Design, Build, Test, Iterate

DIY PCB Etching: Colors of Compounds and Helpful Links

UPDATE 11/10/11: Although most of these colors are accurate when dealing with concentrated solutions of salt, some are not (e.g., descriptions of copper (I) and (II) chlorides are mixed up). Take it with a pinch of… salt.

Yellow? Copper hydroxide is supposed to be green!

If you have already been at this for hours and days (or even weeks) and are simply frustrated and confused because you don’t know what all those brown and green solutions are and why there are yellow and black flakes everywhere, here’s a list to save you some time:

  • Iron (II) chloride (aq): yellow/green solution in water.
  • Iron (III) chloride (aq): brown solution.
  • Copper (I) chloride (aq): nearly opaque, brown solution in concentrated HCl or other solutions like a saturated salt solution with suitable donor molecules.
  • Copper (II) chloride (aq): aqueous solutions can range in color (yellow, green, blue) due to various copper (II) complexes depending on concentration, temperature, and the presence of extra chloride ions.
Solids and Precipitates
  • Iron (II) chloride (s): off-white anhydrous, green tetrahydrate.
  • Iron (III) chloride (s): dark green by reflected light, purple-red by transmitted light.
  • Iron (II) hydroxide (s): dark green/grayish gunk, almost insoluble in water.
  • Copper (I) chloride (s): white.
  • Copper (II) chloride (s): light brown anhydrous, slowly absorbs moisture to form blue-green dihydrate.
  • Copper (II) hydroxide (s): blue-green gelatinous solid.

Copper (II) chloride also reacts with extra chloride ions to form complex ions:

  • CuCl3 (aq): red.
  • CuCl42- (aq): yellow.

Other notes:

In my electrolysis setup, the main source of resistance in the system was fairly consistently the salt bridges. Bigger salt bridge = faster electrolysis. Even with layers upon layers of napkins, the bridge got hot enough to melt plastic.

Electrolysis will produce the lower chloride (i.e., copper (I), not (II), chloride) because the copper (II) chloride and copper comproportionate to form copper (I) chloride. This solution of cuprous chloride must then be chlorinated in order for cupric chloride to be formed.

Helpful resources:

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  1. […] I then tried it on the entire batch, but even at 7 amps of current (high enough to heat up the solution and start melting the bottle), it produced no immediate results and only produced a lot of copper precipitate that soon dissolved away. When I tried using a salt bridge, I contaminated the negative cell and the sodium hydroxide within with copper ions, which didn’t make me any happier. At least now I can tell different compounds apart. […]

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