Acid salt

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.

Formation

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]

NH₃(aq) + HCl(aq) → [NH₄]⁺Cl⁻(aq)

Examples of acid salts

Example of acid salts

Name	Sodium bisulfate	Monosodium phosphate	Disodium phosphate
Structural formula
Chemical formula	NaHSO4	NaH2PO4	Na2HPO4
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

Acid 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 acid salt can be mixed with certain base 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, 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 K_a value dominates the chemical reaction. It serves as a better contributor of protons (H⁺). A comparison between the K_a and K_b indicates the acid–base property of the resulting solution by which:

1. The solution is acidic if K_a > K_b. It contains a greater concentration of H⁺ ions than concentration of OH⁻ ions due to more extensive cation hydrolysis compared to that of anion hydrolysis.
2. The solution is alkaline if K_a < K_b. Anions hydrolyze more than cations, causing an exceeding concentration of OH⁻ ions.
3. The solution is expected to be neutral only when K_a = K_b.^[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 K_a value compared to that of water molecules; K_a of NH+4 is 5.6×10⁻¹⁰, and K_w of H₂O 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 K_b value is very low (K_b of Cl⁻ is 7.7×10⁻²¹).^[13]

Hydrolysis of ammonium at room temperature produces:

${\displaystyle {\ce {NH4+_{(aq)}\ + H2O_{(aq)}<=> NH3_{(aq)}\ + H3O+_{(aq)}}}}$

${\displaystyle K_{\mathrm {a} }={\frac {\mathrm {[NH_{3}][H_{3}O^{+}]} }{\mathrm {[NH_{4}^{+}]} }}={\frac {K_{\mathrm {w} }}{K_{\mathrm {b} }}}}$

${\displaystyle K_{\mathrm {a} }={\frac {[1.0\times 10^{-14}]}{[1.8\times 10^{-5}]}}={5.6}\times 10^{-10}}$

See also

• Base salt
• Salt (chemistry)
• Oxoacid
• Sodium bicarbonate
• Sodium bisulfate: an example of acid salt
• Disodium phosphate: an example of acid salt
• Monosodium phosphate: an example of acid salt