If my wife slaps me first and I slap her back, can I be arrested?

The short answer to your question is yes, you can be arrested.  While you may be able to assert that you were acting in self defense, that would be a question for the finder of fact (i.e. the Jury).  The law allows for you to use reasonable force to protect yourself from imminent harm.  If you slapped your wife back in order to prevent her from continuing to attack you, for example, then your actions might be justifiable self defense.  However, if you slapped her back for retribution (i.e. you hit me, so now I'm going to hit you), that would not be a legal defense to the charge of battery.  In order to determine whether you have a strong defense to the charge, I would need to know more of the details of the incident.  However, as stated, the answer to your question is that you can absolutely be arrested under the circumstances you've set forth.