LaMotte SMART Spectro Spectrophotometer User Manual

Page 125

APPLICATION:

Domestic and industrial wastes.

RANGE:

0–1500 mg/L COD

MDL:

20 mg/L

METHOD:

Dichromate in the presence of silver salts, at high

temperature in a closed system, oxidizes most organic

compounds to 95-100% of the theoretical amount. This

process is called digestion. As dichromate oxidizes the

organic compounds, a green complex is formed. The

concentration of the green complex is measured at

620 nm and is directly proportional to the COD of the

sample.

SAMPLE HANDLING &

PRESERVATION:

Collect samples in glass and test as soon as

possible. If samples must be stored, preservation is

accomplished by the addition of concentrated H

to adjust the pH below 2. Samples with suspended

solids should be homogenized in a blender (100 mL

for 30 seconds) and then stirred gently with a magnetic

stirrer.

INTERFERENCES:

Volatile organic compounds are not oxidized to the

extent that they are in the vapor above the digestion

solution. Therefore, they do not contribute to the

COD reading. Chloride concentrations above 10% of

COD interfere with the mercury free tubes. Chloride

above 2000 ppm will interfere with the mercury tubes.

Nitrite gives a positive interference of 1.1 ppm O

per ppm NO

–N which is insignifi cant unless nitrite

concentrations are very high. Other reduced inorganic

compounds are stoichiometrically oxidized, causing

a positive interference. Corrections can be made for

these compounds based upon their stoichiometry and

concentrations.

COD, Standard Range

SMART Spectro Test Procedures 2.11

Test P

rocedures