Every day we use personal care products like shampoos, soaps, and lotions. We trust that these substances will clean and beautify us without giving us rashes. The scientists who design shampoos and face creams need to make sure their products work without making us break out in hives.
First of all, you need to understand pH. The pH scale is a measurement system designed to show the concentration of H+ ions in a solution. An ion is an atom that has either a positive or negative charge. H stands for the element hydrogen, and because the H has a + next to it, we know it is positively charged. The more H+ ions a substance has, the more acidic it is, and the lower the pH. For instance, lemon juice tastes sourer than orange juice. This is because lemon juice has more H+ ions, making it more acidic. The pH of lemon juice is around 2. Orange juice is also an acidic substance, but less so, because it has less H+ ions and its pH is around 3. Acidic substances all have pH values below 7.
A neutral substance on the pH scale has a pH of 7. Pure water has a pH of 7. Substances that have low H+ concentrations are called basic substances. Unlike acidic substances, basic substances don’t taste sour, they taste bitter. If you have ever accidentally gotten soap in your mouth, you have tasted a base. The pH of soap is around 10. Of course, you aren’t going to do any tasting in this experiment. The pH of any basic substance is above 7. The strongest bases have the highest pH. For example, oven cleaner, which you should never get on your bare hands, has a pH of 13.
Another concept that is important in the design of personal care products total alkalinity. Alkalinity is the measure of the capacity of a liquid to neutralize acids. Personal care product designers sometimes add alkaline compounds to products to help remove H+ ions and make the products less acidic. One familiar example of an alkaline compound is baking soda. Alkaline compounds usually work by picking up the extra H+ ions and making new compounds.
Now that you know about two concepts important to the chemical design of personal care products, you can test for them. Have fun, but protect your eyes, and no tasting!
What are the pH and total alkalinity of personal care products?
- Wide range pH strips
- Total alkalinity test strips (available at pool supply stores)
- Lemon juice or vinegar
- Baking soda
- Very clean cups
- Wide range of personal care products
- Following the directions for your pH paper, measure the pH of lemon juice or vinegar and tap water. Why would you want to find the pH of substances you already know about?
- Following the directions on the package, measure the pH of the personal care products you are interested in. Record data in table.
Baking soda solution
Personal Care product #1
- Following the directions on your total alkalinity test strips, measure the total alkalinity of a glass of water.
- Add a spoonful of baking soda to the glass of water. Measure the total alkalinity again. Why would you want to measure the total alkalinity of water and water with baking soda?
- Measure the total alkalinity of the personal care products you are interested in. Record data in table.
The lemon juice should have a pH of about 2. The tap water should have a pH of 7. If the pH values you find are widely different, you might have a problem with the cleanliness to your cup. Pure water will measure close to 0 in alkalinity, unless you have very hard water. The water with baking soda will have a much higher alkalinity. Many of the personal care products will have pH values close to 7, which is neutral. Products that are used for cleansing and have some soap in them may be on the basic side, but probably much closer to neutral than normal soap. Many products will have significant alkalinity values.
You measured the pH of the lemon juice and water to make sure you were using the pH paper correctly and to double check that factors like old soap on your glassware weren’t affecting your results. You measured the alkalinity of the water and baking soda solutions for the same reasons. The reason many of the products you tested have a pH close to neutral is because both high and low pH values can irritate skin, especially the eyes. Many of the products contain alkaline materials to absorb any extra acid and keep the product’s pH stable.
Total alkalinity and pH are only two of many factors personal care product designers have to consider. Find about what ingredients are likely to cause allergic reactions and how product safety is tested.