To determine a substance’s pH value—a measure of hydrogen ion concentration—chemists and researchers use specialized chemical indicators. Two common tools for this purpose are litmus paper and universal pH paper, which serve different roles due to their distinct compositions and capabilities. Understanding the difference between these two indicators is crucial for selecting the correct tool for a given task.
The Basic Indicator: Litmus Paper
Litmus paper is one of the oldest and simplest forms of acid-base indicators, historically used since the 17th century. The paper is treated with a natural, water-soluble dye extracted primarily from lichens. This dye mixture is sensitive to changes in hydrogen ion concentration.
The function of litmus paper is purely to provide a qualitative, binary result: Is the substance acidic or basic? Blue litmus paper turns red when exposed to an acidic solution, while red litmus paper turns blue in the presence of an alkaline (basic) solution. The color change occurs over a broad pH range, typically between 4.5 and 8.3. If the paper does not change color, it only indicates that the substance is not of the opposite nature.
The Precision Tool: Universal pH Paper
In contrast to litmus, universal pH paper is designed to provide a more detailed, quantitative assessment of a solution’s acidity or alkalinity. This paper is impregnated with a universal indicator, which is a blend of several different indicator dyes. Each dye in the mixture changes color at a specific pH range, allowing the combined indicator to cover the entire pH scale from 0 to 14.
When universal pH paper is exposed to a solution, the resulting color is compared to a standardized color chart printed on the packaging. This chart provides a spectrum of colors, with each color corresponding to a specific numerical pH value. For instance, a strong acid may turn the paper bright red, a neutral solution will appear green, and a strong base will result in a violet or indigo color. This multi-dye system transforms the simple acid/base test into an approximate numerical measurement.
Key Functional Differences
The most significant difference between the two papers lies in the precision of the result they deliver. Litmus paper is a binary indicator, providing only a “yes or no” answer to the question of whether a substance is acidic or basic. It cannot distinguish between a weakly acidic solution and a strongly acidic solution.
Universal pH paper, by utilizing its blend of indicators, offers a numerical result, allowing for the determination of an approximate pH value. This quantitative capability is important when a precise reading is needed, such as monitoring the acidity of a swimming pool or the pH of soil for agriculture. Furthermore, litmus paper is ineffective at identifying a neutral solution, as water causes little to no color change in either red or blue strips. Universal pH paper, however, clearly indicates neutrality with a distinct color, typically green.
Another difference is the cost and application of the two indicators. Litmus paper is generally less expensive to produce and is best suited for quick, preliminary checks where only the general nature of the substance needs to be confirmed. Universal pH paper, being more complex to manufacture, is reserved for applications where an approximate numerical value is necessary, justifying the higher cost. The range of litmus paper is also limited by the fact that its color change interval is narrow, roughly 4.5 to 8.3, which means it cannot accurately reflect the nature of solutions outside this range.
Using Indicators Correctly
Accurate results from both types of indicator paper depend on proper technique. The most reliable method involves applying a drop of the solution to the paper using a clean glass rod, rather than dipping the entire strip into the bulk sample. Dipping the strip risks contaminating the solution with the paper’s dye. For solids, a small amount must first be dissolved in distilled water to create a solution for testing.
Reading the result immediately is important, particularly with universal pH paper, to prevent the color from changing as the paper dries or reacts with air. A common pitfall for both types of paper is improper storage; exposure to humidity or acidic/basic fumes in the air can cause the dyes to degrade or prematurely change color. Indicators should always be kept in their tightly sealed containers to ensure their accuracy remains intact.