PCTM 4C
ISSUED-
1996
Moisture
content of tall oil by Karl Fischer titration
Scope
This test method covers the
determination of water in liquid organic materials containing 0.5 to 10% water.
It is based on the reduction of iodine by sulfur dioxide in the presence of
water. The moisture content of tall oil is determined by automatic, volumetric
titration with Karl Fischer reagent. In the Karl Fischer reaction, iodine
reacts with water in the presence of sulfur dioxide, an organic nitrogen base
like pyridine and an alcohol:
I2+H2O
+ SO2+
3C5H5N→2C5H5N•HI +C5H5N•SO3C5H5N•SO, +ROH→C5H5N•HSO4R
The Karl Fischer reagent is the
titrant. It normally consists of iodine, pyridine and sulfur dioxide, in a
molar ratio of 1 : 10 : 3, dissolved in anhydrous 2-metboxyethanol.
Safety precautions
Always operate the titrator in a
fume hood. Most of the reagents, including some of the pyridine-free systems
are noxious or toxic and inhalation or direct skin contact with them should be
avoided.
Apparatus
1. Automatic Karl Fischer titrator.
2. Sample vials, 8-mL or 4-dram.
3. Medicine droppers.
4. Disposable plastic syringes, 1-cc
with 16, 18 or 20 gauge needles.
5. Syringe, 10- and 25-uL.
6. Weighing pipet (optional).
Reagents
Karl Fischer reagent or Karl Fischer
pyridine-free reagent Reagents are available from the manufacturer of the
apparatus.
Instrument preparation
Prior to sample analysis the analyst
must become familiar with the operation of the titrator which differs slightly depending
on the manufacturer of the apparatus.
1. Prepare and calibrate the
apparatus as specified in the operating manual supplied by the manufacturer.
This generally includes the following steps:
a.Clean and assemble the titration
chamber.
b.Introduce a volume of anhydrous
titration solvent into the titration vessel. This solvent is normally ethylene
glycol- pyridine and is supplied by the manufacturer.
c.Titrate the solvent to the
instrument end point with Karl Fischer reagent. This insures that all water due
to the solvent and due to moisture on the surfaces of the apparatus is
consumed.
d.Now standardize the Karl Fischer
reagent.
2. Standardize the Karl Fischer
reagent each day. Inject exactly 10 uL or exactly 25 uL of water below the
surface of the titration solution. Larger volumes of water are undesirable
because they require large volumes of titrant which fills the vessel so that
the solvent has to be poured out and new solvent introduced. A weighing pipe
may also be used to introduce the water.
3. Put the instrument in the TITRATE
or RUN position and record the volume of titrant used when the END light comes
on or a message appears that the titration is complete.
Property of the Pine Chemicals
Association. All Rights Reserved.
Reproduction Restricted to Personal
Use.
Moisture content of tall oil / 24. Calculate the weight of water in milligrams per mL of
titrant. This is the equivalency factor and will be used later in the
calculation of the water content of the sample:
F =where:
F = equivalency factor, mg of water
per mL of titrant
A = weight of water used, mg (10 or
25)
B = volume of Karl Fischer titrant
used, mL
Procedure
Add the sample to the titrator after
the standardization has been completed.
1. Put approximately 4 mL of tall
oil into a small sample vial. Do not allow the sample to touch the outside of
the vial.
2. Put a medicine dropper into the
vial and weigh the vial, sample and medicine dropper together on an analytical
balance to the nearest 0.001 g. Record this weight.
3. Using the medicine dropper,
introduce 0.5 to 1.0 mL of the sample into the titrator.
4. Put the medicine dropper back
into the vial and weigh to the nearest 0.001 g. Record this weight.
5. Begin the titration and record
the end point in mL as before.
6. Calculate the water content of
the sample as shown below.
Calculation
Moisture, %v = (V x F)
10 x W
where:
V = volume of Karl Fischer reagent
required to titrate the sample, mL
F = equivalency factor, mg of water
per mL of titrant
W= weight of sample added to the
titrator, g
Report to the nearest 0.01%.
Alternate methods
PCTM 4A, PCTM 4B
References
AB
ASTM D890 "Water in Liquid
Naval Stores."
ASTM D1364 "Water in Volatile
Solvents."
Property of the Pine Chemicals
Association. All Rights Reserved.
Reproduction Restricted to Personal
Use.
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