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CONDUCTIVITY MEASUREMENT
All solutions containing water conduct electricity to some extent. The measure of the ability of a solution to conduct electricity is called “conductance” (the reciprocal of resistance). Addition of electrolytes such as salts, acids or bases to pure water will increase the ability of the liquid to conduct electricity. This increases the solution’s conductance (decreases the resistance).
An electrolytic conductivity measuring system measures solution conductance by using an analyzer interconnected with cable to a sensor immersed in the solution. The conductivity sensor is composed of a temperature sensor and two electrodes in contact with the solution. The analyzer circuitry impresses an alternating voltage between the two electrodes and the magnitude of the resulting current is linearly related to the solution conductivity.
Conductivity Sensor Design
A conductivity sensor generally consists of two electrodes that are insulated from each other. Electrode materials are typically 316 stainless steel, titanium-palladium alloy or carbon. Theoretically, any conductive material may be used if it will not dissolve in the solution. In practice, however, this does not always apply. Unexpected results may occur when current is impressed between the two electrodes. The magnitude of the voltage and current may have an effect on electrode life and measurement accuracy. No electrode material exists that suits all applications.
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DISSOLVED OXYGEN MEASUREMENT
Dissolved oxygen is defined as the measure of water quality indicating free oxygen dissolved in water. The quantity of dissolved oxygen in water is typically expressed in parts per million (ppm) or milligrams per liter (mg/l). Since oxygen is soluble in water, the amount of dissolved oxygen in water is in the state of dynamic equilibrium. The solubility of the dissolved oxygen is proportional to the temperature and pressure of the water.
The most common application for dissolved oxygen measurement occurs in wastewater treatment. Biochemical breakdown of sewage is achieved by bacterial attack in the presence of oxygen. This process typically takes place in an aeration basin of a wastewater treatment plant, and is accomplished by aerating or bubbling air (or pure oxygen) through the wastewater. Maintaining the proper concentration of dissolved oxygen in the aeration basin is necessary to keep microorganisms alive and allow break down of organic waste. These microorganisms turn organic wastes into inorganic byproducts; specifically, carbon dioxide, water and sludge. When the measured dissolved oxygen decreases below a desired concentration, the plant control system automatically adds air to the aeration basin to provide life-sustaining oxygen for the microorganisms, and to facilitate thorough mixing of the organic waste. Without enough dissolved oxygen concentration, beneficial microorganisms will die while troublesome filamentous microbes proliferate, causing sludge settling problems. Conversely, aeration is the largest single operating expense, and oxygen levels greater than the required optimum concentrations are wasteful and inefficient.
pH MEASUREMENT
OVERVIEW
The determination of pH is one of the most common process chemical measurements made today. This booklet explains the principles behind the measurement and discusses ways of avoiding common pitfalls. The booklet also discusses industrial ORP (oxidation-reduction potential) measurements. Although the determination of ORP is not nearly as common as pH, certain industries make valuable use of the measurement.
1.1 INTRODUCTION
pH is a measure of the relative amount of hydrogen and hydroxide ions in an aqueous solution. In any collection of water molecules a very small number will have dissociated to form hydrogen (H+) and hydroxide (OH-) ions:
H2O = H+ + OH
The number of ions formed is small. At 25°C fewer than 2 x 10-7 % of the water molecules have dissociated. In terms of molar concentrations, water at 25°C contains 1 x10-7 moles per liter of hydrogen ions and the same concentration of hydroxide ions. In any aqueous solution, the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is constant. Stated in equation form:
Kw = [H+] [OH-] (1)
where the brackets signify molar concentrations and Kw is the dissociation constant for water. The value of Kw depends on temperature. For example, at 25°C Kw = 1.00 x 10-14 and at 35°C Kw = 1.47 x 10-14. Acids and bases, when dissolved in water, simply alter the relative amounts of H+ and OH- in solution. Acids increase the hydrogen ion concentration, and, because the product [H+] [OH-] must remain constant, acids decrease the hydroxide ion concentration. Bases have the opposite effect. They increase hydroxide ion concentration and decrease hydrogen ion concentration. For example,suppose an acid is added to water at 25°C and the acid raises the H+ concentration to 1.0 x 10-4 moles/liter. Because [H+] [OH-] must always equal 1.00 x 10-14, [OH-] will be 1.0 x 10-10 moles/liter. pH is another way of expressing the hydrogen ion concentration. pH is defined as follows:
pH = -log [H+] (2) Therefore, if the hydrogen ion concentration is 1.0 x 10-4 moles/liter, the pH is 4.00.
The term neutral is often used in discussions about acids, bases, and pH. A neutral solution is one in which the hydrogen ion concentration exactly equals the hydroxide ion concentration. At 25°C, a neutral solution has pH 7.00. At 35°C, a neutral solution has pH 6.92. The common assertion that neutral solutions have pH 7 is not true. The statement is true only if the temperature is 25°C.
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