By Peter S. Cartwright, PE, CWS-VI

Halogen chemicals are all oxidants and, with the exception of fluorine, are used as water disinfectants.

The basic chemistry fundamentals presented each month are not intended to be a comprehensive chemistry course, but rather basic instruction on chemistry as it relates to water and water treatment. It is hoped that your interest will be piqued and induce you to want to learn more. The desired outcome is that it will help you become a more effective and valuable water treatment professional. Please get back to us with any questions or concerns; we welcome your input!

Last month we talked about elements. Most chemicals found in water treatment are molecules and compounds.

A molecule is defined as the smallest particle of an element or compound that retains all of the characteristics of that element or compound. It is made up of one or more atoms.

A compound is a chemical substance composed of two or more elements whose composition is constant. Several compounds are simply two of the same element that combine to form a ‘diatomic’ compound (see Figure 1). They are all gases and include:

  • Hydrogen (H2)
  • Oxygen (O2)
  • Nitrogen (N2)
  • Chlorine (Cl2)
  • Bromine (Br2)
  • Iodine (I2)
  • Fluorine (F2)

Figure 1.

Since these are often encountered in water treatment, everyone in the industry should remember them. The mnemonic acronym ‘HONClBrIF’ might help.

The last four of these gases (Cl2, Br2, I2 and F2) are called the ‘halogen’ chemicals. They are all oxidants and, with the exception of fluorine, are used as water disinfectants. Fluorine is such a powerful oxidant that it presents handling problems and cannot be used for this purpose.

Compound formation

Compounds are formed when two or more elements combine in a definite ratio. Some examples include:

  • K+ + NO3- KNO3
  • 2 Na+ + CO32- Na2CO3

Compounds exhibit different properties than the elements that make them (Cl2 vs. NaCl). They can be as simple as table salt (NaCl) or complex like DNA.

Sodium combines with fluorine by sharing one electron to form a sodium fluoride (NaF) compound. (See Figure 2).

Figure 2.

Primary compounds key to the understanding of water treatment include:

  • H2O (Water)
  • CO2 (Carbon Dioxide)
  • H2SO4 (Sulfuric Acid)
  • CH4 (Methane)
  • NH3 (Ammonia)
  • Na2SO4 (Sodium Sulfate)
  • Al2 (SO4)3 (Aluminum Sulfate)

Combined elements
Most of the compounds encountered in water treatment are not comprised of the same elements but specific combinations of elements. These molecules will have much different properties than the basic elements from which they are formed.

For example, the compound sodium chloride (common table salt) is much different than both elemental sodium and chlorine. Examples of compounds familiar to those of us in the water treatment industry include:

  • H2O (water)
  • CO2 (carbon dioxide – a gas)
  • H2SO4 (sulfuric acid)
  • CH4 (methane – a gas)
  • NH3 (ammonia – a gas)
  • Na2SO4 (sodium sulfate)
  • Al2 (SO4)3 (aluminum sulfate)

Weight and moles
The sum of the atomic weights of all of the elements in a compound or element is its molecular weight, which has units measured in Daltons. The molecular weight of an element or compound in grams is known as a mole of that substance (m).

A mole of one substance always contains the same number of particles as a mole of any other substance. This number is known as Avogadro’s Number and is 6.02 x 1023 particles/mole.

A mole of any substance dissolved in a liter (L) of solution is known as molarity (M). The formula for such is: M= m (solute) ÷ L (solution)

About the author
Peter S. Cartwright, CWS-VI, President of Cartwright Consulting Company, of Minneapolis is a registered Professional Engineer in Minnesota. He has been in the water treatment industry since 1974, has authored over 125 articles, presented over 125 lectures in conferences around the world and has been awarded three patents. Cartwright has chaired several WQA committees and task forces and has received the organization’s Award of Merit. A member of WC&P Technical Review Committee since 1996, his expertise includes such high-technology separation processes as RO, UF, MF, UF electrodialysis, deionization, carbon adsorption, ozonation and distillation. Cartwright is also Technical Consultant to the Canadian Water Quality Association. He can be reached by phone (952) 854-4911; fax: (952) 854-6964; email: or on his website



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