Guide for Titration Test

Transcription

1 Guide for Titration Test A titration experiment is used to determine the quantity of an acid/base in a solution. This section explains what you need to execute a titration, and how to perform a titration to estimate the purity of your Sodium Pentobarbital powder. Requirements A set of scales with an accuracy of at least 0.01 grams 0.1M HCl Solution (0.1N) Methyl Orange Indicator Solution (0.1%) Pipettes that allow you to accurately measure 1 ml. I used disposable 3 ml graduated pasteurpipettes. 1 bottle of distilled or de-ionized water 1x 200 ml erlenmeyer (small conical flask) 2x laboratory beaker of 150 or 250 ml Glass or metal stirring rod Optional: ph Indicator paper You may wonder where to buy some of these materials. Distilled water is sold in most supermarkets. I got most of the other items in a single order from a website that sells laboratory supplies. Any country will have some of these websites, use Google to find them. Alternatively, you can look on ebay. Practice: Titration of Sodium Carbonate If you have no experience with titration it is advisable to do a few practice runs. By practicing with washing soda you can learn the experiment without wasting valuable Nembutal powder. Sodium carbonate (Na₂CO₃) is available in every supermarket as washing soda or soda ash. It freely dissolves in water to form a basic solution. Note that you can also use a different base or even plain water for practicing. But sodium carbonate is a good option because it is a base very similar in strength to sodium pentobarbital powder. Preparing the 0.02M sodium carbonate solution Fill a beaker with 100 ml of distilled water. Weigh out approximately 0.57g of sodium carbonate. Add the sodium carbonate to the beaker with water, and stir continuously until fully dissolved. This should create a solution with a concentration of 0.02M Na₂CO₃, although the exact concentration will vary between different brands of washing soda. If you test the ph it should come out at around 11.

2 Performing the titration Put 30 ml of your 0.1M sodium carbonate solution into an erlenmeyer. You can use a scale to weigh out 30 grams of solution, or use a graduated pipette. Add 4 drops of methyl orange to the erlenmeyer. The solution should take on a yellow colour. Pour around 50 ml of 0.1M HCl solution in a glass beaker. This is your titrant. Use a pipette to add 1 ml of 0.1M HCl solution to the erlenmeyer. You will see some purple color where it hits the solution. Swirl the erlenmeyer around to mix the solution until the color stabilizes. Repeat steps 4-5. When you start to see a hint of color change, switch to adding smaller quantities of HCl. You have reached the endpoint of the titration when addition of one drop or less of solution causes the solution in the flask to turn orange-red and stay orange-red. Scroll down to see what the endpoint should look like. Note down the total volume of HCl. You should see a color change after adding around 12 ml of HCl solution, but this may vary depending on your brand of washing soda. The important thing is to learn to recognize the color change. Click here for a video. The real thing: Titration of Sodium Pentobarbital powder Preparing the sodium pentobarbital solution Fill a beaker with 30 ml of distilled water. Put a piece of tinfoil on the scale, and zero the scale. Weigh out around 250 mg of sodium pentobarbital powder on the tinfoil. Record the exact number of milligrams! Add the powder to the beaker with water. Put the piece of tinfoil back on the scale. Subtract the milligrams you see now from the value recorded at step 2. Now you know the exact amount of powder in the solution: write that number down somewhere. Stir the beaker continuously until all powder is fully dissolved. If you test the ph it should come out at around 10. Performing the titration Pour the 30 ml of sodium pentobarbital solution into an erlenmeyer. Use some distilled water to wash out the remaining solution from the beaker, and to wash the solution down the wall of the erlenmeyer. Add 4 drops of methyl orange to the erlenmeyer. The solution should take on a yellow colour. Pour around 50 ml of 0.1M HCl solution in a glass beaker. This is your titrant. Use a pipette to add 1 ml of 0.1M HCl solution to the erlenmeyer. You will see some white-purple precipitation where it hits the solution. Swirl the erlenmeyer around to mix the solution until the color stabilizes and the liquid is clear. Repeat steps 4-5. When you start to see a hint of color change, switch to adding smaller quantities of HCl. You have reached the endpoint of the titration when addition of one drop or less of solution causes the solution in the flask to turn orange and cloudy, and stay that way. Scroll down to see what the endpoint should look like. Note down the total volume of HCl. You will see the solution in the flask turn white/purple where the HCl solution hits because the solution will be acidic there temporarily. But the color should return to a clear yellow quickly as you swirl the flask. As you continue adding more and more HCl solution during this titration you will notice that the purple color and cloudiness will take longer and longer to fade away. This is a signal that you are approaching the endpoint of the titration and need to proceed more slowly and more carefully in order to avoid overshooting the endpoint. While it is fine at the start of the titration to add 1-2 ml of HCl solution at a time, toward the end of the titration (when the orange color and cloudiness are lingering for long periods of time) you might want to add only a drop or a fraction of a drop at a time.

3 Interpreting the result 1 ml of HCl solution corresponds to mg of sodium pentobarbital. Multiply the number of milliliters you found by 24.83, and compare it with the actual number of milligrams powder used. This will yield a purity percentage. Example: If you required 9.0 milliliters of HCl solution to reach the endpoint, this equals: 9.0 * = milligrams When 250 milligrams of sodium pentobarbital powder was used, this means the purity of the powder is: 223.5/250*100 = 89.4% Tips You will know when you are getting close to the endpoint because addition of a small amount of HCl (as little as a drop) will produce a red-ish color that will take a long time to fade as the flask is swirled. Using a sheet of white paper or other white background under or behind the erlenmeyer makes it easier to detect the first hint of a color change. If your mixture becomes bright red you have overshot the endpoint. Thorough mixing of the solution is very important. If you don't have an erlenmeyer, stir gently until the color stabilizes every time after adding the HCl. While titrating, it is a good idea to wash down any HCl solution that sticks to the walls of the flask. Wash it down with distilled water so that all of the titrant gets into the reaction mixture. You can use a pipette to do this. Addition of small to moderate amounts of distilled water will not change the results of your titration. It is very important that your glassware be clean before use. An easy way to clean your glassware is to rinse it once or twice with distilled water and then once or twice with the solution to be used. Be careful with the chemicals, direct contact with your skin will cause skin irritation. What does the endpoint look like? Endpoint in washing soda titration: Endpoint in sodium pentobarbital titration:

4 Guide for Melting Point Test After dissolving, sodium pentobarbital can be converted to pentobarbital by adding an acid. In this melting point test you will check the temperature at which pentobarbital crystals melt. Good quality powder should result in a melting point of around 130 degrees Celcius. Requirements A set of scales with an accuracy of at least 0.01 grams A digital thermometer that with a range of at least 140 degrees Celcius 0.1M HCl Solution (0.1N) or a bottle of white vinegar 1 bottle of distilled or de-ionized water 1x 250 ml or 150 ml laboratory beaker (heat resistant) Capillary tubes for melting point testing A teapot warmer with a large candle in it (or other heat source) Sunflower oil A coffee filter and filter holder A smooth surface to dry the crystals on, i.e. a plate. Glass or metal stirring rod Testing steps Creating pentobarbital crystals with HCl solution Fill a beaker with around 30 ml of distilled water. Weigh out roughly 250mg of sodium pentobarbital powder Add the powder to the beaker with water, and stir continuously until fully dissolved. Add 9-10 ml of 0.1M HCl solution to the beaker. You will immediately start seeing white precipitation. Stir well and let the mixture sit for a few minutes. Now it is time to isolate the pentobarbital crystals from the solution. Pour the solution from the beaker into a coffee filter. You can use some distilled water to wash out the residue from the beaker, and to wash the precipitate in the filter. Leave to dry for a while, then take the residue out of the filter and spread it out thinly on a plate to dry at room temperature. The liquid that dripped through the filter can be discarded. After 24 hours the crystals will have dried out enough to perform the melting point test. Alternatively, you dry the crystals in an oven at 100 degrees Celcius.

5 If you don't have HCl solution, you can use 4 milliliters of white vinegar instead. Vinegar contains other ingredients that can contaminate your sample, so it is better to use HCl solution if possible.i did a succesful melting point test using vinegar, but others have reported failure with vinegar - and success with HCl. So I guess it depends on the type of vinegar you are using. In either case, make sure you do not add too much acid, because the precipitate will disappear again when that happens! Watch the solution carefully and stop adding acid when it becomes really cloudy. Note that after you have completed the titration experiment, you can without problems use the crystals formed in the erlenmeyer. And don't get confused about the term 'crystals': pentobarbital crystals are not the form of crystals that precious stones appear in, but a somewhat coarse white powder. After filtering you may be left with a white sludge, or with a crystalline substance. This varies depending on exactly how much acid you added, how long you let the solution sit, and how long you filtered it. In my experience it doesn't matter if you have sludge or crystals: as long as you dry it out properly the results should be the same. Performing the melting point test Scrape some of the dried crystals from the plate, and crush them up a bit. Press the open end of the capillary tube into the white powder. Do this until you have 2-3 millimeters in the tube. This is important: an overpacked tube gives inaccurate test results! Gently bounce the capillary tube on a hard surface until all powder has dropped to the bottom of the tube. Attach the capillary tube to the thermometer probe with a piece of iron wire. Make sure the bottom of the capillary tube is placed next to the thermometer probe head. Fill a laboratory beaker with 70 ml of sunflower oil. Put the beaker on a heat source. I used a teapot warmer with a large candle. Hang the thermometer in the center of the beaker. Make sure the probe head is a few cm from the bottom of the beaker. Warm up the oil to 100 degrees Celcius, while stirring regularly. When you reach 100 degrees Celcius, set the heat source to a very low setting. Heat the oil further at a rate of 1 or 2 degrees per minute, while stirring constantly. Doing it slowly and evenly is important. Watch closely to see when the powder in the tube starts to melt, and when it is finished melting. The crystals turn from white to transparent when they melt, and you may see some signs of condensation on the inside of the tube. Sodium pentobarbital with good purity should result in a melting point of around 130 degrees Celcius. If your results are in the range C, you can consider the test a success. When using a candle as the heat source, make sure the candle flame is close to the bottom of the beaker, in order to generate enough heat. I had some problems with too much / too little heat generated by the candle, so it is probably better to heat up the oil on a stove. To do this, suspend the beaker with oil in a pan with more sunflower oil (au bain marie), and put that on the stove. Two videos about melting point testing can be found here and here.

6 My test results Using powder from Cherry Chemicals, I performed both the titration test and the melting point test. Titration test: With 256 mg of powder, the titration endpoint occurred after 9.5 ml of HCl solution. This indicates a purity of around 92%. Melting point test: I created three batches of crystals: once I used the crystals created during the titration experiment, once I used HCl solution, and once I used white vinegar. All three batches melted between 129 and 130 degrees Celcius. So both tests indicate the powder is of good quality. 0

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