About pH meter
- Jan 04, 2018 -
pH stands for "potential of hydrogen" and is a measure of hydrogen ion concentration. A high concentration of hydrogen ions results in acidity while a low concentration results in alkalinity. The concentration of hydrogen ions is represented on a scale from 0 to 14 with 7 being neutral (i.e. an equal number of hydrogen (+) ions and hydroxide (-) ions to cancel each other out). Values below pH 7 are acidic and values above pH 7 are alkaline. The scale is logarithmic, meaning each full point increases or decreases by a factor of 10x. Some extreme substances can score lower than 0 or greater than 14, but most fall within the scale.
pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment & water purification, and many other applications.
pH meters generally consist of a glass electrode connected to an electronic meter that measures and displays the pH reading. Meters can range from units that combine all the parts into a simple, handheld unit or more complex, laboratory-quality instruments that provide highly accurate readings and a host of other functions.
The electrode itself is comprised of a measuring electrode, a reference electrode, and, often, a temperature sensor. Electrode components are usually combined into one device called a combination pH electrode. The reference electrode is filled with an electrolyte of a known value which reacts with the medium being tested. That reaction is what determines the potential of hydrogen. Since pH is sensitive to temperature fluctuations, a temperature sensor is included so readings can be properly compensated. Temperature compensation can be achieved manually or automatically. Manual temperature compensation is usually achieved by entering the temperature of the fluid being measured into the instruments menu. Automatic temperature compensation requires input from a temperature sensor and constantly sends a compensated pH signal to the display.
There is a wide range of pH electrode styles to suit particular applications. All electrodes include a junction which separates the electrolyte within the reference electrode from the medium being tested. Double junctions are available for applications which risk contaminating the electrolyte. There are also a number of electrode tip styles which are suitable for particular materials being tested.
Maintenance and Calibration of pH Instruments
Accuracy of pH measurements is dependent upon well maintained and properly calibrated instruments. The main factors that degrade the performance of an electrode are coating, contamination and aging. Necessary maintenance and calibration depends on these factors and, of course, of the accuracy required from measurements.
All glass electrodes have a bulb which must be kept hydrated and a reference junction which must be kept wet to prevent excess leakage of the internal electrolyte solution from the reference junction. It is recommended keeping the electrodes in a storage solution between uses. Special cleaning solutions are also available to ensure electrodes are free from contamination or build-up which can affect performance and shorten the lifespan of the electrode. The electrolyte within some reference electrodes must also be refilled periodically. Other electrodes are gel-filled or "low-maintenance" and do not require refilling.
All pH meters require calibration from time to time. They should be calibrated at a minimum of two points. Most manufacturers offer buffer solution at specific pH levels for calibration. For most applications, pH 7.00 and 4.00 (or 7.01 and 10.01 for alkaline values) are suitable for calibration.
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