pH Measurement and pH Meters
The approximate display of pH can be obtained using pH paper or indicators. These substances change color when the pH changes. However, the drawback of using indicators is their poor accuracy, and it can be difficult to detect the color change accurately in samples with color. A more accurate method for measuring pH is using a pH meter.
Basis of pH Measurement
pH is the unit of measure for acidity or basicity. It ranges from 0 to 14. The quantitative information provided by the pH value reflects the level of dissociation of hydrogen ions (H+) in an acid or base. The pH value of a compound is directly related to the ratio of hydrogen ions (H+) and hydroxyl ions (OH-). If the concentration of H+ is greater than OH-, the environment is acidic, and the pH will be less than 7. If the concentration of OH- is greater than H+, the environment is basic, and the pH will be greater than 7. If the concentration of H+ is equal to OH-, the environment is neutral, and the pH will be 7. Acids and bases have free H+ and OH- ions, respectively. The relationship between H+ and OH- ions in any solution is constant under all conditions, so knowing one of these quantities will allow you to deduce the other.
Structure of a pH Meter
A pH meter is a device used to measure the acidity or basicity of a solution (referred to as pH). A pH measurement system consists of three parts: 1) the pH electrode, 2) the reference electrode, and 3) the high input resistance meter. The pH electrode can be thought of as a battery, with a voltage that changes depending on the pH of the solution being measured. The pH electrode is a glass bulb sensitive to H+ ions, with an output signal (in millivolts) that changes based on the concentration of H+ ions inside and outside the bulb. The signal from the reference electrode does not change, even when the concentration of H+ ions varies. The pH electrode has a high resistance, which makes the voltage fluctuation hard to measure. Therefore, the input resistance of the pH meter and the output resistance are crucial factors. Essentially, the pH meter is a high-gain amplifier that can measure the small voltage of the electrodes and display the result directly in pH units on a digital or analog display. In some cases, the voltage can also be read for special applications or for the use of ion-selective electrodes or oxidation-reduction potential electrodes.
pH Electrodes
The production of pH electrodes is an art. The electrode body is made of special glass, which is shaped through glassblowing techniques. The thickness of the glass determines the resistance and affects the output signal.
Buffer Solutions
A buffer solution is one that has a constant pH value and resists changes in pH. It is used to calibrate the pH measurement system (electrodes and meter). There are slight differences in the output signal between two electrodes and changes in the signal over time. Therefore, the system must be calibrated periodically. These buffers can be in liquid form or more conveniently in powder-filled capsules. Most pH meters require calibration at a few specific pH values. The first calibration is done at the isoelectric point (the signal from an electrode at pH = 7 is 0 mV at 25ºC), and subsequent calibrations are performed at pH = 4 or pH = 10. It is best to choose a buffer with a pH value close to the actual pH of the sample being measured.
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