Acid salt

Examples of acid salts

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 salts (such as sodium bicarbonate (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 even 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 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.

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).

Hydrolysis of ammonium at room temperature produces:

${\displaystyle {\text{NH}}_{4(\text{aq})}^{+}+{\text{H}}_2^{}{\text{O}}_{(\text{aq})}^{}\stackrel{-\rightharpoonup }{\leftharpoondown -}{\text{NH}}_{3(\text{aq})}^{}+{\text{H}}_3^{}{\text{O}}_{(\text{aq})}^{+}}$

${\displaystyle K_{\mathrm{a}}=\frac{[\mathrm{N}{\mathrm{H}}_3][{\mathrm{H}}_3{\mathrm{O}}^{+}]}{[\mathrm{N}{\mathrm{H}}_4^{+}]}=\frac{K_{\mathrm{w}}}{K_{\mathrm{b}}}}$

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