The chromate ion is kinda fun. Maddeningly yellow or orange, it has a strong color. It is a reasonable oxidizer. Many of its salts are insoluble, allowing corrosion-resistant coatings (such as the golden-iridescent zinc dichromate finish on many steel parts) and strong pigments (lead chromate a prime example). And to top it all off, it's a known carcinogen, so it's biologically exciting too!
Too bad it's not something found at the hardware store. I could probably find it, but screw that, I like making things. So how to do it? Well, you need a strong oxidizer to start with, which isn't much of a problem since we're all breathing an excellent one. Oxygen doesn't do chemistry very well though. Hypochlorite is a more accessible one, and works quite nicely. What of the chrome in the first place? There's about 10-20% in any random chunk of stainless steel, but bleh, what a mess to seperate. Can always go grab some Cr2O3 at the pottery store. Happens that I have some on hand from that source. But uh, thing is, this shit is refractory. It doesn't dissolve worth jack. I need something a lot more aggressive. How about a bath of molten hydroxide? MMMMM that might do it!
My chosen reaction is: Cr2O3 + 4 NaOH + KClO3 + 3 KCl = 2 K2CrO4 + 4 NaCl + 2 H2O. The atomic weights are 152 + 122.5 + 223.5 + 160 = 388 + 234 + 36, respectively. I chose to use this combination because I have NaOH and not KOH, and KClO3 or NaClO3 but no KNO3. I chose KClO3, and added extra KCl since I want K2CrO4 to come out of solution, either after fusing or after recrystallizing from water. The mixture needs to be fractionally crystallized (salt will crystallize first), or modified for dichromate instead (potassium dichromate's solubility is on par with potassium chlorate).
Here are the weighed chemicals (give or take 0.1 g) laid out on a sheet of paper. 14.9 g Cr2O3, 15.1 g KClO3, 20.0 g KCl and 17.5 g NaOH. The Cr2O3 is greener in person, I just didn't get a great picture of it.
All the chemicals, mixed and waiting in the crucible. This is one application of my still-in-development induction heater. I can't get much power into this relatively large crucible, but it's enough to melt this mixture.
These three pictures show the green slop. As the temperature rises, first the NaOH melts and the chrome disperses, making a thickly green liquid. Then things start dissolving. For instance, sodium chromite may be formed: Cr2O3 + 2 NaOH = Na2Cr2O4 + H2O (or other stoichiometries). As the reaction progresses, steam is liberated, which must be controlled carefully. The reaction appears to be slightly exothermic, so I started this fusion with no kaowool surrounding the crucible. I controlled the temperature carefully (with the heater's voltage control) and stirred constantly to make sure it didn't boil over. In the middle picture, the mixture is bubbling smoothly (but not to be left unattended without stirring!). After about ten minutes at the same temperature, the steam is mostly gone and it begins to thicken. The rightmost picture shows the globby product at this point.
Once it thickens, the steam is just about done and stirring only needs to be done to keep the mixture evenly heated. Since it's not in danger of boiling over, I cranked the heat. Pretty soon, it spontaneously starts turning orange, probably a more exothermic reaction (especially with a chlorate oxidizer). In the left picture you see what looks like orange glow, but is actually reflected light from the chromate that is forming. In the middle picture, the reaction is essentially complete, a few minutes after the mixture thickened. Throughout the oxidation, the walls don't heat up much (being solid steel), so I heat it to the melting point to homogenize the reactants. (I put some kaowool back in around the crucible to help it heat up.) The remarkable thing is, chromate is thermochromic: it melts to a deep red, dense liquid. The color is almost blood red, but blood is too bright; this is much darker. I can tell it is dense because the coarsely ground potassium chloride I added floats on top, until it dissolves at a somewhat higher temperature (maybe 1000°F, around the threshold of visible incandescence, which has about the same color as the reflected light from this liquid!).
At that peak temperature, the liquid is quite mobile (not as mobile as NaCl fortunately) and pours easily.
Some of the cooled product. It breaks up (due to the stress, loudly!) if left alone while cooling. The most dramatic thing to be seen in these two pictures is the color change, back from deep red, to orange, to a canary yellow with almost a tint of green.
What remains? I will dissolve this in water and recrystallize it. I may add a little HCl to form the deep orange dichromate instead. The optimal reaction for dichromate, using these reagents, uses only one mole of KCl, since the final product has the formula K2Cr2O7. No need for excess potassium ion.