PREPARING GAS SAMPLES IN LARGE PLASTIC SYRINGES

Transcription

1 PREPARING GAS SAMPLES IN LARGE PLASTIC SYRINGES A wide variety of gases can be prepared safely inside a 60 ml plastic syringe. Here you will practice making carbon dioxide, so that you know the technique when you make more difficult gases. The general strategy of the method is to react two substances in a 60-mL syringe. The limiting reagent is always used in solid form and is placed in a small vial cap. The second reagent is prepared as an aqueous solution. For example, one could generate CO 2 (g) from excess aqueous acetic acid and solid NaHCO 3, as the limiting reagent. When you take the written portion of the lab final, you will be allowed to use your lab notebook pages. Take good notes and make careful observations! Making Carbon dioxide. 1. The solid reagent. Measure out 0.21 g NaHCO 3 directly into the vial cap. Hint: Cut the top off of a plastic pipet at an angle to make an inexpensive spatula. Fill the syringe barrel with water. Place your finger over the hole to form a seal. Fill completely to the top. Float the vial cap containing the solid reagent on the water surface. This is easiest if the syringe barrel is filled completely to the top with water. Lower the cap by flotation. Release the seal made by finger to lower the cap into the syringe barrel without spilling its contents. Allow the syringe to drain into a wide mouth beverage container. When successfully completed, the cap should rest upright on the bottom of a syringe with all reagent still in the cap. The syringe should always be held in a vertical position during these first steps. Install the plunger while maintaining the syringe in a vertical position. The plunger should fit snugly against the rim of the vial cap. 1

2 The aqueous reagent. The liquid reagent is drawn into the syringe as described below. We will use 5 ml vinegar, HC 2 H 3 O 2, to make carbon dioxide. The liquid reagent is always the reagent in excess. Draw aqueous reagent into syringe. The aqueous reagent, measured into a small weighing boat, is drawn into the syringe while maintaining the vertical position of the syringe. The vial cap with the solid reagent should float on the solution. Push the syringe cap over the syringe fitting. It simply pushes on! Generating the Gas. The fun part is generating the gas and you are ready to do that now! Shake the device up and down in order to mix the reagents. As the liquid reagent splashes into the vial cap, gas generation will commence and the syringe plunger should rise. It is sometimes necessary to gently help the plunger move up the barrel. Remove cap to stop the reaction. After the plunger has reached the desired mark (usually 50-mL), tip the syringe so that it is positioned with plunger downward and syringe cap upward. Carefully remove the syringe cap assuming that the syringe may be under positive pressure. Discharge reagents. Turn the syringe 180 o and discharge the liquid reagent into the plastic cup. Caution: Never remove the syringe cap with the cap end of the syringe directed downward: Reagents will spray out of the syringe. Immediately cap the syringe with the syringe cap to prevent loss of gas by effusion or reaction with air. Prepare CO 2 and show TA for approval (Required before NO Prep) 2

3 Other useful gas syringe techniques. There are a several other techniques that come in handy when working with gases in syringes. Here are the ones you will find useful in the lab practical final. Practice these techniques today. A. Syringe-to-syringe Transfer procedure. It is often useful to transfer a gas from the syringe in which it was prepared to another clean, dry syringe. In the third step shown below, note that the clean, dry syringe is on top. 1. Connect a short piece of tubing to a clean syringe. 2. Connect gas-filled syringe to tubing: 3. Transfer the gas with clean, dry syringe on top: B. Controlled discharge of gas from a syringe. Plungers do not always move smoothly in their syringe barrels. As a result, gases may be discharged in large unintended portions (such as 40-mL all at once). This is more likely to occur if the method shown in the left diagram below is used. Instead, grasp the syringe by its plunger (right figure) and pull the barrel towards your hand. This simple technique will give you excellent control of gas delivery. INCORRECT WAY CORRECT WAY 3

4 C. Preventing unwanted discharges of noxious gas. Some of the gases that can be generated by the in-syringe method are noxious and should not be discharged into breathable air. These include nitrogen monoxide, NO, nitrogen dioxide, NO 2, both prepared in the lab practical final next week. The use of syringes to generate such gas samples works exceptionally well and far better than any other method in preventing undesired discharges. There are two simple considerations to keep in mind whenever handling noxious gases: (1) Whenever opening the syringe (by removing the syringe cap), do so with the plunger slightly withdrawn so the contents are under a slight negative pressure. Use your thumb to maintain the plunger in this position as shown in the drawing. This will allow a bit of air to enter the syringe but no gas will escape out; and (2) After the gas sample has been generated, discharge the used reagents into a large cup of water to dilute them and prevent further reaction. D. Safety First. Cap Up and Off! The solid reagent is the limiting reagent. If you use too much of the solid, there is a chance that you will generate more than 60 ml gas. This should not happen if you measured your mass of reagent carefully. However, If it seems that more than 60 ml gas is going to be generated, follow these steps. 1. Hold the syringe so the cap is directed upward. 2. Remove the cap by twisting. 3. Discharge the solution to prevent further gas collection. 4. Recap syringe. E. Clean-up and Storage At the end of the experiments, clean the syringe parts, caps and tubing with water. Rinse all parts with distilled water. Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel unless both are completely dry. 4

5 Preparation of nitrogen monoxide and its reaction chemistry. In this experiment, you will prepare nitrogen monoxide gas in a syringe and perform a series of reactions. The common name for nitrogen monoxide is nitric oxide. Take thorough notes in your laboratory notebook and make sure you understand everything as you go. Record all masses, volumes, and observations! When you are finished, you will take a quiz over the experiment. You will be allowed to use your laboratory notebook. A periodic table will be provided. You will not be allowed to use these instructions or any other materials. You will need a calculator. The preparation of nitrogen monoxide utilizes the following reaction. Using the quantities stated below, sodium nitrite is the limiting reagent. 2 NaNO 2 (s) + 2 FeSO 4 (aq) + 3 H 2 SO 4 (aq) à 2 NO(g) + Fe 2 (SO 4 ) 3 (aq) + 2 NaHSO 4 (aq) + 2 H 2 O(l) Part A. Preparation of nitrogen monoxide Each NO(g) preparation uses 0.25 g solid NaNO 2 and about of 5 ml of the acidic ferrous sulfate solution. You will not be given this solution until you have successfully prepared CO 2 and have instructor s initials. 1. Wear your safety glasses! 2. Measure out approximately 0.25 g solid NaNO 2 using the analytical balance. Place the solid directly into the vial cap to prevent loss. 3. Fill the syringe barrel with water. Place your finger over the hole to form a seal. 4. Float the vial cap containing the solid reagent on the water surface. 5. Lower the cap by flotation. Release the seal made by finger to lower the cap into the syringe barrel without spilling its contents. 6. Install the plunger while maintaining the syringe in a vertical position, supported by the wide-mouth beverage bottle or flask. 5

6 7. Fill the weighing dish with the Acidic Ferrous Sulfate Solution. Draw 5 ml of this solution into the syringe. 8. Push the syringe fitting into the syringe cap. 9. Shake the device up and down in order to mix the reagents. Gently help the plunger move up the barrel. Upon mixing the reagents in the syringe with vigorous shaking, gaseous NO is quickly produced. Pure nitrogen monoxide is colorless. A trace of reddish NO 2 is observed at first but soon disappears. The aqueous solution turns black in color. 10. Fill your plastic cup 1/4 full with tap water. With the plunger slightly withdrawn (about 2 ml past from where it stopped) to assure reduced pressure inside the syringe, and with the syringe held cap-up, remove the syringe cap. 11. Discharge the liquid reagent into a cup of water. Immediately cap the syringe to prevent loss of gas. Transfer the liquid into the recovered acids container. 12. With the aid of a short length of Latex tubing, transfer the nitric oxide to a clean, dry syringe. Remember: Clean, dry syringe is on top! 13. (5 pts) Show the gas to your instructor who will evaluate your gas sample. [ ] Gas is colorless [ ] At least 40 ml produced Points: [ ] Syringe clean, dry [ ] Passes NO test Initials: Part B. Preparation of nitrogen dioxide Do not start this section until you have completed Step 13 above with a score of 4 or better. Nitrogen dioxide is prepared by reacting nitrogen monoxide with oxygen gas: 2 NO(g) + O 2 (g) à 2 NO 2 (g) 1. Your nitrogen monoxide sample must be in a clean, dry syringe. Nitrogen monoxide is not water soluble, but nitrogen dioxide is very soluble in water. 2. Clean and dry the syringe used to prepare NO(g). Bring it to your instructor who will fill it with oxygen. 6

7 3. Connect the oxygen syringe to the nitrogen monoxide-filled syringe. 4. Hold the plunger of the nitrogen monoxide syringe firmly in place (so it cannot easily move in the barrel), and gently push the plunger of the oxygen syringe inward. Sometimes the oxygen syringe plunger will move inward by itself. Eventually the oxygen syringe plunger will not easily move any further. The pressure inside the two syringes is about the same as the external pressure. 5. When the reaction is complete, pull the plunger of the syringe now containing nitrogen dioxide outward about 3 ml and then remove the connector tube and immediately cap the NO 2 (g) syringe. 6. (3 pts) Show the gas to your instructor who will evaluate your gas sample. [ ] Gas passes inspection [ ] At least 40 ml produced Points: [ ] Syringe clean, dry Initials: Part C. Preparation of nitric acid Nitric acid is prepared by reacting nitrogen dioxide with water: 3 NO 2 (g) + H 2 O(g) à 2 HNO 3 (aq) + NO(g) 1. Draw 5 ml distilled water into your syringe filled with nitrogen dioxide. Shake the mixture. How do you know a reaction occurred? How do you know it is over? 2. Draw some air (10 ml) into the syringe. Are these results expected? 3. Discharge the solution into a cup containing 20 ml distilled water. Add a drop of phenolphthalein and titrate to a pink endpoint by adding drops of 1 M NaOH. Count the drops. You can assume there are about 20 drops per ml. 4. Dispose of contents down the drain. 7