Let's say I have some Star San that has been sitting around for a long time, and let's also say it was exposed to the air the whole time. What does it become after losing its effectiveness?

Is it water? Is it still slightly like a sanitizer? Is it an encouraging home for yeast and bacterial growth?

Not trying to be cheap here, so don't worry about its effectiveness as a sanitizer. I'm just curious about the changes that happen over time in one of our everyday products.

I will probably have phosphate water from the phosphoric acid. The other compound in star San I do not know what it would break down to. Though the breakdown process has to be extremely dependent on your water since the disolved minimal content needs to act as your base in the reaction. If that is so then using distilled water would allow the star San to remain active for a long period of time.

If I recall correctly from the Charlie Talley show, he said it is basically an acidified soap. Once the pH climbed above 3.5, it was ineffective. You could "rejuvenate" it by adding acid to lower the pH.

So I guess it would be slightly acidic, soapy water.

The 'soap' (and it is technically considered a soap) is the extremely popular detergent dodecyl benzene sulfonic acid. Given that it works at cell membrane lysis by having its non polar tail dissolve in the membrane lipids while its negatively charged head is attracted to water molecules one migth conclude that it would be more effective at higher pH as it is more charged at higher pH and this may be true but then remember that the killing is actually done by the protons released from the phosphoric acid and therfore the system is not as effective as pH rises. Exposure to air should not, by itself, be detrimental. Oxygen (translation - acid generator) isn't going oxidize the phosphoric acid and I doubt it would hurt the DDBSA either but I really don't know that.

What will do in the phosphoric acid is alkali. When phosporic acid is mixed with alkali the following chain of reactions takes place

H3PO4 + 3(OH-) ---> H2PO4- + H2O + 2(OH-) ---> HPO4-- + 2H2O + OH- ---> PO4--- + 3H2O

with the pH rising as you go across the page. If the water is hard (contains lots of calcium) as soon as the pH gets high enough calcium phosphate (hydroxyl apatite) will precipitate as well and this is responsible, I believe, for the reports one sees that the solution turns cloudy. Precipitaton removes phosphate from the solution.

So I think you will get more mileage out of it if you dilute with RO or DI water. I guess you could 'rejuvenate' old solution with phoshphoric acid but unless the stuff is really expensive I'd just throw it out at that point.

If you have very hard water, Star San will get cloudy right away - especially if the solution comes in contact with anything that has lime scale deposits on it. I use RO water to dilute and store it in a used bucket that I got from the doughnut shop on the corner. I use it a few times and then make a new batch.

When I think the PH of the solution has risen enough to not be an effective sanitizer anymore, I use it to shine up metal stuff around the kitchen - my copper saucepan, and some Japanese carbon (non-stainless) knives. It leaves the copper shiny like a new penny and it takes rust spots off the knives and replaces them with a black phosphate patina.

So at my house Star San turns into metal pickling solution when it ceases to be an active sanitizer.

this technical talk is making my painus soft!

i just add a bit more every few months to keep it viable. i've been using the same 5g for 1.5 years... down to about a gal in the water bottle.

Thanks for the replies.

As I try to wrap my head around some of them, allow me to ask a follow up question along the same lines...

Let's say you drop your cleaned and sanitized racking cane onto the ground. On one side of the room you have an open bucket of VERY, VERY old Star San. On the other side, you have a bucket of tap water. Into which do you swish your racking cane before you siphon your precious beer? For this question, assume that there are no other options.

the "star san" or boiling water. tap water is as bad as the floor as far as contamonants goes. leave in the ss longer, if suspect.