A pH meter is an electronic instrument used to measure the pH (acidity or basicity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances, such as cheese). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading. The correct title of this article is . ... An acid (often represented by the generic formula HA) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a pH of less than 7. ... Look up base in Wiktionary, the free dictionary. ... Glass electrode is a potentiometric sensor made from glass of f specific composition. ...

A simple pH meter with its probe immersed in a mildly alkaline solution. The two knobs are used to calibrate the instrument.

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History

The first commercial pH meters were built around 1936 by Dr. Arnold O. Beckman in the United States and by Radiometer in Denmark. Beckman was an assistant professor of chemistry at the California Institute of Technology when he was prompted to build a pH meter by his old college friend Glen Joseph who was a chemist at a research laboratory run by the California Fruit Growers Exchange (Sunkist). The Exchange’s citrus growers wanted to get an accurate and rapid measure of the acidity of lemon juice. Beckman's invention helped him to launch the Beckman Instruments company (now known as Beckman Coulter). In 2004 the Beckman pH meter was designated a ACS National Historical Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.^[1] Arnold Orville Beckman (born April 10, 1900 in Cullom, Illinois; died May 18, 2004) was an American chemist who founded Beckman Instruments based on his invention of the pH meter, a device for measuring acidity, in 1934. ... The California Institute of Technology (commonly referred to as Caltech)[1] is a private, coeducational university located in Pasadena, California, in the United States. ... Sunkist Growers, Incorporated is a citrus growers non-stock membership cooperative comprised of 6000 members from California and Arizona. ... Sunkist Growers, Incorporated is a citrus growers non-stock membership cooperative comprised of 6000 members from California and Arizona. ... Binomial name Citrus × limon (L.) Burm. ... Beckman Coulter Inc. ... The ACS National Historic Chemical Landmarks Program was launched by the American Chemical Society in 1992 and has recognized over 50 landmarks to date. ...

The Probe

The pH probe measures pH as the concentration of hydrogen ions surrounding a thin-walled glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter. For more information about pH probes, see glass electrode. The correct title of this article is . ... Glass electrode is a potentiometric sensor made from glass of f specific composition. ...

The Meter

The meter circuit is fundamentally no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance—approximately 20 to 1000MΩ (Megaohms see ohm) —of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about -17. The inverting amplifier converts the small voltage produced by the probe (+0.059 volt/pH in basic solutions, -0.059 volt/pH in acid solutions) into pH units, which are then offset by 7 volts to give a reading on the pH scale. For example: Two digital voltmeters. ... Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Ohm may refer to: The scientist Georg Ohm. ... Glass electrode is a potentiometric sensor made from glass of f specific composition. ... A 741 operational amplifier in a TO-5 metal can package An operational amplifier, usually referred to as an op-amp for brevity, is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ...

• At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 * 17 + 7 = 7.
• At alkaline pH, the voltage at the probe's output ranges from > 0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units from neutral), 3 * 0.059 = 0.18 volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
• At acid pH, the voltage at the probe's output ranges from -0.7 volts to < 0. So for a sample of pH 4 (also 3 pH units from neutral, but in the other direction), 3 * -0.059 = -0.18 volts, the output of the meter's amplifier is -0.18 * 17 + 7 = 4.

The two basic adjustments performed at calibration (see below) set the gain and offset of the inverting amplifier.

Calibration and Use

Calibration with at least two, but preferably three, buffer solution standards is usually performed every time a pH meter is used, though modern instruments will hold their calibration for around a month. One of the buffers has a pH of 7.01 (almost neutral pH) and the second buffer solution is selected to match the pH range in which the measurements are to be taken: usually pH 10.01 for basic solutions and pH 4.01 for acidic solutions (It should be noted that the pH of the calibration solutions is only valid at 25°C). The gain and offset settings of the meter are adjusted repeatedly as the probe is alternately placed in the two calibration standards until accurate readings are obtained in both solutions. Modern instruments have completely automated this process and only require immersing in each solution once, or at worst, twice. Calibration refers to the process of setting the magnitude of the output (or response) of a measuring instrument to the magnitude of the input property or attribute within specified accuracy and precision. ... Buffer solutions are solutions which resist change in hydronium ion and the hydroxide ion concentration (and consequent pH) upon addition of small amounts of acid or base, or upon dilution. ...

The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit)^[2] with the pH scale. After calibration, the probe is rinsed in distilled, deionized water to remove any traces of the buffer solution, blotted with a clean tissue to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in the sample. Between uses, the probe tip, which must be kept wet at all times, is typically kept immersed in a small volume of storage solution, which is an acidic solution of around pH 3.0. Alternatively, the pH 7.01 calibration solution can be used, but this results in a need for more frequent calibration. In an emergency, tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water 'sucks' ions out of the probe, which degrades it. The correct title of this article is . ...

Occasionally (about once a month) the probe should be cleaned using pH electrode cleaning solution, a strongly acidic solution with a pH of around -1.5 to -2.

Types of pH Meters

pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator (ion-sensitive, redox), reference electrodes, and temperature sensors such as thermoresistors or thermocouples. Cheaper models sometimes require that temperature measurements be entered to adjust for the slight variation in pH caused by temperature. Specialty meters and probes are available for use in special applications, harsh environments, etc. Pocket pH meter are readily available today for a few tens of dollars that automatically compensate for temperature. An ion selective electrode (ISE) is an electrode designed to respond to only one type of ion. ... A redox electrode is an electrode made from electron-conductive material and characterized by high chemical stability in the solution under test. ... Reference electrode is an electrode which has a stable and well-known electrode potential. ... A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ... In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ...

History

The first commercial pH meters were built around 1936 by Dr. Arnold Orville Beckman in the United States and by Radiometer in Denmark. Beckman was an assistant professor of chemistry at the California Institute of Technology when he was prompted to build a pH meter by his old college friend Glen Joseph who was a chemist at a research laboratory run by the California Fruit Growers Exchange (Sunkist). The Exchange’s citrus growers wanted to get an accurate and rapid measure of the acidity of lemon juice. Beckman's invention helped him to launch the Beckman Instruments company (now known as Beckman Coulter). In 2004 the Beckman pH meter was designated a ACS National Historical Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.^[3] Arnold Orville Beckman (April 10, 1900 – May 18, 2004) was an American chemist who founded Beckman Instruments based on his invention of the pH meter, a device for measuring acidity, in 1934. ... The California Institute of Technology (commonly referred to as Caltech)[1] is a private, coeducational university located in Pasadena, California, in the United States. ... Sunkist Growers, Incorporated is a citrus growers non-stock membership cooperative comprised of 6000 members from California and Arizona. ... Sunkist Growers, Incorporated is a citrus growers non-stock membership cooperative comprised of 6000 members from California and Arizona. ... Binomial name Citrus × limon (L.) Burm. ... Beckman Coulter Inc. ... The ACS National Historic Chemical Landmarks Program was launched by the American Chemical Society in 1992 and has recognized over 50 landmarks to date. ...

Building a pH Meter -- pH Meter Schematic Diagram

Because the circuitry of a pH meter is fundamentaly quite simple, it is possible to build a serviceable pH meter or pH controller with parts available at a neighborhood electronics retailer. (pH probes, however, are not so easily come by and must usually be ordered from a scientific instrument supplier.) For a schematic diagram of a simple pH meter, see the application note for the LM6001 chip at the National Semiconductor web site. In this circuit, a pH probe is connected to the input of and the output can be read with any inexpensive voltmeter. Although the application note is for a specialty chip, serviceable pH meters can be built from any operational amplifier with a high input impedance, such as the common and inexpensive National Semiconductor TL082 or its equivalent. For a detailed walkthrough of the entire process of building a simple pH meter, see A pH Measurement & Control System for the Planted Aquarium. Categories: Electronics companies of the United States | Companies based in California | Corporation stubs ...

See also

• pH measuring section
• pH indicator

The correct title of this article is . ... A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily. ...

References

1. ^ [1]
2. ^ [2]
3. ^ [3]

External links

• Introduction to pH - Excellent overview of pH and pH measurement at the Omega Engineering website
• The story of pH made easy - An introduction to the basics of pH and pH measurement
• www.ph-meter.info- Theoretical and practical information about pH meters
• The Beckman pH Meter - National Historic Chemical Landmark of the American Chemical Society
• CurTiPot software - All-in-one freeware for pH and acid-base equilibrium calculations and for simulation and analysis of POTentiometric TItration CURves