Why is a solution of naoh which was standardized yesterday unsuitable to be used as a standard solution today?

It is essential that the standard solution, in this case NaOH, must be prepared daily because a standard has to have a high percentage purity. Aside from high purity, standards should also have accurate measurements. If we will use the same standard solution, there is a great tendency for the solution to get contaminated. In turn, readings using this solution will no longer be accurate.