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Disodium pyrophosphate

Disodium pyrophosphate, also known as sodium acid pyrophosphate (SAPP), is an with the Na₂H₂P₂O₇ and a molecular weight of 221.94 g/mol. It appears as a white, odorless crystalline powder or granules that is highly soluble in , with a of approximately 4 in a 1% , and decomposes at around 220°C. This sodium salt of is produced industrially by neutralizing with or carbonate followed by thermal dehydration at about 250°C. In the , disodium pyrophosphate serves multiple functions as a (GRAS) additive, including acting as a in baked goods like cakes and pancakes by releasing upon reaction with baking soda. It also functions as an emulsifier in processed cheeses to prevent separation and as a sequestrant in canned or potatoes to inhibit discoloration and chelate metal ions that could affect quality. Beyond food, it finds applications in industrial processes such as , metal cleaning, and as a in drilling muds, as well as in as a chelating and buffering agent. Disodium pyrophosphate is regulated as GRAS by the U.S. (FDA) for use in human food and animal feeds under good manufacturing practices, with specific limits in standardized foods such as up to 3% in cheese products and 0.5% in canned tuna. The U.S. Environmental Protection Agency (EPA) exempts it from tolerances when used as an inert ingredient, and it is approved internationally by bodies like (INS 450(i)) and the (E 450(i)), with a recommended intake limit of 70 mg/kg body weight per day. Safety assessments indicate low toxicity, with an oral LD50 of 2650 mg/kg in mice, though it can cause mild irritation to skin, eyes, and upon direct exposure to dust.

Chemical identity and properties

Nomenclature and identifiers

Disodium pyrophosphate is systematically named disodium dihydrogen diphosphate according to IUPAC nomenclature. It is commonly referred to as sodium acid pyrophosphate (SAPP) or disodium diphosphate in industrial and food contexts. Key chemical identifiers for disodium pyrophosphate include the following:
IdentifierValue
CAS Number7758-16-9
EC Number231-835-0
E Number (food additive)E450(i)
The compound has the molecular formula Na₂H₂P₂O₇ and a molar mass of 221.94 g/mol. It typically appears as a white, odorless crystalline powder.

Molecular structure and formula

Disodium pyrophosphate has the molecular formula \ce{Na2H2P2O7}, comprising two sodium cations (\ce{Na+}) and the dihydrogen anion (\ce{H2P2O7^2-}). The core structural feature of the anion is a P-O-P bridge connecting two \ce{PO3} groups, where each phosphorus atom adopts a tetrahedral with four surrounding oxygen atoms. This bridging oxygen in the P-O-P linkage is distinct from the terminal oxygens, contributing to the overall diphosphate architecture. The two acidic hydrogen atoms are attached to terminal oxygen atoms, rendering the anion as a partially protonated form of the fully deprotonated ion \ce{P2O7^4-}. A simplified representation of the structure highlights the ion as [\ce{O3P-O-PO3}]^{4-}, protonated to \ce{[H2O3P-O-PO3H2]^2-} with sodium counterions. In terms, each exhibits five electrons forming four bonds to oxygen, with the bridge oxygen shared between the two phosphorus centers, and double-bond character often implied in terminal P=O bonds for stability. This configuration arises from the of two orthophosphate units (\ce{HPO4^2-}), where eliminates to forge the P-O-P .

Physical and chemical properties

Disodium pyrophosphate appears as a , odorless crystalline powder. Its is 1.86 g/cm³. The compound does not have a defined , instead decomposing at approximately 220 °C. It exhibits moderate in , with approximately 11.5 g dissolving in 100 mL at 20 °C, and is insoluble in . A 0.1 M has a of approximately 4.3, reflecting its mildly acidic nature. Chemically, disodium pyrophosphate functions as a chelating agent, forming stable complexes with divalent metal ions such as Ca²⁺ and Mg²⁺, which helps sequester these ions in solution. It possesses buffering capacity primarily in acidic ranges due to the values of its phosphate groups (around 0.9, 2.1, 6.7, and 9.4). The compound undergoes slow in water to form orthophosphate ions, with the rate increasing with temperature and decreasing . Thermally, it decomposes above 220 °C to yield sodium metaphosphate. Disodium pyrophosphate is non-flammable and non-explosive under standard conditions.

Synthesis and production

Laboratory preparation

Disodium pyrophosphate, also known as sodium acid pyrophosphate, is commonly prepared in laboratory settings through the thermal dehydration of monosodium dihydrogen phosphate (NaH₂PO₄). This method involves heating the dihydrogen phosphate to temperatures between 200 and 250 °C, facilitating the represented by the equation: $2 \mathrm{NaH_2PO_4} \rightarrow \mathrm{Na_2H_2P_2O_7} + \mathrm{H_2O} The process requires careful control of heating to prevent over-dehydration, which may result in the formation of higher polyphosphates such as . Under optimized laboratory conditions, this reaction typically yields 80-90% of the desired product, depending on the purity of the starting material and precise temperature regulation. Following synthesis, the crude product is purified by recrystallization from hot water or ethanol. The mixture is dissolved in the minimum amount of boiling solvent, filtered to remove impurities, and then cooled slowly to promote crystal formation, yielding colorless, odorless crystals suitable for analytical or experimental use.

Industrial manufacturing

Disodium pyrophosphate, also known as sodium acid pyrophosphate (SAPP), is primarily produced on an industrial scale through a two-step process involving partial neutralization followed by dehydration. In the first step, food-grade , typically derived from the process to ensure high purity and minimize impurities like heavy metals, is partially neutralized with or (soda ash) to form (NaH₂PO₄). This reaction is carried out in under controlled conditions to achieve the desired , often around 4.5, yielding a solution or slurry of monosodium phosphate. The monosodium phosphate is then dehydrated in the second step via calcination, where two molecules condense to form disodium pyrophosphate: 2 NaH₂PO₄ → Na₂H₂P₂O₇ + H₂O. This thermal treatment occurs in a rotary kiln or furnace at approximately 250°C, with residence times adjusted to promote dehydration while limiting side reactions. The process is energy-intensive, requiring precise temperature control to avoid excessive formation of byproducts such as sodium trimetaphosphate. Spray-drying may precede calcination to produce a dry feed for efficient heating. Feedstocks for this production are sourced from wet-process (purified for food-grade applications) and soda ash, which is obtained from natural deposits or synthetic processes. Global demand for disodium pyrophosphate was approximately 120,000 metric tons as of 2022, driven primarily by demand in the and sectors, with major manufacturing hubs in the United States, , and . Process variations enhance efficiency and scalability, including continuous furnace heating systems for steady-state operation and reactors, which improve and uniformity by suspending the material in a gas . These methods reduce compared to batch processes and allow for higher throughput in large-scale facilities. Quality control is critical, particularly for food-grade material, which must meet purity standards exceeding 98% disodium pyrophosphate content, with limits on impurities such as (e.g., <3 ppm) and insoluble matter. Analytical techniques like and ensure compliance, while process monitoring minimizes byproducts through optimized conditions.

Applications

Food and beverage uses

Disodium pyrophosphate, also known as sodium acid pyrophosphate (SAPP), serves multiple functional roles in and beverage , primarily as a , sequestrant, emulsifier, and acidity regulator. It is recognized as generally safe (GRAS) by the U.S. for use in various edible products at specified levels. In the , it constitutes a significant portion of additives employed to enhance product quality, texture, and without altering nutritional profiles substantially. As a , disodium pyrophosphate is commonly incorporated into double-acting baking powders, where it reacts with (NaHCO₃) to produce gas. This reaction occurs in two stages: an initial release at during mixing and a slower release at higher baking temperatures, contributing to uniform rise and tender crumb in products like cakes, muffins, and refrigerated doughs. The key reaction is: \mathrm{Na_2H_2P_2O_7 + NaHCO_3 \rightarrow Na_3HP_2O_7 + CO_2 + H_2O} In and , disodium pyrophosphate acts as a color and texture stabilizer by chelating metal ions that cause oxidation and discoloration. It is added to canned at levels up to 0.5% by weight to inhibit crystal formation, maintaining product clarity and appearance. Similarly, in processed and other cured meats, it is used at a maximum of 0.5% in the finished product to enhance retention and prevent darkening. For potato products, such as and , it functions as a sequestrant at typical levels around 0.1-0.5%, binding iron and other metals to preserve natural color during storage and cooking. Disodium pyrophosphate also serves as an emulsifier in dairy-based foods, particularly processed cheeses, where it is limited to 3% of the finished weight to promote smooth melting and prevent fat separation by dispersing proteins. It is similarly applied in instant puddings to stabilize emulsions and achieve creamy textures. As an acidity regulator, it adjusts in beverages and other formulations, ensuring consistent flavor and stability without excessive sourness.

Industrial and other applications

Disodium pyrophosphate serves as a key ingredient in detergents and cleaners, functioning as a chelating agent that softens water by binding calcium and magnesium ions, thereby enhancing cleaning efficiency and preventing scale buildup. It also acts as a corrosion inhibitor in formulations for dairy equipment and metal surfaces, protecting against degradation in hard water environments. These properties make it valuable at low concentrations in industrial cleaning solutions, where it helps maintain equipment integrity without excessive foaming. In chemical processing, disodium pyrophosphate stabilizes solutions by chelating metal s that catalyze decomposition, extending shelf life in bleaching and disinfection applications. It is similarly employed in baths as a buffering to control and prevent metal precipitation, ensuring uniform deposition during metal finishing processes. Beyond these roles, disodium pyrophosphate finds use as a tanning auxiliary in the leather industry, where it removes iron stains from hides and aids in the sulfitation of extracts to improve preservation and flexibility. In , it disperses clays in muds, reducing and strength to facilitate smoother operations in oil wells. Additionally, it controls tartar formation in toothpastes by inhibiting crystallization on dental surfaces, contributing to products. In manufacturing, it enhances palatability, particularly in formulations, by improving and perception without altering nutritional profiles.

Safety, health, and regulation

Toxicity and health effects

Disodium pyrophosphate demonstrates low via oral exposure, with a reported LD50 of 2,650 mg/kg in mice. Dermal absorption is minimal, as evidenced by an LD50 greater than 2,000 mg/kg in rabbits, though the compound can act as an to skin and eyes in its powder form, potentially causing redness or discomfort upon direct contact. At high doses, primary effects are limited to gastrointestinal , including , , or , due to its content and solubility in aqueous environments. Chronic exposure primarily occurs through dietary sources, where exposure to from phosphate additives, including disodium pyrophosphate, contributes to overall consumption. EFSA's 2019 re-evaluation estimated mean exposure to from phosphate additives at 71–537 mg P/day for adults, with high chronic exposure (95th percentile) up to 1,071 mg P/day, generally below the group (ADI) of 40 mg/kg body weight (expressed as ) for healthy individuals. In vulnerable groups, such as those with , excessive intake from additives like disodium pyrophosphate may contribute to overload, potentially leading to , which can exacerbate cardiovascular risks and mineral imbalances. With no evidence of or carcinogenicity in available studies, the compound shows low concern for tumor promotion. Animal studies indicate no reproductive or developmental associated with disodium pyrophosphate at doses up to the group ADI for of 40 mg/kg body weight expressed as . Human data from comprehensive reviews, including those by the (EFSA) and the U.S. (FDA), affirm its safety when used at approved levels in food, with no observed adverse effects in general populations under typical exposure scenarios.

Regulatory status and environmental impact

Disodium pyrophosphate, known as E450(i) in the , is authorized as a with maximum permitted levels varying by food category, typically ranging from 500 to 20,000 mg/kg depending on the product. In the , it is permitted in baked goods such as and fine wares at levels up to 2,500 mg/kg expressed as , either alone or in combination with other phosphates, to function as an acidity regulator, raising agent, and sequestrant. The General Standard for Food Additives permits its use in baked goods up to 5,000 mg/kg in certain wares, with provisions for up to 9,300 mg/kg in specific formulations under . In the United States, disodium pyrophosphate holds (GRAS) status under 21 CFR 182.1087, allowing its use in food in accordance with without specified numerical limits. Globally, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) has established a maximum tolerable daily intake (MTDI) of 70 mg/kg body weight for from all sources, including pyrophosphates like disodium pyrophosphate, based on evaluations of . Environmentally, disodium pyrophosphate undergoes rapid in aqueous environments to orthophosphate, rendering it biodegradable but contributing to loading in wastewater. This release can exacerbate in receiving waters by promoting algal blooms when discharged from or municipal effluents. However, it exhibits low bioaccumulation potential, as neither the compound nor its dissociated ions are expected to concentrate in organisms due to their ionic nature and solubility. Regulatory restrictions on disodium pyrophosphate stem from broader phosphate controls to mitigate environmental impacts. In the , phosphate bans in detergents, including phosphorus compounds like pyrophosphates, were implemented progressively, with a limit of 0.3 g phosphorus per standard dosage in detergents and 0.5 g per standard dosage in laundry detergents effective from January 2013 and 2017, respectively, under Regulation () No 259/2012. This phase-out targets eco-detergents to reduce risks, though exemptions apply for industrial uses. In applications, its use is monitored to prevent excessive phosphorus discharge, with ongoing assessments by bodies like the to evaluate cumulative environmental loading.

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