LaMotte CHEMICAL OXYGEN DEMAND AB-COD - Manual (Page 3)

5. Add 5-10 glass beads (0889) to the flask.
CAUTION:
The reflux mixture should be thoroughly mixed before
heating.
6. Attach the reflux flask to the condenser (0888). Connect the
rubber tubing to the condenser and to the faucet.
NOTE:
Be sure that the condenser and reflux flask are held securely
on a hot plate by the clamps and adjust the water flow through the
condenser so that water pressure does not build up within the
condenser.
7. Boil contents of reflux flask for 2 hours.
NOTE:
Shorter time periods may be used for some samples which are
low in organic matter if the COD gives the maximum value using the
shorter time periods. Samples containing a high concentration of
organic matter will consume all the dichromate: as a result, no
endpoint can be obtained with the titration. If this is the case, the
untreated sample must be diluted with deionized water until there is
an excess of titratable dichromate. Repeat Steps 1-7 on the diluted
sample and multiply the results by the dilution factor.
8. Wait for mixture to cool. Pour the contents of the reflux flask into the
rinsed 50 mL graduated cylinder (0418). Wash down the insides of the
condenser and the reflux flask with Deionized Water (5115PS) adding
the washing to the sample in the graduated cylinder. Dilute to 50 mL
with Deionized Water (5115PS). Mix. Pour the diluted sample into a
125 mL Erlenmeyer flask (0431).
9. Add drops of *Ferroin Indicator (7584) until the color of the sample is
bluish-green. Usually 3-5 drops of indicator are sufficient.
10. Fill the buret (0427) to the 10 mL mark with *Ferrous Ammonium
Sulfate, 0.05N (7585). While gently swirling the flask, add *Ferrous
Ammonium Sulfate, 0.05N until solution turns reddish-brown.
NOTE:
The first color change is the endpoint.
CAL CU LA TION
COD, chemical oxygen demand from dichromate:
mg/L COD =
(A - B) x C x 8,000
mL sample
A = mL *Ferrous Ammonium Sulfate, 0.05N used for blank
B = mL *Ferrous Ammonium Sulfate, 0.05N used for sample
C = Normality of *Ferrous Ammonium Sulfate, 0.05N