Analyze Immediately

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Analyze Immediately

• Dissolved Oxygen

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Topics of Discussion

• Definition Pointers
• ApplicationsWater 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

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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

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Hold Times

• 15 minutes - note time of collection time of
  analysis!

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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.

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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

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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.

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Theory

• 2. The temperature of the water.
• 3. The amount of other substances dissolved in
  the water.

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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

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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.

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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).

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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.

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Winkler Method

• Stu Nagourney covers Winkler Method in greater
  detailhowever.

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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)

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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.

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So..what else is there???
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NJAC Regulations!!

• NJAC 718-5.2(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

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...NJAC Regs, continued

• ii. Meters shall be capable of temperature
  compensation.
• Also...

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...NJAC Regs, continued

• NJAC 718-5.5(c)1
• The lab shall calibrate dissolved oxygen
  instruments against air or air saturated water
  before each use or weekly, whichever is less
  frequent

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...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.

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...NJAC Regs, continued

• The values from the DO meter are compared to the
  values obtained from the Winkler Titration
• the difference must be lt0.2 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

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In closing..

• Dissolved Oxygen - useful tool
• inter-related to and inter-dependent on other
  parameters (pH, BOD, temperature, to name a few)