Question:  What are the criteria for a good primary standard?

Answer:

A primary standard is a solution which can be used to determine the concentrations of other solutions to an extremely high accuracy.  They're different than other standard solutions in that they're more stable to decomposition than other solutions and can be stored for a very long period of time without changing their concentrations.

Generally, a good primary standard solution should maintain 100.00 +/- 0.05% purity for periods of several years.  They should be not only stable to decomposition but also stable with regard to weight changes associated with gaining or losing water.  Primary standards generally have high molar masses, and are relatively inexpensive (though this requirement is for economic reasons rather than chemical reasons)

Primary standard solutions are rarely used directly to find the concentrations of unknown solutions.  Rather, they're used to determine the concentrations of other standard solutions to very high levels of accuracy.  These secondary standard solutions should have the following properties to be most useful:

• They should either contain a primary standard or be standardized with a primary standard.
• They should be stable for reasonable periods of time.
• They should react quickly with the solutions you intend to analyze.
• They should react in definite and unchanging proportions with the solutions you're analyzing.
• They should react completely with the solutions you're analyzing - the equilibrium should lie very far to the right.
• They should be specific for the solution you're analyzing to minimize interference from other contaminants.
• There should be some clear endpoint to tell you when the equivalence point has been reached.
• The standard should be fairly soluble, typically around 0.1 M.