University of Virginia
Physics Department

An Acid-Base Reaction in Photography

A Physical Science Activity

2003 Virginia SOLs



Students will:


Motivation for Learning

Demonstration-Discrepant Event


*** The photo chemicals and paper can be obtained from a local camera/hobby shop or your school's (or the high school's) photography department.  They are not very expensive.  Approximate costs are:



  1. Hold up the developed black and white photo for the class to examine.  Ask the class how a photograph is made.  Students will probably state that an image is "developed" onto the paper.  Ask if any particular paper will work.  Probe students until it is revealed that light reacts with a sensitized emulsion on the photo paper to produce an image.  If the students are more advanced, you can discuss the specifics of the chemical reaction.  (See the explanation in the background section below).
  2. Now hold up a blank piece of the photo paper.  Tell the students that you are now placing the paper into a finishing chemical called Dektol.  Using the tongs, place the paper into the Dektol that you have placed in a plastic container for 90 seconds.  Bring the paper out of the solution and the paper should be totally darkened (black in color).  Ask the students why this happened.  Magic?  No, the light exposed all the areas of the paper.  Then mention that photographers work in dark rooms where the light will not expose or darken all of the paper.  If appropriate,  discuss that the only "safe light" in dark room is a red or amber light,  with wavelengths that are long enough that they will not affect the paper for a certain period of time (it will allow about four minutes of working time).


Suggestions and Possible Modifications for the Teacher

  1. If expense is an issue, rather than using the photo paper and placing it in both solutions,students could count the drops or measure the volume of Dektol needed to neutralize the Stop Bath.  Approximately 25 drops of Dektol are needed to neutralize 10 mL of stop Bath.
  2. If you could make your classroom or an area of the school "light-proof" by sealing out the light and working with red lights then you could actually have students develop images that have been captured on the paper by photography students.  This could be a collaborative lesson with the art department.  Otherwise, when working in regular lighting the entire sheet of paper will turn black when placed in the Dektol.
  3. If the technology is available at your school,  you could replace the pH paper with the pH probes and use the computer or CBL to collect data.
  4. The teacher should pre mix the chemical according to the instructions on the package.
    • Dektol - 1 pkg to 3.75 L of water, water needs to be a certain temperature when mixing
    • Stop Bath - 2 oz. to 3.75 L of water


Background Information

This activity is designed to show students how acid and base solutions are used in photography.  When photos are developed, several chemical reactions occur, and students should have a basic understanding of the reactions.  Light reacts with the silver halide crystals in the paper to form a latent image.  Photon energy from the light excites particles from the emulsion on the paper, kicking out electrons.  Then the paper is placed in a developing solution that magnifies the reaction causing the silver to clump together and darken in certain areas forming the image.  However, when the photo paper is exposed to light and then placed in a developing solution it will cause the entire sheet to be exposed, or turn black.  That's why photographers work in specially lighted dark rooms.  In the developing process, the photos are immersed in a basic solution such as Dektol (the one that will be used in this activity), and then in an acidic solution, the Stop Bath.  The Stop Bath contains an indicator (similar to phenolphthalein) that will change color once the acid has been neutralized.  The photographer then knows that it's time to change the solution.  The students will measure how much Dektol it takes to neutralize the Stop Bath. 

Suggested web sites that contain more information on the photo processes and products  (many contain links to specific topics):





Student Activity

To print out the Student Copy only, click here.

Materials (for each lab group)



  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.

    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.



  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.


Students with Special Needs

For students with processing difficulties it may be helpful to read one step at a time and have all groups complete that step before going further.  Then the teacher could be sure that the students are doing what they should and getting proper results.  Another strategy would be to have students physically check off the procedure steps as they complete them.

Click here for further information on laboratories with students with special needs.




To print a copy of the assessment only, click here.

  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

  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.





Answers to Assessment

  1. The starting pH of the Dektol solution is about 12, classifying it as a strong base.  The starting pH of the Stop Bath is about 4, classifying it as a weak acid.  The pH of the Dektol should not change,  but the ending pH of the Stop Bath should be close to neutral (7).
  2. The entire sheet turned black because the light from the room exposed all areas of the paper, causing all of the silver to clump and darken giving a solid black image.
  3. Students should be able to develop about 7 sheets, using approximately 12 mL.
  4. Results will vary.
  5. The amount of Dektol transferred to the Stop Bath with each sheet of paper is slightly different, so results will be different.  Also, it is hard to catch the Stop Bath at the exact point when it is neutralized or turns lavender. 
  6. Greater variance is expected in counting the number of sheets because it is a less exact method of measurement. 
  7. The Stop Bath changed colors because it contains an indicator that causes the solution to change colors when it reaches a neutral pH.