Acid-Soluble Chloride Analysis Test in Soil.
Principle.
For the determination of acid-soluble chlorides, which
includes chlorides not extracted by water, chlorides are extracted from dry
soil sample with dilute nitric acid. Results are expressed as the chloride
content.
The acid-extract method is applicable to the determination
of the chloride content of soils from desert areas or where the origin of the chlorides
is uncertain.
Reagents and Apparatus.
- Burette, 50 mL
- Weighing balance
- Stoppered conical flask, 500 mL
- Beaker, 500 mL
- Measuring cylinder
- Hot plate
- Filter paper, coarse grade
- Filter funnel
- Nitric Acid
- N Silver Nitrate Solution
- N Potassium Thiocyanate Solution
- 3-5-5-Trimethylhexan-1-ol
- Ferric alum indicator
Procedure.
Standardization of potassium thiocyanate solution.
Transfer 25 mL of the silver nitrate solution into a 250 mL
conical flask, using a pipette, and add
5 mL of the nitric acid solution and 1 mL of ferric alum indicator solution.
Add thiocyanate solution from a burette
until the first permanent color change occurs, that is from colorless to pink. Record the volume of
thiocyanate solution added V1 (in mL). Calculate the concentration C (in mol/L)
of the solution from the following equation: C =2.5/ V1.
Preparation of test specimen.
Obtain an initial sample. Dry this sample in an oven at 105
°C to 110 °C, and cool to room
temperature in the desiccator. Sieve the sample on a 150 µm test sieve.
Crush all retained particles to pass the
150 mm sieve and mix thoroughly with the material already passing the sieve. Divide the material by successive
riffling to produce test specimens each of about 10 g. Dry the test specimens in the oven at 105
°C to 110 °C and allow to cool in the
desiccator.
Preparation of acid-soluble chloride extract.
Weigh 5+0.005 g of soil sample in a 500 mL beaker. Add 50 mL
distilled water, followed by 15 ml
nitric acid. Heat the sample to near boiling and kept warm for 10-15 minutes.
Filter through coarse grade filter paper
into a conical flask. Wash the filter paper with hot water and collect the filtrate and allowed to cool.
Add silver nitrate solution until all the chloride has been precipitated, then add little excess of
silver nitrate. Add 2 mL 3-5-5-trimethylhexan-1-ol and shake the flask vigorously to coagulate
the precipitate. Add 5 mL ferric alum indicator
and titrate with standardized Potassium Thiocyanate solution until the
first permanent color change occurs,
that is from colorless to red. Record the volume V3 (in mL) of thiocyanate solution added.
Calculation.
Chloride Content= 0.07092(V2-10CV3)
Where: V2= volume of silver nitrate solution added
V3= volume of standardized
potassium thiocyanate solution added
C= concentration of standardized
potassium thiocyanate solution
Reference.
BS 1377 Part 3:1990 Amd 9028 :1996.