Titration of Unknown Acid

Titration of Unknown Acid

[Two images of a drop counter titration setup: The apparatus of clamp, burette, drop counter, pH probe, ring stand, magnetic stir bar, and beaker. The clamp attaches the burette to the ring stand above the drop counter. The pH probe fits through a hold in the drop counter. The magnetic stir bar is in the beaker resting on the built in stirring apparatus. A cord attaches to the Vernier apparatus [not in photo] and to an electrical outlet [not in photo].]

Objective:

1. Determine the concentration of an unknown acid.

2. Use a titration curve to determine the endpoint of a titration.

3. Calculate the molarity of solutions of acids and bases.

Prerequisite Knowledge:

·  Acids and Bases

·  Neutralization

·  Concentration [Molarity)

·  pH

·  Titration curve

·  Equivalence point

·  Mole quantity

NSTA Science Process Skills:

·  Observe – Students observe base added to acid.

·  Measure – Students measure the pH using an electronic sensor.

·  Infer – Students calculate the concentration of the acid indirectly.

·  Experiment – Students create a titration experiment and set up the apparatus to determine the unknown.

·  Communicate – Students prepare a titration curve to calculate the equivalence point of the titration.

Accessibility: [Specific changes to procedure in blue parentheses]

·  Calibrate the drop counter by using a pie pan to hear the drops.

·  Drop tray to contain spills.

·  Notched syringe for measuring volume of unknowns.

What You Need:

·  Goggles – 1 per

·  Vernier Stir Station – 1 per 2

·  Ring stand post – 1 per 2

·  Clamp, two prong, to hold plastic reagent reservoir and valves – 1 per 2

·  Stirring bar, 4 cm – 1 per 2

·  6 Volt DC power adapter OR 4 C batteries – 1 per 2

·  Vernier Drop Counter – 1 per 2

·  Plastic reagent reservoir – 1 per 2

·  Plastic valve with double stopcock fitting – 1 per 2

·  Vernier pH probe – 1 per 2

·  pH probe storage solution – 1 per 2

·  Vernier LabQuest – 1 per 2

·  Distilled water – 100 mL per 2

·  Wash bottle, to contain distilled water – 1 per 2

·  Unknown acid, can be lemon juice, orange juice, soda, energy drinks, cleaning solutions, tomato juice, coffee, and others [choose one or several, based on student relevance) [Note: Soda must be flat) – 60 mL per student group

·  100 mL beaker – 1 per

·  Sodium hydroxide – 100 grams

·  Graduated cylinder, 50 mL capacity – 1 per 2

·  Transfer pipet – 1 per

·  Plastic bucket, for collecting base waste – 1 per 4

·  (Metal pie pan, 6” – 1 per 2)

·  (Notched syringe, 25 mL capacity – 1 per 2)

·  (Accessible calculator [scientific?) (could be a mobile app, a piece of hardware, or an app on a Braille notetaker))

·  (Tactile graph paper OR Talking graphing calculator – 1 per 2)

If Calibrating the pH Sensor:

·  Buffer solution, pH 4 – 50 mL

·  Buffer solution, pH 7 – 50 mL

·  Buffer solution, pH 10 – 50 mL

·  Beaker, 100 mL – 3

Preparation:

Vernier pH probes come precalibrated. Calibration varies by software type. For more information follow this [http://www.vernier.com/manuals/ph-bta/) link to the online user manual. Calibration is typically unnecessary unless a sensor is producing erratic results.

Setup the Apparatus:

1. [Put a drop tray down first.] Plug in 6 Volt power adapter or add C batteries to the Stir Station. Attach the ring stand post to the base of the stir station.

2. Attach the plastic valve with double stopcock fitting to the plastic reagent reservoir. Turn both stopcocks to be horizontal [closed).

3. Clamp the plastic reagent reservoir and plastic valve with double stopcock fitting to the ring stand.

4. Attach the drop counter to the ring stand post with its built in clamp.

5. Place a 100 mL beaker beneath the drop counter on the Stir Station. Add the magnetic stir bar to the beaker.

6. Adjust the height of the drop counter so that the pH sensor is at the bottom of the beaker. Move the stir bar so that it does not hit the probe while spinning.

7. Turn on the Vernier LabQuest. Attach the pH sensor and drop counter cords. Check to be sure each registers for students.

- Set the remaining beaker, cylinder, and wash bottle near the apparatus.

- Create a location in the classroom for the unknowns.

- Prepare 0.1 M NaOH solution by dissolving 4 g of sodium hydroxide in 1 L of water. Prepare about 100 mL per pair of students.

What You Do:

Introduction:

1. Ask, “Raise your hand (and say your name) if you have ever heard your dentist say that soda rots your teeth?” “Does anyone know how this works?” [Typically some students know about the acid.]

2. Explain, “Soda rots your teeth two ways, it contains acid and sugar. Because soda contains concentrated acid, the acid corrodes away your tooth enamel. Because it contains sugar, bacteria on your teeth eat the sugar and produce acid right at the enamel surface. This acid reacts with the enamel in your teeth to corrode it away.”

3. Explain, “Usually the saliva repairs the damage from these bacteria between meals, but in low pH environments, like mouths drinking a little bit of soda at a time, the acid release is more constant and the saliva can’t repair the damage.”

4. Tell, “Today we are going to test different drinks to see which is the most damaging to teeth using a pH meter.” Reveal the titration choices to students.

5. Remind students of the importance of wearing goggles. Tell, “We will be working with substances that could damage your eyes today. Put your goggles on and keep them on.”

Calibrate the Drop Counter:

1. Attach the plastic valve with double stopcock fitting to the plastic reagent reservoir. Turn both stopcocks to be horizontal [closed).

2. Clamp the plastic reagent reservoir and plastic valve with double stopcock fitting to the ring stand. (Place a pie pan beneath it to hear the drops.)

3. Place a 100 mL beaker under the stopcock.

4. Add about 20 mL of distilled water to the plastic reagent reservoir using a wash bottle.

5. Turn the lower stopcock to vertical [open) position.

6. Slowly turn the upper stopcock to let the distilled water out slowly. Adjust it to achieve a drop rate of ~1 drop per 3 seconds.

7. Turn the lower stopcock to the horizontal position. Pour out or remove any remaining distilled water. Note: It may be easier to remove the liquid with a pipet rather than taking apart the entire apparatus.

8. Pour out the distilled water in the beaker. Dry the magnetic stir bar.

Perform the 1st Titration:

1. Put on goggles. Keep them on until directed to remove them by the instructor.

2. Measure 20 mL of the liquid into a 50 mL graduated cylinder. (Use a notched syringe to measure the unknown.)

3. Pour the liquid into a 100 mL beaker.

4. Add the stir bar and place this beaker under the drop counter apparatus.

5. Remove the pH sensor from the pH sensor storage solution bottle. Insert the pH sensor in to the hole in the drop counter. [Do not allow the pH sensor to be exposed to air and dry out).

6. Adjust the location of the beaker so that the stir bar does not hit the pH sensor.

7. Fill the plastic reagent reservoir with 0.1 M NaOH.

8. Turn the Stir Station on. Record the starting value for your pH.

9. Turn the lower stopcock to vertical. The Vernier counts the number of drops and the pH. The pH change will be slow at first and then rapidly accelerate.

10. Record the pH reading after each drop. Continue the titration until the plastic reservoir is empty or until the pH stops changing.

11. Lift the pH sensor above the liquid level and rinse it with distilled water.

12. Dispose of titrated material in a waste bucket.

Perform the 2nd and 3rd Titrations:

1. Measure 20 mL of the substance to be titrated. (Use the notched syringe.)

2. Pour the material into the 100 mL beaker.

3. Refill the plastic reagent reservoir.

4. Adjust the stir bar, pH sensor, and drop counter as before.

5. The procedure is the same, except that the drop rate can be sped up until near the endpoint, rather than collecting pH data at every drop. Add drops of 0.1 M NaOH until within 15 drops of the previous endpoint, then slow the drop rate to 1 drop per 3 seconds for 30 drops.

6. Record the final number of drops below the data table.

Calculations:

1. Look carefully at your pH data. Find the drop with the largest change in pH before it and after it for each trial and circle it on your data sheet.

2. Find the average of these three numbers of drops and use it in the calculations that follow.

3. To calculate the volume of base added, use the assumption that 24 drops = 1 mL.

4. Convert this amount of mL to L by dividing by 1,000.

5. Calculate the amount of moles of base added using the molarity formula. The concentration of the base is 0.100 M.

6. The moles of base added = moles of acid present.

7. Calculate the molarity of the substance you titrated. Its beginning volume was 20 mL.

8. Take the negative logarithm of your result. This is the pH of your substance assuming it is completely dissociated.

Example Calculations:

For an example of 312 drops of base.

312 drops base* 1 mL base 24 drops base=13.0 mL base

13.0 mL base*1 L1,000 mL=0.013 L base

Molarity= molesvolume [L)

0.1= moles0.013

0.1*0.013=moles0.013*0.013

0.0013=moles base

moles base=moles acid

0.0013 moles base=0.0013 moles acid

Molarity= molesvolume [L)

Molarity= 0.00130.020=0.0065 M

pH= -logH+

pH= -log0.0065=1.18

Analysis:

1. Construct a titration curve for your 1st titration on graph paper or a spreadsheet program. The x-axis is drops added and the y-axis is pH.

2. Circle or highlight the drop with the greatest change in pH on your graph.

Student Questions:

1. If a substance has a pH below 5 it can cause damage to teeth. Does your substance damage teeth?

2. What was the final pH of the substance you titrated?

3. Look up the accepted value for the pH of your substance. Calculate a percentage error.

Percentage error=accepted-experimentalaccepted

4. What could have caused the error in the final reading?

5. Look at the titration curve you prepared. What happens to the pH of a substance as it reaches the endpoint?

6. Why does the pH change so slowly before the endpoint? What is happening to the base as it enters the solution?

Differentiations:

1. The second and third titration is optional if student attention wanes or if students have difficulty changing the drop rate.

2. Ease student calculations. An automatically calculating spreadsheet can greatly ease student anxiety about calculations, improve their accuracy, and lower the time spent calculating.

3. Choose the number 1 and number 2 drinks of student choice as the substances to be titrated. Present the activity as a “Which is worse for your teeth?” activity.

4. Add a short presentation of the data at the end of the exercise. Students should present their titration curves, data, report their final result, and include their percent error and explain it. These presentations are most interesting when every group has a different substance, but duplicative results improve confidence in the final result.

5. Ease the graphing difficulty. The Vernier LabQuest can automatically produce a graph of drops vs. pH. This is different on each of the Vernier platforms, but does not require tremendous sophistication to produce. This http://www.vernier.com/training/videos/play/?video=209&autoplay=true link shows how to do it for force vs. acceleration, but includes the relevant concepts.

Next Generation Science Standards Alignment:

5-PS1-4; MS-PS1-2; HS-PS1-7

CCC Standards Alignment:

W.5.7; W.5.8; W.5.9; RST6-8.3; RST6-8.7; WHST6-8.7; 8.EE.A.3; 6.SP.B.4; 6SP.B.5; RST.9-10.7; WHST9-12.7; MP.4; HSN-Q.A.1;