View Full Version : Citric acid-based ZINC test

05-14-2010, 12:18 AM
Here's how to use a citric acid + table salt solution as a test for zinc and zinc alloys. This "zinc test" is easy and safe to perform. It avoids use and storage of much more dangerous or concentrated acids.

Citric acid is fairly safe to handle, but do not rub the powder onto skin or pour the liquid on your hands. Take care to not splash the powder or this solution in your eyes. If you do, just rinse with lukewarm water. This testing solution produces a weak hydrochloric (aka muriatic) acid solution. Use of common sense is implicit and mandatory when mixing and using any chemicals. Use eye protection when working with acids.

OK, now that we're done with the safety lecture, we can move along. Here are the necessary testing materials and items:
Hot water
Citric acid powder
Table salt
Small glass mixing container
Measuring spoons
Small plastic spoon
Rubbing alcohol
Baking soda
Magnifying glass (optional, but may really help older eyes)
Your metal sample for testing

In your glass container, mix 2 tsp(teaspoons) hot tapwater with 1/4tsp citric acid powder, and 1/4tsp table salt. Stir or gently swirl the mix until the powder and salt dissolve. The salt and citric acid will react to produce a small amount of hydrochloric acid. That's what does the work in this test.

Prepare the metal sample surface for testing by cleaning with rubbing alcohol and allow the metal to dry. Using the plastic spoon, place 1 drop of testing solution on your metal test sample. Allow to sit undisturbed for 5 minutes.

In my tests, the zinc samples reacted with the test solution almost immediately to evolve some tiny, silvery bubbles. No huge foaming reaction-- just tiny bubbles that were easily seen with the naked eye. A magnifying glass will help older eyes, and will tell you 'no-question' if you have bubbles or not. The solution also very slightly etched the zinc surface after 10 minutes has elapsed (that's when I removed the solution). Pewter did not produce bubbles.

Here is the full list of metals tested:
Clip-on ww ingots: No visible reaction.
Sn40/Pb60 solder: No visible reaction.
Sn96/Ag4 solder: No visible reaction.
Sn94/Ag6 solder: No visible reaction.
Yardsale silverplate tableware: No visible reaction. Did not remove tarnish.
Polished pewter: Very slightly etched the polished surface. No bubbles.
Unpolished pewter: Left a very weak grayish stain. No bubbles.
Cu70/Zn30 brass: Removes tarnish almost immediately.
6061 aluminum alloy: Solution turned slightly dark and left a weak gray stain. No bubbles.
Zamac alloy: Produced many tiny bubbles, and slightly etched the surface.
Pure anode zinc: Solution produced silvery bubbles and slight etching.
Pure copper: Removes tarnish almost instantly.
Pure tin: No visible reaction.
Pure antimony: Tarnish removed from surface of metal. No bubbles, no etching, and no other visible reaction.

Once done with the testing, the acid solution can be completely neutralized with a 1/2 tsp of baking soda. It will fizz and fuss a lot.

This test easily discriminated between zamac and pewter. It should be of use to those who are scouring yardsales for pewter, or who have "mystery" metal. Be advised that there are many alloys out there that may be confused with pewter. Some may react differently to this test, but it apears that common zinc alloys may be easily distinguished by the formation of bubbles. The hydrochloric acid reacts with the zinc in the sample to produce zinc chloride and tiny bubbles of hydrogen.

The photo below shows a small ingot of zinc with a drop of the testing solution in the center. It's not an easy photo to take, but the tiny silvery bubbles should be visible. They are easily visible up-close during the testing.

Hope this helps, best of luck.