Chlorine, a common disinfectant in our daily lives, seems to be inextricably linked to the pH of water. We will start with the chemical properties of chlorine, analyze the series of reactions that occur in water, and combine them with practical application scenarios to reveal the effect of chlorine on pH.

What is Chlorine?

Chlorine (chemical symbol Cl) is a chemical element belonging to the halogen group, group 17 in the periodic table, with atomic number 17. Chlorine is a yellowish-green gas with a strong odour and corrosive properties, which is gaseous at room temperature and pressure. It is widely used in many fields such as industry, cleaning and disinfection.

Basic Properties of Chlorine

Physical Property

Chlorine is a yellowish green gas with a strong irritating odour at room temperature.
Chlorine has a boiling point of -34.04°C and a melting point of -101.5°C. Therefore, it is a gas at room temperature and liquefies and solidifies easily.

Chemical Properties

Chlorine is a very reactive element that reacts readily with other elements, especially metals and non-metals. It is usually found in nature in the form of the chloride ion (Cl-).
Chlorine can react with almost any element to form chlorides. For example, chlorine reacts with sodium to form sodium chloride (NaCl), which is commonly known as table salt.
Chlorine reacts with water to form hydrogen chloride (HCl) and hypochlorous acid (HOCl), which makes it useful as a disinfectant in water treatment.

What is the Purpose of Chlorine Measurement?

Chlorine measurement is an important task in water quality monitoring and environmental protection, and has a wide range of uses, especially in drinking water treatment, wastewater treatment, industrial water quality management and disinfection processes.

Drinking Water Disinfection and Safety Monitoring

Chlorine Disinfection: Chlorine is commonly used in the disinfection of drinking water to kill pathogens (e.g., bacteria, viruses, and protozoa) in the water to ensure safe water quality. Measuring the chlorine concentration ensures that the level of chlorine in the water meets the specified safety range and avoids residual chlorine in the water that could be hazardous to human health.
Swimming pool water quality control: The water in swimming pools also needs to be measured regularly for chlorine concentration to ensure that it is sufficiently disinfectant and to avoid skin and eye irritation from high chlorine levels.

Wastewater Treatment and Effluent Disinfection

In wastewater treatment, chlorine is used to disinfect the treated water to kill remaining bacteria and pathogenic microorganisms. Measuring the chlorine content in wastewater ensures that the disinfection is up to standard and prevents excess chlorine from having a negative impact on the environment.
Chlorine Residual Monitoring: Chlorine residual is an important indicator of whether chlorine has been completely removed after wastewater treatment. By measuring the concentration of chlorine, it is possible to determine whether further treatment is required.

Industrial Water Treatment And Cooling Water System

Chlorine is used to prevent the growth of bacteria, algae and other microorganisms in industrial processes, especially in cooling water systems and boiler water. Measuring the chlorine concentration helps to regulate the amount of chlorine added, keeping the water hygienic and the system running efficiently.
Optimisation of water treatment processes: By measuring the chlorine concentration, industrial water treatment processes can be optimised to ensure that there is enough chlorine in the water to disinfect it, while avoiding excess chlorine that could be harmful to the system or the environment.

Monitoring of Agricultural Irrigation Water

Chlorine is used as a disinfectant in agricultural irrigation water, especially when the water source may be contaminated. Measuring the chlorine concentration in irrigation water ensures that the water quality is appropriate and prevents excessive chlorine concentrations from affecting soil and crop growth.

Chlorine Leak Monitoring

Chlorine leakage monitoring is used in industrial facilities, at sites where chlorine is stored and during transport. Chlorine leakage is a serious safety hazard that can cause harm to workers and the surrounding environment. By measuring the concentration of chlorine gas in real time, leaks can be detected early and safety measures can be taken.

Environmental Protection and Pollution Monitoring

Measurement of chlorine concentration in industrial wastewater, rivers, lakes and other bodies of water helps to assess pollution levels and ensure that chlorine is not causing long-term harm to ecosystems. Excessive amounts of chlorine can cause damage to organisms in water bodies, especially harming aquatic organisms.

Why is PH Important for Chlorine Measurement?

pH is critical to chlorine measurement because it directly affects the chemical form of chlorine in water and its ability to disinfect. A low pH optimises the disinfecting effect of chlorine, while a high pH reduces the bactericidal ability of chlorine. To ensure that chlorine disinfection is effective and the measurement is accurate, both pH and chlorine concentration in the water must be monitored.

Different Types of Chlorine Products and Their pH Effects

Chlorine Gas (Cl₂)

Chemical Properties:
- When chlorine gas dissolves in water, it forms hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻):
  [ Cl_2 + H_2O \rightarrow HCl + HOCl ]
- Under acidic conditions, chlorine gas produces hypochlorous acid (HOCl), while in alkaline conditions, it transforms into hypochlorite ions (OCl⁻).
pH Impact:
- Low pH (Acidic): At low pH, chlorine is predominantly in the form of hypochlorous acid (HOCl). Hypochlorous acid is a strong oxidizer with better disinfecting properties, making chlorine more effective in acidic environments.
- High pH (Alkaline): At high pH, chlorine converts to hypochlorite ions (OCl⁻), which are less effective as an oxidizer, reducing the disinfecting power.

Sodium Hypochlorite (NaOCl)

Chemical Properties:
- When sodium hypochlorite dissolves in water, it partially dissociates to form hypochlorous acid (HOCl) and hydroxide ions (OH⁻):
  [ NaOCl + H_2O \rightleftharpoons HOCl + NaOH ]
- This means that sodium hypochlorite solution contains both hypochlorous acid (HOCl) and hydroxide ions (OH⁻), where the hydroxide ions make the solution alkaline.
pH Impact:
- Low pH (Acidic): At low pH, the concentration of hypochlorous acid (HOCl) increases, enhancing its disinfecting effect. Thus, sodium hypochlorite is more effective in acidic environments.
- High pH (Alkaline): At higher pH, the hypochlorous acid (HOCl) in the water converts to hypochlorite ions (OCl⁻), weakening its disinfecting power.

Calcium Hypochlorite (Ca(OCl)₂)

Chemical Properties:
- Calcium hypochlorite is a common solid chlorine compound. When it dissolves in water, it forms calcium hypochlorite (Ca(OCl)₂) and releases hypochlorous acid (HOCl):
  [ Ca(OCl)_2 + H_2O \rightarrow Ca^{2+} + 2HOCl ]
- Calcium hypochlorite solutions tend to have a higher pH, making the water alkaline.
pH Impact:
- Low pH (Acidic): Calcium hypochlorite can also produce hypochlorous acid (HOCl) in acidic conditions, but its effectiveness is limited in acidic environments since it is typically more stable and effective in alkaline conditions.
- High pH (Alkaline): Calcium hypochlorite solutions are usually alkaline, helping to stabilize hypochlorite ions (OCl⁻) and thus extending the efficacy of chlorine.

Hypochlorous Acid (HOCl)

Chemical Properties:
- Hypochlorous acid is often used directly as a disinfectant. It is the main active substance formed when chlorine gas reacts with water and is a strong oxidizer.
- The stability of hypochlorous acid in water is closely related to the pH of the water.
pH Impact:
- Low pH (Acidic): At low pH, hypochlorous acid (HOCl) exists in its undissociated form, providing stronger disinfecting properties.
- High pH (Alkaline): At high pH, hypochlorous acid transforms into hypochlorite ions (OCl⁻), which reduces its disinfecting effectiveness.