Oxygen Basic principles Oxygen is not only a constituent of air but also exists in a dissolved state in liquids. A 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. For the sake of physical and chemical correctness, it should be added that 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. Dry, atmospheric air Air over water surface N 2 Air pressure C, Inert gas Water vapor pressure Page 1 of 22
The following condition is met when the air is saturated: p ( T) = 0,2095 ( p p ( )) (1) O T 2 Air W where p O ( T ) is the partial pressure of oxygen, p 2 Air is the air pressure and p W (T ) is the water vapor pressure. (T ) represents temperature-dependent values. Usually, however, the level of the concentration of oxygen β ( ) is required. The concentration is proportionally dependent on the partial pressure of oxygen and, of course, on the type of liquid, as indicated by the Bunsen absorption coefficient a O ( ). T 2 T ao ( T ) po ( T ) M 2 2 O2 β O ( T ) = (2) 2 V M where M O2 is the molar mass of oxygen and V M is the molar volume. Knowledge of the temperature is absolutely imperative when measuring the oxygen concentration. If the result is required as a % of saturation, the current air pressure is also required. The equations show that water can dissolve more oxygen at higher air pressures than at lower air pressures. Water vapor pressure increases as temperature rises, i.e. the partial pressure of oxygen decreases. To illustrate this effect, values can be compared at 20 C and 40 C at an air pressure of 1013 hpa. While 9.09 mg/l oxygen dissolve in water at 20 C, only 6.41 mg/l dissolve at 40 C. The amount by which the volume of a liquid changes with a corresponding change in temperature is dependent on the type of liquid. In water, the effect is minor and negligible. Not so the effect of dissolved substances. They can either reduce or increase the solubility of oxygen. A salt content (sodium chloride) of one percent in water lowers the saturation concentration from 9.09 mg/l to 8.54 mg/l at 20 C. Organic substances, on the other hand, generally increase the solubility of oxygen in water. The maximum saturation concentration increases with the proportion of the organic substance. Pure ethanol, for example, dissolves 40 mg/l of oxygen. Page 2 of 22
Oxygen sensors The basic principle underlying the electrochemical determination of oxygen concentration is the use of membranecovered electrochemical sensors. The main components of the sensors are the oxygenpermeable membrane, the working electrode, the counterelectrode, the electrolyte solution and a possible reference electrode. A voltage is applied between the gold cathode and the anode that consists of either lead or silver, 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. The following electrode reactions take place during the electrochemical determination of oxygen: Oxygen is reduced at the cathode: + 2 H 2 O + 4 e - 4 OH - During this process, the cathode provides electrons and the oxygen that diffuses through the membrane reacts with water to form hydroxide ions. Gold cathode e- e - e - e - e - e - H 2 O + OH - Membrane Page 3 of 22
The metal of the electrode is oxidized at the anode, a process which releases the electrons required for the cathode reaction. The reaction that takes place is either or Ag Ag + + e - Pb Pb 2+ + 2 e - The equations of the anode reaction illustrate how an electrolyte solution works. The components of the electrolyte solution bind the metal ions generated by the anode reaction. The electrolyte solutions must be matched to the electrode type. The galvanic sensors, CellOx 325 or StirrOx G, require an ELY/G solution and polarographic sensors, e.g. TriOxmatic 300 or EO 196, require an ELY/N solution. or Pb 2+ Ag + + Br - AgBr + 2 OH - 2 Pb(OH) 2 Pb(OH) 2 PbO + H 2 O This is shown here for the silver electrode: Ag + Br - e - e - e - e - e - e - e - + AgBr The resulting substances of low solubility prevent the gold cathode from becoming coated with lead or silver, which would occur if the ions were left unbound. With the addition of a silver/silver bromide electrode, the polarographic sensors can be connected to form a three-electrode cell. They no longer have an anode in the classical sense. One of the silver/silver bromide electrodes acts as a counterelectrode (current derivation) and the other one acts as an independent reference electrode. Current does not flow through this electrode and, thus, it can maintain a much more constant potential than a conventional electrode could. The potential of the reference electrode is determined by the bromide ion concentration, i.e. the electrode is ion-selective. Hence, the electrode can be used to monitor the concentration of the electrolyte solution yet another aspect of this type of sensor. WTW Page 4 of 22
sensors incorporate this feature in the so-called AutoReg function of the TriOxmatic sensors for laboratory and online measurements. Furthermore, the polarization periods of the sensors differ and must be observed as stated in the operating manual. The polarization period is the waiting time between the connection of the sensor and the beginning of measurement. It corresponds to the start-up period required to achieve stable measuring values. After the sensor is refilled and the sensor head is exchanged (see Regeneration of Sensors), the new components still contain an indefinite amount of oxygen that must first be electrochemically neutralized. Furthermore, the polarization of the electrodes generates a flow of current that is comparable to the loading of a capacitor. The polarization period is significant not only after the regeneration of a sensor. As explained above, the galvanic sensor is self-polarizing, i.e. even after disconnection of the sensor from the meter, the sensor continues polarizing. Therefore, no waiting period is required when it is reconnected. For polarographic sensors, on the other hand, a polarization period must be observed each time the sensor is disconnected from the instrument (see operating manual). For this reason, field measurements should be carried out with galvanic sensors. However, this continuous readiness comes with a small disadvantage. Because polarization is continuously taking place, the electrolyte solution is consumed even when the sensor is not connected to the meter during periods of inactivity. Therefore, regeneration may become necessary even though measurements were not carried out. The battery is empty. The significance of the sample temperature for oxygen measurements is evident in the dependency of the various variables on temperature (e.g. Bunsen absorption coefficient) in the equations presented at the outset. Furthermore, the oxygen permeability of the membrane is also temperature-dependent. Therefore, in addition to the external temperature probe (sample temperature!), another probe is required and is built into the sensor head. With these two temperature values, the instrument can compensate for the influence of temperature on the oxygen permeability of the membrane (IMT isothermal membrane temperature compensation). Page 5 of 22
Calibration and analytical quality assurance Calibration As for ph measurements, calibration must also be carried out for dissolved oxygen measurements at regular intervals. 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 interval depends on the oxygen sensor used and ranges from two weeks for pocket instruments to 2 3 months for WTW stationary oxygen sensors. Each linear calibration function is defined by at least two points. For dissolved oxygen measurements with WTW instruments, 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. Current flow Water-saturated air Zero point of sensor Oxygen concentration This is called the zero-current point of the sensor. For the experimenter, the calibration of WTW instruments then has the appearance of a one-point calibration. 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. 1.: 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. Page 6 of 22
WTW offers special air calibration vessels for laboratory measurements. The partial pressure of oxygen in air can be calculated from the air pressure using the equation already presented: p ( T) = 0,2095 ( p p ( )) O T 2 Air W This relationship requires the current air pressure p Air to be measured (not the pressure corrected down to sea level as common in weather forecasts!) In the past, the user measured the pressure with a barometer or approximated it by entering the geographic elevation. Modern WTW meters automatically determine the current pressure with an integrated pressure sensor. If the humidity is 100%, the partial pressure of the water vapor p W (T ) is a function of the temperature. To determine this value, the sensor is additionally equipped with a temperature probe. It is important to ensure that there are no water droplets on the membrane. Otherwise, the calibration would partially take place in water! Electrolyte solution in sensor Membrane Water-saturated air It is particularly important to take precautions after the sensor has been stored in the calibration vessel for an extended period of time and condensation droplets may have formed on the membrane. In any case, check the membrane prior to calibration and dry it with a soft paper towel when necessary. It is sufficient for the sponge in the OxiCal to be damp! It should never be wet. Moisten the sponge with distilled water and squeeze it out like a lemon. The amount of moisture that remains is completely adequate. 2. 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. Page 7 of 22
The point at which the water is completely saturated is difficult to estimate. There is a risk of undersaturation. Oxygen-depleting substances lead to undersaturation. Both types of calibration are in accordance with DIN EN 25814. Calibration in watersaturated air is supported by every WTW instrument and, as the above list shows, is clearly preferable to calibration in air-saturated water. The displayed calibration result is the relative slope in na/hpa rather than the absolute slope (as for ph meters). What procedure is used for calibration? The oxygen meter picks up the electrode signal (current flow!) and compares it to the partial pressure of oxygen that is yielded by equation (1) for the prevailing conditions of air pressure p Air and temperature T. The meter obtains a slope factor that allows conversion of the measured current into the partial pressure of oxygen p O ( T ). Concurrently, the 2 actual measured value is compared to the average saturation current of the respective electrode type after refilling (nominal current). This yields the relative slope S. When the average saturation current is used as a reference, slopes exceeding 1.00 may result. A slope value of 0.81 after calibration indicates that the slope is 81% of the nominal value. The slope provides no information about the accuracy of the measurement, but rather serves as an indicator for estimating the remaining operating time, i.e. the time remaining until the next exchange of electrolyte solution becomes necessary. This also applies to the sensor symbols on the display of modern oxygen meters that estimate the period of inactivity of a sensor in the same manner. Sensor Symbol Bewertung Evaluation sehr gut / excellent +++ Steilheit -Sensor/ Slope of D.O. probe S = 0.8... 1.25 gut / good ++ S = 0.7... 0.8 ausreichend / sufficient + S = 0.6... 0.7 schlecht / poor S < 0.6 oder / or S > 1.25 Page 8 of 22
The oxygen concentration is calculated using the internally stored absolute slope in na/hpa. At this point, a measuring problem should be pointed out although it cannot actually be considered as such. The underlying issue is the solubility of oxygen in water as determined by iodometric titration. Some of the table values differ in Europe and the USA. The differences lie in the order of up to 5%. (A good example of the fact that every measurement is subject to an unpredictable flaw.) WTW oxygen meters use the European table according to DIN EN 25814. This could lead to misunderstandings in the USA and in Asia. Page 9 of 22
Checking the sensor function For ph measurements, the calibration data can be used directly to evaluate the quality of the measuring system consisting of the instrument, the electrode and the standard buffer solution. Because of the relative slope, however, this is not possible for oxygen measurements. To still be able to evaluate the sensor function despite this limitation, 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 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. The 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). 102 1 100 2 Meter display [% SAT.] 0 3 0 100 102 Oxygen [% SAT.] Page 10 of 22
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 do 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 WTW successfully sought an alternative to the conventional calibration procedure in air-saturated water. If the sensor does 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 is quite simple: 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 reentering the solution. One minute after submerging polarographic sensors (EO 300, EO 196, 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 zerocurrent-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 counterelectrodes (CellOx 325 and StirrOx G) 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. Page 11 of 22
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 AQA 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 (<50 C) 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 µm and it is easily destroyed. It is best to use a soft paper towel. Do 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 AutoReg function responds or when the slope (S<0.6) has decreased markedly when calibrating. The AutoReg function is only implemented in the three-electrode sensor from WTW and, therefore, only responds in this equipment. It notifies the user when regeneration becomes necessary. 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 toxified 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! Page 12 of 22
Only the gold cathode should be polished; the counterelectrode 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 WTW abrasive film that has a special grain that polishes and does 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 doesn t fit either! Gold cathode Electrolyte solution Membrane Spacing lattice Membrane The spacing lattice is clearly visible when the membrane head is held up against the light. When regenerating three-electrode sensors (e.g. TriOxmatic 300), the third electrode mentioned previously must not come into contact with the cleaning solution while the sensor is submersed (operating manual). The polarization period after regeneration must be observed. Units and display of the measuring result 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. The new inolab instruments provide an additional display of the partial pressure of oxygen in mbar. Page 13 of 22
Polarization periods (start-up 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. This does not apply to galvanic sensors (gold-lead electrode system) because they are self-polarizing and can be used immediately. Drift control (AUTOREAD) As in ph meters, the drift control function tests the stability of the sensor signal. It examines chronological changes in the measured values and evaluates the behavior of the sensor over time. If the drift lies below a defined value, the signal is sufficiently stable and the current measured value is considered to be the true value. β/ t Oxygen concentration [mg/l] β/ t Time [s] This eliminates the need for a subjective evaluation of the stability of the measured values and improves the reproducibility of the measurements. The AUTOREAD function is particularly valuable in the determination of biochemical oxygen demand since precision measurements can be carried out with significantly improved certainty. Page 14 of 22
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. Sensor head Sensor head Approach flow This condition can be met by stirring the sample or moving the sensor in the sample. WTW 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 the Karlsruhe bottles for BOD measurements. The WTW StirrOx G 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 Page 15 of 22
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. WTW 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 undersaturated with oxygen and oxygen can be expelled from or stirred into the sample. Supersaturation with oxygen, for example, can be observed in the summer in stagnant waters when luxuriating algae produce oxygen by photosynthesis. An example of undersaturation with oxygen is the BOD determination in which bacteria lower the oxygen concentration in Karlsruhe 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 (see equation 2) 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 DIN EN 25814 standard also recommends the use of this function for other waters since the deviation is minimal (<2%). Page 16 of 22
Influence of interfering gases The membrane is also permeable to gases other than oxygen. Sensor head H 2 S C N 2 NH 3 H 2 S NH 3 C N 2 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 counterelectrode. 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 meters from WTW 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. Page 17 of 22
Checking the oxygen meter The oximeter is 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. Page 18 of 22
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:! Follow the instructions on the chemical bottles! Always wear protective garments (goggles, gloves,...)! Never point open containers towards other persons! Do not eat, drink or smoke! Ensure the satisfactory disposal of chemicals! Carefully remove or clean up any spilled chemicals! Contact specialist personnel if any serious problems arise We provide these short instructions in the hope that they lead to successful and safe practical studies Safety data sheets 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. Page 19 of 22
The following equipment and facilities must be present: Washable tables Resistant floor coverings Running water Eye bath Checklist for measuring station 1 oxygen meter e.g. Oxi 330/340, inolab Oxi 1 oxygen sensor matched to oxygen meter 1 OxiCal 1 cleaning solution matched to sensor 1 electrolyte solution matched to sensor 1 SF 300 abrasive foil 1 RZ 300 1 Oxi-Stirrer 300 1 Karlsruhe bottle with stopper 1 Oxi-PL sodium sulfite 1 stand to hold the electrode 2 beakers (150 ml) Spatula, washing bottle with dist. water Protective goggles, gloves, smock Ballpoint pen and pad, tissues, lint-free cloths Page 20 of 22
Practical experiments 1. Calibrating the sensor Calibration is carried out in the OxiCal as described in the operating manual. Steps 4 and 5 may be carried out first. 2. Checking the sensor Check the measured value in water-saturated air leaving the sensor in the OxiCal Check the zero-current point of the sensor Carry out a short-term measurement in an unstirred sodium sulfite solution. Then rinse the sensor thoroughly with distilled water. Check the measured value in air-saturated water. Close a Karlsruhe bottle filled with 100 ml distilled water with a stopper and shake it for approx. 5 10 minutes. The water will then be saturated with oxygen. Recalibrate the oxygen sensor with an RZ 300 attached. Subsequently measure the oxygen concentration. 3. Influence of stirring on measured values While the AUTOREAD function is active, measure the oxygen concentration in water in a Karlsruhe bottle both with and without stirring. After measuring the value with an activated stirrer, deactivate the AUTOREAD function and the stirrer. Monitor the displayed oxygen concentration values for the next 2 minutes. Carry out another measurement (without stirrer) under AUTOREAD criteria. 4. Regeneration of a sensor Regenerate an oxygen sensor while paying attention to the most crucial steps, such as the disconnection of the sensor, the types of cleaning and electrolyte solutions, the depth to which the sensor is submersed in the cleaning solution, the prevention of air bubbles when filling the sensor. 5. Influence of polarization period An experiment to calibrate the previously regenerated sensor and additional regeneration experiments at intervals of 5 minutes. Page 21 of 22
Bibliography [1] DIN 38408, Bestimmung des gelösten Sauerstoffs mittels membrangedeckter Sauerstoffsonde, 1986 [2] DIN EN 25814, Bestimmung des gelösten Sauerstoffs Elektrochemisches Verfahren, 1992 Page 22 of 22