Nitric acid assay

Why we Analysis of Nitric acid (HNO3) Assay? plese suggest me.

To check its purity.

I have analyze the assay of Nitric acid against with 1N NaoH and Faoud the Result -77.61% but AR Grade Nitric acid have 69 to 71% purity. so this way is corect or Not???
pocedure-1.0g of sample+50mlwater +2-3 drop of methyle red Indicator and Titrate with 1NaoH

=Consumed volumemolarity of 1N NaoH6.3

Weight of sample