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The Effect of Molecular Weight and Time on the Rate of Diffusion
of Potassium permanganate, Potassium dichromate, and Methylene blue1
scientific paper submitted in partial fulfillment of the requirements in General
Biology I laboratory under Prof. Diana Rose Gonzales, 1st sem., 2013 - 2014
The effect of molecular weight and time on the rate of diffusion of potassium permanganate,
potassium dichromate, and methylene blue was tested
using a petri dish of agar - water gel with three wells.
The three substances were dropped simultaneously in
the petri dish. Potassium permanganate (MW
158g/mole) increased rapidly (14.50 mm) while
Methylene blue (MW 374 g/mole) gradually increased
(9.50 mm) only. Thus, molecular weight and time
affects the rate of diffusion.
Diffusion is a movement of molecules from an area of higher concentration to an
area of lower concentration. The particles will fuse when they are evenly distributed and
have reached equilibrium.
Potassium permanganate, potassium dichromate, and methylene blue are
substances used as indicators and oxidizing agents. Potassium permanganate is used in
organic compounds and used commercially to purify water and sanitizer. It is chemically
used to regulate certain reducing compounds. Potassium dichromate is used to determine
ethanol concentrations in solutions and determine the presence and purity of silver.
Researches also suggest that potassium dichromate functions as an agent that cause
genetic mutation against DNA repair - deficient strains of Escherichia coli. Lastly,
methylene blue is used as dye to identify bacteria and nucleic acids. The dye will have
the deepest shade of blue when in contact with acids.
As indicators and oxidizing agents, the substances stated above must prove that
their diffusion must be fast in order to do their functions. Due to their difference in
molecular weight, a test was conducted to prove what substance is more recommendable
to be used in getting the rate of diffusion. To further verify the experiment rate of
diffusion was also tested with time. To test this, agar - water gel is one of the materials
was used. Agar - water gel is a substance that functions as a thickener, stabilizer, and
emulsifier since it do not melt at room temperature until it is heated to 85oC and more.
Also, with a stopwatch the substances were measured (mm) at a regular three - minute
interval for thirty minutes.
The study aims to determine the effect of molecular weight and time on the rate
diffusion of potassium permanganate, potassium dichromate, and methylene blue. The
specific objectives are
1. to prove that molecular weight affects the rate of diffusion; and
2. to explain why molecular weight must also be observed with time
MATERIALS AND METHODS
In testing the effect of molecular weight and time on the rate of diffusion of
potassium permanganate, potassium dichromate, and methylene blue, agar - water gel
was used. Three bottles with dropper containing the substances and a petri dish with agar
- water gel were given to each group. As three members of the group dropped all
substances together in the wells of the agar - water gel, the stopwatch started
simultaneously with it. One member measured each wells of the agar - water gel with a
ruler to get the initial measurement (mm) of the wells. The group then drew the initial
appearance of the experiment (Figure 4.1.). One member was assigned to signal the
group if three minutes have passed and the member of the group who is assigned to
measure the wells will immediately but carefully measure the area with the ruler. There
was a regular three - minute interval for thirty minutes.
After thirty minutes, the group then drew the final appearance of the experiment
Figure 4.2. Also, the group computed for the average of the substances by adding all the
data that were gathered divided by number of time intervals. To test the effect of time on
the rate of diffusion the data gathered were computed by partial rate. Partial rate is
computed by subtracting the diameter of colored area immediately (di-1) before the
diameter of colored area at a given time (d1) divided by the time when d1 was measured
(t1) minus the time immediately before t1 (ti - 1). Again, the average of each substances
were computed by adding all the date divided by the number of time intervals. A graph
comparing the average rate of diffusion of each substance was plotted against its
molecular weight in Figure 4.3. Also, a graph comparing the partial rate of diffusion of
each substance was plotted against the time elapsed in Figure 4.4
Figure 4.1. Initial appearance of the substances in the agar - water gel wells.
Figure 4.2. Final appearance of the substances in the agar - water gel wells.
RESULTS AND DISCUSSIONS
As seen in Table 4.2, potassium permanganate (MW 158 g/mole) has the highest
average rate of diffusion with 11.32 mm/min, followed by potassium dichromate (MW
294 g/mole) with 10.86 mm/min, and methylene blue (MW 374 g/mole) with 7.95
mm/min. In Table 4.3, potassium permanganate has the highest partial rates of diffusion
with 0.35 mm/min, followed by potassium dichromate with 0.32 mm/min, and methylene
blue with 0.19 mm/min. This results shows that molecular weight has an effect in the rate
of diffusion. When the molecular weight is lower then the rate of diffusion will be higher
meaning they have an indirect relationship. Also, with time the rate of diffusion of the
substances decreased, meaning time and the rate of diffusion has an indirect relationship.
Table 4.2. Change in circumference (mm/minute) of the three substances in thirty minutes.
(MW 158 g/mole)
(MW 294 g/mole)
(MW 374 g/mole)
Table 4.3. The partial rates of diffusion (mm/min) of the three substances.
Average rate of
(MW 158 g/mole)
(MW 294 g/mole)
(MW 374 g/mole)
Average rate of
100 200 300 400
Molecular Weight (g/mole)
Figure 4.3. A bar graph comparing the average rate of diffusion per molecular weight of
the three substances.
Partial rate of 1.2
(mm/min) 1.40 1
612 18 24 30
Time elapsed (min.)
Figure 4.4. A line graph comparing the partial rate of diffusion of each substances
SUMMARY AND CONCLUSION
The effect of molecular weight and time on the rate of diffusion of potassium
permanganate, potassium dichromate, and methylene blue was determined. Each
substance was dropped simultaneously with each other into the petri dish with agar -
water gel. With a stopwatch the time was observed with an interval of three minuets in
thirty minutes. After thirty minutes, the average of the circumference and the partial rates
of diffusion were computed.
Results showed that potassium permanganate (MW 158 g/mole) which has the
least molecular weight had the highest average rate of diffusion compared to methylene
blue (MW 374 g/mole), which has the greatest molecular weight among the three. In
time, the circumference (mm/min) of each substance increased since there was diffusion
in the agar - water gel wells.
Hence, molecular weight and time has an effect in the increase, decrease,
slowness, and fastness of the diffusion of substances. Nevertheless, further
experimentation must be done to improve the results of the experiment. It is
recommended to use other substances, a different medium other than the agar - water gel,
a longer time period to test the diffusion, a more stable environment, and have more trials
to observe better results.
Dean, John A. Lange's Handbook of Chemistry, 12th ed.; McGraw-Hill Book Company: New York, NY, 1979; p 9:4-9:94.
Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; p 137.
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