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B-1 Training: Introduction to Soluble Salts A P P E N D I X B INTRODUCTION TO SOLUBLE SALTS
B-2 Key Learning Objectives This Training Addresses the Following Questions â What are Soluble Salts? â What makes Salts Soluble? â How Do Salts Affect Solution Conductivity? â Why do Soluble Salts Matter for Bridges? What is a Salt? Definition: â A salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base. Skoog, D.A; West, D.M.; Holler, J.F.; Crouch, S.R. (2004). Fundamentals of Analytical Chemistry; Chapters 14, 15 and 16 (8th ed.). Thomson Brooks/Cole. ISBN 0-03- 035523-0 . â This compound is often formed by negatively charged non-metal ions (acids in solution) and positively charged metal ions (bases in solution) In chemistry, a salt is an ionic compound which is made up of two groups of oppositely charged ions. The ion with a positive charge is called a cation, and the one with a negative charge is called an anion. Salts are usually a combination of positive metal ions and negative non-metal ions that when combined have no overall electrical charge
B-3 Everyday Salts Table Salt â NaCl Rock Salt â NaCl De-Icing Salt â NaCl2, CaCl , MgCl2 Sea Salt â evaporated ocean water Epsom salt â MgSO4 Key point is that âsaltâ is a very broad term. Bridge Salts Airborne salts from salt water source De-Icing salts deposited during winter weather Industrial sources Chlorides, Sulfates, Nitrates Salts of concern for bridges are typically from deicing salts, marine environment, or industrial sources. Common ions of concern are chlorides, sulfates, and nitrates
B-4 How are Salts Formed? Neutralization Reaction â HA + BOH BA + H 2O â Two oppositely charged ions lead to one neutrally charge salt and water â Cation -> positively charged -> HA â Anion -> Negatively charged -> BOH â To form NaCl: + + Hydrolysis Reaction â BA + H2O HA + BOH â Salt + water = cation and anion in solution â Salt is solubilized onto the surface â To dissolve NaCl: + + Formation and solubility of salts
B-5 Solubility Rules Relating to Salts Soluble Salts containing: â Lithium (Li) â Sodium (Na) â Potassium (K) â Caesium (Cs) â Nitrate (N03) â Chloride (Cl) â Bromide (Br) â Sulfate (SO4-2) Insoluble Salts containing: â Silver (Ag) â Transition metals (Fe, Zn, Mg, etc) â Carbonates (CO3) â Chromates (CrO4) â Phosphates (PO4) â Fluorides (F) General solubility rules, there are some exceptions Bolded common salt contributors Sodium, Nitrate, Sulfate Iron Insoluble salts include most silver except silver nitrate, AgNO3 Aside from this, usually insoluble cations will take precedence over soluble ions, i.e., AgCl is insoluble. Some of these insolubilities are used to measure the level of salts in a solution, like the above mentioned AgCl http://www5.csudh.edu/oliver/chemdata/solrules.htm â Organized list https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Equilibria/Solubilty/ Solubility_Rules â List with References
B-6 Precipitation of Salts Salts will precipitate under the following conditions: â An increase of salt concentration past the solubility limit â A decrease in water present causing the concentration to exceed the solubility limit â The formation of new salts from ions in solution that are insoluble Precipitated salts may draw moisture from the air, keeping the surface wet which can either create a corrosion cell or stress the protective coating. How Salts Affect Conductivity All dissolved ions (cations and anions) contribute to solution conductivity based on their ionic conductivity as shown in the table from Physical Chemistry, P.W. Perkins, W.H. Freeman and Company, San Francisco, CA, 1978 Key point is that both the cation and anion (e.g., Na and Cl) contribute to solution conductivity when they are dissolved. Also, different ions contribute differently to the conductivity.
B-7 Where do Salts Occur on Bridges Salts may precipitate on surfaces, be contained in debris, or collect in crevices formed between steel members Consequences of Soluble Salt Contamination Why Soluble Salt Measurements are important Higher levels of soluble salts lead to: â Increased rates of corrosion from defect sites â Increased metal loss due to corrosion â Poor adhesion of coating systems â Blistering of coatings â Pre-mature coating breakdown â Increased discoloration due to rust staining When moist, salts disassociate into ions in solution â¢ Dissolved ions will increases conductivity of the solution and may de-passivate steel surfaces, both of which will increase the rate of corrosion reactions Under a painted surface, salts will draw water through coatings through osmosis â¢ May cause a coating to blister
B-8 Situations Where Salt Testing may be Useful The following situations may benefit from soluble salt testing: â Determine if a prepared surface is sufficiently free of soluble salts to be painted. â Determine whether a bridge washing provides a thorough salt remediation â Evaluate the severity of the environment around the structure being tested, and how this environment may be changing over time â Document how different winter salt brines compositions adhere to the surface, and whether seasonal washing removes them from surfaces The most common use of soluble salt detection techniques on bridges is to determine if a surface is sufficiently clean prior to painting. Detection techniques may also be used to document and/or understand what salts are on a bridge to inform future maintenance decision-making.