MIL-DTL-757C
as shown in figure 1. The solution need not be standardized, as it is compared with the standard
ferric ammonium sulfate each time an analysis is made.
4.4.11.2.4 Ammonium thiocyanate solution (20 percent). Dissolve 20 g of NH4CNS in
80 g of distilled water.
4.4.11.2.5 Sodium acetate solution (20 percent). Dissolve 20 g of CH3COONa in 80 g of
distilled water.
4.4.11.2.6 Stannous chloride solution. Dissolve 50 g of SnCl2 2H2O in 100 mL of
concentrated hydrochloric acid and dilute to 1 liter with distilled water.
4.4.11.2.7 Diphenylamine sulfonate indicator solution. Add 0.095 g of the barium salt of
diphenylamine sulfonic acid to 100 mL of distilled water and stir until solution is complete. Add
10 mL of dilute sulfuric acid (1 to 1), let stand several hours, and filter. Add 500 mL of 85
percent phosphoric acid and dilute to 1 liter.
ferric ammonium sulfate (FeNH4(SO4)2 ∙12H20 in one liter of 5 percent sulfuric acid. The
4.4.11.2.8 Ferric ammonium sulfate solution (0.2N). Dissolve approximately 100 g of
normality is determined as follows: Pipet 25 mL of the ferric ammonium sulfate solution into a
500 mL wide mouth Erlenmeyer flask and add 25 mL of concentrated hydrochloric acid. Heat
nearly to boiling point and add the stannous chloride solution drop-wise and with stirring until
the yellow color of ferric chloride is discharged and then add 1 drop stannous chloride solution in
excess (no more). Cool and add 10 mL of a saturated mercuric chloride solution. Dilute to about
200 mL and occasionally shake for 5 minutes. Add 15 mL diphenylamine sulfonate indicator
solution and titrate with a standard 0.2000 N potassium dichromate solution. The change at the
end point is from light green to purple. The normality of the ferric ammonium sulfate solution is
calculated as follows:
A× N
Normality of ferric ammonium sulfate solution =
B
where: A = potassium dichromate solution, mL.
N = normality of potassium dichromate solution.
B = ferric ammonium sulfate solution, taken by titration, mL.
4.4.11.3 Procedure. Transfer approximately 0.6 to 0.8 g of the dry sample weighed to
nearest 0.1 mg to a 100 mL beaker. Add 50 mL of 25 percent acetic acid solution and stir to
dissolve. Transfer to a 250 mL volumetric flask and dilute to the mark. Attach the source of
inert gas (oxygen-free carbon dioxide or nitrogen) to the titration flask as shown in Figure 1 and
allow the gas to flow for 5 minutes to displace the air. Continue the flow of gas during the entire
titration procedure. Pipet a 25 mL aliquot of the solution of the sample into the titration flask
(Figure 2). Add 30 mL of 20 percent sodium acetate solution and 20.00 mL of 0.2 N titanous
chloride solution and swirl the flask for about 20 seconds. Add 25 mL of 15 percent hydrochloric
acid. Titrate with ferric ammonium sulfate solution until the purple color begins to fade, then
add 5 mL of 20 percent ammonium thiocyanate solution and continue the titration to a faint
permanent pink. A blank run shall be made using the quantities of reagents as above, but
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