No, diet coke will NOT impede weight loss. This is coming from someone who drinks 6-8 diet cokes daily, and has managed to lose 110 lbs in the past. I did make sure that I was drinking my water, also.
People like to say that diet coke has "so many chemicals and sodium" and these will hurt your weight-loss efforts. Quite the contrary: 1 diet coke, on average, only contains 40-70 mg of sodium (you're allowed around 2,000 per day). As you can see, in the big-sodium picture, so to speak, diet cokes really do very little damage. It is those processed, microwaveable foods that contain unbelievably high amounts of sodium. For example, one of my Lean Cuisine meals has over 600 mg of sodium! Now THAT'S high.
So don't worry too much about splurging on a diet coke. If it weren't for diet drinks, I think I would go nuts! x(
Sherry