Objectives & Procedures: Colorimetry – Spectroscopy

2.1 – Quantitative Analysis w/ Light

Learning Objectives

  • Learn to match the observed spectra of colored solutions with spectrophotometric charts of percent transmission and wavelength.
  • Learn to predict the visible results of color mixing.
  • Learn to make a set of solutions of known concentrations by dilution of a standard.
  • Use colorimetry to determine the concentration of an unknown colored solution.
  • Colorimetrically determine the concentration of chlorine in pool and tap water samples.

Procedure Overview

  • The observed spectra of colored solutions projected on the screen by an overhead projector are matched with MicroLAB Spectrum Profiles of percent transmission and wavelength.
  • Observe the origin of color using colored solutions and predict the visible results of color mixing
  • Make a set of five standard food dye solutions of known concentrations by dilution of a given standard.
  • Beer’s law is derived and used to determine the concentration of an unknown colored solution.
  • Colorimetrically determine the concentration of chlorine in pool and tap water samples.

2.2 – Crystal Violet: Beer’s Law Investigation

Learning Objectives

  • Illustrate the basic principles of colorimetry.
  • Demonstrate the components of a colorimeter and how a colorimeter is interfaced to a computer.
  • Discover the Beer’s Law relationship and apply it to the analysis of an unknown solution.

Procedure Overview

  • “Ideal” simulated data is manually entered into the MicroLAB spreadsheet and a series of plots using different functions operating on the current readings are constructed in order to discover a linear relationship between the current and concentration variables.
  • A series of standard crystal violet solutions are prepared and analyzed using the MicroLAB colorimeter. A Beer’s Law plot of these concentrations is constructed using the relationship discovered with the simulated data.
  • An unknown solution of crystal violet is analyzed in the colorimeter and the Beer’s Law plot is used to determine its concentration.

2.3 – The Formation Constant for a Complex Ion

Learning Objectives

  • Illustrate how colorimetric measurements are made using the MicroLAB interface.
  • Use Beer’s Law to measure the equilibrium concentration of a complex ion.
  • Calculate the equilibrium constant for the formation of a complex ion.

Procedure Overview

  • A set of standard solutions is prepared.
  • The MicroLAB colorimeter is used to determine the absorbency of the five standard solutions and the molar absorptivity constant for FeSCN2+.
  • The MicroLAB spreadsheet is used to calculate the equilibrium concentrations of Fe3+, HSCN, and H+ from the equilibrium concentration of FeSCN2+.
  • [f] – The value of K is calculated for the complex formation reaction from the collected data.

2.4 – Spectrophotometric Determination of an Equilibrium Constant

Learning Objective

  • To determine the equilibrium constant governing the formation of Fe(SCN) from iron(2+ III) and thiocyanic acid by using the MicroLAB Colorimeter to measure the concentration of Fe(SCN)2+.

Procedure Overview

  • Colorimetric measurements for the absorbance of four different solutions of a complex ion, [Fe(SCN)2+], are made using the MicroLAB Interface.
  • The equilibrium constant for the formation of the complex is determined.

3.2 – Do Ni2+ & Cu 2+ Form BIS- or TRIS- Complexes

Learning Objectives

  • To understand how a simple calorimeter is used to determine the maximum number of ethylenediamine (en) molecules that will complex to aqueous Ni2+ and Cu2+.
  • To understand the effect of structure of a coordination compound on its reactions.

Procedure Overview

  • The equimolar amounts of [Ni(H2O)6] or [Cu(H2O)6] and ethylenediamine are reacted and the heat 2+ 2+ of reaction is determined calorimetrically.
  • The reaction mixture is then cooled down to the initial temperature, and a second equivalent of “en” is added. The process is repeated until the addition of the next equivalent of “en” fails to produce a significant temperature change. The small temperature increase observed when further replacement is not possible is due to the heat of dilution of ethylenediamine.
  • By measuring the evolved heat, it is possible to determine the maximum number of ethylenediamine molecules that have complexed in each reaction.

8.1 – Reaction of Crystal Violet w/ NaOH

Learning Objectives

  • Study the rate of reaction of crystal violet with NaOH using the MicroLAB interface colorimeter.
  • Determine the order of reaction with respect to both reactants.
  • Calculate the rate constant for the reaction at room temperature.

8.2 – Reaction of Crystal Violet w/ NaOH a Kinetic Study w/ Ea Calculation

Learning Objectives

  • Study the rate of reaction of crystal violet with NaOH using the MicroLAB interface colorimeter.
  • Determine the order of reaction with respect to both reactants.
  • Calculate the rate constant for the reaction at room temperature.
  • [a] – Determine the activation energy, E , for the reaction from supplied data. (Students will not take this data since most general chemistry labs do not have the necessary equipment for doing these experiments.)

8.3 – Kinetic Studies of the Ferroin Complex

Learning Objectives

  • Determine the rate of a chemical reaction.
  • Determine the rate law for a chemical reaction.
  • Propose a mechanism for the reaction under study.
  • Determine the activation energy for the reaction.

10.1 – Quantitative Analysis w/ Light

Learning Objectives

  • Identify band and line spectra, and relate the physical state of a light-emitting substance to the type of spectrum observed.
  • Determine the relationship between the colors of the visible spectrum and wavelength and frequency.
  • Determine the relationship between the energy, frequency, and wavelength of light waves.
  • Examine the fingerprint nature of spectra.
  • Construct a spectrograph calibration chart and identify an unknown element by measurement of its emission spectrum.
  • Use an energy-state model to explain the atomic spectra of hydrogen gas.