INTRODUCTION TO THE SPECTROMETER COLOR OF AN ACID-BASE INDICATOR

Transcription

1 INTRODUCTION TO THE SPECTROMETER COLOR OF AN ACID-BASE INDICATOR The main purpose of this experiment is to introduce you to one of the most fundamental and th important instruments in the chemical laboratory, the spectrophotometer (Silberburg, 5 Ed., p ). A spectrophotometer is an instrument that measures the amount of light that is absorbed by a sample. As you will see in the next experiment, this amount or absorbance is directly proportional to the concentration of liquid samples and it is this feature that makes the spectrometer so useful in the lab. Spectrophotometers (or devices that incorporate them) are probably the most common means for determining concentrations of solutions and we will be using them for this purpose in several experiments this semester. However, in this experiment, we will not measure concentration. Instead the goal will be to measure the visible absorption spectrum of colored solutions. The visible absorption spectrum is the amount of light that a compound absorbs at each wavelength of visible light. Note that the color you see is the color of light that is not absorbed by a sample. For example, a compound will appear blue if it absorbs red and yellow light. (You may want to check out Figure 7.8 on p.276 of your text to remind yourself what the visible spectrum looks like.) The colored solutions you will be studying in this experiment will be solutions of an acidbase indicator at two different ph s. Acid-base indicators are compounds that change color when the ph changes. This occurs because the molecular structure of the indicator changes when the ph changes. Indicators are commonly used to roughly measure the ph of solutions or detect the endpoint of an acid-base titration. OBJECTIVE Examine the color change of an acid-base indicator by using a spectrophotometer to measure the visible absorption spectrum of the indicator at two ph s, one strongly acidic, the other strongly basic. PRINCIPLES A basic outline of a simple spectrophotometer is shown in Figure B7.3 on p.282 of your text. There is a light source that produces light over the desired wavelength range. This light goes through a monochromator which separates the light into its component wavelengths and only lets light of set wavelength through. The monochromatic light then passes through the sample, and the intensity of the light that gets through is measured by a detector on the other side. The relative amount of light that gets through is described by a quantity called transmittance, T, which equals the intensity of light hitting the detector in the presence of the sample, I, divided by the intensity of the light in the absence of the sample, I : 0 revised 1/25/12 1.1

2 More light will get through the sample (be transmitted) if less light is absorbed by the sample. This means that the absorbance, A, of the sample should be inversely related to the transmittance, T. The exact relationship is given below. One thing that is important to realize is that spectrophotometers really measure transmittance although they can be set to read out in absorbance units, which is what we will be doing. A consequence of this is that there is a limitation to how large of an absorbance that can be measured accurately. This is because if the absorbance is large, then not much light is getting to the detector and it won t be able to measure the intensity of that light very accurately. In general, absorbance values above 2.0 are suspect. If you measure absorbance values above 2.0, the sample should be removed, diluted and measured again. For best results, the maximum absorbance should be in the range of 0.10 to 1.0 absorbance units. All instruments used for chemical analysis need to be calibrated in some manner before use. With spectrophotometers the minimum calibration that needs to be performed is to set the zero absorbance reading by using an appropriate BLANK solution. The blank solution should ideally contain all of the components of the sample solution (the one that is being measured) EXCEPT for the compound or compounds of interest. SAFETY Wear approved eye protection, a lab coat/apron and closed toe shoes at all times while working with chemical solutions in the laboratory. The acids and bases used in this experiment are fairly dilute, but they may be irritating to the skin. Wash the affected area thoroughly with water if you accidentally spill any of the solutions on your skin. revised 1/25/12 1.2

3 EQUIPMENT AND MATERIALS Each group will need the following equipment: QTY Equipment QTY Chemicals 1 Spectrophotometer 10 ml ph 2 aqueous soln 4 Cuvettes 10 ml ph 11 aqueous soln 4 drops acid/base indicator The following acid/base indicators will be available in the lab: bromocresol green, bromophenol blue, bromothymol blue, methyl orange, phenolphthalein, thymol blue. The spectrophotometers will be set up on the lab benches. Spectrophotometer cuvettes, ph 2 and ph 11 stock solutions and the different indicator solutions will be provided in the lab. Each group will be assigned one indicator. WASTE Create a local waste container by labeling the largest beaker available and using it to collect your rinses and waste solutions. At the end of the lab, pour the collected waste into the main Hazardous Waste container located under the hood at the back of the lab. The amount of water for this experiment should be ml per group. PROCEDURES Your TA will divide the class into groups for this experiment. Each group of students will be assigned a particular indicator to investigate. Sample Preparation 1. Rinse the cuvettes twice with the solution to be measured. Fill two of the cuvettes two-thirds full with ph 2 (acidic) solution and the other two cuvettes two-thirds full with ph 11 (basic) solution. 2. Add 2 drops of the indicator to one of the cuvettes containing the ph 2 solution and 2 drops of the indicator to one of the cuvettes containing the ph 11 solution. Mix thoroughly. This can be done by covering the opening with a small piece of parafilm and shaking. Remove the parafilm and dislodge any air bubbles by gently tapping on the side of the cell. Wipe off any finger prints or liquids on the outside of the cuvette. Place the cuvettes on a paper that you have marked to indicate which cuvette is which. Be careful to not mix them up! The cells which don t contain indicator will be the BLANKS. revised 1/25/12 1.3

4 Operation of the Spectrophotometer 1. If the instrument is not already on, turn on the instrument with the Power Switch (rocker switch on the back of the instrument, left side). Allow the spectrophotometer to warm up for at least 15 minutes to stabilize. 2. After the warm-up period, set the desired wavelength with the Wavelength Control buttons [(nm ) or (nm )]. 3. Set the display mode to ABSORBANCE by pressing the MODE control button [ (A/T/C) ] until A shows on the right side of the display. The instrument will now automatically convert transmittance to absorbance for you. 4. Place the sample cell containing the BLANK in the sample compartment and align the clear walls with the arrow on the sample compartment. It is important to place the cuvettes in the same position to reduce variables in the readings; usually they have a mark on them for this purpose. Press the cell firmly in the sample compartment and close the lid. 5. Adjust the display to 0.0 A by pressing the zero adjust button. 6. Remove the BLANK cell and insert the cell containing the sample to be measured. Align the clear walls with the arrow on the sample compartment. Press the cell firmly in the sample compartment and close the lid. 7. Read and record the absorbance value in your notebook. Data Collection Follow the directions above to measure absorption values for the ph 2 and ph 11 solutions at wavelengths between 400 and 700 nm. Start at 400 nm and increase the wavelength by 10 nm increments. Each time you change the wavelength you need to first insert the appropriate BLANK solution and zero the absorbance as described in steps 4 and 5. Then put in the corresponding sample and measure and record both the wavelength and the absorbance value in your notebook. REPORT Follow the general outline given on pp of this manual. In the Procedures section, refer to the notebook pages that contain the pre-lab for this experiment and give the indicator you used for your experiment, along with any other changes you made in the procedure. The Experimental Data section should consist of graphs of the measured absorption values (y axis) versus wavelength of light (x axis) for the indicator at ph 2 and ph 11. These are the visible absorption spectra for the indicator at these ph s. No Calculated Results or Sample Calculations are required for this report. The Discussion section of your report should also follow the general format. In this case you can summarize your experimental data by indicating the observed color and the wavelength of maximum absorption at each ph for your indicator. (If the indicator was colorless at one of the ph s, you don t revised 1/25/12 1.4

5 need to give a wavelength, just say it was colorless.) Discuss the reasonableness of your results by explaining why (or why not) the observed color agrees with the absorption spectra. (Note that the relationship between color and ph depends on the indicator so this is not something you can use to decide whether the results are reasonable. Many students, having used litnus paper to test for acid/base character, think base is blue and acid is red, but this is only true for the indicators in litnus paper, it is not generally true.) Also, mention any clear problems there are with the data such as negative absorption values. Conclude with a discussion of possible sources of error. revised 1/25/12 1.5