Question

Assume that you are given a solution of an unknown acid or base. How can you tell whether the unknown substance is acidic or basic?

Short answer

Determine the pH; if it's <7, it's acidic. If >7, it's basic.

Step-by-step solution

Gather Materials

Before you begin, make sure you have a pH meter or pH paper, clean beakers, and the unknown solution. A pH meter provides a digital reading of pH, while pH paper changes color depending on the pH of the solution it is dipped into.

Calibrate the pH Meter

If you are using a pH meter, calibrate it according to the manufacturer's instructions using standard buffer solutions, typically of pH 4, 7, and 10. Make sure the meter reads accurately for each buffer.

Test the Solution

If you are using pH paper, dip a small strip into the unknown solution and observe the color change. Use the color chart provided with the pH paper to match the color to a corresponding pH value. If you are using a pH meter, immerse the electrode in the solution and wait for the reading to stabilize.

Analyze the Results

For both pH paper and a pH meter, determine the pH value of the solution. If the pH is less than 7, the solution is acidic. If the pH is greater than 7, the solution is basic. A pH of exactly 7 indicates a neutral solution.

Key concepts

pH Meter

A pH meter is a precise device used to measure the acidity or alkalinity of a solution by providing a digital reading. This tool offers a quantitative insight into the pH level, which is a measure of how acidic or basic a solution is. Using a pH meter is straightforward, but it must be handled with care. To get an accurate reading, you must first

• Calibrate the Meter: This ensures accuracy by adjusting the meter to known pH values before use. Calibration is an essential step, often using buffer solutions of known pH values like 4, 7, and 10.
• Immerse Correctly: Ensure that the electrode is clean and fully immersed in the solution you are testing.
• Stabilize: Wait for the reading to stabilize before taking your final measurement.

Ultimately, a pH meter helps to identify if a solution is acidic (below 7), basic (above 7), or neutral (around 7). By being consistent with the calibration and use, a pH meter is a reliable source of accurate pH information.

pH Paper

pH paper is a simple and quick tool used to get an overview of a solution's pH level. It is a strip of special paper that changes color when dipped into a solution due to the presence of indicators that react with hydrogen ions.
These indicators are substances that change color at specific pH values, providing a visual queue of the solution’s acidity or basicity.

• Dip the Strip: Immerse a strip into the unknown solution briefly.
• Observe the Color Change: Quickly match the strip's color to a chart provided with the paper.
• Determine the pH Value: Use the chart to find out what pH value matches the color of your strip.

Take note that while pH paper is easy to use, it is less precise than a pH meter. It doesn’t provide a numerical output but rather an approximation based on the color change. This is generally adequate for many educational purposes or when a rough estimate is acceptable.

Calibration

Calibration is the process of configuring an instrument to provide results within an acceptable range. For pH meters, this step is crucial for ensuring accurate and reliable measurements. Without calibration, a pH meter might drift from true readings, leading to incorrect conclusions.
This process entails using buffer solutions, which are mixtures with a known and stable pH. These solutions typically have pH values of 4, 7, and 10 and serve as a reference point to adjust the pH meter.

To calibrate a pH meter:

• Prepare the Buffers: Use fresh and clean buffer solutions for accuracy.
• Follow Manufacturer Instructions: Every pH meter might have slightly different calibration steps, so refer to the instructional manual for precise steps.
• Adjust to Read Correctly: Immerse the electrode in the buffer, and adjust the meter so it reads the exact pH of the buffer.

Calibration helps the pH meter maintain precision by aligning the device's readings with the known values from the buffers. It's advisable to calibrate the meter often, particularly before each use, to account for any deviations that might occur over time due to various environmental or usage factors. By doing so, you ensure that your pH measurements are trustworthy and accurate.