Analyze Immediately Dissolved Oxygen. Topics of Discussion *Definition & Pointers...
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Transcript of Analyze Immediately Dissolved Oxygen. Topics of Discussion *Definition & Pointers...
Topics of Discussion
* Definition & Pointers* Applications:Water Quality & Sewage
Treatment* Theory
* Partial Pressure of Oxygen on Dissolved O2
* 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
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….
Effect of Partial Pressure of Oxygen on Dissolved O2
• Water in contact with air will absorb air (O2)
• …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.
But first, a word or two about the Winkler Method
• Na2SO4 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. (*MgSO4 & 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.
• O2 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
• 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