d. Suppose you mistakenly acidify the filtrate with HCl instead of with HNO3. What would you obser after completing the halide test, and what would that observation lead you to conclude? Organic Laboratory Technique Primer Sodium Fusions and lon Testing Background: The elements that commonly occur in organic compounds along with carbon, hydrogen, and oxygen, are: nitrogen sulfur, and the halogens. The detection of these latter three elements depends on converting them into water soluble lons (CNS , and X respectively. This may be accomplished by heating the compound with sodium metal, which reduces the covalently bound atoms to these ions. The solution resulting from the sodium fusion may then be tested using specific tests for each ion Sodium Fusion: The elemental sodium performs the following reduction reactions on the elements for which we will be testing: Na US Na Saa) Na + C+ N NaN(aq) Na + X-NaX(wa) Figure 1: Set up ready to begin the sodium fusion Sodium Fusion Procedure: ADJUST YOUR HOOD so that it is about 18″-2′ above the bench top where you plan to work. Place a 50-ml beaker about half-full of distilled water underneath the hood. Set up your Bunsen burner to the side of the hood. Next, under your hood, add a few milligrams of the solid sample for a drop or two if a liquid) on top of the hot sodium. Expect a bit of a reaction! Heat the test tube again, continuing to heat towards the bottom on the side of the test tube, add more sample, allowing it to react thoroughly with the sodium metal. The heating and addition of sample may be repeated for a third time if you wish. You want to have a reasonable amount of sample so that the solutions are concentrated enough to give observable results for the ion tests. Obtain a small (0.25 g or so) amount of the sample to be tested. If it is a solid, grind it up on your clay tile in preparation for reacting it with the sodium. Next, put the tube back into the flame and heat it until the tube is a dull red so that it will shatter and the contents will be thoroughly mixed with the water. CLAMP a DRY SMALL (10 X 75 mm) test tube in a test tube holder and hold it vertically, with the clamp as close to the top of the test tube as possible. WEARING GLOVES (sodium metal will react with the moisture on your skin and burn you), obtain a piece of clean sodium metal (about 4 mm on each side). You may need to cut the available pieces of sodium metal with a spatula to obtain the right size. Dry the solvent off the sodium metal using a paper towel. WHILE THE TUBE IS STILL VERY HOT, PLACE IT – (DO NOT DROP IT) into the beaker of water. CAUTIONII BE SURE THAT THE TUBE IS NOT POINTED AT ANYONE AT THIS TIME!! The tube should shatteril Allow a moment for the initial vigorous reaction to subside. If the tube is still intact, tap it on the bottom of the beaker and break it up with a stirring rod. Place the sodium metal into the clamped small test tube. Angle the test tube and heat the lower part of the tube with a small flame until the sodium melts (98″C). When heating a test tube, move the tube in and out of the flame slowly, changing the position of the test tube in the flame in order to heat the sample evenly and to avoid hot spots (hot spots can cause the tube to crack – this is cheap glass). Next, heat the solution to boiling for a few minutes, using either your Bunsen burner or a hot plate. Then, gravity filter the solution to remove broken glass and bits of solid residue. The filter and broken glass bits may be disposed of in the broken glass container The clear almost colorless) filtrate is used for the tests described below. Set up a series of clean, numbered test tubes from the tubes in your lab drawer to conduct these tests. Be sure to take notes in your la notebook for each test & note the numbers on the test tubes indicates the presence of cyanide los no cyanide ons are present, the solution should be a pale yellow color TEST FOR SULFUR: NOTE: A positive test for cyanide is good evidence for the presence of nitrogen in the original sample, but the presence of nitrogen should never be ruled out solely on the basis of a negative test for cyanide lons. Nitrogen in a high oxidation state for example, as a nitrogroup.is especially difficult to detect. The test for sulfur involves sodium nitroprusside and creates a purple freddish violet) color when sulfur is present The waste from the nitrogen test may be washed down the drain with plenty of water. Na, + Na[Fe(CN),NO) Sodium nitroprusside Naa[Fe(CN),NOS) violet color TEST FOR HALOGENS: One may also test for sulfur using lead salts to create the insoluble lead sulfide precipitate, however, we are doing just the nitroprusside test since it does not generate hazardous waste. The test for halogens is the most difficult to perform correctly. This test reacts the filtrate with silver nitrate solution, but if any s’ or CN lons are present, they will form precipitates with silver ion. Therefore, the filtrate is first treated with acid to convert any sulfur into H and any cyanide anion into HC . These byproducts can then be boiled off. IIIIIIIIIII111111 Sulfur Test Procedure: Measure out a milliliter or two of the filtrate produced from the fusion and place it in one of the (clean!) larger test tubes from your lab drawer. Add 2 drops of the sodium nitroprusside solution. Mix by shaking the test tube from side to side. If needed, you can obtain a rubber stopper from the shelves near the main lab door/side bench. A deep reddish-violet coloration indicates the presence of sulfur. The desired reaction is to observe the precipitates of insoluble silver halides to test for the presence of halide anions: Ag + X – AgX. (ppt) The waste from the sulfur test may be washed down the drain with plenty of water. The silver halides have characteristic colors: AgCl is white; AgBr is pale yellow, and Agl is yellow. TEST FOR NITROGEN: The undesirable reactions with S. shown: , or so. are as The test for nitrogen utilizes iron sulfate. The reaction of sodium cyanide first produces sodium ferrocyanide and then reacts further in the presence of acid to produce Prussian Blue (ferric ferrocyanide) 2 Ag Ag 2 Ag +S + CN + SO ARS (black ppt) _ AgCN (white ppt) _ Ag.S04 (white) 6NaCN + Feso. – Na.[Fe(CN).] + Na,so. Sodium ferrocyanide Na.[Fe(CN)] + Fet -Fe [Fe(CN)J Ferric ferrocyanide “Prussian Blue” Halogen Test Procedure: Take about 2 mL of the filtrate, and make it strongly acidic using dilute nitric acid. Remember the proper way to test pH is to dip your stirring rod in to your solution and then touch that to the alkacid paper, this avoids contaminating the solution with the pH indicator reagents. Nitrogen Test Procedure: To 2 mL of the filtrate in a medium-sized test tube (from your lab drawer), add 0.1 – 0.2 g of powdered ferrous sulfate. Heat the mixture cautiously and with shaking until it boils. (Keep the test tube moving in and out of the flame and slightly tilted (pointed away from anyone)!!) Then, while still HOT, add just enough 30% sulfuric acid, with shaking, to dissolve the gelatinous hydroxides of iron. The appearance of a blue precipitate of ferric ferrocyanide (Prussian Blue) Add a few boiling stones to keep this solution from bumping, and boil this solution gently for a several minutes (keep it moving) to expel any HCN or HS that may be present if the sample had tested positive for nitrogen and/or sulfur (See note below for why this step is important). Hy smells like rotten eggs, so waft the fumes from the test tube to help ensure that you have removed all of the H.S. Continue to boil for a couple of minutes
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