Calibrating a pH Meter and Measuring the pH of a Solution
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Calibrating a pH Meter and Measuring the pH of a Solution
Overview
pH is the quantitative measure of the acidity or basicity of aqueous or other liquid solutions. The pH is determined by measuring the concentration of the hydrogen ion, denoted [H+]. [H+] ranges between approximately 1 and 10-14 gram-equivalents per liter. In pure water, which is neutral, the [H+] is 10-7 gram-equivalents per liter, which corresponds to a pH of 7. A solution with a pH less than 7 is considered acidic; a solution with a pH greater than 7 is considered basic, or alkaline. pH is calculated as the negative log of the H+ concentration ( pH = -log[H+] ).
Procedure
A. Calibration of the pH Meter
1. Remove the pH meter probe from the pH 7 Standard Solution (see Hint #1).
2. Rinse the probe with a gentle stream of ddH2O.
3. Remove any excess liquid from the probe tip (see Hint #2).
If the solution to be measured is acidic follow Steps #A4 to #A9
4. Place the probe in the pH 7 Standard Solution.
5. Adjust the meter to read a pH of 7.0 (see Hint #3 and Hint #4).
6. Remove the probe from the solution, rinse the probe with a gentle stream of ddH2O, and remove any excess liquid from the probe tip (as in Steps #A1 to #A3).
7. Place the probe in the pH 4 Standard Solution.
8. Adjust the meter to read a pH of 4.0 (see Hint #3 and Hint #4).
9. Following the instructions accompanying the pH meter in use, calibrate a new linear regression curve (see Hint #5) and continue with Section B.
If the solution to be measured is basic, follow Steps #A10 to #A13
10. Place the probe in the pH 7 Standard Solution.
11. Adjust the meter to read a pH of 7.0 (see Hint #3 and Hint #4).
12. Remove the probe from the solution, rinse the probe with a gentle stream of ddH2O, and remove any excess liquid from the probe tip (as in Steps #A1 to #A3).
13. Place the probe in the pH 10 Standard Solution.
14. Adjust the meter to read a pH of 10.0 (see Hint #3 and Hint #4).
15. Following the instructions accompanying the pH meter in use, calibrate a new linear regression curve (see Hint #5) and continue with Section B.
B. Measuring the pH of a Solution
1. Rinse the probe with a gentle stream of ddH2O and remove any excess liquid from the probe tip (as in Steps #A1 to #A3).
2. Place the probe into the solution (see Hint #6).
3. Use the meter to measure the pH of the solution.
4. Record the measured pH.
Solutions
pH 7 Standard
Purchase an ACS pH standard
pH 10 Standard
Purchase an American Chemical Standard (ACS) pH standard
pH 4 Standard
Purchase an ACS pH standard
BioReagents and Chemicals
pH 10 Standard
pH 7 Standard
pH 4 Standard
Sodium Chloride
Protocol Hints
1. The pH meter probe must be stored in a balanced salt solution such as the pH 7 Standard Solution. If the probe is not stored in a balanced salt solution, then the exchange surface of the probe can dry out. It is imperative that there is ample supply of a saturated salt (NaCl) solution inside the pH meter. Visually inspect the inside solution of the pH probe and verify that there are salt crystals and water (see Hint #4).
2. Never touch the probe tip surface. To remove excess liquid from the probe tip, use the corner of a Kimwipe and touch the corner to the liquid that is hanging from the probe tip. The glass membrane at the probe's tip in a glass electrode is designed to allow partial penetration by the analyte ion (i.e. H+). Touching the probe tip even lightly will impact tissue into the membrane and impede the flow of H+, resulting in inaccurate pH measurements.
3. Before using the meter to measure the pH of the solution, allow the measured pH of the solution to equilibrate (depending on how clean the pH probe is, this can take from 1 min to several minutes). Follow the meter manufacturer's instructions on how to "tell" the meter the pH of the solution.
4. A pH meter consists of a voltmeter attached to a pH-responsive electrode and a reference, or unvarying, electrode (both electrodes are inside a pH meter probe). The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride electrode or a silver-silver chloride electrode. When the two electrodes are immersed in a solution, they act as a battery. Therefore, a saturated salt (NaCl) solution inside the meter is critical to allow an electrical current between the two probes. The glass electrode develops an electric potential (charge) that is directly related to the hydrogen-ion activity in the solution, and the voltmeter measures the potential difference between the glass and reference electrodes.
5. The electronics of modern pH meters contain a linear regression curve algorithm. All pH measurements made after calibrating the meter will use this newly created regression curve to calculate the pH of solutions. If the regression residue (r2) is not sufficiently close to 1.000, then repeat the calibration steps.
6. Do not stir the solution while measuring the pH. If there is excessive fluid motion, the exchange of H+ across the glass electrode and the measured pH will not be accurate.
