Spectronic 20 Calibration Procedure
1. Calibration Materials
1.1. Determine the ID No. for the Spectronic 20 instrument you are to calibrate, and obtain the Instrument Calibration Record Book.
1.2. Using a clean 50 mL beaker, rinsed with a few mL of the standard Cobalt solution, obtain the quantity of cobalt solution required to perform the following calibration procedures from the stock Cobalt solution in the laboratory. (Normally about 25 mL.; 40 mL if test-tube matching is done.)
1.3 Also obtain from the stock Cobalt bottle label the certification data for the stock solution (date prepared; measured concentration); enter in your Lab Notebook and the Instrument Calibration Record Book.
2. Measurement Procedures.
m2.1 Turn on the Power Switch and wait 15 minutes before making any measurements.
2.2. Select and mark a set of matched test tubes. (If a matched set is already available, proceed to 2.3.)
a. Check out 8 of the special Spectronic 20 sample test tubes from the Service Center
b. Set the wavelength at 510 m
c. Perform operation 2.3.c - e, with one selected test-tube
d. Perform operation 2.3.f for each test-tube, transferring the stock solution from one clean, dry test-tube to the next, and determine which two test-tubes provide readings most nearly identical. Select and mark these two test-tubes with the ID No. of the instrument. Use this pair for subsequent measurements. Return the rest to the Service Center.
2.3. Measure transmittance (stock solution) at different wavelengths
a. Fill one test tube with deionized water, and one with stock solution. (If not already labeled, each test tube should be marked to contain either water or absorbing solution. For periodic calibrations, the same test tubes should be used for the same solution each time.)
b. Turn the Wavelength Control to 500 mµ.
c. Adjust the Zero Control to zero reading.
(No test tube in the instrument and the sample holder cover must be closed; no light will be transmitted.)
d. Insert the test tube* with deionized water into sample holder and close the cover.
e. Adjust the Light Control to 100% transmittance.
f. Insert the test tube* with the absorbing solution into the sample holder (removing test tube with deionized water) and close the cover. Record wavelength and % transmittance (to 3 significant figures).
g. Turn the Wavelength Control to 505 mµ. Repeat 2.3.c - f, six times.
h. Turn the Wavelength Control to 510 mµ. Repeat 2.3.c - f, six times.
i. Turn the Wavelength Control to 515 mµ. Repeat 2.3.c - f, six times.
j. Turn the Wavelength Control to 520 mµ. Repeat 2.3.c - f, six times.
k. Compute the average absorbance and standard deviation at each wavelength and enter in both Notebooks with appropriate significant figures.
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* Align the mark on the test tube with the line on the sample holder.
2.4. Measure transmittance (stock solution) at maximum absorbance
wavelength (minimum %T).
Set the Wavelength Control at the maximum absorbance wavelength and repeat 2.3.c - f, six times.
2.5. Prepare dilute stock solution (see Addendum 1)
2.6. Measure transmittance (dilute stock solution) at maximum absorbance wavelength (minimum %T).
Set the Wavelength Control at the maximum absorbance wavelength and repeat 2.3.c - f, (using the dilute stock solution), six times.
2.7. Compute the average absorbance and standard deviation for each solution and enter in Notebooks.
3. Finishing Procedures
3.1 Turn off power switch
3.2 Clean, dry matched test-tubes and return to the Service Center.
3.3 Complete entries in Lab Notebook and Instrument Notebook
a. Date, Time, Operator
b. Instrument I.D.#
c. Stock Solution Preparation Date & certified concentration
d. Wavelength for maximum absorption
e. Absorbances, Std. Deviations, for Stock and Dilute Stock Solutions at maximum wavelength
Addendum 1.
Preparation of Solutions (If Needed).
1. Cobalt Chloride Stock Solution.
Weigh out accurately 20.1881 g of pure CoCl2×6 H2O (F.Wt=237.93), and transfer to a 250 mL beaker. Dissolve with about 100 mL 1 % hydrochloric acid (see A.4). Transfer to a 500.0 mL volumetric flask and dilute to volume with deionized water. (Label flask with date and name of preparer. Record weighing data and concentration calculations in Cobalt Standard Notebook and your Lab Notebook, date and sign both Notebooks.)
2. Dilute Stock Solution.
Pipet 10.0 mL cobalt stock solution to a 25.0 mL volumetric flask and dilute to volume with deionized water.
3. 0.12M Hydrochloric Acid.
Place about 200 mL deionized water in a 500.0 mL volumetric flask. Add, slowly and with caution, 5 mL concentrated hydrochloric acid. Make to volume with deionized water. (Label flask with date and name of preparer).
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