SALT SLEUTHS

PROBLEM:
You are a food scientist working for a company that manufactures various types of butter and margarine. When the federal government passed legislation requiring companies to report the amount of salt on labels, you were ask by the corporate president to test the company's products and determine the amount of salt present.

BACKGROUND INFORMATION:
The Nutrition Labeling and Education Act of 1990 has made nutrient and health claims on food labels legal under some conditions. Two minerals that are specifically identified as relating to nutrient and health claims are calcium and sodium. Sodium chloride content in foods can be determined by titrating the chloride ion with silver, and excess silver ions with potassium chromate. The end point occurs when all the chloride ions are complexed and the excess silver ions react with the chromate ion to form the orange color of silver chromate. The endpoint in this reaction is therefore at the first hint of an orange color. When preparing reagents for this assay, use boiled water to avoid interference from carbonates in the water.

Equation: Ag + + Cl - AgCl (until all Cl - is complexed)
2Ag + + CrO 4 2- Ag 2 CrO 4 (orange only after all Cl- is complexed)

OBJECTIVE:
To determine the NaCl content of butter using the Mohr titration method.

MATERIALS:

  • 5% solution of potassium dichromate
  • 0.1 N silver nitrate
  • Balance
  • Hot plate
  • Thermometer
  • Burette
  • 250 ml Erlenmeyer Flasks
  • Various types of butter and margarine

    SAFETY AND DISPOSAL:
    Potassium dichromate should be handled with care in the fume hood. Avoid contact with skin or clothing. Use of safety gloves is mandatory while handling the solution. Avoid inhalation, which can cause respiratory irritation.

    TIME:
    1 class period

    PROCEDURE:

    1. Accurately weigh 5g ( ± 10 mg) of butter into a 250 ml Erlenmeyer flask, in duplicate.
    2. Add 100 ml of boiling water. Let stand, swirling occasionally, for 5-10 minutes while cooling to 50-55C.
    3. Add 2 ml of potassium dichromate indicator, and titrate with 0.1N silver nitrate until the orange brown color persists for 30 seconds.
    4. Calculate % NaCl

      % NaCl = ml of AgNO 3 solution x 0.585
      % NaCl = ml g of sample (butter)

      [0.585 = (58.5 g /mole for NaCl)/100]

    EXTENSIONS:

  • Have students bring in various types of butter to determine the salt content of various brands.
  • Have students compare their results to the package information, if given. They could also calculate their percent error.
  • Have the students devise methods such as these for other products (for example, potato chips).
  • Have students compare butter with margarine.

    QUESTIONS:

    1. Express the data in ppm NaCl and moles NaCl/gram of butter.
    2. Was margarine or butter higher in NaCl? Why?
    3. What other types of food products could use this method to determine salt content?

    FOR ADDITIONAL INFORMATION CONTACT:
    Dr. Andrew Proctor

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