[wineguy26]

Hello,
My name is Mark.  This is my first post and I'm glad to have found this forum.  I'm seeking help with the classic "Ripper" test in wine, which uses iodine as a titrant to reduce SO₂.  The reaction is:
SO₂+I₂+2H₂O :rarrow:H₂SO₄+2HI

The reaction is usually conducted visually with 1% starch solution.  When the SO₂ is consumed, excess iodine complexes with the starch to produce a blue-ish-purple-ish endpoint, but that is difficult to sense in a red wine. 

However, this reaction can be sensed with a platinum ORP electrode and pH meter.  The redox endpoint is seen as a rapid jump in millivolts, usually +50-100mV.  I once worked at a winery that used the ORP electrode and a Orion model 301 analog pH meter.  The drop of the needle indicates the endpoint.  But something was different.  When we immersed the wine sample in the electrode, the meter needle usually pegged to -400mV (full negative scale).  After titrating and reaching the end point, the needle dropped and fell smoothly all the way to +400mV (full positive scale).

I have the same set-up in my lab.  The needle moves about 50-60 mV signaling the endpoint, but it doesn't drop all the way to 400mV (0 pH). 

What's different? 

[Arkcon]

Briefly, what you're doing is a potentiometric titration, to determine free SO₂.  I don't really understand what the question is -- but briefly, its the rate of change of conductivity, that lets you know the endpoint, not where the titration starts or ends up.  http://en.wikipedia.org/wiki/Potentiometric_titration  This Wikipedia article is pretty difficult for a novice to follow, but do look at the pictures at the top -- the first is what you're seeing.  By taking the second derivative, you can determine exactly when you have consumed all the SO₂ with titrant.

[Borek]

briefly, its the rate of change of conductivity, that lets you know the endpoint

Not of the conductivity, but of the redox potential in the solution.

But I am not sure what the question is about either. Expected change in the potential around the equivalence point can be calculated, but I don't have enough time right now to try. Several hundreds mV doesn't look off to me.

[wineguy26]

I probably failed in my layman's chemistry terms to explain things.   So, quoting from Zoecklein et. al Wine Analysis and Production, "the analysis for free and total SO₂ is dependent on the redox reaction: H₂SO₃ + I₂  :arrow: H₂SO₄ + 2HI.  Completion of the reaction is signaled by the presence of excess iodine in the titration flask.  This excess iodine can complex with starch (blue-black endpoint) or be sensed with a platinum electrode" (Zoecklein p.495). 


One can use either an analog or digital pH meter.  The analog is more "user friendly", as you are only looking for the needle inflection (mV) rather than the constantly changing numbers of a digital display.  When I do the titration via analog or digital meter, the endpoint is usually a surge in voltage around 100 mV.  I've sent samples to an outside lab, so I know my results are correct. The Orion 301 analog pH meter that I have has a scale of -400 to 400 mV, so a change of ~100mV, moves the needle about .75". 

According to the Nernst equation, shouldn't the endpoint be far greater than 100mV?  I don't understand how to calculate the potential with the Nernst equation.  Help appreciated.  Thank you.

[Borek]

First of all, can you check if the meter is properly calibrated? I don't know this particular model, but I suppose it has some kind of switch for choosing the amplification/scale/range - are you sure it is set up properly and working as expected?

What are concentrations of sulfur dioxide and iodine? Is the sample diluted before titration? All these data are important, as the voltage change depends on them.

[wineguy26]

The meter is functioning properly.  A 50 ml wine sample is acidified with 5 mls of 25% H₂SO₄ and is titrated by .0156N I₂ to the endpoint as described by Zoecklein.

[Borek]

What is the determined SO₂ level?

[wineguy26]

What is the determined SO₂ level?

It varies from wine to wine.  Generally, wines measure between 25-40 ppm free SO2.  I don't really notice any difference in "endpoint" voltage of various samples.  It always seem to be ~ 50-100 mV regardless of SO₂ level.

[Borek]

If I am not wrong it should be around 250 mV jump from 99% to 101%.

Because of the sulfuric acid presence (around 0.3 M) solution has ionic strength of at least 0.4, which makes exact results questionable. But it is definitely something between 200 and 300 mV.

But then, if you observe the jump between ±0.1%, it will be definitely smaller, by more or less 59 mV.

[wineguy26]

If I am not wrong it should be around 250 mV jump from 99% to 101%.

Because of the sulfuric acid presence (around 0.3 M) solution has ionic strength of at least 0.4, which makes exact results questionable. But it is definitely something between 200 and 300 mV.

But then, if you observe the jump between ±0.1%, it will be definitely smaller, by more or less 59 mV.

Thank you, Borek.  I do have a question.  Do you think an ISE Iodide electrode could be used for the test, and if so, would it cause a huge mV spike?  I've just recently learned that in the Ripper titration, it is not actual I₂ which reacts with SO₂, but it is I₃^-, trioxide.

And one last question...does the reference electrode input on a pH meter output a polarizing current?
Thanks.

[Borek]

Concentration of iodidies doesn't change rapidly near end point, so ISE is of no use.

No idea what reacts with SO₂. Iodine in solution containing iodides is present mostly as I₃^-, but there is always some equilibrium concentration of I₂, so there is no problem with the reaction being between I₂ and whatever is being oxidized.

I am not sure what the polarizing current is. Reference electrode should be connected in such a way it doesn't have to produce any current (technically it always has to, but the only circuit in which it should produce a current is the one going through the voltmeter, and good voltmeters have pretty high input resistance).

[wineguy26]

Thank you Borek.  With your help and the help of others, I've confirmed that there is no electrochemical principle that explains why that pH meter that I used at a previous employer functioned in such a manner.  There is no way the reaction can go from -400mV to +400mV, and peg the needle in both directions. 

My suspicion now is that someone "modified" that analog pH meter.  The only reason why I'm interested is that if the needle moves a further distance, it makes determination of the endpoint easier.  It makes the test more "user friendly".  I've browsed some electrical forums, and there is a wealth of information on modifying a voltmeter to measure within a specific range.  Onward.

[Borek]

As I already hinted at earlier, you can always change the sensitivity of the meter so that small changes look as if they were large. Sometimes broken switch is all that is needed to make things look this way. And as long as you are using the meter for detecting titration end point not by titrating to some particular value, but just to the fast potential change, it doesn't interfere with the result.

[wineguy26]

As I already hinted at earlier, you can always change the sensitivity of the meter so that small changes look as if they were large. Sometimes broken switch is all that is needed to make things look this way. And as long as you are using the meter for detecting titration end point not by titrating to some particular value, but just to the fast potential change, it doesn't interfere with the result.

Understood sir.  I have attached an image of the Orion 301.  It's a simple unit with a BNC input, REF input, calibration control, and temp control.  I do have the owner's manual with wiring schematic.  There is no reference on how to change the sensitivity of the meter.  I have messed with the calibration and temp knobs every which way to try to get it to function as previously described.  I'm not sure if the wine lab I used to work at modified the meter just using the knobs or if they opened up the back, took a trip to Radioshack, and went to town. 

[wineguy26]

Got it figured out with help from an electronics forum.  In my analog meter, the calibration knob and temp knob work to change needle gain/travel.  By shorting the cal knob to the temp knob, it gets you pretty much infinite possibilities of needle travel.  I can make a 50mV change in sample potential correspond to a -400 to +400 mV change on the meter (full scale).