Central Texas Wine Guild

So, if we're going to do SO2 measurements, let's assume that we want to *do* something as a result of the new information!

I am in the middle of a round of my Free SO2 measurements here on the two batches that I started 9 months ago. Both my Malbec and Carmenere were pretty darn low a month or two ago. So I'm bumping up their levels some.

Naturally, large portions of my additions are immediately becoming bound. That means I'm not getting the protection level I want yet, and am having to re-check after a small wait.

For example, I checked my Carmenere about a month ago. It was like, less than 5 ppm. That's likely purely residual from fermentation or experimental error. Too low for me. With my pH at 3.60 on 23 liters, I targeted 50 ppm and threw in 20ml of 10% SO2 solution. I gave it a gentle swirl and waited.
About 10 days later I checked again and measured pH=3.58 and FreeSO2 at 11 ppm. Roughly 40 ppm became bound. It was better, but not what I wanted yet (30 ppm Free). So I added another 20 ml of 10% solution and waited some more. Yesterday I checked it and found 34 ppm Free and pH=3.51.

So I bottled it!

Heck, it wasn't so much that I got 30 ppm as it was I needed the carboy!

So at pH=3.51 I had:


Total 100 ppm = 66 ppm bound + 34 ppm free


Typically, you see:


pH bound free
---------------------
3.5 50% 50%
3.1 25% 75%


So here's a good point about Total and Free SO2: It changes over time and it is pH sensitive. (Anyone have a reference to this complete graph up their sleeve?)

In any event, a good guideline is to try to keep total additions SO2 under 100 ppm.

Let's talk about doing the additions.

First, potassium metabisulphite (K-meta) is 57% SO2. That's the basis fact for everything that follows here.

I make a 10% K-meta solution by adding enough distilled water to 100 grams of potassium metabisulphite to make a total volume of 1.00 liters. (Really, I add 75 grams K-Meta to a wine bottle,
fill it to 750ml, and stopper it.)

Combining the previous two paragraphs, that means the 10% K-meta solution is 5.7% SO2. Here's the confusing thing now:

Some wine literature calls this 10% K-meta solution a 6% SO2 solution.

Yeah. I know. They are both essentially correct (5.7 ~= 6), but it is pretty darn confusing! To er, compound the problem, some people call their 10% K-meta solution a "10% SO2 solution" because they know that the purpose of their solution is to add *SO2*, not K-meta. Ah well.

In any event, I recommend that you make your SO2 additions using this 10% K-Meta solution using a small "oral" syringe calibrated in ml.

So how much to add?

I get confused by so-called Sulfite addition charts. They generally look something like this (made up):


Sulfite to add to get:

Wine 10ppm 20ppm 50ppm
Volume
--------------------------------------
1 liter 1.5ml 3.0ml 7.5ml
--------------------------------------
1 gallon
--------------------------------------
5 gallon
19.1 liters etc...
--------------------------------------
6 gallon
22.7 liters
--------------------------------------

So I went to direct basic math. The question I always want answered is this:

How many ml of 10% solution should I add to my L liters in order to get P ppm of SO2?

Surprisingly, the answer is the simple equation:


Add ml = 1/57 * P ppm * L liters


The constant 1/57 comes from the same 57% above. So for example, if you want 50ppm in 23 liters:


Add ml = 1/57 * 50ppm * 23L
= (50 * 23) / 57
= 20.17


So, I need to add 20 ml of 10% K-Meta solution. For me, that's exactly two draws (10 ml each) of my dosing syringe.

If you need to convert, to use say, gallons instead, just subtitute in L = G * 3.7854


Add ml = 1/57 * P ppm * (G * 3.7854 g/L)


I find this direct equation approach easier than having to locate the dosing table and trying to figure out how to interpret it each time.

Some reading material:

• http://www.bcawa.ca/winemaking/so2use.htm
• http://www.homebeerwinecheese.com/SO2.html
• http://www.vawa.net/winemaking-articles/using-so2.html