AGENT ACID MEETS BASE THE BULLY
PROBLEM:

One early morning you get up, get ready for work, go out to your car and find that it won't start. You notice that your battery cables appear to be corroded. Being a food chemist, you know certain household products contain acid, which can be helpful in removing the buildup from the cables. However, you only have three products at your disposal: vinegar, lemon juice and 7-Up®. You must determine which product has the highest acidity level.

BACKGROUND INFORMATION:

A standard acid is a solution of precise normality made from a pure, dry, accurately weighed organic acid (usually KHP, potassium acid phthalate.) A standard base is an alkali solution whose normality has been precisely determined by titration against a standard acid. Standard acids are more stable than standard bases because atmospheric carbon dioxide does not dissolve in standard acids.

Often it is necessary to determine the titratable acidity of a food product. One example is measuring the maturity of a fruit. A base standardized from a standard acid is used to accurately measure the amount of acid in a food product.

There is a close relationship between measuring pH and measuring titratable acidity, but they are not the same. Titratable acidity is a measure of the total acidity in a sample, both as free hydrogen ion and as hydrogen ions still bound to undissociated acids. In contrast, pH is the hydrogen ion concentration, expressed on a logarithmic scale, of the free of dissociated hydrogen ions only.

The techniques needed to measure pH and titratable acidity are quite different. A hydrogen ion sensitive electrode connected to a digital or analog meter measures pH. Titratable acidity is measured by careful additions of a standardized base to the sample until a predetermined end point is reached. The endpoint may be an indicator color change at a particular pH. The color change at pH 8.1 for phenolphthalein is commonly used. A pH meter can be used as well.

For more information about pH and food chemistry see Appendix D.

OBJECTIVE:

  • To prepare a standard acid (0.1N KHP)
  • Determine the precise normality of or standardize a base by titration with the standard acid.
  • Determine the titratable acidity of vinegar, lemon juice and 7-Up®.
  • Determine the pH of vinegar, lemon juice and 7-Up®.

    MATERIALS:

  • Potassium Acid Phthalate (KHP)
  • Distilled water
  • 50 ml Volumetric Flask
  • Sodium Hydroxide pellets
  • Carbon dioxide free water
  • 1 L Volumetric Flask
  • 10 ml Pipette
  • Phenolphthalein Indicator
  • Lemon juice
  • Vinegar
  • 7-Up ®
  • Hot plate
  • pH meter
  • 500 ml Beaker
  • Burette

    SAFETY AND DISPOSAL:

    In preparing the dilute HCl be careful to avoid spilling any of the concentrated acid on your skin or clothes. ALWAYS ADD ACID TO WATER. If you do accidentally spill a concentrated acid or base, flush the area immediately with large amounts of water. Never try to neutralize the spill with base or acid because heat would be generated and the burn would be aggravated.

    TIME:

    Approximately one class period

    PROCEDURE:

    1. Prepare a Standardized Base ( 1 liter of 0.1 N NaOH)

      TEACHER NOTE: A saturated NaOH solution can be prepared by mixing equal weights of NaOH pellets and distilled water. The saturated NaOH should stand for several days to allow contaminating carbonates to precipitate. Only the clear carbonate free solution should be diluted for the standard solution.

      1. Obtain a saturated solution of NaOH from your instructor. Measure 5.4 ml (measure with a graduated cylinder, do not pipette) and dilute to one liter. Mix well and transfer to a storage bottle.
      2. Since NaOH is extremely hygroscopic and cannot be dried to a constant weight, potassium acid phthalate (KHP) is used to standardize the basic solution. Weigh exactly 0.5 grams of the previously dried KHP into each of two Erlenmeyer flasks. Dissolve the acid using 30 ml of distilled water. The molecular weight of potassium acid phthalate is 204.23 grams/mole.
      3. Add two drops of phenolphthalein solution to the acid.
      4. Titrate the acid in duplicate samples to the end point using your standard base. The endpoint is reached when the faintest pink color cannot be swirled away. For more information about how to set up a titration apparatus, see Appendix D.
      5. Calculate the precise normality of the base as follows:

        Knowing:

        1. The moles of acid titrated.
        2. At the equivalence point, molesof acid are equal to moles of base.

          N base x L base = g KHP x 1 mol KHP
          N base x L base = g KHP x 204.23 g KHP

          N = equivalents/L and for NaOH N = M = moles/L

          N base = 1000ml x 0.5g KHP x 1 mol KHP
          N base = 10 1L ml x 0 ml base Px 204.23 g KHP

      6. Label the standardized base carefully with the normality (average of two samples), the date and your initials.

    2. pH and Titratable Acidity
      1. Measure the pH of lemon juice, vinegar and 7-Up® using a pH meter.
      2. Titrate 5 ml of lemon juice (in duplicate samples) with your standard base to a phenolphthalein end point. Record ml of base used.
      3. Titrate 5-ml vinegar (in duplicate samples) with your standard base to a phenolphthalein end point. Record ml of base used.
      4. Boil approximately 350 ml of 7-Up® to remove carbon dioxide. Cool it to room temperature. Measure the pH of the cooled 7-Up®. Titrate 5 ml of the cooled 7-Up® solution with your standard base to a phenolphthalein end point. Record ml of base used.

        Calculate the normality of your three household acids using the following equation: Milliliters of acid x N acid = Milliliters of base x N of base

    QUESTIONS:

    1. Was the pH of the 7-Up® different before and after heating?



    2. Why is carbon dioxide contamination a problem with standardized base but not standardized acid?



    3. Which of the three household acids is the strongest according to pH?



    4. Which of the three household acids is strongest according to titratable acidity?



    5. Which acid would you use to remove the buildup on your battery cable? Why?



    FOR ADDITIONAL INFORMATION CONTACT:

    Dr. Andrew Proctor

    REFERENCES:

    Nielsen, S. 1998, 2 nd ed. Food Analysis. Aspen Pub. ISBN 0-8342-1203--X

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