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Dissolved Oxygen (DO) The Fundamentals

Presented By N.E.M Business Solutions ©2001

Introduction

Dissolved oxygen (DO) is the term commonly used for the measurement of the amount of oxygen dissolvedin a unit volume of water.

In a water quality application, for example, where we want to maintain a fresh water stream fit forrecreational purposes such as swimming and fishing and as a source of potable water, we must keep the DOcontent high. If the DO level falls too low, the fish will suffocate and conditions will become favorable forthe growth of harmful bacteria.

In sewage treatment, solids are allowed to settle in large basins to which are added solutions rich in bacteriato speed the decomposition of the solids. There is an optimum DO level for this process and the level ismaintained by mechanically aerating the "activated sludge" as the bacteria impregnated content of the basinsis called. If the DO level falls too low, the bacteria die and the decomposition ceases; if the DO level isexcessive, more power is used than necessary for aeration and the process becomes unnecessarily costly.Another important application of DO is the control of the quality of boiler make up water. In this case, sincethe presence of oxygen in the water will enhance corrosion and cause the build up of boiler scale that inhibitsheat transfer, it is very desirable to hold the DO concentration to a minimum.

Theory

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

Effect of Partial Pressure of Oxygen on Dissolved OxygenA volume of water in contact with air will absorb air and hence oxygen untilthe pressure the absorbed oxygen exerts at the air-water interface equals thepressure exerted at the same interface by the oxygen in the air. At this point,the water is said to be saturated with oxygen. The amount of oxygen actuallyabsorbed is quite small being of the order of about five or ten parts of oxygento one million parts of water.

Effect of Temperature on Dissolved OxygenAs everyone who has ever watched a pan boil knows, bubbles form on theside and bottom of the pot. The number and size of the bubbles increase with temperature. These are bubblesof air that have been dissolved in the water. Figure 1-A represents a beaker of oxygen saturated water at room

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temperature. The partial pressures of oxygen above and below the water surface are equal. Figure 1-B showswhat happens when we begin to heat the system. In essence, we are putting energy into the system. Theoxygen molecules which have a low solubility become readily energized by the infusion of energy into thesystem. The more sluggish water molecules step up their molecular activity at a slower pace. As a result,more oxygen molecules break through the air-water interface to the space above the water surface than watermolecules do, leaving fewer oxygen molecules dissolved in the water. When the water boils as is shown inFigure 1-C, all of the oxygen molecules have been driven out of the water and now the water molecules arecoming out of the beaker so fast that they form a layer of water vapor immediately above the water surface.This layer effectively isolates the water from atmospheric oxygen and the DO content of the water is zero.

You can think of a volume of water as if it were a homogeneousmedium with a number of holes interspersed throughout thevolume. The pressure of the air immediately above the watersurface will cause these holes to be filled with air. If now asubstance such as salt is dissolved in the water, the dissolved saltwill occupy some of the holes used by the oxygen molecules asthey constantly shift from air to water and water to air in anequilibrium situation. The amount of oxygen the water can holdat given temperature has now decreased, but the partial pressureof the dissolved oxygen remaining in the solution must still equalthe partial pressure of the atmospheric oxygen above the watersurface. This situation is graphically portrayed in Figure 2 where curve A represents fresh water and curve Brepresents salt water.

Measurement TechniquesBasically there are two general techniques for measuring DO. Each employs an electrode system in which thedissolved oxygen reacts at the cathode producing a measurable electrochemical effect. The effect may begalvanic, polarographic or potentiometric.

One technique uses a Clark-type cell which is merely an electrode system separated from the sample streamby a semi-permeable membrane. This membrane permits the oxygen dissolved in the sample to pass throughit to the electrode system while preventing liquids and ionic species from doing so. The cathode is a hydrogenelectrode and carries a negative applied potential with respect to the anode. Electrolyte surrounds theelectrode pair and is contained by the membrane. In the absence of a reactant, the cathode becomes polarizedwith hydrogen and resistance to current flow becomes infinite. When a reactant, such as oxygen that haspassed through the membrane is present, the cathode is depolarized and electrons are consumed. The anodeof the electrode pair must react with the product of the depolarization reaction with a corresponding releaseof electrons. As a result, the electrode pair permits current to flow in direct proportion to the amount ofoxygen or reactant entering the system; hence, the magnitude of the current gives us a direct measure of theamount of oxygen entering the system.

Membrane probes readily lend themselves to conditions of high interfacial turbulence. In the case of the

thallium probe, a high degree of turbulence may decrease the life of the probe because high turbulence willsweep away the thallium ions, thus causing electrode depletion.

Although dissolved organic materials are not known to interfacewith the output from dissolved oxygen probes, inorganic salts are afactor in the performance of the probes. As we saw in Figure 2, we

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must apply a correction factor to the output of probes withmembranes when used in salt solutions. The thallium proberequires the presence of salts in concentrations which provide aminimum conductivity of approximately 200 micromhos.

The two principle gaseous interferers of membrane probemeasurements are chlorine and hydrogen sulfide. Sulfurcompounds such as hydrogen sulfide, sulfur dioxide andmercaptans cause erroneous outputs from the thallium probe.Halogens do not interfere with the thallium probe. Figure 3 - Velocity, Agitation, and/or Turbulence Effectsof Interfacial Dynamics on Probe Output. Figure 2 - Typical Curves Concentration of Dissolved Oxygen atSaturation

At low dissolved oxygen concentrations, pH variations below pH 5 and above pH 9 interfere with theperformance of the thallium probe. This interference amounts to about ±0.5 mg/L DO per pH unit. Theperformance of membrane probes is not affected by pH changes. A serious limitation of the thallium probe isthe fact that thallium is quite toxic and you must exercise care in using it. Membrane probes do not have thisdrawback. Since all of the oxygen that passes through the membrane reacts and since the amount of oxygenthat passes through the membrane is a function of the partial pressure of the oxygen in solution, thistechnique actually measures the partial pressure of the oxygen in solution. It does not measure the actualconcentration of the oxygen in the solution. For this reason, we must correct the readings of DOconcentration given by this technique when some substance, for example salt, is dissolved in the water. As wesaw above, the dissolved salt will reduce the number of holes available for carrying oxygen and hence reducethe actual concentration of oxygen without changing its partial pressure in the solution. If the electrodematerials are selected so that the difference in potential is -0.5 volts or greater at the cathode, an externalpotential is not required and we have what is called a galvanic system. Some workers in this field, instead ofrelying on their selection of electrode materials to give them the required -0.5 volts difference of potential atthe cathode, use an external potential source to give them the required potential difference. This system isknown as a polarographic system. In either case, since the partial pressure of dissolved oxygen is a functionof the temperature of the sample, we must either hold the temperature of the sample constant or compensatefor varying sample temperature. Generally, the former is impractical so the latter is the more popularapproach. A suitably selected thermistor or resistance thermometer in a properly designed electric circuit

does a fair job of temperature compensation.

The second basic measuring technique uses an electrode system thatconsists of a reference electrode and a thallium measuring electrode. Nosemi-permeable membrane is used; the electrode system is immerseddirectly into the sample. Oxygen concentration is determined bymeasuring the voltage potential developed, in relation to the referenceelectrode, when dissolved oxygen comes in contact with the thalliumelectrode. At the surface of the electrode the thallous-ion concentration isproportional to the dissolved oxygen. The voltage potential developed bythe cell is dependent upon the thallous-ion concentration in this layer andvaries as the dissolved oxygen concentration changes. The cell outputrises 59 millivolts for each decade rise in oxygen concentration. This technique uses a potentiometric system.The method measures directly the concentration of oxygen in the sample. As in the first technique,temperature compensation is a must and is achieved in about the same way. In both techniques, interfacialdynamics at the probe-sample interface are a factor in the probe response. A significant amount of interfacial

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turbulence is necessary and for precision performance, turbulence should be constant. This situation isportrayed in Figure 3. As long as the operating point remains above the knee of the curve, small changes inturbulence can be tolerated.

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