Advanced projects in chemistry

Advanced Projects in Chemistry Author: Anjali Gharpure

Advanced Projects in Chemistry Dissolved Oxygen levels in water bodies

For students of the K-11 or K-12 grades with a background in biological sciences, the dissolved oxygen tests would be both a worthwhile and a learning effort. Measuring dissolved oxygen levels is important to determine the level of contamination of groundwater, an important  water quality parameter. Dissolved oxygen levels are important as they indicate ground water pollution.

Oxygen is vitally important for healthy aquatic life and low dissolved oxygen levels are indicative of polluted waters. The optimum value for dissolved oxygen determined should be between 4-6 mg/L.

Tests commonly monitored for determining the oxygen content in water masses are-Biological oxygen demand (BOD), Chemical oxygen demand (COD) and Dissolved oxygen (DO) by Winkler’s method.

Biochemical Oxygen Demand (BOD) is an empirical test determining the relative oxygen requirement of effluent and polluted waters. BOD tests consist of a 5 day period in which a sample is placed in an airtight bottle under controlled conditions temperature (20ºC ± 1ºC), under dark conditions to prevent photosynthesis. The dissolved oxygen (DO) in the sample is measured before and after the 5 day incubation period. In contrast, Chemical Oxygen Demand (COD) is often preferred for daily analysis. The COD analysis is reproducible and requires a shorter period of time for completion. It measures both the biologically oxidizable and biologically inert organic materials present in sewage. Waste water sample is refluxed with excess potassium dichromate in dilute sulfuric acid. In the presence of silver sulfate as catalyst and mercuric sulfate. The organic matter of the sample is oxidized to water, carbon dioxide and ammonia. The excess dichromate remaining unreacted in the solution is titrated against standard Ferrous ammonium sulfate. The Chemical Oxygen Demand (COD) test measures the oxygen equivalent consumed by organic matter in a sample during strong chemical oxidation. Potassium dichromate is used as the oxygen source with concentrated sulfuric acid added to yield a strong acidic oxidizing medium. Other reagents added, in the analysis serve to drive the oxidation reaction to completion. These include mercuric sulfate, silver sulfate and sulfamic acid. Mercuric sulfate is added to remove complex chloride ions present in chlorinated water samples. Without the mercuric sulfate the chloride ions would get oxidized form chlorine compounds in the strong acidic medium. These chlorine compounds would in turn oxidize the organic matter in the water sample. The resulting COD value determined would then be lower than the actual value. Silver sulfate is added as a catalyst for the oxidation of short, straight chain organic compounds and alcohols. In the absence of silver sulfate the COD of the sample would be lower than the actual value. Sulfamic acid prevents interference of the nitrite ions. In the absence of sulfamic acid, the COD reading of the sample determined would be higher than the actual value.

However, the oxidation of the organic matter is not always 100% complete with the COD test. Volatile organics, ammonia and aromatic hydrocarbon are not oxidized completely by this method. Despite these observations, the COD test has a number of merits over the BOD test. COD results are available much sooner. COD tests require very little sample preparation. The COD test oxidizes a wider range of chemical compounds and can be standardized more easily. The major disadvantage of the COD test is that the results are not directly applicable to the 5-day BOD results without correlation studies over a long period of time. The Winkler Method is a technique used to measure dissolved oxygen in freshwater systems. Dissolved oxygen is used as an indicator of the health of a water body, where higher dissolved oxygen concentrations are correlated with high productivity and little pollution. This test is performed on-site, as delays between sample collection and testing may result in an alteration in oxygen content. The Winkler Method also uses titration to determine dissolved oxygen in the water sample. The sample is filled completely with water in an airtight bottle. The dissolved oxygen in the sample is then "fixed" by adding a series of reagents- managnous sulfate, alkaline azide iodide till a floc forms. The floc is allowed to settle after which concH2SO4 is added to form an acid compound. It is titrated with a neutralizing compound that results in a color change. The point of color change, the "endpoint," coincides with the dissolved oxygen concentration in the sample. Dissolved oxygen analysis is best done in the field, as the sample is left unaltered by atmospheric equilibration. The liberated iodine is titrated against a standard solution of sodium thiosulfate, from which the amount of dissolved oxygen is calculated.

Water pollutants include organic and inorganic effluents from industries, radioactive materials and thermal pollutants. A K-12 grade project that includes these experiments tests the students learning outcome and awareness in environmental chemistry. It gives the students the chance to determine and analyse for themselves the level of water contamination in their city or town.