Experiment #8 / Unit 1
Chemical Analysis by Acid-Base Titration
Introduction:
Titration is a common method of determining the amount or concentration
of an unknown substance. The method is easy to use if the quantitative
relationship between two reacting solutions is known. The method is particularly
well-suited to acid-base and oxidation-reduction reactions. Titrations are routinely
used in industry to analyze products to be sold. Many manufacturers are under
strict standards of quality control because their products are sold for public
consumption. In this experiment, we will analyze a number of commercial
products and, in some cases, test the validity of the information given on their
labels and/or the claims made in television commercials. The products to be
tested include antacid tablets, vinegar, fruit juice, and candy.
Outline:
I. Standardization of a base (NaOH) using a primary standard (KHP)
II. Standardization of an acid (HCl) with the standard base
III. Titration analysis of unknown acids and bases:
A. Antacid tablets (Tums vs. Rolaids)
B. Vinegar
C. Fruit juice (apple)
D. Sour Candy
I. Standardization of NaOH
Titrations permit the concentrations of unknown acids/bases to be
determined with a high degree of accuracy. In order to analyze unknown
acids/bases, we must have a "standard" solution to react with the unknowns. A
standard solution is one in which the concentration is known accurately. We will
first prepare a standard solution of NaOH. One way to prepare a standard
solution is to dissolve an accurately massed amount of the substance and dilute
it to a measured volume (like we did with the MnO4- solution in Expt. #1.6). In this
way, the concentration can be calculated exactly. However, it is usually
impossible to obtain NaOH of sufficient purity to use it as a primary standard. An
indirect method is more practical for obtaining a standard solution of NaOH. We
will prepare a solution of a approximate molarity and standardize it against a
primary standard of known purity.
A primary standard should:
 be of high purity
 remain unchanged in air during massing and remain stable during storage
 have a high molar mass to reduce massing errors
 react with the solution to be standardized in a direct, well-defined reaction
Potassium acid phthalate will serve as our primary standard. This is a large
molecule (KHC8H4O4) with a molar mass of 204.2 g/mol. Instead of writing the
whole formula, we abbreviate it as KHP, where "P" stands for the phthalate ion,
C8H4O42- (not for phosphorus). KHP is an acidic substance, with the ionizing
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hydrogen being set forward in the formula for emphasis. Therefore, KHP is
monoprotic and will react with NaOH in a simple 1 to 1 relationship according to
the following equation:
NaOH(aq) + KHC8H4O4(aq) ---> KNaC8H4O4(aq) + H2O(l)
Here is a structural representation of the ionization that occurs (notice that
they left out many lone pairs which is typical to reduce clutter):
After we standardize our base with KHP, we will use this standard base to
standardize our acid so that we will know the exact concentrations of both
solutions and can then easily titrate and analyze the unknowns.
Procedure:
1. Dilute enough 1 M NaOH to make 1 L of 0.1xx M NaOH. This will be your
titrant. Rinse a buret with water and then with a small amount of the NaOH
solution. Fill the buret with NaOH solution. Fill the buret tip by momentarily
opening the stopcock. Label this buret BASE. Read the initial volume.
2. Mass approx. 0.8xx g of KHP into a 250 mL Erlenmeyer flask. Add
about 100 mL of water and swirl the flask until the sample is dissolved. Add 3
drops of phenolphthalein indicator (colorless in acidic solution; pink in basic
solution).
3. Titrate the KHP solution with the base solution to be standardized.
Titration should proceed until the faintest pink persists for 30 sec. after
swirling. The color will fade upon exposure to the air (WHY?). After
completing a trial, breathe into the flask and swirl. What has happened?
4. Make duplicate determinations and calculate the average molarity of the NaOH.
For excellent work, the molarities need to be within 1% of one another (calculate it
as a % error).
Data (for each trial):
 mass of KHP in sample flask
 initial buret reading
 final buret reading
 error log
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Calculations:
 volume of NaOH required to neutralize the KHP
 molarity of NaOH for each trial
 avg. concentration
Question:
1. Explain why the pink color of a phenolphthalein endpoint will disappear after
exposure to atmospheric air (or the air we exhale). Include 2 balanced equations.
2. Explain why the high molar mass of KHP is an advantage to its being used as
a primary standard. Make up a scenario with numbers and % errors.
Now you are ready to standardize your acid with your known concentration of base.
II. Standardization of HCl Solution
Procedure:
1. Obtain about 50 mL of 0.1xx M HCl solution in a beaker. Rinse a second
buret with water and then a small amount of HCl. Fill the buret with HCl
solution. Fill the buret tip by momentarily opening the stopcock. Label this
buret ACID. Record the initial volume and then deliver approx. 10.xx mL (as
long as you know accurately how much you used) into a clean 250 mL
Erlenmeyer flask. Add about 100 mL of water and 3 drops of phenolphthalein
indicator to this sample flask.
2. Read the initial level in the standard NaOH buret. Titrate the acid with the
NaOH standard to the faint pink endpoint.
3. Repeat the titration with a second sample of HCl. The calculated molarities of
HCl should be within 1% of one another.
Data (for each trial):
 volume of acid sample
 initial buret reading
 final buret reading
 molarity of standard NaOH
 error log
Calculations:
 volume of NaOH used
 molarity of HCl for each trial
 avg. concentration of standardized HCl
We can now create an ACID buret to go along with our BASE
buret. This means that we’re ready to titrate any and every
unknown acid or base now that we’re armed with known
concentrations of both acid and base (standard solutions).
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III. Analysis of Unknown Acids and Bases
A. Analysis of Antacid Tablets
Procedure:
1. Obtain an antacid tablet and mass the whole tablet. Grind the tablet with
mortar and pestle. Mass two portions of approx. 0.6xx-0.7xx g each.
2. Add the samples to two 250 mL Erlenmeyer flasks. From your acid buret, add
approx. 60.xx mL (must be known accurately) of the standard HCl to the
antacid powder. Swirl the flask to dissolve the solid. All of it may not dissolve,
since there may be some insoluble components.
3. Add 4-5 drops of methyl orange indicator to the sample flask. The solution should
turn pink-red since the HCl was added in excess. If the solution is yellow, add
approx. 10.xx mL more acid, measuring the volume carefully.
4. The antacid has neutralized a portion of the acid. Our job is to figure out
exactly how much HCl was neutralized by titrating the acid left over with our
standard base. "Back-titrate" the excess acid in the flask with the standard
NaOH. As you approach the endpoint, add the base drop by drop. The
endpoint of the titration occurs when the solution turns from red to pale orange.
5. Repeat with the second sample. Now analyze the other brand.
Data (for each trial):
 mass of whole antacid tablet
 mass of antacid sample
 volume of HCl added
 concentration of standard HCl
 initial buret reading of NaOH
 final buret reading of NaOH
 concentration of standard NaOH solution
 error log
Calculations (for each trial):
 volume of NaOH required
 mmol of HCl added
 mmol of NaOH added
 mmol of acid neutralized by the antacid
 mmol of acid neutralized per gram antacid
 avg. mmol acid neutralized/g antacid
Data and Calculations (based on class data):
 class avg. mass of tablet for each brand
 class avg. mmol acid neutralized per gram antacid for each brand
 cost of bottle for each brand
 cost of antacid per gram (cents/g) for each brand
 best buy (cents/mmol acid neutralized)
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Questions:
1. Rolaids once ran a national advertising campaign in which it was claimed that
Rolaids consumes "forty-seven times its own weight of excess stomach acid".
Can this be true?!!
*Assume that stomach acid is 0.100 M HCl and that its density is 1.00 g/mL.
2. Rolaids is running a national advertising campaign in which it touts "Rolaids
Regular neutralizes 44% more acid than Tums Regular". Is this true?!
B. Analysis of Vinegar
Percent acidity is the basis for determining the legality of vinegar. The
percent acid in a sample is calculated as g/mL x 100. Vinegar is regulated by the
Food and Drug Administration (FDA) and must have a minimum of 5% acidity.
The acid involved is acetic acid.
Procedure:
1. Pipette 5.00 mL of an unknown vinegar sample into a 250 mL Erlenmeyer flask.
Add about 100 mL of water and 3 drops of phenolphthalein indicator. Titrate with
your standard base. Complete a duplicate determination.
Data (for each trial):
 volume of vinegar sample
 initial buret reading
 final buret reading
 error log
Calculations:
 volume of base required
 avg. volume of base
 the acidity of the vinegar expressed as molarity
 the acidity of the vinegar expressed as a percent (is the vinegar legal?)
C. Analysis of Fruit Juice (Apple Juice)
Procedure:
1. Pipette 10.00 mL of fruit juice into a 250 mL Erlenmeyer flask. Add
about 100 mL of water and 3 drops of phenolphthalein. Titrate with your
standard base. Due to the yellow color of the juice, the endpoint will
appear peach instead of pink. Complete a duplicate determination.
Data (for each trial):
 volume of fruit juice sample
 initial buret reading
 final buret reading
 concentration of standard base
 error log
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Calculations:
 volume of base required
 avg. volume of base
 the acidity of the juice expressed as molarity
 the acidity of the juice expressed as a percent
 the juice is fortified with vitamin C. Determine the percent acidity from
ascorbic acid versus the native acid.
Extension:
 build the acids (acetic, ascorbic, citric, malic, and tartaric) in ChemSite (see
separate instructions on web site). Include a ball & stick model, a space-filling
model, a Lewis Structure, and a structural model with functional groups
labeled (see me for example). Section 11.7 of your book discusses carboxylic
acids. Ascorbic acid is an exception in that it is not a carboxylic acid.
D. Analysis of Candy (Sweet Tarts, Shock Tarts, Warheads, etc.)
Procedure:
1. You determine your method. The candy must be a product that uses a single
acidulant for tartness.
Data (for each trial):
 ????
Calculations:
 percent acidity in the candy
Lab Report #1.8: (150 pts)
 title page (1 pt)
 abstract (12 pts)
 procedure (2 pts)
 data, calculations, and questions by section (net ionic equations under each
data table) (92 pts)
 extension (15 pts)
 class spreadsheet (2 pts)
 results and discussion - separate paragraphs for each part. Read about
R + D on lab report format handout and consider the following: variation in
duplicates, comparisons to class means and standard deviations, and comparison
to labels, claims, or legal standards? Remember that you have data for the
accuracy and precision of your pipetting from a previous lab. (26 pts)
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W.G. Rushin
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