Fats are usually determined only on sewage sludge, but some mud deposits
contain small quantities due to the presence of trade wastes.
_Procedure._—Weigh 0.5 to 25 grams of dry material according to the
quality of the sludge or mud. Add water to the weighed portion in a
porcelain dish and acidify the mixture with N/50 sulfuric acid in the
presence of litmus tincture or azolitmin solution as indicator. Avoid
adding too much acid as an excess gives too high results on account of
fatty acid residues. Evaporate the acidified mixture to dryness on the
water bath, and heat it in the hot air oven at 100° C. two to three
hours. Extract the dry residue with boiling ether, rubbing the sides and
bottom of the dish to insure complete solution of the fat. Three
extractions with ether are usually sufficient. Filter the ether solution
through a 5 cm. filter paper into a small flask. Evaporate the ether
slowly, dry the fatty extract for half an hour at 100° C., cool in a
desiccator, and weigh. If it is desirable, particularly with certain
industrial wastes, to determine the quantity of saponified fat determine
the fats with and without the addition of acid. The difference between
the quantities found by the two determinations is the content of
saponified fat.