Dissolved Oxygen Determination - PDF

DETERMINATION OF DISSOLVED OXYGEN (DO) IN WATER To survive, we need oxygen in the air we breathe. Oxygen is also essential for most aquatic organisms, but there is much less oxygen available in water than in air. Dissolved oxygen - the amount of oxygen dissolved and freely available in water...

DETERMINATION OF DISSOLVED OXYGEN (DO) IN WATER To survive, we need oxygen in the air we breathe. Oxygen is also essential for most aquatic organisms, but there is much less oxygen available in water than in air. Dissolved oxygen - the amount of oxygen dissolved and freely available in water - it is important for aquatic life. Like land animals, fish and shellfish require oxygen to survive. When oxygen levels fall below 5 milligrams per liter (mg/L), fish are stressed. At oxygen levels of 1-2 mg/L, fish die. Colder water can hold more oxygen than warmer water. - an important characteristic of water quality in many industries (hydroponics, food and beverage industries, aquariums, environmental sampling, wastewater, etc.). Main Sources of dissolved oxygen: 1. Air - at surface water, oxygen from the air equilibrates with oxygen dissolved in the water. An equal number of oxygen molecules are leaving and entering the water. - if water temperature increases, the water can't hold as much oxygen as before—the water is oversaturated with oxygen. 2. Photosynthesis - the fundamental biological process that uses light energy to produce sugar from carbon dioxide and water. - oxygen is a by-product of photosynthesis. Both algae (phytoplankton, seaweeds) and plants can be found in natural bodies of water. These organisms are net producers of oxygen in the daytime, but at night become net consumers of oxygen. What causes dissolved oxygen levels to vary? Amount of dissolved oxygen will be determined by: how much oxygen the water can hold (temperature-dependent), how much surface area is available for diffusion from the air, how much oxygen is produced by photosynthesis, and how much oxygen is consumed by respiration. How dissolved oxygen measured? Titrimetric Method Titrations - use one liquid where the concentration is already determined (titrant), to identify the concentration of another (your sample). Iodimetry titrations - use iodine as an indicator; the iodine indicator will either appear or disappear at the end of the titration. Electroanyalytical (Galvanic & Polarographic Probes) Method Two types of electroanalytical sensors:  Galvanic Probes/Sensors - these are membrane probes that have two parts that produce a voltage, acting as a battery (the metals have different electrode potentials). A thin semi-permeable membrane inside the cap of the  electrode allows gasses to pass through and block anything else. When oxygen diffuses across the membrane, it dissolves into the probe cap that contains a buffered electrolyte. This allows the oxygen to react with the cathode (usually silver) in the electrode, gaining an electron. The electron given to the oxygen molecule comes from the anode (usually zinc or lead) in the electrode, creating a voltage between the anode and cathode in the probe. It is when this current is formed the meter can convert the reading taken from the probe into a DO concentration value.  Polarographic Probes/Sensors - contain a thin semi-permeable membrane allowing oxygen into an unbuffered electrolyte. However, instead of acting like a battery, a voltage is applied between the silver anode and gold cathode in the probe. The voltage acts as a catalyst driving an oxygen reaction. When oxygen hits the cathode, an electron is added creating a current, determining the DO concentration. Optical Dissolved Oxygen Method - also uses a probe with a semi-permeable membrane to test DO in water, but the probe and meter monitor luminescence instead of monitoring a reaction. - the probe emits blue light Colorimetric Method - this method measures color. Intensity of the color is proportional to how much DO is in the sample. Indigo Carmine is, whereas Rhodazine D is used to measure much lower DO concentrations (ppb). Variations of Colorimetric Method  Indigo Carmine - produces a blue color where the intensity is proportional to the DO concentration. - used for measuring DO concentrations between 0.2 and 15 ppm - if using this method, keep reagents away from bright lighting, as this can deteriorate the Indigo Carmine. - results are obtained between 30 seconds (low-range tests) and 2-minutes (high-range tests).  Rhodazine D - produces a rose-colored or pink solution. - oxidizing agents (chlorine, cupric copper, and ferric iron) can interfere with results creating higher DO readings - this method is time-dependent, ensure you analyze the water sample within 30 seconds of adding the reagent. Importance of Dissolved Oxygen in water  to monitor water quality for healthy aquatic ecosystem  wastewater treatments  brewing beers Factors that Can Affect Dissolved Oxygen in Water Temperature - temperature is one of the biggest and common factors that directly affects dissolved oxygen in water. - colder water contains more oxygen than warmer water - the lower the DO, the higher the temperature, and inversely the DO concentration increases as temperature decreases. Salinity - freshwater contains more oxygen than saltwater because of the charge a salt molecule carries-  - as salinity levels increase in a solution, DO decreases. Atmospheric pressure - as atmospheric pressure decreases, the concentration of DO increases. - as altitude or atmospheric pressure increases, the number of DO molecules absorbed in water decreases as there is less pressure forcing the oxygen to be diffused in the water, increasing the partial pressure of oxygen. Humidity - when humidity levels increase, the level of DO also increases