CHELATOR: Adding a Phosphate/Carbonate-Buffer resp. a new Chelator

Hi!

Searching for an easy way to do some calculations of concentration of
calcium in physiological solution (ACSF) i found the CHELATOR-Program
by Schoenmakers.
It seems to work fine but unfortunately i' dont manage to add buffers
to the internal list. The solution I'm working with is based on a dual
buffering system.
(1) a carbonate buffer (by adding NaHCO3)
(2) a phosphate buffer (by adding NaH2PO4)
When I try to add these buffers to the programms list, i am asked to
enter the "delta pka (25??C)" values. I must confess, i have no idea
what a "delta pka" is.
Is there anybody who might help me out?
(a) what is the "delta pka" ? And what tells me this data?
(b) what is the correct delta pka for HCO3 resp. H2PO4 (or where can i
find these values)?

My second problem deals with something similar. Since my solution also
contains GTP, i think i'll have to consider it in a calculation of
free calcium. When i do so i'm finally ending up to a point where a
menu opens and i have to select between 12 possible binding kinetics
and enter its rate constants. At this pont i choose the first
equation and select Ca as binding metal. But i have no idea what to
enter for the following specifications.

I'd be very grateful if there is someone out there who might help me
out with this.

Regards
Thomas