CHEMISTRY : Qualitative Analysis

Identification of metals in 1 Cent Maltese coins

INTRODUCTION AND PRINCIPLE

This is a qulitative analysis test to determine the metals present in alloys, specifically metal coins. The test is carried out on cheap 1cent Maltese coins (not more circulated whne Malta joined EU). It can however be applied to any other coins or differnt types of metal alloys.

Basically, the coins are dissolved in strong acid, namely nitric acid (which dissolves almost all common metals). The excess acid is neutralised and a set of precipitation reactions are carried out on the remaining solution. From the pattern of results, the metal (or metals) present can be determined from the specific reactions that metal cations produce with different anions (refer to the Cation ID section of this section)

SAMPLE PREPERAION

About 12 one cent coins were put in concentrated nitric acid one by one. Initially the reaction was very vigourous and highly exothermic. The reaction began to slow down on adding further coins, and heat was applied to increase the rate of reaction. The slow or non reacting coins where left in the nitric acid + salt solution for one week, heating it frequently at regular intervals. This is however not required and leaving the coins in warm nitric acid for an hour is enough.

The mixture solution, which now was deep blue in colour, was added sodium Hydroxide to neutralize the xs, unreacted nitric acid, until some ppt remained.

The mixture was filtered from unreacted metal pieces and the little hydroxide precipitate. Precipitation tests will be carried out on this filtrate solution which is assumed to be salts of the metal(s) present in the sample coins without any remaining Nitric Acid. Xs Acid would interfere with these precipitation reactions, hence it is important that the solution is neutral. The solution was diluted 1:5.

Conclusions part 1:

1. The blue solution produced is a characteristic colour for a copper salt.
2. However more metals having colourless salts can be also present, and so more tests are required.

PRECIPITATION TESTS

To a the diluted solution the following reagents in the table below were added and the results obtained are given accordingly :

TEST No.	TEST APPLIED	RESULTS
2.01	Sodium Hydroxide	Blue ppt which turns black on heating
2.02	Ammonium Hydroxide	Blue ppt which becomes deep blue sol on xs
2.03	Sodium carbonate	Sky Blue ppt (unchanged on heating)
2.04	Sodium ThioSulphate	Green sol, becomes black ppt on heating
2.05	Sodium Sulphite	Yellow/green ppt, turns brown on heating.
2.06	Sodium Sulphide	Dark Brown/Black ppt formed
2.07	Sodium Chloride	Faint white pt developed
2.08	Sodium Iodide	Brown Iodine sol + cream ppt on heat
2.09	Potassium Iodate	Some green/blue ppt was formed
2.10	Ammonium Phosphate	Sky blue gelatinous ppt.
2.11	Sodium Salicylate	Green solution, but no ppt even on heating
2.12	Sodium Benzoate	Light sky blu ppt immediately formed.
2.13	Tannic Acid	Grey brown ppt
2.14	Pot. HexaCyanoferrate(II)	Intense red brown ppt
2.15	Pot. HexaCyanoferrate(III)	Dirty Brown/yellow ppt
2.16	Sodium Vanadate	Yellow ppt + yellow solution (masked to a green colour with xs blue copper nitratre sol.)
2.17	Potassium Dichromate	Traces of Red brown ppt
2.18	Ammonium Molybdate	Light blue ppt formed only on heating
2.19	Sodium Selenite	Light green (mint) coloured ppt was formed
2.20	Potassium Thiocyanate	Green solution (or yellow masked with xs blu Cu2+ sol) and a white/lilac ppt on heating.

Conclusion Part 2

1. COPPER is CONFIRMED; all tests give same results with a copper solution.
2. The most significant confirmatory test is the deep blue sol with xs ammonia. (2.02).
3. With xs ammonia, no ppt was left over, hence if other trace metals are present, it's hydroxide is soluble in xs ammonia. These are Silver, Cadmium, Zinc, and Nickel. However, these may be so dilute that, no ppt resulted.
4. It is difficult to say if there are trace ammounts of other metals since their ppt will be masked with the intense copper precipitates.
5. The faint brown ppt with the dichromate is also given by Tin(II), Iron (actually yellow/brown ppt), and Silver. (2.17).
6. If other metals exist (hence an alloy with copper), they exist in trace ammounts, less from 5%
7. The presence of other metals than Cu relies on the faint ppt with the chloride, and dichromate, together with the considerable ammount of ppt with the molybdate after heating.

QUANTITAVE ANALYSIS FOR OTHER TRACE METALS AFTER REMOVAL OF COPPER IONS

The copper is precipitated out from solution leaving other metals in solution and seperated by filtering off the copper precipitate. There is no ideal reagent which forms a ppt only with copper, but the Thiosulphate will ppt out copper as black Copper Thiosulphate but not many other metals namely the ones shown below:

Thiosulphate will not form a precipitate with the following metal ions in solution:
Mg, Ca, Ba, Sr, Cd, Zn, Fe, Sn(II), Mn, Ni and Co

However Thiosulphate forms a precipitate with Aluminium cations in solution. Al 3+ can be seperated from Cu2+ by Tannic Acid or Potassium Hexacyanoferrate(III) which both ppt Copper ions out but not Aluminium ions which do not react.

So Sodium thiosulphate was added in excess (exact procedure below) to the sample solution to precipitate Copper Thiosulphate out which then was filtered away. By now, the filtrate contain, excess Thiosulphate, Sodium Nitrate as a bi-product of the precipitation reaction, together with any other unprecipitated metal ions, if present in the sample. These two will not interfere with the consequent precipitation reaction tests in order to detect other metal ions apart from the eliminated Copper ones.

Sodium Thiosulphate solution was added to 50ml sample solution to form a black insoluble precipitate of Copper Thiosulphate.

The ppt is filtered off and should be a clear solution without any traces of blue CU2+ ions. To make sure that all Copper has been removed, few drops of dilute Thiosulphate solution is added to the collected filtrate. If some ppt is again formed (could be brown instead of black when in trace amounts), more thiosulphate is added and solution is refiltered.

Since the trace metals exist in dilute form, the filtrate should be evaporated to around 10mls

2ml of the filtrate is placed in 4 small test tubes

A control set of 4 test tubes with Sodium Thiosulphate and another four with Sodium nitrate solution are also prepared, and serves as a control to ensure that any colour changes formed iin the coming tests are due the presence of metal cations, not these compounds also present in the filtrate solution (as mentioned above)

To the set of 4 sample tubes (and controls), Sodium Hydroxide, clear Sodium Carbonate, Sodium Sulphide, and Potassium hexocyanoferrate(II) are added in each tube respectively.

TEST No.	TEST APPLIED ON FILTRATE	RESULTS
3.01	Sodium Hydroxide	No precipitate was formed
3.02	Sodium Carbonate	No precipitate was formed
3.03	Sodium Sulphide	No precipitate was formed
3.04	Pot. HexoCyanoFerrate(III)	No precipitate was formed

Conclusion part 3:

The lack of precipitate or reaction indicates that no or negligible amounts of other metal ions are present in the sample metal, apart from Copper.

CONCLUSION:

Since of the lack of precipitates with the solution of sample metal, with removed copper salt, and the above excellent pripitating agents, one can conclude that the sample of 1c Maltese coins consists of opper metal. Although it seems that the metal is pure copper, one must consider that some other metals in trace ammounts may be present, of which their precipitate cannot be detected. Hence it is safe to conclude that the 1c coins consists of a minimum of 98% Copper.

1C MALTESE COINS ARE MADE ONLY OF COPPER

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