Purpose: The purpose of this lab is: to observe differences in the shapes of

titration curves when various strengths of acids and bases are combined, to learn about the function and selection of appropriate acid-base indicators, and to experimentally determine the equivalence point of strong and weak acid and base titrations.

Background: An acid/base titration is performed by carefully adding one solution

from a buret, the titrant, to another substance in a beaker until all of the substance in the flask has reacted. To show when the titration is complete, a dye called an indicator (an acid or base whose conjugate acid or conjugate base has a color different from that of the original compound,) is added to the solution to show via color change when the equivalence point, or when the chemical equilibrium in which the moles of the acid and the moles of the base are the same, is reached.

This experiment consists of reacting various combinations of strong and weak acids and bases. In each trial, a pH Sensor will be placed in the acidic solution, and a solution of a base will be dripped slowly from a buret at a constant rate into the acid solution that will also have phenolphthalein indicator added to it. As the titration occurs, there will be a gradual change in pH until the solution gets close to the equivalence point. Near the equivalence point, a rapid change in pH occurs. Beyond the equivalence point (where mols of base is greater than mols of acid in the solution), there will be more gradual changes in pH. In this experiment, the titration curve will be a plot of pH versus time, assuming that time is proportional to volume of base.

Materials: CBL 2 interface TI Graphing Calculator

with DataMate program pH sensor 0.10M NaOH solution 0.10M NH3 solution 0.10M HCl solution 0.10M HC2H3O3 solution Hand-held timer Phenolphthalein indicator

Ring stand (2) utility clamps 50mL buret 250ml beaker Graduated cylinder Wash bottle with distilled

water Magnetic stirrer and stirring

bar

Procedure:1) Obtain and wear safety goggles.

2) Place 8mL of 0.10 M HCL solution into a 250mL beaker. Add about 100mL of distilled water, and 3 drops of phenolphthalein indicator as well.3) Place the beaker onto a magnetic stirrer & add a small stirring bar.4) Plug the pH sensor into channel 1 of the CBL 2 interface. Use the link cable to connect the TI Graphing Calculator to the interface. 5) Use a utility clamp to suspend a pH Sensor on a ring stand. Position the pH Sensor in the HCl solution and adjust its position toward the outside of the beaker so that it will not be struck by the stirring bar.6) Obtain a 50mL buret, rinse it with a few mL of 0.10 M NaOH solution, and fill it with NaOH to the 0mL mark.7) Turn on the calculator and start the DataMate program. Press CLEAR to reset the program.8) Set up the calculator and interface for the pH Sensor.

a. Select SETUP from the main screenb. If CH 1 displays PH, proceed directly to step 9. If it does not,

continue with this step to set up your sensor manually.c. Press ENTER to select CH 1.d. Select PH from the SELECT SENSOR menu.

9) Set up the data-collection mode.a. To select MODE, press ^ once and press ENTER.b. Select TIME GRAPH from the SELECT MODE menu.c. Select CHANGE TIME SETTINGS from the TIME GRAPH

SETTINGS menu.d. Enter “3” as the time between samples in seconds.e. Enter “80” as the number of samples. The length of the data

collection will be 4 minutes.f. Select OK twice to return to the main screen.

10) Select START to begin data collection; start the hand-held timer at the same time. Carefully open the buret stopcock to provide a dripping rate of about 1 drop per second. 11) Watch to see if the phenolphthalein changes color before, at the same time, or after the rapid change in pH at the equivalence point. If phenolphthalein is a suitable indicator for this reaction, it should change from clear to read at about the same time as the jump in pH occurs. In your data table, record the elapsed time when the phenolphthalein color change occurs.12) When data collection stops after 4 minutes, turn the buret stopcock to stop the flow of NaOH titrant. As you move the cursor right or left on the displayed graph, the time (x) and pH (y) values of each data point are displayed below the graph. Determine the approximate time for the equivalence point; that is, for the biggest jump in pH in the steep vertical region of the curve. Record this time in the data table. Rinse and dry the 250mL beaker for the next trial.

13) Before printing the graph of pH vs. time, rescale the y axis from 0 to 14 pH units. To do this:

a. Press ENTER to return to the main screen.b. Select GRAPH from the main screen.c. Press ENTER then select RESCALE.d. Select “Y SCALE” from the RESCALE menu.e. Enter “0” as the min pH.f. Enter “14” as the max pH.g. Enter “1” as the pH increment.

14) Print a graph of pH vs. time. Label the graph with the acid and base used for that reaction. Press ENTER to return to the main screen.15) Repeat Steps 10-14 using NaOH titrant and acetic acid solution. Add 8 mL of 0.10 M acetic acid (H2C3O2) solution to the 250mL beaker. Then add about 100mL of distilled water and a squirt of phenolphthalein. Rinse the pH sensor and position it like in Step 5. 16) Repeat Steps 10-14 using NH3 titrant and HCl solution. Drain the remaining NaOH from the buret. Rinse the buret with a few drops of 0.10 M NH3 solution. Fill the buret with NH3 solution to the 0mL mark. Add 8mL of 0.10 M HCL solution to the 250mL beaker. Add about 100mL of distilled water and a squirt of phenolphthalein to the beaker. Rinse the sensor and position it like in Step 5.17) Repeat Steps 10-14 using NH3 titrant and HC2H3O2 solution. Add 8mL of 0.10M HC2H3O2 solution to the 250mL beaker. Rinse the pH sensor and position it like in Step 5.18) When finished, rinse the pH sensor with distilled water and return it to the pH storage solution.

Questions:1. In Trials 1 and 2, the indicator changed color at about the same time as the large increase in pH occurred at the equivalence point. In Trials 2 and 3, there was a significant difference in these two times.

2. Phenolphthalein can be used effectively to determine the equivalence point of a strong base and weak acid titration, which has an equivalence point pH around 9.0 that coincides with the color change that phenolphthalein, will indicate when a solution’s pH reaches 9.

3. By examining a titration curve, you can pinpoint the pH at a titration’s equivalence point, which will allow you to choose an acid-base indicator that changes color within the appropriate pH range.

5. Methyl red could be used to determine the equivalence point of the titration of a strong acid/weak base combination because its transition pH range is between 4.4-6.2, which makes it appropriate because the pH of the titration of a strong acid and a weak base will be under 7.0 and fall into this range.

6. The strong acid and strong base combination had the longest vertical region of the equivalence point, and the weak base and weak acid combination had the shortest vertical region of the equivalence point.

7. An acid-base indicator that changes color at pH 5.0 or 9.0 works just as well for a reaction between HCl and NaOH that produces a solution with a pH of 7.0 because 7.0 pH falls in between the range in which this indicator is useful for indicating the pH at the equivalence point of the titration.

8. In general the shape of a curve with a weak specie differs from the shape of a curve with a strong specie in that the initial pH will be higher and the peak pH will be lower.

Analysis:The color changes when the solution contains a 1:1 mixture of the

differently colored forms of the indicator (weak acid and conjugate base.) As you know from the Henderson-Hasselbalch equation, the pH equals the pK a of the indicator at the endpoint of the indicator.

However, if a strong base is used to titrate a weak acid, the pH at the equivalence point will not be 7. There is a lag in reaching the equivalence point, as some of the weak acid is converted to its conjugate base. You should recognize the pair of a weak acid and its conjugate base as a buffer. In , we see the resultant lag that precedes the equivalence point, called the buffering region. In the buffering region, it takes a large amount of NaOH to produce a small change in the pH of the receiving solution.

Conclusion:Graphing the titrations of different combinations of strong and weak

acids and bases revealed that the pH of a strong acid and a strong base is very close to 7.0 (actual pH 7.124), the pH of a strong base and a weak acid is above 7.0 (actual pH about 8.5), the pH of a weak base and a strong acid is below 7.0 (actual pH about 6,) and the pH of a weak acid and a weak base depends on which acid or base is stronger. Also, as for general shape of the titration curves, the initial pHs of each titration correlated with the strength of the acid initial pHs of titrations involving strong acid HCl were 1.0 and 1.5; initial pHs of titrations involving the weak acetic acid were 2.0 and 3.3) and the peak of the pH coincided with the strength of the base used in the titration. (initial pHs of titrations involving strong base NaOH were both approx. 12.0; initial pHs of titrations involving the weak base ammonia were 9.0 and 10.0) The equivalence point for a strong acid-strong base titration curve occurs at exactly 7.0 pH because the salt produced does not undergo any hydrolysis reactions. However, when a strong base is used to titrate a weak acid, the pH of the solution at the equivalence point is above 7.0 because some of the weak acid is converted to its conjugate base, which is basic by nature. In conclusion, the pH of an acid-base titration is dependent upon the strength of each acid or base used. It is important to note that the pH of a solution at the equivalence point has nothing to do with the volume of titrant used to reach the equivalence point; it is a property inherent to the composition of the solution.