Analytical Methods--Recommended procedures for calibrating dissolved oxygen meters March 14, 1979 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 79.10 Subject: Analytical Methods--Recommended procedures for calibrating dissolved oxygen meters Dissolved oxygen meters have been in routine use by the Water Resources Division for a number of years for measuring dissolved oxygen (DO) concentrations in fresh and saline waters. However up to this point there has been no documentation of recommended procedures for calibrating these meters. Several calibration procedures, based largely on manufacturers' recommendations and on instructions given at various WRD training courses,are in current use within the Division. These procedures vary widely from district to district. In an attempt to develop a standardized and recommended DO meter calibration methodology the Quality of Water Branch requested the California District to conduct a detailed evaluation of several calibration procedures. The evaluation was carried out over temperatures ranging from 0.5 to 30 deg. C and at altitudes ranging from sea level to over 8,000 feet. Based on the results of this evaluation the attached method describing four DO meter calibration procedures was prepared. The four procedures are: (l) air calibration chamber in water, (2) calibration by Winkler titration, (3) calibration with air saturated water, and (4) air calibration chamber in air. All four procedures will give comparable results (within +/- 0.1 mg/L when compared with a Winkler DO measurement) provided the procedures are followed carefully. Use of one of the above four procedures is recommended for calibration of DO meters used in collection of data for WRD programs. Of the four procedures, the air calibration chamber in water (procedure l) is preferred. It is the most convenient to use and the most accurate under a variety of conditions. In situations where this calibration procedure cannot be used, one of the other three should be used. Note 6 on page 5 of the DO calibration method points out a problem in using the YSI DO probe model 5739 with the air calibration chamber. This probe has a pressure compensating diaphragm which will not fit inside the air calibration chamber and can lead to erroneous calibration. Quality of Water Branch Technical Memorandum No. 79.08(B), dated March 7, 1979, describes a modification which can be made to the air calibration chamber to accomodate the DO probe and diaphragm, thereby eliminating this problem. Any questions or comments regarding these DO meter calibration procedures should be addressed to the Chief, Quality of Water Branch, with copies to your Regional Water Quality Specialist. R. J. Pickering Chief, Quality of Water Branch Attachment WRD Distribution: A, B, FO, PO Oxygen, dissolved, electrometric, polarographic probe (I-1576-78) Parameters and Codes: Oxygen, dissolved (mg/L): 00300 Oxygen, dissolved (percent of saturation): 00301 1. Application 1.1 This method may be used to analyze waters and wastewaters containing between 0.1 and 20 mg/L of dissolved oxygen. 2. Summary of method 2.1 Dissolved oxygen (DO) is determined by an electrode system consisting of a sensor or probe containing a gold cathode and silver anode surrounded by an electrolyte solution of potassium chloride. A thin oxygen-permeable membrane is stretched across the end of the probe to isolate the electrolyte and sensor elements from the environment. When a potential of 0.8 volt is applied across the cell, oxygen is reduced at the cathode causing current to flow in an amount proportional to the partial pressure of oxygen at the external membrane surface. Inasmuch as oxygen is consumed at the gold cathode, the oxygen pressure inside the membrane is effectively zero, and the force causing oxygen to diffuse through the membrane is therefore proportional to the absolute pressure of oxygen outside the membrane. A linear relationship exists between the cell current and external oxygen pressure or concentration of oxygen in the sample solution. Four procedures for calibrating the oxygen meter are given. They are listed in order of preference. 3. Interferences 3.1 The membrane is permeable to gases, other than oxygen. Halogens, hydrogen sulfide, sulfur dioxide, and helium interfere. Hydrogen sulfide poisons the electrode. 4. Apparatus 4.1 Oxygen meter, Yellow Springs Instrument Co., Model no. 54 or no. 57, equipped with model 5739 probe, or equivalent. 4.2 The oxygen meter selected should meet the following general requirements: Range of dissolved oxygen: 0.1 to 20 mg/L. Accuracy: +/- 1 percent of maxiumum range. Operating temperature range: -5 deg. to +45 deg.C. Minimum scale readability: 0.05 mg/L 02 Temperature-compensated, portable, and rugged for field and laboratory use. 4.3 Calibration chamber, Yellow Springs Instrument Co., Model no 5075, or equivalent. 4.4 Pocket altimeter-barometer, capable of being read to at least the nearest 5 millimeters of mercury. USGS warehouse supply stock number 01321, or equivalent. 5. Reagents 5 1 Sodium sulfite solution: Dissolve 1 gram of sodium sulfite (Na2S03) and a few crystals (about 1 mg) of cobalt chloride (CoC12) in 1 liter of distilled water. 5.2 Reagents for Alsterberg-Azide method: See method I-1575. These reagents are required when using calibration procedure 6.3.1B. 6. Procedure 6.1 Prepare oxygen meter for calibration in accordance with the manufacturers instructions (Notes 1. 2. 3. 4. and 5). Note 1. A number of precautions should be observed when using the oxygen meter, particularly when the instrument is used for field measurements. (l) Check mechanical zero before turning meter on and adjust if necessary, (2) turn instrument on to any operational position 1 hour before use to insure stability of operation; meter should be left on until final measurement for day is completed. (3) Check REDLINE and ZERO and adjust as needed; If meter will not adjust to REDLINE recharge or replace batteries, (4) Protect instrument from temperature changes that may result from exposure to direct sunlight or contact with ice or cold wind. Note 2. The DO probe requires periodic replacement of electrolyte and membrane. Follow manufacturers directions but observe these points: (l) There should be no bubbles of air behind membrane. Electrodes with pressure compensators should have the pressure compensation membrane moved in and out with the eraser end of a pencil to insure that all bubbles are removed from pressure compensation chamber. (2) the membrane should be smooth with no wrinkles (3) excess membrane material should be trimmed to within 2 to 3 mm of the o-ring. Note 3. O-rings stretch with use and must be replaced before they become loose. A common cause of electrode malfunction is leakage of electrolyte around the O-ring seal. To check for this problem, rinse the probe unit to remove traces of electrolyte remaining from membrane replacement. Place the probe in a 250-mL beaker of distilled water; Measure the specific conductance with an appropriate instrument. An increase in specific conductance indicates leakage of the electrolyte around the o-ring. Check the probe for physical damage. Replace the membrane and repeat the test. It may be necessary to replace the o-ring also. Note 4. Keep DO probe in water when not in use to prevent drying out of the membrane. Note 5. Periodically check the thermistor in the DO probe against a certified mercury thermometer over the normal operating temperature range of the instrument. If the two readings are not in agreement a correction curve should be constructed or an electronic adjustment should be made to the instrument by the manufacturer or qualified service center. 6.2 Once daily, check that the instrument will read zero in a dissolved-oxygen free solution. This may be accomplished by inserting the probe into the sodium sulfite solution and measuring the dissolved oxygen. If instrument reading exceeds 0.2 mg/L the probe needs repair or the membrane and electrolyte need replacement. 6.3 Calibrate the DO meter in accordance with one of the following four procedures. Procedure A is the preferred procedure. Procedures B, C and D may be used in situations where procedure A cannot be used. These four procedures are given in order of preference. 6.3.1A Air Calibration Chamber in water : The air calibration chamber (Yellow Springs Instrument number 5075, or equivalent) permits calibration of the oxygen meter at the temperature of the water in which the DO content is to be measured, thereby minimizing errors due to temperature differences. Dip the calibration chamber into the water; pour out excess water, and then insert DO probe into the wet chamber. This insures that the air inside the chamber is saturated with water vapor. Caution: Be sure that no water can leak into the calibration chamber and that the membrane has no droplets of water adhering to it since this would reduce the rate of oxygen diffusion through the membrane and would produce erroneous results. 6.3.2A Place calibration chamber in the water in which the DO content is to be measured. Allow 10-15 minutes for the temperature of the air inside the chamber to equilibrate with the water. Note 6. The calibration chamber (model 5075) is designed to allow the membrane surface of the DO probe (Model 5739) to be at ambient atmospheric pressure while in the chamber, but the chamber is not large enough for the entire DO probe and pressure compensator to fit inside. Since the pressure compensator must remain at atmospheric pressure care must be taken to insure that the pressure compensator is not submerged below the water surface if method 6.3.1A is used. 6.3.3A Read true atmospheric pressure from pocket altimeter- barometer to the nearest 5 mm of mercury. 6.3.4A Recheck the REDLINE and ZERO reading on the oxygen meter and adjust if necessary. 6.3.5A Measure temperature in the calibration chamber to the nearest 0.5 deg. C, using the thermistor in the DO probe. 6.3.6A Using the oxygen solubility table from section 7.2 determine the DO saturation value at the measured water temperature and true atmospheric pressure. Apply a salinity correction if appropriate. Note 7. The solubility of oxygen in water decreases as salinity increases. Oxygen saturation values may be corrected for salinity using the correction factors based on either chloride concentration or specific conductance. Correction factors based on chloride concentration are given in the oxygen solubility tab1e in section 7.2. Correction factors based on specific conductance are also given in a table in section 7.2. The correction based on specific conductance may be more useful since specific conductance can be easily determined from a field measurement. The solubility of oxygen at 25!C and 760 mm of mercury in water containing 5,000 mg/L of chloride is calculated as follows using the chloride correction factors: 8.3 mg/L 5,000 mg/L Cl x 0.008 = 7.9 mg/L (Saturation DO at 100 (Difference per O mg/L chloride 100 mg/L chloride and 760 mm Hg from from section 7.2) oxygen solubility table) If, in the above example, specific conductance of the water was 15,000 micromhos per cm at 25 deg. C the calculation could be made as follows using the specific conductance correction factors: 8.3 mg/L x 0.951 = 7.9 mg/L (Saturation DO at (Correction factor from O mg/L chloride and table in section 7.2 for 760 mm Hg from oxygen specific conductance of solubility table) 15,000 micromhos and at a temperature of 25 deg. C) In both of the above examples the DO meter would be adjusted to 7.9 mg/L rather than 8.3 mg/L. The YSI model 57 DO meter has a manual salinity correction built into the instrument. Follow the manufacturer's instructions in its use. The salinity value required for use with the YSI model 57 meter may be estimated from a field measurement of specific conductance by the following equation: Salinity, in parts per [Specific Conductance] - 1.025 thousand = 0.677 l000 6.3.7A Select the scale (0-10 mg/L or 0-20 mg/L) to be used for the DO measurement and adjust CALIBRATION control until meter reads the DO saturation value determined in 6.3.6A. Do not change scales without recalibrating meter or verifying that identical readings are obtained on both scales. 6.3.8A Remove probe from calibration chamber and place in water to be measured. Move probe through water to insure a velocity of at least 1 foot per second past the probe (alternatively, a submersible stirrer, YSI Model 5491, or equivalent may be used) to prevent localized depletion of DO at the membrane-water interface. 6.3.9A Switch meter to TEMPERATURE; after meter reading has stabilized record temperature to the nearest 0.5 deg. C. 6.3.10A Switch meter to the proper DO scale; after meter reading has stabilized record DO concentration to the nearest 0.1 mg/L. 6.3.1B Calibration by Winkler titration : The DO meter is calibrated with distilled or deionized water in which the DO concentration has been determined by the Winkler method. 6.3.2B Place 1 to 2 liters of distilled or deionized water in a large container. The water should preferrably be near saturation with respect to DO and the water temperature should be reasonably close to the ambient temperature. These conditions should insure that the DO concentration in the distilled water remains constant. 6.3.3B Place DO probe in the distilled water. Maintain a velocity of at least 1 foot per second past the DO probe with a magnetic stirrer or other means. 6.3.4B Check ZERO and REDLINE, and adjust if necessary. 6.3.5B Switch meter to the appropriate scale and adjust reading to the approximate DO concentration with the CALIBRATION control. 6.3.6B After DO meter has stabilized determine the DO concentration of two aliquots of the distilled water by the Winkler titration procedure (Method I-1575). If results of the two Winkler titrations do not agree within 0.1 mg/L, the titration should be repeated. 6.3.7B Without delay recheck ZERO and REDLINE then adjust CALIBRATION control until DO meter reads DO concentration determined in step 6.3.6B. Meter is now calibrated and ready for use. 6.3.8B Place DO probe in water to be measured. Allow sufficient time for the probe to equilibrate with the water temperature. Move probe through water at a velocity of at least 1 foot per second (alternatively, a submersible stirrer, YSI Model 5491, or equivalent may be used) to prevent localized depletion of DO at the membrane-water interface. 6.3.9B Switch meter to TEMPERATURE; After meter has stabilized record temperature to nearest 0.5 deg. C. 6.3.10B Switch meter to the appropriate DO scale. After meter has stabilized record DO concentration to nearest 0.1 mg/L. The alternate salinity correction procedure (See note 7) should be applied if appropriate. 6.3.1C Calibration with Air Saturated Water: The DO meter is calibrated against water which is saturated with oxygen at a known temperature and true atmospheric pressure. Obtain approximately 1 liter of distilled water or water from the water body to be measured. The temperature of the water used for calibration should be close to ambient temperature so there is no heating or cooling. 6.3.2C Place DO probe and the calibration water in a large beaker or open-mouth container and allow it to come to thermal equilibrium. Shield beaker from direct sunlight and wind to minimize temperature variations. 6.3.3C Aerate the water for 5-10 minutes. This is most readily accomplished with a small battery-operated aquarium pump or minnow bucket aerator and a short section of tubing. A gas diffusion stone should be attached to the end of the tubing and placed at the bottom of the beaker of calibration water. Note 8. It is extremely important to insure that the water is exactly 100 percent saturated with oxygen. An indication of 100 percent saturation can be determined as follows. Switch DO meter to the 0-10 mg/L scale and adjust meter reading to approximately 8 mg/L with the CALIBRATION control. Observe meter while aerating calibration water. When no change in DO reading on meter is observed for a period of 4-5 minutes the water can be assumed to be saturated. 6.3.4C Read true atmospheric pressure from pocket altimeter- barometer to the nearest 5 mm of mercury. 6.3.5C Recheck the REDLINE and ZERO reading on the oxygen meter and adjust if necessary. 6.3.6C Measure temperature of the calibration water to the nearest 0.5 deg. C. 6.3.7C Using oxygen the oxygen solubility table from section 7.2 determine the DO saturation value at the measured calibration water temperature and true atmospheric pressure. Apply a salinity correction if appropriate (Note 7). 6.3.8C Select the scale (0-10 mg/L or 0-20 mg/L) to be used for the DO measurement. Turn off aerator and adjust CALIBRATION control until meter reads the DO saturation value determined in 6.3.7C. Do not change scales without recalibrating meter or verifying that identical readings are obtained on both scales. Note 9. It is essential that calibration be completed with the temperature of the calibration water at the value measured in 6.3.6C, otherwise the calibration water may be undersaturated or oversaturated with oxygen. If the temperature changes or later recalibration of the meter is necessary the calibration procedure must be repeated beginning with step 6.3.2 C. 6.3.9C Remove probe from calibration container and place in water to be measured. Move probe through water at a velocity of at least 1 foot per second (alternatively, a submersible stirrer, YSI Model 5491, or equivalent may be used) to prevent localized depletion of DO at the membrane-water interface. 6.3.10C Switch meter to TEMPERATURE; after meter reading has stabilized record temperature to the nearest 0.5 deg. C. 6.3.11C Switch meter to the proper DO scale; after meter reading has stabilized record DO concentration to the nearest 0.1 mg/L. 6.3.1D Air Calibration Chamber in Air: This procedure is similar to the procedure described in 6.3.1A, except that calibration is performed with the air calibration chamber in air rather than in water. This procedure can only be used with probes that have the temperature sensing thermistor located adjacent to the membrane. The probe must be capable of automatically compensating for temperature changes. However, it is recommnded that this procedure not be used if the air temperature differs from the water temperature by more than 10 degrees. Also, the calibration chamber must be shielded from direct sunlight and wind to avoid large temperature fluctuations. 6.3.2D Wet the inside of the calibration chamber with water; pour out the excess water and insert the probe into the wet chamber. This insures 100 percent humidity. Allow sufficient time (10-15 minutes) for the DO probe and air inside the calibration chamber to equilibrate. Observe precautions stated in 6.3.lD. 6.3.3D Read true atmospheric pressure from pocket altimeter- barometer to the nearest 5 mm of mercury. 6.3.4D Recheck the REDLINE and ZERO readings on the oxygen meter and adjust if necessary. 6.3.5D Measure temperature in the calibration chamber to the nearest 0.5 deg. C using the thermistor in the DO probe. 6.3.6D Using oxygen the oxygen solubility table from section 7.2 determine the DO saturation at the measured temperature and true atmospheric pressure. Apply a salinity correction if necessary. (Notes 7) 6.3.7D Select the scale (0-10 mg/L or 0-20 mg/L) to be used for the DO measurement and adjust CALIBRATION control until the meter reading is at the DO saturation value determined in 6.3.6D. Do not change scales without recalibrating meter or verifying that identical readings are obtained on both scales. 6.3.8D Remove probe from calibration chamber and place in water to be measured. Move probe through water at a velocity of at least 1 foot per second (alternatively, a submersible stirrer, YSI Model 5491, or equivalent may be used) to prevent localized depletion of DO at the membrane-water interface. 6.3.9D Switch meter to TEMPERATURE; after meter reading has stabilized record temperature to the nearest 0.5!C. 6.3.10D Switch meter to the proper DO scale; after meter reading has stabilized record DO concentration to the nearest 0.1 mg/L. 7. Calculations 7.1 Read the concentration of dissolved oxygen in milligrams per liter directly from the meter of the instrument. 7.2 The following table of values for the solubility of oxygen in water exposed to water-saturated air should be used to calibrate the DO meter. The table may also be used to determine dissolved oxygen as percent saturation at the measured temperature and true atmospheric pressure as follows. DO (percent saturation) = Measured DO (mg/L)/saturation DO (mg/L) x 100 8. Report 8.1 Report dissolved oxygen concentrations to the nearest 0.1 mg/L. 8.2 Report dissolved oxygen as percent saturation to the nearest percent. 9. Precision 9.1 Precision data are not available for this method. References American Public Health Association and others, 1976, Standard methods for the examination of water and wastewater (14th ed.): New York, Am. Public Health Assoc., Inc., p. 446 Brown, Eugene, Skougstad, M. W. and Fishman, M. J., 1970, Methods for the Collection and Analysis of Water Samples for Dissolved Minerals and Gases: Techniques of Water-Resources Inv. of the U.S. Geological Survey, book 5, ch. Al, p. 129. Green, E. J. and Carritt, D. E., 1967, New Tables for Oxygen Saturation of Seawater: Journal of Marine Research, p 140.