Thanks for responding in the comment section as to whether this is for full-time or contract employment.
I've done contract work (mostly) on and off for about 20 years now. One rule of thumb is that whatever you offer in the beginning as your hourly rate, you're usually going to have to suck it up unless the responsibility level is drastically different. One example of a higher responsibility level might be you being hired as a developer, but being asked to function as the team lead or manager. Generally, if it's just a few more tools to use each day, you're not going to get anywhere with the argument, because you're being paid by the hour.
This brings me to point #2. Don't low-ball, because you'll get stuck, as with this!!! If your skill in the market would get you, say, $75/hr in most places, then don't even bother offering yourself up for a gig paying only $50/hr unless you're desperate and you can keep cool about making less-than-market. Sometimes, recruiters won't even bother if there's a huge variance between their offer and your normal rate, even if you're pressing for it -- because there's likelihood that you'll quit and get a job with your normal rate anyhow. Contracting takes a lot of nerve and patience.