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ffe OXYGEN, DISSOLVED (Cont°d) APPENDIX Le. B. A sample pretreatment necessary for activated sludge samples follows: 1. Add 10 mL of Copper Sulfate-Sulfamic Acid Inhibitor Solution to a clean 1000•mL graduated cylinder. 2. Fill the cylinder with the sample to be tested,using a tube that empties near the bottom of the cyl- inder,and allow the sample to overflow by about 200 mL. 3. Swirl the cylinder to mix the contents and allow the suspended solids to settle. 4. Siphon the relatively clear,top layer into a BOD bottle through a siphon tube extended to the bonam of the bottle.Withdraw the siphon tube while the weler is flowing,making certain that no air bubbles are trapped in the bottle.Continue with Steps 2 through 10. C. Allowing the floc to settle twice ensures reaction of the chemicals with all of the dissolved oxygen present.The floc will settle very slowly in ash water and usually will require an additional five minutes before proceeding with Step 5.Results will not be affected if the floc refuses to settle or if some of the reagent powder does not dissolve. D. The PAO Tivant,0.025N, is more stable than conventional solutions of sodium thiosulfate and is not af- fected by bacterial action.However,the PAO Titrant is affected by ultraviolet radiation and should be 5 protected from direct sunlight.The strength of the solution can be checked as a titrant for dissolved oxy gen,us g an Iodate-)odidc Standard Solution which is equivalent to 10 mg/Las dissolved oxygen.The check can be performed by adding one Sulfamic Acid Powder Pillow to 200 ML Iodate-Iodide Standard Solution and titrating as described in Steps 7 through 10. The volume of titrant used should be 10 mL.If x more than 10.5 mL is necessary to reach the end point(colorless),the titrant should be discarded. E. Standard AFHA solutions fordissolved oxygen can be used in Place of the powder piliow reagents by suit- "`� atituting 2 mL of Manganous Sulfate Solution and 2 mL of.Alkalinelodide Azide Reagent,respectively; / in Step 2,and 2 mL of Sulfuric Acid(concentrated)in Step 5.These solutions must be dispensed below '• the surface of the liquid.Sodium Thiosulfate Standard Solution also is available for use in place of the PAO Titrant. & F. An alternate method for determining dissolved oxygen content is the Membrane Electrode Method.No reagents are needed and most interfering substances experienced with othermethods have little effect on the electrode de.ermination.The Model 16046 Portable Dissolved Oxygen Meter by Hach is the ideal in- ' strument for measuring DO in the field. G. Referlo the Hach Digital Titration Manual for determining oxygen concentrations above I0 rng/L 02. c jS SJ - F '�']w - -