Science Project:

Don't Get Soap in Your Eye!

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The goal is to have the student formulate and test a hypothesis about pH and eye irritation in commercial soaps and detergents.

  • What is the relationship between the pH values of commercial soap and detergent products and the product manufacturers’ claims about potential for eye irritation?
  • Is pH a reasonable measure of potential eye irritation in soaps and detergents?
  • Is there a pH at which soaps and detergents no longer irritate the eye?

Soaps and detergents are both cleansing products. Soaps are made of natural products (fats and alkali); detergents are made from synthetic compounds. The FDA requires that the product ingredients of detergents (but not soaps) be listed on the package.

All soaps and detergents break up oils and dirt and wash them away. The main differences are in pH, whether they contain bleach, and the types of surfactants they use. [Surfactants are long chain molecules with a hydrophilic (water-loving) molecular group at one end a hydrophobic (water-hating) group at the other.]

Most dirt is acidic, so an alkaline product makes a good cleanser. A pH of 7 is neutral, reflecting a balance of acidity and alkalinity. Acidic solutions have pH’s in the range of 0 to 6, with 0 very acidic; and alkaline solutions have pH’s between 8 and 14, with 14 very alkaline.

Automatic dishwasher detergents break down stains and food with high pH and chlorine bleach. Detergents used for hand dishwashing tend to have neutral pH’s and use blends of surfactants that are mild to the skin. Laundry detergents fall in between. Traditional detergent formulations use high pH for cleaning, while newer detergents have pH’s in the neutral range and rely more on surfactants to help disperse dirt and oils. A mild soap has a pH of about 8, while a harsh one has a pH closer to 10.

Soap and detergents cause a burning sensation when they enter the eye. This is usually attributed to their alkalinity, although added ingredients such as fragrances may also be eye irritants. Studies have found that the pH in the healthy eye is about 7.5.

  • pH tester
  • Commercial soaps and detergents
  • Eye goggles

These items can be found in the following places: Internet (pH tester, safety goggles), Wal-Mart type store (soaps/detergents, distilled water)

  1. Read about soaps and detergents and formulate a hypothesis that predicts whether pH should correlate closely with the potential for eye irritation.
  2. Place 1 teaspoon of the soap or detergent to be tested in a 16-oz.cup.

Soaps and detergents may irritate the eyes. Always wear safety goggles when working with them.

  1. Place 8 oz. of distilled water in the cup.
  2. Mix the solution for two minutes.
  3. Measure the pH of each solution using a digital pH tester.
  4. Rinse the tester with distilled water.
  5. Repeat these steps for each of the soaps/detergents to be tested.

Be sure to include products marketed as “tear-free” in the study.

  1. Tabulate your results.
  2. In a separate column in the table, list the manufacturer’s claims or warnings about the potential of the products to cause eye irritation.

Contact the manufacturer if this information does not appear on the package.

  1. Look for a correlation between measured pH and the manufacturers’ claims/warnings.
  2. Evaluate your hypothesis using the data. If necessary, modify or revise it and then conduct additional experiments.

pH Meter Test Results. Source: http://biology.clc.uc.edu/scripts/phresl.pl

Substance

Avg. pH

Detergent‚ Bubble Bath‚ Avon

4.70

Detergent‚ Rinse‚ Jet Dry

6.16

Detergent‚ Bubble Bath

7.04

Detergent‚ dishwashing liquid

7.14

Soap‚ Liquid

7.16

Detergent‚ dishwashing liquid‚ Ajax

7.61

Detergent‚ dishwashing liquid‚ Joy

7.91

Soap‚ Bar

8.05

Detergent‚ dishwashing liquid‚ Ivory

8.08

Detergent‚ dishwasher‚ Calgon

9.31

Detergent‚ laundry‚ Tide

9.55

Detergent‚ laundry

9.67

Detergent‚ dishwasher‚ Cascade

9.92

Detergent‚ Ajax

10.10

Detergent‚ Gain

10.52

Detergent‚ dishwasher‚ Electrasol

11.00

Soap‚ Anti-Bacterial

11.40

Terms/Concepts: Soap; Detergent; Surfactant; Acidic; Alkaline; pH; Eye irritants

References:

Author: Randall Frost, Ph.D.
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