FED-STD-791D
6.11 Using a 50-mL pipet, pipet a 50-mL aliquot into a 400-mL beaker, add 2
drops of methyl orange indicator and neutralize the excess ammonium hydroxide in the
filtrate aliquot with drops of concentrated hydrochloric acid. Neutralization will be
complete at the first sign of pink or red color in the solution.
6.12 Add 30 mL of ammonium acetate to the solution and 10 mL of glacial acetic
acid. Heat solution to near boiling.
6.13 Add a previously calculated amount of lead acetate solution (31 to 33
mL/0.26 to 0.28 g of sample) in aliquot respectively, (see Note 3), to the sample solution
from a buret, dropwise at the rate of 30 mL in 2 min. While stirring the solution
mechanically add an excess of 3 mL of lead acetate and stir for 5 min after complete
addition.
NOTE 3.
For an accurate check on the quantity of lead acetate solution required, the
tannic acid solution can be used as an outside indicator. The end point is
reached when a drop of the neutralized aliquot solution added to 0.1 mL of
the tannic acid indicator solution on a spot plate fails to give a brown
coloration.
6.14 Place sample solution on a hot plate and heat until near boiling and hold at
this point for at least l hour and allow the precipitated lead molybdate to settle
completely.
6.15 Decant the bulk of the solution through a Whatman No. 42 (or equivalent)
paper containing a small amount of paper pulp. Transfer the precipitate to the filter paper
and wash 4 times with 50-mL portions of hot ammonium nitrate solution. The filtrate can
be discarded.
6.16 Place filter paper with residue in a previously cleaned, ignited, cooled and
weighed porcelain crucible. Ignite carefully in a muffle furnace at 550 to 600C
(1,020 to 1,110F) for l h.
6.17 Remove crucible and place in desiccator for 30 to 60 min and allow to cool
to room temperature. Weigh crucible. Repeat ignitions to constant weight. Calculate the
weight percent MoS2 (see 7.2).
7. CALCULATIONS
7.1 The percent total insoluble matter may be calculated as follows:
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