Detection of Nickel Cations in Coins

still image from video of a reaction taking place between Ni ion and DMG

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) + 2C4H8N2O2 (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 .Andy Brunning also created a similar poster for C&EN .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

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).


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. , created by Global Safety Management Ink. (accessed 5/5/19)
  2. Kauffman, G.B., Platinum Metal Rev., 1973, 17, (4).
  3. , Truman State University CHEM 222 Lab Manual (accessed 5/5/19)
  4. , andycapo123 YouTube Channel, Published 4/25/19 (accessed 5/5/19)
  5. , 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.,, Education in Chemistry magazine, March 9, 2015.
  9. , Andy Brunning, Compound Interest, March 27, 2014. (accessed 5/6/19)
  10. , 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.

Concepts: 

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

NGSS

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.

Summary:

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.

Assessment Boundary:
Clarification:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Summary:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Assessment Boundary:
Clarification:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

Join the conversation.

Comments 4

Pasha Veres | Fri, 10/25/2019 - 23:31

I tried dissolving 1g of DMG in vodka (водка) and it looks like little, if any, actually went into solution. Even after 24 hours, at room temp. Is there another way? Thank you. (Admittedly I used the cheapest vodka they had; didn't spring for столичная or anything. ;) )

My intent is to determine if a substance reacted with HCl contains any Ni (pass/fail)? I already know the sln contains ferrous or ferric chloride, does it also contain any Ni? Additionally it would be good to know if it's a little or a lot.

Andres Tretiakov's picture
Andres Tretiakov | Tue, 10/29/2019 - 08:05

Hi Pasha,

Thank you for trying out the procedure. I'll try to answer your questions as best as I can.

It doesn't matter if the DMG does not fully dissolved because the cotton buds or Q-tips will have absorbed most of the DMG during the soaking time and the solvent evaporates anyway over time. The quality of vodka does not matter only the amount of alcohol in it. The higher the percentage of ethanol the more DMG will dissolve, but as I said the important thing is to transfer as much of the DMG to the cotton head. These will later be covered with a few drops of dilute ammonia solution.

I'm not sure if if I understand your question. But yes, if you dissolve an unknown solid in dilute HCl and it turns the alkaline cotton bud cherry red then yes you have nickel ions in that solution. How much? I don't know because that's simply a qualitative test. In order to know how much or little (trace amounts in the ppm range) of it you have (quantitative test) you could try gravimetry analysis, spectrophotometric techniques such as atomic absorption/emission, ICP-AES, spectroscopic analysis such as UV-Vis and colorimetry detection to determine the concentration of nickel ions using a standard nickel chloride solution. The iron ions (ferrous and/or ferric) might interfere (mask your results) and produce a false positive but as I said in the article those are produced in alkaline conditions only when iron hydroxides precipitate. Try adding some citric acid to dissolve the hydroxides produced and then if you observe the characteristic cherry red then you know you also have nickel present.

Hope this helps,

Andres

 

Pasha Veres | Thu, 10/31/2019 - 03:09

Thank you so much, Andres. I appreciate your getting back to me.

I went ahead and bought EVERCLEAR 190 proof (azeotropic?); the DMG dissolved readily in all that alcohol. Since I don't drink, and had to buy a 750ml bottle, it'll be a lifetime supply. lol

Does it matter that it be a 1% sln DMG? Can I increase it to 2%? 5%? More? 

Is the intensity of the red any indication of the amount of Ni? 

Likely gravimetric analysis is the only option available to me. I really want to know only gross amounts of Ni; is it just a few molecules? Molar amounts? Maybe like orders-of-magnitude as opposed to any specific amount.

I'm still deciphering the final instructions regarding the hydroxides and citric acid, fortunately I have plenty of citric acid.

Thank you so much for your help.

Aloha,

Pasha

 

Andres Tretiakov's picture
Andres Tretiakov | Thu, 10/31/2019 - 10:13

Hi Pasha,

Thanks again for more questions. I'm glad you have all these questions but I don't have all the answers. All I can do is guide you and encourage you to search more in depth or detailed procedures and explanations in a literature search. I think this will be of immense help.

Regarding your questions:

No, I don't think it matters if you increase the concentration of DMG since it's already in excess assuming you want to detect trace amounts of nickel. I would encourage you to try out your own experiments and observe any differences. Note: check solubility of DMG in ethanol.

Yes, correct. The intensity of the colour is an indication of amount or concentration of nickel ions but since it's subjective (by observation) and different observers might have different ways to describe how intense the colour is, the method is qualitative only. The intensity/absorption can be measured using colorimetry for example which will give you accurate measurable data.

Gravimetry is a fantastic method to measure the percentage of Ni in a sample. If you follow the method from reference number 3 you will obtain the percentage of Ni in your initial sample. Assume you started with 100g of an unknown mixture and you obtained 6% Ni in the end. This means you have 6g of Nickel in your sample. From there you can calculate the number of moles, etc.

Once you have your red precipitate, try adding a dilute solution (about 5%) of citric acid. Any iron hydroxide precipitates will react to form citrates which are soluble in water. You can know filter, and keep the residue.

Hope this helps,

Andres