Background:

Many fruits are naturally acidic. The juice of a lemon is a fairly strong acid; it can be as low as 2.3 on the pH scale.

The Questions:

What happens to pH when a base is added to an acid?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Wear protective equipment while handling chemicals.

Materials:

Procedure:

Connect the pH sensor to your computer - if you are using a USB sensor, your apparatus will look something like this:

Step 2:

Configure the data collection software to collect pH data over time.

The following is an example DataStudio graph setup;

It would also be wise to choose a digital display of the data, so you can clearly see the pH value at any moment:

Step 3:

Measure 100 ml of lemon juice with the graduated cylinder and pour it into a 500 ml beaker. Rinse the cylinder with water.

Step 4:

Add 50 ml (dry measure - approximately equal to 3 tablespoons) of NaHCO₃ to the other 500 ml beaker and add distilled water to bring the water level up to 500 ml. Stir this mixture until the NaHCO₃ is mostly dissolved (some may settle to the bottom, this is normal).

Step 5:

Place the pH probe in the lemon juice, and record data. Write down the pH value in an appropriate table, such as the one below. Rinse the pH probe with distilled water.

Place the pH probe in the NaHCO₃ solution, and record data. Write down the pH value. Rinse the pH probe with distilled water.

Step 6:

Predict how much NaHCO₃ solution you will have to add to the lemon juice in order to neutralize it (bring the pH to 7).

Pour NaHCO₃ solution from the beaker into the graduated cylinder until it reaches the 100 ml mark. You will use this cylinder of NaHCO₃ solution to pour into the lemon juice.

Place the pH probe into the lemon juice and begin recording data. Very gently swirl the pH sensor (do not bang it against the walls of the beaker) and then VERY SLOWLY add NaHCO₃ solution (do not add it quickly or it will foam over the sides of the beaker). Observe the pH change while slowly adding NaHCO₃ solution, and continue adding until the pH has reached 7.0 - stop as close to pH 7 as possible. If you have to refill your graduated cylinder with 100 ml of NaHCO₃ solution, then do so. Make note of the total amount of NaHCO₃ solution you added.

Stop recording data.

Step 7:

You are now ready to analyze your pH data. Re-scale the graph so the data fills the whole window. If possible, print out the resulting graph. Your data may look something like this example graph - this was done using limejuice:

Find the change in pH you recorded. (If you are using DataStudio software, click on the “Smart Cursor” button. The cursor becomes a crosshair which you can drag over the graph in order to more easily determine the coordinates at any particular point.) Write down this pH change in your notes, or on the graph printout.

Carefully dispose of the beaker contents as instructed by your teacher.

Forming Conclusions:

Based on the data you have collected, answer the following questions:

1.	What happens to pH when a base is added to an acid?
2.	Was the NaHCO3 successful at neutralizing the lemon juice? Explain.
3.	Recall that you started with 100 ml of lemon juice. Was there a difference in the amounts of lemon juice and NaHCO3 added together to reach a pH of 7? Explain why the amounts were different.

Extending:

• Discuss with your classmates how you might repeat this experiment to make the results more reliable. If you think improvements could be made, make changes to the procedure, and repeat the experiment.