What is 'STPP' ??
[Information first posted in the Appliances Forum by Rich (spambdamn_rich) on 24 April 2003 - thanks, Rich!]:
"STPP" is a term you may see mentioned in threads here on THS. It stands for Sodium Tripolyphosphate, one of the group of compounds known as complex phosphates.
Why is STPP important? Well, after WWII, synthetic detergents gradually replaced natural soap products for use in washing machines. The detergents had the advantage of using surfactants, or sudsing agents, that were not as liable to being inactivated by hard water mineral ions as soaps were. However, surfactants are only a fraction of the mixture we call a laundry detergent. To replace other actions of a natural soap, compounds known as "builders" are added to detergent formulations.
These builders function in several ways. They increase the alkalinity of the wash solution, which helps the surfactant activity and also helps to emulsify fats and oils in the soiled fabrics. They also help to "break" clay-types of dirt from the fabrics, and combine with it to help prevent it from redepositing on the fabrics. They also function to combine with hard water mineral ions, thus "softening" the water.
As with many things, some compounds are better at "breaking" function than others. A common builder is sodium carbonate. It has high alkalinity and also is good for breaking soil from fabric. However, it will form an insoluble compound with the hard water mineral ions, and also with mineral ions in the soil they release from fabrics. This insoluble compound gets redeposited on fabrics and washer parts. On fabrics it can look like white lint or powder. On washer parts, it can form a rock-like scale which can be harmful to the washer mechanisms.
Here is where complex phosphates come in. These are cool because not only do they have all the virtues of water softening and breaking of precipiting breakers like sodium carbonate, but they also do not form a precipitate with mineral ions. Rather, they form what is known as a "complex" with them, which stays in solution and is easily rinsed away.
Complex phosphates can occur in several different types of compounds. The most common used in powdered detergent formulations has been STPP or sodium tripolyphosphate. This compound works well to lift minerals like calcium, magnesium, and iron from fabrics as well as soften water, without forming a precipitate. STPP has one weakness, and that is that over time, with exposure to water, it will decompose into a mono-phosphate, or "orthophosphate", called trisodiumphosphate, or TSP. TSP is often used for cleaning hard surfaces where a precipitate is not a problem, but due to its precipitate formation is not favored for laundry use. Many dishwashing detergents contain complex phosphates; for this reason the boxes usually are tightly sealed in foil faced packages, and have advice printed on them to store them in a cool dry place. This is intended to help prevent moisture...