The Measurement of Chlorine
The addition of chlorine into aqueous systems is used in many different industries for a variety of purposes. In the chemical processing, pharmaceutical, textile, and pulp & paper industries, chlorine is commonly used as a bleaching or oxidizing agent. The food and beverage processing industry, as well as municipal water plants, use chlorine as a disinfectant or sanitizer to control odors, improve the taste of drinking water and eliminate pathogens. Power plants and petrochemical refineries use chlorine to control the growth of microorganisms, algae, and biofilms in the cooling water used in their cooling towers, as these microorganisms and biofilms reduce the efficiency of the cooling towers. Chlorine is also a popular sanitizing agent used in the swimming pool industry to prevent health problems and keep the water sparkling and clear. This paper will focus on the measurement of chlorine in its use as a disinfectant, as this is the application in which chlorine is most widely used.
Chlorine is typically produced by the electrolysis of a sodium chloride solution, often referred to as “brine” solution. This process is a fairly expensive one, so proper measurement and control of chlorine levels is essential to keeping maintenance costs under control. If chlorine concentrations are too high, unnecessary costs are introduced into the process. If chlorine levels are too low, your disinfection process will be ineffective.
Here is a quick list of some common industry terms and their concise definitions regarding chlorine measurement. The following terms will be discussed in further detail below.
Free available chlorine – The portion of total chlorine that has not yet reacted with contaminants, and is therefore “free”, or “available” to kill bacteria and other contaminants.
Combined available chlorine – The portion of chlorine in the water that has reacted and combined with ammonia, nitrogen-containing contaminants, or other organics.
Total residual chlorine – The sum of Free available chlorine and Combined available chlorine. This term is also sometimes referred to as total chlorine.
Chlorine can be added to water as chlorine gas, aqueous sodium hypochlorite solution (liquid bleach), solid calcium hypochlorite (granular or tablets), lithium hypochlorite, or chlorinated isocyanurates (a family of compounds which add cyanuric acid – a stabilizer).
When chlorine is added to water in any of these forms, it creates hypochlorous acid, a very potent bactericide:
Cl2 + H2O à HOCl + HCl
Hypochlorous acid (HOCl) is a weak acid that dissociates into hypochlorite ion (OCl-) according to the following equation:
HOCl (Hypochlorous acid) ó H+ + OCL- (hypochlorite ion)
Together, HOCl and OCl- are referred to as free chlorine. These two species exist in an equilibrium which is both pH and temperature dependent. At 25 °C and a pH of 7.5, half of the chlorine is present as HOCl and the other half as OCl-. At pH values below 7.5, HOCl is the dominant species. At pH values above 7.5, OCl- is the dominant species. At pH 5, nearly all the chlorine is present as HOCl, while a pH value of 10 drives nearly all the chlorine to be present as OCl-. See Figure 1 below:
Figure 1. Ionization Curve of HOCl as a function of pH at 25 °C
As a disinfectant, hypochlorous acid (HOCl) is more effective than hypochlorite ion. By controlling the pH, we can ensure that the more effective bactericide, HOCl, remains the dominant species in solution.
Free chlorine readily reacts with Ammonia and other ammoniated compounds to form what are known as “chloramines”. These chloramines are known as monochloramine, dichloramine, and trichloramine. Chloramines are also referred to, in the industry, as combined chlorine. While chloramines do have some bactericidal properties, they are 80 to 100 times less effective than free chlorine. The rate of formation of chloramines is dependent on the ratio of free chlorine concentration to ammonia concentration. Optimum pH, temperature, and mixing conditions are also a factor.
When chlorine is added to water containing ammonia (NH3), chlorine will replace one hydrogen ion on the ammonia molecule with a chloride ion, resulting in the formation of monochloramine:
HOCl + NH3 à NH2Cl (Monochloramine) + H2O
If the free chlorine to ammonia ratio is allowed to fall below 5:1 by weight (either by chlorine loss or by the addition of ammonia to the sample), all free chlorine will be converted to monochloramine. Between the pH range of 7-8, this reactions takes place almost instantaneously.
If the chlorine to ammonia ratio is increased back above 5:1, the additional chlorine will displace a second hydrogen ion from the monochloramine molecule and replace it with another chloride ion, resulting in the formation of dichloramine:
HOCl + NH2Cl à NHCl2 (Dichloramine) + H2O
Dichloramines are notorious for their bad smell, and are usually the prime suspects for “chlorine smell” and eye irritation in the pool and spa industry.
The third member of the chloramines group, trichloramine, is similarly formed. Additional chlorine reacts with dichloramine to form the tri-substituted trichloramine (commonly referred to as Nitrogen trichloride):
HOCl + NHCl2 à NCl3 (Trichloramine) + H2O
(Trichloramine is both volatile and unstable and is usually not allowed to form in most industries).
At this point, if enough chlorine is added to bring the chlorine to ammonia ratio up to 10:1, the mono- and dichloramines are almost completely destroyed and converted back into less offensive nitrogen compounds and chloride salts:
2 NHCl2 + 2 HOCl à N2 + 6 Cl- + 2 H2O
This last reaction is generally referred to as “Breakpoint chlorination”. A constant state of breakpoint chlorination is required to prevent the formation of chloramines. This means maintaining the 10:1 chlorine to ammonia ratio in the water at all times.
Organic Chloramine Formation
Chlorine is not specific in reacting only with ammonia. Chlorine will also readily react with compounds containing carbon and nitrogen. These compounds are called organic nitrogen compounds and are generally described by the formula R-N, where R represents a carbon chain of some undefined length and contains at least one reactive nitrogen group. Chlorine reacts with these compounds in similar fashion as it does with ammonia, and the product of these reactions are referred to as organic chloramines, or organochloramines. They have little to no disinfectant properties. While these organochloramines are usually only present in low levels, it is important to realize that they can react as chloramines. This can lead to false readings in many tests used for controlling mono- and dichloramines.
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