Dissolved Oxygen William R. Stagg Colgate Un~vers~ty Ham~lton,New York 13346
A relevant experiment for the introductory laboratory
T h e plea by students for courses which are relevant is one which strikes a sympathetic chord in many of us. I interpret relevance to mean that which has utility in meeting or describing any threatening event. hlany students are concerned over the apparent threat to our environment today. I have used this concern to provide a means of teaching some practical aspects of oxidation-reduction reactions and volumetric analysis. The fundamental reactions are those of the thiosulfate-iodine titration but with a twist (1). The demand for precision is low, 1% deviation being exceptionally good. Quantitative accuracy cannot generally be assessed. An importnnt part of thc experiment involves going into the field to collect samples for analysis. This brings a feeling of reality to an otherwise sterile experiment. The experimental instructions as presented to the student arc given below. The experiment follows a classroom discussion of redox reactions, reduction potentials, and spontaneity of reaction. Dissolved Oxygen in Natural Waters
Myriad forms of life exist in our lakes, streams, and oceans. These creatures vary in size greatly but all depend on dissolved oxygen (D.O.) in the water for their life support. Occasionally something will happen which depletes the oxygen content of a natural water system. If it happens slowly one simply observes that the kinds of life able to survive change and ultimately all life disappears. Thus one observes the degradation of a trout stream by a diminution of trout and an increase of "trash" fish, those which can survive a t lower concentrations of dissolved oxygen. Ultimately no fish survive at all. If the depletion occurs
rapidly there is a "fish kill" with many carcasses floating downstream. Causes for these depletions are many but two may be listed. As a lake undergoes eutrophication, algae grow rapidly, then die. The decaying algae are oxidized by the dissolved oxygen, thus depleting it. A more rapid depletion occurs when an easily oxidizable material is dumped into a stream from an industrial process or when a drought partially dries up a small lake. Gases which are dissolved in water obey Henry's law to a first approximation. z, = kxP, where x. = mole fraction of gas in solution (= n,/(n, nl). kH = Henry's law constant, P, = partial pressure of the gas, n. = number of moles of gas in solution, nl = number of moles of solvent in solution. For oxygen at 20°C, ka = 2.5 X atm-I and Po, = 0.21 atm for air. Also it is approximately true that zo, = no,lnx,o since no,
