How to calculate first derivative of time series

Question

I would calculate the first derivative (dpH/dtime) of time series using two variables, time and pH.

Are there any kind of functions to do this

Answer 1

Here's a link to "Numerical Differentiation".

http://en.wikipedia.org/wiki/Numerical_differentiation

Here's a link describing a method based on Taylor Series Expansions:

http://ocw.usu.edu/civil_and_environmental_engineering/numerical_methods_in_civil_engineering/ODEsMatlab.pdf