dissolved oxygen (do) lecture. introduction air we contains about 20% oxygen fish and other...

Dissolved Oxygen Dissolved Oxygen (DO) Lecture(DO) Lecture

IntroductionIntroduction

Air we contains about 20% oxygenAir we contains about 20% oxygen Fish and other aquatic organisms require oxygen as Fish and other aquatic organisms require oxygen as

wellwell Term Dissolved Oxygen (DO or D.O.)Term Dissolved Oxygen (DO or D.O.)

Refers to amount of free oxygen dissolved in waterRefers to amount of free oxygen dissolved in water Which is readily available to aquatic organisms Which is readily available to aquatic organisms Because fish and other aquatic organisms cannot survive Because fish and other aquatic organisms cannot survive

without oxygenwithout oxygen DO is one of most important water quality parametersDO is one of most important water quality parameters DO is usually expressed as a concentration of oxygen in a DO is usually expressed as a concentration of oxygen in a

volume of watervolume of water Milligrams of oxygen per liter of water, or mg/LMilligrams of oxygen per liter of water, or mg/L

IntroductionIntroduction

Liquid and Air state of equilibrium is Liquid and Air state of equilibrium is reached whenreached when Partial pressure of oxygen (part of total Partial pressure of oxygen (part of total

pressure that is due to oxygen) is equal in pressure that is due to oxygen) is equal in air and in liquidair and in liquid

Liquid is then saturated with oxygen Liquid is then saturated with oxygen

Total pressure in air is the sum of all of Total pressure in air is the sum of all of partial pressures (Dalton’s Law)partial pressures (Dalton’s Law)

Dalton’s LawDalton’s Law

Total pressure in air is sum of all of Total pressure in air is sum of all of partial pressures (Dalton’s Law)partial pressures (Dalton’s Law) Atmospheric pressure of 760 mmHg in dry Atmospheric pressure of 760 mmHg in dry

air will contain a partial pressure of oxygen air will contain a partial pressure of oxygen (pO(pO22) of approximately 159 mmHg) of approximately 159 mmHg

Changes in atmospheric pressure will cause Changes in atmospheric pressure will cause a directly proportional change in partial a directly proportional change in partial pressure of oxygen in the airpressure of oxygen in the air

Dalton’s LawDalton’s Law

PPtotaltotal = P = P11 + P + P2 2 + . . . P+ . . . Pnn

Where PWhere P11, P, P22, and P, and Pnn are the partial pressures of are the partial pressures of the gases involvedthe gases involved

ExampleExample A mixture of oxygen, hydrogen and nitrogen A mixture of oxygen, hydrogen and nitrogen

gases exerts a total pressure of 278 kPa. If the gases exerts a total pressure of 278 kPa. If the partial pressures of the oxygen and the partial pressures of the oxygen and the hydrogen are 112 kPa and 101 kPa respectively, hydrogen are 112 kPa and 101 kPa respectively, what would be the partial pressure exerted by what would be the partial pressure exerted by the nitrogenthe nitrogen

AnswerAnswer

PPtotaltotal = P = P11 + P + P22 + . . . P + . . . Pnn

278 kPa = 112 kPa + 101 kPa + P278 kPa = 112 kPa + 101 kPa + Pnitrogennitrogen

PPnitrogen nitrogen = 278 kPa - (112 kPa + 101 kPa)= 278 kPa - (112 kPa + 101 kPa) PPnitrogennitrogen = 65 kPa= 65 kPa

DO TheoryDO Theory

Meter uses polarographic method of Meter uses polarographic method of determining [DO] determining [DO] Voltage is maintained across 2 electrodesVoltage is maintained across 2 electrodes Gold cathode and silver anodeGold cathode and silver anode An electrolyte separated from sample solution by An electrolyte separated from sample solution by

oxygen permeable membraneoxygen permeable membrane Oxygen diffuses from sample across the membraneOxygen diffuses from sample across the membrane And it gets reduced at the cathodeAnd it gets reduced at the cathode Reduction current is directly proportional to partial Reduction current is directly proportional to partial

pressure of oxygen in samplepressure of oxygen in sample

DO TheoryDO Theory

Oxygen cathode reduction equationOxygen cathode reduction equation OO2 2 + 2H+ 2H22O + 4eO + 4e-- → 4OH→ 4OH--

Current generated (I) by electrode is proportional to Current generated (I) by electrode is proportional to [oxygen] of sample[oxygen] of sample

Rate of oxygen reduction at cathode is significantly Rate of oxygen reduction at cathode is significantly faster than rate of diffusion to cathodefaster than rate of diffusion to cathode

So the rate of oxygen reduction is diffusion limitedSo the rate of oxygen reduction is diffusion limited I = kD (I = kD (OO22)) I = current measuredI = current measured k = proportionality constantk = proportionality constant D = diffusion coefficient of oxygenD = diffusion coefficient of oxygen

Equation is a simplified description of current Equation is a simplified description of current generated by oxygen electrodegenerated by oxygen electrode

DO TheoryDO Theory

Many other factors integral to this process:Many other factors integral to this process: Electrode surface areaElectrode surface area Membrane thicknessMembrane thickness Membrane permeability coefficientMembrane permeability coefficient Oxygen partial pressureOxygen partial pressure All incorporated into constant k and DAll incorporated into constant k and D

As pOAs pO22 in water is equal to that of atmosphere in water is equal to that of atmosphere Driving force bringing oxygen to the electrodeDriving force bringing oxygen to the electrode Is pOIs pO22 differential btw sample and electrolyte inside differential btw sample and electrolyte inside

electrode electrode

DO ProbeDO Probe

Silver

Gold

Meter OperationMeter Operation

New probes are shipped with dry membrane cap to New probes are shipped with dry membrane cap to protect electrodesprotect electrodes

New membrane cap must be installed before useNew membrane cap must be installed before use 1. Remove stir paddle from probe. Pull it straight out1. Remove stir paddle from probe. Pull it straight out 2. Unscrew old membrane cap from the probe2. Unscrew old membrane cap from the probe 3. Before installing new membrane, clean probe tip with DI 3. Before installing new membrane, clean probe tip with DI

water to remove any contaminantswater to remove any contaminants 4. Hold membrane cap and fill it half full with electrolyte soln 4. Hold membrane cap and fill it half full with electrolyte soln

providedprovided 5. Screw cap moderately tight (small amount of electrolyte 5. Screw cap moderately tight (small amount of electrolyte

should overflow)should overflow) 6. Rinse off excess electrolyte with DI water6. Rinse off excess electrolyte with DI water 7. Reinstall the stir paddle7. Reinstall the stir paddle

CalibrationCalibration

Calibration can be done quickly and convenientlyCalibration can be done quickly and conveniently 1. Air calibration1. Air calibration

Make sure no water droplets on membrane (cause low Make sure no water droplets on membrane (cause low calibration reading)calibration reading)

So shake probe downward then clean paper towelSo shake probe downward then clean paper towel 2. Water-saturated air2. Water-saturated air

Under equilibrium, pOUnder equilibrium, pO22 in air-saturated water is equal to pO in air-saturated water is equal to pO22 in in water-saturated air, i.e., air at 100% relative humiditywater-saturated air, i.e., air at 100% relative humidity Probe calibrated in water-saturated air will correctly read pOProbe calibrated in water-saturated air will correctly read pO22 in in

water samplewater sample Diffusion rate of Diffusion rate of oxygenoxygen in water and air differs slightly in water and air differs slightly Advanced meters apply a correction factor to water-saturated air Advanced meters apply a correction factor to water-saturated air

calibration value to obtain correct air-saturated water valuecalibration value to obtain correct air-saturated water value

Gold Cathode/Silver Gold Cathode/Silver AnodeAnode

Gold Cathode:Gold Cathode: Can become tarnished or plates with silver after Can become tarnished or plates with silver after

extended useextended use Can be cleaned with adhesive sanding disc (come Can be cleaned with adhesive sanding disc (come

with probe)with probe) Silver Anode:Silver Anode:

Normal for dark layer of silver chloride to cover Normal for dark layer of silver chloride to cover silver anodesilver anode

Cleaning:Cleaning: Soak probe in 14% ammonium hydroxide for 2-3 minSoak probe in 14% ammonium hydroxide for 2-3 min Or soak probe in 3% ammonium hydroxide overnightOr soak probe in 3% ammonium hydroxide overnight Then rinse with DI waterThen rinse with DI water Recharge electrode and install new membraneRecharge electrode and install new membrane

Cleaning of SensorsCleaning of Sensors

Component of sensor that is sensitive to Component of sensor that is sensitive to contamination is contamination is membranemembrane Contamination results in:Contamination results in:

Lower readings when measuring or Lower readings when measuring or Lesser slopes when calibratingLesser slopes when calibrating

Because a portion of membrane surface is not Because a portion of membrane surface is not available for diffusion of oxygenavailable for diffusion of oxygen

Attempt to compensate for contamination by Attempt to compensate for contamination by adjusting instrument does not fix anythingadjusting instrument does not fix anything

Cleaning of SensorsCleaning of Sensors

Preferable to clean membranePreferable to clean membrane 1. Acetic or citric acid with a concentration of 5-10% 1. Acetic or citric acid with a concentration of 5-10%

(percent in weight!)(percent in weight!) Used for calcium and iron oxide depositsUsed for calcium and iron oxide deposits

2. Warm (<50C) household detergent2. Warm (<50C) household detergent Used for fats and oilsUsed for fats and oils

Avoid strong mechanical treatment of Avoid strong mechanical treatment of membrane during all cleaning activitiesmembrane during all cleaning activities Because its easily destroyedBecause its easily destroyed

Best to use a soft paper towelBest to use a soft paper towel

Probe StorageProbe Storage

Store DO probe with membrane covered Store DO probe with membrane covered by distilled waterby distilled water Covering membrane with DI water prevents Covering membrane with DI water prevents

KCl fill solution from evaporating through KCl fill solution from evaporating through membranemembrane

Polarization periods Polarization periods (startup periods) prior to (startup periods) prior to measurementmeasurement

If sensor was disconnected from meterIf sensor was disconnected from meter Appropriate polarization period must elapse Appropriate polarization period must elapse

after polarographic sensors are reconnected after polarographic sensors are reconnected (gold-silver electrode system) before start of (gold-silver electrode system) before start of measurementsmeasurements

About 30 minutesAbout 30 minutes Sometimes longer time is necessarySometimes longer time is necessary

Trouble ShootingTrouble Shooting

Unable to CalibrateUnable to Calibrate Allow 30 min for probe to polarizeAllow 30 min for probe to polarize Change membraneChange membrane Clean probeClean probe

Unstable readingUnstable reading Allow one minute for reading to stabilizeAllow one minute for reading to stabilize Change membraneChange membrane Clean probeClean probe

Motor not workingMotor not working Check power supplyCheck power supply Manually turn stir paddle to help start the motorManually turn stir paddle to help start the motor Replace motorReplace motor

Loud motorLoud motor Check that paddle is in all the wayCheck that paddle is in all the way Replace motorReplace motor

Amount of DO in Water is Amount of DO in Water is Function of:Function of:

1. Temp1. Temp Higher temp less oxygen water holdsHigher temp less oxygen water holds Relationship is non-linearRelationship is non-linear

2. Atmospheric pressure2. Atmospheric pressure Oxygen pressure reduce at higher altitudeOxygen pressure reduce at higher altitude Solubility reduces with increase in altitude (or reduction Solubility reduces with increase in altitude (or reduction

in atmospheric pressure)in atmospheric pressure)

3. Concentration of dissolved salts or salinity3. Concentration of dissolved salts or salinity More salts less oxygenMore salts less oxygen

Temp Temp °°CC

Solubility Solubility of Oxygen of Oxygen

mg/Lmg/L

00 14.6014.60

2020 9.089.08

2525 8.248.24

4040 6.416.41

5050 5.495.49

Oxygen SolubilityOxygen Solubility

Oxygen solubility is critical parameter in many Oxygen solubility is critical parameter in many areas:areas: BiochemistryBiochemistry Industrial processesIndustrial processes Environmental science and engineeringEnvironmental science and engineering

DO importance:DO importance: Sufficient dissolved oxygen is an absolute Sufficient dissolved oxygen is an absolute

requirement for good water qualityrequirement for good water quality Essential for aerobic lifeEssential for aerobic life

Oxygen SolubilityOxygen Solubility

DO concentrations:DO concentrations: Below 5 ppm will result in substantial Below 5 ppm will result in substantial

damage to aquatic ecosystemdamage to aquatic ecosystem Below 2 ppm results in fish kills and growth Below 2 ppm results in fish kills and growth

of harmful bacteriaof harmful bacteria

Practical ConsiderationPractical Consideration

1. Stirring1. Stirring 2. Membrane2. Membrane 3. Calibration3. Calibration

StirringStirring

Consumption of Consumption of oxygenoxygen by the probe by the probe Can cause a lowering of [Can cause a lowering of [oxygen]oxygen] at at

boundary layer btw sample and probe boundary layer btw sample and probe membranemembrane

For this reason, sample stirring is For this reason, sample stirring is recommendedrecommended

MembraneMembrane

Two types are commonly used:Two types are commonly used: 1. Loose membranes1. Loose membranes 2. Membrane cap assemblies2. Membrane cap assemblies

1. Loose membranes:1. Loose membranes: Less expensive but they are more difficult to installLess expensive but they are more difficult to install Give lower precision in resultsGive lower precision in results Stretch of membrane determines how thick electrolyte layer Stretch of membrane determines how thick electrolyte layer

adjacent to cathode is, which affects time response of probeadjacent to cathode is, which affects time response of probe 2. Precision manufactured membrane cap assemblies 2. Precision manufactured membrane cap assemblies

gives:gives: Reproducible electrolyte layer thicknessReproducible electrolyte layer thickness Speed up probe servicingSpeed up probe servicing Eliminate assembly problemsEliminate assembly problems

DO and TemperatureDO and Temperature

Temperature affects DO readings in two Temperature affects DO readings in two ways:ways: 1. Changes permeability of membrane1. Changes permeability of membrane 2. Temp affects amount of oxygen that can 2. Temp affects amount of oxygen that can

be dissolved in waterbe dissolved in water


1. Changes permeability of membrane1. Changes permeability of membrane As temp increases so does permeability of As temp increases so does permeability of

membranemembrane As temp decreases, oxygen permeability As temp decreases, oxygen permeability

through membrane decreasesthrough membrane decreases


2. Temp affects amount 2. Temp affects amount of oxygen that can be of oxygen that can be dissolved in waterdissolved in water As temp increases, As temp increases,

oxygen saturation point of oxygen saturation point of water decreaseswater decreases So less DO in waterSo less DO in water

Most meters have ATC to Most meters have ATC to compensate for temp compensate for temp error due to changes in error due to changes in permeability of permeability of membrane membrane Water forms a layer of water vapor

above water surface, layer isolates water from atmospheric oxygen DO at boiling is zero

Influence of interfering Influence of interfering gases: gases:

Membrane is also permeable to gases Membrane is also permeable to gases other than oxygenother than oxygen Nitrogen does not react and is irrelevantNitrogen does not react and is irrelevant High pH value of electrolyte solution protects High pH value of electrolyte solution protects

measurement from interfering influence of measurement from interfering influence of ammoniaammonia

Carbon dioxide is problematicCarbon dioxide is problematic

Carbon dioxideCarbon dioxide

Buffering capability of electrolyte solution Buffering capability of electrolyte solution is sufficient for short-term exposureis sufficient for short-term exposure

During long-term exposure carbon During long-term exposure carbon dioxide shifts pH value into acidic range dioxide shifts pH value into acidic range and leads to increased valuesand leads to increased values

ApplicationsApplications

A. Foods and BeveragesA. Foods and Beverages Many foodstuffs are packed in a Modified Atmosphere Many foodstuffs are packed in a Modified Atmosphere

Packaging where a low or controlled oxygen level is necessaryPackaging where a low or controlled oxygen level is necessary DO levels in some drinks, such as beer, should be kept in DO levels in some drinks, such as beer, should be kept in

specific rangespecific range Practice of adding oxygen under pressure to bottled water to Practice of adding oxygen under pressure to bottled water to

make oxygenated water has become more commonmake oxygenated water has become more common B. Environmental monitoringB. Environmental monitoring

DO fluctuations in lakes, rivers etc gets recordedDO fluctuations in lakes, rivers etc gets recorded Deep sea oxygen probes are used in oceans and deep lakesDeep sea oxygen probes are used in oceans and deep lakes At different depths to give a total picture of the state of area At different depths to give a total picture of the state of area

concerned concerned


C. Water treatment (Re-circulating)C. Water treatment (Re-circulating) Water is cleaned and filtered through mechanical and Water is cleaned and filtered through mechanical and

biological filtersbiological filters Ozone can be added to "burn off" pollutantsOzone can be added to "burn off" pollutants pH of water is measured and controlled using a pH meterpH of water is measured and controlled using a pH meter DO content is measured and pure oxygen is injectedDO content is measured and pure oxygen is injected This oxygen injection can also be used to strip off carbon This oxygen injection can also be used to strip off carbon

dioxidedioxide Often only a small proportion of water is oxygenated at high Often only a small proportion of water is oxygenated at high

pressurepressure resulting super-saturated water is mixed with main flow to give resulting super-saturated water is mixed with main flow to give

healthy DO levels in growth tankshealthy DO levels in growth tanks


D. Waste Water TreatmentD. Waste Water Treatment It is no longer enough just to filter the water and It is no longer enough just to filter the water and

dump the detritus in the seadump the detritus in the sea The larger part of the waste is mainly organic, and The larger part of the waste is mainly organic, and

this must be broken down in sludge tanks and the this must be broken down in sludge tanks and the effluent water controlled and treated as necessaryeffluent water controlled and treated as necessary

E. Measuring biochemical oxygen demandE. Measuring biochemical oxygen demand A suitable detection limit is 1 mg/lA suitable detection limit is 1 mg/l

Biochemical Oxygen Biochemical Oxygen Demand (BOD)Demand (BOD)

BOD testBOD test Measure degree to which water is contaminated Measure degree to which water is contaminated

with organic materialswith organic materials Organic materials are decomposed by aerobic bacterial Organic materials are decomposed by aerobic bacterial

processesprocesses Important test for municipal and industrial Important test for municipal and industrial

dischargesdischarges Sample gets diluted with oxygen saturated waterSample gets diluted with oxygen saturated water DO content is measured immediately after dilutionDO content is measured immediately after dilution And after 5-day incubation periodAnd after 5-day incubation period BOD is expressed as the mg/L of DO consumed BOD is expressed as the mg/L of DO consumed

during incubationduring incubation

BOD calculations BOD calculations

Steps to calculate BODSteps to calculate BOD Based on addition of a nutrient source (carbon - glucose - Based on addition of a nutrient source (carbon - glucose -

glutamic acid) and no nutrient sourceglutamic acid) and no nutrient source 1.1. BOD of blanks (no nutrient source) = DO Final – DO Initial BOD of blanks (no nutrient source) = DO Final – DO Initial 2.2. BOD of nutrient added samples = (DO Final – DO Initial) BOD of nutrient added samples = (DO Final – DO Initial)

times dilution factor per 300mltimes dilution factor per 300ml * 300 ml is based on the volume contained in BOD bottles* 300 ml is based on the volume contained in BOD bottles

BOD of sample and standards are calculated by BOD of sample and standards are calculated by subtracting the final DO from the initial DO and subtracting the final DO from the initial DO and multiplying this factor by dilution factormultiplying this factor by dilution factor

Final value is determined by subtracting out BOD for Final value is determined by subtracting out BOD for blank from BOD that has been nutrient enrichedblank from BOD that has been nutrient enriched

This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and does not discriminate on the following basis: •against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political affiliation or belief; and•against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or his or her participation in any WIA Title I-financially assisted program or activity.

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dissolved oxygen (do) lecture. introduction air we contains about 20% oxygen fish and other...

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