Olfactory Titration



[This laboratory exercise was adapted from an article in the March 1996 edition of the Journal of Chemical Education, page 257, by John T. Wood and Roberta M. Eddy.]

The endpoint of an acid base titration is usually signaled by a visual indicator. Two of the most common visual indicators are phenolphthalein and bromthymol blue. However, visual indicators are not the only means to signal the endpoint of a titration. Just as a visual indicator changes color in response to a change in the pH of a solution, an olfactory indicator will change either its odor or odor intensity as the pH changes. Vanillin and eugenol have previously been identified as olfactory indicators. The purpose of this experiment is to investigate the odor of raw onion for its usefulness as an acid base indicator.

The odor and flavor of raw onions are due to sulfur containing compounds. Many compounds are present but the principle ones are: 1-propenyl thiosulfinate, propyl thiosulfinate, and methyl thiosulfinate.

The experiment will consist of three parts. The first part will attempt to identify which solution, acid or base, causes the odor of onion to be most intense. The second part will consist of extracting the indicator from onions. The actual titration will be done in the third part.

Procedure Part A: Indicator Investigation

  1. Contaminate three pieces of cloth with the odor of raw onion. This can be accomplished two ways. One method is to rub each piece of cloth with a fresh raw onion until the odor is very strong. The second method is to place three pieces of cloth in a sealed plastic bag containing chopped onion in a refrigerator overnight.
  2. Fill three 400 mL beakers so that one beaker contains 200 mL of 0.1 M NaOH, another contains 200 mL of distilled water, while the third contains 200 mL of 0.1M HCl. Place one of the pieces of cloth prepared in step 1 above into each beaker. Stir with a stirring rod for 3 minutes.
  3. With forceps lift out each piece of cloth and rinse with distilled water to remove traces of NaOH and HCl. Smell each piece of cloth and note the presence or lack of the odor of onion.

Procedure Part B: Indicator Extraction

  1. Place about one-teaspoon of chopped onion in about 30 mL of the base to be titrated (0.1 M NaOH). Let the chopped onions soak for about 10-15 minutes with occasional stirring.
  2. Filter using a "fast" filter paper like Whatham #1.

Procedure Part C: The Titration

  1. Fill a buret with 0.1 M HCl. Accurately measure out exactly 10.00 mL of the onion-indicator-laden (OIL) base prepared in part B above into a Erlenmeyer flask. Using your observations in part A above, titrate until the correct olfactory endpoint is reached.
  2. To a second sample of the OIL base add 2 drops of phenolphthalein. Repeat the titration until the visual endpoint of phenolphthalein has been reached. (Note: Comparisons will be easier if you make sure that the volume of base used in each titration is the same.)


  1. Determine the molar concentration of the base for each titration.
  2. Determine the class values listed, determine the average concentration of the base for each indicator.
  3. Compare the relative effectiveness of the olfactory indicator to the standard visual indicator, phenolphthalein. (You should mention in your discussion not only whether the olfactory indicator is useful, but the relative error, if any, to be expected in the procedure.)
  4. Lets assume that in step 2, part C untreated base was used instead of the OIL base. Why would a comparison now be more difficult?

Questions? Comments??