1 Principle
Dissolve the sample with oxidizing acid; at nitric acid acidity of 2.2mol/L, add ammonium molybdate to generate ammonium phosphomolybdate deposition; after filtering, dissolve with excessive standard sodium hydrate solution; taking dihydroxyphthalophenone solution as indicator, back-titrate the excessive sodium hydrate with dihydroxyphthalophenone solution till the pink just disappears (the end point) (about pH8).
The general reaction formula of dissolving ammonium molybdate precipitation with sodium hydrate is as:
(NH4)2H(PMo12O40)•H2O+240H-=HPO +12MoO +2NH +13H2O
Arsenic of less than 100μg, tantalum <500μg, zirconium, vanadium or niobium <1mg, wolfram <8mg, titanium < 10mg and silicon < 20mg contained in the test solution, may not interfere the determination; exceeding the limits above, remove arsenic by hydrochloric acid and hydrobromic acid; mask zirconium, niobium, tantalum, titanium and silicon by fluohydric acid; reduce vanadium by hydroxylamine hydrochloride; remove wolfram (in ammonia solution at presence of EDTA) by beryllium carrier.
2 Reagents
