How does pH depend on pH?

Answer 1

pH is a measure of the acidity/or alkalinity of a substance. Substances that are more acidic have a lower pH value. It may help you to remember if you think of the pH scale as a measure of alkalinity (the opposite of acidity), that way, as the numbers get bigger the alkalinity increases. Acidity is due to the presence of hydrogen ions within the substance, and a greater concentration of hydrogen ions will cause something to be more acidic.

pH is commonly measured by introducing the substance to a pH strip, and comparing the color the strip turns against a scale provided with the strip. A pH strip is a piece of paper in which is embedded some substance that reacts to a certain level of acidity by changing color. Litmus is perhaps the most famous pH indicator (thus the expression of applying a litmus test) and will turn red at a pH below 4.5 and blue above 8.3. There are a number of other substances that change color at a specific acidity. A very accurate pH test strip works by embedding some of these substances in the strip so that different substances will change color, thus reveling the acidity of the substance.

Answer 2

pH paper is blotting paper that has been treated with a mixture of different indicators. Each indicator changes color over a different pH range.

Let's take a simplified example. Suppose we use two indicators: one that is red below pH 5, orange from 5 to 6, and yellow above 6; and one that is yellow below pH 7, green from 7 to 8, and blue above 8.

• At 4.5, we have red and yellow -> orange
• At 5.5, we have orange and yellow -> yellowish-orange
• At 6.5, we have yellow and yellow -> yellow
• At 7.5, we have yellow and green -> yellowish-green
• At 8.5, we have yellow and blue -> green

By adding additional indicators we can get even more color variation.