Summer Research Program for Science Teachers & Partners in Science Program

How can the concentrations of a solution be determined?

Annelisse Falzone, Bronx High School of Science - 1997

Aim: How can the concentrations of a solution be determined?

Student Objectives: 1. Students will prepare a set of Co(II) standard solutions.

2. Students will measure %T (% transmittance) of all known standard solutions.

3. Students will measure the %T for at least one unknown solution and determine its concentration from the calibration curve (%T vs concentration).

4. Students will use a computer graphing application to graph the calibration curve (e.g. Criket Graph). [Content Standard A- Use of technology/mathematics] [Content Standard E- Understandings about science/technology]

Materials: 0.010M stock solution of (CoNO3)26H2O, 5 labeled test tubes, test rack, graduated cylinder, and a Spec 20 spectrophotometer

Discussion: Colored ions either absorb or transmit light from the visible spectrum.Teachers may use solutions containing Co (II) complexes, Fe(II) complexes or Cu complexes. [Content Standard B- Properties of matter]

Beer-Bouger Law: A= abc

where, A = absorbance

a = absorbance coefficient

b = cell thickness, 1cm

c = concentration of the solution

Assume that a and b are the same for the standard and the unknown solution then by equating A1= a1b1c1 where A1 equals the absorbance of the standard solution to

A2 = a2b2c2, where A2 equals absorbance of the unknown solution solution:

A2 / A1 = c2 / c1.

The spectrometer should be set at the maxmium wavelength for the compound in question ((CoNO3)26H2O) where maximum absorbance 510 nm is achieved while executing the A or %T measurments on the standard solutions.

Preparation of Standard Solutions : Students will dilute a stock solution of 0.01M CoNO3)26H2O solution prepared by the teacher.Students will then take their graduated cylinders and measure 2ml of the stock solution and dilute to the 10mL mark with distilled water. Transfer 6mL of the diluted solution into a labeled test tube #1. Discard the remaining 4mL into a waste bottle. Students will then pour out into the graduated cylinder respective amounts(4mL, 6mL, and 8mL) of the stock solution into the graduated cylinder and diluted each of the amounts to the 10ml mark. Transfer 6mL of each diluted solution into test tubes labeled #2, 3 and 4. Test tube #5 contains 6mL of the stock solution.

Preparation of the Spec 20 or any visible absorption spectrometer : Use the instruction manual or set the machine to zero %T by having the sample slit empty and closed adjust the %T knob to zero if necessary.

Then fill a test tube with distilled water and place it into the sample slit and adjust the %T knob to 100 %T with the right knob if necessary.

Data Table :

test tube 1 2 3 4 5 Unknown

dilution factor 2:10 ? ? ? ? ----------

concentration(molarity) ? ? ? ? ? ?

%T ? ? ? ? ? ?

*Ask students/teams to fill in this section before beginning the experiment. Have a student /team put up their dilution factors on the board have students explain their answers.

Students will construct a calibration curve %T vs concentration. The unknown concentration can be found by finding the %T of the unknown and determining its concentration from the calibration curve. [Content Standard Unifying Concepts- Change, constancy, and measurement]

Questions : 1.What is the concentration of your unknown solution at %T of (left to teacher)? [Teaching Standard B- Orchestrate scientific discourse]

2. What is the percent error for your unknown compared to its true concentration?

3. How can we improve our dilution techniques ?

4. How can we improve this experiment?