PRECISION SPECTROPHOTOMETRY

Background

The purpose of this experiment to investigate various procedures for optimizing the precision of quantitative analyses in optical absorption spectroscopy. The `trick' is to use the instrument's amplifiers to expand the measurement scale. The expanded scale results in a more precise measurement. However, the instrument no longer reads out in absolute absorbance or transmittance. Thus calibration curves must be made. The accuracy of your results is dependent on the accuracy of your calibration.

In this experiment you will measure low concentrations of chromium. Chromium is currently on the Environmental Protection Agencies (EPA) list of toxic substances. It is produced in large quantities by the semiconductor manufacturing industry. Improperly disposed of, this toxic waste often finds its way into the ground water and thus into municipal water supplies. California, in particular silicon valley, has a problem. Let's not let it happen here. Dispose your waste in labeled containers.

Apparatus

Bausch and Lomb Spec-20 Spectrophotometer, or equivalent, or better. 1 set cuvettes.

Reagents

0.2500 M chromium(III) nitrate [Cr(NO3)3]; chromium unknown.

Procedure

  1. Carefully prepare 25 mL each of standard chromium solutions, by volumetric dilution of the stock solution. Make solutions with concentrations of 0.0500 M, 0.0400 M, 0.0300 M, 0.0200 M, and 0.0100 M. Make up standards in volumetric glassware and transfer to labeled plastic bottles for storage.
  2. Perform 1:10 volumetric dilutions of each of the above standard solutions, to give solutions of 0.0050 M, 0.0040 M, 0.0030 M, 0.0020 M, and 0.0010 M. Store solutions in volumetric flasks.
  3. Obtain two chromium unknowns, one for the low-absorbance technique and one for the high precision technique.
  4. Low-Absorbance Method:

a. Measure and record %T at 575 nm of each of the chromium solutions prepared in step 2 (the dilute set) in the usual manner (setting 0% T with no cuvette, 100% T with distilled water).
b. Readjust the zero using the most concentrated solution (0.0050 M) instead of no cuvette. Readjust 100% T using distilled water. Recheck the zero and 100% settings. Record the %T for each of the remaining dilute solutions (0.0040, 0.0030, 0.0020, and 0.0010 M). Record %T for the more dilute unknown.

  1. High Precision Method:

a. Measure and record %T at 575 nm of each of the chromium solutions prepared in step 1 (the more concentrated set) in the usual manner (setting zero T with no cuvette, 100% T with distilled water).
b. Readjust the zero using the most concentrated solution (0.0500 M) instead of no cuvette. Readjust 100% T using the least concentrated solution (0.0100 M) instead of distilled water. Recheck the 0% and 100% settings. Measure and record the %T for each of the remaining dilute solutions (0.0400, 0.0300, and 0.0200 M). Measure and record the %T for the more concentrated unknown.

Calculations

Low-Absorbance Method Convert all transmittance readings to absorbance by taking the log. Make two plots of the absorbance as a function of concentration data obtained in step 4, one for the data obtained in the usual manner and one for the data obtained with the zero adjusted using the 0.0050 M solution as the "blank". Determine and report the concentration of the unknown using the latter plot.

High Precision Method Convert all transmittance readings to absorbance. Plot transmittance versus concentration, and absorbance versus concentration from the data obtained in step 5. Determine and report the concentration of the unknown using each plot.

Questions

  1. Which technique is most suitable for "trace analysis?" Why?
  2. How would an error in making up one of the standard solutions be detected and corrected for?
  3. Why are calibration curves necessary in this method of analysis?

Report

Report concentrations in M of the two unknowns. Your grade will be based 50% on each of the two reported concentrations.

Reference

Friday, October 03, 2003