# 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. 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

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:
    NH$_3$(aq) + HCl(aq) → [NH$_4$]$^+$Cl$^−$(aq)

## Examples of acid salts

Example of acid salts :
| Name | Sodium bisulfate | Monosodium phosphate | Disodium phosphate |
| Structural formula | | | |
| Chemical formula | NaHSO$_4$ | NaH$_2$PO$_4$ | Na$_2$HPO$_4$ |
| IUPAC name | Sodium hydrogen sulfate | Sodium dihydrogen phosphate | Disodium hydrogen phosphate |
| Other name | Sodium acid sulfateBisulfate of soda | Monobasic sodium phosphateSodium acid phosphateSodium biphosphate | Disodium hydrogen orthophosphateSodium phosphate dibasicdisodium phosphate |
| Molecular weight | 120.054 g/mol | 119.976 g/mol | 141.957 g/mol |
| Formal charge | zero | zero | zero |
| Odour | Odourless | Odourless | Odourless |
| Appearance | White crystals or granules | White crystalline powder | White, hygroscopic powder |
| Structure | triclinic (anhydrous)monoclinic (monohydrate) | Monoclinic crystals | Monoclinic crystals (anhydrous) |
| Solubility | Soluble in waterInsoluble in ammonia | Soluble in waterInsoluble in ethanol or ether | Soluble in waterInsoluble in ethanol |
| Density | 2.742 g/cm$^3$ (anhydrous)1.8 g/cm$^3$ (monohydrate) | 0.5–1.2 g/cm$^3$ | 1.7 g/cm$^3$ |
| Decomposition (through heating) | | Emits toxic fumes of phosphoxides and sodium oxide | Emits toxic fumes of phosphorus- and sodium oxides |
| Uses | Bleaching agentsPlating agents and surface treating agentsCleaning and Furnishing Care Products | Treat constipationClean the bowel before a colonoscopyBleaching agents | A source of phosphorusVisicol tablets are indicated for cleansing of the colonCorrosion 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." 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. 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, Na$_2$HPO$_4$, is used in foods and monosodium phosphate, NaH$_2$PO$_4$, 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:
* The solution is acidic if K$_a$ > K$_b$. It contains a greater concentration of H$^+$ ions than concentration of OH$^−$ ions due more extensive cation hydrolysis compared to that of anion hydrolysis.
* The solution is alkaline if K$_a$ < K$_b$. Anions hydrolyze more than cations, causing an exceeding concentration of OH$^−$ ions.
* The solution is expected to be neutral only when K$_a$ = K$_b$.
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 H$_2$O 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$^{−21}$).
Hydrolysis of ammonium at room temperature produces:
    ${\ce {NH4+_{(aq)}\ + H2O_{(aq)}<=> NH3_{(aq)}\ + H3O+_{(aq)}}}$
    $K_{\mathrm {a} }={\frac {\mathrm {[NH_{3}][H_{3}O^{+}]} }{\mathrm {[NH_{4}^{+}]} }}={\frac {K_{\mathrm {w} }}{K_{\mathrm {b} }}}$
    $K_{\mathrm {a} }={\frac {[1.0\times 10^{-14}]}{[1.8\times 10^{-5}]}}={5.6}\times 10^{-10}$