in my country Indonesia, it's hard to find Disodium phosphate, can change it to trisodium posphate?
as far as I know, 1 % solution of TSP has pH of 12,1.
compared to pH around 9 of 1 % solution of disodium phosphate (smewhere it's said it is neutral).
maybe it can be substitued, but then you have to adjust the pH by adding some acid. i'm not a chemist, sorry.