Jul 24, 2013

Make Your Own pH Paper

Objective
The goal of this project is to make your own pH indicator paper, and use it to measure the acidity and alkanity of various solutions from around your house.
Introduction
In this project you’ll learn how to make your own pH paper that you can use to find out if a solution is acidic or basic (alkaline). What does it mean for a solution to be acidic or alkaline?
It all has to do with hydrogen ions (abbreviated with the chemical symbol H+). In water (H2O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxyl ions (OH−). The “lost” hydrogen ions join up with water molecules to form hydronium ions (H3O+). For simplicity, hydronium ions are referred to as hydrogen ions H+. In pure water, there are an equal number of hydrogen ions and hydroxyl ions. The solution is neither acidic or basic.
An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxyl ions is shifted. Now there are more hydrogen ions than hydroxyl ions in the solution. This kind of solution is acidic.
A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxyl ions shifts the opposite way. Because the base “soaks up” hydrogen ions, the result is a solution with more hydroxyl ions than hydrogen ions. This kind of solution is alkaline.
Acidity and alkalinity are measured with a logarithmic scale called pH. Here’s why: A strongly acidic solution can have one hundred million million (100,000,000,000,000) times more hydrogen ions than a strongly basic solution! The flip side, of course, is that a strongly basic solution can have 100,000,000,000,000 times more hydroxide ions than a strongly acidic solution. Moreover, the hydrogen ion and hydroxide ion concentrations in everyday solutions can vary over that entire range. In order to deal with these large numbers more easily, scientists use a logarithmic scale, the pH scale. Each one-unit change in the pH scale corresponds to a ten-fold change in hydrogen ion concentration. The pH scale ranges from 0 to 14. It’s a lot easier to use a logarithmic scale instead of always having to write down all those zeros! By the way, notice how one hundred million million is a one with fourteen zeros after it? It’s not coincidence, it’s logarithms!
To be more precise, pH is the negative logarithm of the hydrogen ion concentration:
pH = log 1/[H]+ = −log [H+] .
The square brackets around the H+ automatically mean “concentration” to a chemist. What the equation means is just what we said before: for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are alkaline (basic). The table below has examples of substances with different pH values (Decelles, 2002; Environment Canada, 2002; EPA, date unknown).
Table 1. The pH Scale: Some Examples
pH ValueH+ Concentration
Relative to Pure Water		Example
010 000 000battery acid
11 000 000sulfuric acid
2100 000lemon juice, vinegar
310 000orange juice, soda
41 000tomato juice, acid rain
5100black coffee, bananas
610urine, milk
71pure water
80.1sea water, eggs
90.01baking soda
100.001Great Salt Lake, milk of magnesia
110.000 1ammonia solution
120.000 01soapy water
130.000 001bleach, oven cleaner
140.000 000 1liquid drain cleaner
In this project you will make your own pH paper from a colored indicator that you will extract from red cabbage by cooking it in water. Once you have the indicator solution, you can soak some coffee filter paper in it, then allow the paper to dry. When the paper is dry, you can cut it into strips, and you’ll have pH paper that will change color. It will turn greenish when exposed to bases, and reddish when exposed to acids. How green or how red? That’s your job! Use different solutions that you have around the house to find out how the color change corresponds to changes in pH.
Terms, Concepts and Questions to Start Background Research
To do this project, you should do research that enables you to understand the following terms and concepts:
  • Acids
  • Bases
  • Logarithms
  • pH
  • pH indicators
Questions
  • What value of pH is neutral?
  • What range of pH values is acidic?
  • What range of pH values is basic?
  • What color is red cabbage pH paper when dipped in acidic solutions?
  • What color is red cabbage pH paper when dipped in basic solutions?
Bibliography
Materials and Equipment
To do this experiment you will need the following materials and equipment:
  • Red cabbage leaves
  • 1-quart cooking pot
  • Water
  • 1-quart bowl
  • Strainer
  • White coffee filters (cone-shaped ones are good)
    • Alternatively, you can use filter paper or chromatography paper.
  • Acidic and basic solutions to test, for example:
    • Lemon juice, vinegar
    • Orange juice, soda
    • Tomato juice, acid rain
    • Black coffee, bananas
    • Milk, saliva
    • Pure water
    • Sea water, eggs
    • Baking soda solution
    • Milk of magnesia
    • Ammonia solution
    • Soapy water
Experimental Procedure
Safety Notes:
  • Adult supervision required.
  • Do not mix strong acids and bases.
  • Use appropriate caution when testing the pH of household cleaning solutions (like ammonia). Avoid skin contact, and follow all precautions on the product label.
  1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
  2. Prepare a red cabbage indicator solution (the “Experiments with Acids and Bases” webpage (Carboni, 2004) has great pictures illustrating all of the steps)
    1. Slice a head of cabbage at approximately 3 cm (1 in) intervals, or peel the leaves from the head and tear them into pieces.
    2. Place the leaves in the cooking pot and cover with water.
    3. Cook on medium heat for half an hour (low boil is good).
    4. Allow the cooked cabbage to cool, then pour off the liquid into a bowl. You can pour through a strainer to catch the cabbage pieces, or hold them back with a large, flat ladle with holes—see the photographs on the “Experiments with Acids and Bases” webpage (Carboni, 2004).
    5. The solution is a deep blue, but will change color when the pH changes. (You can experiment with using the liquid as a pH indicator.)
  3. Here’s how to make pH paper using the red cabbage solution and coffee filters:
    1. Soak the white coffee filters in the red cabbage solution for about 30 minutes.
    2. Drain the excess solution from the filters, and set them out in a single layer on some paper towels to dry overnight. To speed up the drying process, you can put them on a cookie sheet and put them in your oven at low temperature (150–200°F.
    3. When the coffee filters are dry, cut them into 3 cm × 8 cm (about 1 in × 3 in) strips.
    4. The strips are now ready to test the pH of various solutions. They start out blue, but will turn green in basic solutions and red in acidic solutions.
  4. Use the strips to test the acidity/alkalinity of various solutions around your house. For example:
    • Lemon juice, vinegar
    • Orange juice, soda
    • Tomato juice, acid rain
    • Black coffee, bananas
    • Milk, saliva
    • Pure water
    • Sea water, eggs
    • Baking soda solution
    • Milk of magnesia
    • Ammonia solution
    • Soapy water
    • Note: if you test the pH of saliva, do not put the pH paper in your mouth! Instead, spit some saliva into a clean container and dip the paper into the saliva.
  5. After testing, put the pH strips in order of increasing pH of the solution tested.
    1. You can use the table in the Introduction as a guide.
    2. The Variations section has some additional suggestions for independent confirmations of the pH readings.
  6. Do you see a gradual change in color as the pH of the tested solutions varies? Can you match specific colors to certain pH levels? Over what range of pH does the color continue to change? How accurately do you think you can determine the pH of a solution with your test papers? Within 1, 2, or 3 pH units?
Variations
  • Compare the performance of your homemade pH paper with commercial pH paper (can be found in a well-stocked tropical fish store). Or, buy an inexpensive pH meter and use it to calibrate your homemade pH paper. Use the table in the Introduction to make a series of different solutions, form low to high pH. Measure the pH of each solution with the pH meter (rinse off the tip between solutions), and write down the results. Now check each solution with your pH paper. Can you see color differences that correspond to the measured changes in pH? Over what pH range do you see color changes? How large does the shift in pH need to be in order to see a change in color?
  • Try making pH indicator solutions (and/or indicator papers) from other natural dyes: for example beet juice, phenolphthalein, or turmeric powder (Beckham, date unknown; Krampf, 2006). Test your household solutions with each of the indicators. Does the additional information from multiple indicators give you a better measure of the pH of your solutions?
  • Does the pH of your saliva change after eating various types of food? If so, how much time does it take to return to normal? Design an experiment to find out. Again, do not put the pH paper in your mouth. Instead, spit some saliva into a clean container and dip the pH paper into the saliva. Also, don’t try changing the pH of your saliva with anything non-edible!
  • What is the pH of rainwater in your area? Can you measure it with your pH paper or pH indicator solutions?
Posted by samkri at 9:14 AM

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