountain. If any of these were the case, then the water would be highly unlikely to enter any run-off systems and end up in the Mississippi or the Gulf of Mexico. In the experiment though, there may have been some sources of experimental error, for instance, there was no way to obtain perfectly clean cuvettes and dirty cuvettes may have caused fluctuations in absorption that would not have otherwise existed. After adding the tin(II) chloride the solution almost immediately changed color to blue, so there was no way of knowing for sure if the reaction was complete except for waiting between 5 and 15 minutes5, this may have caused the concentration to be either high or low for all of the concentrations and absorbance values including those in the dilutions, but there may have been no effect at all.

Conclusion

The results suggest that the water in Showalter Fountain would have highly consequential effects on the environment if it were to be released in run-off. The concentrations were both above average and above the threshold for causing high amounts of eutrophication indicating toxicity to marine life. However, the water in the fountain is likely closed off from being able to enter the outside environment so it is not likely that the water would have any damaging effects. So the water might have high levels of phosphate, but because it is likely closed off from the outside and because of how small of a body of water it is, there is very little chance that it will affect marine life in any way.

References

1. Kundu, S.; Coumar, M. V.; Rajendiran, S.; Ajay; Rao, A.S. (2015). Phosphates from detergents and eutrophication of surface water ecosystem in India. Current Science, 108(7), 1320–1325.
2. Blomberg, L. Dead in the Gulf. E-The Environmental Magazine, Sep/Oct 2011, p 18-19.
3. Dodds, W. K.; Welch, E.B. Establishing Nutrient Criteria in Streams. Journal of the North American Benthological Society. 2000, 19, 186-196.
4. Prieto, P.; Pineda, M.; Aguilar, M. Spectrophotometric quantitation of antioxidant capacity through the formation of a phosphomolybdenum complex: specific application to the determination of vitamin E. Anal. Biochem. 1999, 269, 337-341.
5. Spencer, B. C127 Environmental Chemistry: Determining the Phosphate Level of Water Handout. Indiana University, Bloomington, IN. 2019.