Free SO2 by Aeration / Oxidation

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David Violante

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I've been trying to ensure that I have enough free SO2 in my wine by following the recommendations here (1/4 tsp per 5-6 gallons every three months) and love Bryan's idea of dividing that amount by a third for monthly top-offs of barrels. Having watched a few presentations from Cornell on 'Preventing Spoilage Mechanisms' and 'Malolactic Fermentation Methods', and from watching the dialogue here, it seems like assessing free SO2 especially in barrels doesn't seem so difficult or expensive. That being said...

The process of assessing Free SO2 by Aeration / Oxidation seems pretty straight forward as described in several places online including this one by Enartis. There's also a video series on the MoreWine! website with their process.

There are also several videos on the internet, and one such video I found related that you didn't need to use the indicator solution of methyl red & bromothymol blue (although helpful) and that you could just use your pH meter and ensure you're starting and ending at the same number (he used 5.0). Using a pH meter would also reduce the chemicals needed, as wine would be below 5.0, brought up to 5.0 by a base solution, go through the aeration/oxidation process and then see how many ml of the base solution it would take to get back to 5.0. Has anyone assessed their free SO2 this way?

Follow up question, instead of using a vacuum to pull the vapors through the setup, or an air pump to push them through, could I use compressed oxygen delivered at the recommended 1L/min flow rate? I have a pretty good supply of compressed oxygen available to me. 10-15 minutes at 1 L/min is quite minimal.

Thank you much....
 
I have an AO setup in my home lab:

AO method.jpg

The sample goes in the sidearm flask on the left (in an ice bath for FSO2) and the evolved SO2 is drawn up the tube and into the small tube with the violet indicator. The vacuum pump is that big thing on the right with the red light.

It is pretty straightforward but it does take a while: assemble the apparatus, neutralize the indicator (ie add NaOH until it turns from violet to grey-green), load sample + phosphoric acid and run the vacuum for 10 minutes. Then, take the indicator flask and titrate with NaOH until the color goes back to the grey-green starting point (though the actual endpoint color is slightly different in practice).

I think it would be a lot more cumbersome to do this with a pH meter; the indicator is quite easy to read and reproducible. The cost is not really a factor since you use only 3 drops at a time. I recently compared my results with those from a professional lab; they measured 30ppm, I measured 29ppm so I am confident that I am getting reliable data.

I'm not sure about pushing rather than pulling the gas through the system. One possible complication would be getting liquid up into the tubing, which would be better avoided. I got my vacuum pump for free, but I think you could also use a fairly cheap aquarium pump.

I think this method is great for getting relatively accurate numbers but it is fairly slow. Running one or two tests is fine, but if you have a dozen samples to run you will be in the lab all afternoon. If you have a burette and a flask or beaker, I think the Ripper method is a viable alternative for running lots of samples. It's not quite as accurate - in general, slightly overestimates SO2 - but for practical purposes it will still give you the information you need to make an informed estimate of how much SO2 to add.

I made a longer post on this here. Also of note, I run the Ripper method routinely at a commercial winery, typically testing 20 or 30 samples at a time. The rate limiting factor is collecting the samples from the various barrels/tanks/etc...
 
I have an AO setup in my home lab:

View attachment 112119

The sample goes in the sidearm flask on the left (in an ice bath for FSO2) and the evolved SO2 is drawn up the tube and into the small tube with the violet indicator. The vacuum pump is that big thing on the right with the red light.

It is pretty straightforward but it does take a while: assemble the apparatus, neutralize the indicator (ie add NaOH until it turns from violet to grey-green), load sample + phosphoric acid and run the vacuum for 10 minutes. Then, take the indicator flask and titrate with NaOH until the color goes back to the grey-green starting point (though the actual endpoint color is slightly different in practice).

I think it would be a lot more cumbersome to do this with a pH meter; the indicator is quite easy to read and reproducible. The cost is not really a factor since you use only 3 drops at a time. I recently compared my results with those from a professional lab; they measured 30ppm, I measured 29ppm so I am confident that I am getting reliable data.

I'm not sure about pushing rather than pulling the gas through the system. One possible complication would be getting liquid up into the tubing, which would be better avoided. I got my vacuum pump for free, but I think you could also use a fairly cheap aquarium pump.

I think this method is great for getting relatively accurate numbers but it is fairly slow. Running one or two tests is fine, but if you have a dozen samples to run you will be in the lab all afternoon. If you have a burette and a flask or beaker, I think the Ripper method is a viable alternative for running lots of samples. It's not quite as accurate - in general, slightly overestimates SO2 - but for practical purposes it will still give you the information you need to make an informed estimate of how much SO2 to add.

I made a longer post on this here. Also of note, I run the Ripper method routinely at a commercial winery, typically testing 20 or 30 samples at a time. The rate limiting factor is collecting the samples from the various barrels/tanks/etc...
I still frequently reference your post that you wrote several months ago on this topic. You have definitely helped my winemaking and lab analysis
 
I have posted quite a few threads here on the Morewine A/O kit a bought years ago and just how well it works. You do need to calibrate your eye to the color change endpoint but once that is done your good to go and numbers are very repeatable. While the Morewine kit is currently OOS it should be only a short time before you can grab one again. Supply chain issues I would imagine.

The one downside to the A/O method is the use of Phosphoric acid. That stuff can be expensive and can be hard to find sometimes. I worked in an analytical chemistry lab and we always had a bottle or two of that under a fume hood so I never had an issue sourcing it until I retired 5 years ago. LOL It does seem to be more available these days on Amazon in many different concentrations. IIRC you only need like 5ml of 25% Phosphoric acid so if you can source 85% its easy to dilute it down to 25% with some distilled water and a liter of the 85% stuff should last you for years.
 
The one downside to the A/O method is the use of Phosphoric acid. That stuff can be expensive and can be hard to find sometimes.
I came across this article a while ago - it describes an inexpensive homemade AO apparatus. For phosphoric acid he uses tile grout cleaner from the hardware store :)

I've had good luck using Lab Alley for lab chemicals. $35 for 1L phosphoric acid. They seem OK shipping to residential addresses (which can not be said for many of the chemical suppliers), I've also got sulfuric acid from them which seems relatively hard to source.
 
Morewine is still showing $120 for a liter (plus shipping) of 25% Phosphoric and it now shows "discontinued" so who knows if you can even order it from them. The kit they sell only comes with about 30mls IIRC. Enough to run 4-5 test I think but the kit works great and their online videos are excellent on both setup and analysis.
 
Just bought this after getting tired of having to constantly buy small bottles from Morebeer. It's 85% which I plan to dilute to 25% so this should last me a very long time. I haven't used it yet but don't see any reason why it won't work.

https://www.amazon.com/dp/B06Y1B82TG

-Adam
 
I came across this article a while ago - it describes an inexpensive homemade AO apparatus. For phosphoric acid he uses tile grout cleaner from the hardware store :)

I've had good luck using Lab Alley for lab chemicals. $35 for 1L phosphoric acid. They seem OK shipping to residential addresses (which can not be said for many of the chemical suppliers), I've also got sulfuric acid from them which seems relatively hard to source.
Do you know if the 24" of glass tubing is necessary? Could he have just purchased two 12" tubes and been good to go?

Also, do you know what size aquarium pump this calls for? 5, 10, 20 gallons, etc.
 
Do you know if the 24" of glass tubing is necessary? Could he have just purchased two 12" tubes and been good to go?

Also, do you know what size aquarium pump this calls for? 5, 10, 20 gallons, etc.
I don't really know since I haven't done this, but it seems it would be easier to just start with 2x12" glass tubes rather than cut 24" in half. Easier still, maybe you can get tubes that already have a tapered end?

I'm not an expert on aquarium pumps but (at least in my setup) you want a flow rate of 1000-1500mL/min. Maybe that can be divined from thepump specs?

edit: L = mL
 
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Thank you all for the help, I'll post some pictures this weekend. I have just about everything I need. I did get the 25% phosphoric acid from Presque Isle Wine Cellars as it was closer to me, 1L for $33. Our local college where I also adjunct was quite helpful with a couple stoppers and pipettes.

I see in the Enartis (and other) directions the flow rate should be 1000mL/min. That's really different than 1000L/min, is that just a typo?

Also, I found this gem!
https://www.winemakingtalk.com/threads/my-aeration-oxidation-free-so2-setup.28646/
 
Here we go! Supply line to the flask on the left is compressed air flowing at 1Lpm. I used a pH meter as I’m waiting for the indicator solution. It started at 5.5, went to 4.3 and I added 3ml of 0.01 NaOH to get back to 5.5. If I’ve done it all right, that puts me at 48ppm free SO2. This is for a 10G barrel of Zin.

IMG_3499.jpeg
 
Here we go! Supply line to the flask on the left is compressed air flowing at 1Lpm. I used a pH meter as I’m waiting for the indicator solution. It started at 5.5, went to 4.3 and I added 3ml of 0.01 NaOH to get back to 5.5. If I’ve done it all right, that puts me at 48ppm free SO2. This is for a 10G barrel of Zin.
Nicely done! How much (%) H2O2 did you use? Most protocols say to use 3% but, bearing in mind that it does go off over time I always use 30% straight from the (drugstore) bottle. I only need 10mL at a time so a bottle lasts me a while...
 
Nicely done! How much (%) H2O2 did you use? Most protocols say to use 3%

Thank you! I used 3% from a new bottle. It was interesting checking out from the drugstore. I had two 1 L bottles of Mountain Dew… to try the MD wine… distilled water, hydrogen peroxide, a bag of sugar, and a snickers. The gal at the checkout eyed me over like, “and what are you doing with these things…”? A few sources say that placing the bottle in the fridge helps.
 
Ugh, I just got out my SO2 test kit from MoreWine and realized I am out of phosphoric acid… they have it on their site, so I ordered the little bottle. I wish I had looked at that last week when ordered all of my yeast, malo, yeast nutrients, oak etc from them. Shipping is more than the acid.
Amazon has a liter of phosphoric acid 25% for $24, but that’s a lot of tests, I only do a couple a year.
 
Ugh, I just got out my SO2 test kit from MoreWine and realized I am out of phosphoric acid… they have it on their site, so I ordered the little bottle. I wish I had looked at that last week when ordered all of my yeast, malo, yeast nutrients, oak etc from them. Shipping is more than the acid.
Amazon has a liter of phosphoric acid 25% for $24, but that’s a lot of tests, I only do a couple a year.
Yea but it does not go bad!
 

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