Purpose

To purify groundwater or surface water using distillation and evaluate its effects.

Expected Learning Outcomes

• Purify a substance using distillation and vacuum filtration
• Evaluate the purity using experimental data.

Introduction

Distillation

Distillation is the process in which a liquid is boiled and its vapor is re-condensed elsewhere to separate it from substances with a higher boiling point. When you vaporize a liquid, each component will have a different partial vapor pressure as determined by Raoult's Law

[latex]P_A = \chi_A P_A^\circ[/latex]

where [latex]P_A^\circ[/latex] is the vapor pressure of the pure liquid, and [latex]\chi_A[/latex] is the mole fraction.^[1]  The more volatile compound (lower boiling point) will have a higher vapor pressure for the pure liquid, thus the more volatile compound will make up a larger proportion of the molecules in the vapor phase.

The liquid condensate is then collected.  Since this was condensed from the vapor phase, it would have the same composition as the vapor phase (and hence contain more of the more volatile component.

The technique requires a heat source, a vessel containing the mixture to be separated, a short vapor pathway to a cooled condenser and a receiver for collecting the condensed liquid.  A typical laboratory distillation apparatus is shown on the following figure.  The condensed liquid collected is referred to as the distillate.

This is a traditional approach used to purify many different chemicals, including how things with a high alcohol content like rum, vodka, and whiskey are created from beer/wine^[2]. But the most commonly used distilled chemical in the chemistry lab is distilled water^[3]. In this lab, we will learn how to distill water from a natural source and then evaluate the purity of our distilled water by using a standard test kit from fish stores.

Determining Concentration of Impurities

To see how well our distillation worked to remove the impurities from our natural water sample, we are going to use several methods to measure the identity and concentration of common impurities. First, we can use the conductivity meter we used in week 2 to measure the total dissolved solids in the sample before and after distillation. However, this doesn't tell us anything about the identity of the impurities, so for that we will need different tests. Fortunately, because of the pool and aquarium industry, there are a lot of very simple commercial resources available to measure the identity and concentration of solutes likely to contaminate water samples. Most of these resources are simple dip-stick tests where you dip a strip of paper that has been treated with proprietary reagents that cause the paper to change color in the presence of a specific chemical. The intensity of the color can then be compared to provided standards to estimate the concentration of that chemical in the sample. This is the way all of the test strips we will be working with work except for the phosphate test strip. When you put the phosphate test strip into the water sample, instead of the test strip changing color, the water changes color and you compare the color of the water to provided color standards to estimate the concentration of the phosphate in the water.

Experimental Procedure

Special Equipment/Supplies

• 100 mL round-bottomed flask
• distillation head
• Condenser
• bent adapter
• 2 Keck clips
• Alcohol thermometer
• Fish water test strips
• 150 mL of natural water
• Conductivity meter

Filtration and Preliminary Observations

1. You will be provided with a sample of water obtained from somewhere in the area to purify. Invert the bottle a few times to thoroughly mix the water sample, then measure out approximately 150 mL of this water. Record the location and appearance of the water.
2. Following the directions in the back matter, set up and vacuum filter approximately 100 mL of water (use a transfer pipet and wet the filter with a few mLs of your natural water sample). Record the appearance and smell (being careful to waft the fumes to your nose rather than to smell the product directly) of the filtrate (the water that passed through the filter) and anything stuck on the filter.
   
3. AFTER starting the distillation, follow the directions on the test kits/strips to determine and record the concentrations of different impurities (sulfate, nitrate, nitrite, carbonate, ammonia), hardness, and pH in the excess filtrate (~50 mL). You should also find the concentration of total dissolved solid (in ppm) using the conductivity meter and the concentration of phosphate using the procedure specified below.
   • See your lab notebook and the lab manual from the Week 2 Significant Figures lab for detailed instructions on how to use the conductivity meter. Note that a large test tube has to be used with the conductivity meter as they are too thick to fit in the small test tubes.

Distillation

1. Create a distillation apparatus as seen above.
   • Keck clips are directional, put the larger side of the clip on the larger part of the joint.
   • Make sure the 3-prong clamp on the condenser has the 2 prongs underneath the condenser, is centered on the condenser, and that the base of the ring-stand is beneath the condenser.
   • use a clean and dry 50 mL graduated cylinder as a receiving container (you can use a 25 mL graduated cylinder and occasionally transfer the liquid to an Erlenmeyer flask if your 50 mL graduated cylinder is too tall
   • Remember to connect the rubber tubing connected to the lower end of the condenser to the water supply and place the other end of the rubber tubing connected to the upper end of the condenser in the sink (if you have trouble getting the rubber tube to stay in the sink, tape it down).
2. Make a note of the temperature in the room according to your thermometer.
3. Add 2 boiling chips to a 100 mL round-bottom flask and measure its mass (use a cork ring to keep the flask from rolling off the balance and breaking). Make a note of which balance you use for this step and be sure to use the same balance to weigh the round-bottom flask at the end of the distillation.
4. Measure out approximately 50 mL of the filtered water into a graduated cylinder (record how much you measured out to the correct number of sig figs) and transfer it into the round-bottomed flask.
5. Turn on the water supply and make sure that there is a continuous flow of water through the condenser, and that the outer jacket is almost completely filled with water. Check with your instructor to ensure that the distillation setup is assembled correctly before proceeding. Make note in your lab notebook about any changes your instructor has you make.
6. Start by heating the flask at a setting of 8 on the Variac (if the Variac has a switch on the side, have this switch set to the bottom setting). Adjust the Variac setting every ~5 minutes by 0.5-1 until you have a drip rate of 2-3 mL per minute. Record the setting, temperature range, drip rate, and how much distillate you collected (and its appearance) during each 5-minute increment.
   • To help keep track of the volume, you can use the rule of thumb that 20 drops = 1 mL, but at some point you will need a more precise measurement of the volume.
   • If your water sample hasn't started boiling after 5 minutes, something is wrong and you should ask your instructor for help.
7. Turn off the Variac and remove it from the round-bottom flask by lowering the lab jack when all of the water has been boiled out of the round-bottom flask.
   • Note that when distilling organic compounds, it's important never to let the flask you're distilling boil dry as some organic compounds can slowly form peroxides over time, which can explode if too much heat is applied (which can only happen after all of the solvent has boiled away). Since we're distilling water, this isn't an issue for us.

Measuring Phosphate

Following the directions below (which are a slight modification of the instructions on the packet for the phosphate test strips) to estimate the phosphate concentration of your natural water sample before and after distillation by comparing the intensity of the blue color in each sample (after adding the test strips) to a provided set of color standards and to estimate the phosphate concentration (you will learn how chemists quantitatively measure concentration next week).

1. Label 2 small test tubes with appropriate names (e.g., before and after distillation). (Put them in a test tube holder found under the sink when you're done.)
2. Measure out 5.0 mL of each sample into the appropriate test tube. Start with your filtrate (pre-distillation), then rinse your 10 mL graduated cylinder thoroughly with house distilled water and let it air dry for a little while before measuring out your distilled natural water sample (move on to "Characterizing the Product" while waiting for the graduated cylinder to dry).
3. Put 1 test strip in each test tube, seal the top with parafilm (ask your instructor if you're not sure how to do this), invert to mix for 10 seconds. Let the test tube sit (with the test strip in it) for 2 minutes, then take out the test strip.
4. Observe and describe the color and appearance of each solution in your lab notebook. Estimate the concentration of each solution in ppb using color standards provided on the test kit bottle by placing the the bottom of the tube on the white circles to the right of the blue circles and looking through the top of the test tube.

Characterizing the Product

1. Describe the appearance and smell (wafting again) of the distillate.
2. Measure the total volume of the distillate using a 50 mL graduated cylinder to the correct number of sig figs.
3. Use the conductivity meter and the provided test strips to determine and record the concentrations of different impurities (total dissolved solids, sulfate, nitrate, nitrite, carbonate, ammonia), hardness, and pH in the distillate the same way you did for the filtrate earlier. Use the procedure above to determine the amount of phosphate present in the distillate as well.
4. After letting it cool to room temperature, describe the appearance and smell (wafting again) of the residue in the round-bottomed flask and measure its mass using the exact same balance you used to measure its mass before distilling.

Waste

The waste from this experiment can be washed down the drain with plenty of water.