Detection of Nickel Cations in Coins

The chelation reaction of nickel ions with the organic bidentate ligand dimethylglyoxime (DMG)¹ in an alkaline ammonia medium producing nickel dimethylglyoxime, Ni(DMG)₂, a red cherry or raspberry colour precipitate has been known since 1905 when it was discovered by Russian chemist Lev Aleksandrovich Chugaev (see figure 1). It was the first organic spot test reagent used to detect a metal ion and as a result DMG is known as Chugaev’s reagent.² 

The balanced ionic equation for this reaction:   

Ni²⁺_(aq) + 2C₄H₈N₂O_{2 (aq)}→ Ni(C₄H₇N₂O₂)_{2 (s)} + 2H⁺_(aq)

Figure 1: Structural equation from The Gravimetric determination of Nickel, Truman State University CHEM 222 Lab Manual³

This reaction is still very much in use today for the detection of nickel metal ions due to its striking colour formation. Both qualitative and gravimetric determinations of nickel are part of many chemistry courses. The reaction involves two dimethylglyoxime molecules acting as chelating agents to form the nickel dymethylglyoxime square planar complex. This reaction is very sensitive and can be used as a confirmation test for the presence of nickel II cations even in very low concentrations.

The same procedure can be easily applied to metal objects that regularly come in contact with the skin, for example coins, jewelry, earrings, spectacle frames, watch straps, etc. for the benefit of people that suffer from a kind of skin sensitivity or dermatitis called Nickel itch.

This contact allergy or allergic contact dermatitis (ACD) usually flares up when sweating. A red itchy rash develops on the skin that has been in contact with the metal. It is believed that the acids present in sweat dissolve a little of the nickel and it is the release of nickel ions that are responsible for causing nickel sensitization and ACD⁴ that can accentuate conditions such as skin erythema, eczema, etc. Only a certain amount of released ions will cause a reaction. For example, alloys such as many stainless steels contain nickel but do not release sufficient amounts of nickel ions to cause someone to become nickel sensitized or have nickel ACD reactions if they are already sensitive to nickel. In contrast, nickel plated jewellery will release vast amounts of nickel ions in contact with sweat from the skin and from everyday brushing, friction, abrasion, etc. especially when worn for extended periods of time.

It is not known why people develop allergies to nickel and there is no cure. The best course of action to prevent an allergic reaction is to avoid contact with products containing nickel and in this post we will to show how to make a cheap and quick nickel detection device at home or in the lab based on the reaction described above.

A video describing how to test for the presence of nickel in various coins using such a device can be watched below (no sound).⁵

The video shows a few drops of DMG, C₄H₆(NOH)₂, being added to a dilute solution of nickel II sulfate with a few drops of ammonia solution. Immediately, an insoluble bright red solid called nickel dimethylglyoxime, Ni(C₄H₈N₂O₂)₂, precipitates out of solution. Shortly after that, the cotton bud or Q-tip containing DMG and ammonia can be seen being rubbed against the surface of a coin. If nickel is present in the coin then the Q-tip turns red.

Even though commercial products exist to test metals for the presence of nickel⁶ a much simpler and cheaper version can be made at home or in the lab with readily available household ingredients.⁷ First, obtain some cotton buds or Q-tips and dip them in a solution of 1% dimethylglyoxime (DMG) in ethanol (vodka will work just as well) for 10 minutes. NOTE: DMG is a flammable irritant and can be difficult to obtain but a very small amount (1 g approx.) is needed to make a large stock of prepared buds. Once the buds have been soaking for at least 10 minutes, remove them and leave them to air dry. Once dry, they are ready to be used or can be stored in an air tight container labelled appropriately.

To test for the presence of nickel in the desired metal object add a few drops of dilute ammonia solution (household ammonia will work depending on concentration, higher concentrations produce faster results) to just wet the tip of the cotton. Clean the article with a moist tissue first and rub the tip on the coin or object for about a minute and observe if any pink-red colour appears. The intensity of the colour will depend on how much nickel is present or if it is freely available on the surface of the object (dark pink-red if made completely out of nickel or nickel plated) or light pink if alloyed with other metals. Interestingly, even when bound tightly in an alloy, the test will detect at least some of the nickel cations present. Coins made out of copper or zinc will show no red colour being formed but iron and steel coins/objects might produce a false positive as Fe²⁺ and Fe³⁺ ions interferes with the detection of Ni²⁺ using DMG because it forms a red-brown coloured iron hydroxide complex in alkaline conditions. To avoid this situation, a few drops of citric acid (lemon juice) can be added to convert the iron complex to a water-soluble colourless complex.⁸

For an excellent infographic on the composition of UK coins and to use as a comparison please see Compound Interest’s The Metals in UK Coins.⁹ Andy Brunning also created a similar poster for C&EN Periodic graphics: the compositions of U.S. coins.¹⁰

Since the majority of nickel on Earth is present in the molten iron core of the planet, the percentage mined each year comes mainly from outer space as iron/nickel meteorites from millions of years ago. Metallic meteorites have a nickel content ranging from 5 to 35% so it is possible to differentiate meteorites from other rocks and manmade iron samples also using this simple reaction.⁸

For an in depth analysis a control could be obtained in the form of a coin that is known to have no nickel metal at all. Secondly, a standard nickel II sulfate solution could be prepared to compare the coloured test samples visually followed by colorimetry analysis to determine the concentration of nickel in each sample.

To conclude the procedure shows an easy, convenient and cheap way for people suffering from skin irritation to limit and/or eliminate the risks of exposure to sources of nickel metal using some interesting classic chemistry.

EXTENSION

crystal_of_ni_compound.png

Figure 2: Needle-shaped crystals of Ni(DMG)₂ under 40x magnification

A drop of water containing Ni(DMG)₂ precipitate was placed on a microscope slide and examined under the microscope (see figure 2). Red needle-like crystals of the complex were observed. In order to grow larger crystals the precipitate was dissolved in hot dilute hydrochloric acid and left to evaporate slowly. Long brown needle-shaped crystals appeared after a day. When the crystals were crushed using a glass rod the familiar characteristic red cherry colour was observed (see figure 3).

crushed_ni_compound.png

Figure 3: Crushed crystals exhibiting the characteristic brilliant red colour of Ni(DMG)₂

Aknowledgements: I would like to thank Deanna Cullen and Tom Kuntzleman for reading a previous draft version of this post providing advice, suggestions and writing constructive comments to improve it. Their experience and support is invaluable to someone like me who does not post or write scientific articles regularly.

References

1. Dimethylglyoxime (DMG) Safety Data Sheet, created by Global Safety Management Ink. (accessed 5/5/19)
2. Kauffman, G.B., Platinum Metal Rev., 1973, 17, (4).
3. The Gravimetric determination of Nickel, Truman State University CHEM 222 Lab Manual (accessed 5/5/19)
4. Quick and easy detection of nickel metal ions in coins, andycapo123 YouTube Channel, Published 4/25/19 (accessed 5/5/19)
5. Fact Sheet 1: Nickel Allergic contact dermatitis, The Nickel Institute. June 2016. (accessed 5/6/19)
6. The kit can be bought on Amazon: https://www.amazon.co.uk/gp/product/B00EWUS86C/ref=ox_sc_act_title_6?smi... or as MQuant Nickel test strips sold by Merck
7. DMG can be purchased from chemical suppliers but the author bought 10 g of DMG 99% from Ebay costing £6.50 (approx. $8.50). Enough to make hundreds of testing cotton buds.
8. Zamora, L.L., Zamora, S.L., Romero, M., Identifying extraterrestrial materials, Education in Chemistry magazine, March 9, 2015.
9. The Metals in UK Coins, Andy Brunning, Compound Interest, March 27, 2014. (accessed 5/6/19)
10. Periodic graphics: the compositions of U.S. coins, Andy Brunning, Chemical and Engineering News, Volume 94 Issue 28, July 11, 2016. (accessed 5/6/19)

For Further Information:

Brandl, Herbert, Trickkiste Chemie, Bayerischer Schulbuch Verlag, München, 1998.

Emsley, John, Nature’s Building Blocks An A-Z guide to the elements, Oxford University Press, 2003.