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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.

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    $\begingroup$ Off-topic, but important: why do you poor chemicals like these in the sink and not in the appropriate chemical waste container?! $\endgroup$ – Michiel Sep 24 '13 at 16:40
  • $\begingroup$ We don't have an "appropriate chemical waste container". Though I hope all the sink outlets are connected to a separate tank. $\endgroup$ – user80551 Sep 24 '13 at 16:42
  • $\begingroup$ @Michiel Safety and environmental concern is never off-topic. $\endgroup$ – Martin - マーチン♦ May 21 '14 at 5:35
  • $\begingroup$ @Martin That's why I still mentioned it, but with the 'disclaimer' $\endgroup$ – Michiel May 22 '14 at 5:12
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[...] diluted $\ce{H2SO4}$ and $\ce{HNO3}$ [...] $\ce{NH4OH}$ [...] and ammonium molybdate.

The yellow precipitate indicates that phosphate was present too. Under these conditions a yellow ammonium molybdatophosphate is formed:

$\ce{H2PO4^- + 22 H+ + 12 MoO4^2- +3 NH4+ -> \underset{yellow}{(NH4)3[P(Mo3O10)4]} +12H2O}$

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$\ce{PbMoO4}$ 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 $\ce{PbMoO4}$.

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Wulfenite is a lead molybdate mineral with the formula PbMoO4. It can be most often found as thin tabular crystals with a bright orange-red to yellow-orange color, sometimes brown, although the color can be highly variable. In its yellow form it is sometimes called "yellow lead ore".

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  • $\begingroup$ Permeakra already wrote about that, years ago... $\endgroup$ – Mithoron Mar 10 '18 at 23:31

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