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Indicators, Paper, and Meters
pH is a measure of the concentration of hydronium (H3O+) ions in an aqueous solution. It is measured on a negative logarithmic scale from 0 to 14. Acidic solutions are below pH 7, with 0 being the most acidic. Basic solutions are above pH 7, with 14 being the most basic. A solution with a pH of 7 is considered neutral. An example of a neutral solution is pure water at room temperature.
In the science classroom, there are many activities that require pH testing. They include chemistry titrations, environmental science water quality testing, and biological processes labs. As science teachers plan these activities, they may be overwhelmed by the variety of testing options available, including liquid pH indicators, pH test papers, and pH meters. Some activities clearly specify the type of measurement tool to be used, while others do not. Read on for information about pH indicators, pH test papers, and pH meters.
Liquid acid-base indicators are weak organic acids or bases that present as different colors in their acid and base forms. An indicator has a specific pH range over which it changes from its acid form to its base form. An indicator is not useful outside its pH range because the indicator does not change color over these pH values.
Some of the most widely-used pH testing tools are pH indicators, including phenolphthalein (range pH 8.2 to 10.0; colorless to pink), bromthymol blue (range pH 6.0 to 7.6; yellow to blue), and litmus (range pH 4.5 to 8.3; red to blue).
Universal indicators are mixtures of several different pH indicators that extend the pH range over which they operate. Bogen universal indicator is a mixture of methyl red, bromthymol blue, and phenolphthalein, and indicates on a wide pH range of 4.0 to 10.0 in discrete color steps.
Liquid indicators are especially useful in acid-base titrations, where a noticeable pH change occurs near the equivalence point. Select a pH indicator whose pH range falls within the pH change of the reaction. pH indicators are also commonly used to perform quick checks on the pH of water samples (aquaria, pools, drinking water). This method of measuring pH is quick, inexpensive, and easy.
There are drawbacks to using pH indicators. The test sample should be fairly colorless to clearly see the indicator's color change. Also, the indicators inherently measure pH at a low accuracy.
Litmus paper is probably the most familiar pH paper. It is used to broadly test whether a solution is acidic or basic and comes in 3 types—red, blue, and neutral. Red litmus turns blue in basic solutions, blue litmus turns red in acidic solution, and neutral litmus (usually purple) turns red in acidic solutions and blue in basic solutions. To find the specific pH of a sample, you will need a pH test paper or strip that is more precise than a litmus strip.
More precise pH test papers or strips can give test results down to 0.2 pH units. There is a large selection of pH test papers and strips available, from wide pH range, low sensitivity to narrow pH range, high sensitivity.
There are many benefits to using pH strips and papers. Like indicator solutions, they are quick and easy to use, and compared to pH meters, they are much less expensive. pH papers are portable, easy to store, and well suited for field work. In classrooms, pH strips can be pasted into a lab notebook to retain experimental results.
While pH strips and papers give more accuracy and precision than liquid indicators, they are still inferior to pH meters. Solution color and turbidity are also concerns when using pH strips and papers—colorless solutions give the best results.
The most precise of the 3 test options, pH meters measure a solution's pH by measuring the electrical potential difference between the pH electrode and a reference electrode. The meter then coverts this potential to a pH reading. They offer readings to 0.01 pH unit, and are useful for advanced science, college, or research work that requires this level of precision.
pH meters require calibration and more upkeep than other pH testing equipment. It is imperative that the meter electrodes are kept clean and maintained or replaced according to the manufacturer's instructions.
There are some added bonuses of using a meter. Many meters also measure dissolved solids, electrical conductivity, and temperature. The convenience of these multi-taskers can often justify the price point for classroom use.
If you're conducting wet labs in your science classroom, you'll likely need to test pH at some point. Consider cost, precision, accuracy, portability, and convenience when choosing a pH testing method.
Whether you choose a pH indicator, pH strips, or a pH meter, be sure to store the measurement tool properly. Always store test strips in their original containers or in another airtight container. Do not expose them to moisture or extreme temperatures. Liquid indicators and some test strips may have an expiration date. Also note and follow any special storage instructions. Before using, test your indicator or strips in a solution of known pH (other than water) to make sure they work properly. If they do not give the expected pH value, discard and order a fresh supply.
For long-term storage of handheld meters, remove the batteries to minimize risk of corrosion, battery leakage, or explosion and damage to the unit. Neither handheld units nor bench meters should be stored under extreme conditions or in wet environments. Always refer to the manufacturer's guide for proper storage.