Sodium pyrosulfate
Sodium pyrosulfate is an inorganic compound with the chemical formula of Na2S2O7. It is a colorless salt. It hydrolyses in water to form sodium bisulfate with a chemical formula of NaHSO4 which has a pH of around 1. 

It is usually prepared by the dehydration of sodium bisulfate into sodium pyrosulfate:                2NaHSO4 —> Na2S2O7 + H2O

Preparation
Sodium pyrosulfate is obtained by the dehydration of sodium bisulfate:

 2 NaHSO4 → Na2S2O7 + H2O

Temperatures above 460 °C further decompose the compound, producing sodium sulfate and sulfur trioxide:

 Na2S2O7 → Na2SO4 + SO3

Applications
Sodium pyrosulfate was used in analytical chemistry. Samples are fused with sodium  pyrosulfate to ensure complete dissolution before a quantitative analysis.

See also
Pyrosulfate
Potassium bisulfate
Potassium pyrosulfate

References

Sodium compounds
Pyrosulfates