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.
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
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
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
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
Relative to Pure Water
||10 000 000
||1 000 000
||lemon juice, vinegar
||orange juice, soda
||tomato juice, acid rain
||black coffee, bananas
||sea water, eggs
||Great Salt Lake, milk of magnesia
||bleach, oven cleaner
||0.000 000 1
||liquid 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.