In measured observations, odor determinations are much
to be preferred to taste determinations. There is no
method for measuring tastes quantitatively.
The threshold odor test is the most widely used
method of determining odor levels. It consists of
comparing different dilutions of the sample (diluted
with odor-free water) to an odor-free standard. The
dilution at which the odor can just be detected is called
the threshold point. The odor at the threshold point is
expressed quantitatively by the threshold number. This
is simply the number of times the odor-bearing sample
is diluted with odor-free water. For example, if odor-
bearing water requires dilution to ten times its volume
with odor-free water to make the odor just perceptible,
its threshold number will be 10, A more concentrated
odor-bearing water will require dilution to 100 times
its volume to make the odor just perceptible; its
threshold number will be 100. Here are some basic
principles of measuring odor values consistently.
1. Some practice with the test is desirable to
develop consistent threshold sensitivity. The
consistency can be developed readily in most
individuals. An acute sense of smell is not essential.
2. An adequate supply of freshly prepared odor-
free water must be available before starting the test.
3. All glass must be clean and free of odor. Rinse
all glassware several times with odor-free water before
each test and between dilutions.
4. Tests should be run in a room free from foreign
odors. Odors caused by fresh paint, volatile solvents,
tobacco smoke, food, and the like, will decrease the
accuracy of the observations.
5. Each dilution should be compared with the
odorless standard to check judgment and minimize
reliance on odor memory.
The following items of equipment are needed to
carry out the threshold odor test:
Six 500-ml Erlenmeyer flasks with ground glass
Two thermometers (0°C-110°C)
One 250-ml graduated cylinder
One 100-ml graduated cylinder
One 50-ml graduated cylinder
One 25-ml graduated cylinder
One 10-ml Mohr pipette
One large hot plate
One odor-free water generator (fig. 7-17)
Several large flasks for collecting and heating
In carrying out the threshold test, determine first
the approximate range of the threshold odor number.
Carefully follow these steps.
1. Add 250-ml, 63-ml, 16-ml, and 4-ml portions of
the odor-bearing water to separate 500ml glass-
stoppered Erlenmeyer flasks.
2. Dilute the last three to 250 ml with odor-free
3. Add 250 ml of odor-free water to another flask
that will be the reference for comparison.
4. Heat the flasks to 140°F (60°C) on a hot plate.
5. Shake the odor-free flask, remove the stopper,
and sniff the vapors.
6. Do the same with the flask containing the least
amount of odor-bearing water and observe by
comparison whether it contains an odor, and, if so, what
type of odor. (See table 7-2.)
7. Repeat steps 5 and 6 and use the sample
containing the next higher concentration of the water
8. Continue the process until all dilutions have
9. Record which flasks contain an odor and which do
not. Experience will enable an operator to estimate the
Figure 7-17.Odor-free water generator.