No idea of the actual pH but it will be be considerably higher than D23, possibly > p11 compared to around maybe pH 8.5 for D23. So the two developers work quite differently, D25 reduces the pH of the developer even closer to pH7 by adding Sodium Metabisuphite to D23.

A stock solution has a very short life, and is not recommended for storage by Ilford with Hyfin, or Beutler, Crawley etc who's developers are all quite similar. There's insufficient sulphite compared to the Carbonate to preventthe the Metol oxidising in solution. Crawley suggested a 2 part stock solution for a developer like this keeping the Carbonate separate.

Ian