2.3.1 Toluene Determination of Moisture
References:
Moisture in Animal Feed by Distillation with Toluene (925.04)
Official Methods of Analysis. 1990. Association of Official
Analytical Chemists. 15th Edition.
Dewar, W.A. and P. McDonald. 1961. Determination of dry matter in
silage by distillation with toluene. J. Sci. Agric.
12:790-795.
Fenner, Heinrich. 1984. Methods for determining fermentation
products in acid preserved feeds and forages. Massachusetts
Agricultural Experiment Station. Res. Bull. 691. 14pp.
Scope:
This procedure is recommended for determination of moisture in
fermented feeds (haylages and silages) which contain high levels
of volatile acids. Volatile acids and alcohols have low
vaporization points and are lost when the sample is oven dried.
Basic Principle:
Water is distilled from sample and trapped under a layer of
toluene.
Equipment:
250 mL pyrex flask Bidwell-Sterling Moisture Receiver, calibrated
to 0.01 mL by distilling known amounts of water into graduated
column and estimating column of water to 0.01 mL. Clean tube and
condenser with chromic acid cleaning mixture, rinse thoroughly
with water, then alcohol, and dry to prevent undue amount of
water from adhering to inner surface.
- 500 mm Liebig condenser
- Toluene distillation hood
Reagents:
Toluene, reagent grade
Safety Precautions:
- Use standard precautions when working with electrical
equipment or glassware.
- Toluene is flammable. Observe proper precautions for
flammable solvents.
- Avoid inhaling vapors.
- Avoid skin contact.
Procedure:
- Weigh sufficient wet sample (at least 25g ) to yield a
minimum of 5 mL water, record weight (W1) to nearest 1
mg, and transfer to 500 mL flask.
- Add sufficient toluene to cover sample completely.
- Immediately fill receiving tube with toluene, pouring it
through top of condenser.
- Bring to boil and distill slowly, ca. 2 drops/sec, until
most of the water passes over, then increase rate of
distillation to about 4 drops/sec. Distill 1hr (or longer
if necessary for wetter samples) to obtain clearing at
the top of the condenser.
- When all water has apparently been distilled, wash down
condenser by pouring toluene in at top and continue
distillation for a short time (approximately 15 min) to
ensure all water is distilled. If 0.1 mL of additional
water is distilled in 15 minutes, repeat this step.
- If water remains in the condenser after distillation is
completed, wash it down with toluene.
- Let receiving tube come to room temperature and read
volume of water in lower layer of receiver and record
volume (V) to nearest 0.01 mL.
Comments:
- To be accurate, distilled water should be analyzed for
volatile acids and alcohol that can co-distill with water
(see references).
- Toluene dry matter is approximately equivalent to partial
drying (section 2.2.1.1
or 2.2.1.2) which
leaves 3 to 5% moisture in the sample (which is roughly
equivalent to volatile acids lost during drying) and is
adequate for dry matter adjustment of crude protein and
fiber.
Calculation: Percent Total Dry Matter
é V ù
% Total DM = 1 - ----- X 100
ë W1 û
- Where V = Volume of water in mL
- W1 = weight of sample in grams
Calculation: Percent Total Moisture:
% Total Moisture = 100 - % Total DM
Quality Control:
Include at least one set of duplicates in each run if single
determinations are being made.
An acceptable average standard deviation among replicated
analyses for moisture or dry matter is about ±0.10, which
results in a warning limit (2s) of about ±0.20 and a control
limit (3s) of about ±0.30. Plot the results of the duplicate
analyses on an R-control chart (Appendix D) and examine the chart
for trends. Results outside the 95 percent confidence limits warn
of possible problems with the analytical system. Results outside
the 99 percent confidence limits indicate loss of control, and
results of the run should be discarded. If more than five or six
points in succession fall on one side or the other of the 50
percent line, it is a strong indication that something has
changed and is cause for investigation.
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