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Potentiometric Titration
of Unknown Acids with a pH Meter |
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Introduction In this experiment we will
determine the identification and concentration of a series of acids using a
pH meter. Determining acid or base
concentration with a pH meter is called potentiometric titration. In an
acid-base titration, the important information to obtain is the equivalence
point. If there are a given number of moles of acid in the titration
flask, the equivalence point is reached when that same number
of moles of base have been added from the buret.
The molarity of the base can then be calculated
since the number of moles of base added is the same as the number of moles of
acid in the flask, and the volume of the base added is also known. Similarly,
if the number of moles of acid in the titration flask is unknown, it can be
calculated for the equivalence point if the molarity
of the base and the volume of base added are known. Often the pH
of the solution will change dramatically at the equivalence point. An
acid-base indicator works by changing color over a given pH range. If an
indicator which changes color near the equivalence point is chosen, there is
also a dramatic change in the color of the indicator at the equivalence point
because the pH changes so rapidly. In a potentiometric acid-base titration, an indicator is not
necessary. A pH meter is used to measure the pH as base is added in small
increments (called aliquots) to an acid solution. A graph is then made with
of pH versus volume of base added. |
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Introduction In this experiment we will
determine the identification and concentration of a series of acids using a
pH meter. Determining acid or base
concentration with a pH meter is called potentiometric titration. In an
acid-base titration, the important information to obtain is the equivalence
point. If there are a given number of moles of acid in the titration
flask, the equivalence point is reached when that same number
of moles of base have been added from the buret.
The molarity of the base can then be calculated
since the number of moles of base added is the same as the number of moles of
acid in the flask, and the volume of the base added is also known. Similarly,
if the number of moles of acid in the titration flask is unknown, it can be
calculated for the equivalence point if the molarity
of the base and the volume of base added are known. Often the pH
of the solution will change dramatically at the equivalence point. An
acid-base indicator works by changing color over a given pH range. If an
indicator which changes color near the equivalence point is chosen, there is
also a dramatic change in the color of the indicator at the equivalence point
because the pH changes so rapidly. In a potentiometric acid-base titration, an indicator is not
necessary. A pH meter is used to measure the pH as base is added in small
increments (called aliquots) to an acid solution. A graph is then made with
of pH versus volume of base added. |
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Objectives 1. To perform a potentiometric
titration of an acidic solution of known molarity. 2. To graph the volume of base added vs the pH and to determine the equivalence point. 3. To calculate the molarity of the
basic solution. Procedure 1. Obtain about 100 mL
of 0.1 M NaOH solution. 2. Obtain 100 mL
of one of the unknown acid solutions in the 250 mL
beaker. 3. Place the pH electrode and the
stirring bar in the unknown acid solution and start the solution stirring (the
solution should NOT vortex nor should the stirring bar strike the pH electrode). 4. Record the pH from the meter. Add a 5 mL
aliquot and again record the pH.
Repeat until the recorded pH is greater than 9.0. 5. Perform the titration on the other
two unknowns. 6.. Prepare
a table and a graph of your results. Use
the shape of the titration curve and the equivalence point to determine the concentration and the
identification of the acid. |
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Questions 1.
Describe how the pH electrode works. Cite your source. 2. Draw a
titration curve for phosphoric acid.
Label each of the equivalence
points and each of the pKa’s. 3. Sulfuric
acid is used in the manufacture of of many
chemicals and lead acid storage batteries.
Describe how it is made and how many metric tons were produced in
2009. Cite your source. 4. Nitric
acid is used to produce explosives and fertilizer. Describe how it is made. 5. Draw a
titration curve for the amino acid glycine. Label each of the equivalence points and each of the pKa’s. Extra
Credit: Proteins are composed of amino
acids, some of which contain either acid or base R-groups. If a protein contained twice as many lysines as aspartic acids and no other ionizable R-groups, what would be the expected pH of a
solution of the protein dissolved in pure water? |
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