Why in the calculation of related substances we don't consider the moisture content of standard used? In other words, why don't we calculate the related substances on anhydrous basis?

Several reasons:

- It's a lot easier to simply use area% at the absorbance maximum of the compound; assuming that all related substances have the same chromophore as the parent, what you then get is the impurities as a percentage of the parent.

- Generally the related substances are there at low level, with a commensurately poor (relative) precision. In most cases, the number will be the same within experimental error whether or not you fudge for the water content.

- Not all compounds are hydrates, so the definition based on area percent is more general

- Finally, they had to define it *some* way.

If what you're trying to do is to make a borderline failure into a pass by fudging for water content, I suspect you're outta luck.