The Ripper Titration: Recent Improvements in Measuring SO2

handrawing of an old wine barrel

In the effort to create high quality wines, it is critical to maintain proper free sulfur dioxide (SO2) levels during all steps of the winemaking process. Keeping SO2 levels at optimum values is key to preventing the discoloration, off-flavors and unpleasant aromas resulting from oxidation and microbial contamination. Suffice to say that all those award winning wines going for $200 a bottle or more have the correct levels of sulfites added to them!

The problem with the task of maintaining SO2 levels is the tendency for this component to disappear over time. The SO2, typically added as a sulfite like potassium metabisulfite, undergoes an equilibrium between free and bound forms which is pH- and temperature-dependent. In addition, oxidation by atmospheric oxygen continually reduces SO2 levels especially when the wine is briefly exposed to air as occurs during racking and bottling. Therefore it is important to have a reliable method handy for measuring free SO2.

There are Several Approaches one can take to determine free SO2. However, as a large number of winemakers rely on the simple Ripper titration, we thought it a good idea to elaborate on the virtues and cautions of using this method.

In the Ripper titration, standard iodine is titrated into an acidified wine sample. The iodine reacts with free SO2, and when the SO2 has all been titrated, free iodine appears at the endpoint. Originally described in 18981, the original Ripper method is fast but its accuracy and precision are notably poor, primarily because it relies on the appearance of a dark blue starch-iodine complex to detect the end point, which is difficult to see (especially in red wines) and subject to errors of interpretation2.

However, the method’s speed and simplicity are attractive; many winemakers have claimed that they can train themselves to read the endpoint reproducibly.

Some modifications to the Ripper method improve the reliability of detecting the endpoint. For example, the use of electrochemical sensors that respond to the appearance of free iodine removes the burden of trying to visualize the starch endpoint against a dark background in red wines.

The Hanna Instruments autotitrators like the HI 84100 (also sold by Milwaukee Instruments) use the ORP electrode in an automated device. This electrode relies on a change in the oxidation status of the solution that occurs as free iodine appears. Vinmetrica’s SC-100 uses an amperometric probe to detect appearance of free iodine. This provides robust, sensitive results that compare favorably with other methods.

There have been numerous reports describing the various sources of potential error and interferences in the Ripper method. The acidification step (also required in some other methods) induces slow dissociation of bound SO2 to form more free SO2, leading to a systematic overestimation of free SO2. This effect can be minimized by performing the Ripper titration quickly (within 2 minutes), such that it probably contributes less than a few percent error.

Another source of error that can lead to overestimation of free SO2 is the presence of other oxidizable compounds in the wine. These can react with the iodine titrant, again leading to overestimation of the SO2 level. In some white wines, there are appreciable levels of ascorbic acid which react readily with the titrant. In red wines, the presence of phenolic compounds can also lead to overestimation.

In this case the error may be estimated by a modified Ripper method3 in which duplicate samples are titrated, one having been pretreated with a drop of 3% hydrogen peroxide, the other untreated. The difference between the two values provides an estimate of the “true” SO2 ppm value (since peroxide primarily removes only the free SO2).

In our hands, using the Vinmetrica SC-100 (or the SC-300 in SO2 mode), we have not seen significant differences between the “true” and uncorrected value of free SO2 in the wines we have worked with to date. This implies that phenolic compounds are not significant sources of interference with Vinmetrica instruments.

Indeed, the aeration oxidation (AO) method, which is theoretically free of these interferences, has been shown in one independent study to give identical results to those of the SC-1004 . Although it seems to be generally accepted that Ripper methods produce higher values and are less accurate5 than other methods, it is possible that the Vinmetrica’s improved endpoint detection may change this perception

In summary, the Ripper method is quick and can be made reliable and accurate enough to guide the winemaker in maintaining proper levels of sulfur dioxide. The SC-100A and SC-300 are simple and inexpensive SO2 meters for accurate SO2 analysis.

Other references:
A summary of an ASEV meeting discussing a comparison of methods
Capillary electrophoresis vs. AO vs. Ripper
More SO2 stuff; good overview.

1 M. Ripper, Die Schwelflige Saure im Wein und deren Bestimmung, J. Praia. Chem. 46, 428-73 (1898)

2 James E Vahl and Jean E. Converse, Ripper procedure for determining sulfur dioxide in Wine: Collaborative study. J Assoc Off Anal Chem 63, 194-9 (1980)

3 http://www.gencowinemakers.com/docs/Measuring%20Free%20Sulfur%20Dioxide.pdf

4 https://vinmetrica.com/Souder_Eval.pdf

5 J.W. Buechsenstein  and C. S. Ough, SO2 Determination by Aeration-Oxidation: A Comparison with Ripper (1978) Am. J. Enol. Vitic. 29:3:161-164

An Encouraging Start in the Wine Analysis Business

hand drawing of grapes on a wine barrell

…continued from “How It All Began“.

In July of 2008, a year after my former company (Molecular Devices) was acquired, my position as Vice President of reagent R&D was phased out.  Soon after that I was able to wrap up my affairs in Palo Alto and join my wife Marcia in Encinitas near San Diego, where we had conducted a commuting marriage for the last 7 years.

Jobs were not plentiful at that time, especially in my field.  I started Sportsman Consulting and had a few clients, but it wasn’t much.  I also worked on a startup called Ascent Biosciences, but in December of 2009 the company moved away.

I talked with my wife about the sulfite meter.  She encouraged me to pursue it.  About this time I decided to open a lab in some space within a warehouse that I rented from a friend of mine in Carlsbad.  Sportsman Consulting had just enough income to pay the rent.  I built a simple prototype sulfite meter and sent it to Dave Donofrio, a former colleague of mine who had been a marketing director at Molecular Devices, and who is a serious winemaker in the Santa Cruz mountain area.

Dave had just purchased a Hanna 84100 autotitrator himself, after struggling for years to keep his sulfites under control by other methods.  Dave endorsed my idea of a simple and accurate sulfite test and agreed readily to be the first guinea pig. Dave did some comparisons and reported that the prototype gave results that were quite close to the Hanna.  (Dave is now advising Vinmetrica on Marketing.)  Dave suggested that we could support a sales price that would make a viable business.

We went on from there.  I retained Bill Ewing after seeing his name on the San Diego Electrical Engineers web site.  I picked him because he lives nearby me in Encinitas.  But what a lucky choice that was!

Bill was enthusiastic from the start and we hit it off right away.  We put together the SC-100 prototype in about 4 months. In the meantime I worked out the details of the chemistry to make it a stable product, and found a manufacturer that could make the electrodes to my specification.

More comparative data was forthcoming.  Dave really liked the new prototype and showed it to some colleagues in the winemaking business, and I began to get some feedback from other winemakers.

One of my first early demos was with three Ramona Valley winemakers who were getting ready to bottle their Merlot.  We did a side by side comparison between the SC-100 and their manual Ripper method.

The way they did their assay was illuminating:  take a 10 mL wine sample.  Dilute it with about 100 mL of water. (Gasp!  You’re going to make the endpoint very fuzzy!)  They explained that by so diluting it, they could reduce the opacity of the red color to see the blue endpoint better.  They added starch and acid in the usual way.

Then they took a 10 ml graduated pipette and filled it to the zero mark with 0.02N iodine.  (Gasp!  This is your burette?!).  They then titrated from this pipette.  It took 0.3 mL to reach an endpoint that they all agreed to after some squinting.  This calculated out to about 20 ppm of free SO2.

At the same time, I did an assay on a 25 mL sample in the usual way using the SC-100.  I determined 6 ppm.

We talked about this, and they agreed to repeat their method, this time using a 25 mL sample.  They titrated to the endpoint that they felt was correct:  also 0.3 mL (mind you, reading 0.3 mL off a 10 mL pipette isn’t exactly giving yourself much resolution!)  Hmmm.  But of course now the calculation gave 8 ppm, since they had taken 25 mL instead of 10 ml wine.

Then I suggested we check the strength of their iodine solution.  I produced a vial with pre-weighed amounts of ascorbic acid  – I use these as a quick check of my SO2 Titrant and they are accurate to 5%.

Turned out that their 0.02N iodine solution, which was near its expiry, was actually 0.015N.  So their 8 ppm was actually 6 ppm, and in fact we had numbers that agreed exactly!

With several examples like this under my belt, I knew we were ready to release the SC-100. Our assessment of the competition showed that inexpensive sulfite tests available for winemakers were not practical.

Chemetrics Titrets don’t work in red wines (according to the manufacturer).
People report struggling to use the test-strip indicator kits offered by some competitors. Aeration-oxidation (AO) is time consuming and somewhat complicated.

Other options were sending out samples for analysis (~$20 a pop) or buying an autotitrator of some kind ($600 or more and significant yearly maintenance.) And even the autotitrators require calibration each time you use them, adding significant time to get the job done.

In the 18 months since we released the SC-100, we have sold over 500 of them.  I think this is a testimony to our commitment to bring low cost, simple and fast testing to winemakers.  Most importantly, what we hear from our customers is satisfaction.

“Great product!”

“I love my Vinmetrica”

“My autotitrator is now a door stop!”

Now we are on the next adventure.  With the release of the SC-200 and SC-300, we have added pH and TA measurement to the product line.  The SC-300 is great for those who want to get reliable testing for pH,TA and SO2 without spending a fortune and sacrificing large amounts of bench space.

I’d rather see you spend your money on some new oak barrels!

Rich Sportsman Junior drinks from a remarkable big barrel of wine
Wine is Big in Brazil!

We will continue to be in the business of quality products for wine testing.  In the works is a test for malic acid. We hope that the malic acid test is as simple and affordable as SO2 now is through Vinmetrica!

 

How It All Began

"a line drawing of grapes being crushed"

I founded Vinmetrica in December of 2009 on the belief that I could put together products that would help winemakers like me.

This belief  crystallized over several years and several events.  In 1993 I moved to Palo Alto with my family after living in Indianapolis for 8 years working as a research scientist at Eli Lilly.

We bought a house that had a venerable apricot tree in the back yard.  This tree was probably a remnant of the fruit orchards that used to occupy most of the Silicon Valley area in the early part of the 20th century.

apricots in a tree
Too Many Apricots?

It was a very productive tree, and after making several cases of apricot jam one summer, I still had several pounds of apricots.

It occurred to me to try to make wine out of them.   It wasn’t long before I was fussing over 5 gallon batches of apricot wine, and gradually I found myself learning the ropes of fermentation chemistry.

Over the succeeding years I graduated from fermenting fruit to condensed juices, then to wine kits and finally grapes (though I still like the dry apricot wine one can make from sugar, water and halved apricots).   Inevitably I was confronted with the task of measuring my sulfite levels.

Now, as an analytical chemist by training, I understood the basics of measuring sulfite by iodometric (or “Ripper”)  titration.  You take a measured quantity of wine, add acid and starch solution, and titrate with an iodine solution of known strength until the color turns blue.

My local winemaking supply guy recommended the Chemetrics Titrets(r) which essentially minimize this procedure into handy measuring ampoules.  Despite the manufacturer’s admonition that these are not suitable for red wines, I kept trying to use the Titrets for a whole winemaking season before finally giving up.  You just can’t see the blue endpoint reproducibly.

Next I went on to doing the full Ripper method with starch.  But again the red wines defeated me. As a scientist I had to admit that I would be fooling myself in claiming that I could see that endpoint in any kind of reproducible way.

I remembered the iodine titrations I did as an undergraduate.  In the experiment we determined the endpoint via a cobbled-together battery-powered electrode system with a microammeter as a readout.  I recall remarking to my professor that I could see the faint yellow iodine color about the same time that the meter started to indicate.

I asked my professor,”Couldn’t you just use the color as the endpoint?” He quipped, “Oh sure, Rich; even better, we could just add some starch and it would turn deep blue!  But the point of the experiment is to make you aware that you can use electrochemistry to detect the endpoint. “

So I thought I would cobble together one of these as best as I could remember.  Two platinum electrodes from ah, disused equipment at work, a few components from Fry’s Electronics, and an output that I read with my voltmeter.  Wasn’t much to look at, but I found I could get reproducible results in minutes.  This was about 1995.  I used this setup for years in my garage.

Little did I know what I was getting myself into…