Experiment #6: Elemental Analysis Co, Faith Abigail K., Salindo, Elysse S. Group # 6, Chem 31.1, HEJ1, Sir Jason Vedad February 9, 2012 I. Abstract Sodium fusion is a technique that enables the detection of nitrogen, sulfur, and halogens in organic compounds by converting them into into soluble salts. P-chloroaniline and sulfanilic acid acid were fused with elemental sodium at high temperatures. P-chloroaniline was negative for nitrogen and positive for halogen. Sulfanilic acid was negative for halogen a nd positive for nitrogen. Both samples were negative for sulfur, II. Keywords: Keywords : elemental analysis, sodium fusion, qualitative tests, sulfur, nitrogen, halogens III. Introduction
observed to condense on the walls. Immediately after the the flam flame e was was remo remove ved, d, abou aboutt 0.3 0.3 gram grams s of pchloroaniline was added. The test tube was heated until the reacti reaction on notabl notably y subside subsided. d. The tube tube was was again again heated to redness for 3 minutes. A few drops of ethyl alcoho alcoholl were were added added in the test tube. The tube tube was was droppe dropped d into into a beaker beaker containi containing ng 10 mL of distil distilled led water, which was quickly covered with gauze. After the reaction has subsided, the solution in the beaker was boiled for 2 minutes. After letting it cool, the solution was filtered and the filtrate was used as a test solution for part B. The same same proced procedure ure was applie applied d using using the sulfanilic acid sample.
Sodium fusion, also known as Lassaigne’s test, is a method used in elemental analysis to qualitatively dete determ rmin ine e the the pres presenc ence e of nitr nitrog ogen en,, sulf sulfur ur,, and and halogens in organic compounds. It involves melting the sample with elemental sodium that turns the sample into into a water water-so -solub luble le salt. salt. The sodium sodium compou compound nd is dissolved in water, and qualitative tests are performed on the result resultant ant soluti solution on for possib possible le consti constitue tuents nts.. Sodium Sodium metal is highly highly reacti reactive, ve, has a low melti melting ng point, and a near universal solubility of its compounds. The elements like nitrogen, nitrogen, sulfur, sulfur, and the halogens halogens cannot be detected in organic compounds due to their stable stable covalen covalentt nature nature.. Becaus Because e of this, this, sodium sodium is “fused “fused”” to the compou compound nd to conver convertt them them into into ionic ionic B. Qualitative Tests forms, making qualitative tests possible. The following following tests tests were performed performed using pQualit Qualitati ative ve tests tests are used used to determ determine ine the chloroaniline and sulfanilic acid fusion solutions. presence of a particular compound in a solution. These tests typically make use of color changes in solutions or B.1 Sulfur Test precip precipit itate ates s in order order to indic indicate ate specif specific ic chemic chemical al Five drops of fusion solution and 3 drops of reactions. These are different from quantitative tests in water were placed in a 5 ml test tube. Two drops of 2% that that these these are only conside considered red with with identi identifyi fying ng the aqueous sodium nitroprusside solution were added to compounds compounds present present in solution. solution. The The objective objectives s of this the solution thereafter. Observations were made. The experiment are to be able to fuse elemental sodium format formation ion of a deep deep blue-v blue-vio iolet let color color indica indicates tes the with with other other compou compounds nds and analyz analyze e the presen presence ce of presence of sulfur. nitrogen, sulfur, and halogen in those compounds. B.2 Nitrogen Test IV. Experimental In a test test tube, ube, a pinc pinch h of ferro errous us sulf sulfat ate e (FeSO4) was was adde added d to 1 mL of the the fusi fusion on solu soluti tion on A. Sodium Fusion obtained in A. Five drops of a 10% solution of KF were An apparatus as shown in Figure 1 was set-up added thereafter. thereafter. The resulting mixture was boiled for 5 prior to the experiment. seconds and added with a sufficient amount of 6 M sulfuric acid (H 2SO4) that dissolved the insoluble iron Test tube hydr hydrox oxid ides es and and made made the the solu soluti tion on acid acid to litm litmus us.. holder Observations were made. The formation of a deep blue 5-mL test tube precipitate indicates the presence of nitrogen. In the event that the color of the mixture makes burner the the dete detect ctio ion n of a blue blue colo colora rati tion on impo imposs ssib ible, le, the the mixture is to be filtered and the residue examined for blue coloration. Fig. 1 Sodium fusion set-up
A piece of sodium was placed in a small test B.3 Halogen Test tube. tube. The The tube tube was heated heated until until sodium sodium vapor was Ten drops of the fusion solution was placed into Chem 31.1, Elemental Analysis
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a small test tube. Note that the sample, once positive for nitrogen or sulfur, is to be acidified by adding 2 M nitric acid (HNO 3). The resulting solution was heated to a gentle boil for about 1 minute to expel any hydrogen cyanide (HCN) or hydrogen sulfide (H2S) present and cooled to room temperature. Four drops of 0.1 M AgNO3 solution were added to the fusion solution. Observations were made. A faint turbidity is a negative test, while a white, pale yellow, or yellow precipitate indicates the presence of AgCl, AgBr, or AgI, respectively. A solubility test was conducted using dilute ammonium hydroxide to differentiate silver bromide (AgBr) from silver iodide (AgI). One-half mL of 2 M NH4OH was added to the precipitate and stirred to determine solubility. Observations were made. AgI is insoluble in the solution while AgBr is slightly soluble. A preliminary test could also be used prior to the solubility test. Using a piece of copper wire, a small loop 3 mm in diameter is to be made at one end and inserted with a cork in the other end to serve as a holder. The coil is to be heated in the hot portion of a Bunsen flame until it no longer imparts color to the flame. The wire is to be cooled and dipped into a small portion of the original sample. The affected portion of the wire is to be heated. A green color indicates the presence of halogen.
other hand, showed negative for both sulfur and halogen test. While a dark blue precipitate formed in the nitrogen test which is evidence that the organic compound contains nitrogen. VI. Discussion The sodium fusion test, otherwise known as Lassaigne’s Test is used to detect the presence of halogens (Cl, Br, and I), sulfur, and nitrogen in organic compounds. These compounds are covalently bonded to the compound and so through heat (bond breaking is an endothermic process), and exposure to sodium metal; inorganic, water-soluble, sodium salts forms – NaCN, NaSCN, Na 2S, or NaX (X=Cl, Br or I). As such, p-chloroaniline fused with sodium formed NaCl and NaCN whilst the fusion of sulfanilic acid separately formed Na2S and NaCN. Normally, a compound that contains both sulfur and nitrogen (like sulfanilic acid) fused with sodium would have formed NaSCN; and a different test altogether would have been used. But in this case, it is possible to perform individual tests for nitrogen and sulfur because during the preparation of the fusion solution, sulfanilic acid was fused with excess sodium metal so instead of forming NaSCN, it formed Na 2S and NaCN separately. NaSCN + 2Na → NaCN + Na2S
V. Results A. Sodium Fusion The solution had a light brown color at first due to the insoluble black particles remaining in the beaker along with the glass shards. After filtration, the filtrate that remained (the fusion solution) was clear in color. B. Qualitative Tests After performing all the qualitative tests, the results observed are as follows: Table1. Results of Qualitative Tests for Both Samples
Sample
Sulfur Test
Nitrogen Test
Halogen Test
p-chloroaniline
-
-
+ (white ppt)
Sulfanilic acid
-
+ (blue ppt)
The solution in the beaker was filtrated and the colorless filtrate is called the Lassaigne’s or sodium extract. This extract is then subject to different qualitative tests to determine whether the organic compound contains any of the elements mentioned above. In the Sulfur Test or the Sodium Nitroprusside Test, formation of a deep blue violet color indicates the presence of sulfur in the compound. The change in color is caused by sodium sulfonitroprusside which is violet in color which is the product of the reaction between sodium sulfide and sodium nitroprusside; as seen in the reaction below: Na2S + Na2 [Fe(CN)5NO] →
Na4[Fe(CN)5NOS] (violet)
-
Both the p-chloroaniline and sulfanilic acid fusion solution showed a negative for this test but The fusion solution for p-chloroaniline showed theoretically, the sulfanilic acid should have showed a negative for both sulfur and nitrogen test (there was no positive result. In the Nitrogen Test, the presence of blue color observed deep blue-violet coloration and blue precipitate respectively). The halogen test came out as indicates the presence of nitrogen in the compound due positive though because of the white turbidity or to the blue complex ferri-ferrocyanide formed. The sodium extract is most likely basic due to precipitate that formed after adding the silver nitrate the NaOH formed by unreacted sodium and water. solution. The fusion solution for sulfanilic acid, on the Upon adding FeSO4, the solution turned brown-green Chem 31.1, Elemental Analysis
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due to the insoluble ferrous hydroxide formed (1). The insoluble to ammonium hydroxide. ferrous ions then react with sodium cyanide and It is also possible to perform a quick produces sodium ferricyanide (2). preliminary, flame test for halogen (called the Beilstein’s Test for Halogen). A green flame indicates the presence (1) FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4 of chlorine, a blue-green flame for bromine, and blue (2) Fe(OH) 2 + 6NaCN → Na4Fe(CN) 6 + 2NaOH for iodine. Fluorine, though part of the halogen group, cannot be detected because copper fluoride is not 3+ Upon boiling the solution, some ferric (Fe ) ions are volatile therefore no indicative flash of flame appears. produced through aerial oxidation. This along with the Unfortunately, this test was not performed in the Fe3+ ions from the added FeCl 3 forms ferric salt. Finally, experiment due to lack of a Bunsen burner. sodium ferrocyanide reacts with the ferric salt to form In the test for halogen, p-chloroaniline showed ferri-ferrocyanide complex which has a Prussian blue a positive result (indicated by the formation of white color. precipitate due to AgCl) while sulfanilic acid showed a negative result. These results are in line with the 3Na4Fe(CN) 6 + 2Fe2(SO4)3 → Fe4[Fe(CN) 6]3 + 6Na2SO4 theoretical result. (Prussian blue) Table2. Theoretical Data for Qualitative Tests for Both Samples
The 6M sulfuric acid was added to dissolve any Sample Sulfur Nitrogen Halogen remaining iron oxide in the solution while the 10% Test Test Test solution of potassium fluoride was added to make the p-chloroaniline + (blue ppt) + (white blue coloring more distinguished and easier to spot. ppt) The p-chloroaniline compound tested negative Sulfanilic acid + (violet + (blue ppt) for the nitrogen test while sulfanilic acid tested positive. ppt) Theoretically though, both compounds should have showed a positive to the nitrogen test. Halogens can be detected through 2 ways – As seen in the table above, only the results silver nitrate test and layer test. For this experiment, the from the Halogen test is consistent with the theoretical first method was performed. Upon adding AgNO 3 to the data. Several factors may have generated the fusion solution, a heavy-curdy type precipitate indicates discrepancies but it is usually because of poor fusion. the presence of halogens – white for AgCl ppt, pale In the nitrogen test, even if the fusion solution was yellow for AgBr, and yellow for AgI. A faint turbidity is a excellently prepared, it may still have failed because negative test. some nitro compounds, like proteins, do not yield In the event that the organic compound is cyanide ions easily. For the sulfur test, it may have positive for nitrogen or sulfur, the fusion solution must been because nitroprusside salts are very unstable and be acidified by 2M HNO 3 first then boiled to expel any so the violet color faded easily. Though the halogen test HCN or H 2S that forms. This is because cyanide and was consistent with the theoretical data, it is still sulfur ions interfere with this test due to Ag + reacting important to be aware of factors that may cause with either CN- to form AgCN (a white precipitate); or erroneous results. Not boiling the fusion solution reaction with S2- to form Ag2S (a black precipitate). enough could cause errors because the HCN or H2S After expelling the interfering ions and was not completely expelled, which would interfere with precipitate still forms, that would indicate a positive for the test. Lastly, erroneous data could have also been halogens. A white precipitate would point to presence of caused by human error or impure reagents. chloride in the compound (1). When yellow precipitate forms, it would indicate that either bromide (2) or iodide VII. Guide Questions and Answers (3) is present in the organic compound. (1) NaCl + AgNO 3 → AgCl(s) + NaNO3 (white ppt) (2) NaBr + AgNO 3 → AgBr(s) + NaNO3 (pale yellow ppt) (3) NaI + AgNO 3 → AgI(s) + NaNO 3 (yellow ppt)
1. Outline the procedure for detecting the presence of oxygen and phosphorus in organic compounds.
The presence of phosphorus can be detected by fusing the compound with an oxidizing mixture such as sodium carbonate and potassium nitrate; or even with sodium peroxide alone. This forms alkali phosphate and is extracted and filtered. The fusion The yellow precipitates AgBr or AgI are easily solution containing sodium phosphate is heated with differentiated by adding excess ammonium hydroxide. concentrated nitric acid and excess ammonium Silver bromide is slightly soluble while silver iodide is molybdate. The formation of yellow precipitate Chem 31.1, Elemental Analysis
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ammonium phosphomolybdate (NH4)3[P Mo12O40] is important to perform the sodium fusion phase precisely indicative of presence of phosphorus. for an easier and more accurate detection of the target Percent of Oxygen in an organic compound is elements. usually estimated by difference between the total But as mentioned before, some compounds percentage composition (100%) and the sum of cannot easily part with the target elements (like in percentage of all other elements. Another method protein) so it is recommended that the analyst find a would be to decompose a compound by heating it. A more appropriate qualitative test for said compounds. mixture of gas products is formed (O 2 among them) Also, due to the highly reactive nature of sodium to which will then be passed over red hot coke to convert water, it is recommended that the fusion test tube be oxygen to carbon monoxide. This will then be reacted properly dried so as to prevent accidental explosions. with warm iodine pentoxide to convert CO to CO2. And as always, pure reagents are a must because impurities will definitely interfere with the reaction and will result in erroneous results. IX. References Cheronis, N.D. (2005). Semimicro and Macro Organic Chemistry. New York: Thomas Y. Crowell Company from Fig 2. Series of Reactions to Derive Oxygen Percent Composition http://library.sciencemadness.org/library/books/ Semimicro_and_Macro_Organic_Chemistry Oxygen percent composition can then be derived from _by_Cheronis_1942.pdf the amount of carbon dioxide or iodine formed. Bhagat, P. (2011). Detection of Extra Elements in the Organic Compound. Retrieved Feb. 7, 2012, from University of Delhi website: http://eduframe.net/andc/chemistry/Che02/extr Aside from the method employed in this %20elements.pdf experiment, sulfur may be detected by first acidifying the fusion solution with dilute acetic acid. Lead acetate may then be added. A black precipitate of PbS Brooklyn College: Department of Chemistry. (2010). Chapter 27 - Qualitative Organic Analysis: The indicates the presence of S. The acetic acid is added in Identification of Unknowns. Retrieved Feb. 7, order to suppress the ionization of the acetate from 2010, from The City University of New York lead acetate. (CUNY), Brooklyn College, Department of Nitrogen may be detected by mixing 5 drops of Chemistry website: 1.5% p-nitrobenzaldehyde in 2-methoxyethanol, 5 academic.brooklyn.cuny.edu/chem/howell/ drops of 1.7% o-dinitrobenzene in 2-methoxyethanol QualOrg3rdEdition.pdf and 2 drops 2% NaOH (aq) with 1 drop of the fusion solution. A deep purple color that is imported to the solution indicates that it is positive for CN - ions, and Nair, S.V. (2007). Qualitative Analysis of Organic Compounds. Retrieved Feb. 8, 2012 from hence for N. http://chemmaster.co.in/showchapter.php? The addition of sulfuric acid will enable the id=20&id2=168&title=Hydrogen detection of halogens. Chlorides react with sulfuric acid to produce a pungent odor and white fumes. Bromides react to produce a reddish-brown solution. Iodides I hereby certify that I have given substantial react to produce violet vapor. contribution o this report. 2. Discuss other methods of detecting the presence of sulfur, nitrogen and halogen in organic compounds.
VIII. Conclusion and Recommendations Sodium fusion or Lassaigne’s Test is one of the Faith Abigail K. Co more widely used methods of qualitative analysis. Normally, the target elements (S, N, Cl, Br and I) are covalently bonded to the organic compound hence it is very difficult to detect. But through fusion with sodium Elysse S. Salindo (thereby converting it to inorganic, sodium salts), the target elements can now be easily detected through their individual qualitative test. Therefore, it is severely Chem 31.1, Elemental Analysis
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