AP Chemistry
NAME:
AP Lab Investigation #4: How much acid is in fruit juices and soft drinks?
Record all procedures, data, question answers, and conclusions in your lab notebook.
Introduction:
Common beverages may be either acidic or basic. Fruit juices, for examples, get their sweet taste
from sugars and their sour or tart taste from weak acid such as citric acid. If the juice contains too
much sugar, it will taste bland, but too much acid and the juice will taste sour. The concentration of
acid in various consumer beverages may be determined by titration with a known concentration of
sodium hydroxide. Titrations are used to quantitatively analyze the unknown concentration of a
solution or the amount of a substance by comparing it to a solution of known concentration.
Background Information:
The main acids present in fruits and fruit juices are critic acid (in citrus fruits), tartaric acid (in grapes),
and malic acid (in apples). All of these are characterized as weak acids.
Figure 1. Organic Acid in Fruit and Fruit Juices. Citric acid (left), Tartaric acid (middle), Malic acid (right)
The amount of citric acid in citrus fruit juices can be determined by titration with a standard solution of
sodium hydroxide. A standard solution is one whose concentration is accurately known, usually to
three significant figures. Citric acid is a tricarboxylic acid - it has three ionizable or "active" hydrogen
atoms in its structure. One mole of citric acid therefore reacts with three moles of sodium hydroxide
via the acid-base neutralization shown in Equation 1.
Acid-base titrations are an extremely useful technique to determine the concentration of an acid or
base in a sample. In titrating beverages such as orange juice, apple juice, and sodas that contain
weak acids, the juice is called the analyte and a strong base is used as the titrant.
In the titration procedure, a sodium hydroxide solution of known molarity is carefully added using a
buret to a measured volume of fruit juice containing an indicator. The exact volume of sodium
hydroxide that must be added to reach the indicator endpoint is measured and then used to calculate
the concentration of citric acid in the juice.
The sodium hydroxide used for this lab was standardized with KHP (potassium hydrogen phthalate).
This was done because sodium hydroxide is hydrophobic. Sodium hydroxide will pick up water
molecules making it difficult to measure its mass accurately. To standardize, a solution of sodium
hydroxide is made as accurately as possible then this solution is titrated with a KHP solution to
determine the concentration more accurately.
A sample set up for a titration is shown in to right, where a buret containing the
titrant is clamped to the support stand and a beaker or flask containing the analyte
is set atop a stir plate. If a pH probe is inserted into the solution, a titration curve
can be constructed by plotting the pH of the solution on the y-axis versus the
volume of titrant added on the x-axis. The shape of the titration curve may be used
to distinguish strong and weak acids in the analyte, and also permits graphical
analysis of the equivalence point. At the equivalence point, moles of the added
titrant are stoichiometry related to moles of analyte in the sample.
Choosing a suitable indicator for a titration is important for accurate results. Indicators signify the
endpoint of a titration when a sudden change in the color of the analyte solution occurs. Indicators
have different pH transition ranges and exhibit different colors in acidic versus basic solutions. The
color changes arise because indicators are weak acids for which the acidic form HIn and the
conjugate base In- have different colors. An appropriate indicator for a titration is one whose color
change occurs close to the theoretical pH of the equivalence point. Examples of indicators provided
in this activity are shown in the following table:
Pre-Lab Questions:
1. Write the complete chemical equation for the reaction of a solution of sodium hydroxide and
hydrochloric acid.
2. How many mL of 0.1 M HCl are required to react completely with 5 mL of 0.1 M NaOH?
3. If equal molar amounts of NaOH and HCl are mixed, when the reaction is completed what will
be the chemical species in the resulting solution?
4. What will be the pH of the mixture in question 3, acidic, neutral, or basic? Explain.
5. Write the complete chemical equation for the reaction of a 0.1 M solution of acetic acid
(CH3COOH) with a 0.1 M solution of NaOH?
6. How many mL of the 0.1 M NaOH solution will be required to react completely with 5 mL of a
0.1 M acetic acid solution? Explain.
7. When the reaction is complete what will the pH (acid, neutral, basic) of the solution in question
6 be? Explain.
8. A 10.0 mL sample of pineapple juice was titrated with 0.100 M sodium hydroxide solution. The
average volume of NaOH required to reach the endpoint was 12.8 mL
a. Calculate the number of moles of sodium hydroxide required to reach the endpoint.
b. Using the mole ratio for the neutralization reaction found in equation 1, determine the
number of moles of citric acid in 10.0 mL of pineapple juice.
c. Multiply the number of moles of citric acid by its molar mass to calculate the mass of
citric acid in 10.0 mL of the juice.
d. The concentration of acid in juices is usually expressed in grams of acid per 100. mL of
juice. What is the concentration of citric acid in pineapple juice?
9. How is it possible to determine when an acid-base reaction is complete when the
concentration of one of the reactants is unknown? In other words, how do we know if we have
reached the endpoint of the titration?
10. Using the table provided in the introduction, explain how indicators are chosen and used
during titrations.
11. The titration curves for hydrochloric acid and acetic acid with sodium hydroxide are shown
below. Distinguish between the strong and weak acid in terms of the initial pH, the pH at the
equivalence point, and the overall shape of the titration curve.
Experiment Overview:
The purpose of this inquiry lab is to conduct acid-base titrations and determine the concentration of
acid in common beverages such as orange juice or pineapple juice. The beverages contain weak
acids, which will be titrated with a strong base, sodium hydroxide (NaOH). We will use the titration
that we performed last week as a model to design a titration procedure that will allow us to obtain a
titration curve data and calculate the molar concentration of acid in a beverage. The titration curve will
be analyzed and the amount of acid in a typical serving size or bottle may also be determined. The
identity of the acid in the beverage may be derived by reviewing the titration curve and reference
information, and by consulting the ingredients label.
In summary, you will determine a question about acidity of beverages that you would like to
investigate. Your task is to write a procedure that will allow you to determine the concentration of
acid in a beverage and answer your research question. You will need to collect enough data points
that you can also construct a titration curve for the titration you perform.
Question: (what is the question that you will be researching?)
Materials Available:
General lab equipment (beakers, flasks, burets, volumetric pipets, graduated cylinders)
Acid-base indicators: methyl orange, bromothymol blue, phenolphthalein
pH meters
Various fruit juices
Standardized 0.10 M NaOH and 0.25 M NaOH
Procedure:
Determine a procedure to answer your question and record it in your lab notebook.
Data:
Construct data tables in your lab notebook
Post-Lab Tasks and Questions
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Determine the acidity of each of the beverages you examined.
Construct a titration curve for at least one of the titrations you performed.
Were you able to answer your research question? What conclusion did you draw?
Suppose a fellow student chose to measure solution volumes using the beakers or graduated
cylinders provided. What effect would this have had on calculated acid concentration? How
might this affect the number of significant figures in your final answer? Explain your answers.
A fellow student rinsed the buret with water, but neglected to rinse the buret with titrant before
conducting the experiment. What effect would this have on the calculated acid concentration
in the beverages? Why?
What problem would arise if you titrated a cola (brown in color)? What would you do to be able
to see the endpoint of the titration?
Imagine that a lab team consistently added base past the first appearance of a pale pink color.
What would happen to the average calculated acid concentration of the beverages? Explain
your answer.
Sketch a titration curve and at each of the following points draw a figure of what is happening
at the molecular level (in other words, draw a picture of all the species present)
a. When 0 mL of base is added
b. When 5 mL of base is added
c. At the equivalence point
d. When an excess of base has been added
Please, write a conclusion for this lab. Write this conclusion as if you were writing it as part of
a lab report. Please, type your conclusion and include it in your lab notebook. A rubric for this
conclusion is below.
Minimum length of two to three paragraphs (5 points)
Professional tone
Summary of entire lab (12 points)
Brief description
Your results described
Keq given (actual value)
Problems / sources of error (specific)
Connection back to the purpose of the experiment (5 points)
Explanation of the science theory behind the experiment (10 points)