Monosodium Diphosphate (NaH3P2O7) Preparation
$\ce{NaH3P2O7}$ is an intermediate acid-salt formed during the neutralization of pyrophosphoric acid ($\ce{H4P2O7}$) which becomes unstable as pH rises and leads to formation of trisodium diphosphate, $\ce{Na3HP2O7}$:
$$\ce{H4P2O7 + NaOH -&gt;[pH &lt; 3] NaH3P2O7 + H2O}$$
$$\ce{NaH3P2O7 + NaOH -&gt;[pH = 3-9] Na2H2P2O7 + H2O}$$
$$\ce{Na2H2P2O7 + NaOH -&gt;[pH &gt; 9] Na3HP2O7 + H2O}$$
This acid-salt can also be produced by the reaction of trisodium phosphate and pyrophosphoric acid:
$$\ce{Na3PO4 + H4P2O7 -&gt; NaH3P2O7 + Na2HPO4}$$
Notes and references:

Study of crystallization processes in some rare earth vanadate, tungstate and phosphate systems under hydrothermal conditions, K Byrappa and B Nirmala, Indian J. Phyx. 73A (5), 621-632 (1999) (PDF)
Its CAS number is 13847-74-0
The first dissociation of pyrophosphoric acid gives the anion, $\ce{H3P2O7^-}$. The dissociation constant is pretty large compared to 2nd, 3rd, and 4th dissociation of the acid.

$$\ce{H4P2O7 + H2O &lt;=&gt; H3O+ + H3P2O7¯ ~~~~ K_{a1} \sim 10^{–1}}$$