needs. For example, some calculations may be assigned as student exercises, particularly the last one if time is short. As an aid to the student, I pass out an outline of the procedure, with space for calculations. I believe that this demonstration-calculation is preferable to either one without the other.
SUBMITTED BY
Marvin Gold California Stale University, Chica Chica. CA 95929 CHECKEDBY
Herbert Slotnick Central Connecticut State College New Britain. CT 06050
Although demonstrations of buffer capacity using indicators are readily ~ e r f o r m e dand , always enjoyed, students often have considerable difficulty handling buffer calculations. The purpose of this demonstration is to carry out a visual demonstration followed (or preceded) immediately by calculations based on the actual materials used. The entire procedure can he completed in a 50-min lecture period. Only standard glassware is required.
A Simple, Effective Introduction to the Beer-Lambert Relationships SUBM~T~ED BY
Leo H. Bowman Arkansas Tech University Russellville, AR 72801 CHECKED BY
Kenneth Lothrop Marshfield High School Marshfield, MA
Procedure 1) Preparation of acetic acid solution: dissolve 26 ml6M HOAc in 800 ml solutian. Calculate the [HOAc].Divide into two 400-ml portions (labeled A and B ) . 2) Preparation of buffer: add methyl orange to A, followed hy addition of 6.6 g NaOAc. Note color change. Add 6.6 g NaOAc toB and set aside. 3) Calculate pH of'the HOAc and HOAciNaOAc solutions and
While the uarameters of the Beer-Lamhert Law are almost intuitively obvious, a simple classroom inquiry-demonstration vividlv illustrates andlor demonstrates the realities of the lieht absorbing phenomenon.
methyl orange to C, and set D aside. 5) Demonstrate buffer capacity: gradually add 5 ml 6 M HCl to A (little color change) and 1drop to C (large color change).Discuss reactions and calculate approximate final pH in each solutian. (Take 1drop = 0.05 ml) 6) Repeat #5 with 6 M NaOH and solution I3 (with methyl red) and D (with alizarin yellow).
Procedure Prior to projection, three dishes are placed on the overhead projector surface. Two of the dishes are partially filled with dichromate solution; the third with methyl orange solution. Care is taken to make sure the depth of the dichromate solutions vary. The initial question posed is: "Which dish contains the most concentrated solution?" Invariably the "deepest orange colored sample" will he the immediate response, hut more perceptive students will almost instantly begin to question that conclusion. Eventually, skillful handling of the discussion will lead the class to meaningful conclusions concerning the absorption dependence on the identity of the absorber, a , the depth of the absorber, b, and the concentration. c.
Summary of calculated and observed pH changes: Calculated pH change
(A)
4.8 to 4.4
(C)
5.0 to 3.1
(B)
4.8 to 5.1
(D)
5.0 to 10.9
Obserued
Indicator Interual
Change
methyl orange (3.1-4.4 red~vellow methyl orange (3.1-4.4) red-yellow methyl red (4.4-6.2) red-yellow alizarin yellow (10.0-12.0) yellow-red
small (yellow) red to yellow
small (red) yellow to red
Remarks The observed changes match the calculated values quite well. T h e procedure is easily varied to suite individual class
Materials Overhead projector Culture dishes Dichromate solution Methyl orange solution
distilledwater will reinforce the conclusions relative to the factor. c. ~ i k e w i s ethe display of several other highly colored solutions allows one to enaaee in conversation about the oriain of color, light absorption, etc.
Volume 59
Number 2
February 1982
155
