Determining diffusion using cftool and error function

I'm trying to determine the diffusion constant for an experiment I ran. I have the appropriate equation but don't know too much about MATLAB. I eventually found that trying to fit a custom equation using cftool is the best way to go about this.

So into MATLAB I've loaded 2 vectors, one being time(x) and one being the concentration(y).

The equation is input is: a/2 * erf(1 + (1 / sqrt(4 * b * x)))

where: a is the initial concentration (0) and b is the diffusion constant (the parameter I've trying to solve). When I put this in however, I get a b value I know to be wrong (I roughly expect it to be ~E-07 - E-12), also I get no line to fit my data unlike the in-built solutions.

I've attached an image of my cftool window

Any help is greatly appreciated

[Erik Schiferle]

Hi Marko,

Did you ever figure this out? I am trying to do the same thing...

Erik