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2015-11-12T21:49:17+05:30

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Primary standard solutions should be such that their concentration values should be known exactly.  They should not attract gases or water vapour from air and react with them.

They should remain in the same concentration as they are made.  They should not evaporate and emit gases into air.

HCl, Na OH do not satisfy these conditions.  Sodium hydroxide attracts water vapour from air.    Hydrogen chloride turns into gas and its concentration changes over a period, when the bottle is opened and closed frequently.

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