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PHOSPHATE ANALYSIS
ONE WEEK LATER

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The phosphomolybdate solution you measure in this analysis is a clean bright yellow solution which strongly absorbs blue light (380 nm).

Curiously, after a few days, some of these solutions turn  greenish and then a deep dark blue.  The tubes on the left reflect these changes. 

Q. Do all of them turn blue?

A. No.  Only the ones containing regular soda (not diet).

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The PARTIAL SOLUTION

The yellow  phosphomolybdate ion has the formula PO₄(MoO₃)₁₂^-3

The molybdenum is in the +6 oxidation state.  The blue color is caused by the reduction of some of the molybdnum to the +5 or +4 oxidation state.

Figure out what is happening and e-mail your answer to me  kluiber@genchem.rutgers.edu