Litmus paper is paper that has been treated with a specific indicator—a mixture of 10-15 natural dyes obtained from lichens (mainly Roccella tinctoria) that turns red in response to acidic conditions (pH 7).
- Litmus paper is a tool used to test whether a substance is an acid or base. When a substance is dissolved in water, the resulting solution causes the litmus paper to change colour.
- The acidity or alkalinity of a solution is determined by the concentration of hydrogen ions, or power of hydrogen, expressed as a pH value.
- A litmus test provides a quick result but cannot determine the level of acidity or alkalinity of a solution.
Performing Litmus Test
- It gives the user a general indication of acidity or alkalinity as it correlates to the shade of red or blue that the paper turns.
- To test the pH of a substance, dip a strip of litmus paper into the solution or use a dropper or pipette to drip a small amount of a solution onto the litmus paper.
- Blue paper can indicate an acid with a pH between 4 and 5 or lower. Red paper can show a base with a pH greater than 8. If a solution has a pH between 5 and 8, it will show little color change on the paper.
- A base tested with blue paper will not show any color change, nor will an acid tested with red paper register a change in color
Limitations
- Using a litmus test is a quick and easy way to determine if a solution is acidic or alkaline.
- Litmus paper is inexpensive, portable and can test acidity and alkalinity using only with a small volume of a solution.
- However, it can’t provide the actual pH for a substance, other than indicating if the pH is roughly less than 5 or greater than 8.
- Litmus paper is not useful for testing substances with a pH that is closer to neutral.
