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 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 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.

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 & 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.

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)