Could it be possible to make a buffer of pH 12.3 from solutions of disodium hydrogen phosphate and trisodium phosphate?
Yes, it could.
One type of buffer consists of a weak acid and its conjugate base.
In this question, the hydrogen phosphate ion is the weak acid and the phosphate ion is its conjugate base.
The chemical equation is
Per the Henderson-Hasselbalch equation, the pH of the buffer is given by
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Thus, you should be able to use solutions of