C-2. Turbidimetric Method
a. Reagents. Reagents include barium chloride crystals (Grind analytical
reagent grade barium chloride to pass a 1-millimeter sieve.); ammonium
acetate solution (0.5 N) (Add dilute hydrochloric acid until the solution has
a pH of 4.2); and distilled water.
b. Apparatus. Apparatus used are a moisture can; oven, 200-milliliter
beaker; burner and ring stand; filtering flask; Buchner funnel, 90 millime-
ter; filter paper, Whatman No. 40, 90 millimeter; vacuum source; spectro-
photometer and standard tubes (Bausch and Lombe Spectronic 20 or
equivalent) and pH meter.
(1) Take a representative sample of air-dried soil weighing approxi-
mately 10 grams, and weight to the nearest 0.01 grams. (The moisture
content of the air-dried soil must be known for later determination of dry
weight of the soil.)
(2) Add the ammonium acetate solution to the soil. (The ratio of soil to
solution should be approximately 1:5 by weight.)
(3) Boil for about 5 minutes.
(4) Filter through Whatman No. 40 filter paper. If the extracting
solution is not clear, filter again.
(5) Take 10 milliliter of extracting solution (this may vary, depending
on the concentration of sulfate in the solution) and dilute with distilled
water to about 40 milliliter. Add about 0.2 gram of barium chloride crystals
and dilute to make the volume exactly equal to 50 milliliter. Stir for 1
(6) Immediately after the stirring period has ended, pour a portion of
the solution into the standard tube and insert the tube into the cell of the
spectrophotometer. Measure the turbidity at 30-second intervals for 4
minutes. Maximum turbidity is usually obtained within 2 minutes and the
readings remain constant thereafter for 3-10 minutes. Consider the turbidity
to be the maximum reading obtained in the 4-minute interval.
(7) Compare the turbidity reading with a standard curve and compute
the sulfate concentration (as SO4) in the original extracting solution. (The
standard curve is secured by carrying out the procedure with standard
potassium sulfate solutions.)
(8) Correction should be made for the apparent turbidity of the
samples by running blanks in which no barium chloride is added.
d. Sample calculation.
Weight of air-dried sample = 10.12 grams
Water content = 9.36 percent
Weight of dry soil = 9.27 grams
Total volume of extracting solution = 39.1 milliliters
10 milliliters of extracting solution was diluted to 50 milliliters after
addition of barium chloride (see step 5). The solution gave a transmission
reading of 81. From the standard curve, a transmission reading of 81
corresponds to 16.0 parts per million. (See fig C-l)
Concentration of original extracting solution = 16.0 x 5 = 80.0 parts per
80.0 x 39.1 x 100
Percent SO4 =
= 0.0338 percent
1,000 x 1,000 x 9.27
e. Determination of standard curve.
( 1 ) Prepare sulfate solutions of 0, 4, 8, 12, 16, 20, 25, 30, 35, 40, 45,
and 50 parts per million in separate test tubes. The sulfate solution is made
from potassium sulfate salt dissolved in 0.5 N ammonium acetate (with pH
adjusted to 4.2).