Since I have been using Purigen for along time and regenerated well over 100 times and read more Purigen threads than I can remember maybe I can add my thoughts about Purigen and the regeneration process.
First off let me say I really love the product which I always purchase in the pre-bagged 100ml size (as compared to the loose stuff) and I always have a few 100ml bags soaking (always store wet after its been used) in a container with some Seachem Prime and Neutral Regulator in the water ready for use.
I just looked at my generic Chlorox Unscented Bleach bottle that I purchase at the Supermarket and it is 8.25% bleach so I am unclear what brands contain a lower percentage as noted by someone else.
I use the 100ml bags (which are rated at 100 gallons for six months) in my Eheim canister filters. I place one bag in each canister and since I have over a dozen canisters running that is a lot of bags that need to be regenerated during the year.
Remember that Purigen loses about 10% of its efficiency for each regeneration according to Seachem so it does have finite lifespan but I suppose that depends somewhat on how exhausted it is before each regeneration.
Purigen works best in a high water flow environment and does NOT remove Nitrates but removes the fresh organics in the water that will become Nitrates. I only mention this as many people that have high Nitrates already get frustrated when they put it in their system and it does not seem to have much effect. It works best in a system with low Nitrates to start with and then helps more from forming.
As Purigen 100ml bags get exhausted they turns tan and then a dark brown and if left in long enough actually hardens into clumps and then almost a solid brick. Obviously it is not doing much (if anything) by the time it gets that hard.
When it has been removed for regeneration rinse it off good (which might lighten it somewhat) and keep it wet until actual regeneration is attempted. Once it has been in use Seachem recommends never letting it dry out again.
Depending on how exhausted the Purigen 100ml bag is to start with determines how big of a plastic container I use for regeneration. I find that if the 100ml bag is really exhausted that I might have to go through the bleach step multiple times in a small container before I can get the Purigen totally regenerated back close to its original color. A small container seems to saturate out at some point and if I empty it out and start again the Purigen regeneration continues. I also try to remember to prod the 100ml bag with a blunt instrument (breaking up any clumps) and flip it through out the generation process so that all the internal beads get fluid contact.
Once I am satisfied that the Purigen 100ml bag is done regenerating I rinse it VERY good under the sink for a few minutes. Some people claim they rinse for as long at 15 minutes but that seems like overkill to me. At this point as you squeeze and rinse the Purigen bag you should NOT see any more tan or brown beads.
I have never personally tested the PH of a regenerated Purigen 100ml bag at this point but it is my understanding that the Bleach throws off the PH of the Purigen so it now needs to go through a Prime/Safe soaking to changes the PH or stop some chemical process started by the Bleach.
I personally use Seachem's directions once for each 100 ml bag so if I am doing two 100ml bags at this point I double their directions. After the 100ml bags have soaked for at least four hours (usually I do longer) I rinse them out VERY good again under the faucet.
Currently Seachem no longer notes a third Buffer step like it did for years earlier. This is because Purigen can retain this Buffering effect short term and cause issues in smaller tanks.
Originally Seachem recommended using one of their three PH Buffering products (Acid/Neutral/Akaline) at this point to finalize the regeneration process. You have to remember that a 100 ml bag is meant to treat a 100 gallon tank. If the PH Buffering of the Purigen at this point is incorrect and you place that 100ml bag is a very small system (20 gallons or less especially if the water has low buffering already) it can dramatically change the system's PH in a short period of time.
This is exactly what happened to some who complained in the Seachem forums years ago. They used an Acid buffer for the third step them immediately (more on that later) placed it in a 10 gallon system (which had a Akaline PH) and the PH plummeted and killed all their fish. Remember they were using 10 times the recommended the dose for this sized system. Seachem should have made clearer in their instructions that if you do a buffer step that you should use the appropriate PH buffer for the destination system.
Numerous people have noted before that the PH of the Purigen after the Prime (second) step rinse seems to change over the next 8-16 hours even without any Buffering. So its a good idea not to use it in a small system setup until it has rested for awhile.
Seachem seems to be of the opinion at this point that NO buffering step is better than an incorrect buffering step. Buffering should not really an issue for people with medium or larger tanks with buffers in the water already as the system can handle the short time that the Purigen is possibly off PH.
Personally I soak my Purigen 100ml bags in a strong Seachem Neutral Regulator solution as a third step just to give them Neutral PH for a few hours and then store them in container containing water with Prime and Neutral Regulator so they are all ready to go.
For those looking for a cheaper solution than using Seachem's liquid Prime product you can use their powdered Safe product. At one time it was very hard to locate online but now it seems to be more widely available. Safe is Drastically Cheaper to use as it is VERY concentrated but more finicky to dose.
Safe is NOT suppose to be pre-mixed and kept as it does NOT contain stabilizers like Prime and goes bad relatively fast. The bigger issue is because of its Very high concentrations you have be very careful about its very small dosages. Seachem has LOWERED its recommended Safe dosages by a factor of SIX in recent years so here are some current dosages.
030 gallons - 1/40 teaspoon
040 gallons - 1/33 teaspoon
065 gallons - 1/18 teaspoon
075 gallons - 1/16 teaspoon
125 gallons - 1/10 teaspoon
As with Prime you can supposedly safely overdose by a factor of 5 for system usage so these calculated dosages are approximate. For Purigen regeneration purposes you would use a VERY VERY small amount of Safe for the size containers I personally use.
Here is a link to the Seachem Safe product page if you want more information:
Seachem's Support Forums are a valuable place to learn a lot more about all their products including Prime/Purigen/Safe and I have never had an unanswered question.