For some reason last night I started questioning my procedure for running TA. After referring to my books and searching the internet I am more confused than ever. Here is what I have been doing:
25 ml of juice or wine
titrating to a pH of 7 with 0.1N NaOH
Volume of tritrant x .3
So 25 ml of NaOH would result in 7.5 gm/liter (.75gm/100ml)
It seems every thing I read was different…some use 5ml of sample and some 15 ml, some called for dilution with demineralized water that have been titated to a given pH to neutralize the acid caused by CO2. Most procedures used 0.1N NaOH but some used 0.2Normal.
Additionally, most procedures say to titrate to a pH of 8.2 or 8.3 because that is where the end point would be using phenolphthyne (sp). But if the titration is about neutralizing the acid why would you not titrate to a pH of 7 (neutral)?
Anyway, I like the way I have been doing it because it’s simple and the sample is large enough to accommodate a magnetic stirrer and temperature compensating probe along with the pH probe. The question I have after wading through all the BS…does the procedure I have been using provide accurate results of TA?
Where is your location (you don’t have it listed on your profile)? That could change your procedure.
In the US we titrate to a pH 8.2 endpoint because we use a tartaric acid reference point. In Europe they titrate to a pH 7.0 endpoint because their reference acid is sulfuric. It really has nothing to do with “neutrality”.
I start with 2-3 mL of wine in 100mL water. Titrate with 0.1 N NaOH to pH 8.2. This is just to get you to the 8.2 starting point (you want to start and end your titration at the same pH). Then to this you add 5mL wine and titrate to 8.2 again. You can continue using this same solution, since it is already at your pH 8.2 starting point.
I spent some time talking wine with a winemaker from South Africa in the last couple of years, so I guess that is where my procedure came from. I’m in Tennessee.
So let me see if I understand your procedure correctly…
100 ml DW with 2 ml of wine/juice titrated to 8.2
Then to that solution add an additional 5ml of wine/juice and titrate that 107 ml of “solution” back to a pH of 8.2.
Now, by what factor do you multiply the amount of 0.1N NaOH used to determine the TA?
Yes. As I said, the first 2-3mL of wine (the amount isn’t really important) is just to get your solution to an initial pH 8.2. Then yes, to this (it will be more than 102-103mL after you do the initial titration), you add your volumetrically measured 5mL wine. This is the wine you will actually be measuring for TA. Then I do my “real” titration. After that, my calculation is :
TA in g/L= (mL NaOH) (N NaOH)(0.075)(1000)
5mL sample
People titrate either to colormetric endpoints (manually) or those of us who are little more anal use pH endpoints to be consistent and remove the factor of who might be judging when the color change occurs. In Europe, they often titrate to an endpoint of 7.0 because that is the color-change of the indicator they use, which is bromothymol blue (at least, this is true in Italy). We titrate here to 8.2 b/c that is the color-change of the indicator we most often use (phenolphtalein). The difference should be slight, because pH shift is rapid at that point, but there really are nothing more than approximate equations for equating one to the other due to differing buffering capacities, etc.
Also, in some parts of Europe, as has been pointed out, acidities are expressed in concentrations of sulfuric acid, while we express acidities in terms of tartaric acid equivalents. Using molecular weights, it should be easy enough to equate one to the other here. I think it’s mostly France that uses sulfuric acid equivs.
Thanks Linda, I can’t believe that I never realized you can keep testing more samples without starting over with each sample. It seems obvious once you point it out.
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