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An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. Glucose is the analyte.

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CH 103: ACID-BASE TITRATIONS An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. – PowerPoint PPT presentation

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Title: An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. Glucose is the analyte.


1
CH 103 ACID-BASE TITRATIONS
  • An analyte is the substance that is being
    analyzed. For example, the concentration of
    glucose in blood is commonly analyzed by
    diabetics. Glucose is the analyte.
  • A titrant is a solution of reagent that reacts
    with the analyte. The concentration of this
    reagent is accurately and precisely known.
  • In a titration, incremental volumes of titrant
    are added to the analyte until the reaction is
    complete.
  • A buret is often used to measure the volume of
    titrant added to the analyte.

2
REQUIREMENTS OF A TITRATION
  • The reaction must be stoichiometric. For
    example, the net ionic equation for the reaction
    of potassium hydrogen phthalate (KHP,
    HOOC-C6H4-COOK) and sodium hydroxide (NaOH) is
    quantitative. Exactly 1 mole of KHP reacts with
    exactly 1 mole of NaOH.
  • HOOC-C6H4-COO- OH- ? -OOC-C6H4-COO- H2O
  • The reaction should be rapid.
  • The reaction should be specific that is, there
    should be no competing reactions. Systematic
    error caused by interferences must be eliminated
    or reduced. For example, HOOC-C6H4-COO- should
    be the only acid in the above reaction.
  • There should be a marked change when the reaction
    is complete. For example, there is a marked
    increase in pH when the above reaction is
    complete. This increase in pH can be observed
    with an indicator that changes color immediately
    after the reaction between the titrant and
    analyte is completed.

3
EQUIVALENCE POINT, END POINT, AND INDICATORS
  • The equivalence point occurs when the volume of
    titrant added to the analyte is the exact
    stoichiometric amount that is needed to bring the
    reaction to completion.
  • The end point occurs when the indicator changes
    color.
  • We want to measure the equivalence point. We
    actually measure the end point. We need to
    select an indicator that has the same end point
    as the equivalence point.
  • The indicator is added to the analyte. It is NOT
    added to the titrant.

4
EQUIVALENCE POINT, END POINT, AND INDICATORS
  • Titration using Phenolphthalein as an Indicator
  • When do you stop adding titrant to the analyte?
  • At the end point.

5
EQUIVALENCE POINT, END POINT, AND INDICATORS
  • The titration of 0.100 M acetic acid (CH3COOH)
    with 0.100 M NaOH has an equivalence point at pH
    8.73. What indicator should you use for this
    reaction?
  • Phenolphthalein.

6
STANDARDIZATION
  • Today we will standardize NaOH against potassium
    hydrogen phthalate (KHP, HOOC-C6H4-COOK).
  • KHP is a primary standard.
  • For example, 25.71 mL of a NaOH solution are used
    to titrate 1.095 g of KHP to a phenolphthalein
    end point. The molar mass of KHP is 204.2 g/mol.
    One mole of KHP reacts with 1 mole of NaOH.
    What is the molarity (M) of this NaOH solution?
  • Next week you will use your NaOH solution to
    determine the molecular weight of an unknown
    acid.
  • This NaOH solution is a secondary standard.

7
REQUIREMENTS OF A PRIMARY STANDARD
  • A primary standard should be 100.00 pure
    although a 0.01 to 0.02 impurity is tolerable
    if it is accurately known.
  • A primary standard should be stable at drying
    temperatures, and it should be stable
    indefinitely at room temperature. (A primary
    standard is always dried before weighing, unless
    it is a hydrate.)
  • It should be readily available.
  • It should have a relatively large formula weight.
    Therefore, a relatively large mass of it will be
    weighed for titration. This will reduce error.
  • Explain this last point.

8
TITRATION OF A STRONG ACID WITH A STRONG BASE
  • Where is the equivalence point?
  • At pH 7.
  • Where is the end point?
  • Between pH 8.0 and 9.5.
  • Is there a significant difference between the
    volume of titrant used to reach the equivalence
    and end points?
  • No. Therefore, phenolphthalein is an appropriate
    indicator for this titration.

9
TITRATION OF A STRONG BASE WITH A STRONG ACID
  • Where is the equivalence point?
  • At pH 7.
  • Could phenolphthalein be used as an indicator for
    this titration?
  • Yes.

10
TITRATION OF A WEAK ACID WITH A STRONG BASE
  • Where is the equivalence point?

11
TITRATION OF A WEAK BASE WITH A STRONG ACID
  • Where is the equivalence point?

12
STANDARD DEVIATION
  • Standard deviation is a measure of precision. It
    is used to quantify the agreement between
    repeated measurements of the same sample.

13
SAFETY
  • Give at least 1 safety concern for the following
    procedure.
  • Using HOOC-C6H4-COOK, NaOH, phenolphthalein, and
    an unknown acid.
  • These are irritants. Wear your goggles at all
    times. Immediately clean all spills. If you do
    get either of these in your eye, immediately
    flush with water.
  • Your laboratory manual has an extensive list of
    safety procedures. Read and understand this
    section.
  • Ask your instructor if you ever have any
    questions about safety.

14
SOURCES
  • Christian, G.D. 1986. Analytical Chemistry, 3rd
    ed. New York, NY John Wiley Sons, Inc.
  • Harris, D.C. 1999. Quantitative Chemical
    Analysis, 5th ed. New York, NY W.H. Freeman
    Company.
  • Traverso M. 2006. Titration using Phenolphthalein
    as an Indicator. Available www.chemistry.wustl.ed
    u/.../AcidBase/phph.htm accessed 14 September
    2006.
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