Skills to build:

Using a burette

Using a micropipette

Using acid-base stoichiometry

Using titration to determine unknown acid concentration
Introduction
Acids and bases chemically react with each other by transferring a proton from the acid to the base. This is known as a neutralization reaction. A typical neutralization reaction involves an acid and a hydroxide base. The products are water and a salt. For a monoprotic acid (HA) and a Group 1 metal hydroxide (MOH), the general reaction is
HA(aq) + MOH(aq)
H O(l) + MA(aq)
The following shows the molecular reaction between nitric acid (HNO
3
) and OH
water and the nitrate ion (NO
3
-
).
to give
Neutralization reactions are used in the laboratory to determine the amount of acid or base in a chemical sample. This experimental procedure is referred to as a titration.
Acid (or base) of known concentration is added to the chemical sample containing a base (or acid) of unknown concentration until the equivalence point signals complete neutralization. At the equivalence point, the number of equivalents of acid (or base) used to complete the titration equals the number of equivalents of base (or acid) in the

Titration of Citric Acid chemical sample. For an acid, one equivalent is equal to one mole of H
3
O
+ base, one equivalent is equal to one mole of OH
−
. Oxalic acid, H
2 or H
+
. For a
C
2
O
4
, contains two protons (H
+
) in its chemical formula and furnishes two equivalents or moles of acid.
Sodium hydroxide, NaOH, contains one hydroxide ion in its chemical formula and furnishes one equivalent or mole of base.
In a titration, an indicator is employed to signal the equivalence point. The indicator is usually a weak organic acid or base which changes color near the pH of the equivalence point. If HIn represents the general formula for an indicator, then the ionization reaction is
HIn(aq) + H
2
O(l)

H
3
O
+
(aq) + In-(aq)
During a titration the solution changes from acidic to basic (or basic to acidic), and, according to Le Châtelier's principle, the above equilibrium shifts from left to right (or right to left). The solution changes from the color of HIn (or In-) to the color of In- (or
HIn). The faint appearance of the color associated with In- (or HIn) signals that the neutralization of the acid (or base) is complete.
Citric acid is a tricarboxylic acid that is widely distributed in plants, especially citrus fruits, and animal tissue and fluids. The three carboxylic acid groups are shown in red below. Lemon and grapefruit juices contain 5 to 8 m/v% and 1-2 m/v% citric acid, respectively. Citric acid is a weak acid with K a
values of 8.2 x 10
-4
for the removal of
O H
O
C
O H
O H
C
C
C
O
H
C
2
OH
C
H
2
O
citric acid the first proton, 1.77 x 10
-5
for the second, and 3.9 x 10
-6
for the third. The complete neutralization of citric acid requires three moles of base per mole of acid.
COOH COO
-
Na
+
HOOC H
C
2
C
OH
C
H
2
COOH
+ 3 NaOH Na +
-
OOC H
C
2
C
OH
C
H
2
COO
-
Na
+
+ 3H
2
O
2

Titration of Citric Acid
In this experiment, a solution of NaOH of known molarity is used to titrate a sample of citrus juice to determine the molarity and m/v% of citric acid. The molarity is calculated from the stoichiometry of the neutralization reaction above. The molarity is then converted to m/v%. The indicator of choice will be phenolphthalein, which is colorless in acidic solution and pink in basic solution.
3D Molecules
(Source: http://homepages.ius.edu/dspurloc/c121/week11.htm)
color in acid color at endpoint too much base
Go to the web site and click on the Citric Acid and Phenolphthalein links. Use the radio buttons to show different views and properties of each molecule. Answer the questions based on the manipulation of the molecules.
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Titration of Citric Acid
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for: citric acid phenolphthalein sodium hydroxide
Preparation of the burette
Rinse the burette twice with a few milliliters of the standardized NaOH solution .
Fill the burette above the zero mL mark with the standardized NaOH. Record the concentration on the bottle label. Drain until the tip is filled and all air bubbles are removed. Record the average normality for the two values given for the standardized base and the initial burette reading (to the nearest hundredth mL) on your report .
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Titration of Citric Acid
Preparation of the juice sample
Clean and thoroughly rinse with distilled water three Erlenmeyer flasks . Using a micropipette , pipet 5 mL of grapefruit or 2 mL of lemon juice into each of the three
Erlenmeyer flasks. Add 25 mL of distilled water to each flask and swirl gently to mix.
Measure the pH of the solution and record on your report. Add 4 drops of phenolphthalein indicator to each flask.
Titration
Place a magnetic stir bar into your flask and place on a magnetic stir plate. Place a sheet of white paper under your Erlenmeyer flask to make the color change more noticeable. Place the tip of the burette inside the neck of the Erlenmeyer flask. Turn on the magnetic stir plate. Adjust the speed to prevent splashing of the mixture up the sides of the flask. Slowly add base from the burette to the Erlenmeyer flask until the pink color of the indicator lasts for a few seconds when the base first enters the solution.
Adjust the flow of base to a drop at a time. When the pinkish color persists for 30 seconds, the titration is at the endpoint. Record the final burette reading on your report.
Measure the pH of the solution and record on your report. Repeat the procedure for the other two samples.
Use the correct number of significant figures in all calculations. Calculate the molarity of each of the three samples. (See Solution Stoichiometry under Lab Report
Calculations at the web site for a sample calculation.) Using dimensional analysis, convert the average molarity to m/v%. (See Concentration Terms and Calculations at the web site for a sample calculation. Note m/v% = g citric acid/100 mL juice.) Which of the juices has the higher concentration of citric acid? Acidity is a measure of sourness.
Does your answer reflect what you know about the taste of grapefruit versus lemon juice? Compare the initial and final pHs for grapefruit and lemon juice. Are they what you expected?
5

Titration of Citric Acid
Laboratory Report
Answers to the following questions should appear in the discussion section of your report:
1. Determine the class average and standard deviation for the m/v% of the two juices.
2. Explain what the pH indicates about the acidity of the two juices relative to each other. (Acidity increases as pH decreases.) Is this reflected in the m/v% for the juices?
3. Explain the differences in the initial and final pH values of the juice samples. Is the acid or base component higher in concentration before the titration? Is the acid or base component higher in concentration at the end of the titration?
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