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Title: Identifying acids and bases with a pH indicator
Principle(s) Investigated:
The use of a homemade pH indicator (coffee filter saturated with red cabbage juice) to identify 5 common substances that are acidic, basic, and neutral.
Grade 8 Focus on Physical Sciences
5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
e. Students know how to determine whether a solution is acidic, basic, or neutral.
Grade 9 through 12 Chemistry
Acids and Bases
5. Acids, bases, and salts are three classes of compounds that form ions in water solutions. As a basis for understanding this concept:
a. Students know the observable properties of acids, bases, and salt solutions.
b. Students know acids are hydrogen-ion-donating and bases are hydrogen-ion-accepting substances.
d. Students know how to use the pH scale to characterize acid and base solutions.
Materials:
Each group of 5 would need,
- 5 containers of solutions, #1 - #5 being tested. (Students may also bring in their own solutions to test.)
- 5 droppers (optional)
- Red cabbage juice litmus paper strips (prepared earlier)
- Paper to place the soaked strips
Procedure:
- Dip one litmus strip into solution #1.
- Observe the color change.
- Determine whether it is acidic, basic, or neutral.
- Repeat steps 1-3 for solutions #2, #3, #4, and #5
- Tabulate changes and reasoning in the Quickwrite table.
Student prior knowledge:
Students need to know that pH (Power of Hydrogen) refers to the Hydrogen ion concentration of aqueous solutions. Students need to know the basic characteristics of acids and bases, such as many acids have a tangy taste and a pH <7, and bases feel soapy when touched and have a pH of >7. They need to know that a pH right in between is referred to as neutral.
Explanation:
This demonstration requires the pH indicator to be prepared ahead of time. In this case coffee filters have been soaked in a saturated solution of red cabbage juice, dried, and cut into strips for ease of use. The cabbage is boiled in distilled water (so as to not be affected by the varying pH of tap water), cooled, filtered and placed in a container. The coffee filters are soaked inside this juice until it is well absorbed. Once dry, they are cut into strips and stored. The strips are then dipped into each solution and observed for a color change.
As a variation, cabbage juice itself may be used as the indicator, and substances may be added to it to produce color changes. The use of pre-prepared strips eliminates the need for excessive juice, reducing the chances of spills in the classroom etc.
An acid-base indicator is a weak acid or a weak base that changes color over a range of hydrogen ion concentrations known as the pH range. The pigment molecule in red cabbage juice, flavin contains anthocyanin which turns red in acids and greenish-yellow in bases. Acids give away hydrogen ions in aqueous solution, having a lower pH, and bases accept hydrogen ions to have a higher pH.
The term pH, which is an abbreviation of "potential for Hydrogen" measures the relative concentrations of hydrogen ions (H+) and hydroxide ions (OH-). Because these ions combine to form water, they are always present in inverse proportions; an excess of one leads to a reduced amount of the other. It turns out that in an aqueous solution, the product of the concentrations of these two ions is 10-14. So if you have 10-7 moles per liter of H+ you will necessarily have 10-7 moles per liter of OH- as well (10-7 x 10-7 = 10-14), but if you have 10-3moles per liter of H+ (which is a lot) you will then have only 10-11 moles per liter of OH-. And that gives you the pH; it is the exponent of hydrogen (but without the minus sign). A concentration of 10-3 moles per liter of hydrogen ions means a pH of 3.
Below is a table illustrating the approximate pH based on the colors seen.
Questions & Answers:
- What other common home and garden chemicals could be used to determine pH instead of red cabbage juice?
Anthocyanin is a plant based indicator found in a variety of fruit and vegetable plants such as beets, berries, turmeric
powder, rose petals, strawberries, tea, etc. Baking soda, vanilla extract, and washing soda also react to acids and bases making them common indicators.
- Name one natural acid-base indicator that can turn back into it's original color when an acid/base is added to it.
Turmeric and rubbing alcohol. (Adding this to a base, such as basking soda would turn this solution from yellow to red,
and adding an acid such as vinegar would turn it back to yellow.
- How can the neutralizing capacity of an antacid be measured?
A standardized solution of 0.01 M HCl would have a pH of 1, similar to stomach acid. Similarly, a solution of 0.01 M NaOH could be used as the base. Determine what volume of base solution is necessary to neutralize 1) a measured volume of fresh acid, and 2) the same volume of acid which has a weighed, crushed antacid tablet dissolved in it. The difference between volumes of base in 1) and 2) is a good, reproducible measure of how much acid was actually neutralized by the antacid.
Applications to Everyday Life:
- pH is critical in maintaining healthy bodies. By staying hydrated and eliminating toxins, a neutral pH in the body is maintained. The use of antacids such as Pepto Bismol and Tums, which are basic relieve symptoms of acidity such as heart burn, nausea etc. thereby keeping a steady stomach pH. Similarly using toothpaste prevents tooth decay caused by excess acidity in the mouth.
- Soil pH is critical in determining whether certain plants can grow in it or not. Similarly pools must maintain a certain pH to prevent algae formation.
- In the food industry, most ingredients must maintain a specific pH in order for the food to turn out right. For example, food coloring in cookies.
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