Acidity in a wine... has me confused!

I have always seem to have a problem determining acidity in a wine, especially the whites. Sauvignon Blanc in my opinion tends to show very good acidity but quite a few times when tasting, say a Mosel or a Chenin Blanc, I tend to think/say "this is pretty soft, needs more acidity".
"Nah Bob, has plenty...what are you tasting?' and so on.

Where is my thinking going wrong here?

I think the tricky part about assessing acidity is that the perception of acidity is changed by sweetness. When you have for example a typical Mosel Kabinett is will have very high acidity but the perception of that acidity is buffered by a fair amount of residual sweetness. The same is often true of Chenin Blanc.

[David M. Bueker]

Oliver has it right. I will say that even in dry wines that I find Chenin Blanc carries much more palate weight (extract perhaps?) than most Sauvignon Blanc, and so hides acidity.

Cola is actually very acidic (pH < 3). It's just hidden by the sugar.
Everyone perceives acid differently. The same juice can seem "refreshing" to one person and "bitter" or "sour" to another.

I think Cola Zero is more acidic than Pepsi Max.

Acidity in wine can be confusing. Grapes contain primarily 2 different acids, tartaric and malic. Tartaric is the stronger of the two acids. As grapes ripen, the level of malic acid decreases as the sugar content rises, but the tartaric acid level remains the same. There are two tests usually performed on grapes for acidity. The simplest is pH, which measures the strength of the acid solution. The second is titration, which measures the total amount of acid in the wine and the result is listed in grams per liter of titratable acid (TA). The TA and pH are not related in a linear manner. It depends on the proportions of the acids present. It's possible to have a wine with both a high TA and a high pH. During fermentation and stabilization, the acid levels drop as the level of tartaric will be reduced as tartrate crystals fall out of solution (wine diamonds). In red wines (and Chardonnay) a bacterial fermentation is frequently encouraged which converts the malic acid into the milder lactic acid (malo-lactic fermentation, or ML). Most whites do not go through this. In tasting for acidity, not only does residual sugar mask some of the acidity, but so will higher alcohol levels.

I think the tricky part about assessing acidity is that the perception of acidity is changed by sweetness. When you have for example a typical Mosel Kabinett is will have very high acidity but the perception of that acidity is buffered by a fair amount of residual sweetness. The same is often true of Chenin Blanc.

Thanks Oliver, nicely summed up for me.