In this post we’re going to make a silver testing solution to determine the authenticity of silver metal.This is the same solution sold commercially and online.We’re going to reproduce it here as well as explore the chemistry.
Warning: Nitric acid is corrosive, while dichromates and chromates are carcinogenic. Wear gloves when handling them. Greetings fellow nerds.
First we start 0. 5g of a dichromate or chromate salt. You can use just about any salt including potassium dichromate, ammonium dichromate, or sodium chromates. They’ll all work. To this we add 10 mL of deionized or distilled water. Now give it a shake. To this solution we add 7 mL of concentrated 70% nitric acid. Now cap it up and shake it until all the chemicals dissolve. We want a solution of dichromate ions but we can use chromates because under the highly acidic conditions the chromate ions convert into dichromate ions. And there is our test solution. So let’s give it a try.
This here is an authentic piece of silver. At least I’m pretty sure it is. To test it all we do is apply a drop of the testing solution to the surface. The reaction takes a minute or two so you have to be patient. If this is authentic silver then what happens is the nitric acid in the solution dissolves a small amount of silver to form silver nitrate. This silver nitrate reacts with the dichromate ions to form insoluble red silver dichromate particles. This is a very strong indication we have authentic silver. We can wipe it off and you see the particles come off on the tissue.
Okay let’s do this again but close up so you can see it better. There we go, now we wait. A drawback of this test is that it because it dissolves a small amount of silver it damages the otherwise pristine finish of the silver object. So you should run this test on an inconspicuous place on the object or use a scratching stone and scratch off a small quantity of silver from the edge.
Usually the edge is less noticeable than a face. The test solution can be applied to the scratch marks to get the same result. Okay now you can see the red silver dichromate crystals precipitating from the solution. Let me wipe it. You can see the crystals on the tissue as well as unsightly damage to the silver surface. Okay so that is how it works with silver metal. Let’s try some non silver metals and see how they compare. First let’s try aluminum foil. As you can see the aluminum foil has no visible change. Also You Can try a good metal detector to detect fake silver.
Aluminum metal is what somewhat resistant to nitric acid due to an effect called passivation. Any aluminum that does dissolve forms aluminum nitrate which is clear and doesn’t affect the color of the dichromate very much. Okay let’s try zinc. Interesting, zinc metal is reacting very quickly and vigorously with the nitric acid to form zinc nitrate. This in turn reacts with the dichromate ions to form yellowish green zinc chromate. Zinc chromate was actually used a pigment before it was discovered that it was highly carcinogenic and killing people with lung cancer from the dust. Now let’s try some steel from this solvent container.
In case you’re wondering, the silver testing solution is very stable as long as you don’t let it evaporate and should last indefinitely if stored properly. So feel free to make up a batch and keep it handy for silver testing. As for the iron, you can see we’re getting more of a yellow brown spot. No hint of red like in silver. Let me do this close up so you can see it better. What we’re getting is a mixture of iron and chromium nitrates. We’re also getting nitrates of whatever other metals are in the steel.
Now if you are making or buying silver test solution to test for silver. I recommend trying a whole bunch of metals as well as known pure samples of silver so you can get used to the subtle color differences. At first glance an inexperienced eye might not be able to tell a real silver spot from a steel spot. Additionally use other tests like magnetism and density to get further evidence of a metal’s identity. Okay let me try another precious metal, platinum. Now no moron in the universe will sell you platinum as fake silver, but I’m going to run the test anyway. Platinum metal is very inert as i’ve proven in my chemical resistance of platinum video. And even this solution of nitric acid and dichromate shouldn’t be able to affect it. At least i hope not. I bought this coin for this video and I intend to return it when i’m done. If it comes back damaged i might be on the hook for the value of the coin. Okay it’s been about five minutes and it still looks the same.
Let me wipe it. Looks like absolutely no effect, platinum is totally immune to silver testing solution. Well that’s a big relief. Okay so that was how you make silver testing solution from nitric acid and a dichromate or a chromate salt.