Sodium Hypochlorite

The following is excerpted from Powell's Sodium Hypochlorite General Information Handbook. A free copy of the entire Sodium Hypochlorite General Information Handbook and other information about Sodium Hypochlorite are available in the Technical Information section.

1.0 Introduction

The consumer of sodium hypochlorite requires an understanding of the product from a chemical and handling perspective. The information below is a brief summary about Sodium Hypochlorite and is intended to assist the consumer to buy the best product and to store and handle it correctly.

2.0 Chemistry of Sodium Hypochlorite

Reacting chlorine and sodium hydroxide will produce Sodium Hypochlorite
Cl2 +2NaOH = NaOCl + NaCl + H2O
(Chlorine + Sodium Hydroxide = Sodium Hypochlorite + Sodium Chloride + Water)

2.1 Relationship between Oxidizing Power of Chlorine and Sodium Hypochlorite

Many consumers are currently replacing chlorine with sodium hypochlorite as the oxidizing agent. In order to calculate how much sodium hypochlorite is required to replace the oxidizing power of chlorine, the following example is provided.

If sodium hypochlorite is used to oxidize iodide in a solution of acetic acid, the following reaction occurs:

NaOCl + 2Kl + 2HAc = l2 + NaCl + 2KAc + H2O

If chlorine is used to react with the same amount of iodide, the following reaction occurs:


Cl2 + 2Kl = I2 + 2KCl

Therefore, a molecule of sodium hypochlorite will oxidize the same amount of iodide as a molecule of chlorine.

2.2 Terms Used to Define the Strength of Sodium Hypochlorite

In various parts of the world, sodium hypochlorite strength is identified using five common definitions that result in different numbers for the answer although the oxidizing power is the same (i.e., the sodium hypochlorite strength is the same). Although there are other definitions used, due to the infrequency of use, these definitions will not be discussed.

The terms to define the sodium hypochlorite strength commonly used in the industry are as follows:

2.2.1 Grams per Liter of Available Chlorine

Grams per Liter of Available Chlorine: the weight of available chlorine in grams in one liter of sodium hypochlorite solution (see Bleach Strength Test). The Bleach Strength Test method was developed to help reduce the typical problems associated with the procedures.

2.2.2 Grams per Liter of Sodium Hypochlorite

Grams per Liter of Sodium Hypochlorite: the weight of sodium hypochlorite in grams in one liter of sodium hypochlorite solution.

It can be calculated by converting the grams per liter available in chlorine into its equivalent as sodium hypochlorite by multiplying the ratio of their respective molecular weights:

Grams Per Liter Available in Chlorine x NaOCl/Cl2 or x 74/71 or 1.05 = Grams Per Liter Sodium Hypochlorite.

2.2.3 Trade Percent of Available Chlorine

Trade Percent of Available Chlorine: a term often used to define the strength of commercial bleaches. It is identical to grams per liter of available chlorine except the unit of volume is 100 milliliters not one liter. Therefore, the result is one tenth of the grams per liter.

Trade % Available Chlorine = GPL Available Chlorine/10

2.2.4 Weight Percent of Available Chlorine

Dividing the trade percent by the specific gravity of the solution gives weight percent of available chlorine. Typically, density measurements will result in errors of 0.5 to 1.0% (i.e. 10% could be 9.90% to 10.10%) when converting from GPL Available Chlorine to Weight Percent Available Chlorine.

Weight % Available Chlorine = GPL Available Cl2 /(10 x specific gravity) OR Trade % Available Cl2/specific gravity

Note: When measuring the specific gravity, measure it at the same temperature as the temperature of the bleach sample used in the bleach strength test.

2.2.5 Weight Percent of Sodium Hypochlorite

The weight percent of sodium hypochlorite is the weight of the sodium hypochlorite per 100 parts of solution. It can be calculated by converting the weight percent of available chlorine into its equivalent as sodium hypochlorite by multiplying the ratio of their respective molecular weights:

Note: When measuring the specific gravity, measure it at the same temperature as the temperature of the bleach sample used in the bleach strength test.

  • Weight percent available chlorine x NaOCl/Cl2 or x 74/71 or 1.05 weight percent NaOCl
  • Weight percent sodium hypochlorite = (gpl available chlorine x 1.05)/(10 x specific gravity)
    • or = (trade % Available Cl2 x 1.05)/specific gravity
    • or = weight percent available chlorine x 1.05

Since sodium hypochlorite is sold based on the strength of the product, it is critical to specify exactly which term is used to define the strength of the product.

2.3 Ratio of Gallons of Sodium Hypochlorite to Pounds of Chlorine Used

In order to calculate the volume and the strength required to replace the oxidizing power of existing chlorine applications, the strength of the sodium hypochlorite purchased must be converted to the equivalent pounds of available chlorine.

For Example:

Using the definition of GPL of available chlorine (weight of available chlorine in grams per liter of bleach), the following conversion is useful:

120 GPL available chlorine = 120 gpl Av. Cl2 x 3.785 liters/gallon x 2.205 pounds/1000 grams = 1 pound/gallon available Cl2

Therefore, one gallon of sodium hypochlorite at 120 GPL available chlorine strength will have the oxidizing power as one pound of chlorine.

Other equal terms:

  • 120 GPL available chlorine = 12 Trade percent
  • or 12/1.168* = 10.27* weight percent available chlorine
  • or 10.27* x 1.05 = 10.79* weight percent sodium hypochlorite

*Caution: Each manufacturer will produce sodium hypochlorite with different specific gravity due to the variation in the amounts of excess caustic, chlorates, and salt. Therefore, all test procedures by both the producer and the consumer should calculate the bleach strength in grams per liter available chlorine. Therefore, if gpl available chlorine is used as the indication of sodium hypochlorite strength, the accuracy of this measurement is not dependant on the accuracy of the specific gravity measurement of the product.

If other strengths of sodium hypochlorite are utilized, such as 160 gpl, the amount of available chlorine per gallon is the ratio of the new strength versus 120 gpl. Therefore, 160 gpl to 120 gpl (160/120) is 1.333 and the amount of available chlorine per gallon is 1.333 pounds per gallon since 120 gpl has 1#/gallon of available chlorine.

In summary, if the process uses one pound of chlorine, the process will use one gallon of sodium hypochlorite at strength of 120 GPL available chlorine.

2.4 Sodium Hypochlorite Decomposition

The consumer must understand the reasons for decomposition of sodium hypochlorite to successfully purchase and utilize the product and to eliminate "oxygen locking" and piping systems plugging.

There are two decomposition pathways of sodium hypochlorite.

The dominant pathway is as follows:

3NaOCl = 2NaCl + NaCLO3 (Chlorate)

This decomposition can be created two major ways:

2.4.1 Sodium Hypochlorite Decomposition by Chlorate Formation Path #1

If during production of the sodium hypochlorite the reaction of chlorine and caustic occurs in a low pH region of the reactor (typically less than 10 pH), hypochlorous acid is formed. This will result in chlorate formation. Refer to Section 5.0 References.

In most batch production systems for sodium hypochlorite using the original methods common in the 1950 and 1960's, high levels of chlorate can be produced during the reaction process. During the 70's, 80's and 90's, most manufacturers have converted to continuous production of sodium hypochlorite resulting in good control of the pH at the reaction point and thus reduced chlorate formation. However, it should be noted that within the continuous sodium hypochlorite manufacturing group, individual methods of operation will greatly affect the levels of chlorate produced during the reaction. For example, if packed towers are utilized for the reaction of caustic and chlorine, high levels of chlorates can be expected if the excess caustic in the column drops below approximately 1.5% by weight excess.

It should also be noted that the strength of sodium hypochlorite produced during the reaction will also affect the levels of chlorate. Regardless of the method used in the sodium hypochlorite production, that production method will create high levels of chlorates if higher strengths of sodium hypochlorite are produced.

More Information:

This article is available as a PDF. Click here to download it. For more information about sodium hypochlorite, visit the Technical Information section.

Additional Information Contained in this Handbook includes:

  • Sodium Hypochlorite Decomposition by Chlorate Formation Path #1
  • Sodium Hypochlorite Decomposition by Chlorate Formation Path #2
  • Method #1 to Reduce Sodium Hypochlorite Decomposition Chlorate Formation Path #2
  • Method #2 to Reduce Sodium Hypochlorite Decomposition Chlorate Formation Path #2
  • Method #3 to Reduce Sodium Hypochlorite Decomposition Chlorate Formation Path #2
  • Minor Decomposition Pathway for Sodium Hypochlorite
  • Sodium Hypochlorite Quality (Strength, Excess Sodium Hydroxide (Caustic), Sodium Carbonate, Specific Gravity, Suspended Solids & Heavy Metals Removal, Parts per million of chlorate, Parts per million of nickel & copper, Parts per million of iron)
  • Transportation, Storage, and Handling of Sodium Hypochlorite
  • Tanker Trailers
  • Flat Bed Trailer Tanks
  • DOT Exempt Polyethylene Tanks
  • 55 & 5 US Gallon Drums and Containers & Other Containers
  • Materials of Construction
  • Installation and Design Considerations
  • Polyethylene
  • Rubber Lined Steel
  • Titanium
  • Materials of Construction
  • Pumps
  • Piping
  • Valves
  • Gaskets
  • Instrumentation