Manual SO2 test

[Bruce]

My manual SO2 tests are puzzling because the color change is going right to dark purple after adding even just a few drops (ie., 1/4mL) of iodine. I'd love to solve this puzzle before considering one of the Vinmetrica testers.

Our wine supplier sells "Stabilized Iodine/Iodate 0.0156N. Also known as Vinodine." Can't find any reference online to 'Vinodine' so have taken their word that its the same as iodine. Using 25% sulfuric acid, not the 10% I was shown how to use, so tried adjusting it @ 40% (ie.  2ml rather than 5ml). Likewise, the vinodine is 0.0156N and not the 0.01N, so tried adjusting it using a chemistry calculator online.

Completed the SO2 test with everything as-is (not adjusting reagents), with adjusted acid, with adjusted vinodine, with adjusted both, with different batches of this winter's cider, with various commercial & craft ciders, etc... all with the same result. Total SO2 test does the same thing. I thought using tap water for dilution would affect things, but that wouldn't explain the undiluted test results.

Testing steps:
25ml cider
5ml 10% sulfuric acid -->haven't tried increasing the acid
0.5g starch
stir
add 0.01N iodine until stable blue/grey for 15sec

The cider (pH 3.5) was started with 100 mg/L SO2 and then another 50 mg/L when racking after fermentation - - after getting the first of these SO2 test results. Had some HS2 issues that seem to be resolved so that's why I tried testing other commercial & craft ciders to see if its my cider.

[Andrew Lea]

On 24/04/2015 23:46, Bruce wrote:

>
> Our wine supplier sells "Stabilized Iodine/Iodate 0.0156N. Also
> known as Vinodine." Can't find any reference online to 'Vinodine' so
> have taken their word that its the same as iodine.

It should be referred to as "iodide / iodate". The chemistry is rather
complex but essentially it is a stable form of iodine which liberates it
'in situ' in the presence of acid. However, if you use it the quantities
and calculations are different from using regular iodine solution (see
below).

> Using 25% sulfuric acid, not the 10% I was shown how to use, so
> tried adjusting it @ 40% (ie. 2ml rather than 5ml). Likewise, the
> vinodine is 0.0156N and not the 0.01N, so tried adjusting it using a
> chemistry calculator online.

You are getting in an awful muddle here. I suggest you follow the steps
below...

50 ml of cider
Add 5 ml of 25% sulphuric acid (anything weaker will not do)
5 - 10 drops of starch solution indicator (freshly made from 2.5 g
soluble starch in 100 ml boiling water and allowed to cool)

Rapidly titrate with the standard 0.0156N 'Vinodine' solution to a blue
endpoint that remains stable for 20 seconds.

Multiply the titre value by 10 to get the Free SO2 in ppm.


To get the Total SO2, pre-treat the 50 ml cider sample with 25 ml of 1N
sodium hydroxide and leave to stand for 10 minutes to dissociate the
bound forms. Then add 10 ml (not 5) of the 25% sulphuric acid, the
starch indicator, and immediately titrate as before (so the bound forms
don't have time to recombine).

Multiply the titre value by 10 to get Total SO2 in ppm.

I suspect you were not adding enough acid in what you were doing
originally. Also bear in mind that after fermentation the amount of
free SO2 will be very low. The detection limit of the method is regarded
as around 10 ppm i.e. 1 ml of titre, so you can't really measure much
lower than that.

Hope this helps.

Andrew


--
near Oxford, UK
Wittenham Hill Cider Portal
www.cider.org.uk

[Claude Jolicoeur]

Andrew wrote:

50 ml of cider
Add 5 ml of 25% sulphuric acid (anything weaker will not do)
5 - 10 drops of starch solution indicator (freshly made from 2.5 g
soluble starch in 100 ml boiling water and allowed to cool)

Question here, Andrew - about the soluble starch, is this any starch?
For example would the « corn starch » we have in the kitchen work for this purpose?
 
Claude

[Andrew Lea]

On 25/04/2015 19:21, Claude Jolicoeur wrote:

> Question here, Andrew - about the soluble starch, is this any starch?
> For example would the « corn starch » we have in the kitchen work for
> this purpose?

It wouldn't work very well. Kitchen starch is intended for thickening.
Soluble starch is made from raw starch by acid hydrolysis, it has a much
shorter chain length and doesn't form a gel. It will also give a sharper
end point.

Anyway with your lovely amperometric electrode you don't need starch!
You measure your end point electronically.

[Claude Jolicoeur]

Andrew wrote:

Anyway with your lovely amperometric electrode you don't need starch!
You measure your end point electronically.

Yes, I know. I was asking out of curiosity. I was wondering if this starch had to be bought from specialized supplier. I guess the answer to this is yes!

[Bruce]

Thanks very much for this info Andrew... And thanks too to Claude for the starch question. That was on my list of things I can't find consensus on.

I'm sure you two in particular hear it a lot, but your guidance on this forum is greatly appreciated.

[Wes Cherry]

You can prolong the shelf life of starch solution by adding salicylic acid at 1g/100mL.   I suppose a ~3 crushed up aspirin would work as well.   

Aspirin often uses starch as a binder/filler, but it’s probably the wrong kind.

I have some starch solution I bought from http://wine.awscientific.com/ that is 3+ years old and still working fine with no mold.  I wonder if it has been preserved with something more antimicrobial.   mercury?.  

-Wes

On Apr 25, 2015, at 3:35 PM, Bruce <geekr...@gmail.com> wrote:

Thanks very much for this info Andrew... And thanks too to Claude for the starch question. That was on my list of things I can't find consensus on.

I'm sure you two in particular hear it a lot, but your guidance on this forum is greatly appreciated.

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[Andrew Lea]

Just for interest the geek here has done an online tour of the MSDS info for various brands of pre-prepared soluble starch solution. Salicylic acid is the preservative most often quoted but I also see mercury iodide (sic), acetic acid and thymol are used in some brands. Take your pick!

Andrew

Wittenham Hill Cider Portal

www.cider.org.uk