Chapter 6   Dissolved Oxygen return to Table of Contents

 

                 In order to maintain metabolic processes, grow and reproduce, all (there are exceptions found in deep oceans) living organisms must have oxygen in one form or another.  The organisms that live in anaerobic (without free oxygen) conditions get their oxygen from the materials which they break down for food by biochemical action.  The organisms that live in aerobic (with free oxygen present) conditions must have a plentiful supply of oxygen readily available.  This oxygen is in the form of Dissolved Oxygen (DO) in the fluid (air or water) in which the organisms live.  The presence of DO is the factor which determines whether the biological changes in a wastewater will be accomplished by aerobic or anaerobic organisms.

                 Oxygen interchange among air, water, and soils is limited by biological, chemical, and physical factors related to oxygen availability.

                 a.  Air consists of about 20% oxygen by volume (21% by weight).

                 b.  Oxygen content of water is limited to about 11.3 milligrams dissolved oxygen per liter at 10º C,

                      9.2 mg/L @ 20º C, and 7.6 mg/L @ 30º C.  The presence of 9.2 mg/L DO represents 0.00092%

                      of oxygen in water on a weight basis.

c.  Surface soils may contain air in voids and variable amounts of dissolved oxygen in water to provide some available oxygen.  Most of the oxygen in soils is in a combined form and relatively  unavailable.

                 d.  Atmospheric pressure (higher pressure = more O₂ dissolved).

                 e.  as ‘b’ states, higher temperature = less O₂ dissolved).

 

                

Oxygen concentration of less than 16% causes blackouts and concentrations above 40% causes the formation of toxic oxygen radicals that damage cell structure and function. This process is known as oxygen toxicity and can lead to death. Thus, life is is greatly affected by the concentration of oxygen (we humans prefer about 20% - most of the hazardous gas analyzers will alarm below that amount).

Two methods for DO analysis are:  the Winkler Method, or iodometric, and the electrometric method.  The Winkler is a wet chemistry, titrimetric procedure based on the oxidizing property of dissolved oxygen.  The electrode method measures the rate of molecular oxygen diffusion across a membrane.  Procedure selection considerations include:  interferences present, desired accuracy, time or convenience.

                 The analysis for DO is a key test in water pollution control activities and waste treatment process control.  The DO test provides information about the condition of the wastewater for the operator to

make process control decisions.  Two methods of DO determination are commonly used today:  A chemical reaction process and an electrometric method.  The chemical Winkler method and its modifications (to cope with numerous interferences) is still considered to be the more reliable of the two, although slower, and used as the reference method.  Certain oxidizing agents liberate iodine from iodides (positive interference) while some agents reduce iodine to iodides (negative).  Most organic matter is oxidized partially when the oxidized manganese precipitate is acidified which will cause negative errors.  Some of the modifications are:  Azide for nitrate interference, permanganate for ferrous iron, alum flocculation for suspended solids and copper sulfate - sulfamic acid for flocculation of activated sludge mixed liquor.  These modifications eliminate or minimize known interference effects.

                 The generalized principle is that iodine will be released in proportion to the amount of dissolved oxygen present in the sample.  By using sodium thiosulfate with starch as the indicator, one can titrate the sample and determine the amount of dissolved oxygen.

                 The  electrometric method uses membrane electrodes in which the rate of diffusion of molecular oxygen across a membrane is measured.  DO electrodes are sensitive to the temperature and atmospheric pressure of the sample. The BOD and CBOD  tests will be completed by using the DO test.  Since each method has different advantages and disadvantages, the operator must apply the appropriate method or both in order to get data which reflects the true condition of the wastestream.

                 Proper sample collection is of utmost importance when determining DO.  This does include using the ‘probe’.  Placing the probe in a location where air and water are being mixed, will give an erroneously high reading (even higher than a supersaturated DO condition).  The probe efficiently profiles a large body of process water by reading the DO at selected depths at many locations within a short span of time.  However, questions can be raised about the prevailing conditions at the sampling area at the moment of reading.  The dissolved oxygen content of the water being tested can be expected to change with depth, turbulence, temperature, sludge deposits, light, microbial action, mixing, travel time and other factors.  A single DO test rarely reflects the condition of a body of water.  Several samples taken at different times, locations and depths are recommended for more reliable information.

                 Grabbing the sample for the Winkler requires careful techniques. The sample container must be lowered into the flow and filled carefully so that the air in the bottle does not bubble up through the entering water, then the stopper is added while the container is completely submerged.  Pouring a sample into a BOD bottle from an open dipper or pail will almost always aerate the sample.  A special sampler called a DO dunker is sometimes used to obtain a subsurface sample without aeration.  A siphon is used to transfer the sample without aeration.  Because of these problems, EPD strongly suggests using a DO meter and field electrode to determine the value in the effluent stream without removing a sample for laboratory analysis.  Samples should be analyzed immediately upon collection.  If they must be transported a distance to the laboratory, the sample is fixed on site by adding manganous sulfate, alkaline iodide azide and sulfuric acid.

                 However, when measuring DO in conjunction with BOD₅ testing, much more precise and accurate numbers for the BOD₅ are obtained when the Winkler titration is used.  The main drawback for the Winkler titration is that 5 days elapse between the start of the analysis and the end of the analysis, frequently Georgia experiences substantial weather changes during that time.  The high dilutions required in BOD₅ analysis of wastewaters magnify any small changes in the calibration of the DO electrode.  A 0.2 difference in DO because of calibration error can result in a 200 mg/L BOD₅ difference at a 0.1% concentration.  Careful attention to detail must be the guiding rule for the laboratory analyst.

                Knowing the temperature and barometric pressure, the concentration of oxygen is a reasonably dependable number. The probe is placed in a BOD bottle with about an inch of distilled water for a saturated environment.  Most people feel comfortable assuming that a DO meter will be linear over the normal response range.  You would probably never calibrate a pH meter at only one point.   If your different BOD dilutions don't give consistent results, the reason might be nonlinear response of your DO probe.  Cleaning procedures of using an ink eraser (or the abrasive paper manufacturers now supply with packages of membranes) on the gold electrode, and using the ammonia soak procedure for renewing the silver/silver chloride electrode may improve the linearity response.   Always keep the membrane in the tip of the probe from drying because the probe can lose its accuracy until reconditioned.  Constant care and calibration of the DO probe/meter is important.

 

 

Winkler Titration Checklist (4500-O C, Standard Methods 18th Edition)

                                                       Reagents

          ___ 1.  0.0250 M Sodium thiosulfate:  3.1025 g sodium thiosulfate pentahydrate (Na₂S₂O₃ • 5H₂O) in reagent water.  Add 0.2 g sodium hydroxide (NaOH, 1 to 2 pellets) and dilute to 500 mL.  Alternate:  Dissolve exactly 6.206 grams sodium thiosulfate crystals in freshly boiled and cooled reagent water and make up water to a volume of 1 liter.  For preservation, add 0.4 gm or 1 pellet of sodium hydroxide.  Solutions of “thio” should be used within two weeks to avoid loss of accuracy because of decomposition of the solution.  Alternate:  Phenylarsine Oxide solution (PAO) may be used instead of “thio” [0.025 N PAO available and standardized from commercial sources.]

          ___ 2.  Alkaline Iodine Azide (AIA) solution:  50 g sodium hydroxide (NaOH and 15 g potassium iodide (KI) are dissolved in reagent water and diluted to 100 mL.  Add 1 g sodium azide (NaN₃) dissolved in 4 mL reagent water.

          ___ 3.  [0.00210 M Potassium Biiodate:  dissolve 406.2 mg potassium biiodate (Primary Standard grade) in reagent water and dilute to 500 mL.  (Source and reason unknown)]

          ___ 4.  Starch solution:  2 g starch and 0.2 g salicylic acid dissolved in 100 ml boiling water, cool.  [Make a thin paste of 6 gm of potato starch in a small quantity of distilled water.  Pour this paste into one liter of boiling, distilled water, allow to boil for a few minutes, then settle over night.  Remove the clear supernatant and save; discard the rest.  For preservation, add two drops toulene (C₆H₅CH₃) or use 1.25 grams salicylic acid.]

          ___  5.  Manganous Sulfate solution:  dissolve 40 g manganous sulfate dihydrate (MnSO₄ • 2H₂O) in reagent water, filter and dilute to 100 mL.

          ___  6.  Sulfuric acid.  Use concentrated reagent grade acid (H₂SO₄).  Handle carefully, since this material will burn skin and clothes.

          ___  7.  Potassium Iodide (KI) crystals.

 

                                         Winkler DO Standardization

          ___  1.  Dissolve 2 g potassium iodide in approximately 100 mL in an Erlenmeyer flask.

          ___  2.  Add 1.0 mL concentrated Sulfuric acid.

          ___  3.  [Add 20.00 mL 0.0021 M potassium biiodate solution and dilute to approximately 200 mL. (Source and reason unknown)]

          ___  4.  Add a magnetic stir bar.

          ___  5.  Fill buret with the sodium thiosulfate solution and lower level to 0.00 mark. Be sure that the tip of the buret is filled with solution and not air.

          ___  6.  Begin stirring and add sodium thiosulfate solution to the flask until a light yellow color is reached.

          ___  7.  Add about 2 mL of starch solution.  The color will be deep blue or purple.

          ___  8.  Continue addition of sodium thiosulfate until the solution clears permanently.  Stop the titration and record the number of mL of sodium thiosulfate used.

          ___  9.  If exactly 20.00 mL of sodium thiosulfate solution is used it can be recorded as standardized at 0.0250 M.  If less than 20.00 mL are needed, add some reagent water to the sodium thiosulfate solution (5 mL reagent water to 100 mL titrant for each 1.00 mL that the titrant is low).  If more than 20.00 mL are required, add solid sodium thiosulfate to the titrant solution (310 mg per 1000 mL for every 1.00 mL the titrant is high).  Recheck the standardization.

          ___ 10.  Repeat the standardization and record both determinations and the average in the notebook.

 

                                 Winkler DO Procedure

  The reagents are to be added in the quantities, order and methods as follows:

          ___  1.  Collect sample in 300 mL BOD bottle taking special care to avoid aeration of the liquid being collected.  Fill bottle completely (no air under cap).

          ___  2.  Insert stopper in bottle, then remove.

          ___  3.  Add, under the surface of the sample, 1.00 mL manganous sulfate solution.

          ___  4.  Immediately add 1.00 mL alkaline iodide azide solution below the surface of the liquid.

CAUTION:  When working with alkaline azide and Sulfuric acid, keep a nearby water faucet running for frequent hand rinsing.

          ___  5.  Insert stopper in bottle, avoid trapping air bubbles, and invert 5 times to mix well.  Repeat this shaking after the floc has settled halfway. 

          ___  6.  Allow floc to settle to half the volume of the bottle a second time, then open and add 1.0 mL concentrated Sulfuric acid by allowing the acid to run down the neck of the bottle above the surface of the liquid.

      ___  7.  Insert stopper and invert to mix until the floc dissolves.

          ___  8.  Using a graduated cylinder, transfer 201 mL of the solution to an Erlenmeyer flask with magnetic stirbar for the titration procedure.

          ___  9.  Titrate with the 0.0250 M sodium thiosulfate solution until the solution is pale yellow.  Variation:   EPA may favor the "full bottle" method, rather than the 201 mL volume.  The titrant is 0.0375 instead of 0.0250 N sodium thiosulfate

          ___ 10.  Add 2 mL starch and continue titration until the color is permanently discharged.  Record the volume of sodium thiosulfate used.

          ___ 11.  Each 1.00 mL of 0.025 M sodium thiosulfate used is equivalent to 1.00 mg/L DO.  Record the answer.

          ___ 12.  Repeat determination on a duplicate sample, then calculate RPD (Relative % Difference) and update control chart.

 

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