Question

APPLICATION!

1. A nonscientist brings a water sample to your laboratory and asks you to determine why there was a fish kill in the nearby lake. Having recently finished this experiment, what might you tell that person about the legitimacy of a test for dissolved oxygen? What reasoning would you use to maintain the integrity of your laboratory?

2. Fish kills are often found near the discharge point of water from cooling waters at electrical generating power plants. Explain why this occurrence may occur.

3.Fish kills are often found in streams following heavy rainfall in a watershed dominated by farmland or denuded forestland. Explain why this occurrence may occur.

4.Explain how the dissolved oxygen concentrations may change starting at the headwaters of a river and ending at the ocean. Account for the changes.

5.Salt (ocean) water generally has a lower dissolved oxygen concentration then freshwater at a given temperature. Explain why this is generally observed.

Answer 1

1. A test to determine dissolved oxygen in water sample is winkler test. It involves titration to do the same. A sample bottle filled completely with water, and filled with reagents to form acid compound with dissolved oxygen, is titrated with a neutralising compound until colour change.The colour change(end point) concludes with dissolved oxygen concentration in the sample. SInce a dead fish indicate the amount of dissolved oxygen in the water is low, the same can be shown by the test.

2. This phenomenon arises due to thermal pollution. Powerplants are the main source of thermal pollution.Cool water is used from streams to cool the generators, and then returned to the streams at elevated tempertaures.Such rapid changes in temprature often kill fishes due to thermal shock.

3.This may also occur due to low oxygen content in water. As stratification occur during summer, cold, stale water is accumulatedon the pond bottom, unable to mix with the surface. These layers contain no oxygen and can develop toxic species like hydrogen sukfide. However as the storm burst out, this stagnant water mix with the fresh water supply, resulting in oxygen depletion and fish kill.

4.The dissolved oxygen (do) concentrations decreases when moving from headwaters of rivers to oceans. Colder, deeper fresh waters have the capability to hold higher concentrations of dissolved oxygen, but due to microbial decomposition, lack of atmospheric contact for diffusion and the absence of photosynthesis, actual DO levels are often far below 100% saturation . Warm, shallow saltwater reaches 100% air saturation at a lower concentration, but can often achieve levels over 100% due to photosynthesis and aeration.

5.  The solubility of oxygen decreases as temperature increases . This means that warmer surface water requires less dissolved oxygen to reach 100% air saturation than does deeper, cooler water. Since oceans have a greater temperature range, the dissolved oxygen content is often less in oceans as compared to riverheads.