Some samples of sewage and water mixed with trade wastes and mine
drainage contain so much iron that it is preferable to use the
volumetric method described on page 57 for the determination of both
total and dissolved iron, rather than to work with quantities small
enough to permit application of the colorimetric methods just described.
If iron is present in large quantities in suspension, as in some sewages
and septic tank effluents, it may be filtered off and the residue
washed, ignited, and fused with potassium and sodium carbonate. The
fusion is then extracted with hydrochloric acid and the iron determined
as on page 57.
Samples containing much organic matter should be evaporated to dryness
with 0.5 cc. of concentrated sulfuric acid and the residue then ignited
before estimation of iron.