I just need you to write one part of the report, which is (Procedure) ONLY.

Investigation

How Much Cobalt Is in the Soil?

Introduction

A group of dairy farmers has recently reported to its county cooperative extension service that its animals appear sick. Based on the symptoms, the county suspects that the cows are suffering from a cobalt deficiency. Cobalt(II) is required for production of vitamin B12. Proper animal diet requires the soil to contain 0.13 to 0.30 mg cobalt per kg of soil. Soils that do not meet this condition can be amended with careful addition of cobalt nitrate or other cobalt compounds.

Because it lacks the proper equipment, the county extension service has contracted for your team to determine the amount of cobalt(II) in a solution sent to your lab. To get representative samples, the extension service collected 2.00 kg of soil from 20 different farms in the county. The cobalt was then extracted from the soil, dissolved in water, and concentrated by a factor of 10,000 to prepare 1 L of solution. Because the cobalt ion is a colored species in water, spectroscopic techniques can be used to determine the concentration of cobalt(II) in an unknown solution.

Goals

As you complete this investigation you will:

1. Prepare a standard plot of absorbance versus known cobalt(II) nitrate concentrations.
2. Determine the concentration of cobalt(II) in the solution obtained from a soil sample.
3. Determine if cobalt nitrate should be added to the soil. If so, how much will be required to meet the necessary nutritional needs of the animals?
4. Report your findings to the cooperative extension service as directed by your instructor.

Materials
0.1 M* Co(NO₃)₂(aq)                           Cuvettes
Unknown cobalt(II) solution                 Tissues (preferably lint free)
Volume measuring equipment              Other supplies by request
Device to measure light absorbance
Obtain exact concentration from the reagent bottle.

Getting Started

For information about the basics of spectroscopy, see Appendix B of this manual. In Appendix B the quantity, percent transmittance (%T), is described as a relationship between the intensity of light before and after passing through a solution. Another quantity known as absorbance (A) is related to %T by the following equation:

A = log(100/%T)

The absorbance is related to concentration of a light-absorbing species by the Beer-Lambert Law, A = ebC, where e is a proportionality constant called the molar absorptivity that is specific for the absorbing species, b is the thickness of the solution in the light path (usually constant for an experiment), and C is the molarity of the absorbing species in the solution. For a given light-absorbing species, a plot of absorbance versus concentration should give a straight line. This straight-line relationship can be used to determine the concentration in an unknown solution of the species tested. For this investigation, a plot of the absorbance versus concentration of various cobalt(II) solutions can be used to determine the cobalt (II) ion concentration in the unknown sample sent by the county extension service.

Use the instructions in Appendix B for calibrating the spectrometer or colorimeter. (If neither of these devices is available you can use the non-instrumental technique described in Appendix B.)

You will also have to determine the wavelength at which you will run your experiments. A solution of 0.1 M cobalt(II) nitrate is available for preparation of your known solutions. (Make sure you write down the exact concentration of the known solution in your lab notebook.)

Develop a procedure for using the cobalt(II) nitrate solution to make several dilutions (at least seven). You will have to select an appropriate range of concentrations. Once your solutions are prepared, use the spectrometer or colorimeter to collect absorbance data for each concentration. If a graphing program is readily available, construct a plot of your data that you can use to determine the concentration in the unknown. At high concentrations (that vary from one chemical species to another) there is deviation from the Beer-Lambert Law. Use your plotted data as a guide for identifying the point at which deviation occurs. If you do not see any deviation from linearity, your concentrations could be in the correct range.

Determine if the amount of cobalt(II) in the soil meets the nutritional needs of the animals. If not, you should calculate the amount of cobalt(II) nitrate that must be added to the soil to reach a level of 0.13 to 0.30 mg of cobalt(II) per kilogram of soil.

Report

As usual you should write your report in customary scientific fashion. Include your recommendations to the cooperative extension service regarding amending soil on the county farms. Remember to include appropriate graphs with your report. (Further information about cobalt deficiency can be found by doing an Internet search on the term cobalt deficiency.)

Caution:

While working in the laboratory wear your goggles at all times.