The no longer available kaliumbitartrate replaced by...

One can 2 directions out:

1. use potassium dihydrogen phosphate but the pH of the solution in water (= 4.5) is 1 pH unit higher as with kaliumbitartrate (= 3.5);
2. prepare potassium bitartrate based on the well-available tartar acid and the not available KOH, which is present at each lab.

Personally I prefer NR 2 because tartrate is known for its metallic complexing properties, which are not found in phosphate.