Subject: How does Solochrome black indicator work in an edta- calcium ion titration?

I have carried out an A - Level investigation in to the purity of
limestone. I carried out titrations with the rock disolved in HCl, Gas
collection from disolving, edta titrations and I used a Calcium
electrode machine. I am currently writing up the investigation and would
be very grateful if you could explain how the solochrome black indicator
works to produce the colour change in the edta titration. The colour
change was from wine red to clear blue. The solochrome black was
prepared by mixing 9.9g of potassium nitrate with 0.1g of the solochrome
black powder.
It would also be helpful if you knew how the Calcium electrode works, or
if you could point me in the direction of any site which has this
information.
Many Thanks.

Re: How does Solochrome black indicator work in an edta- calcium ion titration?