The pH scale measures how acidic or alkaline a substance is by quantifying the concentration of hydrogen ions in a solution. Spanning from 0 to 14, a value of 7 is neutral, such as pure water. Substances below pH 7 are classified as acidic, while those above 7 are considered alkaline. pH paper, often called litmus paper or test strips, provides an accessible method to quickly determine this value using chemical indicators that change color upon contact with a sample. The simple color-matching process provides an approximate pH reading, making it a common tool for home testing and science classrooms.
The Step-by-Step Guide to Testing
Gather the substance to be tested, a clean surface, and a fresh strip of pH paper. Avoid touching the reactive end of the paper, as oils or moisture from your skin can contaminate the strip and skew the result. For liquid samples, gently dip one end of the pH paper into the substance for one to two seconds, ensuring the indicator pads are fully saturated.
If testing a semi-solid or surface, such as soil or food, use a small drop of distilled water to create a moist sample, or press the strip directly onto the damp surface. Consult the manufacturer’s instructions for precise timing, but the color change usually happens almost immediately upon contact. Carefully remove the strip and shake off any excess liquid, preparing to read the result without delay.
The waiting time before comparing the color is typically specified as 10 to 15 seconds, though some papers require a shorter duration. Reading the result too early or too late can lead to an inaccurate measurement, as the chemical reaction continues to change slightly over time. Once the recommended time has passed, the color is fixed and ready for comparison against the standardized chart.
Interpreting the pH Color Chart
After the color has developed, hold the strip next to the color chart provided with the pH paper kit. This chart displays a range of colors, each corresponding to a specific whole or half-unit pH value. Comparing the color of the moistened paper to the chart under good lighting helps find the closest match and determine the sample’s approximate pH level.
The universal indicator colors follow a recognizable spectrum, helping to quickly categorize the sample. Strong acids (pH near 0) produce colors in the red to orange range. Neutral substances (pH 7) often result in a green or yellowish-green color.
As the pH increases toward 14, indicating a stronger alkaline substance, the colors progress through shades of blue and purple. Understanding that reds and oranges indicate acidity, while blues and violets indicate alkalinity, assists in quickly assessing the nature of the substance. The specific color intensity provides the final numerical value.
Tips for Accurate Measurement
To maintain the integrity of the test, prevent cross-contamination by using a separate, clean sample for each strip. Store the pH paper in its original, air-tight container away from humidity, as exposure to moisture or air can degrade the chemical indicators and lead to false readings. Always check the expiration date on the packaging, as older paper may not react reliably.
For the most dependable result, the color comparison should be made immediately after the manufacturer’s waiting period has elapsed, as the color can begin to fade or change further once the strip dries out. General pH paper is designed for an approximate, or qualitative, measurement, typically with an accuracy of one pH unit. For applications requiring high precision, such as sensitive scientific research, a calibrated pH meter is necessary.