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.
1CH 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.
2REQUIREMENTS 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.
3EQUIVALENCE 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.
4EQUIVALENCE POINT, END POINT, AND INDICATORS
- Titration using Phenolphthalein as an Indicator
- When do you stop adding titrant to the analyte?
- At the end point.
5EQUIVALENCE 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.
6STANDARDIZATION
- 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.
7REQUIREMENTS 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.
8TITRATION 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.
9TITRATION 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.
10TITRATION OF A WEAK ACID WITH A STRONG BASE
- Where is the equivalence point?
11TITRATION OF A WEAK BASE WITH A STRONG ACID
- Where is the equivalence point?
12STANDARD DEVIATION
- Standard deviation is a measure of precision. It
is used to quantify the agreement between
repeated measurements of the same sample.
13SAFETY
- 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.
14SOURCES
- 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.