Chemistry:Acid salt

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Short description: Salts that produce acidic solutions

Acid salts are a class of salts that produce an acidic solution after being dissolved in a solvent. Its formation as a substance has a greater electrical conductivity than that of the pure solvent.[1] An acidic solution formed by acid salt is made during partial neutralization of diprotic or polyprotic acids. A half-neutralization occurs due to the remaining of replaceable hydrogen atoms from the partial dissociation of weak acids that have not been reacted with hydroxide ions (OH−) to create water molecules.

Acidic solution and examples of acid salts

Structure of ammonium chloride

Acid–base property of the resulting solution from a neutralization reaction depends on the remaining salt products. A salt containing reactive cations undergo hydrolysis by which they react with water molecules, causing deprotonation of the conjugate acids.

For example, the acid salt ammonium chloride is the main species formed upon the half neutralization of ammonia in aqueous solution of hydrogen chloride:[2]

NH
3(aq) + HCl(aq) → [NH
4]+
Cl−
(aq)
Example of acid salts
Name Sodium bisulfate Monosodium phosphate Disodium phosphate
Structural formula
Sodium bisulfate.svg
Monosodium phosphate.png
Disodium hydrogen phosphate.png
Chemical formula NaHSO
4 		NaH
2PO
4 		Na
2HPO
4
IUPAC name Sodium hydrogen sulfate Sodium dihydrogen phosphate Disodium hydrogen phosphate
Other name
  • Sodium acid sulfate
  • Bisulfate of soda
  • Monobasic sodium phosphate
  • Sodium acid phosphate
  • Sodium biphosphate
  • Disodium hydrogen orthophosphate
  • Sodium phosphate dibasic
  • disodium phosphate
Molecular weight 120.054 g/mol 119.976 g/mol 141.957 g/mol
Formal charge zero zero zero
Odour Odourless Odourless Odourless[3]
Appearance White crystals or granules White crystalline powder[4] White, hygroscopic powder[5]
Structure
  • triclinic (anhydrous)
  • monoclinic (monohydrate)
 Monoclinic crystals[6] Monoclinic crystals (anhydrous)[7]
Solubility
  • Soluble in water
  • Insoluble in ammonia
  • Soluble in water
  • Insoluble in ethanol or ether
  • Soluble in water
  • Insoluble in ethanol
Density
  • 2.742 g/cm3 (anhydrous)
  • 1.8 g/cm3 (monohydrate)
 0.5–1.2 g/cm3 1.7 g/cm3
Decomposition

(through heating)

Emits toxic fumes of phosphoxides and sodium oxide[8] Emits toxic fumes of phosphorus- and sodium oxides[8]
Uses
  • Bleaching agents
  • Plating agents and surface treating agents
  • Cleaning and Furnishing Care Products
  • Treat constipation
  • Clean the bowel before a colonoscopy
  • Bleaching agents
  • A source of phosphorus
  • Visicol tablets are indicated for cleansing of the colon
  • Corrosion inhibitors and anti-scaling agents

Use in food

Acidic salts are often used in foods as part of leavening agents. In this context, the acid salts are referred to as "leavening acids."[9] Common leavening acids include cream of tartar and monocalcium phosphate.

An acidic salt can be mixed with an alkali salt (such as sodium bicarbonate or baking soda) to create baking powders which release carbon dioxide.[10] Leavening agents can be slow-acting (e.g. sodium aluminum phosphate) which react when heated, or fast-acting (e.g., cream of tartar) which react immediately at low temperatures. Double-acting baking powders contain both slow- and fast-acting leavening agents and react at low and high temperatures to provide leavening rising throughout the baking process.[11]

Disodium phosphate, Na
2HPO
4, is used in foods and monosodium phosphate, NaH
2PO
4, is used in animal feed, toothpaste and evaporated milk.

Intensity of acid

An acid with higher Ka value dominates the chemical reaction. It serves as a better contributor of protons (H+). A comparison between the Ka and Kb indicates the acid–base property of the resulting solution by which:

  1. The solution is acidic if Ka > Kb. It contains a greater concentration of H+ ions than concentration of OH− ions due more extensive of cation hydrolysis compared to that of anion hydrolysis.
  2. The solution is alkali if Ka < Kb. Anions hydrolyze more than cations, causing an exceeding concentration of OH− ions.
  3. The solution is expected to be neutral only when Ka = Kb.[12]

Other possible factors that could vary pH level of a solution are the relevant equilibrium constants and the additional amounts of any base or acid.

For example, in ammonium chloride solution, NH+
4 is the main influence for acidic solution. It has greater Ka value compared to that of water molecules; Ka of NH+
4 is 5.6×10−10, and Kw of H
2O is 1.0×10−14. This ensures its deprotonation when reacting with water, and is responsible for the pH below 7 at room temperature. Cl− will have no affinity for H+ nor tendency to hydrolyze, as its Kb value is very low (Kb of Cl− is 7.7×10−21).[13]

Hydrolysis of ammonium at room temperature produces:

[math]\ce{ NH4+_{(aq)}\ + H2O_{(aq)} <=> NH3_{(aq)}\ + H3O+_{(aq)} }[/math]
[math]\displaystyle{ K_\mathrm{a} = \frac{\mathrm{[NH_3][H_3O^+]}}{\mathrm{[NH_4^+]}} = \frac{K_\mathrm{w}}{K_\mathrm{b}} }[/math]
[math]\displaystyle{ K_\mathrm{a} = \frac{[1.0 \times 10^{-14}]}{[1.8 \times 10^{-5}]} = {5.6} \times 10^{-10} }[/math]

See also

References

  1. ↑ Cady, H. P.; Elsey, H. M. (1928). "A general definition of acids, bases, and salts". Journal of Chemical Education 5 (11): 1425. doi:10.1021/ed005p1425. Bibcode: 1928JChEd...5.1425C. 
  2. ↑ Dekock, Roger L.; Gray, Harry B. (1989). Chemical bonding and structure (Second ed.). Sausalito, California: University Science Book. pp. 97–98. ISBN 978-0-935702-61-3. https://books.google.com/books?id=q77rPHP5fWMC&q=nh4cl+lewis+structure&pg=PA97. Retrieved 8 February 2018. 
  3. ↑ U.S. Coast Guard, Department of Transportation. CHRIS - Hazardous Chemical Data. Volume II. Washington, D.C.: U.S. Government Printing Office, 1984-5.
  4. ↑ Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 1153
  5. ↑ Lide, D.R. CRC Handbook of Chemistry and Physics 88TH Edition 2007-2008. CRC Press, Taylor & Francis, Boca Raton, Florida 2007, p. 4-90
  6. ↑ Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics. 95th Edition. CRC Press LLC, Boca Raton: FL 2014-2015, p. 4-89
  7. ↑ Somov, N.V.; Chausov, F.F.; Russ, J. (2017). "High-symmetry polymorph of anhydrous disodium hydrogen phosphate". Russian Journal of Inorganic Chemistry 62 (2): 172–174. doi:10.1134/S0036023617020176. 
  8. ↑ 8.0 8.1 Wiley, John; Hoboken, NJ (2004). Sax's Dangerous Properties of Industrial Materials (11th Edition By Richard J. Lewis ed.). Wiley-Interscience. p. 3274. doi:10.1021/ja041002c. ISBN 978-0-471-47662-7. 
  9. ↑ Wallace, David (10 June 2015). "Leavening Acids – Baking Ingredients" (in en-US). https://bakerpedia.com/ingredients/leavening-acids/. 
  10. ↑ "The Many Uses of Cream of Tartar" (in en). https://www.mcgill.ca/oss/article/health-nutrition-quackery/chemistry-lesson-food-babe-and-everyone-else-12-cream-tartars-many-uses. 
  11. ↑ Gabriala, Pop (2007). "RESEARCHES REGARDING THE CHEMICAL LEAVENING AGENTS' ROLE IN QUALITY OF BAKERY PRODUCTS". Journal of Agroalimentary Processes and Technologies XIII, No. 1: 105–112. 
  12. ↑ Raymond, Chang (2010). Chemistry (tenth ed.). Americas, New York: McGraw-Hill. pp. 725–727. ISBN 978-0077274313. https://archive.org/details/Chemistry_10th_Edition_Raymond_Chang. Retrieved 9 February 2018. 
  13. ↑ Lower, S.K., (1999). Introduction to acid-base chemistry. Chem1 General Chemistry Text. Retrieved from http://www.chem1.com/acad/pdf/c1xacid1.pdf



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