For the lab

1 - juice extractor with built in strainers

5 - 400 mL beakers

1 - 50 mL graduated cylinder

5 - 1 mL polycarbonate pipettes

5 - 250 mL beakers

5 - sturdy glass stirring rods

5 - 4-oz amber bottles of Universal Indicator, one for each row,

with polycarbonate pipette attached at side of bottle 3000 mL 0.10 M NaOH in 5 plastic wash bottles, labelled 0.10 M NaOH.

For each student and instructor:

1 wash bottle with distilled water

1 50 mL erlenmeyer flask

1 polycarbonate eye dropper (pipette)

1 sheet white paper

1 50 mL beaker

1 pair safety glasses

For each session

Acidity, according to Aurand & Wells,

Already pre-cut, Food Composition & Analysis, %Acid, as citric

1 - grapefruit 1.58

2 - limes 7.2

2 - lemons 5.07

2 - oranges 0.67

2 - tangerines ---

stawberries (reference) 3.2

Initial procedure. Do the following procedure for all but one of the fruits before the class begins. Do the procedure for the last fruit in front of the class:

1. Squeeze the juice out of two halves of each kind of fruit to be used.

2. Strain the pulp and seeds; collect the juice.

3. Pour enough of the juice into a 50 mL graduated cylinder to fill it to the 40 mL mark.

4. Pour the filtered juice into a 250 mL beaker and add 160 mL water. Stir the contents with a glass stirring rod.

H H

| |

H-C-COOH H-C-COO^- Na⁺

| |

H-O-C-COOH + 3 NaOH --> H-O-C-COO^- Na⁺ + 3 H₂O

| |

H-C-COOH Sodium H-C-COO^- Na⁺ Water

| Hydroxide |

H H

Citric Sodium

Acid Citrate

Indicator(H) + NaOH ---> Indicator^- Na⁺ + H₂O

Sodium Water

Hydroxide

Today we are going to determine the relative acidity of two citrus fruits (tell them which ones).

All citrus fruits have an abundance of citric acid, C₆H₈O₇. Each molecule of citric acid contains three acid "functions", called carboxylic acid groups.

Each "COOH" is a carboxylic acid group. The H atom on each COOH group can attach easily (as an H⁺ ion) to a hydroxide ion, OH^- (a strong base), to become a water molecule, H₂O. This is a process called "neutralization".

If we use a hydroxide solution to neutralize the acid in a constant amount of the juice of an orange or a lemon, we can compare how much hydroxide is needed, and therefore we can find out which of these fruits is the most acidic and which is the least acidic.

But both the reactant, citric acid, and the neutralization product, sodium citrate are colorless in solution. Using only these reactants, there is no way of knowing when the reaction has finished, so we make use of a colored indicator. An indicator is a substance which changes color when it reacts with sodium hydroxide. In the indicator phenolphthalein, the hydrogen which is removed, binds more tightly to the molecule than does the hydrogens in citric acid. When all of the citric acid has reacted, then the indicator reacts with sodium hydroxide to produce a pink solution.

NOTES for getting started:

Important skills a chemist must learn

Separation

Analysis

Qualitative

Quantitative

Synthesis

Some mixtures can be analyzed without separation, but only in a narrow sense.

Today: analysis of acid content

Writing reports: Purpose, procedure, data and observations, calculations, conclusion.

Today: Purpose, data and observations, conclusion.

Today we will determine the relative concentrations of acid in several citrus fruits.

All citrus fruits, like lemons, oranges and grapefruit have an abundance of citric acid, C₆H₈O₇. Each molecule of citric acid contains three acid "functions", called carboxylic acid groups.

H

|

H-C-COOH

|

H-O-C-COOH

|

H-C-COOH

|

H

Citric acid

Each "COOH" is a carboxylic acid group. The H atom on each COOH group can attach easily (as an H⁺ ion) to a hydroxide ion, OH^- (a strong base), to become a water molecule, H₂O. This is the process of neutralization.

If we use a hydroxide solution to neutralize the acid in a constant amount of the juice of a citrus fruit, we can compare how much hydroxide is needed, and therefore we can find out which of these fruits is the most acidic and which is the least acidic.

The instructor has prepared stock quantities of dilute citrus fruit juices for your use in this experiment. He prepared the juice for all but one fruit before you came to class. He will do the final preparation in front of you, and then you can begin the experiment using all five fruits:

Using the final fruit,

1. He will squeeze the juice out of two halves of the final fruit.

2. He will strain the pulp and seeds in order to collect only the juice.

3. He will pour enough of the juice into a 50 mL graduated cylinder to fill it to the 40 mL mark.

4. He will pour the filtered juice into a 250 mL beaker and add 160 mL water. He will stir the contents with a glass stirring rod so as to assure uniformity of the solution.

Now, it is time for you to get to work:

5. Use the plastic pipette next to the 250 mL beaker to transfer 1.00 mL of diluted juice to the 50 mL Erlenmeyer flask.

6. The instructor will give you some 0.10 M NaOH in your 50 mL beaker.

7. The instructor will give you 0.25 mL Universal Indicator in your Erlenmeyer flask. The color of the solution will turn red.

8. Transfer some water from your wash bottle into the red solution until the volume comes up to the 20 mL mark.

9. Fill your plastic pipette with NaOH solution from your beaker.

10. Slowly drip NaOH solution into your diluted juice, while constantly swirling the flask. Count the drops as they go in and swirl the contents of the flask after each drop.

11. When the color of your diluted juice turns yellow-green, or green, you have neutralized all of the acid in your diluted juice.

12. Report the number of drops to the instructor, who will write that number on the board.

13. If you have time left, you may neutralize the diluted juice of the other fruits.

_________________________________________________________

Fruit Number of drops of NaOH

___________________ ______________

___________________ ______________

___________________ ______________

___________________ ______________

___________________ ______________

1. Which is the most acidic fruit?

___________________________________

2. Which is the least acidic fruit?

___________________________________