Where does the blue go?

I presoak my LF negatives before processing, and the presoak water always takes on a nice blue color (at least with FP4+ and TMAX).

The other day I dumped the presoak into a plastic milk jug, then dumped in my first-wash water, and finally my used developer (DK-76B, a D-76 variant, used 1+1). The blue cleared very rapidly as I added the developer.

Just out of curiousity, does anyone know what the reaction is? The working-strength developer is 1.25g metol, 50g sodium sulfite, 1gm sodium metaborate in 1 liter, and the presoak, first wash, and used developer (6 sheets 4x5) were each about 1 liter.

Thanks for any enlightenment. This could turn into a cool chemistry demonstration for kids.

The blue colour you see is due to dyes added to the film (either for sensitization or for an anti-halation effect) that come out during the pre-soak. I'm not sure which specific dyes are used for these application, but rapid clearing of the colour on adding developer is almost certainly a simple acid-base reaction (i.e. the dye is acting as an acid-base indicator) or, less likely, a reduction reaction.

You can confirm this with a control experiment next time you have some waste pre-soak to play with: add some dilute alkali (say, a few g of your sodium metaborate dissolved in water) and see if you get the same colour change. If you don't see any colour change, try with a sodium sulfite solution.