# Yellow precipitate in the presence of ammonium molybdate?

We had analytical chemistry experiments to determine acidic radicals from a mixture of two salts. After the chloroform-chlorine water test for distinction between halides $(\ce{salt\ solution + H2SO4 + CHCl3 + Cl2_{(aq)}})$ when I emptied the test-tube in the sink, all of it suddenly turned yellow.

What reaction could have occurred to cause this? The sink mostly contained diluted $\ce{H2SO4}$ and $\ce{HNO3}$ along with small quantities of $\ce{AgNO3, BaCl2, NH4OH}$, lead acetate and ammonium molybdate.

All of these would mostly be in small quantities since I had rinsed the sink with water before emptying the test-tube.

• Off-topic, but important: why do you poor chemicals like these in the sink and not in the appropriate chemical waste container?! – Michiel Sep 24 '13 at 16:40
• We don't have an "appropriate chemical waste container". Though I hope all the sink outlets are connected to a separate tank. – user80551 Sep 24 '13 at 16:42
• @Michiel Safety and environmental concern is never off-topic. – Martin - マーチン May 21 '14 at 5:35
• @Martin That's why I still mentioned it, but with the 'disclaimer' – Michiel May 22 '14 at 5:12

[...] diluted $\ce{H2SO4}$ and $\ce{HNO3}$ [...] $\ce{NH4OH}$ [...] and ammonium molybdate.
$\ce{H2PO4^- + 22 H+ + 12 MoO4^2- +3 NH4+ -> \underset{yellow}{(NH4)3[P(Mo3O10)4]} +12H2O}$
$\ce{PbMoO4}$ has solubility 0.00001161 g/ 100 g and is yellow. I believe, that with diluting acidity of the solution fell down, leading to significant deprotonation of $\ce{H2MoO4}$ and subsequent sedimentation of $\ce{PbMoO4}$.