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C1 G) Chromatography
C1 G) Chromatography
Mixtures are made from a combination of different substances that are not chemically bonded together. These different substances can be elements or compounds.
An example of a mixture is air. 78% of air is nitrogen, 21% is oxygen and the remaining 1% is other gases like argon, carbon dioxide, methane etc.
Another example of a mixture is an acid such as hydrochloric acid. The acid is a mixture of salt (HCl) and water (H2O).
We can easily separate mixtures so that we get each of the individual substances in the mixture on their own. We will be looking at a variety of different separation techniques in this section and the coming sections. The first separation technique that we are going to look at is chromatography.
Chromatography
We use chromatography to separate mixtures of coloured compounds. For example, we can use chromatography to separate the different dyes in an ink. A diagram of the set up for chromatography is shown below.
We use chromatography to separate mixtures of coloured compounds. For example, we can use chromatography to separate the different dyes in an ink. A diagram of the set up for chromatography is shown below.
We undertake chromatography by doing the following steps:
After the filter paper has dried, we have a chromatogram like what is shown below.
- The first step is to draw a pencil line near the bottom of the sheet of filter paper. We use pencil because pencil is insoluble in most solvents (we do not use pen to draw the line because the ink from the pen will rise up the filter paper as ink is soluble in most solvents). The pencil line is known as the baseline.
- We then add our sample of ink onto the pencil line at the bottom of the filter paper.
- We now place our filter paper into the solvent. When undertaking chromatography at school, the solvent is usually water, but there are a wide range of solvents that we can use, such as ethanol. The solvent that we use depends on the substances in the ink that we are testing; we choose a solvent that most of the substances in our ink dissolves in. When we place the filter paper in the solvent, we need to make sure that the baseline with the sample of ink does not go into the solvent; the baseline needs to be above the solvent. This is because we do not want the ink in our sample to dissolve into the solvent.
- The next step is to place the lid onto the top of the container. This is to stop the solvent evaporating.
- As soon as the filter paper is in the solvent, the solvent will start to rise up the filter paper. Whilst the solvent rises up the filter paper, it will take the different dyes in the ink up with it. The different dyes in the ink will move up the filter paper at different rates, which results in the different dyes separating out creating different spots on the filter paper. Each of the different spots is a different dye.
- Only the soluble dyes will move up the filter paper. Insoluble dyes will remain on the baseline.
- We leave the filter paper in the solvent until the solvent has nearly reached the top of the filter paper (the top part of the solvent on the filter paper is known as the solvent front). As soon as the solvent front is a few centimetres from the top of the filter paper, we remove the filter paper from the solvent and leave it in a place to dry.
After the filter paper has dried, we have a chromatogram like what is shown below.
There are 3 different spots on the above chromatogram, which means that the ink is made out of 3 different dyes.
Comparing Chromatograms
We can add multiple different inks to the baseline on the filter paper and compare the results on the chromatogram. We can compare the number of dyes in each of the different inks, which we do by comparing the number of spots for each of the inks on the chromatogram. We can also look for common dyes in the different inks. Common dyes will rise up the filter paper by the same amount, so if any of the spots for the different inks are at the same level, it tells us that we have a common dye between the inks (this will make more sense when we have a chromatogram to look at).
Question
I undertake chromatography for 3 different inks; ink A, ink B and ink C. The chromatogram is shown below:
We can add multiple different inks to the baseline on the filter paper and compare the results on the chromatogram. We can compare the number of dyes in each of the different inks, which we do by comparing the number of spots for each of the inks on the chromatogram. We can also look for common dyes in the different inks. Common dyes will rise up the filter paper by the same amount, so if any of the spots for the different inks are at the same level, it tells us that we have a common dye between the inks (this will make more sense when we have a chromatogram to look at).
Question
I undertake chromatography for 3 different inks; ink A, ink B and ink C. The chromatogram is shown below:
Comment on the results from the chromatogram.
Answer
The first comparison that I am going to make is about the number of dyes in each of the 3 inks:
I am now going to look for common dyes in the 3 different inks. Common dyes will rise up the filter paper by the same amount. Therefore, if there are any spots at the same level/ height, we can say that there is a common dye in both of the inks. When we look at the chromatogram, we can see that there is a spot for ink A and ink C that is at the same height; this means that there is a common dye in ink A and ink C. I have circled this common dye on the diagram below.
Answer
The first comparison that I am going to make is about the number of dyes in each of the 3 inks:
- There are 2 spots for ink A, which means that ink A is made out of 2 different dyes.
- There is 1 spot for ink B, which means that ink B is potentially a pure substance made out of one dye.
- There are 3 spots for ink C, which means that ink C is made out of 3 different dyes. One of the spots for ink C is on the baseline, which means that one of the dyes in ink C is insoluble in the solvent used.
I am now going to look for common dyes in the 3 different inks. Common dyes will rise up the filter paper by the same amount. Therefore, if there are any spots at the same level/ height, we can say that there is a common dye in both of the inks. When we look at the chromatogram, we can see that there is a spot for ink A and ink C that is at the same height; this means that there is a common dye in ink A and ink C. I have circled this common dye on the diagram below.