LaMotte CHEMICAL OXYGEN DEMAND AB-COD - Manual (Page 2)

SAM PLING AND STOR AGE
Samples are taken with clean water sampling bottles which are free of
organic matter. The COD determination should be made almost
immediately after the sample is taken. If, for some reason, the COD
analysis cannot be made immediately, the sample is made acidic (pH 2 -
pH 3) by adding a few drops of 25% Sulfuric Acid (7588) and stored at
4-8
o
C. For longer storage periods, deep-freezing is recommended. Samples
containing detritus or settleable solids should be homogenized by a blender
to obtain a representative sample. All dilutions should be made with
deionized water and should be measured accurately in volumetric flasks.
AP PA RA TUS
A hot plate is necessary to provide adequate boiling of the reagents in the
reflux mixture. A metal stand is required to hold both the distilling flask
and the reflux condenser. All the other material required to make a COD
analysis is supplied with the LaMotte Chemical Oxygen Demand Outfit.
STAN DARD IZA TION
*Ferrous Ammonium Sulfate, 0.05N (7585) must be restandardized before
a series of analysis is started, due to its unstable nature. To determine the
normality of *Ferrous Ammonium Sulfate, 0.05N (7585), dilute 5 mL of
*Potassium Dichromate, 0.25N (7582) to 50 mL with Deionized Water
(5115). Add 15 mL *COD Reagent
#
3 (7583) and allow to cool. Titrate
with *Ferrous Ammonium Sulfate, 0.05N (7585) using 3-5 drops of Ferroin
Indicator (7584) and substitute the values in the following equation:
Normality of Reagent #5 =
mL Potassium Dichromate, 0.25N
x .25
mL Ferrous Ammonium Sulfate, 0.05N
PRO CE DURE
A blank consisting of 10 mL of distilled water must be run with all the
reagents. The blank is refluxed and analyzed in the same manner as
outlined above.
1.
Fill graduated cylinder (0416) to 10 mL line with sample water.
Transfer to 50 mL flat bottom flask (0887).
2. Use the 0.1 g spoon (0699) to add 0.2 g *Mercuric Sulfate Powder
(7851). Mix.
3. Use a second graduated cylinder (0416) to add 5 mL of *Potassium
Dichromate, 0.25N (7582). Mix.
4. Fill graduated cylinder (0418) to 15 mL line with *COD Reagent
#
3
(7583). Add small portions at a time to the distillation flask, mixing
well with each addition.
CAUTION:
*COD Reagent #3 (7583) is Sulfuric Acid. Read
Material Safety Data Sheets (MSDS).