Welcome to the dissolved oxygen (D.O.) theory Page. Dissolved oxygen from A to Z.

• Basic principles of Polagrography cell (electrode)

• Polar gram

• EID’s dissolved oxygen electrode picture

• Electrode reactions

• Number of electrons involved

• Calibration

• Calibration in air saturated with water vapor

• Calibration in air saturated water

• Checking the sensor function

• Measurement and analytical quality assurance

• Cleaning of sensors

• Regeneration of sensors

• Polarization periods (startup periods) prior to measurement

• Approach flow

• Correction for salt content

• Influence of interfering gases

• Solubility functions

• Checking the oxygen meter and or logger

• Dissolved oxygen Applications

• Practical experiments

Dissolved Oxygen:

The air we breathe contains about 20% oxygen. Fish and other aquatic organisms require oxygen as well. The term Dissolved Oxygen (DO or D.O.) refers to the amount of free oxygen dissolved in water which is readily available to respiring aquatic organisms. State water quality standards often express minimum concentrations of dissolved oxygen which must be maintained in order to support life as well as be of beneficial use. Levels of dissolved oxygen below 4-5 milligrams per liter affect fish health and levels below 2 milligrams per liter can be lethal to fish.

Additionally, biochemical oxygen demand (BOD) is commonly used with reference to effluent discharges and is a common, environmental procedure for determining the extent to which oxygen within a sample can support microbial life. The test for BOD is especially important in waste water treatment, food manufacturing, and filtration facilities where the concentration is crucial to the overall process and end products. High concentrations of DO predict that oxygen uptake by microorganisms is low along with the required break down of nutrient sources in the medium.

Basic principles of Polagrography cell:

Liquid and Air state of equilibrium is reached when the partial pressure of oxygen, i.e. the part of the total pressure that is due to oxygen, is equal in air and in liquid. The liquid is then saturated with oxygen.

Figure 1.1 Air and liquid oxygen equilibrium

Polargram:

When an electrode of noble metal such as platinum or gold is made 0.6 to 0.8 V negative with respect to a suitable reference electrode such as AgAgCl or an calomel electrode in a neutral KCI solution (see Figure 1.2), the oxygen dissolved in the liquid is reduce at the surface of the noble metal.

Figure 1.2 Polarographhy diagram

This above phenomenon can be observed from a current to voltage diagram called a polarogram of the electrode. As shown in Figure 1.3a, the negative voltage applied to the noble metal electrode (called the cathode) is increased, the current increases initially but soon it becomes saturated. In this plateau region of the polarogram, the reaction of oxygen at the cathode is so fast that the rate of reaction is limited by the diffusion of oxygen to the cathode surface. When the negative bias voltage is further increased, the current output of the electrode increases rapidly due to other reactions, mainly, the reduction of water to hydrogen. If a fixed voltage in the plateau region (for example, - 0.6V) is applied to the cathode, the current output of the electrode can be linearly calibrated to the dissolved oxygen (Figure 1.3b). It has to be noted that the current is proportional not to the actual concentration but to the activity or equivalent partial pressure of dissolved oxygen, which is often referred to as oxygen tension. A fixed voltage between -0.6 and -0.8 V is usually selected as the polarization voltage when using Ag/AgCl as the reference electrode or any other EID's dissolved oxygen electrodes.

Additionally for physical and chemical correctness, partial pressure in a liquid actually refers to the fugacity. In the pressure range relevant to the measurements at hand, it is acceptable to equate the two values and this allows us to restrict the following considerations to the partial pressure. In dry, atmospheric air, the partial pressure of oxygen is 20.95% of the air pressure. This value is reduced over a water surface because water vapor has its own vapor pressure and a corresponding partial pressure.

Figure 1.3 (a) Current to voltage diagram at different oxygen tension; (b) Calibration obtained at a fixed polarization voltage of –600 mV.

When the cathode, the reference electrode, and the electrolyte are separated from the measurement medium by a polymer membrane, which is permeable to the dissolved gas but not to most of the ions and other species, and when most of the mass transfer resistance is confined in the membrane, EID’s electrode system can measure oxygen tension in various liquids. This is the basic operating principle of the membrane covered polarographic Dissolve oxygen probe (Figure 1.4).

The basic principle underlying the electrochemical determination of oxygen concentration is the use of membrane covered electrochemical sensors. The main components of the sensors are the oxygen permeable membrane, the working electrode, the electrolyte solution and a possible reference electrode. A voltage is applied between the gold (platinum or silver) cathode and the anode that consists of either lead or silver (AgAgCl), and causes the oxygen to react electrochemically. The higher the oxygen concentration the higher the resulting electric current. The current in the sensor is measured and, after calibration, converted into the concentration of dissolved oxygen.

If the anode is made of silver, the meter applies the required voltage (polarographic sensor). If it is made of lead, the sensor is self-polarizing, i.e. the voltage is generated in the sensor by the electrodes themselves, comparable to the process in a battery (galvanic sensor). The meter merely evaluates the current.

Figure 1.4 Basic Polarographhy electrode

EID’s polaragraphoc dissolved oxygen electrode picture:

EID’s ELECTRODER - ABS body Dissolved Oxygen Sensor (ADO)

EID’s dissolved oxygen, Probe, polaragraphic, ABS body, 12mm * 120mm, with 10K Negative Temperature Compensation

Figure 1.5 Basic Polarographhy-electrode

Electrode reactions:

For our polarographic electrodes, the reaction proceeds as follows:

• Cathodic reaction:    0₂ + 2H₂ 0 + 2e^- à H₂O₂ + 2OH^-

                                     H₂0₂ + 2e^- -> 20H^-

•  Anodic reaction:       Ag + Cl^- à AgCl + e^-

•  Overall reaction:      4Ag + 0₂ + 2 H₂O + 4 Cl^- à 4 AgCl + 4 OH^-

The reaction tends to produce alkalinity in the medium together with a small amount of hydrogen peroxide.

Number of electrons involved:

Two principal pathways were proposed for the reduction of oxygen at the noble metal surface. One is a 4-electron pathway where the oxygen in the bulk diffuses to the surface of the cathode and is converted to H₂O via H₂O₂ (path a in Fig. 1.6). The other is a 2-electron pathway where the intermediate H₂O₂ diffuses directly out of the cathode surface into the bulk liquid (path b in Figure 1.6). The oxygen reduction path may change depending on the surface condition of the noble metal. This is probably the cause for time-dependent current drift of polarographic sensors. Since the hydroxyl ions are constantly being substituted for chloride ions as the reaction starts, KCI or NaCl has to be used as the electrolyte. When the electrolyte is depleted of Cl^-, it has to be replenished.

                                                                             2e^-

                                                        2e^-         à (a)  à  H₂O₂

                                                        O₂ à O₂ à H₂O₂   

                                                                          Diffusion

                                                                     à (b)   à  H₂O₂

Figure 1.6 alternative pathway of oxygen reduction at cathode surface

Calibration:

Calibration must be carried out for dissolved oxygen measurements on a regular base. This is because the measuring process consumes the electrolyte solution in the sensor head, as shown by the electrode reactions presented above. The ions of the electrolyte solution bind the released metal ions, thereby changing the composition of the solution. The recommended calibration period depends on the oxygen sensor used and ranges from one week for pocket instruments to 1-2 months.

Each linear calibration function is defined by at least two points. For dissolved oxygen measurements with EID meter and/or logger, one of the points on the line is the zero point of the sensor. At the zero point, the sensor signal obtained in the absence of oxygen lies below the resolution of the sensor. This point is called the zero-current point of the sensor. The second point of the calibration line can be set as required. Its position is based on the fact that, in a state of equilibrium, the partial pressure of oxygen in liquid and air is equal.

Figure 1.4b Two-point calibration

The rate at which oxygen enters a dissolved oxygen probe is a function of:

• the concentration of oxygen in the sample

• the diffusion coefficient/permeability of the membrane (function of temperature)

As described above calibration routines for dissolved oxygen probes use a two point linear calibration where one point is at zero mg/L oxygen and the second point is at saturation or equilibrium with the atmosphere, C^* . The zero measurement is not zero volts due to the conductivity of the electrolyte between the electrodes as well as any errors in the analog signal conditioning circuit. For the circuit and probe system used in the Environmental lab the zero measurement is approximately 1 mV (where approximately 200 mV corresponds to saturation levels of oxygen) and hence the zero measurement is not significant. Thus a single point calibration is used.

C* is a function of the atmospheric pressure and temperature. The functional relationship with temperature is implemented using a lookup table (based on equilibrium at atmospheric pressure) with interpolation. The effect of atmospheric pressure is implemented as shown (Equation 1 below).

P

C^* =------- f (T)

P_atm

The permeability of the membrane increases about 5% per C degree. I chose to use 25 C as the reference temperature and thus K_membrane(T_ref) has a value of 1. The following (equation 2) creates a coefficient that describes this variation.

K_membrane(T)₌K_membrane(T_ref)e^0.05(T-Tref)

The slope of the linear fit (k) can be calculated after the voltage corresponding to saturation oxygen is measured (Equation 3 below).

    C^*_cal  K_membrane

C =------------------

 V^*_cal

The slope coefficient is placed in the polynomial array.

The equation for the dissolved oxygen concentration illustrates that the predicted concentration is a function of sample temperature because K_membrane varies with temperature.  The coefficient, k, should be independent of temperature but will vary as a membrane fouls (Equation 4 below)

      KV

C=-----------

     K_membrane

Pressure

The constituents of air have been well defined, and it is known that air contains 20.946% oxygen. Since the total pressure in the air is the sum of all of the partial pressures (Dalton’s Law), an atmospheric pressure of 760 millimeters Mercury (mmHg) in dry air will contain a partial pressure of oxygen (pO₂) of approximately 159 mmHg (760 mmHg * 0.20946).  Changes in atmospheric pressure will cause a directly proportional change in the partial pressure of oxygen in the air.  Atmospheric pressures will vary depending upon altitude and local weather conditions.  Some average pressures for varying altitudes are listed in Table 1 bellow. 

The relationship between oxygen partial pressure and total atmospheric pressure should be understood and incorporated into the air calibration in order to minimize calibration error, which could be as high as 5-10% dependent upon altitude and local weather conditions.  Most dissolved oxygen meters that have any sort of advanced air calibration (such as temperature compensation, which will be discussed in a later section) will be based upon an atmospheric pressure of 760 mmHg.  Most tables of oxygen solubility are referenced to this value.  Because of the change in oxygen partial pressure with changes in atmospheric pressure, a correction must be made when the pressure varies from this value.  A simple means of incorporating pressure changes is listed in the “correction factor” shown in Table 1 bellow.  The value listed is a rough multiplier, which can be used once the initial oxygen concentration is determined based upon temperature and relative humidity.  A more accurate calculation for incorporating pressure will be discussed after relative humidity and temperature effects are investigated. 

Some EID’s dissolved oxygen meters contain a pressure sensing device which provides compensation for pressure effects when an air calibration is performed.  If you use our electrode on not-EID-meter, since most meters do not have this, it is usually necessary to note the average pressure in the local vicinity of the probe, which will be mostly altitude-based, and adjust the calibration using the simple correction factor or the more complex calculation performed later.  A mercury barometer located in the immediate vicinity of the meter will give a relatively accurate measurement of the local atmospheric pressure if an older meter with no pressure sensor is used.

Table 1: Oxygen Value Corrected for Pressure (25 °C)

Relative Humidity and temperature effect and Temperature compensation

If desired the Eid's probes can be coupled with a temperature thermistor (10K Ohms) to achieve temperature compensation since K_membrane varies with temperature.

The discussion of pressure effects were based upon atmospheric pressure with dry air (no moisture content).  Whenever air contains a certain amount of moisture, the atmospheric pressure contains another source of partial pressure -- water vapor.  If a comparison of the oxygen partial pressure in air with 100% relative humidity and air with 0% relative humidity is done while both are at the same atmospheric pressure, the air with 100% relative humidity will have a lower oxygen partial pressure due to the presence of the water vapor pressure (pH₂O).  Water vapor pressure in air varies with temperature, and is well defined.  The effect of temperature on oxygen partial pressure in moist air is such that higher temperatures yield lower oxygen partial pressure, while lower temperatures yield higher pressures.  Note that the effects of relative humidity and temperature can cause errors when air calibration is performed in dry air, since most of the current tables and meter temperature compensations are based on air containing 100% relative humidity.  Table 2 bellow shows both the oxygen concentration, which is linear with the partial pressure of oxygen,  that would be present at 100% relative humidity and 0% relative humidity.  The values only differ by a few percent in ambient air conditions, and thus is generally ignored. Most dissolved oxygen meters have temperature compensation for air at 100% relative humidity, and no manual correction is necessary.  However, many older meters do not have temperature compensation included, and therefore this calculation must be done manually.  If temperature is not compensated for in the calibration, the error can be as much as 20 to 30 % for every 10 degrees difference from 25 °C, and therefore temperature compensation is standard on most dissolved oxygen meters today.   Since the effects of relative humidity is minimal at all but the highest temperatures, no current dissolved oxygen meters incorporate any kind of relative humidity sensing device.

In order to ensure an accurate temperature and current reading, the probe must be exposed to the air for enough time to allow thermal equilibrium to occur.  There are often significant temperature differences between the process water and the ambient air.  Larger temperature gradients between the two necessitate additional time for thermal equilibrium to take place.  For instance, a 20 °C difference between ambient air and process water can cause a calibration delay of about 30 minutes in many probes for the probe to fully equilibrate to ambient temperature.  Since most temperature gradients will not be this large, allowing approximately 15 minutes is usually a safe assumption.  It is common for users to calibrate the unit before the dissolved oxygen meter is reading the stabilized temperature and current value, which can cause significant error since a difference of even 5 °C from actual can cause the reading be off by 5 to 10%.  It is often useful to have a calibrated temperature sensor, accurate to 1 °C or better, at the calibration location to know when the probe temperature is reading the correct ambient air temperature. 

It is useful to have an equation which can be used to determine oxygen concentrations in air based upon temperature, relative humidity, and pressure. Since the full equation is quite lengthy and complex, two easier versions are presented to the user, along with Table 2 bellow, to determine the correct oxygen concentration in air.  Equation 5 bellow should be used with air with 100% relative humidity, and Equation 6 should be used for air with 0% relative humidity. 

Equation 5 (100% Relative Humidity): OS = (OS’) * (P - p) / (760 - p)

where:

OS  = Oxygen solubility at barometric pressure of interest

OS’ = Oxygen in saturation at one atmosphere (760 mmHg) at a given temperature

P  = Barometric pressure of interest

p  = Vapor pressure of water at the temperature of interest

Example 1:

The user wishes to calibrate a dissolved oxygen probe in air at an altitude of 3500 feet.  The temperature is 30 °C, and the relative humidity is 100%. 

At an altitude of 3500 feet, the atmosphere pressure will usually be about 668 mmHg (Table 1 above).  The sample temperature is 30 °C, and the relative humidity is 100%.  From water vapor pressure tables, the water vapor pressure at 30 °C is 31.8 mmHg. The oxygen saturation level at 760 mmHg and 30 °C is 7.54 ppm (Table 2 bellow).  Substituting these values in the above (equation 5) gives the following:

OS  =  (7.54)  * (668 - 31.8) / (760 - 31.8) =  6.59 ppm

Example 2:

Assume the same conditions as in example 1, but with a relative humidity of 0%.  In this case, the value used for the oxygen saturation level would be 7.87 (Table 2 bellow), not 7.54.  The calculation will change since there will be no water vapor pressure.

Equation 6 (0% Relative Humidity): OS = (OS’) * (P) / (760 mmHg)  

Substituting the above values into the equation yields the following:

OS = 7.87 * (668) / (760) = 6.92 ppm

Note: that the multiplier of (668) / (760) is actually the simplified correction factor listed in Table 1 above for an altitude of 3500 feet (0.88). Table 3 bellow lists calibration values for varying temperatures pressures at relative humidity levels of 100%.

Table 2 above: Dissolved Oxygen Solubility vs. Temperature

Table 3 above: Oxygen concentration (ppm) for varying pressures (mmHg) and temperatures (degrees Celsius) at 100% relative humidity

Calibration in air saturated with water vapor:

This requirement is met over large water surfaces, such as lakes or the sludge activation basin of a wastewater treatment plant.

Note: EID’s offers special air calibration vessels for laboratory measurements.

Calibration in air saturated water:

The water is aerated until the partial pressure of the oxygen in the water is the same as in the air. This method is accompanied by some inherent risks:

• The air pressure in the aeration tube is always somewhat higher than the normal air pressure and, therefore, the water is always somewhat supersaturated after aeration.

• The water temperature falls during aeration (latent heat!)

•  If the experimenter waits until the temperatures are equal, the water will be somewhat supersaturated.

• The point at which the water is completely saturated is difficult to estimate. There is a risk of under saturation.

• Oxygen depleting substances lead to under saturation.

As you can see from the above list, is clearly preferable to calibration in air-saturated water.

Checking the sensor function:

Relative slope limitation, to evaluate the sensor function despite it, there are three characteristic measuring points in addition to a visual test.

In the visual test, the gold cathode is examined visually. If it has lost its gold or platinum color and is coated with lead or silver, the sensor will yield values that are too high and will generally no longer be zero-current-free. This can be corrected by regenerating the oxygen sensor as described in the operating manual. The gold cathode may only be polished with a moist special abrasive film using a circular motion with little pressure. It is imperative that only this special film be used since a scratched and unpolished electrode surface can harm the sensor and impair its accuracy.

Attention: Anodes of lead or silver cannot be polished at all.

One subjective, visual examination of the sensor can be supplemented by a more comprehensive test, an evaluation at three specific measuring points: in air saturated with water vapor (1), in air saturated water (2) and in oxygen free water (3).

1. Test in air saturated with water vapor:

The sensor should obtain a reading between 100 and 104% oxygen saturation in water-saturated air. If the values lie above this range, the membrane was probably wet during calibration; perhaps there is too much water in the calibration vessel. A value above 100% saturation is due to the differing viscosity of water and air as well as to the surface tension of water. To put it simply, it is easier for oxygen molecules in the air to permeate the membrane than for those in the water to dissolved oxygen so. In the measuring mode, which is the mode in which the test takes place, calculations are based on a liquid sample and this results in a saturation level over 100%.

2. Test in air saturated water:

After calibration, the value in air-saturated water should lie between 97 and 102% saturation. The theoretical value is 100% but is difficult to reproduce. This relatively large tolerance is due not to the sensor but to the saturation procedure. This is also the reason why EID successfully sought an alternative to the conventional calibration procedure in air-saturated water. If the sensor dissolved oxygen not display a reading within this tolerance range, it should be sent back to the manufacturer for tests.

3. Test using zero solution:

This is to test the zero current point of the sensor. When the oxygen content is 0 mg, the maximum reading of the sensor should not exceed the resolution of the meter (1 digit). This test is carried out using sodium sulfite solution. Sulfite reacts with the dissolved oxygen to form sulfate, binding the oxygen dissolved in the water.

Preparation of the solution: Dissolve a teaspoon of sodium sulfite in 100 ml tap water. The solution will be oxygen free after it has stood undisturbed for 15 minutes. It must remain undisturbed to prevent oxygen in the surrounding air from re entering the solution. One minute after submerging paleographic sensors (EID-E-ADO–A001, EID-E-ADO-A002, etc.) Into the solution, the meter should display a maximum reading of 2%; after 15 minutes, the maximum reading should be 0.4%. If not, the sensor is no longer zero current free and must be cleaned or sent to the manufacturer for tests. After the test, the sensor should be rinsed thoroughly with distilled water to remove any remaining traces of sodium sulfite solution.

Galvanic sensors with lead counter may be submersed for no more than 3 minutes. Subsequently, they must also be rinsed thoroughly with distilled water. Cleaning of the sensors is extremely important to prevent toxification and lasting damage.

Measurement and analytical quality assurance:

Measurement of the oxygen concentration is now quite easy to carry out. The sensor is submersed in the liquid to be investigated and the measured value is read from the display. In principle, this is all there is to it, but nevertheless a few important points should be observed and among those is the proper maintenance of the sensors.

Cleaning of sensors:

The component of the sensor that is sensitive to contamination is the membrane. Contamination results in lower readings when measuring or lesser slopes when calibrating because a portion of the membrane surface is not available for the diffusion of oxygen. The attempt to compensate for the contamination by adjusting the instrument does not agree with the water principle. It is preferable to clean the membrane. Acetic or citric acid with a concentration of 5--10% (percent in weight!) is used for calcium and iron oxide deposits and warm (<50C) household detergent is used for fats and oils.

Avoid strong mechanical treatment of the membrane during all cleaning activities because its thickness is on the order of m and it is easily destroyed. It is best to use a soft paper towel. Dissolved oxygen not clean the sensor in an ultrasound bath as this may cause the coating of the anodes to peel off.

Regeneration of sensors:

Regeneration of the sensor becomes necessary when the function responds or when the slope (S) < 0.6 has decreased markedly when calibrating.

Basically, regeneration is required when the electrolyte solution is depleted, when the gold cathode has become coated with lead or silver, when the reference electrode is to xified or when the membrane is damaged or contaminated.

It consists of exchanging the electrolyte solution, cleaning the electrodes and exchanging the membrane head.

It is important to follow the operating manual exactly! Mistakes are then easily avoided.

The following points should be emphasized:

• The sensor must be disconnected from the meter. When the sensor is connected and submersed in the cleaning solution, no chemical reaction takes place between the solution and the oxidized reference electrode surface; instead, the cleaning solution may become electrolyzed!

• Use the cleaning or electrolyte solution suitable for the particular sensor as stated in the operating manual! A solution that is suitable for silver electrodes cannot regenerate lead electrodes!

• Only the gold cathode should be polished; the counter electrode is merely wiped clean with a soft cloth to wipe away easily removable salt deposits! A spotty coating after regeneration of the lead or silver electrodes does not impair measurements!

• When polishing the gold electrode, only use the moistened EID abrasive film that has a special grain that polishes and do not scratch!

• It is also recommended to use a new membrane head since the used membrane cannot necessarily guarantee that the membrane fits correctly against the gold cathode which is ensured by a spacing lattice on the inside of the membrane. Baggy clothing don't fit either!

Please note: The spacing lattice is clearly visible when the membrane head is held up against the light.

The result of an oxygen measurement can be documented in several ways:

•  Display of the concentration: The instrument requires the appropriate data of the calibration curve and uses them to calculate the concentration in mg/L (ppm is identical in this case), allowing for the temperature dependency of the individual parameters

• Display of the percentage of oxygen saturation: The instrument measures the sensor current and calculates the partial pressure of oxygen according to the calibration. The current air pressure is measured for the calculation of the saturation partial pressure. The display corresponds to the quotient, converted into a percentage.

Polarization periods (startup periods) prior to measurement:

If the sensor was disconnected from the meter, an appropriate polarization period must elapse after the polarographic sensors are reconnected (gold-silver electrode system) before the start of measurements. Please be advised that this does not apply to galvanic sensors (gold-lead electrode system) because they are self polarizing and can be used immediately.

Approach flow:

The approach flow to the sensor membrane must be continuous for oxygen measurements to be correct. The diffusion of the oxygen molecules in the sensor head creates an oxygen poor zone that simulates a reduced concentration of oxygen. The concentration at the membrane must always be the equal to the concentration in the remainder of the sample. This condition can be met by stirring the sample or moving the sensor in the sample.

EID offers special stirring attachments that rotate like little turbine blades and continuously supply the membrane with fresh sample. They are driven by an alternating electromagnetic field that is generated by the base of the stirrer.

The major advantage of this unit is the size of the attachment. It has the same diameter as the sensor and is mounted on the sensor head. This simplifies measuring in sample bottles such as bottles for BOD measurements.

The EID-E-ADO–A001 sensor is specially designed for BOD measurements. The sensor shaft contains a propeller similar to a marine screw propeller to maintain a continuous approach flow at the membrane. The stirring effect is sufficiently large to homogenize the sample in addition to generating the approach flow.

If agitators or magnetic stirrers are used, the possible formation of eddies must be taken into account. The oxygen sensor may not be positioned in the eddy because air at the sensor head

Approach flow membrane may falsify readings. This can be prevented by lowering the stirring frequency or positioning the sensor away from the eddy.

When the sensor is installed in pipelines, the sample flows past the sensor head, providing a sufficient approach flow. EID offers stationary measuring systems with special installation assemblies for pipes.

Alternatively, the sensor itself can be moved in the medium being investigated, e.g. by stirring the sensor in a beaker or by swinging it back and forth in a lake. For measurements at great depths, depth armatures for water depths up to 100m are available.

It is important that stirring does not falsify the measured values. This is likely to happen when the investigated sample is supersaturated or under saturated with oxygen and oxygen can be expelled from or stirred into the sample.

Super saturation with oxygen, for example, can be observed in the summer in stagnant waters when luxuriating algae produce oxygen by photosynthesis. An example of under saturation with oxygen is the BOD determination in which bacteria lower the oxygen concentration in bottles through respiration. For this reason, the volume of the sample is important. A measurement taken in a lake or an activation basin is uncritical because of the enormous quantity of the sample. In an open beaker, however, stirring can easily alter the oxygen concentration.

Correction for salt content:

The temperature dependent Bunsen absorption coefficient changes when substances are dissolved in the water. This effect is accounted for by entering the salinity. The salinity can be determined using a conductivity meter and corresponds to the salt content of seawater in g/kg. The standard recommends the use of this function for other waters since the deviation is minimal (<2%).

Influence of interfering gases:

The membrane is also permeable to gases other than oxygen. Nitrogen does not react and is, therefore, irrelevant. The high pH value of the electrolyte solution protects the measurement from the interfering influence of ammonia. Carbon dioxide, on the other hand, is problematic. The buffering capability of the electrolyte solution is sufficient for short-term exposure; during long-term exposure, however, carbon dioxide shifts the pH value into the acidic range and leads to increased values. Polarographic sensors can better regenerate the buffer capability than galvanic sensors because they generate an excessive number of hydroxide ions during the electrode reactions. The buffer capacity of the electrolyte solution in the sensor is insufficient for samples with a high carbon dioxide content (e.g. beer, sparkling wine or soft drinks). The pH shifts into the acidic range and the meter shows higher than normal readings. Hydrogen sulfide presents the greatest danger for oxygen sensors because the sulfide-ions generated by the neutralization reaction toxify the counter electrode. The sensors can withstand small amounts, but continuous exposure markedly shortens their lifetime.  Hydrogen sulfide has the smell of rotten eggs and is easily perceptible at very small concentrations, eliminating the need for complex measurements.

Solubility functions:

In order to determine the concentration of dissolved oxygen in non-aqueous liquids, the appropriate solubility function must be known. High performance dissolved oxygen meters and logger from EID have stored software programs that make this type of determination feasible. If the solubility function is known, oxygen measurements can be carried out similarly to measurements in water.

Checking the oxygen meter and or logger:

The METER-D0 (meter) or PROBER-DO (data logger) are checked using simulators. The simulators are connected to the instrument in place of the sensor. They generate defined current signals that the instrument must display correctly. If the readings lie outside the tolerance indicated by the certificate, the instrument must be sent to the manufacturer for servicing.

Applications:

A. Foods and Beverages
Many foodstuffs are packed in a Modified Atmosphere Packaging where a low or controlled oxygen level is necessary. Dissolved oxygen levels in some drinks, such as beer, should be kept in specific range. Practice of adding oxygen under pressure to bottled water to make oxygenated water has become more common. These dissolved oxygen measurements required dissolved oxygen probes that can be cleaned at elevated temperatures without being removed from the application.

B. Environmental monitoring
EID's dissolved oxygen data loggers can be left to record dissolved oxygen fluctuations in lakes, rivers etc. Deep sea oxygen probes are used in oceans and deep lakes. EId's dissolved oxygen electrodes with fast response are used to map the dissolved oxygen content of lakes and fishing waters. EId's dissolved oxygen probes are not only raised and lowered in the water, but also towed through the water at different depths to give a total picture of the state of the area concerned.

C. Fish Farming - Aquaculture
Fish farmers needed multi-channel dissolved oxygen meters. Additionally, they need dissolved oxygen monitoring and dissolved oxygen control equipment. EID's dissolved oxygen monitoring and logger are encompass alert units with both high dissolved oxygen alarm and low dissolved oxygen alarm. Equipment introduced by EID in 1977 for controlling dissolved oxygen they are been used all over the world.

D. Water treatment (Re-circulating)
The water is cleaned and filtered through mechanical and biological filters. Ozone can be added to "burn off" pollutants, either by direct ozone injection or by UV ozone activation. This process can be controlled using a redox or ORP measurement. The pH of the water is measured and controlled using a pH meter and pH controller. The dissolved oxygen content is measured and pure oxygen is injected. This oxygen injection can also be used to strip off carbon dioxide. Often only a small proportion of the water is oxygenated at high pressure. The resulting super-saturated water is mixed with the main flow to give healthy dissolved oxygen levels in the growth tanks. EID's In-line dissolved oxygen electrodes, Twist and lock mount dissolved oxygen electrodes or flow cell dissolved oxygen electrodes can be used in such high pressure oxygenation systems.

E. Hatchery and growth tanks
Water level as well as dissolved oxygen should be measured in each tank - the water supply to one tank could be cut off. Oxygen level alarms are set on the dissolved oxygen measurements. Aeration or oxygen injection to each tank is not often practiced in smaller indoor tanks, oxygen is added to the inlet or re-circulated water. Aeration or oxygen injection is, however, seen in larger tanks, requiring a separate dissolved oxygen meter with dissolved oxygen controller system for each tank. This is easily done with EID's MultiProbe Technology^TM dissolved oxygen metering, logging and control equipment.

F. Sea cages
Since it is difficult to control the dissolved oxygen content of the sea. Dissolved oxygen measurement is very important because feed uptake and dissolved oxygen levels are interconnected. Intensive feeding after fish have experienced low dissolved oxygen levels can not only be a waste of food, but can actually harm the fish. The measurement of dissolved oxygen levels enables feed to be dosed optimally and, if relayed to the shore can warn that the cage should be moved if extremely low dissolved oxygen levels should occur.

G. Transport tanks
Dissolved oxygen measurement should also be performed during transport to the processing plant. A healthy fish gives a better finished product when contain the right level of oxygen. Another situation requiring dissolved oxygen measurement is the transport of juvenile fish to tanks or cages for growing.

H. Oxygen generator control
Pure oxygen meters and oxygen controllers equipment are also used in aquaculture. The purchase of liquid oxygen in bulk is often the most economic solution, but there are many cases where oxygen generators are installed locally. Two of the many advantages of using pure oxygen are that 1) it is possible to super-saturate the water with oxygen and 2) you save pump energy since pumping air means pumping 79% nitrogen and "only" 20.9% oxygen.

I. Waste Water Treatment
It is no longer enough just to filter the water and dump the detritus in the sea. The larger part of the waste is mainly organic, and this must be broken down in sludge tanks and the effluent water controlled and treated as necessary.

Sludge tank dissolved oxygen measurement and control is kept at approximately 2 mg/l.

Flow measurement, suspended solids measurement, sludge blanket detection, conductivity measurement, nitrate measurement and phosphate measurement utilizing EID's Industrial electrodes are also all used to enable the efficient and effective cleaning of waste water.

J. Safety Monitoring
Both oxygen detection in flammable gas and oxygen monitoring in ambient air are examples of this. Blanket gas is often used where flammable substances occur. Blanket gas is gas that cannot burn or sustain fire, i.e. it does not contain oxygen. Volumetric oxygen measurement is carried out both on the blanket gas and the surrounding air, the latter for worker safety. Special versions of the EID's dissolved oxygen electrodes are approved for use in potentially dangerous atmospheres, i.e. in classified areas.

K. Measuring biochemical oxygen demand

The BOD test requires a commitment of five (5) days from initial sample collection to the end of the analysis. During this time, samples are initially seeded with microorganisms and supplied with a carbon nutrient source of glucose-glutamic acid. The sample is then introduced to an environment suitable for bacterial growth at reproducible temperatures, nutrient sources and light within a 20C incubator such that oxygen will be consumed. Quality controls, standards and dilutions are also run for accuracy and precision. Determination of the dissolved oxygen within the samples can be determined through titration. The difference in initial DO readings (prior to incubation) and final DO readings (after a five (5) day incubation period) predicts the BOD of the sample. A suitable detection limit as per environmental quality control is 1 mg/l.

BOD calculations

Steps to calculate Biochemical Oxygen Demand (BOD). They and are based on the addition of a nutrient source (carbon - glucose - glutamic acid) and no nutrient source.

1. The BOD of the blanks (no nutrient source) = DO_Final - DO_Initial

2. The BOD of the nutrient added samples = (DO_Final - DO_Initial) time dilution factor per 300ml

* 300 ml is based on the volume contained in BOD bottles

The BOD of the sample and standards are calculated by subtracting the final DO from the initial DO and multiplying this factor by the dilution factor. The final value is determined by subtracting out the BOD for the blank from the BOD that has been nutrient enriched.

Practical experiments:

Preparations:

All practical experiments should be carried out in a suitable laboratory to guarantee working safety. This is a general recommendation. In the field of oxygen measurement, the cleaning solutions and the electrolyte solutions may contain caustic substances. A possible danger therefore exists when regenerating the sensor.

Safety instructions. General rules of conduct when handling chemical substances

When working at a work place at which chemicals are handled, the following rules must be observed:

1.       Follow the instructions on the chemical bottles

2.       Always wear protective garments (goggles, gloves...)

3.       Never point open containers towards other persons

4.       Do not eat, drink or smoke

5.       Ensure the satisfactory disposal of chemicals

6.       Carefully remove or clean up any spilled chemicals

7.       Contact specialist personnel if any serious problems arise

We provide these short instructions in the hope that they lead to successful and safe practical research and studies.

Safety data sheets are available for the cleaning and electrolyte solutions. The user must have one of these data sheets. Because they are fairly comprehensive, they cannot be included in every delivery. The manufacturer will, however, provide them on request.

The following equipment and facilities must be present when you are measuring dissolved oxygen:

• Washable tables

• Resistant floor coverings

• Running water

• Eye bath

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