Looking in to Chromatography With Water

Such a preparation I term a chromatogram and the corresponding method, the chromatographic method - Michael Tswett

Those were the words of the Russian Micheal Tswett who first described Chromatography in 1906 and later founded Tswett's column which is one way to separate a substance using a solvent. But what is Chromatography? In simple terms, Chromatography is where you use a solvent to split a mixture into its compounds. This solvent (e.g. water) is absorbed into the medium (stationary phase) (e.g. filter paper) and spreads carrying the fixed mixture (mobile phase) with it. The constituents of the mixture that have different strengths of bond with the medium and so are separated from the rest quicker or slower creating bands of colour on the paper. These can be identified as different chemicals allowing you to know what the mixture is. Of course, there are many different ways to do this, and many different solvents or methods, but they all achieve the same goal.

Even though now over 100 years old, chromatography is still and will be a very important tool in identifying mixtures or separating for example a drug from a substance or even creating new paint colours. It might seem boring or a complicated scientific method, but I will show a device that will help you do Chromatography anywhere and create fun and interesting patterns that will be a good way to pass the time on a hot afternoon.

Some of these supply sizes can be changed to adapt to your set-up:

• Bright and colourful water solvent pens.
• wooden strips- two 7 by 4 cm pieces and one 12.5 by 3 cm piece.
• Some filter paper ( we used coffee filter paper).
• A pipette. We used a glass pipette 18 cm long and 0.8 cm in diameter with a rubber bulb.
• A 3D printer to 3D print support materials (I have attached the files below)
• a clear plexiglass circle to fit inside your petri dish (9 cm diameter in our case). It should have a hole in the middle just enough to fit the bottom of the pipette in
• 9.5 cm diameter Petri dish

This is the openSCAD script that can be edited to adapt to your petri dish or other type of container.

 // Chromatography apparatus supports
 $fn=100;
 
 module Base(){
   cylinder(3,3,3,center=true) ;
   cube([92,2,3],center=true); 
   cube([2,92,3],center=true); 
   difference(){
     cylinder(3,46,46,center=true) ;
     translate([0,0,-1]) cylinder(10,44,44,center=true) ;
   }
 }
 
 module UpperRing() {
   difference(){
     cylinder(3,46,46,center=true) ;
     translate([0,0,-1]) cylinder(10,44,44,center=true) ;
   }
 }  
 
 // uncomment to use them
 Base();
 translate([0,0,20]) UpperRing();

The pipette is held by a simple wooden bridge with a hole at the top to fit in your pipette. The bridge is made by cutting three pieces of wood strips 1 cm thick. Two pieces are 7 by 4 cm and one is 12.5 by 3 cm large. Drill a 1cm wide hole in the centre of the large piece for the pipette. Glue or nail together in the pieces to form the bridge shown in the image above.

3D print the support for the filter paper using the STIL file provided in the Supplies section. If you use filters of different sizes then you can modify them.

First, you must create the setup to start the Chromatography as shown in the image above. Get your Petri dish or a box that fits the filter paper perfectly and put the "Base" ring in it first then you can put in your filter paper with the colour of your choice as a dot in the centre. Place the "UppeRing" ring on top. The rings are to provide a gap between the base of the petri dish and the plexiglass circle so they don't affect the results. The plexiglass circle goes next which is just a clear circle the diameter of the filter paper with a hole in the centre to stabilise the pipette.

Finally put the wooden frame over the Petri dish. You should wash the pipette and fill it with water before putting it in so that it just touches the coloured dot.

If the pipette doesn't have any bubbles in it it should automatically start dispensing water due to the Capillary action and the paint starting to diffuse and show its components. Wait 5 minutes before removing the pipette and the filter paper. Let it dry and then observe your masterpiece!

Now repeat step 1 with different water solvent pens and see what they are made of. Of course, you don't just need to do one colour at a time you can have a dot made of many different colours and see what effect it does. You can even make abstract art out of Chromatography! Get creative and don't be afraid to try different things.

If you want to find out more about amateur chromatography experiments, here are some places you can go to quench your interest:

1. Paper Chromatography, Wikipedia.
2. C.L. Strong. An amateur uses paper chromatography to separate the constituents of mixtures, Scientific American, July 1961.
3. Science Buddies site: Candy Chromatography: What Makes Those Colors?