# Relationship of transmittance and concentration

### Beer's Law - Theoretical Principles

The effect is measured either as Transmittance (T, the percentage of light the absorbance is directly proportional to the concentration of absorbing material. Describe the relationship between Absorbance and Transmittance and the relationship between concentration of a solution and each of Absorbance and. Because there is a mathematical relationship between solute concentration and light absorption/transmittance, we can use spectrophotometry to determine the.

Is the slope of the calibration line at nm greater than, less than, or equal to the slope at nm? You can choose any wavelength to create a calibration plot, the only differerence will be the slope of the line.

When you actually choose your wavelength to create your calibration graph, you would generally like to choose a wavelength where there is room for the concentration to decrease. Look at the spectrum above. Do you think nm would be a good wavelength to use for a calibration graph?

### Difference Between Absorbance and Transmittance

You would not choose that wavelength because when you lower the concentration, you would not be able to see much of a difference in the absorbance, and the calculations would be inaccurate. You would most likely want to choose wavelengths like nm or nm where there is a lot of room for absorbance change. Now for the fun part! Using the calibration plot that YOU made from the data two pages ago. We are going to determing the concentration of an unknown solution.

Make sure you have your plot ready, because here we go!

Here's a typical problem. You take 3mL of your unknown sample and 7mL water and mix them together. Thus for a given absorbing species at a specific wavelength in a given sample holder of fixed path length ba plot of A versus concentration is a straight line if the Beer-Lambert Law is obeyed.

To determine if the Beer-Lambert Law is obeyed over a given concentration range by a given species, measure absorbance as a function of concentration, using the same test-tube for all of the measurements. If the curve is linear, then the slope of the line may be calculated and used to determine the concentration by dividing the absorbance by the slope. An alternative method of determining concentration from the calibration curve is to mark off the measured absorbance on the ordinate, draw a line perpendicular to the ordinate until it intersects the curve, then drop a perpendicular line to the abscissa from the intersection point.

The intersection of the latter line and the abscissa gives the concentration. Level Part I of this activity can be used for all levels of students. For basic students, it might be best for you to collect the data with the assistance of several students.

## What is the relationship between %transmittance and concentration?

For general students, groups of two or three should be able to collect the data for Part I, but Part II might be better done as a class activity. Honor students should be able to do the entire activity. Expected Student Background This activity will probably be the first exposure students have with instrumentation.

The experimental procedure is rather straightforward and should not pose great difficulty to students. By this point in the course, students should have the experience with routine laboratory procedures involving the use of glassware, how to clean glassware, and so forth.

Time Teacher preparation time to mix the 0. The spectrophotometers should be checked prior to use. The estimated time for students to perform the activity is50 min to complete Part I. Additional time up to 50 min may be required for Part II, particularly if students are required to prepare their own calibration curves. Safety goggles should be worn during the activity.

The chromium III nitrate solution is moderately toxic and should not be poured into the drain. Students are instructed to return the solution actually used in the measurements back into the original container. If there is some possibility that the solution supply will be contaminated, then an alternate disposal procedure should be devised.

If it has not been contaminated, the solution may be saved for later use. There are several sources referred to in SourceBook that give recommended disposal procedures see Safety section. Local and state regulations must be followed. In view of the disposal problems, only the actual volume of solution needed with a modest excess should be prepared. Check the spectrophotometers to be certain they are operating correctly.

Allow a 20 min warm-up period prior to use. To prepare the solution, use 2. Weigh the solute carefully. The unknowns are prepared by successively diluting the 0.

Use these solutions to prepare a calibration curve and student unknowns. Use a buret to measure all volumes.

**Transmittance and Absorbance**

Make the dilutions according to the following table. Pre-Laboratory Discussion Demonstrate for students the techniques involved with using a spectrophotometer. In particular show them how to clean, fill, and place the test-tubes or cuvettes in the instrument. A transparency master showing a front view of the Spectronic 20 " with the controls identified is in the Appendix. Show students how to use the calibration curve to determine an unknown concentration.

## What is the relationship between %transmittance and concentration?

Students should have little difficulty in preparing a data table. For Part I a simple three column table is needed. Part II needs a simple table to record the wavelength used, the percent transmittance recorded and the calculated absorbance.