───────────────────────┬───────────────────────┬─────────────────────── Value in standard iron │ Platinum solution. │ Cobalt solution. solution. │ │ ───────────────────────┼───────────────────────┼─────────────────────── _cc._ │ _cc._ │ _cc._ 0.0│ 0│ 0.0 .1│ 2│ 1.0 .3│ 6│ 3.0 .5│ 10│ 5.0 .7│ 14│ 7.5 1.0│ 20│ 11.0 1.5│ 28│ 17.0 2.0│ 35│ 24.0 2.5│ 39│ 32.0 3.0│ 39│ 43.0 3.5│ 40│ 55.0 ───────────────────────┴───────────────────────┴─────────────────────── VOLUMETRIC METHOD.[24f] 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. DISSOLVED IRON. Determine, by the method described for total iron, the iron in the sample after filtration. Iron may precipitate from some samples during filtration. SUSPENDED IRON. The suspended iron is the difference between total iron in the unfiltered sample and dissolved iron in the filtered sample. FERROUS IRON.[24e] Determine the total ferrous iron in an unfiltered sample and the dissolved ferrous iron in a filtered sample. _Reagents._—1. Standard iron solution. Dissolve 0.7 gram of crystallized ferrous ammonium sulfate in a large volume of freshly boiled distilled water to which 10 cc. of dilute sulfuric acid has been added and dilute to 1 liter. This solution should be freshly prepared when needed. One cc. of this standard solution contains 0.1 mg. of Fe.