Three condensed phosphates arrive in the same white-bag packaging, carry similar sounding names, and get quoted interchangeably by suppliers who should know better. Sodium acid pyrophosphate (SAPP, CAS 7758-16-9), tetrasodium pyrophosphate (TSPP, CAS 7722-88-5) and sodium tripolyphosphate (STPP, CAS 7758-29-4) are not variants of one product – they sit at three different points on the same acid-base series, and choosing the wrong one usually shows up as a failed batch rather than a wrong number on a certificate.
This note gives the selection logic, the numbers behind it, and the three questions that settle the choice in most cases.
The one number that separates them
All three are sodium salts of phosphoric acid condensed into P-O-P or P-O-P-O-P chains. What changes along the series is how much sodium has been added, and that single variable determines the solution pH – which in turn determines what the salt can be used for.
| Product | Formula | Na:P ratio | pH (1% solution) | Character |
|---|---|---|---|---|
| Sodium acid pyrophosphate (SAPP) | Na2H2P2O7 | 1:1 | 3.5-4.5 | Acidic salt |
| Tetrasodium pyrophosphate (TSPP) | Na4P2O7 | 2:1 | About 10 | Strongly alkaline |
| Sodium tripolyphosphate (STPP) | Na5P3O10 | 5:3 | About 9.5-10 | Alkaline, chain phosphate |
The practical rule: if the system needs acid, only SAPP qualifies. No amount of blending fixes a pH problem that comes from buying a tetrasodium salt into an acidic process.
Phosphate content per kilogram
Buyers often compare these products on price per tonne without normalising for the phosphorus they actually deliver. Calculated from the formulas, the P2O5 content is:
| Product | Molecular weight | P2O5 content (calculated) | Acidity |
|---|---|---|---|
| SAPP | 221.94 | 63.96% | Acidic |
| STPP | 367.86 | 57.85% | Alkaline |
| TSPP | 265.90 | 53.38% | Alkaline |
SAPP carries the most phosphorus per kilogram of the three because it carries the least sodium. If a process is buying “phosphate” as a nutrient or as a sequestrant and can tolerate acidity, SAPP is the most efficient carrier on a per-kilogram basis. If the process cannot tolerate acidity, the comparison is between STPP and TSPP, and the choice turns on whether chain or ring structure matters.
What each product actually does
SAPP – the acid
- Leavening acid. Reacts with sodium bicarbonate slowly at room temperature and fast in the oven, which is why it is the slow half of a double-acting baking powder. One hundred grams of pure SAPP neutralises 75.7 g of sodium bicarbonate and releases 39.6 g of CO2 – at 96% assay, the working neutralizing value is about 72-73.
- Chelant. Sequesters iron and copper, which is why it controls discolouration in cut potatoes and prevents struvite crystals in canned fish.
- Acid cleaner. Removes rust and scale in metal treatment without the hazards of a mineral acid.
TSPP – the buffer and dispersant
- Buffering. Holds alkaline pH in cleaning, metal finishing and water treatment systems.
- Dispersing. Keeps insoluble particles suspended – used in drilling fluids, ceramics and slurry systems.
- Sequestering. Binds calcium and magnesium in alkaline conditions where SAPP would simply be neutralised.
STPP – the builder and water binder
- Detergent builder. Softens wash water and deflocculates soil; the classic phosphate builder where local regulation still permits phosphates in detergents.
- Water binding. Improves yield and texture in meat, poultry and seafood processing within phosphate limits.
- Ceramics and drilling. Deflocculant and thinner in slurries.
The three questions that settle it
- Is the process on the acid side or the alkaline side? Acid side: SAPP. Alkaline side: TSPP or STPP.
- Do you need gas, chelation, or dispersion? Gas and chelation point to SAPP; dispersion in alkaline media points to TSPP; water binding and detergency point to STPP.
- Is chain length relevant? TSPP is a ring (pyro) phosphate with strong sequestering power at alkaline pH; STPP is a short chain with three phosphorus atoms and better deflocculating behaviour. In most industrial uses the difference is secondary to pH.
Where buyers go wrong
Substituting on assay alone. A certificate showing “96%” is not a licence to interchange products. SAPP at 96% and STPP at 96% are both 96% pure and behave nothing alike.
Ignoring the sodium load. TSPP carries four sodium ions and STPP five per formula unit. In formulations where total sodium is constrained – boiler water, some food systems, glass batch – SAPP’s lower sodium is a real advantage.
Assuming hydrolysis is slow in every application. All condensed phosphates hydrolyse back to orthophosphate in water, faster when hot and faster at extreme pH. A solution made up on Monday is not the same solution on Friday. If a process holds a phosphate solution in a tank, specify the holding time and temperature rather than assuming the certificate follows the liquid.
Product pages
The grades we supply, with their full specifications, are on the sodium acid pyrophosphate, tetrasodium pyrophosphate and sodium tripolyphosphate pages. For the leavening chemistry in context, see our guide to sodium bicarbonate in food and baking.
Disclaimer. P2O5 contents and neutralizing values are calculated from molecular formulas and shown with the working; specification limits come from current product certificates. Regulatory limits on phosphate use differ by market and by application and are not covered here.
Contact PolyBlueChem if you need samples of two grades side by side for a trial – that is usually faster than arguing about certificates.
Related reading.