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.

Equipment

The following items of equipment are needed to carry out the threshold odor test:

Six 500-ml Erlenmeyer flasks with ground glass stoppers

Two thermometers (0C-110C)

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 odor-free water

PROCEDURES

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 water.

3. Add 250 ml of odor-free water to another flask that will be the reference for comparison.

4. Heat the flasks to 140F (60C) 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 sample.

8. Continue the process until all dilutions have been observed.

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.