ASTM D1252-06 (2020) pdf free.Standard Test Methods for Chemical Oxygen Demand (Dichromate Oxygen Demand) of Water.
The amount of dichromate consumed in Test Method Ais determined by titration rather than the spectrophotometricprocedure used in Test Method B.This test method is appropriate where larger sample volumes would provide betterprecision and better representativeness of where equipment orspace limitations exist.
The precision of this test method in standard solutionscontaining low-volatility organic compounds has been exam-ined in the range of approximately 10 to 300 mg/L.
The sample and standardized dichromate solution, in a50 % by volume sulfuric solution,is refluxed for a 2-hdigestion period.
Excess dichromate after the digestion period is titratedwith a standard ferrous ammonium sulfate solution usingortho-phenanthroline ferrous complex as an internal indicator.12. Interferences
The test method does not uniformly oxidize all organicmaterials.Some compounds, for example, are quite resistant tooxidation,while others，such as carbohydrates，are easilyoxidized.A guide to the behavior of various types of organicmaterials is provided in Table 1.Volatile organics that are difficult to oxidize may bepartially lost before oxidation is achieved.Care in maintaining a low-solution temperature (about 40°C) and permitting oxi-dation to proceed at the lower temperature for a period of timebefore reflux is initiated will result in higher recoveries oftheoretical COD of volatile organics.
The apparatus consists of a 50O-mLErlenmeyer or a 300-mL roundbottom flask,made of heat-resistant glass connected to a 300-mm (12-in.) Allihn con-denser by means of a ground-glass joint.Any equivalent refluxapparatus may be substituted，provided that a ground-glassconnection is used between the flask and the condenser，andprovided that the flask is made of heat-resistant glass.
A heating mantle or hotplate capable of delivering sufficient controlled heat to main-tain a steady reflux rate in the reflux apparatus is satisfactory.13.3 Apparatus for Blending or Homogenizing Samples—Ahousehold blender is satisfactory.
Add 8 to 10 drops of phenanthroline ferrous sulfatesolution and titrate the excess dichromate with 0.25 N ferrousammonium solution.The color change at the end point will besharp，changing from a blue-green to a reddish hue. If thesolution immediately turns a reddish-brown upon the additionof the indicator, repeat the analysis on a smaller sample aliquot.ASTM D1252 pdf download.