Chlorine Demand

Definition - What does Chlorine Demand mean?

Chlorine demand is the difference between total chlorine added in the water and residual chlorine. It is the amount which reacts with the substances in water, leaving behind an inactive form of chlorine. Chlorine demand can be caused in a water body due to rain containing ammonia or the addition of fertilizers which can be oxidized by chlorine.

Corrosionpedia explains Chlorine Demand

To purify water supplies and make them suitable for purposes like drinking, cooking and swimming, chlorine is added. Chlorine demand from the total chlorine added can be explained using the following equation:

Chlorine demand = Total chlorine – Chlorine residual

The purity of water can be determined by monitoring the value of chlorine demand. If the value is zero, the water is already free of pathogenic microorganisms. If the value is less than the total chlorine, it shows that the amount of chlorine added initially to the water was sufficient.

Total Chlorine

Definition - What does Total Chlorine mean?

Total chlorine is the sum of combined and free chlorine. In all instances, the level of total chlorine will always be above or equal to free chlorine levels. The most common levels of free chlorine present in drinking water are 0.2 to 2.0 mg/L, but can be up to 5.0 mg/L.

Total chlorine is usually measured in wastewater that has undergone treatment.

Corrosionpedia explains Total Chlorine

Chlorine that is present in water consists of two forms: combined and free. The latter performs the job of oxidizing contaminants and killing bacteria. When chlorine is added to water, it is added as free chlorine. Total chlorine is the sum of both free and combined chlorine.
When free chlorine comes into contact with various forms of contaminants, the chlorine is transformed to combined chlorine, also known as chloramines. This type of chlorine has very little ability to sanitize and oxidize.
When chlorine is added to water, various reactions take place and most of the chlorine initially reacts with metals and organic materials in water that is not accessible for disinfection, referred to as "chlorine demand" of the water. The rest of the chlorine concentration following the chlorine demand is referred to as the total chlorine, that is subdivided to the two types mentioned above.
The levels of total chlorine, free chlorine and combined chlorine should be measured accurately to identify the amount of appropriate solutions that should be added to water in order to maintain the residual of free chlorine present in the water. In a more wide-scale basis, the main goal is the proper disinfection of piped water treatment systems

Free Available Chlorine

Definition - What does Free Available Chlorine mean?

Free available chlorine is the part of the total chlorine measurement that has not yet reacted with contaminants. Therefore, it is called available or free.
The total chlorine in this case is the sum of combined available chlorine and free available chlorine, also called total residual chlorine.

Corrosionpedia explains Free Available Chlorine

Chlorine is usually added to water in different forms such as:

• Chlorine gas
• Sodium hypochlorite or liquid bleach
• Lithium hypochlorite
• Stabilizers

When any of these chlorine forms are added to water, a highly potent bactericide known as hypochlorous acid is created. This is a type of weak acid that can be dissociated into hypochlorite ions.
Hypochlorous acid and hypochlorite ions, when combined, is known as free chlorine. Both of these establish equilibrium and are both temperature and pH dependent at 7.5 pH and 77°F (25°C). Hypochlorous acid is more potent than hypochlorite ions in terms of disinfection. Through proper control of pH, hypochlorous acid remains dominant in the solution of free available chlorine to ensure its effectiveness as a bactericide.

Free available chlorine readily reacts with compounds such as ammonia to produce chloramines, or industrially known as combined chlorine. Although chloramines also have antibacterial properties, these are about 80 times less effective than free chlorine.
In the formation of free chlorine and chloramines, optimum temperature, pH as well as mixing conditions should be taken into consideration.

Breakpoint Chlorination

Definition - What does Breakpoint Chlorination mean?

Breakpoint chlorination is the point where the demand for chlorine has been fully satisfied in terms of chlorine addition to water.

When chlorine is added to water, a reaction is produced in the compounds present in the water. These compounds utilize the chlorine, resulting in zero chlorine residual.

Corrosionpedia explains Breakpoint Chlorination

Once chlorine has been added to water, it is consumed by a type of chemical reaction that has a net effect of increased concentration of chlorine. For typical addition of chlorine, the rate of reaction instantly speeds up, reducing the concentration of chlorine. This is because chlorinated compounds acquire more chlorine.
The pace at which the chlorine atom is added is comparatively slow, but the rates can be faster for the following reactions since chlorinating potentiates an activity called reactivity. Once almost all of the chlorine reactions are accomplished, adding more chlorine leads to permanent residual.
The period wherein the concentration of chlorine goes into an upward slope is called the "breakpoint." In some cases, there can be no breakpoint seen because various organic compounds react at different rates.

Breakpoint chlorination is usually measured to determine when chlorination has been satisfied. This is a common practice in disinfecting water in industrial water systems as well as swimming pools. It is one of the most typical forms of chlorination where adequate chlorine is incorporated into the water to achieve the breakpoint, keeping the water well chlorinated and appropriate for its intended use.

Corrosion

Definition - What does Corrosion mean?

Corrosion is the deterioration and loss of a material and its critical properties due to chemical, electrochemical and other reactions of the exposed material surface with the surrounding environment.

Corrosionpedia explains Corrosion

Corrosion of metals and nonmetals takes place due to the gradual environmental interaction on the material surface. The structures and facilities of different materials are affected by this interaction. Even the ambient air, laden with moisture and oxygen, can start this process, known as rusting, on steel surfaces. In the case of buried structures and pipelines the soil chemistry and moisture determine the rate of damage. Fumes of acids such as sulfuric acid and dust of caustic soda also accelerate corrosion. In the case of aluminum, however, the oxide film formed due to initial corrosive attack protects the surface from further damage. In marine environments, in which airborne salt crystals are deposited onto ships, corrosion of submerged surfaces as well as surfaces floating above water level occurs.

Corrosion affects the microstructure, mechanical properties and the physical appearance of the materials. Rusting and other types of deterioration drastically reduces the capacity of pipelines and equipment, resulting in loss of output as well as loss of equipment, or even life. Hence, anti-corrosive coatings are used to combat the corrosion damage to critical structures and equipment.

Passivation

Definition - What does Passivation mean?

Passivation is the process through which materials are protected from corrosion. Passivation aims to reduce the deterioration of materials (usually metals) caused by their reaction with the environment. The end goal of passivation is to increase the lifespan of the material in question by improving its corrosion-resistant properties.

Corrosionpedia explains Passivation

Passivation involves applying an outer layer to a material to protect it from harmful reactions such as corrosion reactions. The layer may occur spontaneously in nature through a process called self-passivating, or it can be introduced to the material as a micro-coating. In this case, a light coat of material, for instance a metal oxide is used to form a shield against corrosion. In microelectronics, passivation is useful in the enhancement of silicon. The great practical significance of passivation is that it protects structural metals from rapid corrosion that occurs in fresh water, moist atmosphere and aggressive chemical mediums.
In industry, the anticorrosive properties of a metal that result from its natural passive state can be supplemented through passivation where a suitable solvent is used to treat the metal. The action of the solvent takes place as a chemical or electrochemical process. Oxidizing agents are the common passivators although other substances may also be used for passivation. For instance, nitric acid is used to passivate aluminum ware and chromate solutions to passivate zinc coatings. The rate of corrosion is reduced by passivation depending on the type of metal and its environment.