Why is borax a good primary standard?

Yes borax is a hydrate, but the reason it is favored for titration is that it gives consistent results, which to most scientists, is far more important than accuracy. The .10 H X 2 O hydration number will remain constant and will not fluctuate to 11 or 9 within normal temperature and humidity ranges. read more

It's given in my book that borax is a useful primary standard for titrating against acids. I have heard that primary standard is a pure reagent and does not gain any weight by absorbing moisture. (Hence any error during weight measurement is prevented). read more

Borax is a useful primary standard for titrating against acids as the 10H2O hydration number of it will remain constant and will not fluctuate to 11 or 9 within normal temperature and humidity ranges. read more

Sodium hydoxide cannot be used as a primary standard because of its hygroscopic properties as a solid. read more