For the Kid!
Congrats on the new job Junior!
"Sour" refers to acidic, which is what most laundry sours are, being opposite on the pH scale of detergents which are mostly alkaline.
In general weak acids in solution, and are added to the final, or second to final rinse in order to neutralise the high pH levels commonly used with commercial laundry. Some sours also deal with iron and rust in water (to deal with rust stains), bacteriostatic, softening, remove chlorine bleach residue and so forth.
It is very important to follow directions for use and dosage of sours, as some fabrics (mainly cotton, linen and some dyes) can be affected by high acid levels. Some sours go into the final rinse, others added in the next to final rinse.
Souring final rinse or last to final rinse water grew out of an old European laundry trick of adding either vinegar or lemon juice to rinse water. It was would that by doing this laundry appeared whiter and brighter. This was because the weak acid removed soap scum and mineral deposits trapped in fabrics when pure soap was the only "detergent" around for doing laundry.
Many modern commercial detergents are neutral to only slightly alkanline, and thus do not require sour rinses. These detergents are big with some laundries as it saves chemical costs, and depending upon how their systems are set up/type of laundry done, an extra step. Such detergents usually contain low to minimal amounts of washing soda, the major chemical used in high pH laundry detergents.
It looks like your laundry system is using Ecolab's or someother dispensing program. If you are around the next time your chemical supplier is about, ask him to give you all the skinny on various chemicals, and perhaps some reading materials as well. Maybe he will be nice and let you have some samples to play with at home.
L.
