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.
2. Potassium ferricyanide solution. Dissolve 5 grams of the salt in 1
liter of distilled water. Use a freshly prepared solution.
3. Dilute sulfuric acid. Dilute 1 part of sulfuric acid, specific
gravity 1.84, with 5 parts of distilled water.
_Procedure._—Add 10 cc. of dilute sulfuric acid to 50 cc. of the sample,
remove the suspended matter by filtration if necessary, and add 15 cc.
of potassium ferricyanide solution. Dilute the solution to 100 cc. with
distilled water. Compare the color developed in the sample with that in
standards made at the same time from the ferrous iron solution. Place in
100 cc. Nessler tubes, in the following order, 75 cc. of distilled
water, 10 cc. of dilute sulfuric acid, and 15 cc. of potassium
ferricyanide solution, and mix well the contents of each tube. Prepare
as many tubes in this way as are needed. Add various quantities of
standard ferrous iron solution to several tubes, mix well, and compare
the resulting colors with the samples _immediately_.