Define reflux (what is the purpose of refluxing?). How is the process of refluxing different than that of distillation?
What mass of \( \ce{Fe(NH4)2(SO4)2 * 6H2O} \) will you use to prepare a 0.0500 M aqueous solution of \( \ce{Fe(NH4)2(SO4)2 * 6H2O} \)? You will make 100 mL of this solution.
Introduction
In this experiment, the alcohol content (% ethanol by volume) will be determined. The experiment uses a chemical method based on the principles of redox titration. This method can be broken into two parts: the oxidation of ethanol with an oxidizing agent and a back-titration of the excess oxidizing agent. First, the oxidation of the ethanol (\( \ce{C2H5OH} \)) is carried out by the addition of excess potassium dichromate to form products that include \( \ce{Cr^{3+}} \) and acetic acid. The balanced chemical equation for this redox reaction is:
Second, the amount of the excess oxidizing agent is determined by back-titrating the resulting solution of the reaction above with an aqueous solution of ferrous ammonium sulfate, \( \ce{[Fe(NH4)2(SO4)2]} \). The balanced chemical equation for this redox reaction is:
Wear your goggles. All solutions containing \( \ce{Cr} \) and \( \ce{Fe} \) must be disposed of in the appropriate container. All other solutions may be rinsed down the drain.
Experimental
Students will work in pairs during this experiment. One unknown will be provided for each pair. However, each student should submit a laboratory report for grading and indicate the name of their partner.
An unknown sample will be provided by the dispensary. Measure 25 ml and transfer the unknown sample into a clean 100 mL distilling flask. In the fume hood, set the flask in a 400 ml beaker halfway filled with ice, then slowly add 20.00 mL of a 0.2500 M potassium dichromate solution. Very slowly add 15 mL of concentrated sulfuric acid. Slowly swirl the flask in the ice bath. Add a 1 inch magnetic stir bar in the flask.
Set up the reflux apparatus shown in Figure \(\PageIndex{1}\). Apply high vacuum grease around the joint, secure with a keck clip then gently turn on the water flow. Close the fume hood sash then keep the alcohol/water solution at a very low boil for 1 hour. It is critical that the solution is heated carefully and slowly so that vapors are not allowed to escape.You should be able to see the solution condensing near the bottom of the reflux condenser at all times. Refluxing is a method utilized to boil a solution without losing any vapors. Often refluxing is utilized to favor a reaction that is stable to heat, which would otherwise react very slowly at room temperature.
.Figure \(\PageIndex{1}\)
While waiting for the reflux to finish, this will be a good time to start making the titrant. Carefully weigh out the appropriate mass of \( \bf{\ce{Fe(NH4)2(SO4)2 * 6H2O}} \) that you calculated in your pre-laboratory preparation. Record the mass to the fourth decimal place (±0.0001) and place the solid in a volumetric flask (what size do you need?). Fill the volumetric flask two-thirds full with deionized water, stopper or seal the top with Parafilm, and swirl/invert the flask to dissolve the solid. Fill the flask to the volume mark on the neck of the flask; use a wash bottle to adjust the meniscus to the exact volume. Load up the buret with the titrant. 100 mL of titrant is needed for the entire experiment.
After reflux, put the reflux solution into a clean 250 mL volumetric flask and dilute to volume. Transfer 10.00 mL of the contents from the volumetric flask into an 125ml Erlenmeyer flask, add 10 drops of Diphenylamine sulfonate indicator, and titrate with 0.0500 M \( \ce{Fe(NH4)2(SO4)2 * 6H2O} \).
The initial color of the solution is a rather unsightly greenish-brown that turns violet when the titrant is added. The endpoint is indicated by the observation of a light green solution or the disappearance of the violet color. Please note that the rate of this reaction may be slow, so it is important that you do each titration in about the same amount of time. Perform several titrations until good agreement is reached (at least 3 times).
Calculations
Calculate the percent ethanol by volume for each titration. Students commonly become confused by dilutions. Approach the calculation in a careful methodical fashion, and you will get the correct answer! Using a diagram might be helpful to determine when you need to consider dilutions. Additionally, keep in mind that you have performed a back-titration. Ethanol has a density of 0.789 g/mL at room temperature. Report an average, standard deviation, relative standard deviation, and 95% confidence limits.
Clean Up
All solutions containing \( \ce{Cr} \) and \( \ce{Fe} \) must be disposed of in the appropriate container. After emptying the glassware, rinse with a small amount (~5 mL) of water and put the rinse into the waste as well. Also, place any excess dichromate solution in the proper waste bottle in the hood. Wash all glassware with deionized water. Be sure to carefully clean your burette and fill it with deionized water before placing it back in the burette rack.
