I'm finding out that the nitrile O-rings and washers typically sold for plumbing work really aren't the best choice in critical applications.
I've contacted both Parker Hannifin Applications Engineering Department, Elder Rubber in Texas and Apple Rubber in New York, all of which make rubber compound O-rings and talked to people in their Engineering or Technical Support groups, and all seem to be in agreement.
Typically, the nitrile O-rings sold for plumbing applications by companies like Master Plumber or Plumb Shop will be made of nitrile rubber (formerly called "Buna-N" rubber and correctly called acrylonitrile butadiene terpolymer) only because nitrile rubber parts are about 1/4 the cost of EPDM rubber which is considerably more resistant to both fresh water and chlorinated tap water. That term "resistant" means that it's properties don't change when exposed to that fluid, and that means that EPDM O-rings will continue bahaving like they were new for a longer period of time than nitrile O-rings when exposed to chlorinated water.
Most general purpose O-rings are made from nitrile rubber because it has good resistance to water, hydrocarbon based oils and most hydrocarbon based solvents, so it's suitable for a wide variety of applications.
Economics is the crux of the matter here because nitrile O-rings for household plumbing applications will cost anywhere up to 25 cents each, whereas the same size O-rings in EPDM rubber will be anywhere from a dime to a dollar. Companies like Plumb Shop and Master Plumber know that they can get their brand into more stores if their product sells for 1/4 the cost of their competitors, especially considering that nitrile and EPDM rubbers are indistinguishable from each other by simple inspection, and with most people totally ignorant about rubber compounds.
So, for simple easy jobs like replacing the O-ring at the base of a kitchen faucet spout, a cheap replacement that'll last "a long time" is probably more than adequate for most people, and that spells "nitrile" rubber. However, if you want to ensure that the spout lasts the longest possible time without leaking again, then go to any place in your yellow pages listed under Hydraulic & Pneumatic Seals and get an EPDM rubber O-ring in the same size and hardness. (for plumbing applications, the hardness will normally be "70 Durometer")
O-rings come in both Imperial and metric sizes that aren't so close as to cause mistakes to be made in determining the correct replacement size. So any place that sells O-rings will be able to measure your old O-ring and tell what size it was when it was new. However, there will always be some statistical variation in size between the rubber cord available from different manufacturers from which O-rings are made, so the same nominal size of O-ring from two different companies my vary slightly in size, and that may affect the fit a little.
Also, O-rings come in different rubber hardnesses, with 70 durometer the standard for plumbing applications. Both nitrile and EPDM rubber O-rings come standard in 70 durometer harness, so EPDM O-rings are totally compatable with any plumbing application where a nitrile O-ring is used.
Take a look at the following O-ring selection chart:
1. click on the "Rubber Materials" tab in the chart on the right. Under "water resistance", nitrile rubber is rated as Fair to Good, whereas EPDM rubber is rated as Excellent.
2. click on the "Fluid Compatability" tab. Both Nitrile and EPDM rubbers are rated as "1" for "Satisfactory", but EPDM rubber is listed as the best choice.
3. click on the "Temperatures" tab. It is seen that EPDM rubber spans a wider temperature range than Nitrile rubber.
So, now you know. Nitrile rubber is way cheaper and lasts a "long time" in plumbing applications. EPDM rubber is more expensive, but will last the longest time in plumbing applications. In most cases, nitrile rubber will provide the best value in an O-ring, but in critical applications, pay more for EPDM rubber for optimal peformance.