Today, we’re going to perform an experiment comparing the pH of soap and shampoo using various brands. We want to find out which soap and shampoo is the gentlest, if shampoo is gentler than soap, and how it all relates to pH.

Background

soap and shampoo pH, science experiment, pH test strips, shampoo pH, soap pHSoap requires three basic ingredients: oil, water and lye. Other ingredients are often added to provide additional benefits, such as a nice scent or moisturizing the skin. Lye has a pH of 13, so soap is usually alkaline. During the soap-making process, the pH drops to a more tolerable level, and can be anywhere from 7.5-10.

Shampoo is usually made by combining a surfactant, typically sodium lauryl sulfate or sodium laureth sulfate, with a co-surfactant, such as cocamidopropyl betaine, in water. Salt (sodium chloride) is also an essential ingredient, and like soap, other ingredients can be added to provide additional benefits. Ideally, shampoo should have a pH from 5-7. The higher the pH, the harsher it is on your hair.

Human hair, scalp oil and sebum (oil produced by our skin) have a pH balance between 4.5 and 5.5. It is slightly acidic, which prevents fungus and bacteria in the hair and scalp. Our skin is also slightly acidic at a pH of about 5.5. It has a thin, protective layer on its surface, known as the acid mantle, which is made up of sebum, mixed with lactic and amino acids from sweat.

With that information, we can conclude that the more alkaline pH of soap is harsher for our skin than the pH of shampoo. However, depending upon the ingredients, it will vary by brand.

Materials

Note: Try finding very different shampoos and soaps. For example, a natural shampoo, dandruff shampoo, intense moisturizing shampoo, store brand shampoo, baby shampoo, bar soap, liquid soap, natural soap, etc. If you have any homemade soaps or shampoos, use those to compare as well.

Instructions

  1. Let’s start by testing the soap pH. Using the razor blade, scrape off shavings of soap. You’ll approximately 1g of each type or 4mL of a liquid soap.
  2. In a cup, mix the soap shavings with 3mL of distilled water for 60 seconds. Label each cup with the brand of soap.
  3. Using the red and blue litmus paper, determine if each soap solution is alkaline or acidic. Test first with blue litmus paper, then with red. Record the results in your laboratory notebook.
  4. Next, test each soap solution with the pH 4.5-10 3 pad test strips. Record the results.
  5. Repeat steps 1-4, but replace distilled water with tap water. Record the results.
  6. Now, let’s test the shampoos. Fill a pipette with shampoo (5mL).
  7. Label each Petri dish with the shampoo brand. Empty the pipette of shampoo into the petri dish.
  8. Using the red and blue litmus paper, test the shampoo to determine if it is alkaline or acidic. Test first with the blue litmus paper, then with the red. Record the results.
  9. Next, test each shampoo with the pH 4.5-10 3 pad test strips. Record the results.
  10. Note: You may need to dilute the shampoo with a little bit of distilled water if it’s too thick and difficult to get a pH reading.

Review your results. Did different soaps and shampoos from the same brand have similar results? How did soaps and shampoos compare across brands? How did they compare across different types (i.e. natural vs. dandruff vs. baby)? Did the soaps generally have a higher pH than the shampoos? Which soap and which shampoo is the gentlest? If you used both liquid and solid soaps, did you find a difference in pH? When you shower, you’re most likely using tap water, not distilled water. Did you find a difference in the soap pH when using tap vs. distilled water?

To take this experiment a step further, you could compare soap and shampoo pH by age if you can find the manufacture date on the packaging. Does age affect the pH value? Another variation would be to include hand sanitizers in the testing pool. What other variations can you come up with to further expand on this experiment?