Bitartrate, also known as hydrogen tartrate, is an anion formed by the partial deprotonation of tartaric acid (2,3-dihydroxybutanedioic acid), a dicarboxylic acid, resulting in the monovalent ion [C₄H₅O₆]⁻ with a molecular weight of 149.08 g/mol.^[1] This ion represents the conjugate base where only one of the two acidic hydrogen atoms from tartaric acid's carboxyl groups is replaced, distinguishing it from the fully deprotonated tartrate anion.^[2] Bitartrates are thus the acid salts or monoesters of tartaric acid, commonly encountered in chemical and industrial applications due to their acidic properties and solubility characteristics.^[3] The most notable bitartrate is potassium bitartrate (KC₄H₅O₆), a white crystalline powder also referred to as cream of tartar, which forms naturally as a sediment during the fermentation of grape juice in winemaking.^[4] As a byproduct of the wine industry, it occurs in grapes, the primary source of tartaric acid, and is extracted for commercial use.^[4] In food science, potassium bitartrate functions as a leavening agent in baking powders, stabilizes egg whites to increase volume and heat tolerance, and acts as an acidity regulator and humectant in confectionery and syrups.^[4] Beyond culinary roles, it serves as a laxative, buffering agent in analytical chemistry (such as NIST reference buffers), and stabilizer in wine production to prevent tartrate precipitation.^[5][6] Other bitartrates, such as sodium bitartrate, find applications in pharmaceuticals and as intermediates in organic synthesis, leveraging the ion's ability to form coordination complexes and regulate pH.^[7] These compounds are valued for their biocompatibility, optical activity derived from tartaric acid's chirality, and role in processes like crystallization control in the food and beverage sectors.^[8]

Chemistry

Definition and Nomenclature

Bitartrate, also known as the bitartrate ion, is the conjugate base of tartaric acid, formed by the deprotonation of one of the two carboxylic acid groups in the parent molecule, resulting in the monoanionic species with the molecular formula $\ce{C4H5O6^-}$.^[9] The preferred IUPAC name for this anion is 3-carboxy-2,3-dihydroxypropanoate.^[9] It is commonly referred to by other names such as hydrogen tartrate or acid tartrate, reflecting its role as the partially neutralized form of tartaric acid.^[3] Tartaric acid itself is a dibasic carboxylic acid, capable of donating two protons, with the fully deprotonated dianion known as the tartrate ion ($\ce{C4H4O6^{2-}}$).^[10] The nomenclature "bitartrate" historically denotes a salt or ester in which only one of the two acidic hydrogen atoms of tartaric acid is replaced by a metal ion or positive group, distinguishing it from the normal tartrate.^[11]

Molecular Structure

The bitartrate ion, with the chemical formula $\ce{C4H5O6^-}$, is the monoanionic conjugate base derived from tartaric acid.^[1] Its structural formula is $\ce{HOOC-CH(OH)-CH(OH)-COO^-}$, featuring a chain of four carbon atoms where the terminal groups consist of one protonated carboxylic acid ($\ce{-COOH}$) and one deprotonated carboxylate ($\ce{-COO^-}$), flanked by two adjacent hydroxyl-substituted chiral carbons.^[4] The molar mass of the ion is 149.08 g/mol.^[1] This structure includes two chiral centers at the 2- and 3-positions, enabling stereoisomerism analogous to that of its conjugate acid, tartaric acid. The naturally occurring form is the $(2R,3R)$-bitartrate, corresponding to the L-(+)-enantiomer prevalent in biological sources such as grapes. The enantiomer is the $(2S,3S)$-bitartrate, while the meso form, $(2R,3S)$-bitartrate, is achiral due to an internal plane of symmetry. These stereoisomers arise from the configurations at the hydroxyl-bearing carbons, influencing the ion's spatial arrangement and potential interactions.^[12] The bonding in the bitartrate ion is characterized by the polar carboxylic acid and carboxylate groups, which differ in their acidity and charge, along with the vicinal diol moiety formed by the two hydroxyl groups. These features confer significant hydrogen bonding potential, particularly through the $\ce{-OH}$ and $\ce{-COO^-}$ sites, facilitating intermolecular associations in salts and solutions.^[13]

Properties

Physical Properties

Bitartrate salts, such as potassium bitartrate (KHC₄H₅O₆), typically appear as colorless monoclinic crystals or a fine white powder.^[4] These salts exhibit moderate solubility in water, dissolving at approximately 0.57 g per 100 mL at 20 °C, while being practically insoluble in alcohol (1 g requires about 8820 mL).^[4] Potassium bitartrate has a density of 1.95 g/cm³ and decomposes upon heating at approximately 230 °C.^[14] Saturated aqueous solutions of bitartrate salts are acidic due to partial dissociation, with a pH of approximately 3.56 at 25 °C.^[15] For comparison, sodium bitartrate shows higher water solubility, around 30 g per 100 mL at 20 °C.