Liquid Diffusion Coefficient

LAB 5 LIQUID DIFFUSION COEFFICIENT 1.0 OBJECTIVE 1.1 To determine the liquid diffusion coefficient of NaCl solution in distilled / de-ionized water.

2.0 INTRODUCTION The SOLTEQ® Liquid Diffusion Coefficient Apparatus (Model: BP 09) has been designed for students experiment on the technique of determining diffusivity of sodium chloride solution in distilled water. A known concentration of sodium chloride solution is placed in a diffusion cell immersed in distilled water. A magnetic stirrer and a conductivity meter are provided to monitor the progress of diffusion over time. A plot of conductivity against time will allow for the determination of the liquid diffusivity.

3.0 THEORY When a concentration gradient exists within a fluid consisting of two or more components, there is a tendency for each constituent to flow in such a direction as to reduce the concentration gradient. This is called mass transfer. Mass transfer takes place in either a gas phase or a liquid phase or in both simultaneously. The rate of diffusion is given by:

The –ve sign indicates that flow is from high to low concentration.

The appropriate units shall be:

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The concentration at the lower ends chosen and taken to be constant and the concentration at the top end is effectively zero during the experiment. Therefore,

Where,

The slope obtained from the plot of conductivity as a function of time can be used to calculate the diffusivity.

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4.0 EQUIPMENT AND SPECIFICATIONS 4.1 Equipment

Figure 1: Diagram for liquid diffusion coefficient apparatus 4.2 Description and Assembly Before operating the unit and running the experiments, students must familiarize themselves with every components of the unit. Please refer to Figure 1 to understand the process.

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5.0 PROCEDURES 5.1

Experimental Procedure

a) Fill the diffusion vessel with 1.1 liter of distilled/de-ionized water. Make sure that there is no air trap inside the conductivity probe protector. b) Connect the conductivity probe BNC connector into the socket on the conductivity meter. Insert the mini phono jack of temperature sensor (built-in) into the socket on the conductivity meter. Plug the magnetic stirrer mains cable to the electrical supply. Be sure that the voltage of the supply is correct to suit the equipment. c) Press the ON button on the conductivity meter. d) Switch on the magnetic stirrer and set the speed knob to the one and half position. e) Read the conductivity value. Distilled/de-ionized water shall give a very low reading. f)

Fill the diffusion cell with the solution prepare in step 2. Make sure the capillary tubes are in place. Completely fill the cell and wipe off any excess solution and ensure that there is no air trap inside the capillary tube.

g) Carefully immerse the cell into the distilled/de-ionized water and position the cell until the top of the capillaries is about 5 mm below the water level. h) Switch on the conductivity meter. Switch on the magnetic stirrer. Reset and start the stop watch. i)

After 5 minutes record the conductivity reading. Take readings every 5 minute intervals until 30 minutes.

j)

Repeat steps f) to i) for 2M and 4M NaCl solutions.

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6.0 RESULTS AND CALCULATIONS 6.1

Table of data

Volume of water, V Length of capillaries, x Diameter of capillaries, d Number of capillaries, N CM

Time (sec) 600 900 1200 1500 1800 2100 2400

= = = = =

1M NaCl Conductivity, k (μS) 35.1 38.3 41.9 45.2 48.2 50.9 53.8

Concentration NaCl (M) 1 2 4

1.1 L 0.5 cm 0.1 cm 97 4.1 x 105 S/M

2M NaCl Conductivity, k (μS) 63.8 68.1 72.3 76.1 80.9 84.5 88.2 Slope, s 0.010 0.013 0.023

4M NaCl Conductivity, k (μS) 100.9 110.1 118.1 125.2 131.8 138.0 143.9 Diffusion, D (cm2/s) 1.76E-05 1.14E-05 1.01E-05

6.2

Plot Conductivity against Time.

6.3

Determine the liquid diffusivity of sodium chloride solution from the obtained slope, s.

6.4

Show your calculations.

CALCULATION

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V x d N CM

= = = = =

1100 cm3 0.5 cm 0.1 cm 97 4.1 x 105 S/M

For 1 M NaCl solution, from the plot of conductivity vs. time, slope, s = 0.010 S/s Thus, from equation 2,

7.0 CONCLUSIONS 7.1 Based on the experimental procedure done and the results taken draw some conclusions to this experiment.

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