Full Lab Manual
Introduction & Goals
Chemistry & Background
In Your Write-up
In this week's experiment, you will use volumetric glassware, including pipets and volumetric flasks. You will also perform quantitative transfer. Finally you will use a Spectronic 20 colorimeter. Be sure you know how to uses each one, before coming to lab this week.
The Beer's Law test will be performed with a partner on a shared set of solutions. The Cobalt Analysis will be performed individually, on each student's sample. Partners will share a Spec 20, but each must analyze his or her own sample. The gravimetric analysis is done individually, on a separate sample of your product.
Start by measuring the mass of your dried, recrystallized product from last week with the analytical balance and recording its mass in your notebook. You will need at least 0.5 g of pure compound to complete all three cobalt analyses in this and next week's lab. If you have less than 0.5 g final pure product, turn in all your product to your TA and receive a 0.5 g sample for analysis in return, so you can analyze a consistent sample. If you have more than 0.5 g of material, save the remainder for the experiments to be done next week and the following week.
Begin the gravimetric analysis at the start of the lab period. While your sample heats, you can perform the other experiments. Any remaining product should be returned to the sample box, in its labeled vial.
To complete your uncertainty analysis, you will need the uncertainty of measurements you make with a volumetric flask, pipet, and analytical balance. Be sure to record these values in your lab notebook.
Do not heat your sample above 120 °C--explosions could occur! Compounds containing both NH3 and nitrate are often explosive if activated by high temperatures.
Begin this procedure immediately at the start of the laboratory period to insure enough time to finish. First measure and record the mass of an empty, dry, uncapped, and unlabeled vial. Then, accurately weigh between 0.2 and 0.25 g of your cobalt salt as a layer up to 0.5 cm thick in the vial. In addition to accurately determining the mass of your cobalt salt, it is useful to also record the total mass of the vial and the cobalt salt. For your uncertainty analysis, record the uncertainty of each mass measurement you make. The uncapped vial plus a slip of paper with your name then go in a small beaker which you place in a 100-110 °C oven for 45 minutes (record the temperature and times in your notebook). After the heating period is over, place the uncapped vial on the bench top to cool for 5 minutes and then in a glass dessicator jar for dry cooling for about 20 minutes or until room temperature. Don't burn your fingers, use tongs! Weigh accurately when the vial has cooled to room temperature. (Warm objects create air currents and erratic weighings.) Repeat this heating/cooling/weighing cycle reducing the oven time to 20 to 30 minutes. Repeat at least once more, until two weighings agree to within 0.0003 grams. Then you can be confident that the dehydration reaction is complete. Make notes of any changes in physical appearance that occur (small changes in the ligands around the Co can often significantly affect the color, for example). When your gravimetric analysis is complete, and all data are recorded, dispose of your dehydrated sample in the waste drum. Do NOT leave salts in ovens, if you have to continue to week 3, but put them back in the sample box, labeled with your name.
Beer's Law Test
In order to insure that Beer's Law is obeyed under the prevailing instrumental conditions and to obtain an accurate value of the molar extinction coefficient for use in the analysis of your aquopentamminecobalt, a Beer's Law calibration plot will be constructed for the [Co(SCN)4]-2 ion. You will be sharing standard solutions and a Spectronic 20 colorimeter with a partner, producing one Beer's Law plot for both partners to use.
The undiluted stock solution has a concentration of about ~3 x 10-4 M and an absorbance of about 0.7 at 625 nm, the wavelength of maximum visible light absorption. Be sure to write the exact concentration in your lab notebook. With your partner, prepare the dilutions by carefully pipetting 5.0 mL, 10.0 mL, 15.0 mL, and 20.0 mL aliquots of the stock solution into 25.0 mL volumetric flasks and diluting to the mark with the 50 % acetone / 50 % water solution provided. Accurate dilutions are required for accurate determination of the molar extinction coefficient. Calculate and record the final concentrations for each dilution.
Measure the absorbance of the undiluted stock solution and the four dilutions at 625 nm using the Spectronic 20 colorimeter. Remember to calibrate the colorimeter between each absorbance reading with a blank of 50 % acetone / 50 % water solution. It is vital to prepare the graph as the data are recorded to check immediately on the quality of agreement between the measurements and Beer's Law. A good straight line fit to the data should be possible. If your data do not form a straight line, find the problem and fix it before continuing.
Procedure for Cobalt Analysis
Accurately weigh about 20 mg (0.02 g) of pure, recrystalized [Co(NH3)5H2O][NO3]3, dissolve this sample in water, and carefully dilute to 25.0 mL in a volumetric flask. Pipet a 2.00 mL aliquot of the solution into two different 30 mL beakers. The optimum quantity of cobalt in an analysis sample is about 5 x 10-3 mmol. A corresponding quantity of complex is less than 2 mg, which is too small a sample to weigh directly on an analytical balance. Therefore, the procedure calls for weighing 20 mg and obtaining a known fraction of this sample by volumetric means. Even 20 mg must be weighed and transferred with great care to achieve good results. For your uncertainty analysis, record the uncertainty in each mass and volume measurement you make.
Working in a fume hood, add 2.0 mL 1 M NaOH to each of the two sample beakers using a Mohr pipet or graduated cylinder. Heat the solutions on a pre-heated hot plate for 15 minutes. Heat gently, just below boiling, and do not allow your samples to boil dry. At the end of this period, all color should be gone from the solution, and a finely divided brown precipitate should be seen. Add 2.0 mL 6 M HCl and continue heating for 10 minutes during which time the solid should dissolve giving nearly colorless solutions. Again, do not allow your solutions to boil dry. During this waiting time, prepare an ammonium thiocyanate solution by dissolving 4.0 g NH4SCN in 7.0 mL of H2O.
The following procedure should be carried out for each sample, one at a time. Transfer the sample quantitatively to a 25.0 mL volumetric flask, but do not use more than 4 mL of H2O in rinsing the contents of the beaker into the volumetric flask. Add 12.5 mL of neat (100%) acetone and 2.0 mL of the NH4SCN solution and shake the flask vigorously to ensure mixing. Dilute the sample to the mark with water. Mix and transfer to a colorimeter cuvette. Record the absorbance at 625 nm, using the same colorimeter for these samples that was used for the Beer's Law Plot. Note that a dilution factor of 12.5 results from this procedure. This dilution factor of 12.5 arises because only 2/25 of the original solution was taken and then diluted back to 25.0 mL in the preparation of each of the two aliquots for analysis.
Use the absorbance readings of your two samples and the molar extinction coefficient from your Beer's Law Plot to determine the cobalt concentration of your two samples.
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