Determination of phosphorus in gold or silver brazing alloys
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THE
d e t e r m i n a t i o n of phosphorus concentrations at m i c r o g r a m levels in p r e c i o u s metal b r a z e m a t e r i a l s is n o r m a l l y done by solid m a s s s p e c t r o m e t r y . In s e v e r a l g o l d - n i c k e l b r a z e m a t e r i a l s used at the G e n e r a l E l e c t r i c Neutron D e v i c e s D e p a r t m e n t (GEND), the nominal composition is about 81 pct gold and 19 pct nickel with a maximum phosphorus concentration of 0.002 pct. The high nickel concentration produces a double ionized nickel s p e c t r a l i n t e r f e r e n c e with the solid m a s s s p e c t r o g r a p h i c method; and thus, an a l t e r n a t e method was needed. Extensive r e s e a r c h failed to r e v e a l a r e l i a b l e method for d e t e r m i n i n g low phosphorus l e v e l s in high nickel containing gold a l l o y s . A method used for the a n a l y s i s of steel ASTM E30-731 had been modified for use in the past, but the p r o c e d u r e was v e r y tedious, time consuming, and lacked r e l i a b i l i t y . Another m a j o r d i s a d vantage was that a 5 g minimum s a m p l e of the p r e c i o u s m e t a l was r e q u i r e d for a single a n a l y s i s . Inquiry of the b r a z e alloy vendor r e v e a l e d they a l s o use a modified p r o c e d u r e s i m i l a r to the ASTM s t e e l method. The s p e c t r o p h o t o m e t r i c method devised to give more a c c u r a t e r e s u l t s is based on the s e p a r a t i o n of phosphorus f r o m i n t e r f e r i n g s p e c i e s by ion exchange c h r o matography and the e s t i m a t i o n of phosphorus by forming a yellow colored molybdovanadophosphoric acid, which is c o m p a r e d to s t a n d a r d s . EXPERIMENTAL Reagents The following r e a g e n t s a r e used in this method. S t a n d a r d Phosphorus Solution. Dissolve 0.4395 g of p o t a s s i u m dihydrogen phosphate (dried at l l0~ for 4 h) in deionized water and dilute to 1 1. Store in a polyethylene bottle. One m i l l i l i t e r of this solution conrains 100 g of phosphorus. R. J. ANTEPENKO is Chemist, General Electric Company, Neutron DevicesDepartment, St. Petersburg,FL 33733. Manuscript submitted May 7, 1979. METALLURGICALTRANSACTIONS A
Working. Phosphorus Solution. T r a n s f e r a 10.0 ml aliquot of the standard phosphorus solution to a 100 ml v o l u m e t r i c flask and dilute to volume with deionized w a t e r . One m i l l i l i t e r of this solution contains 10 ~tg of phosphorus. Vanadat e Reagent.. D i s s o l v e 2.5 g of reagent grade ammonium vanadate (NH4VO3) in 500 ml of boiling w~ter. Cool the solution, add 20 ml of concentrated n i t r i c acid and dilute to 1 1. Store in a polypropylene bottle. Molybdate Reagent. D i s s o l v e 50 g of reagent g r a d e ammonium molybdate (NH4)6Mo7Oz4 "4H20 in 1 1 of w a r m deionized w a t e r . Cool and s t o r e in a polyproplylene bottle. Ion Exchange R e s i n s . P r e t r e a t a l l r e s i n s for the r e moval of fine m a t e r i a l s . P l a c e the r e s i n in a l a r g e r b
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