jotakah

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So 1oz of Starsan in 5gallons of water is supposed to provide 300ppm of dodecylbenzenesulfonic acid and 780ppm of phosphoric acid. To sanitize at this solution, it is recommended that the solution makes contact with the surface for 30seconds.

Does any chemist on here know if I can use lower concentrations for longer time? Like 150ppm dodecylbenzenesulfonic acid & 390ppm phosphoric acid for, say, 5 minutes?

It would be a great way to save sanitizer but I'm sure there's some sort of threshold below which it would no longer be acidic enough to kill bacteria.

mabrungard

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Very good question. I am not a microbiologist, but I await an answer. At a lower concentration, I'm not sure if it will work like heat deactivation does where say 1 minute at boiling kills 100% and 15 minutes at 150F kills 100%. There is probably a minimum concentration at which the organisms we are trying to kill just thumb their 'noses' at us and laugh.

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dwarven_stout

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I'm guessing the answer is "no", because reliably killing bad bugs takes a certain pH. You could always email the company and find out, though!

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DarthMalt

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From what I understand, Star San loses effectiveness above a PH of 3.5. Maybe you could mix up a batch using the dilution ratio you're questioning and test the PH?

ajdelange

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The dodecylbenzene sulfonic acid is just a surfactant. It is the phosphoric acid which is doing the job and its effectiveness depends on the pH it is able to establish. The recommended dose would result in a pH of about 2.33. Halving the recommended dose would result in a pH of 2.56 i.e about 0.23 pH higher. This doesn't seem like much and only amounts to a decrease in hydrogen ion activity of 40%. All this assumes distilled water. If water of alkalinity 80 (more or less typical) were used the recommended dose would give a pH of 2.44 and the product must be designed to work with water at that level of alkalinity. So I'm guessing that it would work at half strength if mixed with DI or RO water.

Note that articles on acid washing of yeast seem to indicate that the pH should be 2.2. I found one pro brewer who advances the rule of thumb: pH 2.2 - 2hrs; pH 2.4 - 4hrs; pH 2.6 - Overnight.

theredben

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To say that the dodecylbenzene sulfonic acid is "just" a surfactant is a bit of an understatement. As it is registered as a sanitizer as a stand-alone product the concentration obviously important. To the OP, I would not be able to say if it was effective at lower concentrations, but I would just remind you that this product is reusable so you should not be needing to use that much of it. Many homebrewers find that their solutions are pH stable for months and so they can keep re-using it.

If Star-san was only a pH adjuster then it would be much cheaper to just buy some phosphoric acid.

ajdelange

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Yes, I guess it is. Saying "just" a surfactant implies that it does not have a role in the sanitization process which it does. But for it to work properly pH must be set to between 2 and 3 and that's the job of the phosphoric. The ddbsa is an anionic surfactant and it is the attraction of the anion to the surface of positively charged bacteria that make it effective but at pH 2.5 only half of it is ionized. At pH 3.5 more would be ionized but the mix is then no longer effective so it is clear that the low pH established by the phosphoric acid plays a major role here. I guess it takes care of the gram negative bacteria which are negatively charged.

As the ddbsa has a molecular wt. triple that of phosphoric acid and a pK of 2.55 as compared to phosphoric's first pK of 2.15 it will be the phosphoric that has the major effect on solution pH so the calculations in the previous post are probably valid and OP should be OK as long as his dilution does not raise pH > 3. Should he increase contact time for the same log kill? I'm guessing if he diluted by a factor of 2 he should double contact time i.e. maintain same CxT. But I agree that the manufacturer should be consulted.

And that's exactly what I do.

josephus23

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I think you should use the recommended dosage to achieve the best results. On the other hand, why not use 2.5 gallons of water and .5 oz. of Starsan for a 5 gallon carboy. I would rather use less solution than use the wrong dose.

I wouldn't think contact time would make a difference at a lower concentration since the process isn't a chemical reaction.

ajdelange

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The effectiveness of sanitizers is frequently expressed in terms of CxT for a specified log kill ratio. Thus, for example, EPA requires 1850 min-mg/L chloramine against giardia at 10 °C for a 3 log kill (99.9%) whereas 23 min-mg/L is sufficient for chlorine dioxide and 1.4 for ozone. This means that if you have a chlorine dioxide concentration of 2.8 mg/L a half a minutes exposure will take care of giardia but if you halve the concentration to 1.4 you must expose for a minute. For 4 log inactivation of viruses the numbers for those 3 disinfectants are, respectively 1491, 35.1 and 1.

With an anionic surfactant/acid sanitizer there is a limit once the pH rises appreciably above 3, but I don't see any reason why the same principle (Chick's law) shouldn't apply to this type of sanitizer but this is hardly my area of expertise.

The process isn't a chemical reaction? Sounds like one to me but wonder what sort of mechanism, other than chemical you have in mind.

Homercidal

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I'm no chemist, but I can tell you that using Starsan is VERY cost effective. The reason is that a slightly stronger solution of it will kill just as well, and will keep for a very long time, and reused over and over (as long as the pH is kept below the required level.)

I normally mix 1/4 ounce in 3 quarts of distilled water. That's stronger than required for a gallon! But for me it's just easier to measure. I keep this in a 3-quart jug (the reason I am mixing 3 quarts is because I like these jugs) and use it to top off a spray bottle.

I normally use a spray bottle to apply it. Sometimes I will pour some into a carboy or bucket and use a cloth to wipe it around and then pour the excess back into the jug.

1/4 ounce of Star San will last me a couple of MONTHS.

I highly recommend using distilled water since alkaline water can neutralize the Starsan solution after a short time.