Using pH Paper and the Indicator Bromothymol Blue (BTB) In this exercise, you will determine the approximate pH

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Exercise 1 Using pH Paper and the Indicator Bromothymol Blue (BTB)

In this exercise, you will determine the approximate pH for 16 different chemicals using pH paper and BTB.

Common names for chemicals may vary by manufacturer. For this reason, always compare the chemical formulas listed in this lab to those printed on the pipet and bottle labels in your kit.

Procedure

Gather gloves, a sheet of paper, a pencil, two disposable pie pans, paper towels, scissors, the 24-well plate, wide-range pH paper, a trash container, and the Acid-Base Chemistry chemical bag for this experiment.

Create labels for the 24-well plate on the sheet of paper as follows:

Fold the paper in half.

On half of the sheet of paper, place the 24-well plate on the paper and use a pencil to draw around the 24-well plate.

Use the pencil to mark where the wells are positioned in each row and column, marking each side of the wells on the paper drawing. See Figure 4.

Use the edge of the well plate to draw a straight line, connecting the positions of where each well row and well column begins and ends. See Figure 5.

Label the drawing, starting with the first well square in the upper left portion of the grid.

Label the wells 11A–11H and 12A–12H, as shown in Figure 6.

Figure caption is an adequate description of the image.

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Figure 6. Labeling the grid 11A–11H and 12A–12H.

 

 

 

 

Turn the paper so the drawing labeled 11A–11H and 12A–12H is facing upward and place the folded paper in a disposable pie pan. Put the 24-well plate on the piece of paper over the markings. See Figure 7.

Figure caption is an adequate description of the image.

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Figure 7. The 24-well plate on the paper over the drawing labeled 11A–11H and 12A–12H.

Open the Acid-Base Chemistry bag and arrange the chemicals in the 24-well plate, with bulbs facing downward, according to the names of chemicals that correspond to each "ID Number", as listed in Data Table 1. For example, the pipet labeled Acetic Acid (CH3CO2H) will go over the well labeled 11A. See Figure 8.

Figure caption is an adequate description of the image.

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Figure 8. Placing the chemicals, bulb face up, in the well plate according to the corresponding “ID Numbers" in Data Table 1. 

Place the two pipets containing BTB in two unlabeled wells of the 24-well plate, bulb facing downward. See Figure 9 for complete setup.

Photo of a 24-well plate holding chemical pipets bulb side down over a piece of labeled paper for each well. The paper and well plate are resting in an aluminum pie pan.

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Figure 9. Complete setup after step 5.

Before moving on to the experiment, make an educated guess (acid or base) for each chemical by studying the chemical formula, shown in Data Table 1. Record your guess in the column of Data Table 1.

Retrieve the wide-range pH paper, and cut each piece of paper in half lengthwise, and then again in half widthwise. See Figure 10.

Figure caption is an adequate description of the image.

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Figure 10. pH paper that has been cut in half lengthwise and width-wise. The final pieces are shown on the right: four pieces have been cut from the single original piece.

Place the cut pieces of pH paper in a pile in a dry location, such as on a sheet of paper, to be used for Exercises 1, 2, and 3.

Note: View the following video for a demonstration of using and reading pH paper before you continue the procedures.

If needed, a descriptive text transcript is available.

Using and Reading Wide-Range pH Paper

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WARNING!

You will be working with strong acids and bases. Do not continue this exercise without wearing protective equipment. You should also wear old clothing that will cover your arms and legs.

Put on your pair of gloves and safety goggles.

Wet a paper towel and place it on the unused disposable pie pan.

Use a pair of scissors to carefully cut off the tip of the first chemical pipet. Cut the pipet over a trash container so the trash container will catch the tip of the pipet.

Replace the pipet, bulb facing downward, back into the 24-well plate in its proper location.

Use the wet paper towel after cutting the pipet to carefully wipe any residue left on the scissors from the chemical.

Repeat steps 10–12, carefully cutting each pipet, one by one, replacing the pipet back in its respective location in the 24-well plate. Wipe off the scissors between each pipet tip cutting.

Keep the used paper towel on the pie pan, but also lay a new, dry piece of paper towel flat in the same pie pan.

Dispense 2 drops of a chemical into its well and then place the pipet bulb down in a disposable cup.

Note: If an air bubble is caught in the tip of the pipet, expel the first drop onto the wet paper towel located in the pie plate. You want each drop that goes into the well plate to be a full drop.

Use one piece of cut pH paper for one well that contains a chemical to determine the approximate pH of the chemical.

Dip the paper into the well and pull it out immediately. It is important to quickly remove the pH paper from the well to avoid dissolving the chemicals from the pH paper into the chemicals in the well.

Then, immediately compare the color of the paper to the scale that was provided with the pH paper.

Record the pH in Data Table 1.

Repeat for each chemical, and only use one sectioned piece of pH paper for one well.

Dispense one drop of BTB in each well of columns 11 and 12.

Record the color of each well in Data Table 1.

Determine whether each chemical was an acid or a base and record your findings in Data Table 1.

Clean up:

Save the chemical pipets for use in Exercise 2.

Save the unused pH paper for use in Exercises 2 and 3.

Clean and dry the 24-well platefor use in Exercise 2.

Exercise 2 Acid-Base Reactions

In this exercise, you will perform a series of reactions between a strong acid and a strong base (SA/SB), a weak acid and a strong base (WA/SB), a diprotic acid and a strong base, and a triprotic acid and a strong base. Based on the stoichiometry of the balanced chemical equations, you will calculate the volume of base that must be added to the final well of each reaction row to reach the equivalence point. An assessment of neutralization will be made using both a pH indicator and universal pH strips.

Procedure

Gather gloves, safety goggles, a sheet of paper, a pencil, a pie pan, paper towels, scissors, the 24-well plate, pH strips from Exercise 1, a trash container, and the chemical pipets used in Exercise 1.

Create labels for the 24-well plate on a sheet of paper as shown in Figure 11. The labels represent the following reactions:

HCl(aq)

+

NaOH(aq)

→

 

NaCl(aq)

 

+

H

2

O(l)

CH

3

CO

2

H(aq)

 

+

NaOH(aq)

→

NaCH

3

CO

2

(aq)

 

+

H

2

O(l)

H

2

SO

4

(aq)

 

+

2

NaOH(aq)

→

Na

2

SO

4

(aq)

 

+

2

H

2

O(l)

H

3

PO

4

(aq)

 

+

3

NaOH(aq)

→

Na

3

PO

4

(aq)

 

+

3

H

2

O(l)

Figure caption is an adequate description of the image.

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Figure 11. Chart for labeled 24-well plate grid. To save space, the AcOH abbreviation is used for CH3CO2H (acetic acid).

Place the labeled paper and well plate in a pie pan so that the markings show through the bottom of the wells. See Figure 12.

Figure caption is adequate description of image.

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Figure 12. 24-well plate resting on labeled paper inside a pie pan.

Put on our gloves and safety goggles.

Retrieve the following opened pipets from Exercise 1: HCl, CH3CO2H, H3PO4, and H2SO4.

Dispense 4 drops of each acid in each of the labeled wells of the 24-well plate. For example, well A1 contains 4 drops of HCl, while well B1 contains 4 drops of CH3COOH.

Note: If an air bubble is trapped in the tip of the pipet, expel the first drop onto a paper towl. Each drop that is dispensed into the well plate must be a full drop.

Dispense 1 drop of BTB indicator in each of the 20 wells containing the acids.

Note: The BTB indicator is yellow in a pH below 6.0.

Using the chemical equations from step 2, calculate the number of drops of NaOH required to neutralize 4 drops of each acid. Refer to the Background as needed, recalling that 1 mL = 20 drops.

Record the calculated number of NaOH drops to neutralize each of the four acids in the "NaOH drops and pH" fields in Data Table 2.

Dispense 1 drop of NaOH into the well A1 containing the 4 drops of HCl and 1 drop of BTB. See Figure 13.

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Figure 13. Adding NaOH to a well containing acid and BTB. The solution will be stirred with the toothpick.

Use a clean toothpick to stir the solution in the well.

Dispose of the toothpick in the trash.

Repeat steps 10–13 for the remaining 19 wells containing acid and BTB, dispensing the number of drops of NaOH as listed on the labeled grid below each well and also the calculated drops for neutralization recorded in Data Table 2 for wells A4, B4, C6, and D6.

Use a strip of pH paper prepared in Exercise 1 to measure the pH of the solution in well A1.

Record the pH of the solution in Data Table 2.

Note: If the color of the pH strip is not an exact match to the pH color chart, estimate the pH value between the closest two colors on the chart.

Place the used pH strip on a paper towel beside the well plate.

Repeat steps 14–16 for the remaining 19 wells of solution, using a new pH strip for each well.

Take a photo of the well plate to be used as a reference when answering the questions at the end of this exercise.

Upload the image into Photo 1.

Cleanup:

Save the unused pH strips for use in Exercise 3.

Properly dispose of the chemical pipets and used pH strips.

Clean and return the well plate to the lab kit.

Exercise 3 Testing the pH of Household Products

In this exercise, you will use pH paper to classify five household products.

CAUTION!

Do not mix the household products! 

Procedure

Put on your safety glasses and gloves, keep them on for the remainder of this experiment.

Locate 5 household products to be used for pH classification. Possible substances to test include home cleaning products, soft drinks, or liquid food items.

Record the name of one 1 household substance in Data Table 3.

Predict if the household substance is an acid, base, or neutral. Record the prediction in Data Table 3.

Pour a small amount of the household substance in a clean cup.

Use 1 cut piece of pH paper to determine the approximate pH of the chemical.

Dip the paper into the cup containing the household substance and immediately remove it. 

Immediately compare the color of the paper to the scale that was provided with the pH paper.

Determine the pH of the substance and record the pH in Data Table 3.

Use the pH to categorize the substance as an “acid, base, or neutral”. Record the conclusion in Data Table 3.

Repeat Steps 3–7 for the remaining 4 household substances. Use only 1 sectioned piece of pH paper for each substance. Use a clean cup for each substance.

Cleanup:

Clean all items and return to the lab kit for future use.