ACIDITY OF FOODS
Larry Alofs Kenwood Academy
5015 S. Blackstone
Chicago, IL 60615
1-312-536-8850
Objectives:
This set of activities will help the students realize that the concept of
acidity applies to everyday things like foods, as well as to laboratory chemicals.
The procedures also provide extra practice in titration techniques, use of a pH
meter, and using titration results to calculate the acid concentration in a food
being analyzed.
The instruction/data sheet on the next page shows the main activities. At
least 5 burets of .5 M NaOH should be provided, and, if possible, 2 or more pH
meters.
The 4 fruit juices can be ranked by acidity using the results from the
titration and from the meter. Hopefully the two rankings will be the same.
The titration of the vinegar provides a chance to do something quantitative
with the results. Inexpensive white vinegar with 5% acidity is recommended. Since
each molecule of NaOH supplies one hydroxl ion and each acetic acid molecule
supplies one hydrogen, the number of moles of acid in the 10 mL sample of the
vinegar equals the number of moles of NaOH used in the titration.
Example Calculation:
16.9 mL of .5 M NaOH used
.5 moles in 1000 mL
.5 X (16.9 / 1000) = .00845 moles NaOH used
1 mole of acetic acid = 60 g.
.00845 X 60 = .507 g. in the 10 mL sample of vinegar,
or 5.07 g. in 100 mL (about 100 g.)
or 5.07 % by mass
ACTIVITY 1
Titrate each of the 4 fruit juices with the .5 M NaOH. Use 10 mL samples
of the juices. Add 3 to 5 drops of indicator. The phenolphthalein will turn
pink or red when you have added enough base. You may wish to dilute the orange
juice and add more indicator to make the change more visible.
pineapple apple lemon orange
--------------------|---------|----------|----------|----------|
buret finish: | | | | |
readings { -----------|---------|----------|----------|----------|
start: | | | | |
--------------------|---------|----------|----------|----------|
mL NaOH | | | | |
(subtract) | | | | |
--------------------|---------|----------|----------|----------|
ACTIVITY 2
Measure the pH of the 4 fruit juices using the pH meter. The meter should
read about 7.0 in the standard buffer solution. Leave the probe in your sample
for a minute or two and hold it off the bottom of the container when taking a
reading. Rinse the probe after each juice, first in tap water and then
distilled water.
pineapple apple lemon orange
------|--------------|---------|---------|---------|
pH: | | | | |
------|--------------|---------|---------|---------|
ACTIVITY 3
Using the meter, measure the pH of the 2 brands of coffee.
Taster's Choice: _______ Kava: _______
Read the information on the Kava label about how the acidity was changed.
Write the formula for the additive:______ How do you feel about drinking this?
ACTIVITY 4
Titrate 10 mL of the vinegar with the .5 M NaOH. If you have time do more
than one trial and average the results.
---------|---------|---------|----------------
Finish: | | |
---------|---------|---------|----------------
Start: | | |
---------|---------|---------|----------------
mL NaOH: | | |
---------|---------|---------|----------------
The molecular formula for the acetic acid in the vinegar is: HC2H3O2
From your titration results, calculate the percent acid of the vinegar to find
out if it agrees with the 5% stated on the bottle.
ACTIVITY 5
Try using pH Paper as another method of measurement for activity 2 or 3.
Are the results similar to the meter readings?
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