"Acids are very important structural components of wine. If a wine is too low in acid, it tastes flat and dull. If a wine is too high in acid, it tastes too tart and sour." - http://www.wineperspective.com/the_acidity_of_wine.htm
Warmer climate sweeter wines with low acidity (high pH) are usually not as complex as cooler climate dry wines with moderate acidity (lower PH).
"Low acidity (high pH), common in grapes that are too sweet because they were grown in excessively warm climates, lowers the amount of subtle flavors in the grapes and consequently in the wine. As mentioned before, there is an optimum pH for fermentation as well. Prior to fermentation, pH is measured either by titration or more conveniently with a pH meter, and if it is too high, tartaric acid is added." - http://www.emsb.qc.ca/laurenhill/science/wine.html
Achieving the perfect balance can be a big challenge for the wine maker. Each grape varietal usually requires a different TA (total acidity or titratable acidity) and PH.
"TA is a measure of the total acids present in wine. TA is related to pH but the concepts are not identical. While pH measures acid strength, TA measures the amount of acids present.
Acidity is perceived in degrees of tartness and decreases as the grapes become riper. Tartaric acid is the primary acid, but others such as malic and citric can be found as well. As the harvest date draws near, TA in the grapes drops (due to the respiration of malic acid). It is important to pick the grapes with enough TA or an adjustment will need to be made. TA is important, but somewhat less important than pH as most winemaking decisions (including acid additions) are based on pH, not TA.
TA is measured in g/100ml in the US, and values for table wines generally fall somewhere between 0.4 and 0.9 g/100ml." - http://www.crushnet.com/enowiki/titratable-acidity-total-acidity-ta
"A typical premium California Chardonnay has a total acidity of 0.58 grams per 100 mL (0.58%) and a pH of 3.4. It is interesting to compare these values with a total acidity of 1.10 grams per 100 mL (1.10%) and a pH of 2.91 found in a late harvest Johannisberg Riesling with 21% residual sugar. Generally speaking, sweet wines require a higher acidity than table wines to balance the high sugar. This is true for Sauternes, Alsatian SGN and German TBA wines." - http://www.wineperspective.com/the_acidity_of_wine.htm
VOLATILE ACIDITY
"Both tartaric and malic acids are nonvolatile which means that they do not evaporate or boil off when the wine is heated. This is to be distinguished from volatile acidity (VA) in wine that represents acetic acid (vinegar). Acetic acid does boil off when heated, and high VA is undesirable in a wine. A VA of 0.03-0.06% is produced during fermentation and is considered a normal level." - http://www.wineperspective.com/the_acidity_of_wine.htm
