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Elemental analysis of natural compounds identify the aspects present in them however they do not provide the real structure or the functional groups present. Considering that all organic substances consist of hydrogen and carbon, most tests include just the decision of oxygen, sulfur, nitrogen and halogens– elements that are most of the time the only ones involved in organic substances. The experiment focused on the Sodium Combination test, which checked for all aspects other than oxygen. In the test, the test compound is broken down into its aspects, which then bond separately with sodium.
The existence of sulfur was evaluated either with lead acetate or salt nitroprusside. The presence of nitrogen was checked with ferrous ammonium sulfate and potassium fluoride. The existence of any halogen was tested by a flame test with copper wire and the recognition of the exact halogen was done using silver nitrate. Cautious experimentation was needed in order to prevent false favorable and incorrect unfavorable results.
Keywords: elemental analysis, oxygen, sulfur, nitrogen, halogens, salt fusion, lead acetate, sodium nitroprusside, flame test [pic] INTRODUCTION
Organic substances are composed primarily of carbon and hydrogen, and in specific practical groups, oxygen, nitrogen, sulfur and halogens. When provided with an unknown natural compound, it is at times enough to identify the aspects present in the substance and not the precise functional groups. The procedure in which the elements of a compound are figured out is called essential analysis.
Among the tests done to carry out essential analysis is the Sodium Combination Test.
In this experiment, salt blend test is utilized in essential analysis of qualitative decision of essential halogens, sulphur and nitrogen in a sample. Salt is an extremely strong reducing representative that will able to break up the natural substances carbon atom chain. It also will transform the atoms which are covalently bonded to the carbon chain to inorganic ions. The aspects are detected by sodium blend test. The natural compound is merged with metal salt to transform these elements into ionic mixture which liquified in water and the filtrate is used to carry out the tests.
The sodium fusion test, in some texts known as the Lassaigne’s test, was developed by the French chemist, Jean Louis Lassaigne in the 1800’s. It is a general test performed in order to determine the presence of nitrogen, sulfur and halogens. The principle behind the test is the breakdown of the compound into its elements at high temperatures. The breakdown of a compound into its elements is possible for organic compounds as they do not dissociate and form ions when dissolved in a liquid unlike salts and other ionic compounds. These elements then fuse with sodium. These sodium compounds are then the ones tested with different solutions that reveal the presence or absence of the above elements. A limitation of this test is that it cannot test for the presence of oxygen, however, the presence of oxygen can easily be determined through the determination of the actual functional groups in the compound. It is a general test for the detection of halogens, nitrogen and sulphur in an organic compound.
These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne’s extract (Wikipedia, 2012.) When an organic compound is heated strongly with sodium, any halogens, nitrogen, and sulfur will be converted into inorganic sodium salts such as sodium halide (for halides), sodium cyanide (for nitrogen), sodium sulfide (for sulfur), and sodium thiocyanate (for sulfur and nitrogen).The nitrogen is confirmed with ferrous sulfate i.e. iron sulfate.
In this experiment, the Sodium Fusion Test was used in order to determine the elements present in an unknown compound.
MATERIALS AND METHODS
The experiment was divided into 2 parts; the first part was the sodium fusion test while the second part consisted of the tests for specific elements. For the first part of the experiment, a piece of sodium metal was washed with hexane and then placed in a completely dry test tube. It was made sure that the test tube was completely dry because any drop of water in the test tube would result to an explosion due to the highly reactive property of sodium. The test tube with the sodium metal was then heated until the metal has melted. Once the vapor has reached a third of the test tube, it was removed from heating and 0.5g/15ml of the sample was added to it.
The mixture was then reheated until the fusion has been completed. A successful fusion was noted by a small explosion or flash. The mixture was removed from heating. Once the mixture has cooled to room temperature, 1ml of ethanol was added to it and the mixture was again heated until a dull red mixture was obtained. This was done with constant stirring. Once the dull red mixture was obtained, the test tube was immersed in 10ml distilled water and was broken using a glass rod. The mixture was stirred and it was made sure that no residue was left on the stirring rod.; it was then heated to boiling and filtered using an ashless filter paper and the residue was kept aside.
For the second part of the experiment, tests for sulfur, nitrogen and halogens were performed. The test for sulfur could be done in two ways: the lead acetate test and the sodium nitroprusside test. For the lead acetate test, 1ml of the sodium fusion filtrate was first tested with blue litmus paper. Acetic acid was then added dropwise until the solution becomes acidic. A few drops of 1% lead acetate solution were added to the solution and the formation of a black precipitate indicated the presence of sulfur.
The sodium nitroprusside test was done by adding 2 drops of 2% sodium nitroprusside to 1ml of the sodium fusion filtrate, with the formation of a deep blue-violet color an indication of the presence of sulfur.
The test for nitrogen was done through the Lassaigne Test or the Prussian Blue Test. 1ml of the sodium fusion filtrate was adjusted to pH13 through the addition of 10% NaOH solution. 2 drops of each freshly prepared ferrous ammonium sulfate solution and 30% potassium fluoride solution. The mixture was then heated to boiling and acidified through the addition of 3M sulfuric acid drop wise until the iron hydroxides have been dissolved. A positive result was indicated by the formation of a dark blue solution or precipitate.
The test for halogens was done in two parts – the first part, called the Beilstein’s test was the test for the presence of halogens while the second test was the test for the identity of the halogen if it is present.
For the Beilstein’s test, a copper wire with a loop at the end was heated using a Bunsen burner until no color was imparted on the flame. It was then cooled and dipped into the sodium fusion filtrate and then burned again. The appearance of a green flame was the indication of the presence of halogens.
For the test for the specific halogen present, 2ml of the sodium fusion filtrate was tested with blue litmus paper. It was then acidified through the drop wise addition of 5% nitric acid solution and boiled slowly for a few minutes. The solution was then cooled and filtered if a precipitate was formed. 2 drops of 0.1M silver nitrate solution was then added to the filtrate. The formation of white, pale yellow or yellow precipitate indicated the presence of chlorine, bromine and iodine respectively. The results can be further verified through the continuous addition of 0.1M silver nitrate in ethanol in order to precipitate all the halogens. The precipitate was then tested with 2ml of 5% ammonium hydroxide. Dissolving of the precipitate indicated the presence of chlorine. Slight dissolving indicated the presence of bromine while insolubility indicated the presence of iodine.
DISCUSSION OF RESULTS
The following table shows the results of the elemental analysis tests and the flowchart shows the correct steps of the elemental analysis (basing on the identity of the unknown and not on the experimental results):
|Cl |- |S |- | |Br |- |N |Present | |I |- |Metal |- | |F |− | | |
During the sodium fusion test, the unknown organic compound was heated to very high temperatures and as a consequence, was broken down into its elements. In the presence of sodium, the compound can form different products depending on the elements present. These products can be NaCN, Na2S, NaI, NaCl or NaBr, since the most common elements found in organic compounds other than carbon,hydrogen and oxygen are nitrogen, sulfur and the halogens.
For the unknown compound, sulfur was present and so Na2S was formed. If the sulfur was tested with lead acetate, the following reaction occurred:
The reaction leads to the formation of PbS, which is a black precipitate. If sulfur was tested with sodium nitroprusside, the following reaction occurred:
The ionic solution formed was of a deep violet color.
For the unknown compound, nitrogen was presend and so NaCN was formed. For the test for nitrogen, the following reaction occurred:
The resulting product is of the Prussian blue color; it was either a solution or a precipitate.
The unknown compound did not contain halogens but if it did, the following reaction would have occurred for the Beilstein’s Test:
In determining the actual halogen present, the following reactions occurred:
AgCl, AgBr and AgI are while, pale yellow and yellow precipitates respectively.
If the sodium fusion test was done in limiting sodium and both sulfur and nitrogen were present in the compound, the thiocyanate ion would be formed during the fusion instead of the cyanate ion. A blood color would emerge from the nitrogen test due to the following reaction:
The organic compounds to be analyzed consist of basically of a chain of carbon atoms which various other atoms are attached. Since these elements are covalently bonded to the carbon chain, it is unable to dissolve in water to form cations and anions. However, sodium fusion test can be used to reduce those atoms that are covalently bonded to the carbon chain to inorganic soluble ions since sodium is a very strong reducing agent. In the Lassaigne’s test, the nitrogen can be reduced to form cyanide ions, CN-: [pic]
For sulphur, it had been reduced to form sulfide ion, S2- in Lassaigne’s test as shown in the following: [pic]
If both nitrogen and sulphur are present in the organic compound at the same time, then the chemical reaction below will take place in the test: [pic]
If halogens (Cl, Br, I) are present in the compound, the halogens will be reduced to form halide ions (Cl-, Br-, I-) during the sodium fusion test. [pic]
The inorganic ions in aqueous solution could be easily observed after undergo certain tests which can indicates the presence of elements in the particular compounds. In the cyanide test, the filtrate of compound A was added with ferrous sulfate, a dark green precipitate was formed. The formation of ferrous hydroxide was produced from the reaction between ferrous sulfate and sodium hydroxide. [pic]
The sodium hydroxide was formed by the reaction of unreacted sodium metal with water due to incomplete reaction of sodium fusion with compound A. [pic] The FeSO4 solution was added to confirm the presence of NaOH and to react completely with it in the filtrate. At the same time, a small amount of black precipitate was formed at the bottom but it was disappeared after more ferrous sulphate was added. The formation of black precipitate may be due to the ferrous sulphide exists in the mixture. [pic]
The equation below shows that the ferrous sulphate was reacted with the sodium cyanide to form sodium ferrocyanide as the main product. [pic] The sulphuric acid and increase in temperature was used to increase the suitable medium for the formation of complex. As a result, ferric-ferrocyanide complex with the colour of Prussian blue was precipitated out after ferric chloride is added to oxidize the Fe2+ to become Fe3+. This Prussian blue precipitate indicates that the unknown A contains nitrogen in the compound. [pic]
Some of the Fe3+ was formed before the oxidation of ferric chloride. This might be due to the air oxidation of iron(II) ions in the mixture before the ferric chloride is added. For compounds B and C, a negative result is obtained which end up with colourless solution as results. Hence, these shown nitrogen are absent in the both organic compounds. The reduced sulfide ion can be confirmed by using two different tests which were sodium nitroprusside test and lead(II) acetate test. For the first test, the appearance of deep purple solution shows the positive result. The formation of sodium sulphonitroprusside is a complex that was formed between the sodium nitroprusside and sodium sulphide. [pic]
In another test, the black precipitate will be formed if the sulphur is present in the compound. The formation of black precipitate shows a positive result for this test. [pic]
There are various possible sources of errors for the experiment. One of these is the incomplete or improper fusion of the compound with sodium. If this happens, then the proper compounds would not be formed and therefore would give negative or even false positive results in the tests.
Another possible source of error for the test is from the Prussian blue test. The solution must be basic because the test does not run in an acidic medium. If the solution is left acidic, the reaction will not occur and would give a false negative result.
Another source of error is the determination of the flame color in the Beilstein’s test. The color should be blue-green because a blue flame indicates Cu1+ ions while a green flame indicates Cu-(non-halide). If the color of the flame was observed incorrectly, then it may give a false positive if it is observed to be blue-green instead of blue or green or it may give a false negative if it is not observed as blue-green. This was the main source of error for this trial since the presence of halogens was incorrectly deduced.
From the experiment, several conclusions could be made. First, the sodium fusion tests consist of various tests, which determine the presence of nitrogen, sulfur and halogens in an organic compound. The presence of nitrogen was confirmed by the formation of a Prussian blue precipitate or solution. The presence of sulfur was confirmed through the formation of the black precipitate PbS or the formation of a deep violet solution from sodium nitroprusside. The presence of a halogen in the compound was confirmed by a blue-green flame, when the flame test was performed with a copper wire while the exact identity of the halogen was determined by the different colors of precipitate formed with silver nitrate.
It can also concluded that the experiment should be done in accuracy and care since the results can give false positive or false negative results when seemingly minor details are incorrectly deduced, for instance the pH of the solution or the color of the flame. Also, sodium is highly reactive and just a small area of contact with water can cause an explosion.
1. Lancashire, Robert John (2005) Qualitative Analysis of Organic Compounds., http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt25.html 2. Baluyut John Y. G., De Castro Kathlia A., Organic Chemistry Laboratory for Chemical Engineering Students Part 2, 2004 3. Klein, David (2012). Organic Chemistry. Danvers. John Wiley & Sons, Inc., 4. Wikipedia (2012). Sodium Fusion Test. http://en.wikipedia.org/wiki/Sodium_fusion_test
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