How Is Acidity Measured in Water?

Rainfall naturally has some acidity.

Pure water has no acidity, but most water has some traces of other substances in it, which changes its level of acidity. Acidic water has free hydrogen ions roaming through it, while basic water has free hydroxyl ions mixed in.

Identification

The term pH refers to the measurement of a substance's acidity. The pH scale ranges from 0 to 14, with 0 through 6 being acidic, 7 being neutral, and 8 through 14 being basic. Water with a measurement close to 7 has less acidity or basicity than water with a number nearer to the scale's separate ends.

Indicators

The most exact pH measurements come from a pH meter, but pH indicators, usually composed of weak acids themselves, offer approximate values and often change color to indicate pH level. Litmus paper, the most common indicator, shows red when the pH reaches below 4.5, and blue when above 8.2.

Acid Rain

Only distilled water comes close to a pH of 7, and most water, even municipal tap water, has some acidity. Normal rain also has some acidity due to the carbon dioxide in the air. According to the Environmental Protection Agency, most rain has an acidic pH of nearly 5.6.

References

• United States Geological Survey: Common Water Measurements
• Elmhurst College: pH Scale
• Elmhurst College: Acid and Base Indicators
• United States Environmental Protection Agency: Measuring Acid Rain