measurement, 4-2 titration of a strong acid - · pdf filetitrate water containing a strong...

© Festo Didactic 86013-00 71

Titrate water containing a strong acid solution with a strong base solution and describe the effect of the pH variation on a chemical indicator. Plot a graph using the titration data, analyze the titration curve, and calculate the pH of a strong acid solution.

The Discussion of this exercise covers the following points:

Strong acid

Calculation of the pH of a strong acid solution

Strong base

Calculation of the pH of a strong base solution

Recording the titration curve using the recorder

Strong acid

When in an aqueous solution, a strong acid completely dissociates into ions. After the dissociation there is no undissociated acid molecule in the solution. All that is left are hydronium ions and a conjugate base. Since there is complete dissociation, the dissociation constant is almost infinite. It is sometimes said that the reaction goes completely to the right side of the equation:

(4-37)

(4-38)

Figure 4-30 shows the proportion of the different chemical species in solution before and after dissociation (at equilibrium).

Titration of a Strong Acid

Exercise 4-2

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Strong acid Base Conjugate

base

Conjugate acid

Ex. 4-2 – Titration of a Strong Acid Discussion

72 © Festo Didactic 86013-00

Figure 4-30. Proportion of the chemical species before and after dissociation.

Table 4-3 lists common strong acids. If an acid cannot be found in this table, it is probably a weak acid (or it is not a single compound acid). Note that sulfuric acid has two hydrogen atoms, thus each sulfuric acid molecule can be ionized twice. For the first ionization, the sulfuric acid acts as a strong acid, which means that it completely dissociates to form and . Once ionized a first time the

ions can be ionized too, but this time there is no complete dissociation, thus ions act like a weak acid. Acids able to donate more than one proton per molecule are named polyprotic acids; sulfuric acid is a diprotic acid since it can donate two protons per acid molecule. Because it can be ionized twice, sulfuric acid has two dissociation constants. The first acid-ionization constant, , is almost infinite and the second acid-ionization constant, , is equal to 1.2x10

-2.

Calculation of the pH of a strong acid solution

A strong acid completely dissociates in an aqueous solution. Each mole of acid dissolved in water results in a mole of hydronium. For example, a solution

of 0.1 mol/L of hydrochloric acid, , produces 0.1 mol/L of after dissociation. In this case, the equation of the dissociation process is:

(4-39)

As already mentioned, pH is given by the equation:

Therefore, the pH of a 0.1 mol/L solution of hydrochloric acid is:

Note that even if it is very unusual, theoretically, pH can be negative.

Because it has only a small

influence on the pH value of

an aqueous solution, the

contribution of the self-

ionization of water to the pH

of a strong acid or strong

base solution is often ne-

glected in the calculation.

Figure 4-31. Hydrochloric acid (2D).

Figure 4-32. Hydrochloric acid (3D).

Pro

po

rtio

n o

f th

e c

he

mic

al sp

ecie

s

Pro

po

rtio

n o

f th

e c

he

mic

al sp

ecie

s

Before dissociation After dissociation

Ex. 4-2 – Titration of a Strong Acid Discussion


Strong base

A strong base completely dissociates into ions when in an aqueous solution. After the dissociation, the aqueous solution is solely constituted of hydroxyl ions and a conjugate acid. There is complete dissociation of the base, thus the dissociation constant is almost infinite and the equation of dissociation can be written as:

(4-40)

Table 4-5, presented earlier, lists the most common strong bases. As for acids, some bases have more than one hydroxyl group and can be ionized more than one time. They are identified as polybasic. Such polybasic molecules have as many dissociation constants as they have hydroxyl groups. Oxides of metals from the group I are monobasic while oxides of metals from group II are all dibasic.

Calculation of the pH of a strong base solution

Each mole of strong base dissolved into water gives a mole of hydroxyl ions. For

example, a solution of 0.1 mol/L of sodium hydroxide produces 0.1 mol/L of . The dissociation equation for this example is:

(4-41)

To calculate the pH after dissociation, the pOH must be calculated first:

The pH of the solution is then given by:

Recording the titration curve using the recorder

During a titration process, the pH of the solution can change drastically within seconds. To obtain a good titration curve, a recorder is a precious tool. To record a titration curve, the output of the pH transmitter must be connected to a channel of the recorder. Figure 4-35 shows a typical setup that allows recording a titration curve. In this setup, there is only one current loop where the output of the transmitter is connected to channel 1 of the recorder.

Strong base Acid Conjugate

acid

Hydroxyl

Figure 4-33. Sodium hydroxide (2D).

Figure 4-34. Sodium hydroxide (3D).

Ex. 4-2 – Titration of a Strong Acid Procedure Outline


Figure 4-35. Typical setup to record a titration curve.

If available, a digital recorder allowing exporting the recorded data to spreadsheet software should be used. The channel of the recorder should be plotted in units of 0% to 100% of the measured variable range.

The Procedure is divided into the following sections:

Set up and connections

Titration of hydrochloric acid

Neutralizing the process water

Titration curve analysis

Set up and connections

Before using ANY of the chemicals provided with the pH Process Control

Training System, read the chemical MSDS and wear the appropriate personal

protective equipment.

1. Connect the equipment as the piping and instrumentation diagram (P&ID) in Figure 4-36 shows. Use Figure 4-37 to position the equipment correctly on the frame of the training system from the 3532 series or use Figure 4-38 to position the equipment on the frame from the 3531 series. Use the basic setup presented in the Familiarization with the Training System manual. Table 4-13 lists the equipment you must add to the basic setup in order to set up your system for this exercise. The only difference between this setup and the setup of Ex. 4-1 is the added paperless recorder that is used to record the pH variation as a function of time.

PROCEDURE OUTLINE

PROCEDURE

24 V

Ch1 Ch2Out

Ex. 4-2 – Titration of a Strong Acid Procedure


Table 4-13. Equipment required for this exercise.

Name Part number Identification

Volumetric flask 38406

Scopulla 38442

Sodium hydroxide solution 1.0 N 38444

Hydrochloric acid solution 1.0 N 38445

Phenol red 38446

Graduated cylinder 38447

Latex gloves 40331-54

Pipette 40331-65

Safety glasses 40348

pH transmitter 46945 AIT

Metering pumps 46956

Paperless recorder 46972 UR

Water analyzer 46989

Chemical tanks 46994

Figure 4-36. P&ID.



Figure 4-37. Setup (series 3532).



Figure 4-38. Setup (series 3531).

2. Wire the emergency push-button so that you can cut power in case of an emergency. The Familiarization with the Training System manual covers the security issues related to the use of electricity with the system, as well as the wiring of the emergency push-button.

3. Wire the paperless recorder to record the output of the pH transmitter.



4. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup—that is not before step 8.

Hydrochloric acid is corrosive and harmful if swallowed. Do not breathe vapor.

Avoid eye and skin contact. When handling, wear chemical-resistant gloves,

chemical safety goggles, and a lab coat. Refer to the MSDS for more details on

this product.

Sodium hydroxide is corrosive and harmful if swallowed. Do not breathe vapor.

Avoid eye and skin contact. Avoid prolonged or repeated exposure. When

handling, wear chemical-resistant gloves, chemical safety goggles, and a lab

coat. Refer to the MSDS for more details on this product.

5. To titrate a strong acid solution with a strong base, one of the chemical tanks must be filled with a solution of 0.08 mol/L of sodium hydroxide and the other chemical tank with a solution of 0.08 mol/L of hydrochloric acid. Make sure there is enough solution left in both tanks from the previous exercise. If not, follow the procedure of Ex. 3-1 to prepare more of these solutions.

6. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive. Be sure to follow the instructions in the Familiarization with the Training System manual as well.

f

All unused male adapters on the column are capped and the flange is

properly tightened.

The hand valves are in the positions shown in the P&ID.

The chemical tanks are filled with the appropriate solutions and are

carefully labeled.

You are wearing the appropriate PPE.

The vent tube is properly installed.

The paperless recorder is set up and configured to record the output of the

pH transmitter.

7. Ask your instructor to check and approve your setup.



Titration of hydrochloric acid

a Tap water usually contains salts that can act as a buffer solution. Therefore, a titration curve obtained using tap water may have a shape different from the titration curve of Figure 4-10. The shape of the curve strongly depends on the type of tap water used. To obtain a titration curve similar to Figure 4-10 use the setup and procedure of Appendix F to filter the process water before proceeding to the titration exercise. This setup requires the optional conductivity process add-on.

8. Power up the electrical unit. This starts all electrical devices.

9. Make sure the pH probe is properly inserted into the connection port on the process workstation.

10. Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate up to 50% of the maximum flow rate the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair all leaks.

11. Start the pump and set the drive speed to 30 Hz.

This setup uses a telescopic pipe, which can extend if the pump runs at an excessive

speed. Be sure to set the drive speed to a maximum of 30 Hz and secure the tubing with

as much attach brackets, Model 85444, as possible.

12. Fill the column up to 25 cm of water. Then, close HV1 and open HV6 to put the process workstation into recirculation mode.

13. Remove one of the caps from the top of the column and, using a funnel, add about 20 mL of phenol red to the process water.

14. Using the metering pump connected to the chemical tank containing the solution of 0.08 mol/L of hydrochloric acid, start injecting acid solution into the process water. The purpose of this operation is to drop the pH of the process water down to a value around 3.0. Set the metering pump to 50% of its maximum delivery rate.

15. Closely monitor the pH of the process water on the transmitter. Once the pH of the process water has dropped to a value of 3.0, stop the metering pump injecting the acid solution and wait two minutes to make sure the process water is perfectly mixed.



16. Once the pH value is stable, inject alkaline solution into the process water using the second metering pump. Set the metering pump delivery rate at around 50%. This stretches the titration process and provides more data for analysis.

17. Make sure the paperless recorder is recording the output of the pH transmitter.

18. Watch the titration process by observing both the tint of the process water given by the phenol red and the pH value measured by the paperless recorder.

19. Observe the mixing process by noting the color variation within the column as you inject the base into the process water. How could the mixing be improved?

20. Wait until the pH of the process water has increased to a value of 10.0 and stop the metering pump injecting the alkaline solution. The titration of a strong acid solution (hydrochloric acid) with a strong base (sodium hydroxide) is completed.

Neutralizing the process water

21. The process water now contains a strong base solution. Hence, it is very important that you neutralize the process water before draining the system.

22. Use the metering pump connected to the chemical tank containing the solution of 0.08 mol/L of hydrochloric acid to decrease the pH of the process water. If necessary, reduce the delivery rate to avoid injecting too much acid solution.

23. When the pH gets close to 7.5, stop the metering pump and wait about one minute for the pH reading to stabilize.

24. If, after one minute, the pH is between 6.5 and 7.5, it is safe to drain the water in the column into the main tank of the process workstation. Open HV4 and HV5 to do so.

25. If the pH is not between 6.5 and 7.5, inject more acid and base until the pH is within the desired range. Then, drain the column.

Ex. 4-2 – Titration of a Strong Acid Conclusion


26. Follow the procedure in the Familiarization with the Training System manual to transfer the data from the paperless recorder to a computer.

27. Stop the system, turn off the power, and store the equipment. Do not forget to rinse the pH probe and store it in a storage solution as described in the Familiarization with the Training System manual.

Titration curve analysis

28. Plot the data using spreadsheet software.

29. Compare the titration curve you have obtained with the typical curve for the titration of a strong acid by a strong base. List the similarities and the differences between the two curves below.

In this exercise, you learned how to titrate a strong acid solution with a strong base solution. You analyzed the titration curve obtained. You also learned how to calculate the pH of a strong acid solution and the pH of a strong base solution.

1. In the discussion section, an approximation has been made in the calculation of the pH of a strong acid and of a strong base. What is this approximation?

2. What is the pH of a solution of 10 mol/L of hydrochloric acid?

3. What is a diprotic acid?

CONCLUSION

REVIEW QUESTIONS

Ex. 4-2 – Titration of a Strong Acid Review Questions


4. When titrating a strong base with a strong acid, which chemical species are present in solution at the equivalence point?

5. What is the pH of a solution of 0.2 mol/L of potassium hydroxide, ?

measurement, 4-2 titration of a strong acid - · pdf filetitrate water containing a strong...

Documents