witch chemicals use copper needle test
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ammonia ( NH3)
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There is quite a bit of chemistry involved in these tests. First, ammonia can be used to detect copper. This is because Cu2+ reacts with ammonia to form a dark blue colored complex:2
Cu2+(aq) + 4 NH3(aq) à Cu(NH3)42+ (Equation 1)
Thus, when a cotton swab soaked in 15 M ammonia is rubbed over the surface of a copper-containing coin, any Cu2+ on the surface causes the cotton swab to turn a blue color as a result of Equation 1.
Also, when coins that contain silver are rubbed with an ammonia-soaked cotton swab, a dark color often appears on the cotton. This color change is due to the formation of a black-brown colored silver complex on account of following reaction:3
Ag+(aq) + 2 NH3(aq) à Ag(NH3)2+ (Equation 2)
The video above shows the use of the reaction between dimethylglyoxime and nickel ions to detect nickel in coins. The chemistry of this reaction has previously been discussed on ChemEdX, 4 and therefore will not be covered here. The reader is referred to this article for further information about this test.
In addition to these tests, solutions of Cu2+ can be used to detect iron, zinc, and aluminum. Iron reacts with dissolved copper, causing solid copper to plate out on the iron surface:
Fe(s) + Cu2+(aq) à Cu(s) + Fe2+(aq) (Equation 3)
The solid copper formed on the iron surface produces an orange color. It should be noted that while some coins do contain iron, iron-containing coins are treated in a manner to avoid the oxidation of iron. Therefore, testing coins for iron as indicated in Equation 3 will not always yield a positive result. It is better to detect iron in coins by showing that a coin is strongly attracted to a magnet,5 but will not show a positive result upon reaction with dimethylglyoxime.
Zinc also will react with Cu2+, forming a black color on the surface of the zinc metal. This observation can be explained by the formation of black-colored copper oxide on the surface of the zinc:
Zn(s) + Cu2+(aq) + ½ O2(g) à Zn2+(aq) + CuO (Equation )
Dissolved copper can also be used as a test for aluminum, as long as chloride ion is present. Samples of aluminum metal generally have a thin, surface layer of Al2O3, which prevents reaction with dissolved copper. However, chloride ions allow Cu2+(aq) to “breach” this so-called passivation layer, which allows for the following reaction to occur:6
2 Al(s) + 3 Cu2+(aq) à 3 Cu(s) + 2 Al3+(aq) (Equation 5)
Thus, black or orange flecks of solid copper will form on samples of aluminum that are treated with solutions of Cu2+ that contain chloride ions.
Use caution when adding solutions of Cu2+ to coins. In my experiments, I often noticed that several coins were irreversibly tainted by such treatments.
There is another simple method for the detection of silver that I did not mention in the video above. The chemistry of this detection method is very interesting, but unfortunately the method tarnishes silver in a manner that is not easy to remove. Therefore, I did not feature this test in the main video above.
(accessed 9/17/19)In this second method, household bleach, which contains hypochlorite ion (OCl-), is added to a sample of metal. If the metal contains silver, it quickly tarnishes.
Cu2+(aq) + 4 NH3(aq) à Cu(NH3)42+ (Equation 1)
Thus, when a cotton swab soaked in 15 M ammonia is rubbed over the surface of a copper-containing coin, any Cu2+ on the surface causes the cotton swab to turn a blue color as a result of Equation 1.
Also, when coins that contain silver are rubbed with an ammonia-soaked cotton swab, a dark color often appears on the cotton. This color change is due to the formation of a black-brown colored silver complex on account of following reaction:3
Ag+(aq) + 2 NH3(aq) à Ag(NH3)2+ (Equation 2)
The video above shows the use of the reaction between dimethylglyoxime and nickel ions to detect nickel in coins. The chemistry of this reaction has previously been discussed on ChemEdX, 4 and therefore will not be covered here. The reader is referred to this article for further information about this test.
In addition to these tests, solutions of Cu2+ can be used to detect iron, zinc, and aluminum. Iron reacts with dissolved copper, causing solid copper to plate out on the iron surface:
Fe(s) + Cu2+(aq) à Cu(s) + Fe2+(aq) (Equation 3)
The solid copper formed on the iron surface produces an orange color. It should be noted that while some coins do contain iron, iron-containing coins are treated in a manner to avoid the oxidation of iron. Therefore, testing coins for iron as indicated in Equation 3 will not always yield a positive result. It is better to detect iron in coins by showing that a coin is strongly attracted to a magnet,5 but will not show a positive result upon reaction with dimethylglyoxime.
Zinc also will react with Cu2+, forming a black color on the surface of the zinc metal. This observation can be explained by the formation of black-colored copper oxide on the surface of the zinc:
Zn(s) + Cu2+(aq) + ½ O2(g) à Zn2+(aq) + CuO (Equation )
Dissolved copper can also be used as a test for aluminum, as long as chloride ion is present. Samples of aluminum metal generally have a thin, surface layer of Al2O3, which prevents reaction with dissolved copper. However, chloride ions allow Cu2+(aq) to “breach” this so-called passivation layer, which allows for the following reaction to occur:6
2 Al(s) + 3 Cu2+(aq) à 3 Cu(s) + 2 Al3+(aq) (Equation 5)
Thus, black or orange flecks of solid copper will form on samples of aluminum that are treated with solutions of Cu2+ that contain chloride ions.
Use caution when adding solutions of Cu2+ to coins. In my experiments, I often noticed that several coins were irreversibly tainted by such treatments.
There is another simple method for the detection of silver that I did not mention in the video above. The chemistry of this detection method is very interesting, but unfortunately the method tarnishes silver in a manner that is not easy to remove. Therefore, I did not feature this test in the main video above.
(accessed 9/17/19)In this second method, household bleach, which contains hypochlorite ion (OCl-), is added to a sample of metal. If the metal contains silver, it quickly tarnishes.
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