Technical Article

ppm Dissolved Oxygen Measurement

INTRODUCTION aerobic bacteria, robbing other aquatic organisms of the oxygen they require to live. Dissolved oxygen (D.O.) levels are used as a Biological Oxygen Demand (BOD) is a measure general indicator of water quality. Oxygen is of the oxygen used by microorganisms to essential to life and vital for countless aquatic decompose this waste. Water supplies forms. D.O. level control is critical in numerous containing organic waste will also contain applications ranging from the monitoring of bacteria to break down this waste. When BOD industrial waste water to environmental levels are high, dissolved oxygen levels surveillance. decrease because the available oxygen in the water is being consumed by the bacteria. Since WHAT IS DISSOLVED OXYGEN? less dissolved oxygen is available in the water, fish and other aquatic organisms may not Dissolved oxygen (D.O.) is the survive. Hence, measurement of D.O. and BOD amount of oxygen gas dissolved in levels are vital in order to maintain sufficient a given quantity of solvent (usually oxygen levels to sustain the natural ecosystem. water) at a given temperature and The use of aerobic bacteria for the atmospheric pressure. As such, it should not be confused with decomposition of organic waste has become a combined oxygen as found in the fundamental aspect of wastewater treatment.

water molecule, H2O. It is usually Bacterial attack in the presence of oxygen expressed as a concentration in results in the biochemical breakdown of sewage parts per million or mg/l. It can also into inorganic byproducts, specifically, carbon be expressed as percent saturation, dioxide, water and sludge. This oxidation where saturation is the maximum process, known as the activated sludge amount of oxygen that can process, is the most widely used method of theoretically be dissolved in water secondary waste treatment today. The process at a given altitude and temperature. typically occurs in an aeration basin and is achieved by aerating or bubbling pure oxygen through the wastewater. The oxygen, which is Aerobic bacteria plays a key role in the depleted by the bacteria, is replenished to allow measurement of dissolved oxygen levels both the process to continue. When the measured from an environmental and industrial D.O. falls below a specified concentration, air is perspective. Microorganisms such as bacteria added to the basin which provides oxygen to are responsible for the decomposition of sustain the microorganisms and mixes the organic waste. When organic matter such as waste. Without enough dissolved oxygen, dead plants, manure, sewage and food waste beneficial bacteria will die; conversely, too are present in a water supply, the bacteria will much oxygen is wasteful and inefficient begin to break down this waste. Much of the resulting in unnecessary energy expenses. To available dissolved oxygen is consumed by maintain optimal dissolved oxygen levels, on- www.iccontrols.com ©2017 IC Controls Page 1 of 5 Technical Article

line analytical instruments are employed to turn affects the rate of diffusion. Most D.O. measure and control the concentration of D.O. sensors contain a temperature sensor that accounts for these variables in calibration. DISSOLVED OXYGEN SENSOR DESIGN D.O. SENSOR CALIBRATION For the continuous monitoring of dissolved At any given temperature and barometric oxygen, the sensor style most commonly pressure the partial pressure of oxygen in encountered utilizes a membrane- type design watersaturated air is exactly the same as it is in with the following characteristics: airsaturated water. Thus a sensor can be calibrated in water-saturated air, using the ➔ – provide the reaction site 20.9% oxygen available in air as the full-scale needed for the reduction of oxygen standard. Both temperature and barometric molecules and generation of electrons pressure affect the partial pressure of oxygen in ➔ MEMBRANE – a gas permeable air saturated water vapour. Most D.O. membrane separates the measurement microprocessors are programmed with all the cell from the sample and allows only values along with a temperature input. dissolved oxygen to diffuse into the cell Therefore, correct data is automatically obtained to compute the correct gain, and ➔ – facilitates the transfer of calibrate the analyzer. This calibration dissolved oxygen and provides an technique will give a 100% saturation reading electrical path to complete the current for the temperature and pressure which the loop and removes metal oxide by- microprocessor displays as ppm dissolved products from the electrodes in order to oxygen. To calibrate the D.O. sensor, simply keep their surfaces clean to react suspend the probe above fresh tap water and The operation of a membrane sensor begins auto calibrate. with the diffusion of oxygen through the membrane into the electrolyte. Since oxygen D.O. SENSOR TECHNOLOGY: pressure within the measuring cell is essentially ELECTROCHEMICAL METHOD zero (oxygen gas is reduced at the ), This type of measuring sensor is an the oxygen pressure in the sample causes it to similar to a battery that diffuse through the membrane to the cathode. produces current in the presence of oxygen. The amount of oxygen reduced on the cathode The measurement cell consists of a cathode depends upon the amount of pressure exerted (negative ) and an (positive on the membrane by the oxygen in the sample electrode) immersed in a filling solution referred to as the partial pressure of oxygen. separated from the process by a gas permeable Oxygen is only one of a number of gases membrane. This separation provides a present whose individual partial pressures controlled environment for the electrodes and make the total gas pressure in the water. electrolyte while allowing oxygen to enter from Dissolved oxygen must diffuse freely through the sample and react. This article will focus the membrane in order for the sensor to strictly on the electrochemical measurement of function properly. Other factors affecting the membranetype sensors, specifically, galvanic rate of oxygen diffusion include membrane (spontaneous voltage) and polarographic thickness and temperature. A thinner (applied voltage) measuring cells. membrane offers faster diffusion and temperature affects the permeability of the membrane and the solubility of oxygen which in www.iccontrols.com ©2017 IC Controls Page 2 of 5 Technical Article

Polarographic Measuring Cell ➔ The electrode requires an external power source, so as soon as the probe is A polarographic (“Clark” or amperometric) cell disconnected, the power supply is cut consists of a metallic anode (eg. silver) and a off. Upon re-connection, the user must metallic cathode (eg. gold or ) wait for the probe to be polarized (ie. for immersed in an electrolyte, typically an the current loop to be stabilized). This aqueous potassium chloride (KCl) solution. A warm-up time can be up to 1 hour and constant polarizing voltage of -0.7V to -0.8V is any measurement taken before this time applied across the electrodes which causes will typically result in a higher value thus oxygen to be reduced at the cathode (ie. it is giving a false reading. chemically converted to OH-). This applied voltage ensures that only oxygen is reduced at the cathode. The resulting current flow is Galvanic Measuring Cell directly proportional to the dissolved oxygen By using electrodes in contact with an content of the electrolyte. The oxidation electrolyte, a spontaneous chemical reaction reduction reactions for a typical Clark cell are occurs in the presence of oxygen. as follows:

- Voltmeter Cathode: O2 + 2H2O + 4e → 4OH Gold Cathode - - O +2H O+4e-->4OH- Anode: 4Ag + 4Cl → 4AgCl + 4e 2 2

- - Overall: O2 + 2H2O + 4Ag + 4Cl → 4OH + 4AgCl

Sensors of this type require a special meter in Lead Anode Pb-> Pb2++ 2e- order to provide the polarizing voltage. Further inherent disadvantages associated with polarographic sensors are as follows: Electrolyte Solution ➔ Since the net result of the chemical reaction is AgCl, over time, a build up of Figure 1: Basic AgCl will coat the anode. Once the entire anode is covered, reaction stops and the In this reaction, the cathode reduces the oxygen probe stops working. However, oxygen into hydroxide, releasing four electrons the probe may be reactivated by for each oxygen molecule. These electrons flow cleaning the anode to remove the AgCl through the electrolyte, proportional to the deposit. oxygen concentration in the electrolyte. ➔ OH- result from the overall reaction Common electrode materials include gold, silver, copper, and lead, with potassium which alters the pH value of the hydroxide (KOH) as the most frequently used electrolyte. The electrolyte, which has a electrolyte. The cathode must be a noble metal near neutral pH, moves into the alkaline (such as silver or gold) for the cathode potential range. This causes a zero shift, and over to reduce molecular oxygen when the cell time, the electrolyte will need to be circuit is closed. The anode should be a base replaced. metal (iron, lead, cadmium, copper, , or ➔ The net reaction also consumes Cl- ions. silver) with good stability and without the Thus the electrolyte will need to be tendency for passivation. Galvanic technology changed as a result of the Cl- ions being overcomes many of the disadvantages affiliated consumed over time with polarographic sensors, for example: www.iccontrols.com ©2017 IC Controls Page 3 of 5 Technical Article

➔ No warm-up (polarization) time required. Anode: 2Pb → 2Pb2+ + 4e- The probe is ready to measure since it is Overall: O2 + 2H2O + 2Pb → 2Pb(OH)2 'self polarized' as long as electrolyte is present. Less battery drain on meter.

➔ Provides an absolute zero measurement as galvanic current is zero when zero oxygen is present.

➔ Non-depleting electrolyte. The fill solution is water-based with selected IC Controls currently offers three D.O. sensor salts added to carry the charge. H2O is designs; Models 802, 814, and 825. All three consumed and replaced rendering the sensors share the same cell chemistry and offer electrolyte stable and thus reducing three membrane options; standard response, frequency of replacement. fast response, and a ruggedized membrane. ➔ Overall reaction produces water and a The fast response membrane is thinner but less durable whereas the ruggedized membrane is metallic oxide. The metallic oxide flakes thicker but with slower response. Each sensor off the anode leaving an exposed anode employs the same method of calibration which for continuous reaction. Therefore, is simply achieved using air over potable water anode polishing is not required. as the standard. ➔ No zero shift. Metallic oxide does not affect electrolyte chemistry. Water is SUMMARY added back as it is used. Electrolyte There are a variety of waste-water dissolved highly stable. oxygen sensor technologies, each with its own advantages and disadvantages. One sensor IC CONTROLS PPM D.O. SENSORS technology will not be suitable in all Due to the advantages of galvanic cells and applications. However, in comparing limitations of polarographic cells, IC Controls polarographic and galvanic technologies, dissolved oxygen sensors utilize galvanic compelling evidence was presented to technology. substantiate the superiority of a galvanic sensor. Galvanic sensors have no significant The measuring cell has a gold cathode, a lead technical drawbacks whereas polarographic anode and uses potassium hydroxide (KOH) as sensors have many. Galvanic systems are the electrolyte. The chemical reactions within virtually maintenance free with the exception of the cell are as follows: occasional membrane changes and regular - - Cathode: O2 + 2H2O + 4e → 4OH calibration. For these reasons, IC Controls has opted to utilize only galvanic technology for its ppm line of dissolved oxygen measurement.

Gold Cathode

Teflon Membrane

Lead Anode KOH Fill Solution

Sensor Body

Insulator www.iccontrols.com ©2017 IC Controls Page 4 of 5 Figure 2: Galvanic Dissolved Oxygen Sensor Technical Article

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