EXPERIMENT 20: Determination of pH of Common Substances
pH paper and color chart (pH range 3 to 12) or pH meter
distilled water white vinegar household ammonia (or baking soda)
spot plate test or 3 small test tubes stirring rod
solutions / fruits juice (lemon, lime, orange, or melon) beverages (cola, carbonated non-cola, milk)
For most of the science experiments below, you will need a pH indicator, such as wide-range litmus, pH paper, or pH meter. These pH indicators contain a chemical that changes color when it comes in contact with acids or bases. For example, litmus and pH paper turn red in strong acids and blue in strong bases. Because only a few pH indicators measure pH over a wide range of pH values, you will need to find out the pH range of the indicator you use. Typically, the color chart provided with each pH indicator kit will show the pH range of that indicator. Color pH indicators provide only an approximate measure of the pH, or the strength of the acid or base. They are not as accurate as the expensive instruments scientists use to measure pH, but they are adequate for the experiments below.
Indicators, in chemistry, are natural or synthetic substances that change color in response to the nature of chemical environment. Litmus, for example, is a natural dye that turns red in most acidic solutions and blue in most basic solutions. Compounds that undergo color changes when there is a pH change in the solutions in which they are contained are called indicators (Table 1).
Indicators are used to provide information about the degree of acidity of a substance or the state of some chemical reactions within a solution being tested or analyzed.
How to Measure With pH Paper
When measuring pH with pH paper, dip the end of a strip of pH paper into each mixture you want to test. After about two seconds, remove the paper, and immediately compare the color at the wet end of the paper with the color chart provided with that pH indicator. When measuring pH with pH meter , dip the end of electrode of pH meter into each mixture you want to test. Write down the pH value and color. Always use a clean, unused strip of pH paper for each mixture that you test.