Five COD test tube volumes of 15 ml were cleaned with dilute
H2SO4. The test tubes were oven dried for an hour. 0.5 ml of
sewage was transferred to each COD test tube. Standard Potassium
dichromate and concentrated sulphuric acid (16 M) which were
prepared earlier were then added in volumes of 2.5 ml and 3.5 ml
respectively. The tubes were then capped, shook and cooled. The
test tubes were then transferred into a COD digester and heated
for 120 minutes at 150°C. A blank, where distilled water was used
instead of sewage and transferred to the digester to be the control
for the COD test as shown in Figure 2.