Final answer:
A primary standard must have high purity, stability, non-hygroscopic nature, and known reaction stoichiometry; it should also be reasonably soluble for effective titration reactions.
Step-by-step explanation:
The characteristics necessary for a primary standard include: high purity, stability, non-hygroscopic property, being in solid state for ease of weighing, and having a known reaction stoichiometry. It does not necessarily have to be polyprotic, its reaction with the desired acid does not have to be exothermic, nor does it need to have a low melting point. However, it must be reasonably soluble to ensure that it will react completely during the titration process.