By Peter 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!
Salts are chemical compounds composed of soluble components that become electrically charged when in solution. These charged components are collectively known as ions.
Those ions with positive charges (+) called cations, and those with negative charges (-) called anions. When acids and bases (substances that release hydroxide ions into the water and will also produce a pH in water above 7) combine, salts are formed.
Acidity and Alkalinity
Acidity is the measure of dissolved solids that are acidic in nature (pH<7).
Acidic waters contain several traits.
They have a sour taste (like fruit juice and vinegar) and they turn Litmus paper red when tested. They also can be corrosive and tend to dissolve many metals.
A very complex subject, alkalinity is the quantitative capacity of water to neutralize an acid. Alkalinity is not the same as pH, because water does not necessarily have a high pH to have high alkalinity.
Principal contributors to alkalinity in water supplies include the bicarbonate ion (HCO3-) and the carbonate ion (CO32-), the latter being common with natural waters in limestone areas. Alkalinity is also associated with high pH solutions (bases) and is most uncommon as hydroxide (OH-), except in municipal waters softened by soda-lime process.
Alkaline waters have a soda taste and can cause drying of the skin. They tend to oxidize metals and turn Litmus paper blue.
The Langelier Saturation Index (LSI) is a calculated number used to predict the calcium carbonate (CaCO3) stability of a water supply. This determines whether this water will precipitate, dissolve, or be in equilibrium with calcium carbonate.
LSI = pH – pH8
Where: pH = actual pH of the water pH 8 = pH at which water having the same alkalinity and calcium content is just saturated with calcium carbonate.
Some common chemicals with low solubility in water include: red rust ferric iron (Fe2O3), calcium carbonate scale (CaCO3) and calcium sulfate (Ca SO4).
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.