Topics of Discussion *Definition & Pointers *Applications:Water Quality & Sewage Treatment *Theory *Partial Pressure of Oxygen on Dissolved O 2 *Temperature Effect on Dissolved Oxygen *Measurement Techniques *NJAC Regulations: What you need to know
Definition Dissolved Oxygen DO for short measurement of the amount of oxygen dissolved in a unit volume of water indicator of usefulness of water for a specific application
Hold Times 15 minutes - note time of collection & time of analysis!
Applications Useful for maintaining a stream fit for swimming, fishing and/or as a source of potable water… DO level must be kept high (in English - permits have minimum limits; DO levels must be kept above this limit) Low DO levels = can have harmful effects on receiving waters; causes suffocation of fish & promotes growth of harmful bacteria.
Applications Sewage treatment: Specialized bacteria (also known as activated sludge) is added to solids in waste water treatment plants optimum level of DO necessary for process too low DO = bacteria die & decomp. ceases too high DO = process becomes costly
Theory Amount of Oxygen that a given volume of water can hold is a function of: 1. The pressure the atmospheric oxygen is exerting at the air- water interface….
Theory 2. The temperature of the water. 3. The amount of other substances dissolved in the water.
Effect of Partial Pressure of Oxygen on Dissolved O 2 Water in contact with air will absorb air (O 2 ) …until the pressure at air-water interface is equal… said to be saturated - about 5 to 10 parts of oxygen to one million parts of water
Effect of Temperature on Dissolved Oxygen Pot of boiling water… bubbles form on bottom & sides of pot… number & size of bubbles increase with temperature…. These are air bubbles that have been dissolved in water.
2 Basic Measurement Techniques : I. Electrode - an electrode system where DO reacts at the cathode producing a measurable electrochemical effect. Effect can be galvanic (ability to conduct an electrical current), polarographic (electrochemical), or potentiometric (measurement of voltages).
2 Basic Measurement Techniques : II. Winkler method (azide modification) SM 4500-OC (or ASTM method D88- 92(A) or another Winkler method promulgated by the USEPA.
Winkler Method Stu Nagourney covers Winkler Method in greater detail…however….
But first, a word or two about the Winkler Method Na 2 SO 4 is to be standardized quarterly, per NJAC regulations. Winkler test must be run within 8 hours of adding the reagents*, due to instability of solution once reagents are added. (*MgSO 4 & alkaline- iodine-azide solution)
Membrane probe technique…. Electrode system separated from the sample stream by semi-permeable membrane, which permits DO in sample to pass through to the electrode system, but prevents passage of liquids & ionic fluids. Most units are temperature compensated(thermistor or resistance thermometer). Another type of probe does not use semi-permeable membrane; system consists of a reference electrode & thallium measuring electrode. O 2 concentration determined by measuring voltage potential when DO comes in contact with the thallium electrode. Thallous-ion concentration is proportional to the dissolved oxygen in the sample.
NJAC Regulations!! NJAC 7:18-5.2(a)17 Dissolved oxygen meters with membrane electrodes shall meet the following: I. Dissolved oxygen measurements shall be accurate to within 0.3 mg DO/L, and shall be precise to within 0.15 mg DO/L, and;
...NJAC Regs, continued ii. Meters shall be capable of temperature compensation. Also...
...NJAC Regs, continued NJAC 7:18-5.5(c)1 The lab shall calibrate dissolved oxygen instruments against air or air saturated water before each use or weekly, whichever is less frequent……
...NJAC Regs, continued (The Weekly Winkler) The Weekly Winkler The lab shall test dissolved oxygen instruments weekly using the Winkler method (azide modification)4500-OC set forth in SM-18 or ASTM method D88 - 92(A), or another Winkler method promulgated by the USEPA.
...NJAC Regs, continued The values from the DO meter are compared to the values obtained from the Winkler Titration… the difference must be <0.2 mg/L. for example if the meter is off by 0.4 mg/L, must stop & recalibrate… Recalibration can mean - -changing the membrane - cleaning the gold electrode ring
In closing….. Dissolved Oxygen - useful tool inter-related to and inter-dependent on other parameters (pH, BOD, temperature, to name a few)