Microcomputer Simulation of a Spectronic-20 Experiment P a u l B. Kelter and J a m e s D. C a r r Uniuersity of Nebraska Lincoln, N E 68588 In the introductory chemistry course for majors and premeds a t the University of Nebraska, we have found that computer simulations serve as an excellent supplemental tool for learning chemistry. For most students, this serves as a fust experience-with micr&mmputers, and the excitement they feel helps them concentrate on the simulation a t hand. The simulation described herein reviews some basic facts about spectroscopy in the visible region, and is patterned after the colorimetric determination of iron by complexation with 1,lO-phenanthroline. The goals of the exercise are: 1) Review the relationship of observed color to absorption spec-
trum. 2) Make a reasonable decision of wavelength at which analysis
should be performed.
Mole-Ratio. The student is eiven several concentration ratios of [y/[X] and their c ~ r r & ~ o n dabsorbance in~ values for an unknown compound XY,. Based on this plot, he must calculate n, the number of molecules of Y that Eomplen with one molecule of X. Simultaneous Equations. The student is presented with absorbance data from two different compounds which both absorb throughout the visible region. He is then given data for an unknown mixture of the two, and asked to find the concentration of each compound in the unknown solution. Program SPEC116-North Star Version 3 BASIC, 220 statements, no comments. Students run SPEC116 via Hazeltine 1500 CRT. Execution requires 32K of %bit words on a North Star Horizon. Documentation includes listing, flowchart, comments, and several sample executions. Students are eiven instructions ~ r i o rto execution time via written h a d o u t s , and a t execuiion time via the terminal. Copies of the listing and documentation are available a t a cost of $1to cover pos&ge and handling. Check or money order should he made out to University of Nehraska and mailed in care of James D. Carr, Department of Chemistry, University of Nebraska, Lincoln, Nehraska 68588.
3) Utilize Beer's Law to calculate unknown concentrations. 4) Determine molecular stoichiometry using a mole-ratio plot. 5) Solve for unknown concentrationsof two species, bath of which
absorb throughout the visible region. There are four main sections in the simulation: Warm-uo. Several soectra are presented seauentially on the CRT tdrminal. ~ a c spectrum h hatches a different colored solution in a nearhv test-tube rack. The student must select the solution that corresponds t o each spectrum. Unknown Concentration. The student is presented with a visible absorption spectrum again displayed on the CRT (see Fig. 2). He must choose the appropriate wavelength for his analysis based on this spectrum. Based on this choice of wavelength, a plot of ahsorhance versus concentration for his unknown is displayed, and he must calculate the unknown concentration.
Counter-Current Distribution: A Calculator Simulation David Holdsworth University of Papua New Guinea Box 4820,Uniuersity P. 0. Papua New Guinea Counter-current distribution enables complex mixtures of natural or synthetic products with similar distrihution coefficients to be separated. Craig counter-current apparatus is expensive and large volumes of organic solvents are required fo; operation. In consequence cou&r-current demonstrations are rarely shown in schools and colleges. A simulation of an ideal counter-current distribution of two or more substances can be carried out on a pocket programmable calculator. A ~ r o m a mcan be written for the instrument to display the mass o? a solute in each tube in turn after transfers. The student can plot the mass of two or more solutes, of differing distribution coefficients, on graph paper to show the separation of the compounds along each tube. The can he easilv varied t o demonstrate the number -~ ~ ~ - of - transfers ~ effectiveness of the separation. he distrihution is a binomial one and the mass of a solute contained in the rth tube after n transfers using equal volumes of two solvents is equal to ~
~
~
~
~~
~
~
~
(2)-
r! (n - r)! K + 1
Kp
where m is the total massofonesolute in the mixtureand K, the distrihution coefficient, is the ratio of concentration of solute stationary phase. Some programmable calculators, such as the Hewlett Packard HP-67, have a factorial (N!) key. When a factorial key is not available (e.g. HP-25,33E, 29C) the first expression can be programmed as
Figure 2.
Unknown concentration spectrum and instructions.
in considerably fewer steps. A calculator with a printout facilitv can orint the mass of solute in each consecutive tube autdmatic&ly. A student can now use the calculator to simulate countercurrent separations of mixtures of different solutes in a variety of solvent systems, varying the distribution coefficients and the number of transfers. Calculator data can he used to decide the number of transfers necessary to obtain a solute of a certain purity. Interested students may wish to consider the more general Volume 57, Number 9, September 1980 / 621
