Constructing the Isotherm

The first thing to do is to set up an analysis for the material that will be adsorbed.

You need some sort of measurement procedure. An inexpensive and common method uses a colorimeter or spectrophotometer. Light is passed through a sample; the higher the concentration, the less the light that escapes. It would be possible to weigh out material each time to make up solutions of known concentration, but it is quicker and easier to weigh out just once to prepare the solution of highest concentration. The other solutions are made by pipetting from the strongest solution into volumetric f lasks and diluting up to the mark. For the very lowest concentrations, it may be more accurate to pipette from one of the intermediate concentrations rather than have to measure a very small amount of the most concentrated solution.

Here are some typical numbers for the solutions of known concentration:

1) We were trying for 0.1 g, but the amount of powdered dye that we weighed in a dish on the analytical balance was 0.183 g. This was placed in a 100 ml volumetric flask, and water was added to the mark.

0.183 g per 100 ml = 1.83 g per liter

2) 5 ml from #1 was added to another 100 ml flask and diluted to the mark. There are several ways to calculate;

5 to 100 is 1 to 20

1.83 / 20 = 0.0915 g/L


0.005 L (5 ml) times 1.83 g/L = 0.00915 g in 100 ml = 0.0915 g/L

3) 1 ml from # 1 was added to Flask #3 and diluted to the 100 ml mark.

0.001 L times 1.83 g/L = 0.00183 g in 100 ml = 0.0183 g/L

4) 10 ml from Flask #2 was added to Flask #4 and diluted to 100 ml.

0.01 L times 0.0915 g/L = 0.000915 g in 100 ml = 0.00915 g/L

This is not enough known solutions for a good calibration curve; eight or ten would be better. 

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  • Last update 10-Jan-95