Does adding sugar to tomato type sauces affect acidity? Many(most?) tomato based pasta sauces have added sugar, it is generally a very small amount. I have always heard this was done to reduce the acidity. 

My main question: Does this little bit of sugar increase the pH enough to be noticeable?

Or, is this just to cut the sourness and balance the flavor
(have we come to expect a slight sweetness in our pasta sauces)?
Weak organic acids such as those found in fruits and vegetables (citric acid, malic acid, tartaric acid) don't react with sugars. 1 There is no change in acidity, which you correctly defined as measured by the pH. 

At the same time, sweet and sour are two tastes which are real antagonists - adding something sweet actually reducess the sourness we perceive, as opposed to just distracting us from it. So the sugar changes the sourness (the taste), but not the acidity (the chemical property). Hearing it otherwise comes from the fact that most people don't even realize that there is a difference between the two terms, and use them interchangeably. 



1 They can partake in reactions as catalysts, for example to create invert sugar when cooking up sugar syrup, but the acid itself does not react away.