pH Measurement

pH meters and controllers are used to measure and monitor the pH level of a substance.

(Courtesy of Hach Company, Loveland, CO)

About pH

pH is a measure of hydrogen ion concentration. pH is defined as the negative logarithm of the concentration of hydrogen ions in a substance, in moles per liter. For example, pure water has a hydrogen ion concentration of 10 -7 moles per liter at standard conditions (25°C), resulting in a pH of 7. The pH scale usually extends from 0 to 14. When an acid fully ionizes in water, it will normally have a pH of 0.0. Acidity describes a substance with a pH under 7 at standard conditions, where a lower pH value corresponds to a higher concentration of hydrogen ions. When an alkaline, or a base, fully ionizes in water, the water will have a pH of 14.0. Alkalinity describes a substance with a pH above 7 at standard conditions. Since pure water has a pH of exactly 7.0, it is neither an acid nor a base, but a neutral solution. Examples of all three are given below, with some common solutions and their approximate pH value. Note that since pH is measured on a logarithmic scale, each unit increase of pH corresponds to a decrease in concentration by a factor of 10. For example, the concentration of hydrogen ions for pH 3 is 10 times greater than that of pH 4 and 100 times greater than pH 5.

About pOH

A solution's pH is dependent on its net concentration of hydrogen ions [H+] compared to concentration of hydroxide ions [OH-]. These two concentrations are related by [H+] * [OH-]=K W , where K W , the dissociation constant of water, is dependent on temperature and equals 1.0 * 10 -14 at 25°C. Expressing the terms above as logarithms yields pH + pOH = pK W . At 25°C this becomes pH + pOH = 14, which is why the scale for pH usually ranges from 0 to 14. A solution with a pH of 7, such as pure water at 25°C is neutral.

pH Meters

pH meters measure the effective acidity or alkalinity of a fluid.

(Courtesy of Hach Company, Loveland, CO)

General Information

pH meters are used to measure and control pH values. This value identifies a solution as acidic, basic, or neutral. The pH of a substance can vary with temperature, so for total accuracy, pH is usually recorded along with temperature. pH meters almost always include an interior temperature monitoring system. pH meters usually include an analyzer as well. pH analyzers convert the pH-sensor output into a signal or indication to be used in conjunction with a computer or other types of data analysis devices.

Equipment Design

As seen in the animation, the pH meter's probe is placed into the fluid sample. At the tip of the probe there is a thin glass bulb that contains a reference electrode, usually made of a silver/silver-chloride element. This reference element takes the form of a wire submerged in an electrolyte of pH 7.0. The glass bulb itself serves as the pH-sensitive glass electrode. When the probe is placed into a sample, both electrodes are immersed and an electrical circuit is completed, extending a millivolt potential across the glass sensor. This voltage is a function of the hydrogen activity in the sample, or the pH of the sample. A voltmeter in the probe registers the voltage and converts it to a pH value.

pH meters must be calibrated before making a measurement. In order to do this the probe is removed from the process and washed with deionized water, before being placed into a buffer of known pH. The pH meter is calibrated to read the buffer pH. The electrode is then removed from the buffer, washed with deionized water, and placed back into the process.

Note that even after calibration, pH can change due to temperature differences and carbon dioxide absorption. Alkaline buffers drop rather steadily due to carbon dioxide and it is recommended to use other types of buffers, and be cautious when using buffers of pH 10 or greater.

(Courtesy of A. Daigger & Company, Vernon Hills, IL)

pH meters are available in a range of shapes and sizes. From smallest to biggest there are hand held, pen, pocket, and bench top, shown below. There are also stick and waterproof pH meters for more effective use in particular applications. Pen, pocket, and hand held meters are all used for field study.

(Courtesy of A. Daigger & Company, Vernon Hills, IL)

The earliest pH meters were strips of litmus paper. Litmus paper is still used when a high degree of accuracy is not required and manual readings are sufficient. A strip of litmus paper is coated with any one of various dyes that alters their color within a narrow range of pH values.

When dipped into a solution, the change in color is immediate and the color can be compared to the color-coded chart supplied with the litmus paper. Examples of dyes coated onto litmus are chlorine, iron, copper, and peroxide.

Usage Examples

pH meters can be combined with other meters, such as ion concentration, ORP (Oxidation Reduction Potential), conductivity, dissolved oxygen, viscosity, or bacteria meters, within a single instrument, resulting in more efficient measurements.

pH meters are used in many fields, such as in water treatment process. The pH meter being operated in the picture below is used in applications that require high accuracy when measuring pH or ORP in ultrapure water, such as pharmaceutical and microelectronics applications. pH meters are also used in organic chemical manufacturing and in the medical field, where they are used to measure the pH of chemicals produced by the body and that are to be introduced into the body.

(Courtesy of Hach Company, Loveland, CO)

Advantages

Disadvantages

  • Hand held meters can make up to 4000 readings on 4 AA batteries.
  • Pocket meters are cheaper than other pH meters.
  • Most often programmed with common buffer values to eliminate the need for constant calibration.
  • Slow to register, and drifts with final values.
  • Deposits can gather on the bulbs of the electrodes and disrupt processes.
  • Must be calibrated often when dealing with solutions of varying pH.
  • pH calibration can be affected by temperature and carbon dioxide absorption.

pH Controllers

pH controllers are used to monitor and treat pH levels in industrial processes.

(Courtesy of Hach Company, Loveland, CO)

General Information

Without proper pH control, mineral deposits can build and corrode the machinery in a plant.

A pH controller will monitor the pH level of a fluid, and once the pH reaches a certain value, the controller releases a substance to restore the pH to its desired value.

(Courtesy of Chemical Engineering Department, University of Michigan, Ann Arbor, MI)

Equipment Design

A pH controller monitors the fluid in a system with a pH meter, to measure the amount of free hydrogen ions in the stream. As seen in the animation, once a system reaches a certain pH, the controller will signal an on-line computer or the control valve. This valve controls the addition of an acid or a base, to lower or raise the pH level back to desired levels.

As seen in the schematic below, a system can involve many controllers that periodically add a substance into a system. Here, the pH of a fluid feed stream is lowered in preparation for a particular process. After the process, the pH of the fluid is increased to the desired final pH.

The substance most used to lower the pH of an aqueous stream is carbon dioxide. When added to an aqueous stream carbon dioxide will form carbonic acid, lowering the pH of the stream. Sulfuric acid is also commonly used, but can damage pipes at high concentrations. Ferric sulphate, aluminum sulphate, chlorine, and sulfur dioxide are also used to lower the pH.

The simplest and cheapest way to raise the pH of a stream is with lime, a strong base. Most power plants also use ammonia, an even stronger base, but it can damage pipes at high concentrations.

Usage Examples

The control of pH is very important in wastewater treatment, pulp and paper industries, chemical processes, and biochemical processes. Shown here is a pH controller located on a benchtop scale bioreactor. The controller monitors the pH level of the yeast solution and sends information to on-line computers to maintain a constant pH.

(Copyright Chemical Engineering Department, University of Michigan, Ann Arbor, MI)

Advantages

Disadvantages

  • Fast and simple.
  • Available alarms sound during an operating problem condition.
  • Available with pump that works to slow down introduction of acid or base and prevent the overshooting of desired pH.
  • Inaccurate pH control leads to wear on equipment and process losses.
  • Difficult to add acid or base to totally neutralize (pH 7.0) a substance.
  • Can become dysfunctional with a change in chemical process or substance.

Acknowledgements

A. Daigger & Company , Vernon Hills, IL

Hach Company , Loveland, CO

References

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Choi, J. Y., H. G. Pandit, R. R. Rhinehart, and R. J. Farrell. "A Process Simulator for pH      Control Studies." Computers & Chemical Engineering: An International Journal of Computer Applications in Chemical Engineering . 19 (1995): 527-539. Print.

Considine, P. E., Douglas M., and Glenn D. Considine. "pH." Van Nostrand's Scientific Encyclopedia . 8th ed. 1995. Print.

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"The Fizz Factor." The Chemical Engineer . Feb. 1990: 17-18. Print.

Hayes, James W. "Select the Right pH Measurement System." Chemical Engineering Progress 91 (1995): 88-97. Print.

"Instrument And Control." Power 137 (1993): 108-114. Print.

Jenkins, Scott. "Facts at your Fingertips: pH Electrodes and Temperature Dependence." Chemical Engineering . Feb 2012: 22. Print

Maiti, Sachi N., Navneet Kapoor, and Deoki N. Saraf. "Adaptive Dynamic Matrix Control of pH." Industrial and Engineering Chemistry Research . 33 (1994): 641-646. Print.

McMilan, Gregory K. "pH Measurement: The State of the Art." INTECH 40 (1993): 35-39. Print.

Nichols, N. and G. Kalis. "Combine Monitoring Techniques to Advance pH Control." Power . 135      (1991): 84-85. Print.

Price, Valerie A. "pH Control in Sugar Processing." INTECH . 38 (1991): 47. Print.

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