Sodium acid pyrophosphate (SAPP) is the acidic member of the condensed phosphate family. Its CAS number is 7758-16-9, its formula is H2Na2O7P2 and its molecular weight is 221.94. In the food industry it is E450(i), the slow-acting leavening acid that makes a baking powder release its gas in the oven rather than on the bench. In industry it is a controlled acid source, a chelating agent and a scale-control additive.
The two identities share one number that decides almost everything downstream: the pH of a 1% solution, normally 3.5-4.5. Everything SAPP does – raising dough, chelating iron in a potato wash, dissolving rust scale, lowering the pH of a drilling fluid – comes back to the fact that it is a weak acid delivered as a dry, free-flowing, water-soluble salt.
This guide covers the chemistry, the manufacturing route, what the specification numbers actually measure, and what to ask before you buy. The product grades we hold are listed on the sodium acid pyrophosphate product page.
1. What the molecule is
| Property | Value | What it means in practice |
|---|---|---|
| Chemical name | Disodium dihydrogen diphosphate | Two tetrahedra sharing one bridging oxygen – a pyrophosphate, not a chain phosphate |
| Synonyms | Sodium acid pyrophosphate, disodium pyrophosphate, SAPP, sodium dihydrogen diphosphate | All the same CAS 7758-16-9; a supplier quoting “disodium pyrophosphate” is quoting the same molecule |
| Formula / MW | H2Na2O7P2 / 221.94 | Theoretical P2O5 content 63.96% (calculated from the formula) |
| Appearance | White crystalline powder or granule | Granular for dry blends, powder for solution preparation |
| pH, 1% solution | 3.5 – 4.5 | Acidic. This is the functional fingerprint of the product |
| Solubility | Readily soluble in water | Solutions are clear at normal use levels; water insolubles are the only haze source |
| Hygroscopicity | Moisture-sensitive | Absorbed water hydrolyses SAPP back to monosodium phosphate, lowering acidity and neutralizing power |
| PubChem CID | 24451 | Identity reference |
Two structural facts drive the commercial behaviour. First, the bridging P-O-P bond is what makes SAPP a condensed phosphate rather than an orthophosphate: it carries more acidity per unit weight and it hydrolyses slowly in solution. Second, the sodium-to-phosphorus ratio is fixed at 1:1 by the formula. Change that ratio during manufacture and you no longer have SAPP – you have monosodium phosphate (Na:P 1:1 but ortho), tetrasodium pyrophosphate (Na:P 2:1) or sodium tripolyphosphate (Na:P 5:3).
2. How SAPP is manufactured
The production route is a two-stage phosphate condensation, and nearly every quality difference between suppliers comes from the second stage.
Stage 1 – neutralisation to monosodium phosphate. Purified phosphoric acid (typically 75-85% H3PO4) is neutralised with sodium hydroxide or sodium carbonate to the monosodium dihydrogen phosphate endpoint:
H3PO4 + NaOH -> NaH2PO4 + H2O
The pH at the end of neutralisation is the control point. Over-neutralise toward Na2HPO4 and the calcination step will produce tetrasodium pyrophosphate instead of SAPP; under-neutralise and unreacted acid is lost in the dryer.
Stage 2 – drying and calcination. The NaH2PO4 solution is concentrated and dried, then calcined at approximately 200-250 deg C. Two molecules of monosodium phosphate condense with the loss of one molecule of water to form the pyrophosphate:
2 NaH2PO4 -> Na2H2P2O7 + H2O
Temperature and residence time are the levers that set the free orthophosphate content and the amount of higher condensates. Under-calcine and the product still contains unreacted monosodium phosphate, which dissolves fast and behaves like a rapid leavening acid; over-calcine and tripolyphosphate forms. Both show up as a shift in assay and, more importantly, as a shift in the rate at which the powder reacts with sodium bicarbonate.
This is the technical reason two suppliers can both deliver “96% min assay, pH 3.5-4.5” and still perform differently in a bakery: the same headline numbers are compatible with different distributions of ortho, pyro and tripoly species.
3. Grades and specification
SAPP is sold in three broad grades. The chemistry is the same; the trace-metal panel and the documentation package are not.
| Grade | Assay | Distinguishing tests | Documentation |
|---|---|---|---|
| Food grade (FCC / E450(i)) | Min 96% on dry basis | Fluoride max 10 ppm, arsenic max 3 ppm, lead max 1 ppm, cadmium max 1 ppm, mercury max 1 ppm, heavy metals as Pb max 10 ppm, aluminium max 200 ppm | Food-grade COA, halal/kosher on request, allergen and GMO statements |
| Industrial / technical grade | Typically 95-96% | Metal panel reduced or omitted; water insolubles and pH still specified | Standard COA and SDS |
| Granular food grade | Min 96% on dry basis | Same panel as food grade, larger particle size for dry blending | As food grade |
Reading the assay on a dry basis
“Assay 96% min on dry basis” is not the same statement as “96% purity as received”. On a dry basis the figures are corrected to zero moisture, so a lot at 0.4% moisture still reports its dry-basis assay but delivers about 0.4% less active material per kilogram. Because SAPP is hygroscopic, a lot that has sat in a humid warehouse can pass the dry-basis assay and still underperform.
The practical response is to read assay and loss on drying together. Assay on a dry basis of 96% with loss on drying of 0.5% max describes a consistent product; assay of 96% with loss on drying of 1.5% describes a product that has taken up water and partly hydrolysed.
Why the fluoride, aluminium and cadmium lines exist
Phosphate rock contains fluoride and cadmium, and both follow the phosphorus through the process unless they are removed deliberately. Fluoride max 10 ppm and cadmium max 1 ppm are therefore statements about ore selection and purification depth, not about the condensation chemistry. Aluminium max 200 ppm is not a standard FCC item – it is a customer-driven limit that appeared with the aluminium-in-baking-powder discussion, and it is a good discriminator when comparing two quotations that otherwise look identical.
Reading the pH range
A pH window of 3.5-4.5 corresponds to a factor of ten in hydrogen ion concentration. At the acid end of the range the material reacts faster and delivers more available acidity per unit weight; at the alkaline end it behaves like a milder acid with more buffering. If your process is sensitive to leavening rate or to the final pH of a brine, specify the pH you need inside the stated window rather than accepting whatever the lot delivers.
4. The leavening chemistry, with the numbers
SAPP is a slow-acting leavening acid. It reacts with sodium bicarbonate slowly at room temperature and rapidly once the batter reaches oven temperature, which is why it is the second acid in a double-acting baking powder (monocalcium phosphate supplies the bench reaction, SAPP supplies the oven reaction).
The stoichiometry of the reaction is:
Na2H2P2O7 + 2 NaHCO3 -> Na4P2O7 + 2 CO2 + 2 H2O
That equation gives a set of calculable numbers that are more useful than a marketing claim:
| Quantity | Calculated value | How to use it |
|---|---|---|
| Sodium bicarbonate neutralised per 100 g SAPP | 75.7 g (theoretical maximum) | The neutralizing value (NV). Multiply by the assay to get the value for a real lot: 75.7 x 0.96 = 72.7, which is why commercial 96% SAPP is quoted at an NV near 72-73 |
| CO2 released per gram of SAPP | 0.396 g (theoretical) | Sets the maximum gas volume a formulation can expect from a given acid charge |
| SAPP required per kg sodium bicarbonate | 1.32 kg (theoretical) | Dosing starting point; real formulations run the acid slightly short or long to control the finished crumb |
The gap between the theoretical NV of 75.7 and the working NV of about 72 is a direct measure of how much non-acidic material the lot contains. That is the number to compare across suppliers when the assay figures look the same.
5. Applications
| Sector | Function | Typical context |
|---|---|---|
| Baking powder and self-raising flour | Slow leavening acid | Double-acting systems; controls crumb structure and volume in cakes, biscuits and pancakes |
| Potato and vegetable processing | Chelation and anti-browning | Binds iron and copper that catalyse enzymatic and non-enzymatic discolouration; also firms tissue in blanching and canning |
| Canned seafood | Struvite prevention | Prevents glass-like magnesium ammonium phosphate crystals forming in canned tuna and salmon |
| Meat and poultry | Acidulant and phosphate component | Brine ingredient for water binding; dosage limited by phosphate regulations in each market |
| Metal treatment | Acid cleaner and chelant | Rust and scale removal, electroplating bath additive, surface preparation before coating |
| Oilfield | pH and scale control | Drilling and completion fluids; also a component in some cement retarding packages |
| Water treatment | Threshold scale inhibition | Controls calcium carbonate scaling in circulating systems at low dose |
6. Application detail that matters to buyers
Leavening: matching acid to bake profile
Leavening acids are classified by the temperature at which they release their gas. Fast acids react in the mixing bowl and the gas must be held by the batter; slow acids hold their gas until the structure sets. SAPP sits in the slow group, so it is used where the batter has a long bench time or a large volume to fill before setting.
Because SAPP releases its gas late, the total acid charge is normally set so that a small excess remains in the finished product. That excess is deliberate: it lowers the finished pH, which affects colour, flavour and shelf life. Under-dosing leaves a soapy, alkaline crumb; over-dosing gives a pale, acidic crumb and can leave a residual phosphate taste.
Potato processing: why chelation beats acidification
Discolouration in cut potatoes is largely driven by polyphenol oxidase acting on iron and copper centres. SAPP works here less as an acid than as a chelating agent: it sequesters the metal ions that the enzyme needs, and the acidic pH additionally inhibits the enzyme directly. Use levels are in the range of a few tenths of a percent in the dip or blanch water; the exact figure depends on water hardness, which is worth measuring before setting a dose.
Canned seafood: struvite is a physical, not a chemical, defect
Struvite crystals (magnesium ammonium phosphate hexahydrate) form in canned fish from magnesium and ammonium naturally present in the tissue. They look like broken glass and generate complaints even though they are harmless. SAPP sequesters magnesium and shifts the equilibrium away from crystal formation, and it is used at low levels in the pack brine.
7. SAPP in the condensed phosphate family
| Product | CAS | Na:P ratio | Solution pH | Main commercial role |
|---|---|---|---|---|
| Sodium acid pyrophosphate | 7758-16-9 | 1:1 | 3.5-4.5 (1%) | Slow leavening acid, chelant, acid cleaner |
| Tetrasodium pyrophosphate | 7722-88-5 | 2:1 | Alkaline (about 10) | Buffering, dispersing, sequestering in alkaline systems |
| Sodium tripolyphosphate | 7758-29-4 | 5:3 | About 9.5-10 | Detergent builder, meat and seafood water binding |
| Monosodium phosphate | 7558-80-7 | 1:1 (ortho) | About 4.5 | Rapid leavening acid, buffer salt, nutrient source |
The family is best understood as one continuous acid-base series. Moving from monosodium phosphate to SAPP to tetrasodium pyrophosphate to sodium tripolyphosphate raises both the sodium content and the solution pH, and each step changes which application the salt suits. Our comparison of tetrasodium pyrophosphate and sodium tripolyphosphate covers the alkaline end of the series.
8. Buying guide
Six questions separate a usable quotation from a number on a page:
- Is the assay quoted on a dry basis or as received? Ask for the loss on drying on the same certificate so you can convert.
- What is the pH of a 1% solution for this lot? Not the specification window – the actual lot value. It predicts leavening rate.
- What are the fluoride, lead and aluminium figures? If the supplier cannot supply aluminium, they are buying from a source that does not test it.
- Powder or granule, and what mesh? Granular flows in dry blends; powder dissolves faster. Mismatched particle size is a common cause of segregation in a finished baking powder.
- What is the packaging and the shelf life? 25 kg paper bags with PE liner and 1000 kg jumbo bags are standard; food-grade shipments often go in fibre drums. Hygroscopic cargo should not be consolidated with liquids.
- Can they supply the food-additive registration for your market? E450(i) covers the EU; other markets have their own monograph and labelling rules.
9. Safety, handling and regulatory position
SAPP is an acidic salt. In concentrated solution or as a dust it is irritating to skin, eyes and the respiratory tract, and the SDS classifies it accordingly. Handling controls are conventional: local exhaust at the bag dump, dust mask and goggles, and prompt washing after contact. It is not flammable and not classified as a hazardous good for transport in normal packaging.
Store sealed and dry below 30 deg C, away from alkalis and calcium salts. Opened bags should be resealed – ambient humidity is enough to start the hydrolysis that shows up as a falling assay and a drifting pH. Typical shelf life is 24 months in unopened food-grade packaging.
Regulatory status: food additive E450(i) in the EU, listed in the Food Chemicals Codex and in national food-additive standards elsewhere. Permitted use levels differ by food category and market, so the application – not the chemical – determines the limit. This article describes the chemistry and the commercial specification; it is not a regulatory opinion for a specific use or market.
10. FAQ
Is sodium acid pyrophosphate the same as disodium pyrophosphate?
Yes. Sodium acid pyrophosphate, disodium pyrophosphate, disodium dihydrogen diphosphate and SAPP all refer to CAS 7758-16-9, H2Na2O7P2.
What is the difference between SAPP and TSPP?
They are two ends of the same pyrophosphate series. SAPP is the acid salt (Na:P 1:1, pH 3.5-4.5 at 1%) and is used as a leavening acid and chelant; tetrasodium pyrophosphate is the fully neutralised salt (Na:P 2:1, alkaline) and is used as a buffer and dispersant. See the tetrasodium pyrophosphate product page.
Why is the assay specified on a dry basis?
Because the product is hygroscopic. Dry-basis assay separates the true purity of the salt from the water it has picked up, so the number stays comparable between a freshly made lot and one that has been in storage.
Can SAPP be used in aluminium-free baking powder?
Yes – SAPP contains no aluminium. That is one reason it remains the standard slow acid in aluminium-free baking powders. The 200 ppm aluminium limit on the specification is a purity ceiling on the phosphate raw material, not an additive.
How much SAPP reacts with one kilogram of sodium bicarbonate?
On stoichiometry, about 1.32 kg of pure SAPP per kilogram of sodium bicarbonate, releasing roughly 0.396 kg of CO2 per kg of SAPP. Real formulations adjust from that starting point.
How should SAPP be stored?
Sealed, dry, below 30 deg C, away from alkalis and calcium salts. Moisture is the main enemy because it drives hydrolysis to monosodium phosphate.
References and further reading
- Molecular formula, molecular weight and PubChem CID 24451 – verified against PubChem compound records for CAS 7758-16-9.
- Theoretical P2O5 content and neutralizing value derived from the stoichiometry of Na2H2P2O7 and its reaction with sodium bicarbonate; worked values shown in section 4.
- Specification limits as quoted on the current product certificate.
- Related reading: sodium bicarbonate in food and baking, sodium tripolyphosphate, tetrasodium pyrophosphate.
Disclaimer. This article is a technical and commercial overview for procurement and formulation professionals. It does not replace the batch certificate of analysis, the safety data sheet, or the food-additive regulation of your market. Dose rates quoted are starting points for trials, not recommendations for a specific product.
For a current COA, a sample of the food-grade powder or granule, or a quotation on bagged and jumbo-bag quantities, contact PolyBlueChem or start from the sodium acid pyrophosphate product page.
Related reading.