TM 5-822-14/AFJMAN 32-1019
APPENDIX C
DETERMINATION OF SULFATE IN SOILS
GRAVIMETRIC METHOD
C-1. Gravimetric Method.
a. Scope. Applicable to all soil types with the possible exception of soils
containing certain organic compounds. This method should permit the
detection of as little as 0.05 percent sulfate as SO,.
b. Reagents. Reagents include barium chloride, 10 percent solution of
BaCl2 l 2H2O (Add 1 milliliter 2 percent HCl to each 100 milliliter of
solution to prevent formation of carbonate.); hydrochloric acid, 2 percent
solution (0.55 N); magnesium chloride, 10 percent solution of MgCl2 l 6H2O;
demineralized water; and silver nitrate, 0.1 N solution.
c. Apparatus. Apparatus used are a beaker, 100 milliliter; burner and
ring stand; filtering flask, 500 milliliter; Buchner funnel, 90 milliliter; filter
paper, Whatman No. 40, 90 millimeter; filter paper, Whatman No. 42, 90
millimeter; Saran Wrap; crucible, ignition, or aluminum foil, heavy grade;
analytical balance; and aspirator or other vacuum source.
d. Procedure.
(1) Select a representative sample of air-dried soil weighing approxi-
mately 10 grams. Weigh to the nearest 0.01 gram. (Note: When sulfate
content is anticipated to be less than 0.1 percent, a sample weighing 20
gram or more may be used.) (The moisture content of the air-dried soil must
be known for later determination of dry weight of the soil.)
(2) Boil for 1-1/2 hours in beaker with mixture of 300-milliliter water
and 15-milliliter HCl.
(3) Filter through Whatman No. 40 paper, wash with hot water, dilute
combined filtrate and washings to 50 milliliter.
(4) Take 100 milliliter of this solution and add MgCl2 solution until no
more precipitate is formed.
(5) Filter through Whatman No. 42 paper, wash with hot water, dilute
combined filtrates and washings to 200 milliliter.
(6) Heat 100 milliliter of this solution to boiling and add BaCl2
solution very slowly until no more precipitate is formed. Continue boiling
for about 5 minutes and let stand overnight in warm place, covering beaker
with Saran Wrap.
(7) Filter through Whatman No. 42 paper. Wash with hot water until
free from chlorides (filtrate should show no precipitate when a drop of
AgNO3 solution is added).
(8) Dry filter paper in crucible or on sheet of aluminum foil. Ignite
paper. Weight residue on analytical balance as BaSO4.
e. Calculation.
Weight of residue
x 411.6
Percent SO, =
Oven-dry weight of initial sample
where
Oven-dry weight of initial sample
Note: If precipitated from cold solution, barium sulfate is so finely dispersed that it cannot be
retained when filtering by the above method. Precipitation from a warm, dilute solution will
increase crystal size. Due to the absorption (occlusion) of soluble salts during the precipitation
by BaSO4, a small error is introduced. This error can be minimized by permitting the
precipitate to digest in a warm, dilute solution for a number of hours. This allows the more
soluble small crystals of BaSO4 to dissolve and recrystallize on the larger crystals.
C-1
Previous Page
