Litmus paper is a simple tool used to determine the chemical nature of a liquid or gas. It consists of filter paper treated with a water-soluble dye mixture derived from lichens. This dye functions as an indicator, meaning its molecular structure changes when exposed to specific chemical environments, resulting in a visible color shift. When red litmus paper is placed into a solution, only alkalinity will cause it to change color, resulting in a distinct blue hue.
The Role of Alkalinity and the pH Scale
The color change of litmus paper results from a chemical reaction between the indicator dye and the concentration of hydroxide ions (\(text{OH}^-\)) in a solution. Alkalinity describes a base, which has a lower concentration of hydrogen ions (\(text{H}^+\)) and a higher concentration of hydroxide ions (\(text{OH}^-\)) when dissolved in water. The pH scale quantifies this concentration, ranging from 0 to 14, where 7 is neutral and values above 7 indicate increasing alkalinity.
Red litmus paper is treated with a dye that acts as a weak acid, ready to give up a hydrogen ion (\(text{H}^+\)). When the paper is introduced to an alkaline solution, the excess hydroxide ions (\(text{OH}^-\)) react with the hydrogen ions in the dye’s molecular structure. This reaction removes the hydrogen ion from the dye, leaving behind a new molecular structure called a conjugate base.
This conjugate base form of the dye absorbs and reflects light differently than the original acid form, specifically reflecting blue light. The color transition for litmus paper occurs over a limited range of approximately pH 4.5 to pH 8.3. Therefore, any substance testing above pH 8.3 will reliably cause the red litmus paper to turn blue, indicating a basic environment.
Common Household Substances That Turn Litmus Blue
Many common household substances are alkaline and will reliably turn red litmus paper blue. This is often due to compounds that release hydroxide ions in water. A simple example is a baking soda solution (sodium bicarbonate), which is mildly alkaline with a pH of around 8.4, high enough to trigger the color change.
Stronger alkaline substances include various cleaning products, such as ammonia-based glass cleaners (pH up to 11.6) and liquid drain cleaners containing sodium hydroxide (lye), a strong base. Common hand soaps and detergents are also formulated to be slightly alkaline to aid in the breakdown of fats and oils, causing the red litmus paper to shift to blue.
Milk of magnesia, an antacid containing magnesium hydroxide, also demonstrates this effect with a pH of about 10.5. These examples illustrate that the intensity of the blue color change does not indicate the exact strength of the base. Litmus paper only confirms that the substance is alkaline, not how far above pH 8.3 it falls.
The Opposite Reaction: What Turns Litmus Red
The opposing reaction occurs when a substance is acidic rather than alkaline. Acids are defined as substances with a pH below 7, characterized by a high concentration of hydrogen ions (\(text{H}^+\)) or hydronium ions (\(text{H}_3text{O}^+\)) in solution. This high concentration of positive ions drives the chemical interaction that causes a color change in the dye.
When blue litmus paper is introduced to an acid, the excess hydrogen ions bond with the dye molecules on the paper. This bonding process changes the molecular structure of the dye into its acid form, causing the paper to turn red. This color shift is observed when the solution’s pH drops below approximately 4.5.
Common household acids, such as vinegar (acetic acid) or lemon juice (citric acid), will turn blue litmus red, but they will cause no change to red litmus paper. A truly neutral substance, like pure distilled water, has a pH of exactly 7 and will not cause a color change in either red or blue litmus paper. This neutral state confirms that the concentration of hydrogen and hydroxide ions are balanced.