2.6: Acidity-pH Indicators, Acid-base Equilibria, and Acid Dissociation Constant-Home

Learning Objectives

• To prepare a homemade pH indicator and compare it with the standard pH strip available in the market
• To elucidate the equilibrium shift involved in the color change
• To calculate the acid dissociation constant of a weak acid from it pH value

Theory

pH is a scale used to measure the acidity of a substance. The scale ranges from 0 to 14, 0 being the most acidic, 14 being the most basic, and 7 being neutral. A pH value below 7 is considered as acidic, above 7 as basic or alkaline, and 7 as neutral. Plant pigment, Anthocyanin, like that in Red cabbage changes color with the acidity of the medium. It is a good indicator to measure the acidity of a solution. 

It measures the amount of H⁺ or H₃O⁺ (hydronium) ions in water. Acids contain more hydronium ions than in pure water (10^-7 M). Bases contain less than that amount. We can calculate the pH using the following equation. pH = (-) log [H₃O⁺] ion concentration. There are many ways to check the pH of a solution like pH meters, pH strips, pH indicator solutions etc.

In Part A we are going to extract a pH indicator found in red cabbage called anthocyanin and see how it changes color with some household items of varying acidity. We will check the pH of the tested items again with standard pH strips to compare and see how good the cabbage indicator is.

pH	pH less than 7=Acid			pH more than 7=Base
	2	4	6	8	10	12
Color	Red	Pinkish purple	Violet	Blue	Blue-green	Green-yellow

The reactive component of general acids is H⁺ or H₃O⁺ (hydronium) ions. The strength of an acid depends on how well the acid can dissociate to release the H⁺ or H₃O⁺ (hydronium) ions. Strong acid ions almost completely. Weak acids ionize partially or reversibly resulting in an equilibrium as follows. The ionization varies for various weak acids, so also their strengths. The strength of a weak acid is often expressed as its dissociation constant known as Acid Dissociation Constant or Ka.

In Part B we will measure the pH of a weak acid (Vinegar or 5% Acetic acid) of known concentration and use the following equation to calculate its Ka value. Then the calculated Ka will be compared with the true value available from the standard chart (1.8 X 10^-5). The % error will be calculated as

% Error= I (True value – Calculate value) I X 100
                            True value

Acid dissociation constant Ka = [H₃O^+] X [A^-] / [HA]                     No units for Ka
[HA] is the initial concentration of the acid. Here it is 0.087 M Acetic acid (5% acetic acid)

[H₃O⁺ ] can be calculate from pH as  [H₃O⁺]= 10^(-pH) . [A-] will be the same as the [H₃O⁺]  as they are 1:1.

Suppose pH =5.6, H₃O⁺ = 10^ (-5.6) = 2.5 X 10^-6 M

Procedure 

Materials required

pH strips, one red cabbage (1-2 leaves), 100 ml water, vinegar (5% acetic acid),5-7 house hold items to check the pH like Baking soda, soap, lemonade, antacid etc.(if the items in the list are not there at home, substitute with some nonhazardous materials at home), Sauce pan, pipets, stove or microwave, heat resistant gloves.

Note

Do not handle the cabbage solution when it is hot. Use heat resistant gloves to handle hot objects. Stick to heat safety protocols.
Reserve some red cabbage solution for the equilibrium lab. Complete part 2 of equilibrium lab involving cabbage solution after this lab.

Part A: Preparation of pH Indicator from Red Cabbage

 Preparation of the Indicator Solution-Place approximately 1 handful of chopped red cabbage pieces in the cooking vessel. Add 100 ml (1-2 cups) tap water. Keep the beaker and content on a hot plate/stove/microwave. Turn on the heat to boil the water. Careful when handling hot objects. When the plant pigment gets extracted into the water, turn off heating, and let the solution cool down to room temperature (say another 10 minutes). Drain off the cabbage pieces. Take the clear blue solution and divide this solution into 10 equal parts into each of the 10 vials/bottles/tumblers.
Analysis of the pH of various solutions including household items 

Label the vials/bottles/tumblers from 1-10 using a marker. Add 8-10 drops of each chemical into the labeled vials. (Read the safety instructions on each household chemical bottles if there any) Follow the order given in the following table. Observe the color change and record in the table given below. Compare with the color chart on page 1 and assign pH, and predict whether acid/base or neutral based on the color and pH comparison. If the color is too dark, dilute with water. Once you are done with the experiment, dispose the remaining indicator solutions and the solutions in the test tubes down the drain. Dispose the cabbage residue in the trash bin. Wash your hands thoroughly with soap. Hope you enjoyed the lab experiment! You can use different house hold items if you do not have all of the following items. But enter the item in the table. Check the pH of the household items tested using standard pH strips and record in the report sheet. Compare and contract the pH you got from the cabbage indicator and the standard strips.

Part B: Calculating the Acid Dissociation Constant for Acetic Acid of Known Concentration

Using a pH strip measure the pH of a Vinegar (5% acidity Acetic acid). Record this PH, and calculate the Hydronium ion concentration using the equation given in the theory part. Use that to calculate the Ka value. Compare the Ka value with the standard Ka value for acetic acid. Find the % error in your calculation.

Report Sheet

Part A: Preparation of pH Indicator from Red Cabbage

Solutions	Tap water	Vinegar	Baking soda	Lemonade	Cola	Antacid	Liquid soap	Table salt	Sugar
Color of cabbage solution
pH estimated
Acid/base/neutral
pH using pH strips

Part B: Calculating the Acid Dissociation Constant for Acetic Acid of Known Concentration

pH of Vinegar (5% acidity Acetic acid) measured using pH strip=
[H₃O⁺]  =

[A-] =

[HA]=0.87 M

Ka calculated=

Ka from literature (True value)= 1.8 X 10^-5

% Error=

Data Analysis

Questions

1. How good is the cabbage indicator in comparison to the standard strips to measure the pH of the household items. Write you opinion.

2. How would you explain the % error in the Ka value if it is more than 5%.

3. Other than anthocyanins what other plant pigments can change color with pH?

Contributors and Attributions

Manjusha Saraswathiamma, Minnesota State University, Moorhead has developed this experiment to be performed at home with less hazardous and cost-effective lab supplies. Manjusha would like to acknowledge the photographer whose picture is included in this experiment. The photo is by an unknown author licensed under CC-BY-SA.