Which of the following is not treated as primary standard in the volumetric analysis?

A nominal pentahydrate form of sodium thiosulfate is available. However, it can be challenging to identify the water that crystallised because the precise makeup of the pentahydrate is very sensitive to temperature and humidity.
In other words, crystals absorb water when the humidity is high, and they lose water when the humidity is low and the temperature is high.
Thus, we cannot determine the specific composition to obtain the precise molecular weight. One cannot produce a standard solution without knowing the exact molecular weight.