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Sodium hypochlorite (NaOCl), commonly known as bleach, is a solution made from reacting chlorine with a sodium hydroxide solution, also referred to as caustic. These two reactants are the major co-products of most chlor-alkali cells.
The chemical compound formula for sodium hypochlorite is NaOCl.
Reacting chlorine and sodium hydroxide will produce sodium hypochlorite:
Cl2 + 2NaOH = NaOCl + NaCl + H2O
NaOCl
Chlorine, Sodium Hydroxide (Caustic)
Greenish-yellow liquid
Sodium hypochlorite produced by a continuous process will have approximately 0.2% by weight excess sodium hydroxide, resulting in a specific gravity of 1.160 at 120 gpl.
The specific gravity of the sodium hypochlorite solution is the ratio of the weight of the solution with respect to water. If the product has a specific gravity of 1.2, a gallon of this sodium hypochlorite weighs 10.00 pounds (1.2 * weight of water). The specific gravity of sodium hypochlorite with the same strength may vary due to the amount of excess caustic in the solution.
Most tables that show strength and specific gravity of hypochlorite solutions were created 40-50 years ago. These levels are shown with excess sodium hydroxide (caustic) being much higher than the levels of sodium hydroxide typically produced by current manufacturers. Excess caustic levels have decreased over time due to improvements in manufacturing techniques.
The old tables will typically show 120 gpl available chlorine bleach with 0.73 % by weight excess caustic which results in a specific gravity of 1.168 at 20 degrees C. Typically, the sodium hypochlorite produced by a continuous process will have approximately 0.2% by weight excess sodium hydroxide and this would result in a specific gravity of 1.160. This number assumes very small levels of chlorate exist in the solution.
Sodium Hypochlorite is the main ingredient in laundry bleach. It is used extensively as a bleaching agent in the textile, detergents, and paper and pulp industries. It is also used as an oxidizing agent for organic products. In the petrochemical industry, sodium hypochlorite is used in petroleum products refining. Large quantities are also used as a disinfectant in water and wastewater treatment and sanitary equipment. In food processing, sodium hypochlorite is used to sanitize food preparation equipment, in fruit and vegetable processing, mushroom production, hog, beef and poultry production, maple syrup production, and fish processing.
In various parts of the world, sodium hypochlorite strength is identified using five common definitions listed below.
Sodium hypochlorite can be produced in two ways:
Most commonly, sodium hypochlorite is made via chemical reaction. In this case, sodium hypochlorite is prepared by reacting dilute caustic soda solution with liquid or gaseous chlorine, accompanied by cooling, eventually producing sodium hypochlorite, water, and salt.
Sodium Hypochlorite is commonly produced either through a batch process or continuous process.
Sodium hypochlorite can also be made by dissolving salt in softened water, then electrolyzing the solution. When salt is dissolved in softened water, a brine is created; then, when the solution is electrolyzed, the brine forms a sodium hypochlorite solution and hydrogen gas.
The advantage of the salt electrolysis system in the creation of sodium hypochlorite is that no transport or storage of chemicals are required, however, the costs to purchase and maintain the electrolysis system are typically higher than that of a batch or continuous process.
The chart below lists some of the varying strengths of Sodium Hypochlorite and how these solution strengths are typically used.
Note: The higher the sodium hypochlorite strength, the faster the decomposition rate becomes. See Decomposition of Sodium Hypochlorite for more information.
Light, heat, organic matter, and certain transition metals (such as copper, nickel, and cobalt) accelerate the rate of decomposition of sodium hypochlorite. The presence of transition metal ions (copper and nickel) is known to catalyze the decomposition of sodium hypochlorite, contributing to the loss of sodium hypochlorite strength and the formation of oxygen. Loss of sodium hypochlorite strength means more product will be needed when the sodium hypochlorite is used as a disinfectant.
Oxygen build-up can pose problems when storing sodium hypochlorite in storage containers or sodium hypochlorite piping due to pressure build-up. By removing suspended solids to nearly undetectable levels, the rate of decomposition is significantly reduced. In addition, the formation of oxygen is nearly eliminated.
All sodium hypochlorite decomposition is dependent on temperature. For example, a 10°C increase in storage temperature will result in an increased rate factor of approximately 3.5.
Storage of sodium hypochlorite at approximately 60°F (15°C) will greatly reduce the decomposition of the sodium hypochlorite. Therefore, cooling the product before shipment will greatly reduce its decomposition.
It is relatively easy to chill the sodium hypochlorite with a chilled water system and plate heat exchanger. However, it is also possible to located storage tanks inside of an air-conditioned room if the tanks are relatively small.
Many different types of materials are used for the construction of sodium hypochlorite storage tanks. Two common types of materials are linear or cross-linked polyethylene and fiberglass reinforced plastic (FRP). Other choices include chlorobutyl rubber lined steel and titanium. In some countries where these materials are not readily available, or the manufacturing quality is suspect, cubical concrete tanks lined with flexible plastic liners such as PVC have been successfully used. The choice of material depends on available capital, tank location, and required service life. Some tanks may only last 3-5 years. If properly specified and maintained, the tanks could last 10-15 years. The only material noted for over 30 year service life is titanium.
The presence of transition metal ions (copper and nickel) is known to catalyze the decomposition of liquid sodium hypochlorite, contributing to the loss of strength and the formation of oxygen. Loss of sodium hypochlorite strength means more product will be needed when the bleach is used as a disinfectant.
Sodium hypochlorite decomposition rate is dependent on the total ionic strength of the product, temperature of the solution, pH, and transition metal content of the solution.
The primary pathway is 3NaOCl = 2NaCl + NaClO3
The minor pathway is 2NaOCl = 2NaCl + O2
The ideal pH of the solution should be from 11.86-13 pH or approximately 0.25% to 0.35% excess caustic (NaOH).
Sodium hypochlorite has a second order decomposition rate. Therefore, reducing the concentration by half will reduce the decomposition rate factor by 5 assuming the products are at the same temperature.
For a 10°C increase in temperature, sodium hypochlorite at the same starting strength will decompose 3.5 times faster.
Transition metals (such as nickel and copper) increase decomposition. This decomposition produces oxygen. Concentrations below 50 ppb can typically be achieved with sub-micron filtration. Higher concentrations will cause unacceptable rates of decomposition.
The filtering of sodium hypochlorite offers numerous advantages. Some of these advantages are as follows:
