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. 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. Acid salt is an ionic compound consisted of an anion, contributed from a weak parent acid, and a cation, contributed from a strong parent base.
Acidic solution and examples of acid salts
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 hydrochloric acid solution:
|Identity:||Sodium bisulfate||Monosodium phosphate||Disodium phosphate|
|IUPAC Name||Sodium hydrogen sulfate||Sodium dihydrogen phosphate||Disodium hydrogen phosphate|
|Molecular Weight||120.054 g/mol||119.976 g/mol||141.957 g/mol|
|Appearance||White crystals or granules||White crystalline powder||White, hygroscopic powder|
||Monoclinic crystals||Monoclinic crystals (anhydrous)|
||0.5-1.2 g/cm3||1.7 g/cm3|
|Forms sodium carbonate, water, and carbon dioxide||Emits toxic fumes of phosphoxides and sodium oxide||Emits toxic fumes of phosphorus- and sodium oxides|
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." 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. 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.
Disodium phosphate, Na2HPO4, is used in foods and monosodium phosphate, NaH2PO4, 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:
- 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.
- The solution is alkali if Ka < Kb. Anions hydrolyze more than cations, causing an exceeding concentration of OH− ions.
- The solution is expected to be neutral only if Ka and Kb are identical.
- For example, in ammonium chloride solution, NH4+ is the main influence for acidic solution. It has greater Ka value compared to that of water molecules; Ka of NH4+ is 5.6 x 10−10 and Kw of H2O is 1.0 x 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 x 10−21).
- Hydrolysis of ammonium at room temperature produces:
- = 5.6 x 10−10
- Alkali salt
- Salt (chemistry)
- Sodium bicarbonate
- Sodium bisulfate
- Disodium phosphate
- Monosodium phosphate
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