Think about why you got it in the first place. The employer believes that you're the best person available. A great deal of time and effort has gone into your selection, and the last thing the employer wants to do is reopen the search. Speak in terms of the market rate and explain that your salary range is based on your education, experience, and potential. Sell yourself on why you are worth what you are asking.
Employers know the market rate.
You should know it from your research. If they want you, they'll pay. However, be prepared to compromise. That is negotiation.