I know that the first sentence 'Isn't this a standard?' is grammatically correct, but sometimes I have encountered the usage like 'is this not a standard?', putting not behind the subject this. Is the second sentence form acceptable? If it is acceptable, is there any difference in their tone?
(To make an answer out of this since one hasn't been made in two days...)
As @user3169 said, they're both okay. And as @FumbleFingers said, the extra emphasis is on not, which has the function of suggesting surprise at discovering that it's not a standard.
Is this not a standard? (Because I thought it was, but you're telling me it's not.)
By contrast, the more common "isn't" could be used with different intonations for a couple of purposes:
Isn't this a standard? (I want to confirm my understanding.)
Isn't this a standard? (I'm politely reminding you of the fact.)
Isn't this a standard? (Because I thought it was, but you're telling me it's not.)