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ELECTROLYTE DEMONSTRATION

Author(s): 

John Hnatow

Note: This article was scanned using OCR from the Fall 1985 CCCE Newsletter. Please contact us if you identify any OCR errors.
 
The purpose of this demonstration is to replace or to supplement the conventional light bulb conductivity apparatus used in general chemistry demonstrations. If one has access to microcomputers in a chemistry setting, this demonstration will process the collected data in a very efficient and organized way and allow students to conceptualize a very important theory. Most
chemistry instructors not familiar with simple interfacing techniques will be quite surprised at both the simplicity of the programming and the low cost of the materials. I developed this program at the Allentown College/Moravian Academy Microcomputer Interfacing Curriculum Development Institute in 1984 and have had time this school year to refine it.
 
The BASIC program which must be loaded is included with these instructions.
 
1            REM ELECTROLYTE DEMONSTRATION
4            REM BY JOHN HNATOW, EMMAUS HIGH SCHOOL, EMMAUS, PA 18049 
10          HOME
20          HGR
30          HCOLOR = 7
35          HOME : VTAB 24: INPUT "PLOTTING INTERVAL IN SECONDS IS "; A
40          FOR J = 0 to 278
50          X = J
60          Y = POL (1) * 159 I 255
65          HCOLOR = 0: HPLOT X + l,Y: HCOLOR = 7
70          HPLOT X,Y
80          FOR I = 1 TO A * 750: NEXT I
90          NEXT J
95          HOME : VTAB 24: PRINT "PRESS ANY KEY TO CONTINUE."
100        GET A$
110        GOTO 35
120        END
 
Two electrodes are connected to pins 1 and 10 of the game I/O connector of an Apple II+ or IIe. These electrodes may be platinum of nichrome wire (or even paper clips if your budget is limited). The electrodes are now immersed in distilled or deionized water .and a conductivity vs time graph is plotted. Operational definitions for non-electrolytes, weak and strong electrolytes are established as 0.1 M solutions are added at a constant rate to 100 cc volume of pure water. Increases in conductivity or decrease in resistivity are observed as dissociation ¬∑or ionization occurs in solution. The program allows placing each plot over the other to obtain relative differences between the conductivities.
 
Two alligator clips may be connected to a 10 pin DIP header (Radio Shack part no. 276-1980). The DIP header should be inserted into the game I/O connector.
 
The program will indicate a "plotting interval in seconds" prompt line, and the demonstrator selects the time interval. An interval of 0.15 is recommended in order to expedite the demonstration,
and to avoid some potential problems caused by possible redox reactions at the electrodes. Initially, reference lines for no conductivity and highest conductivity can be established on the graph. First, run the program without any solution between them and then, run it while touching the electrodes together.
 
Start with pure water with the electrodes in position. Now, non-electrolytes, such as sucrose, can be added to the pure water by simply adding a spoonful or a small volume of the substance. A nice feature of the program is that a pointer will move across the screen if super position occurs.
 
In preparation for this demonstration, fill four labeled burets with 0.1 M solutions of acetic, boric, phosphoric, and hydrochloric acids and set them aside. The electrodes should be placed at fixed distances in a beaker containing pure water. The use of a magnetic stirrer is recommended. The acids must be added from a buret at a rate of approximately one drop per second. It is very important to rinse and dry the electiodes after each acid'is run. after each new acid addition in a beaker rinsed and filled with pure water. A very weak electrolyte (such as boric acid) may not allow any change in conductivity on the first run. However, running the program a second time while the weak electrolyte is still being added at one drop per second should produce desirable results. A sample of a printout from a typical demonstration shown below.
 
Since the Apple supplies volts DC to the electrodes, in many instances gases may build
up on the electrodes or a prec1p1tate may coat the electrodes and drastically decrease the conductivity. To stop plotting before the plot has progressed to the end of the screen, simultaneously
press CONTROL and C. There are now two opt1ons. If the plot 1s to be continued from the same
place, type CONT and press RETURN. If a new.plot is desired, type RUN 35.
 
After the demonstration is performed for the class, a sample printout is supplied for the students to label and discuss. Strong, weak, and non-electrolytes are more easily understood after operational definitions are developed with the aid of this microcomputer demonstration. More advanced high school classes might attempt to determine the acid dissociation constants of the weak acids from the results, and perhaps additional experiments.
 
Emmaus High School
Emmaus, PA 18049
Date: 
09/16/85 to 09/20/85