PH meter
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- The correct title of this article is pH meter. The initial letter is shown capitalized due to technical restrictions.
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[edit] 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.[1]
[edit] 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.
[edit] 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 1000 MΩ—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:
- 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.
[edit] 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.
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.
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.
[edit] 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 availble 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.
[edit] 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]
[edit] 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. The probe is connected to the input of this circuit 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.
[edit] See also
[edit] 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 CURvesda:PH-meter
