Acid-Base Titration (Lourdes Ramos Quevedo)

Research Question and Hypothesis

How do you determine the concentration of an acid using a titration method?

Standards

Chemistry Standards (5- Acids & Bases)

b. Students know acids are hydrogen-ion-donating and bases are hydrogen-ion-accepting substances.

c. Students know strong acids and bases fully dissociate and weak acids and bases partially dissociate.

d. Students know how to use the pH scale to characterize acid and base solutions.

e.* Students know the Arrhenius, Brønsted-Lowry, and Lewis acid–base definitions.

f.* Students know how to calculate pH from the hydrogen-ion concentration.

Dimension 1: Scientific and Engineering Practices

1. Asking questions (for science)

3. Planning and carrying out investigations

4. Analyzing and interpreting data

5. Using mathematics and computational thinking

6. Constructing explanations (for science)

8. Obtaining, evaluating, and communicating information

Experimental Design

In this experiment,

• Use a pH Sensor to monitor changes in pH as sodium hydroxide solution is added to a hydrochloric acid solution.

• Plot a graph of pH vs. volume of sodium hydroxide solution added.

• Use the graph to determine the equivalence point of the titration.

• Use the results to calculate the concentration of the hydrochloric acid solution.

Independent variable

The independent variable is the base with a known concentration. By adding small volumes of the base to the acid, once equimolar, the concentration of the acid can be calculated.

Dependent variables

The dependent variable is the pH of the acid-base solution. Initially, the pH is acidic because the pH sensor is sitting in the acidic solution. As base is added to the acid, the pH will become more basic.

Series

Study of pH as the volume of base is added to the acidic solution of unknown concentration.

Constants and Controls

The concentration of NaOH is kept constant. The volume of the acid is kept constant. I control would be running the same experiment with a known concentration of acid.

Materials

computer magnetic stirrer (if available)

Vernier computer interface

stirring bar or Microstirrer (if available)

Logger Pro

wash bottle

Vernier pH Sensor

distilled water

HCl solution, unknown concentration

ring stand

~0.1 M NaOH solution

1 utility clamp

pipet bulb or pump

250 mL beaker

Vernier Drop Counter

100 mL beaker

60 mL reagent reservoir

10 mL graduated cylinder

5 mL pipet or graduated

10 mL pipet

Procedures

Using a Vernier Drop Counter to take volume readings. NaOH titrant is delivered drop by drop from the reagent reservoir through the Drop Counter slot. After the drop reacts with the reagent in the beaker, the volume of the drop is calculated, and a pH-volume data pair is stored.

1. Obtain and wear goggles.

2. Connect the pH Sensor to CH 1 of the computer interface. Lower the Drop Counter onto a ring stand and connect its cable to DIG/SONIC 1.

3. Add 40 mL of distilled water to a 100 mL beaker. Use a pipet bulb (or pipet pump) to pipet 5.00 mL of the HCl solution into the 100 mL beaker with distilled water. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin.

4. Obtain approximately 40 mL of ~0.1 M NaOH solution in a 250 mL beaker. Record the precise NaOH concentration in your data table. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.

5. Obtain the plastic 60 mL reagent reservoir. Note: The bottom valve will be used to open or close the reservoir, while the top valve will be used to finely adjust the flow rate. For now, close both valves by turning the handles to a horizontal position. Rinse it with a few mL of the ~0.1 M NaOH solution. Use a utility clamp to attach the reagent reservoir to the ring stand. Add the remainder of the NaOH solution to the reagent reservoir. Drain a small amount of NaOH solution into the 250 mL beaker so it fills the reservoir’s tip. To do this, turn both valve handles to the vertical position for a moment, then turn them both back to horizontal.

6. Prepare the computer for data collection by opening the file “24b Acid-Base (Drop Count)” from the Chemistry with Vernier folder. Check to see that the pH value is between 1.5 and 2.5.

7. To calibrate drops so that a precise volume of titrant is recorded in units of milliliters:

   1. From the Experiment menu, choose Calibrate . DIG 1: Drop Counter (mL).

   2. Proceed by one of these two methods:

      1. If you have previously calibrated the drop size of your reagent reservoir and want to continue with the same drop size, select the Manual button, enter the number of Drops / mL, and click . Then proceed directly to Step 8.

      2. If you want to perform a new calibration, select the Automatic button, and continue with Step c below.

   3. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter, lining it up with the tip of the reagent reservoir.

   4. Open the bottom valve on the reagent reservoir (vertical). Keep the top valve closed (horizontal).

   5. Click the Start button.

   6. Slowly open the top valve of the reagent reservoir so that drops are released at a slow rate (~1 drop every two seconds). You should see the drops being counted on the computer screen.

   7. When the volume of NaOH solution in the graduated cylinder is between 9 and 10 mL, close the bottom valve of the reagent reservoir.

   8. Enter the precise volume of NaOH (read to the nearest 0.1 mL) in the edit box. Record the number of Drops / mL displayed on the screen for possible future use.

   9. Click . Discard the NaOH solution in the graduated cylinder as indicated by your instructor and set the graduated cylinder aside.

8. Assemble the apparatus.

   1. Place the magnetic stirrer on the base of the ring stand.

   2. Insert the pH Sensor through the large hole in the Drop Counter.

   3. Attach the Microstirrer to the bottom of the pH Sensor, as shown in the small picture. Rotate the paddle wheel of the Microstirrer and make sure that it does not touch the bulb of the pH Sensor.

   4. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up with the center of the magnetic stirrer.

   5. Lift up the pH Sensor, and slide the beaker containing the HCl solution onto the magnetic stirrer. Lower the pH Sensor into the beaker.

   6. Adjust the position of the Drop Counter so that the Microstirrer on the pH Sensor is just touching the bottom of the beaker.

   7. Adjust the reagent reservoir so its tip is just above the Drop Counter slot.

9. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.

10. You are now ready to begin collecting data. Click . No data will be collected until the first drop goes through the Drop Counter slot. Fully open the bottom valve—the top valve should still be adjusted so drops are released at a rate of about 1 drop every 2 seconds. When the first drop passes through the Drop Counter slot, check the data table to see that the first data pair was recorded.

11. Continue watching your graph to see when a large increase in pH takes place—this will be the equivalence point of the reaction. When this jump in pH occurs, let the titration proceed for several more milliliters of titrant, then click . Turn the bottom valve of the reagent reservoir to a closed (horizontal) position.

12. Dispose of the beaker contents as directed by your teacher.

13. Print copies of the table.

14. Print copies of the graph.

15. If time permits, repeat the procedure.

Sample data and graphs

Analysis & Conclusions

1. Use your graph and data table to determine the volume of NaOH titrant used in each trial.

Examine the data to find the largest increase in pH values upon the addition of 1 drop of

NaOH solution. Find and record the NaOH volume just before and after this jump.

5.110 mL of NaOH before increase in pH, pH= 3.79

6.968 mL of NaOH after increase in pH, pH= 10.69

2. Determine the volume of NaOH added at the equivalence point. To do this, add the two

NaOH values determined above and divide by two.

(5.110mL + 6.968mL)/2= 12.078 mL/2= 6.039 mL of NaOH

3. Calculate the number of moles of NaOH used.

6.039mL X 1 Liter X 0.1 moles NaOH = 6.039 X 10^-4 moles NaOH

1000 mL 1 Liter

4. Using the equation for the neutralization reaction given in the introduction, determine the

number of moles of HCl used.

Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:

H⁺_(aq) + Cl^–_(aq) + Na⁺_(aq) +OH^–_(aq)  → H₂O_(l) + Na⁺_(aq) + Cl^–_(aq)

6.039 X 10^-4 moles NaOH = 6.039 X 10^-4 moles HCl

5. Calculate the HCl concentration using the volume of unknown HCl you pipeted out for each

titration.

6.039 X 10^-4 moles HCl = 1.208 X 10^-1 M HCl = 0.1208 M HCl

5.0 X 10^-3L

Photos & Movies