# A Simple Question about Diffusion and ion speed

1. Jan 3, 2009

### WarpedWatch

Greetings,
I'm trying to get an intuitive understanding about a process involving an ion mobility that (I'm told) has D = 9.3 x 10-5 cm2/s. I'm assuming that D is a diffusion coefficient.

I'd like to know approximately how long it takes for the ion to travel about 10 cm. Would this be something that happens in a matter of femtoseconds, minutes, hours, weeks, years.... or what? Unfortunately I don't have any more information than that, so is there some way to calculate (or just roughly guess) the ion speed based on D?

many thanks,
Mark

2. Jan 3, 2009

### Mapes

The general approximation for diffusion time is $t=L^2/D$. The exact answer depends upon the boundary conditions, but this estimate is good to within an order of magnitude. In your case: about 12 days.

3. Jan 4, 2009

### WarpedWatch

Mapes,

That's great! Thanks very much for your help on this!

Mark