DM - Colorimetry and visible spectroscopy

Question 1

What are the colours you see from?

Answer 1

They are the complements of those that are absorbed.

Question 2

What happens when white light hits a transition metal ion?

Answer 2

One frequency of the light is absorbed. The rest of the frequencies are transmitted and it’s the transmitted frequencies that correspond to the colour that you see.

Question 3

What are complementary colours?

Answer 3

The colours that are opposite of each other on the colour wheel.

Question 4

What happens if you mix two complementary colours of light together?

Answer 4

You get white light.

Question 5

How do you get white light?

Answer 5

By mixing any two complementary colours together.

Question 6

The colour of the transition metal solution is…

Answer 6

…the complement of the colour that is absorbed.

Question 7

If a colour appears green, what colour must be absorbed?

Answer 7

Red

Question 8

If a colour appears violet, what colour must be absorbed?

Answer 8

Yellow

Question 9

If a colour appears red, what colour must be absorbed?

Answer 9

Green

Question 10

If a colour appears yellow, what colour must be absorbed?

Answer 10

violet

Question 11

If a colour appears orange, what colour must be absorbed?

Answer 11

Blue

Question 12

If a colour appears blue, what colour must be absorbed?

Answer 12

orange

Question 13

What does the colour that a transition metal absorbs depend on?

Answer 13

The oxidation state of the ion, the type of ligand and the coordination number/shape of the complex.

Question 14

What can colorimetry be used for?

Answer 14

To find concentrations of transition metal ions in a solution.

Question 15

What is a colorimeter?

Answer 15

A meter to measure colour.

Question 16

What does a colorimeter measure?

Answer 16

The absorbance of a solution.

Question 17

Why can a colorimeter be used to find the concentrations of transition metal ions in a solution?

Answer 17

Because as their concentration increases so will the absorbance.

Question 18

How does a colorimeter work?

Answer 18

By filtering a source of white light into monochromatic light.

Question 19

Monochromatic light

Answer 19

Has a narrow frequency range, so only has one colour.

Question 20

Explain the process of colorimetry

Answer 20

1) Pick a filter that produces a colour that is absorbed by the transition metal you want to look at (otherwise it won’t be absorbed and you can’t measure any change).
2) Set the colorimeter to zero by measuring the absorbance of a blank sample.
3) After passing through the filter, the light goes through the sample where some of it is absorbed. The samples are each placed in a clean cuvette. The remaining light travels to the detector, which compares the absorbance of the sample to the blank.
4) A high absorbance reading means a lot of light has been absorbed and the sample is very concentrated.

Question 21

What is a blank sample?

Answer 21

A sample of the solvent your metal ions are dissolved in - usually just water.

Question 22

What does a blank sample do in colorimetry?

Answer 22

Tells the colorimeter what zero is, so that it is only measuring the absorbance of the metal ions and not the solvent too.

Question 23

What is a cuvette?

Answer 23

A small, rectangular tube with clear sides.

Question 24

What is a cuvette used for in colorimetry?

Answer 24

Used to hold liquid samples in experiments involving light.

In colorimetry it is designed not to absorb any of the light.

Question 25

What does a high absorbance in colorimetry mean?

Answer 25

That a lot of light has been absorbed and the sample is very concentrated.

Question 26

What does a low absorbance in colorimetry mean?

Answer 26

That not a lot of light has been absorbed and the sample is not very concentrated.

Question 27

What can colorimeters be used to find?

Answer 27

Concentrations of transition metal ions.

Question 28

What do you need to work out the concentration of a transition metal solution?

Answer 28

A calibration graph.

Question 29

How is a calibration graph made?

Answer 29

By measuring the absorbance of samples with a known concentration, usually around the expected concentration of your sample. You can then plot the absorbance of each of these samples against their concentration to get a calibration graph.

Question 30

How can different samples be made to make a calibration graph?

Answer 30

These standard samples can be made by dissolving different amounts of the metal ion in a fixed volume of water or by making consecutive dilutions of a single concentrated solution. They must contain the same metal ion as the unknown sample and be in the same solution or solvent. You should use a burette/pipette to take accurate volume readings.

Question 31

How do you use a calibration graph to find concentrations of transition metal ions?

Answer 31

When you measure the absorbance of the unknown sample you can use the standard curve to read off its concentration.

Question 32

What are the axis for a calibration graph?

Answer 32

X-axis = Concentration of ion Y-axis = Relative absorbance

Question 33

What can visible spectroscopy be used to find?

Answer 33

Concentrations of ions.

Question 34

What does visible absorption spectroscopy use?

Answer 34

A beam of monochromatic light - light of a single colour.

Question 35

White light

Answer 35

A mixture of all the different wavelengths and frequencies of visible light.

Question 36

How can white light be split into monochromatic light?

Answer 36

Using a filter.

Question 37

What machine is used in visible spectroscopy?

Answer 37

A visible spectrophotometer.

Question 38

What is a visible spectrophotometer used in?

Answer 38

Visible spectroscopy.

Question 39

Explain how visible spectroscopy is done using a visible spectrophotometer

Answer 39

1) The beam of monochromatic light is passed through a solution of the complex. A detector measures the intensity of light before and after it's passed through the solution. From this you can then calculate the absorbance (how much light the solution has absorbed). 2) Different frequencies of light are passed through the solution to produce a visible absorption spectrum. The peaks in the graph tell you which colours of light the complex absorbs most strongly. 3) Different complexes absorb different colours, so produce characteristic visible absorption spectra. This allows you to identify the complexes. You can then use a calibration curve to convert absorbance data into concentration data.

Question 40

What is a visible absorption spectrum?

Answer 40

A graph of frequency or wavelength against absorbance.

Question 41

What do the peaks in the absorption spectrum tell you?

Answer 41

Which colours of light the complex absorbs most strongly.

Question 42

What are the axis of absorbance spectra?

Answer 42

X-axis = Either wavelength or frequency. Y-axis = Absorbance.