Acid Caculator - Chemistry / Maths Help Required!

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jamesbsmith

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I have been working on a spread sheet which we will be releasing soon which allows wine makers to determine the acidity of their wines by either measuring acidity by titration or reading pH, which then gives the amount of acid / alkali they need to add to reach their desired acid level as well as the amount of campden needed when bottling.

I think that pH can be determined from titration (and visa versa), as long as the type of acid present in the wine / must is known (and if not an approximate value determined).

I believe that I have found out the way you can convert the values, but I just wanted to verify this before finalising and releasing my calcs.

If any of you are any good at chemistry / maths, would you be able to verify this:

pH from Titration

Determine number moles of NaOH present=
ml NaOH used in titration X Strength (M) of NaOH X Volume of Wine / Must

Determine number moles of Acid present=
2 moles NaOH react with 1 mole of malic acid
2 moles NaOH react with 1 mole of tartaric acid
3 moles NaOH react with 1 mole of citric acid

Therefore the main acid type in the wine needs to be known. Divide number of moles of NaOH by this value

To Calculate the pH from moles of a Weak Acid =
pH = -log (square root of ka [H+])

At present I have put the moles of acid in the place of [H+], or should that be moles of acid X 2 (for malic and tartaric), or moles of acid X3 for citric acid

Concentration of Acid from Known pH
By rearranging the formula above, for calculation of pH of a weak acid, to find [H+], the formula becomes=

Concentration of Acid = (10^-pH)^2 / ka
There are more than one ka value for acids which disassociate more than one H+ (i.e. all of the acids which may be present in wine). We are only interested in the first ka, as we are interested in the pH prior to any alkali being added.

The ka's for our three major wine acids are:
Citric Acid 0.00084
Malic Acid 0.000394
Tartaric Acid 0.001047

Again, I am unsure whether the value of [HA] gained needs to be multiplied by the number of moles of acid that would react with our titrated strong alkali NaOH.

Any help / advise on this would be great. The calcs are to accompany the excellent YoBrew Wine Calcs which I cannot take any credit for, but I use all the time and are a genius piece of work!
 
pH is a measure of acid strength and depends in the amount of dissociated protons. You can not calculate that from a titration. And you don't know what else is in the juice to buffer the hydrogen ion concentration. The titration tells you the amount of bound and free acidic hydrogens.


Sent from my iPhone using Wine Making
 
Hi Salcoco - yes there is - but not one like this! Also, I want to understand what is behind the calculator, not just use it. I hate it when people sit on this just slagging off peoples efforts to try and look smart. Im sorry if this is above your head.

GreginND - I appreciate what you are saying, but if you know the main acid type in the wine / must and you know the acids dissociation constant, you should be able to determine (to some degree of accuracy), the amount of dissociated H+ hence pH? Could additional potential buffers in a juice have such a significant effect on dissociating H+, that it would drastically effect pH?!

Thanks for your comments though!
 
Absolutely. And different varietals have very different buffering capacities. That is the reason why nortons usually have high pH and high TA. Also, how can you determine the ratios of the various acids without HPLC or Gc analysis? Your acid ratios will also change when after fermentation and different yeasts will affect it differently. It seems there are way too many errors possible to make any kind of useful calculation.


Sent from my iPad using Wine Making
 

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