Transcript Analyze Immediately Dissolved Oxygen Topics of Discussion * Definition & Pointers * Applications:Water Quality & Sewage Treatment * Theory * Partial Pressure of Oxygen on Dissolved.

Analyze Immediately
Dissolved Oxygen
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
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 airwater 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 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 D8892(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
• 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 & alkalineiodine-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.
So…..what else is there???
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)