Appendix II General Instructions for the Operation of the Blocktronic Spectrometer
PART 1: Blocktronic Unit This section contains an introduction to the use and operation of the Blocktronic spectrometer. It is intended to be used as a reference and a guide to the operation of the Blocktronic. The Blocktronic is a simple, inexpensive spectrometer that uses a computer to measure light transmitted by a solution. From this information, the concentration of the solution may be obtained using Beer's Law. The Blocktronic can either measure absorbance continuously, or at specific time intervals. The Blocktronic unit consists of a green light emitting diode (LED), which acts as a light source and monochometer and a cadmium sulfide photo cell which is the detector. The sample holder is a 13 x 100 mm test tube. All of the components are held together in a block made from two pieces of 2 x 4 inch lumber. The Blocktronic is connected to the gameport of an Apple II computer via an adapter box. A light shield, usually a cardboard box, covering the entire unit during use completes the physical makeup of the Blocktronic spectrometer. A Project Seraphim computer disk contains the software needed by the computer to make the measurements.
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The Blocktronic works on the same principle as all spectrometers. Light from the diode passes through the sample and strikes the CdS photocell, altering its resistance to the passage of electricity. The amount of change in resistance is dependent on the intensity of light that strikes it. The change in resistance is monitored by the computer. The computer compares this resistance to a reading previously established by the user when no sample was present in the light path and reports the result as relative absorbance. The CdS detector is most sensitive to green colored light, so the LED used is usually green. The maximum emission of the LED is in the range of 500-600 nm. The Blocktronic is essentially a monochromatic instrument. It is useful for only certain colors of solutions (Can you tell which colors?) and cannot be used to obtain absorption spectra.
Specific instructions for using the Blocktronic are given below. The information has been organized into modules describing the individual tasks involved. Read the information carefully. When you need to perform a Blocktronic experiment, refer to the appropriate modules for the step by step instructions.
Extending from the Blocktronic unit you will find 4 colored wires which must be
connected to the adaptor box. The adaptor box is a small box with eight labeled push- button connection ports on top and a multi-wire cord with a plug extending from one side. Find the red and yellow wires on the Blocktronic. Twist together the exposed copper extending from these wires. Push one of the +5 volt buttons on the adaptor box and insert the twisted wire into the port. Release the button to close the port. Connect the end of the green wire, in the same way, to the GC0 port and connect the black wire to the GND (ground) port. Tug gently on the wires to assure that they are tightly held.
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Examine the adaptor box cord. If it has a round cord with a 9-pin plug, it can be plugged into the matching external port on an Apple IIe, IIc, or IIgs computer. If the cord is a length of ribbon wire with a 16 pin dip adaptor, it must be plugged into an internal port, as described below. Remove the lid cover of the Apple II computer. Locate the game port. It is a small rectangle on the bottom of the interior board, usually near the right rear corner. Look at the adaptor box plug. Notice that the prongs on the plug fit the pattern of holes in the game port with the plug turned two different ways. However, the instrument will not operate unless the plug is oriented correctly. On one side of the plug will be a small dot, notch, or arrow. This must be lined up with a similar dot or notch on the game port. Plug the adaptor into the game port with the marks properly aligned. Be very careful as the prongs are thin and easy to bend. Feed the cord through one of the slits in the back of the computer. Before replacing the lid, turn the computer on and check to see that the LED is lit. If not, review the steps you have taken to this point. If you cannot find an error consult your instructor.
With the Project SERAPHIM disk inserted into the disk drive, turn the computer
on. It will "autoboot" the disk. Follow the instructions at the bottom of the screen to step through the title material. A menu will appear titled "Program Entries." Select option 1, General Laboratory Interfacing, by pressing the RETURN key while this option is highlighted. Use the arrow keys to move the highlight if necessary. From the General Laboratory Menu select option 4, Game Port Status, by typing "4." Read the information and follow the instructions at the bottom of the screen. Check to be sure that GC0 is less than 255. If not, the Blocktronic unit may be improperly connected. Consult your instructor. Place the light shield (cardboard box) over the sample opening. The value displayed on the screen should now be approximately 60 units. If it is not, adjust the potentiometer by turning the dial slightly (consult your instructor if you are not sure how to do this). After observing a constant value of about 60 units, type "Q" to return to the menu. When you have returned to the General Laboratory Interfacing menu, select option 2, Blocktronic, by typing "2." Read the material and follow the directions at the bottom of
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the screen to obtain the Blocktronic Main Menu. Type "1" to select the game port control option. Choose game port control, GC0, by typing "1" again. This assures that the computer is monitoring the channel (GC0) on which the signal from the Blocktronic is being sent. When you have returned to the Blocktronic Main Menu, the instrument is ready for use.
The purpose for establishing a blank reading is to let the computer know the intensity of the light when none is absorbed by the sample. Because the sample holder and solvent may absorb some of the light available from the source, the blank should include everything that will be present in the sample solution except the substance being studied.
From the Blocktronic Main Menu, select option 2, Read Solution Blank, by typing "2." Select a clean 13 x 100 mm test tube to serve as the sample holder. Make sure it fits easily into the Blocktronic opening. Using a marking pen, draw straight lines on the top of the Blocktronic extending out from the sample opening and on the test tube, near the opening, as shown in the figures below. (These lines may already have been drawn for you.)
Each time the test tube is placed in the sample holder of the Blocktronic the line on the test tube should be exactly lined up with the mark on the Blocktronic. This is done because the glass used to make the test tube may contain flaws, scratches, or variations in thickness which may absorb light. By keeping the test tube aligned, absorption from these flaws becomes constant and does not cause errors in the experimental data.
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Rinse the test tube with a little of the blank solution. (Any spectroscopic experiment you run in this class will specify which solution is to be used as the blank.) Dispose of the washing and then fill the test tube about 3/4 full with the blank solution. Carefully wipe the outside of the test tube with a Kim wipe or tissue and insert it into the Blocktronic. Cover the Blocktronic unit with a cardboard box to act as a light shield. Press the space bar to begin taking measurements. Words and numbers will flash on the screen as the computer works. When a stable value is obtained, hit the space bar to save the value and return to the Blocktronic Main Menu.
After you have calibrated the Blocktronic with a blank solution, the absorbance of that solution should be 0.0. It is a good idea to check to be sure this is so. Select option 3, Continuous Light Sensing, from the menu by typing "3." The relative absorbance of the solution will be displayed on the screen. The computer will take readings continuously, every few seconds. The absorbance displayed on the screen will flash with each new reading. Wait a few seconds for the readings to reach a constant value. Sometimes it takes longer than a few seconds for the Blocktronic to settle down to a constant reading. If the absorbance of the blank is not zero, you should recalibrate the Blocktronic. Repeat the procedure in this section. It may be necessary to do this two or three times before a zero reading is obtained for the blank. This is perfectly normal and should not be considered cause for alarm. If the Blocktronic is not properly shielded, or if a great deal of intense sunlight is incident on the unit it may be impossible to obtain stable readings.
It is a good idea to recalibrate the Blocktronic before each series of measurements.
In any experiment you must use only one 13 x 100 mm test tube as the Blocktronic
sample holder. Imperfections in the glass cause variations in light transmissions, so it is important to keep this variable constant by using the same test tube and keeping the marks on the tube and the Blocktronic lined up for each solution. Be sure to rinse the test tube very carefully when changing solutions. If another person has used the Blocktronic after you calibrated it, be sure to recalibrate before making any more measurements.
Calibrate the Blocktronic with the blank solution as directed in Section III.
From the menu, select option 3, Continuous Light Sensing, by typing "3."
Remove the test tube containing the blank from the Blocktronic. Save the blank solution for later use. Rinse the test tube with a small amount of the first solution to be tested. An easy way to do this is to add 2 - 3 mL or solution, cover the opening of the test tube with a small piece of parafilm or wax paper and shake vigorously. Dispose of the rinsing. Fill the tube about 3/4 full of the solution. Wipe the outside of the test tube with a Kim wipe or tissue and place it in the Blocktronic. Be careful to line up the mark on the test tube with
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the mark on the Blocktronic. Cover the Blocktronic unit with the box. The relative absorbance of the solution will be displayed on the screen. Wait a few seconds for the reading to stabilize and then record the absorbance of the solution. Sometimes it takes longer than a few seconds for the Blocktronic to settle down to a constant reading. If the fluctuation in readings are small, ±1 or 2 in the last decimal place, this is considered normal. Record the average or most often observed value. If the fluctuations are much larger, remove the sample and recalibrate the Blocktronic with the blank, as directed in Section III. If this does not correct the problem, consult your instructor. Measure the absorbance of the remaining solutions in the same way. Be sure to rinse the test tube thoroughly when changing solutions.
When all of the absorbance values have been recorded, type "Q" to return to the Blocktronic Main Menu.
solution for later use. Rinse the test tube with a small amount of the solution to be tested. Dispose of the rinsing, then fill the test tube about 3/4 full of the solution. Wipe off the outside of the test tube with a Kim wipe or tissue and insert the test tube in the Blocktronic unit. Be careful to line up the mark on the test tube with the mark on the Blocktronic. Cover the unit with the light shield.
From the Blocktronic Main Menu select option 4, Sample Light Levels, by typing "4." Enter the number of seconds to elapse between readings, and then press the RETURN key. The data can be displayed in a table or plotted in a graph. (The table is usually more convenient and provides data which can be plotted later.) Select your choice by typing "1" or "2." Next enter the number of samples desired. Type the number of readings the computer should make and then press RETURN. Note that the total elapsed time will be (time between readings) x (number of reading - 1). Press RETURN when you are ready to begin taking readings. To stop at any time, press "Q." Data will appear on the screen as it is collected. When the run is complete, a bell will sound and a message will appear on the bottom of the screen. To save the data on the computer disk, type "S." If you do not wish to save your data, press any other key to return to the main menu. To save the data, you must enter a name for the file. The name must be unique. If necessary, check the disk catalog to see what names have been used. To do this type the letter "C" followed by RETURN. The name of the file should start with a letter, not a number or symbol. The length should be no more than 30 characters. It is usually best to use your name or initials followed by a number so the your file names can be easily remembered. After entering the file name, press the RETURN key. After the data has been saved on the disk, the main menu will be displayed.
To look at the file or print a hard copy, select option 5, Display a File, by pressing "5." From the Display a File Mode, the data can be printed on the computer screen by typing "1", printed on the printer (if one is attached to the computer!) by typing "2", or you can return to the main menu by typing "3." To print a file, you must enter the correct name of the file followed by RETURN.
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