By Peter S. Cartwright, PE, CWS-VI
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!
There is a strong chemical relationship between water and solute, which makes understanding this essential for work in the water treatment and purification industry.
When an inorganic compound (salt) is dissolved in water, it breaks down into ionic components.
They dissociate into positively and negatively charged particles known as ions. It is the basic concept for ion exchange water softening.
Most dissolved organic compounds do not form ions. Instead, they form covalent bonds, which are created when elements share electrons in order to become stable. These generally do not dissociate in water.
Ions theoretically move freely as independent particles. This water and salt relationship makes a big difference, as they easily bond to form an ionic component.
Such an ionic bond is formed between two elements due to opposite electrostatic forces. Salt (NaCl) can easily dissociate in water, forming into Na + ions and Cl – ions (Figure 1).
NaCl Na+ + Cl-
Differences
Ions differ from an atom or molecule in that they carry an electrical charge. Positively charged ions are called cations. These include: Ca2+, Mg2+, Na+, K+, Fe2+, Mn2+, Al3+
Negatively charged ions are called anions. These include: OH-, NO3-, CO32-, Cl-, PO43-
The sum of all positive ions equals the sum of all negative ions in a water solution; therefore, the solution is electrically neutral. Water is most stable when it is electrically neutral; i.e. when the number of positive charges equals the number of negative charges.
Pure water does not conduct electricity. In a simple experiment, a battery and light bulb with electrodes in pure water do not conduct electricity. If you add salt to the water, the bulb will light up.
Not all compounds that dissolve in water form ions. Generally, only inorganic salts form ions. If you add sugar to the water, the bulb will not light. (Figure 2)
Valence
The number of charges on an ion is known as its valence. It is irrelevant whether this is associated with a cation or an anion. Some common examples have a variety of charges (Figure 3).
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: [email protected] or on his website www.cartwright-consulting.com.