University of Virginia
Physics Department

## An Acid-Base Reaction in Photography

A Physical Science Activity

### Student Activity

Materials (for each lab group)

• Graduated cylinder (capable of measuring 25 mL)
• Two shallow plastic containers (slightly larger than 4 x5 inches)
• 25 mL of Dektol solution
• 25 mL of Stop Bath solution
• Photo paper (pre-cut into 4 x 5 inch pieces)
• Paper towels
• Tongs
• pH paper
• Funnel
• Stop watch

Procedure

1. Measure 25 mL of Dektol and place in the first plastic container (#1).  Take care not to get any chemicals on your skin or clothing.
2. Rinse the cylinder well and measure 25 mL of Stop Bath and pour it into the second plastic container (#2).
3. Place the containers side by side with the Dektol (#1) on the left.  Open paper towels and place them to the right of the Stop Bath (see diagram below).

4. Using the pH paper record the starting pH level of each solution.  Record it under Assessment #1.
5. Classify the solutions you are starting with as either a strong acid,  weak acid,  weak base,  or strong base using the pH diagram below.  Record your answer on Assessment #1.

 Neutral 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Strong Acid Weak Acid Weak Base Strong Base

6. Use the tongs to place the first piece of photo paper into the Dektol (#1) solution for 90 seconds.  Then immerse it in the Stop Bath for 30 seconds,  and finally place it on the paper towels to dry.  Notice that you should be working from left to right.
7. Repeat with the second piece of photo paper and continue until the Stop Bath changes to a lavender color.  Be sure to count the number of photos you were able to "develop."  Record this under Assessment #3. You should stop once the Stop Bath keeps its lavender color for more than a couple seconds.
8. Check the ending pH level of the two solutions and record this under assessment #1.
9. Very Carefully, use the funnel to pour the remaining Dektol (#1) into the empty graduated cylinder.  Measure how much Dektol was left in the container.  Use this to calculate how much Dektol was used by the acid solution.

Example:  If the graduated cylinder reads 10 mL,  then subtract that from the amount you started with (25 mL) to get the amount that was used.

25 - 10 = 15      This amount,  15mL,  is then entered under Assessment #3.

10. Record data from the other groups in class and compare the results.  Record class data in the chart under Assessment #4.

Extensions

1. Search the Internet or the library to find out more about photo chemicals and the photography process.
2. Using what you know, write a paragraph explaining how acids and bases are not only used in science laboratories.

Assessment

1. Starting pH of Dektol:_______
Classification: (from procedure step 5) _______________________
Ending pH of Dektol:_______
Starting pH of Stop Bath:_______
Classification (from procedure step 35) _______________________
Ending pH of Stop Bath:_______
2. Why did the entire sheet of photo paper turn black when it was placed in the Dektol?

3. How many pieces of paper were you able to develop before the Stop Bath turned lavender?________
Amount of Dektol used by the Stop Bath?________
4. Record data from the other groups in your class.  If your class does not have 15 groups, leave some blank.

Data Sheet

 Group Number Number of Photos Developed Amount of Dektol Used 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5. What could explain the fact that results differ from group to group?

6. Do you see greater variance (a wider range of results) between the number of photos developed or the amount of Dektol used?

7. Explain why the Stop Bath changed colors.