Litmus paper provides a rapid, qualitative assessment of a substance’s acidity or alkalinity. This simple indicator strip has been treated with natural dyes, typically derived from lichens, to determine if a sample is an acid or a base. Using the paper correctly ensures you get a reliable indication of the substance being tested.
Understanding the pH Scale and Indicators
The measurement of acidity and alkalinity is organized by the pH scale, which spans from 0 to 14. A substance with a pH below 7 is acidic, while a substance with a pH above 7 is basic or alkaline. Neutral substances, like pure water, have a pH level of 7. Litmus paper functions as an acid-base indicator by chemically reacting to the concentration of ions in a solution.
The dye infused in the paper changes its molecular structure depending on the environment. In an acidic solution, the dye absorbs a hydrogen ion, causing it to reflect red light. Conversely, in a basic environment, the dye releases a hydrogen ion, which shifts the color to blue. This reaction provides a general range, indicating acidity below a pH of 4.5 and alkalinity above a pH of 8.3.
Proper Procedure for Testing Samples
Handle the strips with tweezers or clean gloves to prevent contamination from oils or moisture on your hands. Pour a small amount of the liquid sample into a separate, clean container, rather than dipping the paper directly into the source container.
For liquid samples, dip the very tip of the strip into the solution or use a clean stirring rod to transfer a single drop onto the paper. A quick dip is sufficient; do not soak the paper, as this can wash away the dye. If testing a gas, the paper must first be lightly dampened with distilled water, allowing the gas to dissolve and react with the indicator. Observe the reaction immediately after contact with the sample.
Decoding the Color Change
Interpreting the color change requires understanding the two primary types of litmus paper: red and blue. Blue litmus paper is used to test for acidity; it turns red if exposed to an acid, but remains blue if the sample is basic or neutral.
Red litmus paper tests for alkalinity. If red litmus paper turns blue, the substance is confirmed to be a base. If the red paper shows no change in color, the sample is either neutral or acidic. If both red and blue papers show no change, the substance is likely neutral, with a pH near 7.
Ensuring Accurate Results and Storage
Several factors can compromise the reliability of a litmus test. Touching the paper with bare fingers or accidentally introducing foreign substances can impact the pH reading. Using paper that is too old can also lead to inaccurate results, as the indicator dye degrades over time.
Temperature extremes affect the chemical reaction, so avoid testing samples that are excessively hot or cold. To maintain the paper’s reactivity and shelf life, store it in its original, sealed container in a cool, dark, and dry location. Protecting the paper from moisture and chemical fumes is essential, as these can trigger a false color change before use.