Figure 1: Sample UV-vis Spectrum
Identifying Colors in Water-Based Inks
Learning Goals
1. To compare and contrast two techniques in identifying components in an unknown solution
2. To use paper chromatography and UV-vis spectroscopy to identify the component colors of water-based inks in an aqueous (water) solution.
Abstract
Paper chromatography is a specialized form of chromatography that is relatively fast and inexpensive yet powerful enough to separate a range of different chemicals. This type of chromatography uses a piece of paper and a solvent to separate the components of a mixture. Paper chromatography is used in this experiment to separate and identify different colors of water-based inks in an aqueous solution.
UV-vis spectroscopy is another technique to identify what is in a chemical sample. A UV-vis spectrometer is an instrument used to measure the amount of ultraviolet (UV) and visible light absorbed by a solution. Depending on the substance, the chemical absorbs a certain amount of light, which causes electrons to move from one energy level to a higher one. The amount of light that reaches the instrument's detector is then recorded as a spectrum (Figure 1).
Figure 1: Sample UV-vis Spectrum
Pre-lab Assignment
1. View the video clips on Using the UV-vis spectrometer. You will need Quick Time video player to see them.
In your lab notebook, prepare the following information:
2. A brief (2-3 sentence) introduction to the lab.
3. A table of safety information including the chemicals used in the lab and any safety handling precautions.
4. A data table to record the brand name and color of ink used, and the results of the paper chromatography and UV-vis spectra peaks (See sample table in the Analyzing Data section).
Give this information to your teacher at the beginning of the lab. You will not be allowed to work in the lab without this information.
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Chemicals
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Equipment and Supplies
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Instruments
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| 7 colors of water-based markers | Chromatography paper | UV-vis spectrometer |
| Water | 4--250-mL beaker | |
| 8 UV-vis cuvettes | ||
| 7 wooden splints | ||
| paper towel | ||
| pencil | ||
| ruler | ||
| scissors | ||
| tape | ||
| 3.5" floppy disk for data storage |
Procedure 1: Paper Chromatography
1. Cut 7 strips of chromatography paper about 15 cm long. Tape one end of the strip to a wooden splint. Repeat for each strip.
2. Collect 7 different colors of water-based ink pens. Be sure to choose the three primary colors (red, yellow, blue), three variations of secondary colors (green, purple, orange), and either a brown or black.
3. Using a pencil and ruler, draw a straight horizontal lab approximately 1.5 cm from the bottom of the first strip of chromatography paper. Draw a small 'X' on the line to mark the starting spot for your sample. Label the top of your strip with the brand name and color of the ink.
NOTE: You must use a pencil to label the strip. Ink from a pen or marker may move with the solvent and confuse your results.
4. Gently touch the tip of the pen for the first color on the 'X' on your chromatography strip. Allow the ink to form a small spot about 3 mm in diameter. Allow the spot to dry, then repeat this step 2-3 times to concentrate the ink.
NOTE: Avoid contaminating your strips with other solutions or chemicals in the lab. Place the strips on a clean paper towel. Always hold the strips at the sides with your fingertips.
5. Repeat the spotting procedure with your other colors of ink.
6. While the spots are drying, prepare the solvent. Add approximately 25-50 ml of water to each beaker.
7. Insert one of the prepared chromatography strips into one beaker so that the bottom of the strip just touches the solvent, but the ink spot does not dip into the solvent. The strip should not touch the sides of the beaker. Balance the wooden splint along the top of the beaker. Carefully add a second strip in the same way, being careful that the two strips do not touch.
8. Repeat for your remaining strips.
9. Allow the strips to remain in the solvent for about 5 minutes. Remove each strip and lay it on the clean paper towel. Use your pencil to mark the farthest distance the solvent (water) traveled up the paper. This is called the solvent front. The completed strips are called chromatograms.
10. Dispose of the solutions. All of the samples in this lab are nontoxic, so you may rinse the solutions down the drain. Clean all of your cuvettes and glassware. Return all your supplies to their proper place.
11. Save your chromatograms for your lab report.
Procedure 2. UV-vis Spectroscopy
You should view the video clips on Using the UV-visible spectrometer before you attempt this procedure. You will need Quick Time video player to see them.
1. Obtain 8 clean, dry cuvettes.
2. Fill the first cuvette half full with distilled water. Label the solution "blank" with a small piece of tape on the side of the cuvette with ridges. Set this aside.
3. Cut four strips of chromatography paper about 5 cm long. Cut each strip in half the long way. You should end up with 8 strips, approximately 1 cm wide x 5 cm long.
4. Take one of the colors used in Procedure 1. Color both sides of one strip about 3-4 cm along its length.
5. Place the paper, colored, into an empty cuvette. Fill the cuvette about half full with distilled water. Ink should move from the paper into the water. Gently move the paper within the cuvette to speed the process. Label the solution with tape on its ridged side.
6. Repeat steps 4 and 5 for the other colors of ink.
7. Take the cuvettes along with your data table to the UV-vis spectrometer.
8. Log on to the instrument, using your account name. Insert your disk into the computer.
9. Take the cuvette with the distilled water and use a Kimwipe or similar tissue to remove any water or fingerprints from the clear sides of the cuvette. To blank the instrument, insert the cuvette with distilled water into the instrument holder. Push gently to make sure the cuvette is all the way into the holder. The blank records what the solvent (water) and the cuvette "look like." The instrument then electronically subtracts the cuvette and solvent when it records a sample spectrum. TIP : The clear sides of the cuvette must face the holes in the sides of the sample compartment. The ridged sides should face the front of the machine. Click the 'blank' option.
Scanning samples
10. To scan your ink samples, insert a cuvette into the instrument holder. Click the 'scan sample' option. If the machine is working properly, you should hear a click as the machine "reads" the sample. The sample spectrum should appear on the screen. The data are automatically saved onto your disk. NOTE: You will need to record the filename of each spectrum along with the name and color of the sample. The filename consists of the date (yymmdd) and a sample number (ex. 010922~1).
* Before continuing, check with your instructor to make sure that your sample spectrum is showing a reasonable graph. A 'good' spectrum has peaks that are smooth rather than jagged, the curve returns to the baseline, etc. If your sample did not produce a usable spectrum, try one or more of these tips:
a. Make sure all the components of the computer and spectrometer are turned on.
b. Re-blank the machine, making sure that the clear sides of the cuvette with distilled water are properly aligned in the instrument holder.
c. Try the first sample again, making sure to wipe the cuvette and insert it properly into the holder.
d. If the spectrum has plateaus instead of distinct peaks, the solution may be too concentrated. Dilute the sample and re-scan. Record your dilution.
11. Scan your remaining ink solutions, remembering to record the filename, name and color of each sample.
12. Make sure that you are at the ICN site.
a. If you are using a data disk, insert the disk into the A: drive. Click on the spectral analysis button on the main ICN screen. Enter 'a:' for your account name. Your spectra files should appear.
b. If you are using a direct Internet connection, click the 'Analyze spectra' button. Enter your account name and password. Your spectra should appear.
13. a. Find the wavelength of maximum absorbance from the x-axis for each of the known samples. The maximum absorbance is the absorbance reading (y-axis) for the top of the peak. The wavelength for the maximum absorbance is read from the x-axis. The shape and location of the peaks in the UV-visible spectra are defined by the wavelength and maximum absorbance. If more than one peak is present -- as you should find in the secondary colors -- find the wavelength of maximum absorbance for each peak. Record these values.
b. You can 'zero in' on the spectra by changing the scale of the x- and y-axis. For example, to change the absorbance scale, highlight the '4' and type in '1'. You may change the wavelength scale by highlighting the '750' and typing '700'.
14. Summarize the data in the following data format:
| Name Brand of Pen | Color of Ink | Colors Present (Paper Chromatography results) | Wavelength(s) of maximum absorbance | Absorbance at Wavelength of Maximum Absorbance |
Post-Lab Assignment
Your lab report should include the following information:
1. Your name, partner's name, date and title
2. Data table
3. Spectra printouts
4. Questions: Compare the spectrum of the secondary colors (green, orange, purple) and a black or brown ink with those of the three primary colors.
a. Identify the colors present in the non-primary colors using the wavelengths of the primary colors. Support your answers with specific data from the spectra.
b. How do the peak absorbances compare
1. within a spectrum (if there is more than one peak)?
2. between spectra containing the same colors (ex. the blue peak in blue and green spectra)? What do these differences indicate about the concentration of the different solutions?
