A
primary standard is a substance which can be weighed out and made into a solution of exact, known concentration. The solution can then be used in a
titration to find, accurately, the concentration of another solution. So it must be available in a reliably pure state. It will not, ideally, contain water of crystallisation because this invariably throws doubt on its absolute composition; it might absorb or release unknown amounts of this water. It should not be affected by exposure to air, meaning it should not react with oxygen, carbon dioxide or water vapour.
In practice, very few substances make ideal primary standards and compromises have to be made.
Sodium hydroxide is not suitable because it rapidly picks up atmospheric carbon dioxide and is likely to have done so before it is taken out the bottle.
Now we have a good idea about satandard solutions, so.......!emmmmm 
What is the difference between primary standard and standard solution?
A primary standard in chemistry is a reliable, readily quantified substance. Features of a primary standard include:
1. High purity
2. Stability (low reactivity)
3. Low hygroscopicity and efflorescence
4. High solubility (if used in titration)
5. High equivalent weight
Standard Solution :
Standard solution is a chemical term which describes a solution of known concentration. The concentration of the solution is normally expressed in units of moles per Litre (mol/L, often abbreviated to M for molarity). This unit, which stands for the moles of solute per litres of solution and
The reason am sharing with you this information in English because am intersted in it and I think that we all should have intrests in English as it's the main language in the world, however the recent studies and researches are taught in this langauge
برنامج امتحان كليّة العلوم .. الدورة الصيفيّة .. لكافة السنوات والاختصاصات 
primary and standard solutions 
انتهى 
ســــــــــــلام
