MIL-DTL-23938B(AS)
4.5.1 Determination of iron as Fe2O3. The determination of iron as Fe2O3 shall be as
follows:
4.5.1.1 Preparation of reagents.
a. 0.1 N Potassium dichromate (K2Cr2O7) - Weigh out exactly 4.9037 grams of reagent
grade or analyzed potassium dichromate. Dilute to 1 liter with distilled water.
b. Sulfuric-phosphoric acid mixture - 150 mL of concentrated sulfuric acid (H2SO4) and
150 mL of concentrated phosphoric acid are mixed together. The mixture of acids is diluted to
1 liter with distilled water.
c. Diphenylamine indicator - One gram of diphenylamine is dissolved in 100 mL of
concentrated H2SO4.
d. Stannous chloride solution (SnCl2) - Dissolve 60 grams of SnC12 in 600 mL of
concentrated HCl; add 400 mL of distilled water.
e. Saturated mercuric chloride (HgC12) - Place 100 grams of HgC12 in 1 liter of distilled
water. Heat to dissolve.
4.5.1.2 Procedure.
a. Weigh 0.2 gram to the nearest 0.1 milligram (mg), of sample into a 250 mL beaker. Add
20 mL of concentrated HCl. Heat at 158 °-176 °F (70 °-80 °C) for one hour or until sample is
completely dissolved.
b. Add stannous chloride (SnCl2) solution to the dissolved sample dropwise until the solution
is colorless; then add two drops of SnC12 in excess. Cool the solution in an ice bath.
c. To the cooled solution, add all at one time, 10 mL of saturated HgCl2; let solution stand
for approximately five minutes, then add 15 mL of the sulfuric-phosphoric acid mixture. Dilute
the sample solution to 150-200 mL with distilled water. Add three drops of diphenylamine
indicator and titrate immediately with the potassium dichromate solution. Near the end point, the
green color changes to a dark blue-green. The dichromate is now added dropwise until the color
changes to an intense violet blue.
d. Calculations:
(0.1000) (Assay of Dichromate)
N (Normality of Potassium Dichromate)
=
100
(mL) (N) (7.985)
Percent Fe2O3
=
sample weight
3
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